Intrinsic neuronal determinants that promote axonal sprouting and elongation

Enhancing structural plasticity of PC12 neurons during differentiation and neurite regeneration with a catalytically inactive mutant version of the zRICH protein

BACKGROUND

Studies of the molecular mechanisms of nerve regeneration have led to the discovery of several proteins that are induced during successful nerve regeneration. RICH proteins were identified as proteins induced during the regeneration of the optic nerve of teleost fish. These proteins are 2',3'-cyclic nucleotide, 3'-phosphodiesterases that can bind to cellular membranes through a carboxy-terminal membrane localization domain. They interact with the tubulin cytoskeleton and are able to enhance neuronal structural plasticity by promoting the formation of neurite branches.

RESULTS

PC12 stable transfectant cells expressing a fusion protein combining a red fluorescent protein with a catalytically inactive mutant version of zebrafish RICH protein were generated. These cells were used as a model to analyze effects of the protein on neuritogenesis. Differentiation experiments showed a 2.9 fold increase in formation of secondary neurites and a 2.4 fold increase in branching points. A 2.2 fold increase in formation of secondary neurites was observed in neurite regeneration assays.

CONCLUSIONS

The use of a fluorescent fusion protein facilitated detection of expression levels. Two computer-assisted morphometric analysis methods indicated that the catalytically inactive RICH protein induced the formation of branching points and secondary neurites both during differentiation and neurite regeneration. A procedure based on analysis of random field images provided comparable results to classic neurite tracing methods.

Maturation-Dependent Differences in the Re-innervation of the Denervated Dentate Gyrus by Sprouting Associational and Commissural Mossy Cell Axons in Organotypic Tissue Cultures of Entorhinal Cortex and Hippocampus

Sprouting of surviving axons is one of the major reorganization mechanisms of the injured brain contributing to a partial restoration of function. Of note, sprouting is maturation as well as age-dependent and strong in juvenile brains, moderate in adult and weak in aged brains. We have established a model system of complex organotypic tissue cultures to study sprouting in the dentate gyrus following entorhinal denervation. Entorhinal denervation performed after 2 weeks postnatally resulted in a robust, rapid, and very extensive sprouting response of commissural/associational fibers, which could be visualized using calretinin as an axonal marker. In the present study, we analyzed the effect of maturation on this form of sprouting and compared cultures denervated at 2 weeks postnatally with cultures denervated at 4 weeks postnatally. Calretinin immunofluorescence labeling as well as time-lapse imaging of virally-labeled (AAV2-hSyn1-GFP) commissural axons was employed to study the sprouting response in aged cultures. Compared to the young cultures commissural/associational sprouting was attenuated and showed a pattern similar to the one following entorhinal denervation in adult animals in vivo. We conclude that a maturation-dependent attenuation of sprouting occurs also in vitro, which now offers the chance to study, understand and influence maturation-dependent differences in brain repair in these culture preparations.

Augmented Buyang Huanwu Decoction facilitates axonal regeneration after peripheral nerve transection through the regulation of inflammatory cytokine production

ETHNOPHARMACOLOGICAL RELEVANCE

Herbal formulation Buyang Huanwu Decoction (BYHWD) has been used to treat cardiovascular disorders including cerebral ischemia. Recent studies showed its effects on promoting axonal regeneration after nerve injury. However, compositional reformulation supplemented with herbal components that regulates inflammation may increase its efficacy for nerve repair.

AIM OF THE STUDY

We prepared a new herbal decoction by adding selected herbal components to BYHWD (augmented BYHWD; ABHD) and investigated the effect of ABHD on the production of inflammatory cytokines and axonal regeneration using an animal model of nerve transection and coaptation (NTC).

MATERIALS AND METHODS

A rat model of NTC was performed on the sciatic nerve. The sciatic nerve and dorsal root ganglion (DRG) were isolated and used for immunofluorescence staining and western blot analysis. DRG tissue was also used to prepare primary neuron culture and the length of neurites was analyzed. Sensorimotor nerve activities were assessed by rotarod and von Frey tests.

RESULTS

Three herbal components that facilitated neurite outgrowth were chosen to formulate ABHD. ABHD administration into the sciatic nerve 1 week or 3 months after NTC facilitated axonal regeneration. Cell division cycle 2 (Cdc2) and brain-derived neurotrophic factor (BDNF) proteins were induced from the reconnected distal portion of the sciatic nerve and the levels were further elevated by in vivo administration of ABHD. Phospho-Erk1/2 level was increased by ABHD treatment as well, implying its role in mediating retrograde transport of BDNF signals into the neuronal cell body. Production of inflammatory cytokines IL-1β and TNF-α was induced in the reconnected nerve but attenuated by ABHD treatment. Behavioral tests revealed that ABHD treatment improved functional recovery of sensorimotor activities.

CONCLUSIONS

A newly formulated ABHD is effective at regulating the production of inflammatory cytokines and promoting axonal regeneration after nerve transection and may be considered to develop therapeutic strategies for peripheral nerve injury disorders.

Re-innervation of the Denervated Dentate Gyrus by Sprouting Associational and Commissural Mossy Cell Axons in Organotypic Tissue Cultures of Entorhinal Cortex and Hippocampus

Collateral sprouting of surviving axons contributes to the synaptic reorganization after brain injury. To study this clinically relevant phenomenon, we used complex organotypic tissue cultures of mouse entorhinal cortex (EC) and hippocampus (H). Single EC-H cultures were generated to analyze associational sprouting, and double EC-H cultures were used to evaluate commissural sprouting of mossy cells in the dentate gyrus (DG) following entorhinal denervation. Entorhinal denervation (transection of the perforant path) was performed at 14 days in vitro (DIV) and associational/commissural sprouting was assessed at 28 DIV. First, associational sprouting was studied in genetically hybrid EC-H cultures of beta-actin-GFPtg and wild-type mice. Using calretinin as a marker, associational axons were found to re-innervate almost the entire entorhinal target zone. Denervation experiments performed with EC-H cultures of Thy1-YFPtg mice, in which mossy cells are YFP-positive, confirmed that the overwhelming majority of sprouting associational calretinin-positive axons are mossy cell axons. Second, we analyzed associational/commissural sprouting by combining wild-type EC-H cultures with calretinin-deficient EC-H cultures. In these cultures, only wild-type mossy cells contain calretinin, and associational and commissural mossy cell collaterals can be distinguished using calretinin as a marker. Nearly the entire DG entorhinal target zone was re-innervated by sprouting of associational and commissural mossy cell axons. Finally, viral labeling of newly formed associational/commissural axons revealed a rapid post-lesional sprouting response. These findings demonstrate extensive and rapid re-innervation of the denervated DG outer molecular layer by associational and commissural mossy cell axons, similar to what has been reported to occur in juvenile rodent DG in vivo.

Melatonin Promotes Nerve Regeneration Following End-to-Side Neurorrhaphy by Accelerating Cytoskeletal Remodeling via the Melatonin Receptor-dependent Pathway

Acceleration of cytoskeletal remodeling in regenerated axons is crucial for a fully functional recovery following peripheral nerve injury (PNI). Melatonin plays important roles in cell differentiation and protection of cytoskeleton stability, thus, the present study aimed to investigate whether melatonin can enhance neurite outgrowth and promote cytoskeletal remodeling in a PNI animal model and in differentiated neurons. End-to-side neurorrhaphy (ESN) rat model was used for assessing cytoskeletal rearrangement in regenerated axon. Subject rats received 1 mg/kg/day melatonin injection for one month. The amplitude of compound muscle action potentials and the number of re-innervated motor end plates on target muscles were assessed to represent the functional recovery after ESN. Melatonin treatment enhanced functional recovery after ESN, compared to the saline treated group. Additionally, in spinal cord and peripheral nerve tissue, animals receiving melatonin displayed enhanced expression of GAP43 and β3-tubulin one month after ESN, and an increased number of re-innervated motor end plates on their target muscle. In vitro analysis revealed that melatonin treatment significantly promoted neurite outgrowth, and increased expression of melatonin receptors as well as β3-tubulin in mouse neuroblastoma Neuro-2a (N2a) cells. Treatment with a melatonin receptor antagonist, luzindole, significantly suppressed melatonin receptors and β3-tubulin expression. Importantly, we found that melatonin treatment suppressed activation of calmodulin-dependent protein kinase II (CaMKII) in vitro and in vivo, suggesting that the β3-tubulin remodeling may occur via CaMKII-mediated Ca²⁺ signaling. These results suggested that melatonin may promote functional recovery after PNI by accelerating cytoskeletal remodeling through the melatonin receptor-dependent pathway.

