Axonal protein synthesis in central nervous system regeneration:is building an axon a local matter?

Neurons of the mature central nervous system(CNS,mainly the brain and spinal cord)are unable to regenerate spontaneously after a lesion,in contrast to neurons of the peripheral nervous system(PNS).While the extraneuronal environment was long thought to be limiting,evidence was given less than 15 years ago that neurons themselves are critical players of their own regeneration(Park et al.,2008).Indeed,CNS neurons show a decline of axon growth capacity as they mature and after an injury.Today,the role of axonal translation is actively explored in the paradigm of embryonic neuronal growth and in peripheral nerve injury and regeneration,but less is known about the role of local protein synthesis in regrowth of adult CNS axons.Here we discuss how the current understanding of axonal translation in the CNS may contribute to the development of novel strategies to enhance axon regeneration in the injured CNS.     

Nogo-A expression dynamically varies after spinal cord injury

The mechanism involved in neural regeneration after spinal cord injury is unclear. The myelin-derived protein Nogo-A, which is specific to the central nervous system, has been identified to negatively affect the cytoskeleton and growth program of axotomized neurons. Studies have shown that Nogo-A exerts immediate and chronic inhibitory effects on neurite outgrowth. In vivo, inhibitors of Nogo-A have been shown to lead to a marked enhancement of regenerative axon extension. We established a spinal cord injury model in rats using a free-falling weight drop device to subsequently investigate Nogo-A expression. Nogo-A mR NA and protein expression and immunoreactivity were detected in spinal cord tissue using real-time quantitative PCR, immunohistochemistry and western blot analysis. At 24 hours after spinal cord injury, Nogo-A protein and mR NA expression was low in the injured group compared with control and sham-operated groups. The levels then continued to drop further and were at their lowest at 3 days, rapidly rose to a peak after 7 days, and then gradually declined again after 14 days. These changes were observed at both the mR NA and protein level. The transient decrease observed early after injury followed by high levels for a few days indicates Nogo-A expression is time dependent. This may contribute to the lack of regeneration in the central nervous system after spinal cord injury. The dynamic variation of Nogo-A should be taken into account in the treatment of spinal cord injury.     

Apelin inhibits motor neuron apoptosis in the anterior horn following acute spinal cord and sciatic nerve injuries

Rat models of acute spinal cord injury and sciatic nerve injury were established.Apelin expression in spinal cord tissue was determined.In normal rat spinal cords,apelin expression was visible;however,2 hours post spinal cord injury,apelin expression peaked.Apelin expression increased 1 day post ligation of the sciatic nerve compared with normal rat spinal cords,and peaked at 3 days.Apelin expression was greater in the posterior horn compared with the anterior horn at each time point when compared with the normal group.The onset of neuronal apoptosis was significantly delayed following injection of apelin protein at the stump of the sciatic nerve,and the number of apoptotic cells after injury was reduced when compared with normal spinal cords.Our results indicate that apelin is expressed in the normal spinal cord and central nervous system after peripheral nerve injury.Apelin protein can reduce motor neuron apoptosis in the spinal cord anterior horn and delay the onset of apoptosis.     

Axon guidance in regeneration of the mature central nervous system:step by step

<正>Unlocking axon regeneration in the injured central nervous system: In adult mammals, central nervous system(CNS) neurons fail to regenerate a?ter a lesion, whether it is traumatic – after spinal cord injury for example – or in the case of neurodegenerative diseases. This causes axons to degenerate and neurons to die,     

Mice lacking perforin have improved regeneration of the injured femoral nerve

The role that the immune system plays after injury of the peripheral nervous system is still not completely understood.Perforin,a natural killer cell-and T-lymphocyte-derived enzyme that mediates cytotoxicity,plays important roles in autoimmune diseases,infections and central nervous system trauma,such as spinal cord injury.To dissect the roles of this single component of the immune response to injury,we tested regeneration after femoral nerve injury in perforin-deficient(Pfp^-/-)and wild-type control mice.Single frame motion analysis showed better motor recovery in Pfp^-/-mice compared with control mice at 4 and 8 weeks after injury.Retrograde tracing of the motoneuron axons regrown into the motor nerve branch demonstrated more correctly projecting motoneurons in the spinal cord of Pfp^-/-mice compared with wild-types.Myelination of regrown axons measured by g-ratio was more extensive in Pfp^-/-than in wild-type mice in the motor branch of the femoral nerve.Pfp^-/-mice displayed more cholinergic synaptic terminals around cell bodies of spinal motoneurons after injury than the injured wild-types.We histologically analyzed lymphocyte infiltration 10 days after surgery and found that in Pfp^-/-mice the number of lymphocytes in the regenerating nerves was lower than in wild-types,suggesting a closed blood-nerve barrier in Pfp^-/-mice.We conclude that perforin restricts motor recovery after femoral nerve injury owing to decreased survival of motoneurons and reduced myelination.     

