Beta secretase activity in peripheral nerve regeneration

Carolyn Tallon, Mohamed J. Farah

Department of Neurology at Johns Hopkins School of Medicine, Baltimore, MD, USA

How to cite this article: Tallon C, Farah MH (2017) Beta secretase activity in peripheral nerve regeneration. Neural Regen Res 12(10):1565-1574.

Funding: is work was supported by the Muscular Dystrophy Association and R01NS079339 from the National Institutes of Neurological Disease and Stroke of the National Institutes of Health.

Beta secretase activity in peripheral nerve regeneration

Carolyn Tallon, Mohamed J. Farah＊

Department of Neurology at Johns Hopkins School of Medicine, Baltimore, MD, USA

How to cite this article: Tallon C, Farah MH (2017) Beta secretase activity in peripheral nerve regeneration. Neural Regen Res 12(10):1565-1574.

While the peripheral nervous system has the capacity to regenerate following a nerve injury, it is oen at a slow rate and results in unsatisfactory recovery, leaving patients with reduced function. Many regeneration associated genes have been identified over the years, which may shed some insight into how we can manipulate this intrinsic regenerative ability to enhance repair following peripheral nerve injuries. Our lab has identified the membrane bound protease beta-site amyloid precursor protein-cleaving enzyme 1(BACE1), or beta secretase, as a potential negative regulator of peripheral nerve regeneration. When beta secretase activity levels are abolished via a null mutation in mice, peripheral regeneration is enhanced following a sciatic nerve crush injury. Conversely, when activity levels are greatly increased by overexpressing beta secretase in mice, nerve regeneration and functional recovery are impaired aer a sciatic nerve crush injury. In addition to our work, many substrates of beta secretase have been found to be involved in regulating neurite outgrowth and some have even been identified as regeneration associated genes. In this review, we set out to discuss BACE1 and its substrates with respect to axonal regeneration and speculate on the possibility of utilizing BACE1 inhibitors to enhance regeneration following acute nerve injury and potential uses in peripheral neuropathies.

peripheral nerve; axonal regeneration; beta-site amyloid precursor protein-cleaving enzyme 1

Introduction

One of the many unsolved mysteries in neuroscience is the unknown reasons why the peripheral nervous system (PNS)is able to regenerate its axons following injury while the central nervous system (CNS) cannot.e search for understanding this fundamental difference between the peripheral and central nervous systems has led to many discoveries of the cellular changes, genes, and pathways that regulate how the PNS is able to regenerate. There is now a good understanding of the steps required in order for the peripheral nerve to begin to regenerate and how these steps differ from the CNS.e first step is the clearance of axonal and myelin debris in order to make space for regenerating axons to grow into. In the PNS, macrophages, and to some extent Schwann cells, begin to phagocytose axonal and myelin debris (Stoll et al., 1993; Brück, 1997; Martini et al., 2008; Niemi et al., 2013;Mietto et al., 2015). In the CNS, resident microglia, and an influx of systemic macrophages, are responsible for debris clearance, however, there is some evidence that the CNS response to debris clearance is very slow and contributes to the limited axonal regeneration (George and Griffin, 1994;Ferguson et al., 2008; Kigerl et al., 2009). Next, the supporting cells need to provide aid to the regenerating axons by guiding them towards the correct end target.e Schwann cells of the PNS carry this out by undergoing dedifferentiation and sending out many processes to create tubes, called Bünger bands, for which the regenerating axons can enter and grow through before reaching their target and getting remyelinated (Fawcett and Keynes, 1990; Griffin et al., 2010;Toy and Namgung, 2013). In the CNS, astrocytes become activated and begin to proliferate and secrete inhibitory proteoglycans, which make up a majority of the glial scar(Toy and Namgung, 2013). This process is detrimental to axonal regeneration and provides a significant barrier for repair following CNS trauma. Another step that must occur for successful regeneration is when neurons and supporting cells alter their gene expression profiles to a pro-regenerative state.e cells of the PNS upregulate various growth factors,transcription factors, and adhesion molecules, collectively termed regeneration associated genes (RAGs), which all work together to enhance neurite outgrowth (Seijffers et al.,2007; Huebner and Strittmatter, 2009; Painter et al., 2014;Gordon and English, 2016). On the other hand, CNS cells generally do not express high levels of RAGs. In fact, oligodendrocytes express various myelin-associated inhibitors,such as Nogo-A and myelin-associated glycoprotein, while neurons express axon regeneration inhibitor molecules, like repulsive guidance molecule and Semaphorin A (Huebner and Strittmatter, 2009).

Our lab has discovered a novel role of beta-site amyloid precursor protein-cleaving enzyme 1 (BACE1), or β-secretase, in regulating peripheral axonal regeneration.BACE1 is a transmembrane protease that is involved in the cleavage and processing of a wide variety of membrane bound proteins (Sinha et al., 1999; Vassar et al., 1999; Yan et al., 1999; Hu et al., 2006; Willem et al., 2006; Kuhn et al.,2007; Hemming et al., 2009; Zhou et al., 2012; Pigoni et al.,2016). What is interesting with regards to peripheral nerve regeneration is that many of BACE1’s substrates have been implicated in the regulation of axonal regeneration and neurite outgrowth. Some have even been identified as genes which are upregulated following nerve injury. We have demonstrat-ed, using genetically modified mouse models, that β-secretase levels negatively regulate peripheral axonal regeneration following axonal injury (Figure 1). When β-secretase was genetically knocked out, we observed an increased number of regenerating axons (Farah et al., 2011; Farah, 2012; Liu et al., 2016). Mice which transgenically overexpressed human βsecretase in their neurons had impaired regeneration following a sciatic nerve crush (Tallon et al., 2017).

Despite the fact that peripheral nerves are able to regenerate, the outcome of this regeneration is often poor and insufficient (Seddon, 1942; Sunderland, 1951; Fawcett and Keynes, 1990; Griffin et al., 2010). Peripheral nerve regeneration must overcome many obstacles, including slow regeneration rates, inefficient axonal guidance, and the degeneration of end targets. Currently, the only available treatment option for patients is the surgical reconnection of the nerve (Palispis and Gupta, 2017). Various novel therapeutic options are being explored to improve outcomes such as enhanced nerve conduits, stem cell treatments, and small molecule drugs to enhance regeneration (Gordon and English,2016; Palispis and Gupta, 2017). Our recent data, together with the greater body of research on BACE1’s substrates’ impact on axonal growth, points towards a possible novel application for BACE1 inhibitors currently being investigated.

β-Secretase and Its Many Substrates

BACE1’s role in amyloid precursor protein processing

β-secretase is a transmembrane aspartyl protease which cleaves mostly type I membrane bound proteins, generating both a membrane bound and a soluble fragment. This enzyme has been well studied in the context of Alzheimer’s disease (AD) as it is one of the main enzymes responsible for generating amyloid beta plaques (Aβ), a pathological hallmark of the disease (Sinha et al., 1999; Vassar et al., 1999;Yan et al., 1999; Cai et al., 2001; Luo et al., 2001; Roberds et al., 2001). In order to generate Aβ, BACE1 cleaves amyloid precursor protein (APP) into a membrane bound C-terminal fragment (CTF) and a soluble APPβ fragment.e CTF is then further cleaved by γ-secretase to generate the Aβ fragment that then goes on to aggregate and form plaques. A reduction in β-secretase activity, either genetically or pharmacologically, leads to a decrease in the production of Aβ(Vassar et al., 1999; Cai et al., 2001; Luo et al., 2001; Roberds et al., 2001; Kennedy et al., 2016). The fact that BACE1 is one of two important enzymes involved in generating Aβ plaques has led many drug companies to investigate BACE1 inhibitors as potential therapeutics for AD (Sankaranarayanan et al., 2008; Chang et al., 2011; May et al., 2015;Kennedy et al., 2016; Cebers et al., 2017).is large interest in inhibiting BACE1 has led many to ask the question, what other substrates, if any, does BACE1 cleave?