Newly Generated and Non-Newly Generated "Immature" Neurons in the Mammalian Brain: A Possible Reservoir of Young Cells to Prevent Brain Aging and Disease?

Brain plasticity is important for translational purposes since most neurological disorders and brain aging problems remain substantially incurable. In the mammalian nervous system, neurons are mostly not renewed throughout life and cannot be replaced. In humans, the increasing life expectancy explains the increase in brain health problems, also producing heavy social and economic burden. An exception to the “static” brain is represented by stem cell niches leading to the production of new neurons. Such adult neurogenesis is dramatically reduced from fish to mammals, and in large-brained mammals with respect to rodents. Some examples of neurogenesis occurring outside the neurogenic niches have been reported, yet these new neurons actually do not integrate in the mature nervous tissue. Non-newly generated, “immature” neurons (nng-INs) are also present: Prenatally generated cells continuing to express molecules of immaturity (mostly shared with the newly born neurons). Of interest, nng-INs seem to show an inverse phylogenetic trend across mammals, being abundant in higher-order brain regions not served by neurogenesis and providing structural plasticity in rather stable areas. Both newly generated and nng-INs represent a potential reservoir of young cells (a “brain reserve”) that might be exploited for preventing the damage of aging and/or delay the onset/reduce the impact of neurological disorders.

Recent advances in branching mechanisms underlying neuronal morphogenesis

Proper neuronal wiring is central to all bodily functions, sensory perception, cognition, memory, and learning. Establishment of a functional neuronal circuit is a highly regulated and dynamic process involving axonal and dendritic branching and navigation toward appropriate targets and connection partners. This intricate circuitry includes axo-dendritic synapse formation, synaptic connections formed with effector cells, and extensive dendritic arborization that function to receive and transmit mechanical and chemical sensory inputs. Such complexity is primarily achieved by extensive axonal and dendritic branch formation and pruning. Fundamental to neuronal branching are cytoskeletal dynamics and plasma membrane expansion, both of which are regulated via numerous extracellular and intracellular signaling mechanisms and molecules. This review focuses on recent advances in understanding the biology of neuronal branching.

Stage-dependent remodeling of projections to motor cortex in ALS mouse model revealed by a new variant retrograde-AAV9

Amyotrophic lateral sclerosis (ALS) is characterized by the progressive degeneration of motoneurons in the primary motor cortex (pMO) and in spinal cord. However, the pathogenic process involves multiple subnetworks in the brain and functional MRI studies demonstrate an increase in functional connectivity in areas connected to pMO despite the ongoing neurodegeneration. The extent and the structural basis of the motor subnetwork remodeling in experimentally tractable models remain unclear. We have developed a new retrograde AAV9 to quantitatively map the projections to pMO in the SOD1(G93A) ALS mouse model. We show an increase in the number of neurons projecting from somatosensory cortex to pMO at presymptomatic stages, followed by an increase in projections from thalamus, auditory cortex and contralateral MO (inputs from 20 other structures remains unchanged) as disease advances. The stage- and structure-dependent remodeling of projection to pMO in ALS may provide insights into the hyperconnectivity observed in ALS patients.

Canadian Association of Neuroscience Review: Axonal Regeneration in the Peripheral and Central Nervous Systems – Current Issues and Advances

Injured nerves regenerate their axons in the peripheral (PNS) but not the central nervous system (CNS). The contrasting capacities have been attributed to the growth permissive Schwann cells in the PNS and the growth inhibitory environment of the oligodendrocytes in the CNS. In the current review, we first contrast the robust regenerative response of injured PNS neurons with the weak response of the CNS neurons, and the capacity of Schwann cells and not the oligodendrocytes to support axonal regeneration. We then consider the factors that limit axonal regeneration in both the PNS and CNS. Limiting factors in the PNS include slow regeneration of axons across the injury site, progressive decline in the regenerative capacity of axotomized neurons (chronic axotomy) and progressive failure of denervated Schwann cells to support axonal regeneration (chronic denervation). In the CNS on the other hand, it is the poor regenerative response of neurons, the inhibitory proteins that are expressed by oligodendrocytes and act via a common receptor on CNS neurons, and the formation of the glial scar that prevent axonal regeneration in the CNS. Strategies to overcome these limitations in the PNS are considered in detail and contrasted with strategies in the CNS.

Reduced expression of regeneration associated genes in chronically axotomized facial motoneurons

Chronically axotomized motoneurons progressively fail to regenerate their axons. Since axonal regeneration is associated with the increased expression of tubulin, actin and GAP-43, we examined whether the regenerative failure is due to failure of chronically axotomized motoneurons to express and sustain the expression of these regeneration associated genes (RAGs). Chronically axotomized facial motoneurons were subjected to a second axotomy to mimic the clinical surgical procedure of refreshing the proximal nerve stump prior to nerve repair. Expression of α1-tubulin, actin and GAP-43 was analyzed in axotomized motoneurons using in situ hybridization followed by autoradiography and silver grain quantification. The expression of these RAGs by acutely axotomized motoneurons declined over several months. The chronically injured motoneurons responded to a refreshment axotomy with a re-increase in RAG expression. However, this response to a refreshment axotomy of chronically injured facial motoneurons was less than that seen in acutely axotomized facial motoneurons. These data demonstrate that the neuronal RAG expression can be induced by injury-related signals and does not require acute deprivation of target derived factors. The transient expression is consistent with a transient inflammatory response to the injury. We conclude that transient RAG expression in chronically axotomized motoneurons and the weak response of the chronically axotomized motoneurons to a refreshment axotomy provides a plausible explanation for the progressive decline in regenerative capacity of chronically axotomized motoneurons.

Proteomic identification of the molecular basis of mammalian CNS growth cones

The growth cone, which is a unique structure with high motility that forms at the tips of extending axons and dendrites, is crucial to neuronal network formation. Axonal growth of the mammalian CNS is most likely achieved by the complicated coordination of cytoskeletal rearrangement and vesicular trafficking via many proteins. Before recent advances, no methods to identify numerous proteins existed; however, proteomics revolutionarily resolved such problems. In this review, I summarize the profiles of the mammalian growth cone proteins revealed by proteomics as the molecular basis of the growth cone functions, with molecular mapping. These results should be used as a basis for understanding the mechanisms of the complex mammalian CNS developmental process.

Motoneuron disease

Amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA) represent the two major forms of motoneuron disease. In both forms of disease, spinal and bulbar motoneurons become dysfunctional and degenerate. In ALS, cortical motoneurons are also affected, which contributes to the clinical phenotype. The gene defects for most familial forms of ALS and SMA have been discovered and they point to a broad spectrum of disease mechanisms, including defects in RNA processing, pathological protein aggregation, altered apoptotic signaling, and disturbed energy metabolism. Despite the fact that lack of neurotrophic factors or their corresponding receptors are not found as genetic cause of motoneuron disease, signaling pathways initiated by neurotrophic factors for motoneuron survival, axon growth, presynaptic development, and synaptic function are disturbed in ALS and SMA. Better understanding of how neurotrophic factors and downstream signaling pathways interfere with these disease mechanisms could help to develop new therapies for motoneuron disease and other neurodegenerative disorders.