Transplantation of olfactory ensheathing cells for promoting regeneration following spinal cord injury

OBJECTIVE： To investigate the status of olfactory ensheathing cells （OECs） transplantation in facilitating the regeneration of spinal cord injury. DATA SOURCES： Articles about OECs transplantation in treating spinal cord injury were searched in Pubmed database published in English from January 1981 to December 2005 by using the keywords of ＂olfactory ensheathing cells, transplantation, spinal cord injury＂. STUDY SELECTION： The data were checked primarily, literatures related to OECs transplantation and the regeneration of spinal cord injury were selected, whereas the repetitive studies and reviews were excluded. DATA EXTRACTION： Totally 43 articles about OECs transplantation and the regeneration and repair of spinal cord injury were collected, and the repetitive ones were excluded. DATA SYNTHESIS： There were 35 articles accorded with the criteria. OECs are the olfactory ensheathing glias isolated from olfactory bulb and olfactory nerve tissue. OECs have the characters of both Schwann cells in central nervous system and peripheral astrocytes. The transplanted OECs can migrate in the damaged spinal cord of host, can induce and support the regeneration, growth and extension of damaged neuritis. Besides, transgenic technique can enable it to carry some exogenous genes that promote neuronal regeneration, and express some molecules that can facilitate neural regeneration, so as to ameliorate the internal environment of nerve injury, induce the regeneration of damaged spinal cord neurons, which can stimulate the regeneration potential of the damaged spinal cord to reach the purpose of spinal cord regeneration and functional recovery. CONCLUSION： OECs are the glial cells with the energy for growth at mature phase, they can myelinize axons, secrete various biological nutrition factors, and then protect and support neurons, also facilitate neural regeneration. OECs have been successfully isolated from nasal olfactory mucosa and olfactory nerve. Therefore, autologous transplantation of OECs and objective genes modified OECs carrying various neurotrophic factors may become an effective method to treat spinal cord injury in the future.     

Heterogeneity in the regenerative abilities of central nervous system axons within species: why do some neurons regenerate better than others?

Some neurons,especially in mammalian peripheral nervous system or in lower vertebrate or in vertebrate central nervous system(CNS)regenerate after axotomy,while most mammalian CNS neurons fail to regenerate.There is an emerging consensus that neurons have different intrinsic regenerative capabilities,which theoretically could be manipulated therapeutically to improve regeneration.Population-based comparisons between"good regenerating"and"bad regenerating"neurons in the CNS and peripheral nervous system of most vertebrates yield results that are inconclusive or difficult to interpret.At least in part,this reflects the great diversity of cells in the mammalian CNS.Using mammalian nervous system imposes several methodical limitations.First,the small sizes and large numbers of neurons in the CNS make it very difficult to distinguish regenerating neurons from non-regenerating ones.Second,the lack of identifiable neurons makes it impossible to correlate biochemical changes in a neuron with axonal damage of the same neuron,and therefore,to dissect the molecular mechanisms of regeneration on the level of single neurons.This review will survey the reported responses to axon injury and the determinants of axon regeneration,emphasizing non-mammalian model organisms,which are often under-utilized,but in which the data are especially easy to interpret.     

RhoA as a target to promote neuronal survival and axon regeneration

Paralysis following spinal cord injury (SCI) is due to failure of axonal regeneration. It is believed that the capacities of neurons to regrow their axons are due partly to their intrinsic characteristics, which in turn are greatly influenced by several types of inhibitory molecules that are present, or even increased in the extracellular environment of the injured spinal cord. Many of these inhibitory molecules have been studied extensively in recent years. It has been suggested that the small GTPase RhoA is an intracellular convergence point for signaling by these extracellular inhibitory molecules, but due to the complexity of the central nervous system (CNS) in mammals, and the limitation of pharmacological tools, the specific roles of RhoA are unclear. By exploiting the anatomical and technical advantages of the lamprey CNS, we recently demonstrated that RhoA knockdown promotes true axon regeneration through the lesion site after SCI. In addition, we found that RhoA knockdown protects the large, identified reticulospinal neurons from apoptosis after their axons were axotomized in spinal cord. Therefore, manipulation of the RhoA signaling pathway may be an important approach in the development of treatments that are both neuroprotective and axon regeneration-promoting, to enhance functional recovery after SCI.     

