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.

Ultra-early amplitude decrement after repetitive nerve stimulation supports early neuromuscular junction injury in amyotrophic lateral sclerosis:a prospective cross-sectional study

The dying-back hypothesis holds that the damage to neuromuscular junctions and distal axons in amyotrophic lateral sclerosis occurs at the earliest stage of the disease.Previous basic studies have confirmed early damage to neuromuscular junctions,but it is difficult to obtain such evidence directly in clinical practice.In this prospective cross-sectional study,we recruited 22 patients with early amyotrophic lateral sclerosis with disease duration < 12 months and with clinical symptoms limited to the upper limbs.We also recruited 32 healthy controls.Repetitive nerve stimulation was performed,and patients were followed for 12 months.We found a significant change in the response to repetitive nerve stimulation in amyotrophic lateral sclerosis patients without spontaneous electromyographic activity.Patients that were prone to denervation had an increased decrement response of target muscles after repetitive nerve stimulation.These results suggest that changes in response to repetitive nerve stimulation may occur before denervation in amyotrophic lateral sclerosis patients.The damage to lower motor neurons is more obvious in patients with a higher percentage of repetitive never stimulation-related amplitude decrements.This study was approved by the Institutional Ethics Committee of Peking University Third Hospital(approval No.M2017198) on August 24,2017.

Astrocytes and retrograde degeneration of nigrostriatal dopaminergic neurons in Parkinson’s disease:removing axonal debris

Objective:The dopaminergic nigrostriatal neurons(DA cells)in healthy people present a slow degeneration with aging,which produces cellular debris throughout life.About 2%-5%of people present rapid cell degeneration of more than 50%of DA cells,which produces Parkinson's disease(PD).Neuroinflammation accelerates the cell degeneration and may be critical for the transition between the slow physiological and the rapid pathological degeneration of DA cells,particularly when it activates microglial cells of the medial forebrain bundle near dopaminergic axons.As synaptic debris produced by DA cell degeneration may trigger the parkinsonian neuroinflammation,this study investigated the removal of axonal debris produced by retrograde degeneration of DA cells,paying particular attention to the relative roles of astrocytes and microglia.Methods:Rats and mice were injected in the lateral ventricles with 6-hydroxydopamine,inducing a degeneration of dopaminergic synapses in the striatum which was not accompanied by non-selective tissue damage,microgliosis or neuroinflammation.The possible retrograde degeneration of dopaminergic axons,and the production and metabolization of DA-cell debris were studied with immunohistochemical methods and analyzed in confocal and electron microscopy images.Results:The selective degeneration of dopaminergic synapses in the striatum was followed by a retrograde degeneration of dopaminergic axons whose debris was found within spheroids of the medial forebrain bundle.These spheroids retained mitochondria and most(e.g.,tyrosine hydroxylase,the dopamine transporter protein,and amyloid precursor protein)but not all(e.g.,α-synuclein)proteins of the degenerating dopaminergic axons.Spheroids showed initial(autophagosomes)but not late(lysosomes)components of autophagy(incomplete autophagy).These spheroids were penetrated by astrocytic processes of the medial forebrain bundle,which provided the lysosomes needed to continue the degradation of dopaminergic debris.Finally,dopaminergic proteins were observed in the cell somata of astrocytes.No microgliosis or microglial phagocytosis of debris was observed in the medial forebrain bundle during the retrograde degeneration of dopaminergic axons.Conclusions:The present data suggest a physiological role of astrocytic phagocytosis of axonal debris for the medial forebrain bundle astrocytes,which may prevent the activation of microglia and the spread of retrograde axonal degeneration in PD.

TDP-43 dysregulation and neuromuscular junction disruption in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis(ALS)is a disease characterized by upper and lower motor neuron(MN)loss with a signature feature of cytoplasmic aggregates containing TDP-43,which are detected in nearly all patients.Mutations in the gene that encodes TDP-43(TARBDP)are known to result in both familial and sporadic ALS.In ALS,disruption of neuro-muscular junctions(NMJs)constitutes a critical event in disease pathogenesis,leading to denervation atrophy,motor impairments and disability.Morphological defects and impaired synaptic transmission at NMJs have been reported in several TDP-43 animal models and in vitro,linking TDP-43 dysregulation to the loss of NMJ integrity in ALS.Through the lens of the dying-back and dying-forward hypotheses of ALS,this review discusses the roles of TDP-43 related to synaptic function,with a focus on the potential molecular mechanisms occurring within MNs,skeletal muscles and glial cells that may contribute to NMJ disruption in ALS.