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The adult mammalian central nervous system (CNS) is deficient in intrinsic machineries to replace neurons lost in injuries or progressive degeneration. Various types of these neurons constitute neural circuitries wired to support vital sensory, motor, and cognitive functions. Based on the pioneer studies in cell lineage conversion, one promising strategy is to convert in vivo glial cells into neural progenitors or directly into neurons that can be eventually rewired for functional recovery. We first briefly summarize the well-studied regeneration-capable CNS in the zebrafish, focusing on their postinjury spontaneous reprogramming of the retinal Muller glia (MG). We then compare the signaling transductions, and transcriptional and epigenetic regulations in the zebrafish MGs with their mammalian counterparts, which perpetuate certain barriers against proliferation and neurogenesis and thus fail in MG-to-progenitor conversion. Next, we discuss emerging evidence from mouse studies, in which the in vivo glia-to-neuron conversion could be achieved with sequential or one-step genetic manipulations, such as the conversions from retinal MGs to interneurons, photoreceptors, or retinal ganglion cells (RGCs), as well as the conversions from midbrain astrocytes to dopaminergic or GABAergic neurons. Some of these in vivo studies showed considerable coverage of subtypes in the newly induced neurons and partial reestablishment in neural circuits and functions. Importantly, we would like to point out some crucial technical concerns that need to be addressed to convincingly show successful glia-to-neuron conversion. Finally, we present challenges and future directions in the field for better neural function recovery.

Transforming growth factor-b 1 (TGFb1) has a diverse role in astrogliosis and neuronal survival, but the underlying mechanism remains to be elucidated, especially in traumatic brain injury (TBI). Here, we show that the expression of TGFb1 was increased in the pericontusional region, accompanied with astrogliosis and neuronal loss in TBI rats. Moreover, TGFb1 knockdown not only reduced the number of neurons and inhibited astrogliosis but also resulted in a significant neurological dysfunction in rats with TBI. Subsequently, Smad3, a key downstream signal of TGFb1, was involved in pericontusional region after TBI. These findings therefore indicate that TGFb1 is involved in neuroprotection and astrogliosis, via activation of down stream Smad3 signal in the brain after injury. Refereed/Peer-reviewed

BACKGROUND This study aimed to evaluate the effects of electro-acupuncture (EA) on neuroplasticity associated with the expressions of neurotrophic factors (NTFs) and their receptors in rats subjected to spinal cord transection (SCT). MATERIAL AND METHODS A total of 144 rats were randomly divided into 3 groups (n=48 per group): sham-operated group, SCT group, and EA (electro-acupuncture) group. Rats in SCT and EA groups received spinal cord transection at T10-T11 vertebral levels. Then, EA group rats received EA treatment. Reverse transcription polymerase chain reaction was used to detect NTFs and receptors at the mRNA level. In situ hybridization (ISH) and immunohistochemistry (IHC) were used to detect the expression of NTFs and their receptors. Basso, Beattie, Bresnahan (BBB) scores and cortical somato-sensory evoked potentials (CSEP) were evaluated to assess the recovery of motor and sensory functions. We also measured BDA (Biotinylated dextran amine) axonal tracing, CGRP (Calcitonin gene-related peptide), GAP-43 (Growth-associated protein), and synaptophysin immunohistochemistry (IHC). RESULTS EA treatment led to obvious improvement in hindlimb locomotor and sensory functions. CNTF, FGF-2, and TrkB mRNA were significantly upregulated, while NGF, PDGF, TGF-b1, IGF-1, TrkA, and TrkC mRNA were concomitantly downregulated in the caudal spinal segment (CSS) following EA. Immunohistochemistry demonstrated an increased number of CGRP fibers, GAP-43, and synaptophysin profiles in the CSS in the EA rats. CONCLUSIONS EA may promote the recovery of neuroplasticity in rats subjected to SCT. This could be attributed to the systematic regulation of NTFs and their receptors after EA.

This study evaluated the effect of electro-acupuncture (EA) on the NGF, BDNF and NT-3 expression in spared L6 dorsal root ganglion (DRG) in cats subjected to bilateral removal of L1-L5 and L7-S2 DRG, using immunostaining, in situ hybridization and RT-PCR. The positive products of NGF, NT-3 protein and mRNA in the small and large neurons of spared L6 DRG in EA side increased greatly more than that of control side, while the increased BDNF was only noted in small and medium-sized neurons. RT-PCR demonstrated that the mRNA level for three factors was not influenced by EA in intact DRG, when a significant increase was seen in the spared L6 DRG of EA side. As it has been well known that DRG neurons project to the spinal cord wherein morphological plasticity has been present after DRG removal, the present results might have some bearing to the observed phenomenon.

Neural stem cells (NSC) could promote the repair after spinal cord transection (SCT), the underlying mechanism, however, still keeps to be defined. This study reported that NSC grafts significantly improved sensory and locomotor functions in adult rats with SCT in acute stage after injury. NSC could survive; differentiate towards neurons or glia lineage in vitro and vivo. Biotin dextran amine (BDA) tracing showed that little CST regeneration in the injury site, while SEP was recorded in NSC engrafted rats. Immunohistochemistry and Real time PCR confirmed that engrafted NSC expressed BDNF and increased the level of BDNF mRNA in injured site following transplantation. The present data therefore suggested that the functional recovery following SCT with NSC transplantation was correlated with the expression of BDNF, indicating the usage of BDNF with NSC transplantation in the treatment of SCI following injury.

