Beta secretase activity in peripheral nerve regeneration

Application and underlying mechanism of acupuncture for the nerve repair after peripheral nerve injury: remodeling of nerve system

Peripheral nerve injury (PNI) is a structural event with harmful consequences worldwide. Due to the limited intrinsic regenerative capacity of the peripheral nerve in adults, neural restoration after PNI is difficult. Neurological remodeling has a crucial effect on the repair of the form and function during the regeneration of the peripheral nerve after the peripheral nerve is injured. Several studies have demonstrated that acupuncture is effective for PNI-induced neurologic deficits, and the potential mechanisms responsible for its effects involve the nervous system remodeling in the process of nerve repair. Moreover, acupuncture promotes neural regeneration and axon sprouting by activating related neurotrophins retrograde transport, such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), glial cell-derived neurotrophic factor (GDNF), N-cadherin, and MicroRNAs. Peripheral nerve injury enhances the perceptual response of the central nervous system to pain, causing central sensitization and accelerating neuronal cell apoptosis. Together with this, the remodeling of synaptic transmission function would worsen pain discomfort. Neuroimaging studies have shown remodeling changes in both gray and white matter after peripheral nerve injury. Acupuncture not only reverses the poor remodeling of the nervous system but also stimulates the release of neurotrophic substances such as nerve growth factors in the nervous system to ameliorate pain and promote the regeneration and repair of nerve fibers. In conclusion, the neurological remodeling at the peripheral and central levels in the process of acupuncture treatment accelerates nerve regeneration and repair. These findings provide novel insights enabling the clinical application of acupuncture in the treatment of PNI.

Central Facial Nervous System Biomolecules Involved in Peripheral Facial Nerve Injury Responses and Potential Therapeutic Strategies

Peripheral facial nerve injury leads to changes in the expression of various neuroactive substances that affect nerve cell damage, survival, growth, and regeneration. In the case of peripheral facial nerve damage, the injury directly affects the peripheral nerves and induces changes in the central nervous system (CNS) through various factors, but the substances involved in these changes in the CNS are not well understood. The objective of this review is to investigate the biomolecules involved in peripheral facial nerve damage so as to gain insight into the mechanisms and limitations of targeting the CNS after such damage and identify potential facial nerve treatment strategies. To this end, we searched PubMed using keywords and exclusion criteria and selected 29 eligible experimental studies. Our analysis summarizes basic experimental studies on changes in the CNS following peripheral facial nerve damage, focusing on biomolecules that increase or decrease in the CNS and/or those involved in the damage, and reviews various approaches for treating facial nerve injury. By establishing the biomolecules in the CNS that change after peripheral nerve damage, we can expect to identify factors that play an important role in functional recovery from facial nerve damage. Accordingly, this review could represent a significant step toward developing treatment strategies for peripheral facial palsy.

Review: Myelin clearance is critical for regeneration after peripheral nerve injury

Traumatic peripheral nerve injury occurs frequently and is a major clinical and public health problem that can lead to functional impairment and permanent disability. Despite the availability of modern diagnostic procedures and advanced microsurgical techniques, active recovery after peripheral nerve repair is often unsatisfactory. Peripheral nerve regeneration involves several critical events, including the recreation of the microenvironment and remyelination. Results from previous studies suggest that the peripheral nervous system (PNS) has a greater capacity for repair than the central nervous system. Thus, it will be important to understand myelin and myelination specifically in the PNS. This review provides an update on myelin biology and myelination in the PNS and discusses the mechanisms that promote myelin clearance after injury. The roles of Schwann cells and macrophages are considered at length, together with the possibility of exogenous intervention.