BACE1 impacts peripheral myelinationvianeuregulin-1 type III

As it turns out, BACE1 is a rather promiscuous enzyme and over 60 putative substrates have been identified (Kitazume et al., 2001; Lichtenthaler et al., 2003; Hemming et al., 2009;Gersbacher et al., 2010; Zhang et al., 2011; Zhou et al., 2012).

Fortunately, despite the large number of substrates, BACE1 knock out (KO) mice are viable with some moderate behavioral phenotypes (Cai et al., 2001; Luo et al., 2001; Roberds et al., 2001; Laird et al., 2005; Savonenko et al., 2008). One of the more striking phenotypes observed was the reduction in the myelination of the peripheral nerves in these mice.e thickness of the myelin is markedly decreased, but not completely absent, and the presence of an increase in unmyelinated groups of axons, called Remak bundles, can be seen in nerve bundles in the periphery (Hu et al., 2006, 2008;Willem et al., 2006; Velanac et al., 2012). While reduced peripheral myelination is cause for concern, this phenotype appears to be a developmental issue. When adult mice are given a BACE1 inhibitor, others (Sankaranarayanan et al.,2008) and our lab (unpublished observation) have observed that the myelination of uninjured peripheral axons is not changed.is observation of altered myelination gave a clue as to what other substrates BACE1 may be cleaving. As it turns out, BACE1 is involved in the cleavage of neuregulin-1(NRG1) type III (Willem et al., 2006; Hu et al., 2008). NRG1 type III is an important player in myelinating the peripheral nervous system (Michailov et al., 2004; Taveggia et al., 2005).The cleavage and activation of NRG1 type III is not solely dependent on BACE1, as it is also cleaved by a disintegrin and metalloprotease 17 (ADAM17) (La Marca et al., 2011).is parallel pathway allows for some myelination to occur and may be the reason for the presence of some myelination in total KO mice.

BACE1 activity influences axonal guidanceviamultiple adhesion molecules

Another phenotype that has caused some concern for BACE1 inhibitor use is the potential issues with axonal guidance. In BACE1 KO mice, there appears to be some defects in axonal connections in the central nervous system, most notably in the olfactory bulb and mossy fiber projections (Hitt et al.,2012). This finding has led to the speculation that BACE1 inhibitors may negatively impact memory and learning by impairing synaptic plasticity (Laird et al., 2005; Savonenko et al., 2008). Since some of the potential BACE1 substrates are involved in cell-cell adhesion, such as neural cell adhesion molecule 1 (NCAM1) (Hemming et al., 2009), it likely follows that this may be the reason behind the axonal guidance issues. Indeed, two of BACE1’s identified substrates, L1 and close homolog of L1 (CHL1) (Kuhn et al., 2012), are thought to be involved in axonal guidance (Zhou et al., 2012).ese molecules are members of the immunoglobulin superfamily and have been identified as being important in proper neurogenesis. A loss of L1 or CHL1 leads to behavioral abnormalities and decreased cognitive function in mice (Montag-Sallaz et al., 2002; Pratte et al., 2003).ere are also known mutations in humans which lead to mental retardation as well as schizophrenia (Kurumaji et al., 2001; Weller and G?rtner,2001; Sakurai et al., 2002; Chen et al., 2005).

Another protein that was identified as a BACE1 substrate is contactin-2, also known as Axonin-1 or transiently expressed axonal surface glycoprotein-1 (TAG-1) (Kuhn et al.,2012).is protein is also a member of the immunoglobulin superfamily and is expressed on the surface of axons as well as the Schwann cells of the PNS (Yamamoto et al., 1986;Traka et al., 2002). A loss of contactin-2 led to a reduced axonal growth speed and impaired guidance in zebrafish(Wolman et al., 2008). Following a sciatic nerve crush, contactin-2 was found to be upregulated in Schwann cells near the lesion (Soares et al., 2005). BACE1 cleavage of contactin-2 also appears to have a negative impact on the level of contactin-2 found on the cell surface of primary mouse neurons (Gautam et al., 2014).

One more BACE1 substrate that has been identified as being an important regulator for neurite outgrowth is Seizure-related gene 6 (Sez-6) (Kuhn et al., 2012). It has been implicated as having an important role in regulating neurite outgrowth and connectivity in the developing neocortex. Cultured cortical neurons from Sez-6 null mice showed more extensive neurite branching, however, they also had a reduction in neurite length (Gunnersen et al., 2007). Another group observed a decrease in neurite length when administering a short hairpin RNA (shRNA) against Sez-6 in PC12 cells treated with nerve growth factor (NGF) (Zhang et al., 2011).

As many of BACE1’s substrates appear to be important in regulating axonal outgrowth and neurite branching,regulating BACE1 activity levels may have implications going beyond AD. Our lab has been studying the effects of BACE1 activity levels on the efficacy of peripheral nerve regeneration following acute nerve injury and hypothesize that BACE1 activity levels have an inverse relationship with regenerative efficacy (Farah et al., 2011; Tallon et al., 2017).

How do Peripheral Nerves Regenerate?

Stages of peripheral axonal regeneration following nerve injury

Axonal sprouting from neighboring intact axons contributes to reinnervation

When the distal portion of the axon is unable to regenerate,or the area of denervation is too far away from the distal site, neighboring intact axons can also send out axonal sprouts from its own axon to reinnervate nearby denervated neuromuscular junctions (Gordon and Borschel, 2017).e terminal Schwann cell sitting on the denervated neuromuscular junction (NMJ) will become activated and send out numerous processes (Reynolds and Woolf, 1992; Woolf et al., 1992; Kang et al., 2003, 2014). Eventually, a process will connect with a nearby intact axonal sprout and create a bridge between the intact axonal sprout and the denervated NMJ. This then allows the axonal sprout to be guided towards the denervated NMJ where it can then reinnervate the NMJ.ese axonal sprouts can form from multiple regions along intact axons (Brown et al., 1981). Axonal sprouts,termed nodal sprouts, can emerge along the main axon branch at the nodes of Ranvier where the myelin sheath does not cover the axon.e sprouts can also emerge closer to the NMJs, called pre-terminal sprouts, where the myelin sheath ends just before the NMJ to allow for the terminal bouton to interact with the NMJ. Finally, there can also be terminal axonal sprouts which come directly from the terminal bouton innervating the NMJ. Terminal axonal sprouts are easiest to identify as axonal sprouts as the normal morphology of the terminal bouton at the NMJ only has one axonal connection(Brown et al., 1981). In cases of disease or injury, these terminal to terminal connections can be seen and scored as an axonal sprout. While axonal sprouting to neighboring targets is an important process to maintain connections under conditions of neuronal cell loss, it is a limited process.e size of any particular motor unit is bound by the surviving neuron’s ability to metabolically sustain such a large number of connections (Gordon et al., 2004; Hegedus et al., 2008).Eventually, the motor unit becomes too large and the neuron cannot afford to continue to sprout axons.

