Uncoupling neurotrophic function from nociception of nerve growth factor: what can be learned from a rare human disease?

Research progress of the effective active ingredients of Astragalus mongholicus in the treatment of diabetic peripheral neuropathy

Diabetic peripheral neuropathy (DPN) is one of the most prevalent consequences of diabetes, with a high incidence and disability rate. The DPN's pathogenesis is extremely complex and yet to be fully understood. Persistent high glucose metabolism, nerve growth factor deficiency, microvascular disease, oxidative stress, peripheral nerve cell apoptosis, immune factors, and other factors have been implicated in the pathogenesis of DPN. Astragalus mongholicus is a commonly used plant used to treat DPN in clinical settings. Its rich chemical components mainly include Astragalus polysaccharide, Astragalus saponins, Astragalus flavones, etc., which play a vital role in the treatment of DPN. This review aimed to summarize the pathogenesis of DPN and the studies on the mechanism of the effective components of Astragalus mongholicus in treating DPN. This is of great significance for the effective use of Chinese herbal medicine and the promotion of its status and influence on the world.

Lipid Nanoparticle Delivery of Chemically Modified NGFR100W mRNA Alleviates Peripheral Neuropathy

Messenger RNA (mRNA) carries genetic instructions to the cell machinery for the transient production of antigens or therapeutic proteins and shows enormous potential in vaccine development, cancer immunotherapy, protein replacement therapy, and genome engineering. Here, the synthesis of chemically modified nerve growth factor mutant (NGF^R100W ) mRNA through in vitro transcription is described. After the replacement of the original signal peptide sequence with the Ig Kappa leader sequence, codon-optimized NGF^R100W mRNA yielded high secretion of mature NGF^R100W , which promotes axon growth in PC12 cells. Using lipid nanoparticle (LNP)-delivery of N1-methylpseudouridine-modified mRNA in mice, NGF^R100W -mRNA-LNPs result in the successful expression of NGF^R100W protein, which significantly reduces nociceptive activity compared to that of NGF^WT . This indicates that NGF^R100W derived from exogenous mRNA elicited "painless" neuroprotective activity. Additionally, the therapeutic value of NGF^R100W mRNA is established in a paclitaxel-induced peripheral neuropathy model by demonstrating the rapid recovery of intraepidermal nerve fibers. The results show that in vitro-transcribed mRNA has significant flexibility in sequence design and fast in vivo functional validation of target proteins. Furthermore, the results highlight the therapeutic potential of mRNA as a supplement to beneficial proteins for preventing or reversing some chronic medical conditions, such as peripheral neuropathy.

Improved neurocognitive performance in FIV infected cats following treatment with the p75 neurotrophin receptor ligand LM11A-31

HIV rapidly infects the central nervous system (CNS) and establishes a persistent viral reservoir within microglia, perivascular macrophages and astrocytes. Inefficient control of CNS viral replication by antiretroviral therapy results in chronic inflammation and progressive cognitive decline in up to 50% of infected individuals with no effective treatment options. Neurotrophin based therapies have excellent potential to stabilize and repair the nervous system. A novel non-peptide ligand, LM11A-31, that targets the p75 neurotrophin receptor (p75^NTR) has been identified as a small bioavailable molecule capable of strong neuroprotection with minimal side effects. To evaluate the neuroprotective effects of LM11A-31 in a natural infection model, we treated cats chronically infected with feline immunodeficiency virus (FIV) with 13 mg/kg LM11A-31 twice daily over a period of 10 weeks and assessed effects on cognitive functions, open field behaviors, activity, sensory thresholds, plasma FIV, cerebrospinal fluid (CSF) FIV, peripheral blood mononuclear cell provirus, CD4 and CD8 cell counts and general physiology. Between 12 and 18 months post-inoculation, cats began to show signs of neural dysfunction in T maze testing and novel object recognition, which were prevented by LM11A-31 treatment. Anxiety-like behavior was reduced in the open field and no changes were seen in sensory thresholds. Systemic FIV titers were unaffected but treated cats exhibited a log drop in CSF FIV titers. No significant adverse effects were observed under all conditions. The data indicate that LM11A-31 is likely to be a potent adjunctive treatment for the control of neurodegeneration in HIV infected individuals.