New insights in drug development for Alzheimer's disease based on microglia function

One of the biggest challenges in drug development for Alzheimer's disease (AD) is how to effectively remove deposits of amyloid-beta (Aβ). Recently, the relationship between microglia and Aβ has become a research hotspot. Emerging evidence suggests that Aβ-induced microglia-mediated neuroinflammation further aggravates the decline of cognitive function, while microglia are also involved in the process of Aβ clearance. Hence, microglia have become a potential therapeutic target for the treatment or prevention of AD. An in-depth understanding of the role played by microglia in the development of AD will help us to broaden therapeutic strategies for AD. In this review, we provide an overview of the dual roles of microglia in AD progression: the positive effect of phagocytosis of Aβ and its negative effect on neuroinflammation after over-activation. With the advantages of novel structure, high efficiency, and low toxicity, small-molecule compounds as modulators of microglial function have attracted considerable attention in the therapeutic areas of AD. In this review, we also summarize the therapeutic potential of small molecule compounds (SMCs) and their structure-activity relationship for AD treatment through modulating microglial phagocytosis and inhibiting neuroinflammation. For example, the position and number of phenolic hydroxyl groups on the B ring are the key to the activity of flavonoids, and the substitution of hydroxyl groups on the benzene ring enhances the anti-inflammatory activity of phenolic acids. This review is expected to be useful for developing effective modulators of microglial function from SMCs for the amelioration and treatment of AD.

The influence of BACE1 on macrophage recruitment and activity in the injured peripheral nerve

Following peripheral nerve injury, multiple cell types, including axons, Schwann cells, and macrophages, coordinate to promote nerve regeneration. However, this capacity for repair is limited, particularly in older populations, and current treatments are insufficient. A critical component of the regeneration response is the network of cell-to-cell signaling in the injured nerve microenvironment. Sheddases are expressed in the peripheral nerve and play a role in the regulation if this cell-to-cell signaling through cleavage of transmembrane proteins, enabling the regulation of multiple pathways through cis- and trans-cellular regulatory mechanisms. Enhanced axonal regeneration has been observed in mice with deletion of the sheddase beta-secretase (BACE1), a transmembrane aspartyl protease that has been studied in the context of Alzheimer’s disease. BACE1 knockout (KO) mice display enhanced macrophage recruitment and activity following nerve injury, although it is unclear whether this plays a role in driving the enhanced axonal regeneration. Further, it is unknown by what mechanism(s) BACE1 increases macrophage recruitment and activity. BACE1 has many substrates, several of which are known to have immunomodulatory activity. This review will discuss current knowledge of the role of BACE1 and other sheddases in peripheral nerve regeneration and outline known immunomodulatory BACE1 substrates and what potential roles they could play in peripheral nerve regeneration. Currently, the literature suggests that BACE1 and substrates that are expressed by neurons and Schwann cells are likely to be more important for this process than those expressed by macrophages. More broadly, BACE1 may play a role as an effector of immunomodulation beyond the peripheral nerve.

Axonal regeneration and sprouting as a potential therapeutic target for nervous system disorders

Nervous system disorders are prevalent health issues that will only continue to increase in frequency as the population ages. Dying-back axonopathy is a hallmark of many neurologic diseases and leads to axonal disconnection from their targets, which in turn leads to functional impairment. During the course of many of neurologic diseases, axons can regenerate or sprout in an attempt to reconnect with the target and restore synapse function. In amyotrophic lateral sclerosis (ALS), distal motor axons retract from neuromuscular junctions early in the disease-course before significant motor neuron death. There is evidence of compensatory motor axon sprouting and reinnervation of neuromuscular junctions in ALS that is usually quickly overtaken by the disease course. Potential drugs that enhance compensatory sprouting and encourage reinnervation may slow symptom progression and retain muscle function for a longer period of time in ALS and in other diseases that exhibit dying-back axonopathy. There remain many outstanding questions as to the impact of distinct disease-causing mutations on axonal outgrowth and regeneration, especially in regards to motor neurons derived from patient induced pluripotent stem cells. Compartmentalized microfluidic chambers are powerful tools for studying the distal axons of human induced pluripotent stem cells-derived motor neurons, and have recently been used to demonstrate striking regeneration defects in human motor neurons harboring ALS disease-causing mutations. Modeling the human neuromuscular circuit with human induced pluripotent stem cells-derived motor neurons will be critical for developing drugs that enhance axonal regeneration, sprouting, and reinnervation of neuromuscular junctions. In this review we will discuss compensatory axonal sprouting as a potential therapeutic target for ALS, and the use of compartmentalized microfluidic devices to find drugs that enhance regeneration and axonal sprouting of motor axons.