Effects of targeted muscle reinnervation on spinal cord motor neurons in rats following tibial nerve transection

Targeted muscle reinnervation(TMR)is a surgical procedure used to transfer residual peripheral nerves from amputated limbs to targeted muscles,which allows the target muscles to become sources of motor control information for function reconstruction.However,the effect of TMR on injured motor neurons is still unclear.In this study,we aimed to explore the effect of hind limb TMR surgery on injured motor neurons in the spinal cord of rats after tibial nerve transection.We found that the reduction in hind limb motor function and atrophy in mice caused by tibial nerve transection improved after TMR.TMR enhanced nerve regeneration by increasing the number of axons and myelin sheath thickness in the tibial nerve,increasing the number of anterior horn motor neurons,and increasing the number of choline acetyltransferase-positive cells and immunofluorescence intensity of synaptophysin in rat spinal cord.Our findings suggest that TMR may enable the reconnection of residual nerve fibers to target muscles,thus restoring hind limb motor function on the injured side.     

Induced pluripotent stem cell-derived neural stem cell therapies for spinal cord injury

The greatest challenge to successful treatment of spinal cord injury is the limited regenerative capacity of the central nervous system and its inability to replace lost neurons and severed axons following injury. Neural stem cell grafts derived from fetal central nervous system tissue or embryonic stem cells have shown therapeutic promise by differentiation into neurons and glia that have the potential to form functional neuronal relays across injured spinal cord segments. However, implementation of fetal-derived or embryonic stem cell-derived neural stem cell therapies for patients with spinal cord injury raises ethical concerns. Induced pluripotent stem cells can be generated from adult somatic cells and differentiated into neural stem cells suitable for therapeutic use, thereby providing an ethical source of implantable cells that can be made in an autologous fashion to avoid problems of immune rejection. This review discusses the therapeutic potential of human induced pluripotent stem cell-derived neural stem cell transplantation for treatment of spinal cord injury, as well as addressing potential mechanisms, future perspectives and challenges.     

Axonal regeneration from the spinal cord to peripheral nerve induced by end-to-side neurorrhaphy Evidence from acetylcholinesterase staining and Fluorogold retrograde tracing

BACKGROUND： In recent years, surgeons have advocated root or trunk repair of avulsed nerve roots for overall recovery. However, donor nerves pose a major problem, because they do not contain adequate numbers of axons. Moreover, the procedures lead to nerve deficits in the donor nerve following transplantation. OBJECTIVE： To observe whether axonal regeneration occurs by end-to-side neurorrhaphy in the peripheral nerve and spinal cord. DESIGN, TIME AND SETTING： A neuroanatomical, randomized, controlled, animal study was performed at Functional Anatomy Lab in Nagoya University School of Medicine from May 2002 to July 2003. MATERIALS： Fluorogold was purchased from Fluorochrome, LLC, USA. BX50 light microscope and fluorescent microscope were purchased from Olympus, Japan. METHODS： A total of 21 rats were randomly divided into three groups, and the posterior avulsion injury model （C6-8） of the brachial plexus was performed. In the ventral root graft group, the avulsed C7 ventral roots were reanastomosed to the small anterior lateral aspect window of the spinal cord via nerve grafts. In the dorsal root graft group, the C7 dorsal roots were reanastomosed at the small pia mater window of the posterior lateral aspect of the spinal cord via nerve grafts. In the control group, the avulsed nerve roots were not repaired. MAIN OUTCOME MEASURES： The nerve grafts were collected from the ventral and dorsal root graft groups, and the C7 proximal nerve end was collected from the control group. Acetylcholinesterase staining was performed on the tissue. Fluorogold retrograde tracing technique was applied to determine the origin of the regenerating axons. RESULTS： Results showed that acetylcholine-positive axons existed in nerve grafts of the ventral and dorsal root graft groups. However, axons were not found in the avulsed nerve roots of the control group. Fluorogold retrograde tracing confirmed the presence of fluorogold-containing neurons in the ventral and dorsal horn of the ventral and dorsal root graft groups. Fluorogold-positive neurons were not observed in the control group. CONCLUSION： End-to-side neurorrhaphy induced axonal regeneration from the spinal cord to the peripheral nervous system.     