BASP1 and its N-end fragments (BNEMFs) dynamics in rat brain during development

Protein BASP1 was discovered in brains of mammals and birds. In presynaptic area of synapses, BASP1 is attached to plasma membrane owing to N-terminal myristoylation as well as to the positively charged "effecter domain". BASP1 interactions with other proteins as well as with lipids contribute to membrane traffic, axon outgrowth and synaptic plasticity. BASP1 is present also in other tissues, where it was found not only in cytoplasm, but also in nucleus. Nuclear BASP1 suppresses activity of transcription factor WT1 and acts as tumor suppressor. BASP1 deficiency in a cell leads to its transformation. Previously it was shown that in BASP1 samples prepared from different animals and different tissues, six BASP1 N-end myristoylated fragments (BNEMFs) are present. Together, they amount to 30 % of the whole molecules. BNEMFs presence in different species and tissues demonstrates their physiological significance. However BNEMFs remain unexplored. In this paper, the time of appearance and dynamics of both BASP1 and BNEMFs during rat development from embryo to adult animals were determined. In rat brain, the amounts of all BASP1 forms per cell systematically increase during development and remain at the highest levels in adult animals. BNEMFs appear during embryogenesis non-simultaneously and accumulate with different dynamics. These results say for formation of six BNEMFs in the course of different processes and, possibly, using different mechanisms.

The insulin-like growth factor 1 receptor is essential for axonal regeneration in adult central nervous system neurons

Axonal regeneration is an essential condition to re-establish functional neuronal connections in the injured adult central nervous system (CNS), but efficient regrowth of severed axons has proven to be very difficult to achieve. Although significant progress has been made in identifying the intrinsic and extrinsic mechanisms involved, many aspects remain unresolved. Axonal development in embryonic CNS (hippocampus) requires the obligate activation of the insulin-like growth factor 1 receptor (IGF-1R). Based on known similarities between axonal growth in fetal compared to mature CNS, we decided to examine the expression of the IGF-1R, using an antibody to the βgc subunit or a polyclonal anti-peptide antibody directed to the IGF-R (C20), in an in vitro model of adult CNS axonal regeneration, namely retinal ganglion cells (RGC) derived from adult rat retinas. Expression of both βgc and the β subunit recognized by C20 antibody were low in freshly isolated adult RGC, but increased significantly after 4 days in vitro. As in embryonic axons, βgc was localised to distal regions and leading growth cones in RGC. IGF-1R-βgc co-localised with activated p85 involved in the phosphatidylinositol-3 kinase (PI3K) signaling pathway, upon stimulation with IGF-1. Blocking experiments using either an antibody which neutralises IGF-1R activation, shRNA designed against the IGF-1R sequence, or the PI3K pathway inhibitor LY294002, all significantly reduced axon regeneration from adult RGC in vitro (∼40% RGC possessed axons in controls vs 2-8% in the different blocking studies). Finally, co-transfection of RGC with shRNA to silence IGF-1R together with a vector containing a constitutively active form of downstream PI3K (p110), fully restored axonal outgrowth in vitro. Hence these data demonstrate that axonal regeneration in adult CNS neurons requires re-expression and activation of IGF-1R, and targeting this system may offer new therapeutic approaches to enhancing axonal regeneration following trauma.

zRICH, a protein induced during optic nerve regeneration in zebrafish, promotes neuritogenesis and interacts with tubulin

Mammals do not regenerate axons in their central nervous system (CNS) spontaneously. This phenomenon is the cause of numerous medical conditions after damage to nerve fibers in the CNS of humans. The study of the mechanisms of nerve regeneration in other vertebrate animals able to spontaneously regenerate axons in their CNS is essential for understanding nerve regeneration from a scientific point of view, and for developing therapeutic approaches to enhance nerve regeneration in the CNS of humans. RICH proteins are a novel group of proteins implicated in nerve regeneration in the CNS of teleost fish, yet their mechanisms of action are not well understood. A number of mutant versions of the zebrafish RICH (zRICH) protein were generated and characterized at biochemical and cellular levels in our laboratory. With the aim of understanding the effects of RICH proteins in neuronal axon outgrowth, stable transfectants derived from the neuronal model PC12 cell line expressing zRICH Wild-Type or mutant versions of zRICH were studied. Results from differentiation experiments suggest that RICH proteins enhance neuronal plasticity by facilitating neurite branching. Biochemical co-purification results have demonstrated that zRICH binds to the cytoskeletal protein tubulin. The central domain of the protein is sufficient for tubulin binding, but a mutant version of the protein lacking the terminal domains, which cannot bind to the plasma membrane, was not able to enhance neurite branching. RICH proteins may facilitate axon regeneration by regulating the axonal cytoskeleton and facilitating the formation of new neurite branches.

Fibronectin supports neurite outgrowth and axonal regeneration of adult brain neurons in vitro

The molecular basis of axonal regeneration of central nervous system (CNS) neurons remains to be fully elucidated. In part, this is due to the difficulty in maintaining CNS neurons in vitro. Here, we show that dissociated neurons from the cerebral cortex and hippocampus of adult mice may be maintained in culture for up to 9 days in defined medium without added growth factors. Outgrowth of neurites including axons was observed from both CNS sources and was significantly greater on plasma fibronectin than on other substrata such as laminin and merosin. Neurite outgrowth on fibronectin appears to be mediated by α5β1 integrin since a recombinant fibronectin fragment containing binding sites for this receptor was as effective as intact fibronectin in supporting neurite outgrowth. Conversely, function-blocking antibodies to α5 and β1 integrin sub-units inhibited neurite outgrowth on intact fibronectin. These results suggest that the axonal regeneration seen in in vivo studies using fibronectin-based matrices is due to the molecule itself and not a consequence of secondary events such as cellular infiltration. They also indicate the domains of fibronectin that may be responsible for eliciting this response.

RGMA and neogenin protein expression are influenced by lens injury following optic nerve crush in the rat retina

BACKGROUND

The death and the failure of neurons to regenerate their axons after lesion of the central nervous system in mammals, as in the case of spinal cord injury and optic nerve trauma, remain a challenge. In this study, we focused on the repulsive guidance molecule A (RGMA) and its receptor neogenin. Since it was reported that RGMA+ cells accumulate in lesioned areas after spinal cord injury, brain trauma, and optic nerve crush, and curiously, anti-apoptotic effects of RGMA were also described, we investigated the role of RGMA and neogenin in the retina after optic nerve crush (ONC).

METHODS

We evaluated the spatial and temporal protein pattern of RGMA and neogenin in the rat retina without (non-regenerating model) or with (regenerating model) lens injury (LI). We investigated the presence of RGMA, neogenin and other proteins at up to nine time points (6 h-20 days post-surgery) by performing immunohistochemistry and Western blots.

RESULTS

Independent of the treatment, RGMA protein was present in the nuclear layers (NLs), plexiform layers (PLs), nerve fiber layer (NFL), and in retinal ganglion cells (RGCs) of the rat retina. RGC and nerve fibers were always RGMA+. Further RGMA+ cells in the retina were blood vessel endothelial cells, astrocytes, Müller cells, and some microglial cells. The RGMA pattern for the specific retinal cells resembled those of previously published data. The neogenin pattern was congruent to the RGMA pattern. Western blots of retinal tissue showed further RGMA+ products only in LI animals. Furthermore, a higher amount of RGMA was found in the retinae of ONC + LI rats compared to ONC rats.

CONCLUSIONS

Although a difference in the localization of RGMA is not obvious, the difference in the amount of RGMA is striking, the higher amount of RGMA in the retinae of ONC + LI rats compared to ONC rats indicates a role for RGMA during degeneration/regeneration processes. Our results are consistent with several reported neuroprotective effects of RGMA. Our new data showing the upregulation of RGMA after ONC in our regenerating model (plus LI) confirm these findings conducted in different settings.