A missense point mutation in nerve growth factor (NGFR100W) results in selective peripheral sensory neuropathy

The R100W mutation in nerve growth factor is associated with hereditary sensory autonomic neuropathy V in a Swedish family. These patients develop severe loss of perception to deep pain but with apparently normal cognitive functions. To better understand the disease mechanism, we examined a knockin mouse model of HSAN V. The homozygous mice showed significant structural deficits in intra-epidermal nerve fibers (IENFs) at birth. These mice had a total loss of pain perception at ∼2 months of age and often failed to survive to adulthood. Heterozygous mutant mice developed a progressive degeneration of small sensory fibers both behaviorally and functionally: they showed a progressive loss of IENFs starting at the age of 9 months accompanied with progressive loss of perception to painful stimuli such as noxious temperature. Quantitative analysis of lumbar 4/5 dorsal root ganglia revealed a significant reduction in small size neurons, while analysis of sciatic nerve fibers revealed the heterozygous mutant mice had no reduction in myelinated nerve fibers. Significantly, the amount of NGF secreted from mouse embryonic fibroblasts were reduced from both heterozygous and homozygous mice compared to their wild-type littermates. Interestingly, the heterozygous mice showed no apparent structural alteration in the brain: neither the anterior cingulate cortex nor the medial septum including NGF-dependent basal forebrain cholinergic neurons. Accordingly, these animals did not develop appreciable deficits in tests for brain function. Our study has thus demonstrated that the NGFR100W mutation likely affects the structure and function of peripheral sensory neurons.

Optical control of ERK and AKT signaling promotes axon regeneration and functional recovery of PNS and CNS in Drosophila

Neuroregeneration is a dynamic process synergizing the functional outcomes of multiple signaling circuits. Channelrhodopsin-based optogenetics shows the feasibility of stimulating neural repair but does not pin down specific signaling cascades. Here, we utilized optogenetic systems, optoRaf and optoAKT, to delineate the contribution of the ERK and AKT signaling pathways to neuroregeneration in live Drosophila larvae. We showed that optoRaf or optoAKT activation not only enhanced axon regeneration in both regeneration-competent and -incompetent sensory neurons in the peripheral nervous system but also allowed temporal tuning and proper guidance of axon regrowth. Furthermore, optoRaf and optoAKT differ in their signaling kinetics during regeneration, showing a gated versus graded response, respectively. Importantly in the central nervous system, their activation promotes axon regrowth and functional recovery of the thermonociceptive behavior. We conclude that non-neuronal optogenetics targets damaged neurons and signaling subcircuits, providing a novel strategy in the intervention of neural damage with improved precision.

Most cells have a built-in regeneration signaling program that allows them to divide and repair. But, in the cells of the central nervous system, which are called neurons, this program is ineffective. This is why accidents and illnesses affecting the brain and spinal cord can cause permanent damage. Reactivating regeneration in neurons could help them repair, but it is not easy. Certain small molecules can switch repair signaling programs back on. Unfortunately, these molecules diffuse easily through tissues, spreading around the body and making it hard to target individual damaged cells. This both hampers research into neuronal repair and makes treatments directed at healing damage to the nervous system more likely to have side-effects. It is unclear whether reactivating regeneration signaling in individual neurons is possible.

One way to address this question is to use optogenetics. This technique uses genetic engineering to fuse proteins that are light-sensitive to proteins responsible for relaying signals in the cell. When specific wavelengths of light hit the light-sensitive proteins, the fused signaling proteins switch on, leading to the activation of any proteins they control, for example, those involved in regeneration.

Wang et al. used optogenetic tools to determine if light can help repair neurons in fruit fly larvae. First, a strong laser light was used to damage an individual neuron in a fruit fly larva that had been genetically modified so that blue light would activate the regeneration program in its neurons. Then, Wang et al. illuminated the cell with dim blue light, switching on the regeneration program. Not only did this allow the neuron to repair itself, it also allowed the light to guide its regeneration. By focusing the blue light on the damaged end of the neuron, it was possible to guide the direction of the cell's growth as it regenerated.

Regeneration programs in flies and mammals involve similar signaling proteins, but blue light does not penetrate well into mammalian tissues. This means that further research into LEDs that can be implanted may be necessary before neuronal repair experiments can be performed in mammals. In any case, the ability to focus treatment on individual neurons paves the way for future work into the regeneration of the nervous system, and the combination of light and genetics could reveal more about how repair signals work.

Biological characteristics of dynamic expression of nerve regeneration related growth factors in dorsal root ganglia after peripheral nerve injury

The regenerative capacity of peripheral nerves is limited after nerve injury. A number of growth factors modulate many cellular behaviors, such as proliferation and migration, and may contribute to nerve repair and regeneration. Our previous study observed the dynamic changes of genes in L4-6 dorsal root ganglion after rat sciatic nerve crush using transcriptome sequencing. Our current study focused on upstream growth factors and found that a total of 19 upstream growth factors were dysregulated in dorsal root ganglions at 3, 9 hours, 1, 4, or 7 days after nerve crush, compared with the 0 hour control. Thirty-six rat models of sciatic nerve crush injury were prepared as described previously. Then, they were divided into six groups to measure the expression changes of representative genes at 0, 3, 9 hours, 1, 4 or 7 days post crush. Our current study measured the expression levels of representative upstream growth factors, including nerve growth factor, brain-derived neurotrophic factor, fibroblast growth factor 2 and amphiregulin genes, and explored critical signaling pathways and biological process through bioinformatic analysis. Our data revealed that many of these dysregulated upstream growth factors, including nerve growth factor, brain-derived neurotrophic factor, fibroblast growth factor 2 and amphiregulin, participated in tissue remodeling and axon growth-related biological processes Therefore, the experiment described the expression pattern of upstream growth factors in the dorsal root ganglia after peripheral nerve injury. Bioinformatic analysis revealed growth factors that may promote repair and regeneration of damaged peripheral nerves. All animal surgery procedures were performed in accordance with Institutional Animal Care Guidelines of Nantong University and ethically approved by the Administration Committee of Experimental Animals, China (approval No. 20170302-017) on March 2, 2017.