Does repair of spinal cord injury follow the evolutionary theory?

Lower vertebrates,such as fish and amphibians,and higher vertebrates in embryonic development can acquire complete regeneration of complex body structures,including the spinal cord,an important part of the central nervous system.However,with species evolution and development,this regenerative capacity gradually weakens and even disappears,but the cellular and molecular mechanisms remain poorly understood.We explored the differences in mechanisms of spinal cord regeneration capability between lower and higher vertebrates,investigated differences in their cellular and molecular mechanisms and between the spinal cord structures of lower vertebrates and mammals,such as rat and monkey,to search for theoretical evidence and therapeutic targets for nerve regeneration in human spinal cord.     

Femoral nerve regeneration and its accuracy under different injury mechanisms

Surgical accuracy has greatly improved with the advent of microsurgical techniques. However, complete functional recovery after peripheral nerve injury has not been achieved to date. The mechanisms hindering accurate regeneration of damaged axons after peripheral nerve injury are in urgent need of exploration. The present study was designed to explore the mechanisms of peripheral nerve regeneration after different types of injury. Femoral nerves of rats were injured by crushing or freezing. At 2, 3, 6, and 12 weeks after injury, axons were retrogradely labeled using 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate(Dil) and True Blue, and motor and sensory axons that had regenerated at the site of injury were counted. The number and percentage of Dil-labeled neurons in the anterior horn of the spinal cord increased over time. No significant differences were found in the number of labeled neurons between the freeze and crush injury groups at any time point. Our results confirmed that the accuracy of peripheral nerve regeneration increased with time, after both crush and freeze injury, and indicated that axonal regeneration accuracy was still satisfactory after freezing, despite the prolonged damage.     

Linking axon transport to regeneration using in vitro laser axotomy

正Spinal cord injury has devastating consequences because adult central nervous system(CNS)neurons do not regenerate their axons after injury.Two key reasons for axon regeneration failure are extrinsic inhibitory factors and a low intrinsic capacity for axon regrowth.Research has therefore focused on overcoming extrinsic growth inhibition,and enhancing intrinsic regeneration capacity.Both of these issues will need to be addressed to enable optimal repair of the injured spinal cord.     

The therapeutic potential of targeting exchange protein directly activated by cyclic adenosine 3',5'-monophosphate(Epac) for central nervous system trauma

Millions of people worldwide are affected by traumatic spinal cord injury,which usually results in permanent sensorimotor disability.Damage to the spinal cord leads to a series of detrimental events including ischaemia,haemorrhage and neuroinflammation,which over time result in further neural tissue loss.Eventually,at chronic stages of traumatic spinal cord injury,the formation of a glial scar,cystic cavitation and the presence of numerous inhibitory molecules act as physical and chemical barriers to axonal regrowth.This is further hindered by a lack of intrinsic regrowth ability of adult neurons in the central nervous system.The intracellular signalling molecule,cyclic adenosine 3′,5′-monophosphate(cAMP),is known to play many important roles in the central nervous system,and elevating its levels as shown to improve axonal regeneration outcomes following traumatic spinal cord injury in animal models.However,therapies directly targeting cAMP have not found their way into the clinic,as cAMP is ubiquitously present in all cell types and its manipulation may have additional deleterious effects.A downstream effector of cAMP,exchange protein directly activated by cAMP 2(Epac2),is mainly expressed in the adult central nervous system,and its activation has been shown to mediate the positive effects of cAMP on axonal guidance and regeneration.Recently,using ex vivo modelling of traumatic spinal cord injury,Epac2 activation was found to profoundly modulate the post-lesion environment,such as decreasing the activation of astrocytes and microglia.Pilot data with Epac2 activation also suggested functional improvement assessed by in vivo models of traumatic spinal cord injury.Therefore,targeting Epac2 in traumatic spinal cord injury could represent a novel strategy in traumatic spinal cord injury repair,and future work is needed to fully establish its therapeutic potential.     

Metallic nanoparticles for peripheral nerve regeneration: is it a feasible approach?