Development and characterization of Thy*IκBα-SI transgenic mice

We developed and characterized a new transgenic model where NF-κB activity is inhibited only in mature neurons. Transgenic mouse strain Thy*IκBα-SI was created using trans-dominant super inhibitor NF-κB (IκBα-SI), which is a multimutant form of IκBα inhibitory protein cloned into specific neutral Thy-1.2 cassette. Detailed molecular analysis showed that the transgene and its product (IκBα-SI protein) are expressed in the nervous system of transgenic mice. In situ hybridization showed that Thy*IκBα-SI in the nervous system is expressed exclusively in neurons. The developed model provides wide opportunities for studying functional role of NF-κB in mature neurons in the central and peripheral nervous system in vivo.

Mammalian target of rapamycin (mTOR) activation increases axonal growth capacity of injured peripheral nerves

Unlike neurons in the central nervous system (CNS), injured neurons in the peripheral nervous system (PNS) can regenerate their axons and reinnervate their targets. However, functional recovery in the PNS often remains suboptimal, especially in cases of severe damage. The lack of regenerative ability of CNS neurons has been linked to down-regulation of the mTOR (mammalian target of rapamycin) pathway. We report here that PNS dorsal root ganglial neurons (DRGs) activate mTOR following damage and that this activity enhances axonal growth capacity. Furthermore, genetic up-regulation of mTOR activity by deletion of tuberous sclerosis complex 2 (TSC2) in DRGs is sufficient to enhance axonal growth capacity in vitro and in vivo. We further show that mTOR activity is linked to the expression of GAP-43, a crucial component of axonal outgrowth. However, although TSC2 deletion in DRGs facilitates axonal regrowth, it leads to defects in target innervation. Thus, whereas manipulation of mTOR activity could provide new strategies to stimulate nerve regeneration in the PNS, fine control of mTOR activity is required for proper target innervation.

Neuroproteomics approaches to decipher neuronal regeneration and degeneration

Given the complexity of brain and nerve tissues, systematic approaches are essential to understand normal physiological conditions and functional alterations in neurological diseases. Mass spectrometry-based proteomics is increasingly used in neurosciences to determine both basic and clinical differential protein expression, protein-protein interactions, and post-translational modifications. Proteomics approaches are especially useful to understand the mechanisms of nerve regeneration and degeneration because changes in axons following injury or in disease states often occur without the contribution of transcriptional events in the cell body. Indeed, the current understanding of axonal function in health and disease emphasizes the role of proteolysis, local axonal protein synthesis, and a broad range of post-translational modifications. Deciphering how axons regenerate and degenerate has thus become a postgenomics problem, which depends in part on proteomics approaches. This review focuses on recent proteomics approaches designed to uncover the mechanisms and molecules involved in neuronal regeneration and degeneration. It emerges that the principal degenerative mechanisms converge to oxidative stress, dysfunctions of axonal transport, mitochondria, chaperones, and the ubiquitin-proteasome systems. The mechanisms regulating nerve regeneration also impinge on axonal transport, cytoskeleton, and chaperones in addition to changes in signaling pathways. We also discuss the major challenges to proteomics work in the nervous system given the complex organization of the brain and nerve tissue at the anatomical, cellular, and subcellular levels.

Ectopic growth of hippocampal mossy fibers in a mutated GAP-43 transgenic mouse with impaired spatial memory retention

In a previous study, it was shown that transgenic mice, designated G-NonP, forget the location of a water maze hidden platform when tested 7 days after the last training day (Holahan and Routtenberg (2008) Hippocampus 18:1099-1102). The memory loss in G-NonP mice might be related to altered hippocampal architecture suggested by the fact that in the rat, 7 days after water maze training, there is discernible mossy fiber (MF) growth (Holahan et al. (2006) Hippocampus 16:560-570; Rekart et al. (2007) Learn Mem 14:416-421). In the present report, we studied the distribution of the MF system within the hippocampus of naïve, untrained, G-NonP mouse. In WT mice, the MF projection was restricted to the stratum lucidum of CA3 with no detectable MF innervation in distal stratum oriens (dSO). In G-NonP mice, in contrast, there was an ectopic projection terminating in the CA3 dSO. Unexpectedly, there was nearly a complete loss of immunostaining for the axonal marker Tau1 in the G-NonP transgenic mice in the MF terminal fields indicating that transgenesis itself leads to off-target consequences (Routtenberg (1996) Trends Neurosci 19:471-472). Because transgenic mice overexpressing nonmutated, wild type GAP-43 do not show this ectopic growth (Rekart et al., in press) and the G-NonP mice overexpress a mutated form of GAP-43 precluding its phosphorylation by protein kinase C (PKC), the possibility exists that permanently dephosphorylated GAP-43 disrupts normal axonal fasciculation which gives rise to the ectopic growth into dSO.

Pericontusion axon sprouting is spatially and temporally consistent with a growth-permissive environment after traumatic brain injury

We previously reported that pericontusional extracellular chondroitin sulfate proteoglycans (CSPGs) are profoundly reduced for 3 weeks after experimental traumatic brain injury, indicating a potential growth-permissive window for plasticity. Here, we investigate the extracellular environment of sprouting neurons after controlled cortical impact injury in adult rats to determine the spatial and temporal arrangement of inhibitory and growth-promoting molecules in relation to growth-associated protein 43-positive (GAP43+) neurons. Spontaneous cortical sprouting was maximal in pericontused regions at 7 and 14 days after injury but absent by 28 days. Perineuronal nets containing CSPGs were reduced at 7 days after injury in the pericontused region (p < 0.05), which was commensurate with a reduction in extracellular CSPGs. Sprouting was restricted to the perineuronal nets and CSPG-deficient regions at 7 days, indicating that the pericontused region is temporarily and spatially permissive to new growth. At this time point,GAP43+ neurons were associated with brain regions containing cells positive for polysialic acid neural cell adhesion molecule but not with fibronectin-positive cells. Brain-derived neurotrophic factor was reduced in the immediate pericontused region at 7 days. Along with prior Western blot evidence, these data suggest that a lowered intrinsic growth stimulus, together with a later return of growth-inhibitory CSPGs, may contribute to the ultimate disappearance of sprouting neurons after traumatic brain injury.

Conditioning lesions enhance growth state only in sensory neurons lacking calcitonin gene-related peptide and isolectin B4-binding

A conditioning lesion improves regeneration of central and peripheral axons of dorsal root ganglion (DRG) neurons after a subsequent injury by enhancing intrinsic growth capacity. This enhanced growth state is also observed in cultured DRG neurons, which support a more sparsely and rapidly elongating mode of growth after a prior conditioning lesion in vivo. Here we examined differences in the capacity or requirements of specific types of sensory neurons for regenerative growth, which has important consequences for development of strategies to improve recovery after injury. We showed that after partial or complete injury of the sciatic nerve in mice, an elongating mode of growth in vitro was activated only in DRG neurons that did not express calcitonin gene-related peptide (CGRP) or bind Bandeiraea simplicifolia I-isolectin B4 (IB4). We also directly examined the response of conditioned sensory neurons to nerve growth factor (NGF), which does not enhance growth in injured peripheral nerves in vivo. We showed that after partial injury, NGF stimulated a highly branched and linearly restricted rather than elongating mode of growth. After complete injury, the function of NGF was impaired, which immunohistochemical studies of DRG indicated was at least partly due to downregulation of the NGF receptor, tropomyosin-related kinase A (TrkA). These results suggest that, regardless of the type of conditioning lesion, each type of DRG neuron has a distinct intrinsic capacity or requirement for the activation of rapidly elongating growth, which does not appear to be influenced by NGF.