Regeneration associated genes

Figure 1 Beta-site amyloid precursor protein-cleaving enzyme 1(BACE1) activity levels inversely regulate peripheral axonal repair.

In order to initiate axonal sprouting, the gene expression profiles change to a pro-regenerative state. With the recent advances of microarrays, and now RNA Seq, the number of genes that are differentially expressed following acute peripheral nerve injury is ever growing (Seijffers et al., 2007; Ma et al., 2011; Painter et al., 2014). Many of these regeneration associated genes (RAGs) are anti-apoptotic, transcription factors, or are involved in cytoskeleton remodeling, cell growth, and cell-cell adhesion. In order to accommodate the rapid increase in transcription, many transcription factors become upregulated following injury and have been observed to enhance regeneration, with c-Jun, signal transducer and activator of transcription 3 (STAT3) and activating transcription factor 3 (ATF3) being notable (Schwaiger et al., 2000;Raivich et al., 2004; Seijffers et al., 2007; Lind? et al., 2011).Many kinase cascades, such as the mitogen-activated protein kinase (MAPK) pathway, have been identified as RAGs,however, there is some debate over whether these pathways are involved in adult peripheral regeneration (Liu and Snider, 2001). Neurotrophic factors, such as insulin-like growth factor (IGF) 1, IGF2 and brain derived neurotrophic factor(BDNF), are also increased following injury and have been shown to positively impact axonal regeneration (Kanje et al.,1989; Glazner et al., 1993; Hammarberg et al., 2000). There is also a large upregulation of the expression of cell adhesion molecules following nerve injuries, such as CHL1 (Zhang et al., 2000). Proteins that aid in the interaction between the cell surface and the cytoskeletal components of the cell, such as growth associated protein 43 (GAP43), are upregulated following injury in order to prepare for the large cytoskeleton remodeling involved in axonal outgrowth (Skene and Willard,1981; Frey et al., 2000a; Bomze et al., 2001).e genes listed here are just a snapshot of the large number of genes whose expression levels are altered in order for neurons to undergo the extensive and complicated process of regeneration.

Problems in Peripheral Nerve Regeneration

Peripheral nerves must overcome many obstacles for proper regeneration

Peripheral nerve regeneration is further impaired by age and disease

Despite insufficient repair following axonal insult to the peripheral nervous system, either due to trauma or disease,there are currently no therapeutics available to enhance regeneration. As it stands right now, the best available treatment is surgical reattachment of the distal portion of the nerve to the proximal stump in hopes that the proximal stump can sprout into the vacated Schwann cell tubes to accelerate repair (Palispis and Gupta, 2017). Even with the best surgical techniques however, this still leaves many without full recovery. Current research to enhance regeneration focuses on developing nerve conduits which improve the connection between the proximal and distal stumps (Lin et al., 2013), this approach does not aid those who suffer from severely pinched nerves or those suffering from neurodegenerative diseases, nor does it address the slow rate of axonal growth. Other potential therapeutics have looked into electrically stimulating the nerves to enhance regeneration. Some research has shown that when the proximal portion of an injured nerve is stimulated, axonal growth is enhanced (Xu et al., 2014). However, this strategy has the potential to be an invasive and painful experience for the patient. What is needed is a minimally invasive therapy that can be administered following successful reattachment, most likely together with a nerve conduit, which can accelerate the growth rate.

Figure 2 Problems in peripheral nerveregeneration.

Figure 3 Potential mechanism for how neuronal beta-site amyloid precursor protein-cleaving enzyme 1 (BACE1) activity levels regulate peripheral axon growth.

β-Secretase as a Novel Regulator of Peripheral Nerve Regeneration

BACE1 activity levels inversely control peripheral nerve regeneration

Recently, our lab has identified a novel role for BACE1 in modulating peripheral nerve regeneration following injury.Based on our data, we have determined that BACE1 activity levels have an inverse effect on the efficacy of peripheral nerve regeneration. We observed a significant improvement in peripheral nerve regeneration following a sciatic nerve crush in BACE1 KO mice when compared with their wild type (WT) littermates (Farah et al., 2011).e nerves in the KO mice were able to grow further on top of having a greater number of regenerated axons compared with WT mice.ere was also an improvement in muscle reinnervation in the BACE1 KO mice.ese experiments provided interesting data highlighting a novel role for BACE1 in regulating peripheral nerve regeneration. However, these experiments did not narrow down where BACE1 was exerting its effects.

The enhanced regeneration could be due to increasing the outgrowth of the axons themselves, enhanced macrophage recruitment and activity, or even enhanced Schwann cell activation and axonal support.

In order to explore where BACE1 was acting and to further understand its role in regeneration, we performed similar experiments in transgenic mice overexpressing human BACE1, solely in neurons (Tallon et al., 2017). Following a sciatic nerve crush, we observed a marked decrease in peripheral nerve regeneration in mice overexpressing BACE1.These mice had reduced axonal outgrowth early on and a decrease in the number of regenerated axons. We also observed a significant reduction in the reinnervation of the gastrocnemius muscle in overexpressing mice.is impaired reinnervation was also reflected in electrophysiological experiments where transgenic mice had a significant delay in the recovery of the compound muscle action potential in the plantar muscles. Based on our own data, we have concluded that neuronal BACE1 activity levels inversely influence axonal regeneration in the periphery following injury.

While BACE1 activity’s influence over the neuronal component of peripheral nerve regeneration is an interesting piece of the puzzle, we also need to investigate the role that macrophage and Schwann cell BACE1 activity levels play in enhancing regeneration as well. Our original study using global BACE1 KO mice indicated that macrophages are not only recruited in greater numbers, but also engulf more myelin debris following a peripheral nerve injury (Farah et al.,2011). We also observed an increase in a macrophage signaling receptor, tumor necrosis factor receptor 1 (TNFR1),expression in the distal stump of the injured nerve in both BACE1 KO mice and WT mice treated with a BACE1 inhibitor (Liu et al., 2016). Studies investigating how BACE1 activity levels influence the initial activation of Schwann cells following peripheral nerve injuries has not yet been studied.However, there is evidence which indicates that BACE1 activity is involved in peripheral nerve remyelination following axonal outgrowth and is an important for proper remyelination to occur (Hu et al., 2008, 2015). In order to truly elucidate how BACE1 activity levels influence each component of the peripheral nerve regeneration machinery our lab is currently conducting experiments utilizing conditional KO mice for each of these components.

BACE1 substrates themselves influence axonal regeneration

In addition to our own data, there has been some evidence that some of the substrates of BACE1 are also involved in regulating peripheral nerve regeneration. A few studies have found that the processing of APP itself has an effect on neurite outgrowth. Knocking out APP in cultured neurons has led to enhanced neurite outgrowth (Perez et al., 1997;Young-Pearse et al., 2008), while soluble APPβ has been shown to have an inhibiting effect on neurite outgrowth (Li et al., 1997). While interesting, the effects APP processing has on peripheral nerve regeneration has been inconclusive.The processing of another BACE1 substrate, NCAM, has also been shown to have an impact on neurite outgrowth and branching. Cleavage of NCAM to produce ectodomain shedding reduces neurite outgrowth, which can be rescued by preventing the release of the ectodomain (Hinkle et al.,2006). Additionally, the cleavage of CHL1 by BACE1 also impacts neurite outgrowth. When the release of CHL1’s cleavage product is prevented, the growth cone no longer collapses and promotes neurite extension (Bar?o et al.,2015). Taken together with our own data, neuronal BACE1 activity on its various substrates may play an important role in regulating peripheral nerve regeneration by modulating cell adhesion molecules (Figure 3).