Expression of BACE1 in the Rat Carotid Body

This study explored the expression of BACE1 (β-amyloid precursor protein cleaving enzyme 1) in the rat carotid body and the effect of CIH (cyclic intermittent hypoxia) on the expression of BACE1. We found that BACE1 was expressed in the rat carotid body and located in the nerve endings and type II cells but not in type I cells. CIH reduced BACE1 level in the carotid body, and reoxygenation or ROS scavenger alleviated this reduction. Furthermore, we found that CIH augmented the mRNA level of PGC-1α but attenuated the mRNA level of BACE1 in the carotid body. Taken together, our results suggest that CIH promotes the production of ROS that upregulates the level of PGC-1α, which may in turn inhibits the transcription of BACE1, and that a reduction in the BACE1 level may be related to CIH-induced reversible and ROS-dependent carotid body plasticity. Our study provides a new candidate molecule for further study of the mechanism of carotid body plasticity.

Novel small molecule therapeutic agents for Alzheimer disease: Focusing on BACE1 and multi-target directed ligands

Alzheimer's Disease (AD) is a progressive neurodegenerative disorder that effects 50 million people worldwide. In this review, AD pathology and the development of novel therapeutic agents targeting AD were fully discussed. In particular, common approaches to prevent Aβ production and/or accumulation in the brain including α-secretase activators, specific γ-secretase modulators and small molecules BACE1 inhibitors were reviewed. Additionally, natural-origin bioactive compounds that provide AD therapeutic advances have been introduced. Considering AD is a multifactorial disease, the therapeutic potential of diverse multi target-directed ligands (MTDLs) that combine the efficacy of cholinesterase (ChE) inhibitors, MAO (monoamine oxidase) inhibitors, BACE1 inhibitors, phosphodiesterase 4D (PDE4D) inhibitors, for the treatment of AD are also reviewed. This article also highlights descriptions on the regulator of serotonin receptor (5-HT), metal chelators, anti-aggregants, antioxidants and neuroprotective agents targeting AD. Finally, current computational methods for evaluating the structure-activity relationships (SAR) and virtual screening (VS) of AD drugs are discussed and evaluated.

BACE1 partial deletion induces synaptic plasticity deficit in adult mice

BACE1 is the first enzyme involved in APP processing, thus it is a strong therapeutic target candidate for Alzheimer's disease. The observation of deleterious phenotypes in BACE1 Knock-out (KO) mouse models (germline and conditional) raised some concerns on the safety and tolerability of BACE1 inhibition. Here, we have employed a tamoxifen inducible BACE1 conditional Knock-out (cKO) mouse model to achieve a controlled partial depletion of BACE1 in adult mice. Biochemical and behavioural characterization was performed at two time points: 4-5 months (young mice) and 12-13 months (aged mice). A ~50% to ~70% BACE1 protein reduction in hippocampus and cortex, respectively, induced a significant reduction of BACE1 substrates processing and decrease of Aβx-40 levels at both ages. Hippocampal axonal guidance and peripheral nerve myelination were not affected. Aged mice displayed a CA1 long-term potentiation (LTP) deficit that was not associated with memory impairment. Our findings indicate that numerous phenotypes observed in germline BACE1 KO reflect a fundamental role of BACE1 during development while other phenotypes, observed in adult cKO, may be absent when partially rather than completely deleting BACE1. However, we demonstrated that partial depletion of BACE1 still induces CA1 LTP impairment, supporting a role of BACE1 in synaptic plasticity in adulthood.

Electroacupuncture promotes peripheral nerve regeneration after facial nerve crush injury and upregulates the expression of glial cell-derived neurotrophic factor