Herpes simplex virus type 1 latency-associated transcript inhibits apoptosis and promotes neurite sprouting in neuroblastoma cells following serum starvation by maintaining protein kinase B (AKT) levels

The herpes simplex virus type 1 (HSV-1) latency-associated transcript (LAT) is expressed abundantly in latently infected sensory neurons. LAT-deletion-mutant virus strains have reduced-reactivation phenotypes in small animal models of infection, demonstrating that LAT plays an important role in the latency-reactivation cycle of HSV-1. Previous studies demonstrated that the anti-apoptosis functions of LAT are important for regulating the latency-reactivation cycle because three different anti-apoptosis genes can substitute for LAT. Although LAT inhibits caspase 3 activation, the signalling pathway by which LAT inhibits caspase 3 activation was not identified. In this study, we analysed mouse neuroblastoma cells (C1300) that express LAT stably (DC-LAT6 cells) following serum starvation. As expected, DC-LAT6 cells were resistant to apoptosis following serum withdrawal. Levels of total and phosphorylated AKT (protein kinase B), a serine/threonine protein kinase that promotes cell survival, were higher in DC-LAT6 cells after serum withdrawal than in C1300 cells or a cell line stably transfected with a LAT promoter mutant (DC-DeltaLAT311). A specific AKT inhibitor reduced the anti-apoptosis functions of LAT and phosphorylated AKT levels. After serum withdrawal, more DC-LAT6 cells sprouted neurites and exhibited a differentiated morphology. NeuN (neuronal nuclei), a neuron-specific nuclear protein, was expressed abundantly in DC-LAT6 cells, but not C1300 cells, after serum withdrawal, further supporting the concept that LAT enhanced neuronal-like morphology. Collectively, these studies suggested that LAT, directly or indirectly, maintained total and phosphorylated AKT levels, which correlated with increased cell survival and mature neuronal-like morphology.

Non-viral gene delivery transfection profiles influence neuronal architecture in an in vitro co-culture model

Gene delivery from tissue engineering scaffolds can induce expression of tissue inductive factors to stimulate the cellular processes required for regeneration. Transfected cells secrete diffusible proteins that can create local concentration gradients, depending on the number, distribution, and expression level of transfected cells. These gradients are linked to cellular organization and tissue architecture during embryogenesis. In this report, we investigate neuronal architecture and neurite guidance in response to the concentration gradients achieved by localized secretion of a neurotrophic factor from transfected cells. A co-culture model was employed to examine neuronal responses to multiple transfection profiles, which affects the local concentration of secreted nerve growth factor (NGF). Neuronal architecture, as defined by number of neurites per neuron and length of neurites, was influenced by the transfection profile. Low levels of NGF production by few transfected cells produced longer primary neurites with less branching relative to the higher expression levels or increased numbers of transfected cells. Furthermore, for low NGF production by few transfected cells, the growth cone of the axons was marked by longer extensions and larger surface area, suggesting the presence of a guidance cue. Control studies with varying NGF concentrations did not substantially alter the neuronal architecture, further supporting an effect of localized concentration gradients, and not simply the concentration. Mathematical modeling of NGF diffusion was employed to predict the concentration gradients produced by the transfection profiles, and the resultant gradients were correlated to the cellular response. This report connects the transfection profile, concentration gradients, and the resulting cellular architecture, suggesting a critical design consideration for the application of gene delivery to regenerative medicine.

Observations on the function of nuclear factor kappa B (NF-kappaB) in the survival of adult primary sensory neurons after nerve injury

Peripheral nerve transections cause much more neuronal death in embryonic and neonatal dorsal root ganglia (DRG) than in adult DRG. Here we used transgenic approaches to examine the hypothesis that NF-kappaB is an important intrinsic factor of adult DRG neurons for their in vivo capacity to survive after nerve injury. We generated transgenic mice expressing the NF-kappaB super-inhibitor (IkappaBalpha-SI), a multi-mutant form of IkappaBalpha, specifically in adult neurons. Adult DRG neurons in these transgenic animals are not abnormally susceptible to apoptosis after peripheral nerve injury, although there is a significant inhibition in the ability of NF-kappaB to translocate into their nucleus. We investigated the observed lack of NF-kappaB neuroprotective function at the level of NF-kappaB transcriptional activity using transgenic NF-kappaB/LacZ reporter mice. We show that the expression of the NF-kappaB reporter transgene is restricted in naïve and injured DRG neurons. However, NF-kappaB transcriptional activity in adult DRG neurons is evident upon exposure to Trichostatin A (TSA) which is a specific inhibitor of histone deacetylases. Taken together our results illustrate that the functions of NF-kappaB are limited in adult primary sensory neurons due to a transcriptional repression mechanism mediated by histone deacetylases, and that intrinsic neuroprotective factors other than NF-kappaB are responsible for the resistance of adult DRG neurons to apoptosis in response to nerve injury.

Identification of Phox2b-regulated genes by expression profiling of cranial motoneuron precursors

Background

Branchiomotor neurons comprise an important class of cranial motor neurons that innervate the branchial-arch-derived muscles of the face, jaw and neck. They arise in the ventralmost progenitor domain of the rhombencephalon characterized by expression of the homeodomain transcription factors Nkx2.2 and Phox2b. Phox2b in particular plays a key role in the specification of branchiomotor neurons. In its absence, generic neuronal differentiation is defective in the progenitor domain and no branchiomotor neurons are produced. Conversely, ectopic expression of Phox2b in spinal regions of the neural tube promotes cell cycle exit and neuronal differentiation and, at the same time, induces genes and an axonal phenotype characteristic for branchiomotor neurons. How Phox2b exerts its pleiotropic functions, both as a proneural gene and a neuronal subtype determinant, has remained unknown.

Results

To gain further insights into the genetic program downstream of Phox2b, we searched for novel Phox2b-regulated genes by cDNA microarray analysis of facial branchiomotor neuron precursors from heterozygous and homozygous Phox2b mutant embryos. We selected for functional studies the genes encoding the axonal growth promoter Gap43, the Wnt antagonist Sfrp1 and the transcriptional regulator Sox13, which were not previously suspected to play roles downstream of Phox2b and whose expression was affected by Phox2b misexpression in the spinal cord. While Gap43 did not produce an obvious phenotype when overexpressed in the neural tube, Sfrp1 induced the interneuron marker Lhx1,5 and Sox13 inhibited neuronal differentiation. We then tested whether Sfrp1 and Sox13, which are down-regulated by Phox2b in the facial neuron precursors, would antagonize some aspects of Phox2b activity. Co-expression of Sfrp1 prevented Phox2b from repressing Lhx1,5 and alleviated the commissural axonal phenotype. When expressed together with Sox13, Phox2b was still able to promote cell cycle exit and neuronal differentiation, but the cells failed to relocate to the mantle layer and to extinguish the neural stem cell marker Sox2.

Conclusion

Our results suggest novel roles for Sfrp1 and Sox13 in neuronal subtype specification and generic neuronal differentiation, respectively, and indicate that down-regulation of Sfrp1 and Sox13 are essential aspects of the genetic program controlled by Phox2b in cranial motoneurons.

Adult mammalian neurogenesis and the New Zealand white rabbit

Although the central nervous system is unable to undergo spontaneous repair and is hostile to the integration of exogenously delivered cells, various examples of adult structural plasticity have been shown to occur. It is now widely accepted that endogenous proliferative activity leading to the production of new neurons exists, at least within two restricted brain sites: the hippocampal dentate gyrus and the forebrain subventricular zone. A substantial insight into spontaneous neurogenesis within these allocortical regions in rodents has been obtained, but less is known regarding its occurrence in other mammalian brain regions. In this review, differences in the structural and temporal characteristics of protracted neurogenesis in mammals will be considered. Attention will be focused on the rabbit cerebrum and cerebellum, where unexpected features of structural plasticity have been found to occur despite the relative closeness of the Orders Lagomorpha and Rodentia.