β-Secretase Inhibitors as a Potentialerapeutic Strategy for Enhancing Peripheral Nerve Regeneration

BACE1 inhibitors developed for AD have good safety profiles

As we mentioned earlier in this review, because BACE1 has a large role in generating Aβ plaques in AD, many pharmaceutical companies have made great efforts to develop small molecule BACE1 inhibitors. Early efforts were held back due to the liver toxicity seen with the Eli Lilly BACE1 inhibitor LY2886721 during their phase II trials. This led many to speculate whether BACE1 inhibitors were a feasible target for AD. However, recent advances in the development of these drugs has shown more promise. Recently, Merck announced that their latest BACE1 inhibitor, verubecestat(MK-8931), was well tolerated at various doses in patients enrolled in their phase I clinical trial (Kennedy et al., 2016).Verubecestat also showed a robust decrease in CSF Aβ production, indicative of proper target engagement. In addition to verubecestat, AstraZeneca/Lilly also have a promising small molecule BACE1 inhibitor, AZD3293, currently in phase II/III clinical trials.is drug was also well tolerated in phase I trials and showed a marked decrease in CSF Aβ production (Cebers et al., 2017).

Unfortunately, Merck recently stopped their mild/moderate Phase III trial for verubecestat due to a lack of significant clinical changes observed. Despite being a disappointing setback, this result was unsurprising to those in the field as Aβ levels tend to plateau and neuronal death is widespread by the time clinical symptoms begin to appear. While this is bad news for AD, all may not be lost for those who have sunk billions into developing BACE1 inhibitors. Since high BACE1 activity appears to have a negative impact on peripheral nerve regeneration, we hypothesize that BACE1 inhibitors may be a potential drug to fill the therapeutic void of compounds for enhancing peripheral nerve regeneration.Indeed, when we administered an older BACE1 inhibitor for 7 days following a sciatic nerve crush, we observed an apparent increase in regenerated axons as well as enhanced axonal debris clearance (Farah et al., 2011).is is a promising start for determining whether BACE1 inhibitors may find a new life as peripheral nerve regeneration enhancers.

In addition to the generation of safe, well-tolerated BACE1 inhibitors, treating peripheral nerve injuries with small molecule inhibitors may not have to worry about the blood-brain barrier issue that hinders CNS drug devel-opment. Unlike the blood-brain barrier, the nerve-blood barrier is rather leaky and would most likely allow larger molecules through at lower concentrations (Mellick and Cavanagh, 1968).is may be beneficial and could potentially allow for the development of more potent BACE1 inhibitors that while too large to cross the blood-brain barrier,may be able to cross the nerve-blood barrier. Being able to get a high efficiency with a lower dose would also lower the risk of toxicity issues that has been seen with some BACE1 inhibitors at higher doses and would give a therapeutic advantage to treating peripheral nerve injuries with BACE1 inhibitors.

Potential issues surrounding the use of BACE1 inhibitors following acute peripheral nerve injuries

While utilizing BACE1 inhibitors as a means to enhance regeneration following a peripheral nerve injury has a lot of promise, due to BACE1’s promiscuous nature, potential issues of inhibition need to be closely monitored. Firstly,because BACE1’s cleavage of NRG1 type III is involved in the myelination of peripheral nerves it is important to think about how this may negatively impact peripheral nerve remyelination as well as the existing myelin on uninjured axons. In the case of uninjured nerves, their myelination does not appear to be altered with BACE1 inhibition as we have seen in our own lab (unpublished data) as well as by other groups (Sankaranarayanan et al., 2008). For regenerating axons, remyelination would most likely be reduced for as long as BACE1 activity is being reduced. This hypomyelination would impact the ability of the axons to fire in a coherent manner and may have a negative impact on the effectiveness of the regenerated axons to engage with its targets correctly.How large this deficit would be is a factor that needs to be studied with regards to administering BACE1 inhibitors following a peripheral axon injury. While this deficit could be similar to the hypo-remyelination observed in BACE1 KO mice (Hu et al., 2008, 2015), it may be ameliorated by only administering the BACE1 inhibitors for a short time period,a couple of weeks at most, immediately following the injury.This dosage schedule would induce increased axonal outgrowth during the early stages of repair and once the inhibition of BACE1 is abolished, remyelination would be allowed to occur in a normal capacity and would hopefully allow for proper axonal firing to occur. Additionally, there is some evidence that BACE1 cleavage of NRG1 type III is not essential for the induction of myelination (Velanac et al., 2012).erefore, the amount of remyelination that occurs with BACE1 inhibition may be sufficient for proper nerve repair.

Another issue that could potentially arise when using BACE1 inhibitors is that there may be possible defects with the muscle spindles. In the absence of BACE1, mice have defects in muscle spindle formation and maturation (Cheret et al., 2013). Additionally, when BACE1 was inhibited in adult mice, Cheret et al. (2013) found that muscle spindle maintenance was impaired as was motor coordination. While this finding may be troubling for the use of BACE1 inhibitors, phase I clinical trial reports about adverse responses to BACE1 administration in human patients did not report any issues with motor coordination even at the highest doses administered (Kennedy et al., 2016).e lack of issues seen in human patients following BACE1 inhibitor treatment leads us to believe that many of the issues surrounding possible pitfalls associated with BACE1 inhibition may not be as severe as originally thought.

BACE1 inhibitors may be useful in early stage peripheral neurodegenerative diseases

We also hypothesize that these inhibitors may be helpful in the early stages of peripheral neurodegenerative disorders.In the earliest stages of motor neuron disease, axonal degeneration typically precedes neuronal cell body death (Azzouz et al., 1997; Frey et al., 2000b; Fischer et al., 2004; Schaefer et al., 2005; Pun et al., 2006; Hegedus et al., 2007). These neurons also die back at different rates depending on their susceptibility to degeneration based on multiple factors such as fiber type, size, and length (Frey et al., 2000b; Pun et al.,2006; Hegedus et al., 2007; Saxena and Caroni, 2007; Saxena et al., 2009, 2013; Tallon et al., 2016). We theorize that motor function, and therefore quality of life, may be prolonged in the early stages of these diseases if the surviving motor neurons could be encouraged to sprout and regrow in an attempt to reinnervate denervated muscle. It is true that even with a disease background, peripheral nerves do have the capacity to sprout, however limited it may be (Brown et al.,1981; Sch?fers et al., 2002; Gordon et al., 2004; Tallon et al.,2016). If the rate of axonal sprouting and outgrowth can be enhanced using a BACE1 inhibitor, it may be able to keep up with the rate of degeneration for a time, especially in slowly progressing disorders.