The efficacy of electroacupuncture in the treatment of peripheral facial paralysis is known, but the specific mechanism has not been clarified. Glial cell-derived neurotrophic factor (GDNF) has been shown to protect neurons by binding to N-cadherin. Our previous results have shown that electroacupuncture could increase the expression of N-cadherin mRNA in facial neurons and promote facial nerve regeneration. In this study, the potential mechanisms by which electroacupuncture promotes nerve regeneration were elucidated through assessing the effects of electroacupuncture on GDNF and N-cadherin expression in facial motoneurons of rabbits with peripheral facial nerve crush injury. New Zealand rabbits were randomly divided into a normal group (normal control, n = 21), injury group (n = 45) and electroacupuncture group (n = 45). Model rabbits underwent facial nerve crush injury only. Rabbits in the electroacupuncture group received facial nerve injury, and then underwent electroacupuncture at Yifeng (TE17), Jiache (ST6), Sibai (ST2), Dicang (ST4), Yangbai (GB14), Quanliao (SI18), and Hegu (LI4; only acupuncture, no electrical stimulation). The results showed that in behavioral assessments, the total scores of blink reflex, vibrissae movement, and position of apex nasi, were markedly lower in the EA group than those in the injury group. Hematoxylin-eosin staining of the right buccinator muscle of each group showed that the cross-sectional area of buccinator was larger in the electroacupuncture group than in the injury group on days 1, 14 and 21 post-surgery. Toluidine blue staining of the right facial nerve tissue of each group revealed that on day 14 post-surgery, there was less axonal demyelination and fewer inflammatory cells in the electroacupuncture group compared with the injury group. Quantitative real time-polymerase chain reaction showed that compared with the injury group, N-cadherin mRNA levels on days 4, 7, 14 and 21 and GDNF mRNA levels on days 4, 7 and 14 were significantly higher in the electroacupuncture group. Western blot assay displayed that compared with the injury group, the expression of GDNF protein levels on days 7, 14 and 21 were significantly upregulated in the electroacupuncture group. The histology with hematoxylin-eosin staining and Nissl staining of brainstem tissues containing facial neurons in the middle and lower part of the pons exhibited that on day 7 post-surgery, there were significantly fewer apoptotic neurons in the electroacupuncture group than in the injury group. By day 21, there was no significantly difference in the number of neurons between the electroacupuncture and normal groups. Taken together, these results have confirmed that electroacupuncture promotes regeneration of peripheral facial nerve injury in rabbits, inhibits neuronal apoptosis, and reduces peripheral inflammatory response, resulting in the recovery of facial muscle function. This is achieved by up-regulating the expression of GDNF and N-cadherin in central facial neurons.

Analysis of transcriptome sequencing of sciatic nerves in Sprague-Dawley rats of different ages

An aging-induced decrease in Schwann cell viability can affect regeneration following peripheral nerve injury in mammals. It is therefore necessary to investigate possible age-related changes in gene expression that may affect the biological function of peripheral nerves. Ten 1-week-old and ten 12-month-old healthy male Sprague-Dawley rats were divided into young (1 week old) and adult (12 months old) groups according to their ages. mRNA expression in the sciatic nerve was compared between young and adult rats using next-generation sequencing (NGS) and bioinformatics (n = 4/group). The 18 groups of differentially expressed mRNA (DEmRNAs) were also tested by quantitative reverse transcription polymerase chain reaction (n = 6/group). Results revealed that (1) compared with young rats, adult rats had 3608 groups of DEmRNAs. Of these, 2684 were groups of upregulated genes, and 924 were groups of downregulated genes. Their functions mainly involved cell viability, proliferation, differentiation, regeneration, and myelination. (2) The gene with the most obvious increase of all DEmRNAs in adult rats was Thrsp (log₂ FC = 9.01, P < 0.05), and the gene with the most obvious reduction was Col2a1 (log₂FC = –8.89, P < 0.05). (3) Gene Ontology analysis showed that DEmRNAs were mainly concentrated in oligosaccharide binding, nucleotide-binding oligomerization domain containing one signaling pathway, and peptide-transporting ATPase activity. (4) Analysis using the Kyoto Encyclopedia of Genes and Genomes showed that, with increased age, DEmRNAs were mainly enriched in steroid biosynthesis, Staphylococcus aureus infection, and graft-versus-host disease. (5) Spearman's correlation coefficient method for evaluating NGS accuracy showed that the NGS results and quantitative reverse transcription polymerase chain reaction results were positively correlated (r_s = 0.74, P < 0.05). These findings confirm a difference in sciatic nerve gene expression between adult and young rats, suggesting that, in peripheral nerves, cells and the microenvironment change with age, thus influencing the function and repair of peripheral nerves.