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.