BACE1 inhibitor treatment may avoid axonal guidance issues

BACE1 inhibitors being used to enhance peripheral nerve regeneration may also be able to overcome many of the axonal guidance issues that have been identified. Many of these studies were performed in null mice, either of BACE1 or its substrates, and using an inhibitor that would not completely abolish BACE1 activity levels may lead to a less severe phenotype with regards to axonal guidance. Additionally,these studies were also focusing on how a loss of BACE1 or its substrates affects axonal guidance in the CNS. Due to the wide variety of BACE1 substrates, it may be possible that the substrates involved in impaired axonal guidance in the CNS may not be as important for similar functions in the peripheral nervous system. An alternative hypothesis is that these substrates may interact with different molecules and cells in the periphery which lead to a different phenotype. This makes sense, as axons in the CNS interact with oligodendrocytes and astrocytes while the PNS axons interact with Schwann cells. Indeed, our lab has not noticed any morphological differences in how NMJs are innervated in uninjured BACE1 KO mice. Another issue to note is that many of the null studies may be related to developmental defects, as the mice are lacking BACE1 and its substrates from the beginning. Using BACE1 inhibitors for the treatment of peripheral neurodegeneration would occur after the bulk of the synaptic connections had been formed, and depending on the type of neurodegeneration, would not need to be a very prolonged treatment and would allow proper guidance to occur in normal pruning.

Conclusion

Our current understanding of how BACE1 is involved in regulating axonal regeneration is still in its infancy. However, we have been able to identify an interesting relationship between BACE1 activity levels and how effective the axons are at regenerating following an acute nerve injury. Our own data has determined that BACE1 activity levels inversely regulate axonal regeneration and this finding has interesting clinical implications. If the administration of BACE1 inhibitors following an acute nerve injury can help to increase the rate and efficacy of peripheral nerve regeneration, it would fill a much-needed therapeutic gap for acute nerve injuries.Additionally, BACE1 inhibitors may also be useful in the early stages of peripheral neuropathies as a means for enhancing the rate of axonal regeneration, which may be able to compensate for the progressive loss of axons and leading to prolonged function.

Author contributions: CT and MHF equally cowrote the manuscript.

Conflicts of interest: None declared.

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Peer review: Externally peer reviewed.

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Open peer review reports:

Reviewer 1: Mohammad Reza Farahpour, Islamic Azad University Urmia Branch, Iran.

Reviewer 2: Ben Christensen, University of Utah, USA.

Azzouz M, Leclerc N, Gurney M, Warter JM, Poindron P, Borg J (1997)Progressive motor neuron impairment in an animal model of familial amyotrophic lateral sclerosis. Muscle Nerve 20:45-51.

Bar?o S, G?rtner A, Leyva-Díaz E, Demyanenko G, Munck S, Vanhoutvin T,Zhou L, Schachner M, López-Bendito G, Maness PF, De Strooper B (2015)Antagonistic effects of BACE1 and APH1B-γ-secretase control axonal guidance by regulating growth cone collapse. Cell Rep 12:1367-1376.

Bomze HM, Bulsara KR, Iskandar BJ, Caroni P, Skene JH (2001) Spinal axon regeneration evoked by replacing two growth cone proteins in adult neurons. Nat Neurosci 4:38-43.

Brück W (1997) The role of macrophages in Wallerian degeneration.Brain Pathol 7:741-752.

Brown MC, Holland RL, Hopkins WG (1981) Motor nerve sprouting.Annu Rev Neurosci 4:17-42.

Cai H, Wang Y, McCarthy D, Wen H, Borchelt DR, Price DL, Wong PC(2001) BACE1 is the major beta-secretase for generation of Abeta peptides by neurons. Nat Neurosci 4:233-234.

Cebers G, Alexander RC, Haeberlein SB, Han D, Goldwater R, Ereshefsky L, Olsson T, Ye N, Rosen L, Russell M, Maltby J, Eketjall S, Kugler AR(2017) AZD3293: Pharmacokinetic and pharmacodynamic effects in healthy subjects and patients with Alzheimer’s disease. J Alzheimers Dis 55:1039-1053.

Chang WP, Huang X, Downs D, Cirrito JR, Koelsch G, Holtzman DM,Ghosh AK, Tang J (2011) Beta-secretase inhibitor GRL-8234 rescues age-related cognitive decline in APP transgenic mice. FASEB J 25:775-784.

Chen QY, Chen Q, Feng GY, Lindpaintner K, Chen Y, Sun X, Chen Z,Gao Z, Tang J, He L (2005) Case-control association study of the close homologue of L1 (CHL1) gene and schizophrenia in the Chinese population. Schizophr Res 73:269-274.

Cheret C, Willem M, Fricker FR, Wende H, Wulf-Goldenberg A, Tahirovic S, Nave KA, Saig P, Haass C, Garratt AN, Bennett DL, Birchmeier C (2013) Bace1 and Neuregulin-1 cooperate to control formation and maintenance of muscle spindles. EMBO J 32:2015-2028.

Farah MH (2012) BACE1 influences debris clearance and axonal regeneration in injured peripheral nerve. J Peripher Nerv Syst 17 Suppl 3:30-33.

Farah MH, Pan BH, Hoffman PN, Ferraris D, Tsukamoto T, Nguyen T,Wong PC, Price DL, Slusher BS, Griffin JW (2011) Reduced BACE1 activity enhances clearance of myelin debris and regeneration of axons in the injured peripheral nervous system. J Neurosci 31:5744-5754.

Fawcett JW, Keynes RJ (1990) Peripheral nerve regeneration. Annu Rev Neurosci 13:43-60.

Ferguson AR, Christensen RN, Gensel JC, Miller BA, Sun F, Beattie EC,Bresnahan JC, Beattie MS (2008) Cell death aer spinal cord injury is exacerbated by rapid TNF alpha-induced trafficking of GluR2-lacking AMPARs to the plasma membrane. J Neurosci 28:11391-11400.

Fischer LR, Culver DG, Tennant P, Davis AA, Wang M, Castellano-Sanchez A, Khan J, Polak MA, Glass JD (2004) Amyotrophic lateral sclerosis is a distal axonopathy: evidence in mice and man. Exp Neurol 185:232-240.

Frey D, Laux T, Xu L, Schneider C, Caroni P (2000a) Shared and unique roles of CAP23 and GAP43 in actin regulation, neurite outgrowth, and anatomical plasticity. J Cell Biol 149:1443-1454.

Frey D, Schneider C, Xu L, Borg J, Spooren W, Caroni P (2000b) Early and selective loss of neuromuscular synapse subtypes with low sprouting competence in motoneuron diseases. J Neurosci 20:2534-2542.

Gautam V, D’Avanzo C, Hebisch M, Kovacs DM, Kim DY (2014) BACE1 activity regulates cell surface contactin-2 levels. Mol Neurodegener 9:4.George R, Griffin JW (1994) Delayed macrophage responses and myelin clearance during Wallerian degeneration in the central nervous system:the dorsal radiculotomy model. Exp Neurol 129:225-236.

Gersbacher MT, Kim DY, Bhattacharyya R, Kovacs DM (2010) Identification of BACE1 cleavage sites in human voltage-gated sodium channel beta 2 subunit. Mol Neurodegener 5:61.

Glazner GW, Lupien S, Miller JA, Ishii DN (1993) Insulin-like growth factor II increases the rate of sciatic nerve regeneration in rats. Neuroscience 54:791-797.

Gordon T, English AW (2016) Strategies to promote peripheral nerve regeneration: electrical stimulation and/or exercise. Eur J Neurosci 43:336-350.

Gordon T, Borschel GH (2017)e use of the rat as a model for studying peripheral nerve regeneration and sprouting aer complete and partial nerve injuries. Exp Neurol 287:331-347.

Gordon T, Hegedus J, Tam SL (2004) Adaptive and maladaptive motor axonal sprouting in aging and motoneuron disease. Neurol Res 26:174-185.

Gordon T, Tyreman N, Raji MA (2011)e basis for diminished functional recovery after delayed peripheral nerve repair. J Neurosci 31:5325-5334.