Changes in the platelet derived growth factor (PDGF) in the spared dorsal root ganglia (DRG) and associated spinal dorsal horns were evaluated in cats subjected to unilateral removal of L1-L5 and L7-S2 DRG, sparing the L6 DRG. The number of PDGF immunopositive neurons and protein expression decreased significantly in the spared DRG and associated dorsal horns of the L3 and L6 cord segments at 3 days post-operation (dpo). It bottomed to the lowest level at 7 dpo in the DRG, then returned to the control level at 14 dpo; while in the L6 dorsal horn, it rapidly increased at 7 dpo and exceeded the control level at 14 dpo. This showed a significant upregulation in the spared DRG and associated spinal dorsal horns, especially in the L6 cord segment following a transient decrease. Meanwhile, a significant upregulation of PDGF mRNA was also seen in L6 DRG and L3 and L6 dorsal horns at 3 dpo. The upregulation of the endogenous PDGF in the said structures indicated a potential role of this factor in spinal cord plasticity after partial dorsal root ganglia removal in cats.

It is well known that some growth factors can not only rescue neurons from death, but also improve motor functions following spinal cord injury. However, their cellular distribution in situ and temporal expressions following spinal cord injury have not been determined, especially in primates. This study investigated the temporal changes in the expression of two growth factors--epidermal growth factor (EGF) and transforming growth factor-beta 1 (TGF-beta1) in the injured motoneurons of the spinal cord and the associated precentral gyrus in adult Rhesus monkeys subjected to spinal cord hemisection. Animals were allowed to survive 7, 14, 30 and 90 days post operation (dpo). Functional recovery of the hindlimbs was assessed using Tarlov scale. The immunohistological expressions of EGF and TGF-beta1 in the ventral horn motoneurons decreased sharply at 7 dpo in the cord segments caudal to the lesion site, which was followed by an increase and a peak between 14 and 30 dpo for EGF and at 90 dpo for TGF-beta1. Changes in the expression of EGF in the precentral gyrus were similar to that in the spinal cord. No TGF-beta1 immunoreactive neurons were detected in the precentral gyrus. In the spinal segments rostral to the lesion, the expressions of EGF and TGF-beta1 peaked at 30 dpo. The mRNA of EGF was detected in both spinal motoneurons and the precentral gyrus, while that of TGF-beta1, only in the spinal motoneuons, suggesting that the spinal motoneurons themselves could synthesize both the growth factors. Partial locomotor recovery in hindlimbs was seen, especially after 14 dpo. It was concluded that a possible association existed between the modulation of EGF and TGF-beta1 and the recovery of locomotor function, and their roles differed somewhat in the neuroplasticity observed after spinal cord injury in primates.

In the present study, we investigated the role of endogenous neurotrophin-3 in nerve terminal sprouting 2 months after spinal cord dorsal root rhizotomy. The left L 1-5 and L 7 -S 2 dorsal root ganglia in adult cats were exposed and removed, preserving the L 6 dorsal root ganglia. Neurotrophin-3 was mainly expressed in large neurons in the dorsal root ganglia and in some neurons in spinal lamina II. Two months after rhizotomy, the number of neurotrophin-3-positive neurons in the spared dorsal root ganglia and the density of neurite sprouts emerging from these ganglia were increased. Intraperitoneal injection of an antibody against neurotrophin-3 decreased the density of neurite sprouts. These findings suggest that endogenous neurotrophin-3 is involved in spinal cord plasticity and regeneration, and that it promotes axonal sprouting from the dorsal root ganglia after spinal cord dorsal root rhizotomy.

Neuroplasticity of the spinal cord following electroacupuncture (EA) has been demonstrated although little is known about the possible underlying mechanism. This study evaluated the effect of EA on expression of neurotrophins in the lamina II of the spinal cord, in cats subjected to dorsal rhizotomy. Cats received bilateral removal of L1-L5 and L7-S2 dorsal root ganglia (DRG, L6 DRG spared) and unilateral EA. They were sacrificed 7 days after surgery, and the L6 spinal segment removed and processed by immunohistochemistry and in situ hybridization histochemistry, to demonstrate the expression of neurotrophins. Significantly greater numbers of nerve growth factor (NGF) and neurotrophin-3 (NT-3) positive neurons, brain-derived neurotrophic factor (BDNF) immunoreactive varicosities and NT-3 positive neurons and glial cells were observed in lamina II on the acupunctured (left) side, compared to the non-acupunctured, contralateral side. Greater number of neurons expressing NGF mRNA was also observed on the acupunctured side. No signal for mRNA to BDNF and NT-3 was detected. The above findings demonstrate that EA can increase the expression of endogenous NGF at both the mRNA and protein level, and BDNF and NT-3 at the protein level. It is postulated that EA may promote the plasticity of the spinal cord by inducing increased expression of neurotrophins.

There are numerous studies reporting on the crucial roles of neurotrophins (NTFs) in neuronal survival and sprouting after spinal cord injury (SCI). But studies on endogenous changes of neurotrophins after SCI are few. In this study we explored by means of immunohistochemistry the localization of NGF, BDNF and NT-3 in the normal adult spinal cord (SC) and the changes in the expression of these chemicals in the ventral horn after right cord hemisection at T9-10. The results showed an obvious increase in the numbers of NGF, BDNF and NT-3-immunoreactive neurons in the ventral horn and also an increase in their intracellular optical density (O.D.) at 3, 7 and 21 days after cord hemisection, when compared with sham-operated rats. The expression of NGF peaked at 7 days postoperation (dpo), while BDNF and NT-3 expressions peaked at 3 dpo. Evaluation of hindlimb functions by Basso Beattie Bresnahan (BBB) scoring showed that the hindlimb support and stepping function improved very quickly at 7 dpo. This study indicated that NGF, BDNF and NT-3 could play important but different roles in the mechanisms of spinal neuroplasticity at different times after SCI.