Local events within the injured and regenerating peripheral nerve trunk: the role of the microenvironment and microcirculation

Peripheral nerves respond to injury in a unique fashion. Changes in the local milieu of the injured nerve trunk may determine both the likelihoods of regeneration and the production of neuropathic pain. For example, changes in local blood flow within this microenvironment may reflect several interesting features of the repair process. Crushed and sectioned nerves develop hyperemia, or rises in local blood flow rather than ischemia, and these rises appear to be mediated by one of several mechanisms. Firstly, vasa nervorum, the blood vessels that supply nerve trunks, are innervated by peptidergic fibers that may participate in "neurogenic" inflammation, as occurs in other innervated tissues. Secondly, following a nerve section or crush, early rises in blood flow may be mediated by local deposition of calcitonin gene-related peptide and nitric oxide from axonal end bulbs. Thirdly, brisk angiogenesis accompanies a proliferative phase in the proximal nerve stump that accompanies mast cell proliferation and axonal sprouting. Axonal sprouting, in turn, may be supported by local trophic factors, and the success of subsequent regrowth down the distal nerve stump may be determined by the microenvironment it encounters on its road to recovery. Better understanding of these and other events in injured nerve trunks is needed to help solve the two cardinal problems of peripheral nerve injuries: functional disability from impaired regeneration and the development of disabling neuropathic pain.Biomedical Reviews 1997; 8: 37-54

Beyond Trophic Factors: Exploiting the Intrinsic Regenerative Properties of Adult Neurons

Injuries and diseases of the peripheral nervous system (PNS) are common but frequently irreversible. It is often but mistakenly assumed that peripheral neuron regeneration is robust without a need to be improved or supported. However, axonal lesions, especially those involving proximal nerves rarely recover fully and injuries generally are complicated by slow and incomplete regeneration. Strategies to enhance the intrinsic growth properties of reluctant adult neurons offer an alternative approach to consider during regeneration. Since axons rarely regrow without an intimately partnered Schwann cell (SC), approaches to enhance SC plasticity carry along benefits to their axon partners. Direct targeting of molecules that inhibit growth cone plasticity can inform important regenerative strategies. A newer approach, a focus of our laboratory, exploits tumor suppressor molecules that normally dampen unconstrained growth. However several are also prominently expressed in stable adult neurons. During regeneration their ongoing expression “brakes” growth, whereas their inhibition and knockdown may enhance regrowth. Examples have included phosphatase and tensin homolog deleted on chromosome ten (PTEN), a tumor suppressor that inhibits PI3K/pAkt signaling, Rb1, the protein involved in retinoblastoma development, and adenomatous polyposis coli (APC), a tumor suppressor that inhibits β-Catenin transcriptional signaling and its translocation to the nucleus. The identification of several new targets to manipulate the plasticity of regenerating adult peripheral neurons is exciting. How they fit with canonical regeneration strategies and their feasibility require additional work. Newer forms of nonviral siRNA delivery may be approaches for molecular manipulation to improve regeneration

Ndel1 Promotes Axon Regeneration via Intermediate Filaments

Failure of axons to regenerate following acute or chronic neuronal injury is attributed to both the inhibitory glial environment and deficient intrinsic ability to re-grow. However, the underlying mechanisms of the latter remain unclear. In this study, we have investigated the role of the mammalian homologue of aspergillus nidulans NudE, Ndel1, emergently viewed as an integrator of the cytoskeleton, in axon regeneration. Ndel1 was synthesized de novo and upregulated in crushed and transected sciatic nerve axons, and, upon injury, was strongly associated with neuronal form of the intermediate filament (IF) Vimentin while dissociating from the mature neuronal IF (Neurofilament) light chain NF-L. Consistent with a role for Ndel1 in the conditioning lesion-induced neurite outgrowth of Dorsal Root Ganglion (DRG) neurons, the long lasting in vivo formation of the neuronal Ndel1/Vimentin complex was associated with robust axon regeneration. Furthermore, local silencing of Ndel1 in transected axons by siRNA severely reduced the extent of regeneration in vivo. Thus, Ndel1 promotes axonal regeneration; activating this endogenous repair mechanism may enhance neuroregeneration during acute and chronic axonal degeneration