<正>The primary function of the peripheral nerves is to transmit signals from the spinal cord to the rest of the body,or to convey sensory information from the rest of the body to the spinal cord.In case of injury or a health disorder,this pathway can be partially or totally disrupted,resulting in pain,loss of sensation,reduced muscular strength,poor coordination,or complete paralysis.Even if peripheral nerves can spontaneously regenerate from injury,in the case of a complete nerve transection,a clinical operation must be performed in order to reconnect the portions of the injured axons.Although current clinical strategies include autografts,allografts and nerve guides,the maximum regeneration distance is limited to 25 mm.Researchers are currently focused on finding new methods and materials to improve this nerve regeneration distance in the case of gap     

Human umbilical cord Wharton’s jelly-derived oligodendrocyte precursor-like cells for axon and myelin sheath regeneration

Human umbilical mesenchymal stem cells from Wharton’s jelly of the umbilical cord were induced to differentiate into oligodendrocyte precursor-like cells in vitro. Oligodendrocyte precursor cells were transplanted into contused rat spinal cords. Immunofluorescence double staining indicated that transplanted cells survived in injured spinal cord, and differentiated into mature and immature oligodendrocyte precursor cells. Biotinylated dextran amine tracing results showed that cell transplantation promoted a higher density of the corticospinal tract in the central and caudal parts of the injured spinal cord. Luxol fast blue and toluidine blue staining showed that the volume of residual myelin was significantly increased at 1 and 2 mm rostral and caudal to the lesion epicenter after cell transplantation. Furthermore, immunofluorescence staining verified that the newly regenerated myelin sheath was derived from the central nervous system. Basso, Beattie and Bresnahan testing showed an evident behavioral recovery. These results suggest that human umbilical mesenchymal stem cell-derived oligodendrocyte precursor cells promote the regeneration of spinal axons and myelin sheaths.     

Growth associated protein 43 and neurofilament immunolabeling in the transected lumbar spinal cord of lizard indicates limited axonal regeneration

Previous cytological studies on the transected lumbar spinal cord of lizards have shown the presence of differentiating glial cells,few neurons and axons in the bridge region between the proximal and distal stumps of the spinal cord in some cases.A limited number of axons(20-50)can cross the bridge and re-connect the caudal stump of the spinal cord with small neurons located in the rostral stump of the spinal cord.This axonal regeneration appears to be related to the recovery of hind-limb movements after initial paralysis.The present study extends previous studies and shows that after transection of the lumbar spinal cord in lizards,a glial-connective tissue bridge that reconnects the rostral and caudal stumps of the interrupted spinal cord is formed at 11-34 days post-injury.Following an initial paralysis some recovery of hindlimb movements occurs within 1-3 months post-injury.Immunohistochemical and ultrastructural analysis for a growth associated protein 43(GAP-43)of 48-50 k Da shows that sparse GAP-43 positive axons are present in the proximal stump of the spinal cord but their number decreased in the bridge at 11-34 days post-transection.Few immunolabeled axons with a neurofilament protein of 200-220 k Da were seen in the bridge at 11-22 days post-transection but their number increased at 34 days and 3 months post-amputation in lizards that have recovered some hindlimb movements.Numerous neurons in the rostral and caudal stumps of the spinal cord were also labeled for GAP43,a cytoplasmic protein that is trans-located into their axonal growth cones.This indicates that GAP-43 biosynthesis is related to axonal regeneration and sprouting from neurons that were damaged by the transection.Taken together,previous studies that utilized tract-tracing technique to label the present observations confirm that a limited axonal re-connection of the transected spinal cord occurs 1-3 months post-injury in lizards.The few regenerating-sprouting axons within the bridge reconnect the caudal with the rostral stumps of the spinal cord,and likely contribute to activate the neural circuits that sustain the limited but important recovery of hind-limb movements after initial paralysis.The surgical procedures utilized in the study followed the regulations on animal care and experimental procedures under the Italian Guidelines(art.5,DL 116/92).     

Subcellular localization of Rho GTPases：implications for axon regeneration

<正>Damage to neurons in the central nervous system often leads to a permanent loss of function due to several factors,including reduced capacity of axons to regenerate and an environment that inhibits axon regeneration because of disruption of myelin and the formation of a growth-refractory glial scar around the injury(Yoon and Tuszynski,2012).Strategies to promote axon regeneration include both increasing the capacity of central nervous system axons to regenerate,as well as limiting the post-insult inhibitory environment(Gordon-Weeks and Fournier,2014).Damaged central nervous system neurons do retain some intrinsic capacity to regenerate.If this ability could be enhanced,axon regeneration and re-connection to appropriate targets could