In vitro hypoxic preconditioning of embryonic stem cells as a strategy of promoting cell survival and functional benefits after transplantation into the ischemic rat brain

Hypoxic preconditioning (HP) and stem cell transplantation have been extensively studied as individual therapies for ischemic stroke. The present investigation is an initial effort to combine these methods to achieve increased therapeutic effects after brain ischemia. Sublethal in vitro hypoxia pretreatment significantly enhanced the tolerance of neurally-differentiating embryonic stem (ES) cells and primary bone marrow mesenchymal stem cells (BMSC) to apoptotic cell death (40-50% reduction in cell death and caspase-3 activation). The HP protective effects on cultured cells lasted for at least 6 days. HP increased secretion of erythropoietin (EPO) and upregulated expression of bcl-2, hypoxia-inducible factor (HIF-1alpha), erythropoietin receptor (EPOR), neurofilament (NF), and synaptophysin in ES cell-derived neural progenitor cells (ES-NPCs). The HP cytoprotective effect was diminished by blocking EPOR, while pretreatment of ES-NPCs with recombinant human EPO mimicked the HP effect. HP-primed ES-NPCs survived better 3 days after transplantation into the ischemic brain (30-40% reduction in cell death and caspase-3 activation). Finally, transplanted HP-primed ES-NPCs exhibited extensive neuronal differentiation in the ischemic brain, accelerated and enhanced recovery of sensorimotor function when compared to transplantation of non-HP-treated ES-NPCs. The cell-priming strategy aimed to promote transplanted cell survival and their tissue repair capability provides a simple yet effective way of optimizing cell transplantation therapy.

Pattern of chondroitin sulfate proteoglycan expression after ablation of the sensorimotor cortex of the neonatal and adult rat brain

The central nervous system has a limited capacity for self-repair after damage. However, the neonatal brain has agreater capacity for recovery than the adult brain. These differences in the regenerative capability depend on local environmental factors and the maturational stage of growing axons. Among molecules which have both growth-promoting and growth-inhibiting activities is the heterogeneous class of chondroitin sulfate proteoglycans (CSPGs). In this paper, we investigated the chondroitin-4 and chondroitin-6 sulfate proteoglycan expression profile after left sensorimotor cortex ablation of the neonatal and adult rat brain. Immunohistochemical analysis revealed that compared to the normal uninjured cortex, lesion provoked up regulation of CSPGs showing a different pattern of expression in the neonatal vs. the adult brain. Punctuate and membrane-bound labeling was predominate after neonatal lesion, where as heavy deposition of staining in the extracellular matrix was observed after adult lesion. Heavy deposition of CSPG immunoreactivity around the lesionsite in adult rats, in contrast to a less CSPG-rich environment in neonatal rats, indicated that enhancement of the recovery process after neonatal injury is due to amore permissive environment.

GAP-43 gene expression regulates information storage

Previous reports have shown that overexpression of the growth- and plasticity-associated protein GAP-43 improves memory. However, the relation between the levels of this protein to memory enhancement remains unknown. Here, we studied this issue in transgenic mice (G-Phos) overexpressing native, chick GAP-43. These G-Phos mice could be divided at the behavioral level into "spatial bright" and "spatial dull" groups based on their performance on two hidden platform water maze tasks. G-Phos dull mice showed both acquisition and retention deficits on the fixed hidden platform task, but were able to learn a visible platform task. G-Phos bright mice showed memory enhancement relative to wild type on the more difficult movable hidden platform spatial memory task. In the hippocampus, the G-Phos dull group showed a 50% greater transgenic GAP-43 protein level and a twofold elevated transgenic GAP-43 mRNA level than that measured in the G-Phos bright group. Unexpectedly, the dull group also showed an 80% reduction in hippocampal Tau1 staining. The high levels of GAP-43 seen here leading to memory impairment find its histochemical and behavioral parallel in the observation of Rekart et al. (Neuroscience 126: 579-584) who described elevated levels of GAP-43 protein in the hippocampus of Alzheimer's patients. The present data suggest that moderate overexpression of a phosphorylatable plasticity-related protein can enhance memory, while excessive overexpression may produce a "neuroplasticity burden" leading to degenerative and hypertrophic events culminating in memory dysfunction.

Solution structure of the catalytic domain of RICH protein from goldfish

Regeneration-induced CNPase homolog (RICH) is an axonal growth-associated protein, which is induced in teleost fish upon optical nerve injury. RICH consists of a highly acidic N-terminal domain, a catalytic domain with 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) activity and a C-terminal isoprenylation site. In vitro RICH and mammalian brain CNPase specifically catalyze the hydrolysis of 2',3'-cyclic nucleotides to produce 2'-nucleotides, but the physiologically relevant in vivo substrate remains unknown. Here, we report the NMR structure of the catalytic domain of goldfish RICH and describe its binding to CNPase inhibitors. The structure consists of a twisted nine-stranded antiparallel beta-sheet surrounded by alpha-helices on both sides. Despite significant local differences mostly arising from a seven-residue insert in the RICH sequence, the active site region is highly similar to that of human CNPase. Likewise, refinement of the catalytic domain of rat CNPase using residual dipolar couplings gave improved agreement with the published crystal structure. NMR titrations of RICH with inhibitors point to a similar catalytic mechanism for RICH and CNPase. The results suggest a functional importance for the evolutionarily conserved phosphodiesterase activity and hint of a link with pre-tRNA splicing.

Growth-associated protein-43 expression in the lens of rats and primates

Growth-associated protein-43 is a specific neuronal protein that regulates differentiation, growth and plasticity. In the present study, growth-associated protein-43 expression was studied in the lens of rats and primates (including man) at different postnatal ages by immunoblotting, immunohistochemistry and quantitative real-time polymerase chain reaction. Growth-associated protein-43 was expressed at all ages in primates and in developing, but not in adult rats. We demonstrate that the lens - a tissue that is devoid of nerves - expresses growth-associated protein-43 throughout life in primates, and in rats during development but not in adulthood. These results suggest that growth-associated protein-43 is involved in differentiation processes also outside the nervous system.

Regulation of intrinsic neuronal properties for axon growth and regeneration

Regulation of neuritic growth is crucial for neural development, adaptation and repair. The intrinsic growth potential of nerve cells is determined by the activity of specific molecular sets, which sense environmental signals and sustain structural extension of neurites. The expression and function of these molecules are dynamically regulated by multiple mechanisms, which adjust the actual growth properties of each neuron population at different ontogenetic stages or in specific conditions. The neuronal potential for axon elongation and regeneration are restricted at the end of development by the concurrent action of several factors associated with the final maturation of neurons and of the surrounding tissue. In the adult, neuronal growth properties can be significantly modulated by injury, but they are also continuously tuned in everyday life to sustain physiological plasticity. Strict regulation of structural remodelling and neuritic elongation is thought to be required to maintain specific patterns of connectivity in the highly complex mammalian CNS. Accordingly, procedures that neutralize such mechanisms effectively boost axon growth in both intact and injured nervous system. Even in these conditions, however, aberrant connections are only formed in the presence of unusual external stimuli or experience. Therefore, growth regulatory mechanisms play an essentially permissive role by setting the responsiveness of neural circuits to environmental stimuli. The latter exert an instructive action and determine the actual shape of newly formed connections. In the light of this notion, efficient therapeutic interventions in the injured CNS should combine targeted manipulations of growth control mechanisms with task-specific training and rehabilitation paradigms.

Concepts of nerve regeneration and repair applied to brachial plexus reconstruction

Brachial plexus injury is a serious condition that usually affects young adults. Progress in brachial plexus repair is intimately related to peripheral nerve surgery, and depends on clinical and experimental studies. We review the rat brachial plexus as an experimental model, together with its behavioral evaluation. Techniques to repair nerves, such as neurolysis, nerve coaptation, nerve grafting, nerve transfer, fascicular transfer, direct muscle neurotization, and end-to-side neurorraphy, are discussed in light of the authors' experimental studies. Intradural repair of the brachial plexus by graft implants into the spinal cord and motor rootlet transfer offer new possibilities in brachial plexus reconstruction. The clinical experience of intradural repair is presented. Surgical planning in root rupture or avulsion is proposed. In total avulsion, the authors are in favor of the reconstruction of thoraco-brachial and abdomino-antebrachial grasping, and on the transfer of the brachialis muscle to the wrist extensors if it is reinnervated. Surgical treatment of painful conditions and new drugs are also discussed.

