Introduction
Following a peripheral nerve injury (PNI), both sensory and motor axons are capable of considerable regeneration. However, functional recovery in nerve-injured patients remains very poor (Scholz et al. , 2009; Bekelis et al. , 2015). There is general agreement that the source of this poor recovery is the slow and often inefficient process of axon regeneration. Thus, experimental treatments for PNI have largely aimed at enhancing axon regeneration. The most effective of these treatments in pre-clinical studies have been those that increase the activity of injured neurons, such as exercise (Udina et al. , 2011a; Udinaet al. , 2011b; Boeltz et al. , 2013; English et al. , 2014; Gordon & English, 2016) or low frequency electrical stimulation (ES) (Al-Majed et al. , 2000b; Al-Majed et al. , 2004). The success of these approaches has been tied to an increase in the binding of brain derived neurotrophic factor (BDNF) to its high affinity tropomyosin related kinase (TrkB) receptor in the regenerating axons (Al-Majed et al. , 2000a; Wilhelm et al. , 2012). Indeed, systemic treatments with small molecule TrkB agonists (English et al. , 2013) or prodrugs that generate them (English et al. , 2022) largely recapitulate the success of the activity-dependent therapies.
Asparaginyl endopeptidase (AEP, also known as δ-secretase or legumain), a lysosomal protease implicated in the pathological features of Alzheimer’s disease (AD) (Zhang et al. , 2014; Zhang et al. , 2015), increases rapidly and persistently at the site of nerve injury. There it degrades the axonal microtubule associated protein, Tau, as well as amyloid precursor protein (APP) (English et al. , 2021). In mice treated with one hour of ES following sciatic nerve transection and repair, this degradation of Tau and APP is reduced, implying that the success of ES in promoting axon regeneration might be due to an effect on AEP. In mice null for AEP, axon regeneration after peripheral nerve injury is enhanced markedly and regeneration is not further enhanced by treatment with ES (English et al. , 2021). Thus, inhibition of AEP might be the principal mechanism by which activity-dependent therapies like ES or exercise enhance regeneration. Treatments that produce a direct inhibition of AEP after nerve injury might be a suitable test of this hypothesis.
In the present study, we used a specific AEP inhibitor, compound 11 (CP11) to evaluate this hypothesis. It was developed using high-throughput screening as a potential treatment for AD (Zhanget al. , 2017). In mouse models of AD, oral treatments with CP11 dramatically reduced Tau fragmentation and blocked the formation of Aβ from APP, both of which are products of AEP activity. We show here thatin vivo systemic treatments with CP11 reduce AEP enzymatic activity at the site of nerve injury and promote the successful regeneration of motor and sensory axons after PNI. Using primary cultures of adult dorsal root ganglion neurons, we show further that treatments with CP11 result in a marked increase in the extension of neurites that is TrkB-independent.