Griffin JW, Pan B, Polley MA, Hoffman PN, Farah MH (2010) Measuring nerve regeneration in the mouse. Exp Neurol 223:60-71.

Gunnersen JM, Kim MH, Fuller SJ, De Silva M, Britto JM, Hammond VE, Davies PJ, Petrou S, Faber ES, Sah P, Tan SS (2007) Sez-6 proteins affect dendritic arborization patterns and excitability of cortical pyramidal neurons. Neuron 56:621-639.

H?ke A (2006) Mechanisms of Disease: what factors limit the success of peripheral nerve regeneration in humans? Nat Clin Pract Neurol 2:448-454.

H?ke A (2011) A (heat) shock to the system promotes peripheral nerve regeneration. J Clin Invest 121:4231-4234.

Hammarberg H, Piehl F, Risling M, Cullheim S (2000) Differential regulation of trophic factor receptor mRNAs in spinal motoneurons aer sciatic nerve transection and ventral root avulsion in the rat. J Comp Neurol 426:587-601.

Hegedus J, Putman CT, Gordon T (2007) Time course of preferential motor unit loss in the SOD1 G93A mouse model of amyotrophic lateral sclerosis. Neurobiol Dis 28:154-164.

Hegedus J, Putman CT, Tyreman N, Gordon T (2008) Preferential motor unit loss in the SOD1 G93A transgenic mouse model of amyotrophic lateral sclerosis. J Physiol 586:3337-3351.

Hemming ML, Elias JE, Gygi SP, Selkoe DJ (2009) Identification of beta-secretase (BACE1) substrates using quantitative proteomics. PLoS One 4:e8477.

Hinkle CL, Diestel S, Lieberman J, Maness PF (2006) Metalloprotease-induced ectodomain shedding of neural cell adhesion molecule (NCAM).J Neurobiol 66:1378-1395.

Hitt B, Riordan SM, Kukreja L, Eimer WA, Rajapaksha TW, Vassar R(2012) β-Site amyloid precursor protein (APP)-cleaving enzyme 1(BACE1)-deficient mice exhibit a close homolog of L1 (CHL1) lossof-function phenotype involving axon guidance defects. J Biol Chem 287:38408-38425.

Hu X, Hu J, Dai L, Trapp B, Yan R (2015) Axonal and Schwann cell BACE1 is equally required for remyelination of peripheral nerves. J Neurosci 35:3806-3814.

Hu X, Hicks CW, He W, Wong P, Macklin WB, Trapp BD, Yan R (2006)Bace1 modulates myelination in the central and peripheral nervous system. Nat Neurosci 9:1520-1525.

Hu X, He W, Diaconu C, Tang X, Kidd GJ, Macklin WB, Trapp BD, Yan R (2008) Genetic deletion of BACE1 in mice affects remyelination of sciatic nerves. FASEB J 22:2970-2980.

Huebner EA, Strittmatter SM (2009) Axon regeneration in the peripheral and central nervous systems. Results Probl Cell Differ 48:339-351.

Kang H, Tian L, Mikesh M, Lichtman JW,ompson WJ (2014) Terminal Schwann cells participate in neuromuscular synapse remodeling during reinnervation following nerve injury. J Neurosci 34:6323-6333.

Kanje M, Skottner A, Sjoberg J, Lundborg G (1989) Insulin-like growth factor I (IGF-I) stimulates regeneration of the rat sciatic nerve. Brain Res 486:396-398.

Kennedy ME, Stamford AW, Chen X, Cox K, Cumming JN, Dockendorf MF, Egan M, Ereshefsky L, Hodgson RA, Hyde LA, Jhee S, Kleijn HJ,Kuvelkar R, Li W, Mattson BA, Mei H, Palcza J, Scott JD, Tanen M,Troyer MD, et al. (2016)e BACE1 inhibitor verubecestat (MK-8931)reduces CNS beta-amyloid in animal models and in Alzheimer’s disease patients. Sci Transl Med 8:363ra150.

Kigerl KA, Gensel JC, Ankeny DP, Alexander JK, Donnelly DJ, Popovich PG (2009) Identification of two distinct macrophage subsets with divergent effects causing either neurotoxicity or regeneration in the injured mouse spinal cord. J Neurosci 29:13435-13444.

Kitazume S, Tachida Y, Oka R, Shirotani K, Saido TC, Hashimoto Y (2001)Alzheimer’s beta-secretase, beta-site amyloid precursor protein-cleaving enzyme, is responsible for cleavage secretion of a Golgi-resident sialyltransferase. Proc Natl Acad Sci U S A 98:13554-13559.

Kuhn PH, Marjaux E, Imhof A, De Strooper B, Haass C, Lichtenthaler SF (2007) Regulated intramembrane proteolysis of the interleukin-1 receptor II by alpha-, beta-, and gamma-secretase. J Biol Chem 282:11982-11995.

Kuhn PH, Koroniak K, Hogl S, Colombo A, Zeitschel U, Willem M,Volbracht C, Schepers U, Imhof A, Hoffmeister A, Haass C, Ro?ner S,Br?se S, Lichtenthaler SF (2012) Secretome protein enrichment identifies physiological BACE1 protease substrates in neurons. EMBO J 31:3157-3168.

Kurumaji A, Nomoto H, Okano T, Toru M (2001) An association study between polymorphism of L1CAM gene and schizophrenia in a Japanese sample. Am J Med Genet 105:99-104.

La Marca R, Cerri F, Horiuchi K, Bachi A, Feltri ML, Wrabetz L, Blobel CP, Quattrini A, Salzer JL, Taveggia C (2011) TACE (ADAM17) inhibits Schwann cell myelination. Nat Neurosci 14:857-865.

Laird FM, Cai H, Savonenko AV, Farah MH, He K, Melnikova T, Wen H, Chiang HC, Xu G, Koliatsos VE, Borchelt DR, Price DL, Lee HK,Wong PC (2005) BACE1, a major determinant of selective vulnerability of the brain to amyloid-beta amyloidogenesis, is essential for cognitive, emotional, and synaptic functions. J Neurosci 25:11693-11709.

Li HL, Roch JM, Sundsmo M, Otero D, Sisodia S, Thomas R, Saitoh T(1997) Defective neurite extension is caused by a mutation in amyloid beta/A4 (A beta) protein precursor found in familial Alzheimer’s disease. J Neurobiol 32:469-480.

Lichtenthaler SF, Dominguez DI, Westmeyer GG, Reiss K, Haass C, Saig P, De Strooper B, Seed B (2003)e cell adhesion protein P-selectin glycoprotein ligand-1 is a substrate for the aspartyl protease BACE1. J Biol Chem 278:48713-48719.

Lin MY, Manzano G, Gupta R (2013) Nerve allografts and conduits in peripheral nerve repair. Hand Clin 29:331-348.

Lind? H, Sk?ld MK, Ochsmann T (2011) Activating transcription factor 3,a useful marker for regenerative response aer nerve root injury. Front Neurol 2:30.

Liu L, Fissel JA, Tasnim A, Borzan J, Gocke A, Calabresi PA, Farah MH(2016) Increased TNFR1 expression and signaling in injured peripheral nerves of mice with reduced BACE1 activity. Neurobiol Dis 93:21-27.

Liu RY, Snider WD (2001) Different signaling pathways mediate regenerative versus developmental sensory axon growth. J Neurosci 21:RC164.