PSA-NCAM in mammalian structural plasticity and neurogenesis

Polysialic acid (PSA) is a linear homopolymer of alpha2-8-N acetylneuraminic acid whose major carrier in vertebrates is the neural cell adhesion molecule (NCAM). PSA serves as a potent negative regulator of cell interactions via its unusual biophysical properties. PSA on NCAM is developmentally regulated thus playing a prominent role in different forms of neural plasticity spanning from embryonic to adult nervous system, including axonal growth, outgrowth and fasciculation, cell migration, synaptic plasticity, activity-induced plasticity, neuronal-glial plasticity, embryonic and adult neurogenesis. The cellular distribution, developmental changes and possible function(s) of PSA-NCAM in the central nervous system of mammals here are reviewed, along with recent findings and theories about the relationships between NCAM protein and PSA as well as the role of different polysialyltransferases. Particular attention is focused on postnatal/adult neurogenesis, an issue which has been deeply investigated in the last decade as an example of persisting structural plasticity with potential implications for brain repair strategies. Adult neurogenic sites, although harbouring all subsequent steps of cell differentiation, from stem cell division to cell replacement, do not faithfully recapitulate development. After birth, they undergo morphological and molecular modifications allowing structural plasticity to adapt to the non-permissive environment of the mature nervous tissue, that are paralled by changes in the expression of PSA-NCAM. The use of PSA-NCAM as a marker for exploring differences in structural plasticity and neurogenesis among mammalian species is also discussed.

Intrapancreatic axonal hyperbranching of dorsal root ganglia neurons in chronic pancreatitis model rats and its relation to pancreatic pain

OBJECTIVES

Increase in number of intrapancreatic nerve bundles has been implicated in the generation of persistent pain in chronic pancreatitis. To examine the origin of these nerve fibers and the mechanisms linking neural morphological change to pain generation, we used neuronal tracing techniques in combination with immunohistochemistry in spontaneous chronic pancreatitis in the Wistar Bonn/Kobori (WBN/Kob) rats.

METHODS

For retrograde tracing, horseradish peroxidase was injected into the pancreas, and labeled neurons in the sensory ganglia were counted. For anterograde tracing, biotinylated dextran amine was injected into the dorsal root ganglia (DRGs), and labeled intrapancreatic sensory fibers were histochemically assessed. For assessment of pain generation, we evaluated c-Fos-positive neurons in the spinal dorsal horn and behavioral changes of the animals.

RESULTS

In WBN/Kob rats, the numbers of horseradish peroxidase-labeled neurons were decreased in the DRGs, and the numbers of biotinylated dextran amine-labeled intrapancreatic nerve fibers and terminals were increased. Biotinylated dextran amine-labeled nerve fibers contained growth-associated protein 43. The number of c-Fos-positive neurons in the dorsal horn was also increased and was correlated with intrapancreatic growth-associated protein 43 immunoreactivity. Grooming behavior was reduced in WBN/Kob rats, and this reduction was facilitated by exocrine stimulation.

CONCLUSIONS

Axonal branching in DRG neurons innervating the pancreas increases in WBN/Kob rats, and these morphological changes are likely involved in pain generation in chronic pancreatitis.

Sprouting, regeneration and circuit formation in the injured spinal cord: factors and activity

Central nervous system (CNS) injuries are particularly traumatic, owing to the limited capabilities of the mammalian CNS for repair. Nevertheless, functional recovery is observed in patients and experimental animals, but the degree of recovery is variable. We review the crucial characteristics of mammalian spinal cord function, tract development, injury and the current experimental therapeutic approaches for repair. Regenerative or compensatory growth of neurites and the formation of new, functional circuits require spontaneous and experimental reactivation of developmental mechanisms, suppression of the growth-inhibitory properties of the adult CNS tissue and specific targeted activation of new connections by rehabilitative training.

Characterization of the domains of zRICH, a protein induced during optic nerve regeneration in zebrafish

Teleost fish show a remarkable capability of nerve regeneration in their CNS, while injuries to axon fibers in the CNS of mammals result in degeneration and loss of function. Understanding this difference has biomedical consequences to humans. Both extrinsic factors from the neuronal environment and intrinsic neuronal factors seem to play a role in successful nerve regeneration. Among the intrinsic factors, a number of proteins termed axonal growth associated proteins (GAPs) are strongly induced during axon regeneration. RICH proteins are axonal GAPs that show homology to mammalian myelin marker proteins termed CNPases. Sequence analysis distinguishes three domains in these proteins. In this report, mutant versions of zebrafish RICH proteins were generated to study the roles of the domains of the protein at biochemical and cellular levels. The central CNPase homology domain was sufficient for catalytic activity. The amino terminal acidic domain causes the anomalous electrophoretic migration observed for RICH proteins. The small C-terminal domain bears an isoprenylation motif and is necessary for the interaction of zRICH with cellular membranes. At the cellular level, expression of wild-type zRICH protein in PC12 cells did not induce neurite generation. Additionally, neither the expression of wild-type zRICH nor the expression of mutant versions of the protein interfered with the levels of differentiation of PC12 cells induced by nerve growth factor, suggesting that, at least in this model of neuronal differentiation, zRICH proteins do not participate in the process of generation of neurites.

Requirement of Cul3 for axonal arborization and dendritic elaboration in Drosophila mushroom body neurons

Cul3 belongs to the family of cullin proteins, which function as scaffold proteins of E3 ubiquitin ligase complexes. Here we show cell-autonomous involvement of Cul3 in axonal arborization and dendritic elaboration of Drosophila mushroom body neurons. Cul3 mutant neurons are defective in terminal morphogenesis of neurites. Interestingly, mutant axons often terminate around branching points. In addition, dendritic elaboration is severely affected in Cul3 mutant neurons. However, loss of Cul3 function does not affect extension of the axons that rarely arborize. Function of cullin-type proteins has been shown to require covalent attachment of Nedd8 (neural precursor cell-expressed developmentally downregulated), a ubiquitin-like protein. Consistent with this notion, Cul3 is inactivated by a mutation in its conserved neddylation site, and Nedd8 mutant neurons exhibit similar neuronal morphogenetic defects. Together, Cul3 plays an essential role in both axonal arborization and proper elaboration of dendrites and may require neddylation for its proper function.

Effects of lipopolysaccharide-induced inflammation on expression of growth-associated genes by corticospinal neurons

BACKGROUND

Inflammation around cell bodies of primary sensory neurons and retinal ganglion cells enhances expression of neuronal growth-associated genes and stimulates axonal regeneration. We have asked if inflammation would have similar effects on corticospinal neurons, which normally show little response to spinal cord injury. Lipopolysaccharide (LPS) was applied onto the pial surface of the motor cortex of adult rats with or without concomitant injury of the corticospinal tract at C4. Inflammation around corticospinal tract cell bodies in the motor cortex was assessed by immunohistochemistry for OX42 (a microglia and macrophage marker). Expression of growth-associated genes c-jun, ATF3, SCG10 and GAP-43 was investigated by immunohistochemistry or in situ hybridisation.

RESULTS

Application of LPS induced a gradient of inflammation through the full depth of the motor cortex and promoted c-Jun and SCG10 expression for up to 2 weeks, and GAP-43 upregulation for 3 days by many corticospinal neurons, but had very limited effects on neuronal ATF3 expression. However, many glial cells in the subcortical white matter upregulated ATF3. LPS did not promote sprouting of anterogradely labelled corticospinal axons, which did not grow into or beyond a cervical lesion site.

CONCLUSION

Inflammation produced by topical application of LPS promoted increased expression of some growth-associated genes in the cell bodies of corticospinal neurons, but was insufficient to promote regeneration of the corticospinal tract.