Localized sympathectomy reduces peripheral nerve regeneration and pain behaviors in 2 rat neuropathic pain models

Previous studies have shown that the peripheral nerve regeneration process is linked to pain in several neuropathic pain models. Other studies show that sympathetic blockade may relieve pain in some pain models and clinical conditions. This study examined reduction in peripheral nerve regeneration as one possible mechanism for relief of neuropathic pain by sympathetic blockade. A "microsympathectomy," consisting of cutting the gray rami containing sympathetic postganglionic axons where they enter the L4 and L5 spinal nerves, reduced mechanical hypersensitivity in 2 different rat neuropathic pain models. In the spinal nerve ligation model, in which some functional regeneration and reinnervation of the ligated spinal nerve can be observed, microsympathectomy reduced functional and anatomical measures of regeneration as well as expression of growth-associated protein 43 (GAP43), a regeneration-related protein. In the spared nerve injury model, in which functional reinnervation is not possible and the futile regeneration process results in formation of a neuroma, microsympathectomy reduced neuroma formation and GAP43 expression. In both models, microsympathectomy reduced macrophage density in the sensory ganglia and peripheral nerve. This corroborates previous work showing that sympathetic nerves may locally affect immune function. The results further highlight the challenge of improving pain in neuropathic conditions without inhibiting peripheral nerve regeneration that might otherwise be possible and desired.

Schwann cell interactions during the development of the peripheral nervous system

Schwann cells play a critical role in the development of the peripheral nervous system (PNS), establishing important relationships both with the extracellular milieu and other cell types, particularly neurons. In this review, we discuss various Schwann cell interactions integral to the proper establishment, spatial arrangement, and function of the PNS. We include signals that cascade onto Schwann cells from axons and from the extracellular matrix, bidirectional signals that help to establish the axo-glial relationship and how Schwann cells in turn support the axon. Further, we speculate on how Schwann cell interactions with other components of the developing PNS ultimately promote the complete construction of the peripheral nerve.

No pain, no gain? The effects of pain-promoting neuropeptides and neurotrophins on fracture healing

Neuropeptides and neurotrophins are key regulators of peripheral nociceptive nerves and contribute to the induction, sensitization, and maintenance of pain. It is now known that these peptides also regulate non-neuronal tissues, including bone. Here, we review the effects of numerous neuropeptides and neurotrophins on fracture healing. The neuropeptides calcitonin-gene related peptide (CGRP), substance P (SP), vasoactive intestinal peptide (VIP), and pituitary adenylate cyclase-activating peptide (PACAP) have varying effects on osteoclastic and osteoblastic activity. Ultimately, CGRP and SP both accelerate fracture healing, while VIP and PACAP seem to negatively impact healing. Unlike the aforementioned neuropeptides, the neurotrophins nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) have more uniform effects. Both factors upregulate osteoblastic activity, osteoclastic activity, and, in vivo, stimulate osteogenesis to promote fracture healing. Future research will need to clarify the exact mechanism by which the neuropeptides and neurotrophins influence fracture healing. Specifically, understanding the optimal expression patterns for these proteins in the fracture healing process may lead to therapies that can maximize their bone-healing capabilities and minimize their pain-promoting effects. Finally, further examination of protein-sequestering antibodies and/or small molecule agonists and antagonists may lead to new therapies that can decrease the rate of delayed union/nonunion outcomes and fracture-associated pain.

Adrenomedullin: an important participant in neurological diseases

Adrenomedullin, a peptide with multiple physiological functions in nervous system injury and disease, has aroused the interest of researchers. This review summarizes the role of adrenomedullin in neuropathological disorders, including pathological pain, brain injury and nerve regeneration, and their treatment. As a newly characterized pronociceptive mediator, adrenomedullin has been shown to act as an upstream factor in the transmission of noxious information for various types of pathological pain including acute and chronic inflammatory pain, cancer pain, neuropathic pain induced by spinal nerve injury and diabetic neuropathy. Initiation of glia-neuron signaling networks in the peripheral and central nervous system by adrenomedullin is involved in the formation and maintenance of morphine tolerance. Adrenomedullin has been shown to exert a facilitated or neuroprotective effect against brain injury including hemorrhagic or ischemic stroke and traumatic brain injury. Additionally, adrenomedullin can serve as a regulator to promote nerve regeneration in pathological conditions. Therefore, adrenomedullin is an important participant in nervous system diseases.