Luo Y, Bolon B, Kahn S, Bennett BD, Babu-Khan S, Denis P, Fan W,Kha H, Zhang J, Gong Y, Martin L, Louis JC, Yan Q, Richards WG,Citron M, Vassar R (2001) Mice deficient in BACE1, the Alzheimer’s beta-secretase, have normal phenotype and abolished beta-amyloid generation. Nat Neurosci 4:231-232.

Ma CH, Omura T, Cobos EJ, Latremoliere A, Ghasemlou N, Brenner GJ,van Veen E, Barrett L, Sawada T, Gao F, Coppola G, Gertler F, Costigan M, Geschwind D, Woolf CJ (2011) Accelerating axonal growth promotes motor recovery aer peripheral nerve injury in mice. J Clin Invest 121:4332-4347.

Martini R, Fischer S, López-Vales R, David S (2008) Interactions between Schwann cells and macrophages in injury and inherited demyelinating disease. Glia 56:1566-1577.

May PC, Willis BA, Lowe SL, Dean RA, Monk SA, Cocke PJ, Audia JE,Boggs LN, Borders AR, Brier RA, Calligaro DO, Day TA, Ereshefsky L,Erickson JA, Gevorkyan H, Gonzales CR, James DE, Jhee SS, Komjathy SF, Li L, et al. (2015)e potent BACE1 inhibitor LY2886721 elicits robust central Abeta pharmacodynamic responses in mice, dogs, and humans. J Neurosci 35:1199-1210.

Mellick R, Cavanagh JB (1968)e function of the perineurium and its relation to the flow phenomenon within the endoneurial spaces. Proc Aust Assoc Neurol 5:521-525.

Michailov GV, Sereda MW, Brinkmann BG, Fischer TM, Haug B, Birchmeier C, Role L, Lai C, Schwab MH, Nave KA (2004) Axonal neuregulin-1 regulates myelin sheath thickness. Science 304:700-703.

Mietto BS, Mostacada K, Martinez AM (2015) Neurotrauma and inflammation: CNS and PNS responses. Mediators Inflamm 2015:251204.

Montag-Sallaz M, Schachner M, Montag D (2002) Misguided axonal projections, neural cell adhesion molecule 180 mRNA upregulation, and altered behavior in mice deficient for the close homolog of L1. Mol Cell Biol 22:7967-7981.

Niemi JP, DeFrancesco-Lisowitz A, Roldán-Hernández L, Lindborg JA,Mandell D, Zigmond RE (2013) A critical role for macrophages near axotomized neuronal cell bodies in stimulating nerve regeneration. J Neurosci 33:16236-16248.

Painter MW, Brosius Lutz A, Cheng YC, Latremoliere A, Duong K, Miller CM, Posada S, Cobos EJ, Zhang AX, Wagers AJ, Havton LA, Barres B,Omura T, Woolf CJ (2014) Diminished Schwann cell repair responses underlie age-associated impaired axonal regeneration. Neuron 83:331-343.

Palispis WA, Gupta R (2017) Surgical repair in humans aer traumatic nerve injury provides limited functional neural regeneration in adults.Exp Neurol 290:106-114.

Perez RG, Zheng H, Van der Ploeg LH, Koo EH (1997)e beta-amyloid precursor protein of Alzheimer’s disease enhances neuron viability and modulates neuronal polarity. J Neurosci 17:9407-9414.

Pigoni M, Wanngren J, Kuhn PH, Munro KM, Gunnersen JM, Takeshima H, Feederle R, Voytyuk I, De Strooper B, Levasseur MD, Hrupka BJ, Muller SA, Lichtenthaler SF (2016) Seizure protein 6 and its homolog seizure 6-like protein are physiological substrates of BACE1 in neurons. Mol Neurodegener 11:67.

Pratte M, Rougon G, Schachner M, Jamon M (2003) Mice deficient for the close homologue of the neural adhesion cell L1 (CHL1) display alterations in emotional reactivity and motor coordination. Behav Brain Res 147:31-39.

Pun S, Santos AF, Saxena S, Xu L, Caroni P (2006) Selective vulnerability and pruning of phasic motoneuron axons in motoneuron disease alleviated by CNTF. Nat Neurosci 9:408-419.

Raivich G, Bohatschek M, Da Costa C, Iwata O, Galiano M, Hristova M,Nateri AS, Makwana M, Riera-Sans L, Wolfer DP, Lipp HP, Aguzzi A,Wagner EF, Behrens A (2004)e AP-1 transcription factor c-Jun is required for efficient axonal regeneration. Neuron 43:57-67.

Reynolds ML, Woolf CJ (1992) Terminal Schwann cells elaborate extensive processes following denervation of the motor endplate. J Neurocytol 21:50-66.

Roberds SL, Anderson J, Basi G, Bienkowski MJ, Branstetter DG, Chen KS, Freedman SB, Frigon NL, Games D, Hu K, Johnson-Wood K, Kappenman KE, Kawabe TT, Kola I, Kuehn R, Lee M, Liu W, Motter R,Nichols NF, Power M, et al. (2001) BACE knockout mice are healthy despite lacking the primary beta-secretase activity in brain: implications for Alzheimer’s disease therapeutics. Hum Mol Genet 10:1317-1324.

Sakurai K, Migita O, Toru M, Arinami T (2002) An association between a missense polymorphism in the close homologue of L1 (CHL1, CALL)gene and schizophrenia. Mol Psychiatry 7:412-415.

Sanders FK, Whitteridge D (1946) Conduction velocity and myelin thickness in regenerating nerve fibres. J Physiol 105:152-174.

Sankaranarayanan S, Price EA, Wu G, Crouthamel MC, Shi XP, Tugusheva K, Tyler KX, Kahana J, Ellis J, Jin L, Steele T, Stachel S, Coburn C, Simon AJ (2008) In vivo beta-secretase 1 inhibition leads to brain Abeta lowering and increased alpha-secretase processing of amyloid precursor protein without effect on neuregulin-1. J Pharmacol Exper 324:957-969.

Savonenko AV, Melnikova T, Laird FM, Stewart KA, Price DL, Wong PC(2008) Alteration of BACE1-dependent NRG1/ErbB4 signaling and schizophrenia-like phenotypes in BACE1-null mice. Proc Natl Acad Sci U S A 105:5585-5590.

Saxena S, Caroni P (2007) Mechanisms of axon degeneration: from development to disease. Prog Neurobiol 83:174-191.

Saxena S, Cabuy E, Caroni P (2009) A role for motoneuron subtype-selective ER stress in disease manifestations of FALS mice. Nat Neurosci 12:627-636.

Saxena S, Roselli F, Singh K, Leptien K, Julien JP, Gros-Louis F, Caroni P (2013) Neuroprotection through excitability and mTOR required in ALS motoneurons to delay disease and extend survival. Neuron 80:80-96.

Sch?fers M, Schmidt C, Vogel C, Toyka KV, Sommer C (2002) Tumor necrosis factor-alpha (TNF) regulates the expression of ICAM-1 predominantly through TNF receptor 1 aer chronic constriction injury of mouse sciatic nerve. Acta Neuropathol 104:197-205.

Schaefer AM, Sanes JR, Lichtman JW (2005) A compensatory subpopulation of motor neurons in a mouse model of amyotrophic lateral sclerosis. J Comp Neurol 490:209-219.

Schwaiger FW, Hager G, Schmitt AB, Horvat A, Hager G, Streif R, Spitzer C, Gamal S, Breuer S, Brook GA, Nacimiento W, Kreutzberg GW (2000)Peripheral but not central axotomy induces changes in Janus kinases(JAK) and signal transducers and activators of transcription (STAT).Eur J Neurosci 12:1165-1176.