Neuronal overexpression of tissue-type plasminogen activator does not enhance sensory axon regeneration or locomotor recovery following dorsal hemisection of adult mouse thoracic spinal cord

CNS axons rarely regenerate spontaneously back to original targets following spinal cord injury (SCI). Neuronal expression of the serine protease tissue-type plasminogen activator (tPA) enhances axon growth in vitro and following PNS injury. Here we test the hypothesis that neuronal overexpression of tPA in adult transgenic mice promotes CNS axon regeneration and functional recovery following SCI. Adult wild-type and transgenic mouse spinal cords were subjected to dorsal hemisection at the level of the T10/T11 vertebrae. PCR confirmed incorporation of the transgene. Immunolabeling revealed overexpression of tPA in transgenic mice in neurons, including large-diameter neurons in lumbar dorsal root ganglia that contribute axons to the dorsal columns. Immunolabeling also revealed the presence of tPA protein within axons juxtaposing the injury site in transgenics but not wild types. In situ zymography revealed abundant enzymatic activity of tPA in gray matter of thoracic spinal cords of transgenics but not wild types. Rotorod locomotor testing revealed no differences between groups in locomotor function up to 21 days postinjury. Transganglionic tracer was injected into the crushed right sciatic nerve 28 days postinjury, and mice were killed 3 days later. There was no evidence for regrowth of ascending dorsal column sensory axons through or beyond the injury site. In conclusion, despite neuronal overexpression of tPA in injured neurons of transgenics, neither locomotor recovery nor regeneration of ascending sensory axons was observed following thoracic dorsal hemisection.

The differential distribution of the growth-associated protein-43 in first and higher order thalamic nuclei of the adult rat

Corticothalamic axons from layer 5 of primary and secondary auditory and visual areas have large terminals that make multiple synaptic contacts on proximal dendrites of relay cells in higher order thalamic nuclei and have been termed "driver" inputs. The corticothalamic cells express mRNA for the presynaptic growth-associated protein-43, in the adult rat [Feig SL (2004) Corticothalamic cells in layers 5 and 6 of primary and secondary sensory cortex express GAP-43 mRNA in the adult rat. J Comp Neurol 468:96-111]. In contrast, ascending driver afferents to first order nuclei (e.g. retinal, inferior collicular, and lemniscal) lose growth-associated protein-43 as mature synaptic terminals are established. Levels of immunoreactivity for growth-associated protein-43 are compared for first and higher order visual (lateral geniculate and lateral posterior), auditory (ventral and dorsal divisions of the medial geniculate), and somatosensory (ventral posterior and posterior) thalamic nuclei. At one week postnatal, staining for growth-associated protein-43 is uniform throughout first and higher order thalamic nuclei. By three weeks and thereafter, staining is denser in the higher order than first order thalamic nuclei. Electron microscopy shows growth-associated protein-43 in profiles with characteristics of afferents from layer 5 in LP and medial geniculate nucleus and no such label in retinal afferents in lateral geniculate nucleus. In these nuclei, approximately 25% of the profiles with characteristics of cortical afferents from layer 6 have label for growth-associated protein-43. The superficial layers of the superior colliculus also show growth-associated protein-43 positive profiles with characteristics of terminals from cortical layer 5. Some growth-associated protein-43 positive terminals were also positive for GABA in the thalamic nuclei studied and in the superior colliculus. The data suggest that sensory afferents to first order thalamocortical relays become stabilized once mature synaptic patterns are established, but the higher stages of information processing involving higher order thalamic relays, via cells in cortical layer 5, retain plasticity related to growth-associated protein-43 in the adult.

The ability of axons to regenerate their growth cones depends on axonal type and age, and is regulated by calcium, cAMP and ERK

The processes activated at the time of axotomy and leading to the formation of a new growth cone are the first step in regeneration, but are still poorly characterized. We investigated this event in an in vitro model of axotomy performed on dorsal root ganglia and retinal explants. We observed that the dorsal root ganglion axons and retinal ganglion cell axons, which had grown out on a poly d-lysine/laminin substrate at the time of culture preparation greatly differed in their regenerative response after a subsequent in vitro lesion made far from the cell body. The majority of axons of adult dorsal root ganglia but only a small percentage of axons of adult retinal ganglion cells regenerated new growth cones within four hours after in vitro axotomy, though both kinds of axons were growing before the lesion. The depletion of extracellular calcium and the inhibition of extracellular-signal regulated kinase 1,2 (ERK) and protein kinase A (PKA) at the time of injury significantly impaired the capacity of dorsal root ganglia axons to re-initiate growth cones without affecting growth cone motility. Pharmacological treatments directed at increasing the level of cAMP promoted growth cone regeneration in adult retinal ganglion cell axons in spite of the low regenerative potential exhibited in normal conditions. Understanding the cellular mechanisms activated at the time of lesion and leading to the formation of a new growth cone is necessary for devising treatments aimed at enhancing the regenerative response of injured axons.

Age Influences the Expression of GAP-43 in the Rat Hippocampus following Seizure

BACKGROUND

Normal aging is associated with impairments in learning and memory and motor function. One viable hypothesis is that these changes reflect an age-related decrease in brain plasticity.

OBJECTIVE

The aim of the present study was to identify age-related changes in the time course of expression of the axonal growth associated protein 43 (GAP-43) in a rat model of brain plasticity.

METHODS

We examined by Northern blotting, in situ hybridization, and immunohistochemistry the effects of age on the time course of the expression GAP-43 following pentylenetetrazole-induced seizure in the hippocampus of 3-, 18-, and 28-month-old rats.

RESULTS

In this model of brain plasticity, young rats displayed a decrease in GAP-43 mRNA levels in CA1, CA3, and polymorphic regions, lasting from 10 h to 3 days after seizure. This was followed by recovery, with peak expression between days 10 and 20. The baseline levels of GAP-43 mRNA decreased with age, especially in the CA3 region. Despite lower baseline levels, middle-aged rats showed the same pattern of upregulation of GAP-43 mRNA expression as the young animals. Old rats showed only minimal upregulation, however, and this occurred only in the polymorphic layer. The level GAP-43 protein itself was higher in old control rats than in the other two control groups, a condition that was transiently reversed by seizure activity.

CONCLUSIONS

Middle-aged rats are still capable of a sustained, though diminished, response to seizure activity, while old rats lose this ability. Disruption of the temporal and anatomical coordination of expression of GAP-43 may contribute to the general decline in brain plasticity with age.

Growth-associated gene expression after stroke: evidence for a growth-promoting region in peri-infarct cortex

Stroke induces axonal sprouting in peri-infarct cortex. A set of growth-associated genes important in axonal sprouting in peripheral nervous system regeneration and cortical development has recently been defined. The expression profiles of these growth-associated genes were defined during the post-stroke axonal sprouting response using a model of stroke in barrel field cortex. Stroke induces sequential waves of neuronal growth-promoting genes during the sprouting response: an early expression peak (SPRR1), a mid expression peak (p21, Ta1 tubulin, L1, MARCKS), a late peak (SCG10, SCLIP), and an early/sustained pattern (GAP43, CAP23, c-jun). These expression peaks correspond to specific time points in the sprouting response. The expression of the growth-inhibiting chondroitin sulfate proteoglycans aggrecan, brevican, versican, and phosphacan are induced late in the sprouting process; except neurocan, which is increased during the peak of the growth-promoting gene expression. The developmentally associated growth inhibitors ephrin-A5, ephB1, semaphorin IIIa, and neuropilin 1 are also induced in the early phases of the sprouting response. At the cellular level, chondroitin sulfate proteoglycans, in the form of peri-neuronal nets, are reduced in the region of axonal sprouting, during the peak of growth-promoting gene expression. These results identify a unique profile of growth-promoting gene expression in adult cortex after stroke, the inhibitory molecules that are present during the sprouting response, and a region in which growth-promoting genes are increased, growth-inhibitory proteins are diminished and axonal sprouting occurs. This region may be a growth-promoting zone after stroke.