AI is a viable alternative to high throughput screening: a 318-target study

: High throughput screening (HTS) is routinely used to identify bioactive small molecules. This requires physical compounds, which limits coverage of accessible chemical space. Computational approaches combined with vast on-demand chemical libraries can access far greater chemical space, provided that the predictive accuracy is sufficient to identify useful molecules. Through the largest and most diverse virtual HTS campaign reported to date, comprising 318 individual projects, we demonstrate that our AtomNet® convolutional neural network successfully finds novel hits across every major therapeutic area and protein class. We address historical limitations of computational screening by demonstrating success for target proteins without known binders, high-quality X-ray crystal structures, or manual cherry-picking of compounds. We show that the molecules selected by the AtomNet® model are novel drug-like scaffolds rather than minor modifications to known bioactive compounds. Our empirical results suggest that computational methods can substantially replace HTS as the first step of small-molecule drug discovery

Unleashing Intrinsic Growth Pathways in Regenerating Peripheral Neurons

Common mechanisms of peripheral axon regeneration are recruited following diverse forms of damage to peripheral nerve axons. Whether the injury is traumatic or disease related neuropathy, reconnection of axons to their targets is required to restore function. Supporting peripheral axon regrowth, while not yet available in clinics, might be accomplished from several directions focusing on one or more of the complex stages of regrowth. Direct axon support, with follow on participation of supporting Schwann cells is one approach, emphasized in this review. However alternative approaches might include direct support of Schwann cells that instruct axons to regrow, manipulation of the inflammatory milieu to prevent ongoing bystander axon damage, or use of inflammatory cytokines as growth factors. Axons may be supported by a growing list of growth factors, extending well beyond the classical neurotrophin family. The understanding of growth factor roles continues to expand but their impact experimentally and in humans has faced serious limitations. The downstream signaling pathways that impact neuron growth have been exploited less frequently in regeneration models and rarely in human work, despite their promise and potency. Here we review the major regenerative signaling cascades that are known to influence adult peripheral axon regeneration. Within these pathways there are major checkpoints or roadblocks that normally check unwanted growth, but are an impediment to robust growth after injury. Several molecular roadblocks, overlapping with tumour suppressor systems in oncology, operate at the level of the perikarya. They have impacts on overall neuron plasticity and growth. A second approach targets proteins that largely operate at growth cones. Addressing both sites might offer synergistic benefits to regrowing neurons. This review emphasizes intrinsic aspects of adult peripheral axon regeneration, emphasizing several molecular barriers to regrowth that have been studied in our laboratory

Angiogenesis at the site of neuroma formation in transected peripheral nerve

Investiture of new microvessels within an injured peripheral nerve trunk may determine the success that the local environment has in promoting axonal sprouting and regeneration. We therefore examined microvessel investment of 24 h–14 d proximal nerve stump preparations in rat sciatic nerves. The stumps, later destined to form neuromas, were created by sciatic nerve transection with resection of distal branches to prevent distal reinnervation. Microvessels were studied in the proximal stump in semithin whole mount sections of nerve and by analysis of India ink perfused microvessel profiles. Quantitative image analysis was made of the luminal profiles of vessels perfused with India ink from unfixed sections of the stumps, contralateral uninjured nerves and sham-exposed but uninjured nerves. Evidence of angiogenesis was observed in stumps 7 d after transection, indicated by a rise in the total numbers of perfused microvessels and in the numbers of 2–6 μm diameter perfused microvessels. There was a shift in the histogram of the percentage of perfused microvessels towards the 2–4 μm range and a reduction in the mean microvessel luminal area in the stumps. By 14 d, new microvessels were larger, indicated by an increase in total luminal area. New microvessels were prominent in the epineurial connective tissue or between layers of perineurial cells of former fascicles. Microvessels probably share a battery of trophic signals with other proliferating cellular elements in the milieu of the injured peripheral nerve trunk

Neurotrophins and other growth factors in the regenerative milieu of proximal nerve stump tips

Classic ideas on mechanisms for axon sprouting and nerve regeneration from peripheral nerves suggest that there is a prominent role for neurotrophin support. There has been comparatively less attention towards features of the regenerative process that develop from the proximal nerve trunk without the support of target tissues or the denervated trunk of a peripheral nerve. We studied early (2–14 d) expression of local growth factors in proximal nerve stump tips of transected sciatic nerves in rats. Immunohistochemical labelling was used to address specific deposition of BDNF, NGF, NT-3, bFGF, CNTF and IGF-1. We observed a unique localisation of BDNF, and to a much lesser extent, NGF in mast cells of injured nerve trunks but they were also observed in intact uninjured nerves. Macrophages did not express either BDNF or NGF. CNTF and IGF-1 were expressed in Schwann cells of intact nerves and stumps. We did not observe bFGF or NT-3 expression in any of the samples we studied. Mast cells may represent an important reservoir of BDNF in peripheral nerves