Seddon HJ (1942) A classification of nerve injuries. Br Med J 2:237-239.

Seijffers R, Mills CD, Woolf CJ (2007) ATF3 increases the intrinsic growth state of DRG neurons to enhance peripheral nerve regeneration. J Neurosci 27:7911-7920.

Sinha S, Anderson JP, Barbour R, Basi GS, Caccavello R, Davis D, Doan M, Dovey HF, Frigon N, Hong J, Jacobson-Croak K, Jewett N, Keim P, Knops J, Lieberburg I, Power M, Tan H, Tatsuno G, Tung J, Schenk D, et al. (1999) Purification and cloning of amyloid precursor protein beta-secretase from human brain. Nature 402:537-540.

Skene JH, Willard M (1981) Characteristics of growth-associated polypeptides in regenerating toad retinal ganglion cell axons. J Neurosci 1:419-426.

Soares S, Traka M, von Boxberg Y, Bouquet C, Karagogeos D, Nothias F(2005) Neuronal and glial expression of the adhesion molecule TAG-1 is regulated aer peripheral nerve lesion or central neurodegeneration of adult nervous system. Eur J Neurosci 21:1169-1180.

Stoll G, Griffin JW, Li CY, Trapp BD (1989) Wallerian degeneration in the peripheral nervous system: participation of both Schwann cells and macrophages in myelin degradation. J Neurocytol 18:671-683.

Stoll G, Li CY, Trapp BD, Griffin JW (1993) Expression of NGF-receptors during immune-mediated and lysolecithin-induced demyelination of the peripheral nervous system. J Neurocytol 22:1022-1029.

Sunderland S (1951) A classification of peripheral nerve injuries producing loss of function. Brain 74:491-516.

Tallon C, Rockenstein E, Masliah E, Farah MH (2017) Increased BACE1 activity inhibits peripheral nerve regeneration aer injury. Neurobiol Dis 106:147-157.

Tallon C, Russell KA, Sakhalkar S, Andrapallayal N, Farah MH (2016)Length-dependent axo-terminal degeneration at the neuromuscular synapses of type II muscle in SOD1 mice. Neuroscience 312:179-189.

Taveggia C, Zanazzi G, Petrylak A, Yano H, Rosenbluth J, Einheber S, Xu X, Esper RM, Loeb JA, Shrager P, Chao MV, Falls DL, Role L, Salzer JL (2005) Neuregulin-1 type III determines the ensheathment fate of axons. Neuron 47:681-694.

Toy D, Namgung U (2013) Role of glial cells in axonal regeneration. Exp Neurobiol 22:68-76.

Traka M, Dupree JL, Popko B, Karagogeos D (2002)e neuronal adhesion protein TAG-1 is expressed by Schwann cells and oligodendrocytes and is localized to the juxtaparanodal region of myelinated fibers.J Neurosci 22:3016-3024.

Vassar R, Bennett BD, Babu-Khan S, Kahn S, Mendiaz EA, Denis P,Teplow DB, Ross S, Amarante P, Loeloff R, Luo Y, Fisher S, Fuller J,Edenson S, Lile J, Jarosinski MA, Biere AL, Curran E, Burgess T, Louis JC, et al. (1999) Beta-secretase cleavage of Alzheimer’s amyloid precursor protein by the transmembrane aspartic protease BACE. Science 286:735-741.

Velanac V, Unterbarnscheidt T, Hinrichs W, Gummert MN, Fischer TM,Rossner MJ, Trimarco A, Brivio V, Taveggia C, Willem M, Haass C,Mobius W, Nave KA, Schwab MH (2012) Bace1 processing of NRG1 type III produces a myelin-inducing signal but is not essential for the stimulation of myelination. Glia 60:203-217.

Verdú E, Ceballos D, Vilches JJ, Navarro X (2000) Influence of aging on peripheral nerve function and regeneration. J Peripher Nerv Syst 5:191-208.

Weller S, G?rtner J (2001) Genetic and clinical aspects of X-linked hydrocephalus (L1 disease): Mutations in the L1CAM gene. Hum Mutat 18:1-12.

Willem M, Garratt AN, Novak B, Citron M, Kaufmann S, Rittger A, De-Strooper B, Saig P, Birchmeier C, Haass C (2006) Control of peripheral nerve myelination by the beta-secretase BACE1. Science 314:664-666.

Wolman MA, Sittaramane VK, Essner JJ, Yost HJ, Chandrasekhar A,Halloran MC (2008) Transient axonal glycoprotein-1 (TAG-1) and laminin-alpha1 regulate dynamic growth cone behaviors and initial axon direction in vivo. Neural Dev 3:6.

Woolf CJ, Reynolds ML, Chong MS, Emson P, Irwin N, Benowitz LI(1992) Denervation of the motor endplate results in the rapid expression by terminal Schwann cells of the growth-associated protein GAP-43. J Neurosci 12:3999-4010.

Wu P, Chawla A, Spinner RJ, Yu C, Yaszemski MJ, Windebank AJ, Wang H (2014) Key changes in denervated muscles and their impact on regeneration and reinnervation. Neural Regen Res 9:1796-1809.

Xu C, Kou Y, Zhang P, Han N, Yin X, Deng J, Chen B, Jiang B (2014)Electrical stimulation promotes regeneration of defective peripheral nerves after delayed repair intervals lasting under one month. PLoS One 9:e105045.

Yamamoto M, Boyer AM, Crandall JE, Edwards M, Tanaka H (1986) Distribution of stage-specific neurite-associated proteins in the developing murine nervous system recognized by a monoclonal antibody. J Neurosci 6:3576-3594.

Yan R, Bienkowski MJ, Shuck ME, Miao H, Tory MC, Pauley AM, Brashier JR, Stratman NC, Mathews WR, Buhl AE, Carter DB, Tomasselli AG, Parodi LA, Heinrikson RL, Gurney ME (1999) Membrane-anchored aspartyl protease with Alzheimer’s disease beta-secretase activity. Nature 402:533-537.

Young-Pearse TL, Chen AC, Chang R, Marquez C, Selkoe DJ (2008)Secreted APP regulates the function of full-length APP in neurite outgrowth through interaction with integrin beta1. Neural Dev 3:15.

Zhang J, Zhang J, Wu Y, Zhou Y, Shao Z, Kang X, Ma L, Li M, Liu L,Shi H (2011) Sez-6 may play an important role in neurite outgrowth through the PKCgamma signaling pathways. Z Naturforsch C 66:614-620.

Zhang Y, Roslan R, Lang D, Schachner M, Lieberman AR, Anderson PN(2000) Expression of CHL1 and L1 by neurons and glia following sciatic nerve and dorsal root injury. Mol Cell Neurosci 16:71-86.

Zhou L, Bar?o S, Laga M, Bockstael K, Borgers M, Gijsen H, Annaert W,Moechars D, Mercken M, Gevaert K, De Strooper B (2012)e neural cell adhesion molecules L1 and CHL1 are cleaved by BACE1 protease in vivo. J Biol Chem 287:25927-25940.

＊Correspondence to:

Mohamed H. Farah, Ph.D.,mfarah2@jhmi.edu.

orcid:

0000-0002-4745-8030

(Mohamed H. Farah)

10.4103/1673-5374.217319

Accepted: 2017-09-26