Differential gene expression after complete spinal cord transection in adult rats: an analysis focused on a subchronic post-injury stage

Investigation of efficacy of treatment in spinal cord injury: Erythropoietin versus methylprednisolone

BACKGROUND

Investigation of the expression of platelet-derived growth factor (PDGF)-β and glial fibrillary acidic protein (GFAP) in rats with spinal cord injury as a marker of neurologic recovery between groups treated with erythropoietin (EPO) and methylprednisolone (MP).

METHODS

Thirty adult female rats were randomly divided into three even groups. A laminectomy was applied to thoracic ninth vertebra and contusion injury was induced by extradural application of an aneurysm clip. Group 1 rats received one-time intrathecal administration of normal saline, group 2 rats received MP, and group 3 rats received EPO. Motor neurological function was evaluated by the Basso, Beattie, and Bresnahan (BBB) locomotor rating scale. Thirty days after the surgery, T8-10 segments of the spinal cords were extracted and the immunohistochemical assay revealed the number of PDGF-β- and GFAP-positive cells.

RESULTS

Evaluation of the last control animal showed that BBB score in the EPO group showed an increase from 1 to 12 ( p < 0.05). The immunohistochemical assay revealed that the number of PDGF-β- and GFAP-positive cells was significantly higher in EPO group ( p = 0.000) when compared to MP and control groups. After studying the effect of PDGF-β expression on the locomotor function, we determined that PDGF-β expression and locomotor function after a spinal injury has a strong relationship ( p < 0.05).

CONCLUSION

EPO seems to better increase the expression of PDGF-β, thus produce better results in locomotor functions when compared to MP.

Thermogelling chitosan lactate hydrogel improves functional recovery after a C2 spinal cord hemisection in rat

The present study was designed to provide an appropriate micro-environment for regenerating axotomized neurons and proliferating/migrating cells. Because of its intrinsic permissive properties, biocompatibility and biodegradability, we chose to evaluate the therapeutic effectiveness of a chitosan-based biopolymer. The biomaterial toxicity was measured through in vitro test based on fibroblast cell survival on thermogelling chitosan lactate hydrogel substrate and then polymer was implanted into a C2 hemisection of the rat spinal cord. Animals were randomized into three experimental groups (Control, Lesion and Lesion + Hydrogel) and functional tests (ladder walking and forelimb grip strength tests, respiratory assessment by whole-body plethysmography measurements) were used, once a week during 10 weeks, to evaluate post-traumatic recoveries. Then, electrophysiological examinations (reflexivity of the sub-lesional region, ventilatory adjustments to muscle fatigue known to elicit the muscle metaboreflex and phrenic nerve recordings during normoxia and temporary hypoxia) were performed. In vitro results indicated that the chitosan matrix is a non-toxic biomaterial that allowed fibroblast survival. Furthermore, implanted animals showed improvements of their ladder walking scores from the 4th week post-implantation. Finally, electrophysiological recordings indicated that animals receiving the chitosan matrix exhibited recovery of the H-reflex rate sensitive depression, the ventilatory response to repetitive muscle stimulation and an increase of the phrenic nerve activity to asphyxia compared to lesioned and nonimplanted animals. This study indicates that hydrogel based on chitosan constitute a promising therapeutic approach to repair damaged spinal cord or may be used as an adjuvant with other treatments to enhance functional recovery after a central nervous system damage. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 2004-2019, 2017.

The upregulation of annexin A2 after spinal cord injury in rats may have implication for astrocyte proliferation

Annexin A2 (ANXA2), is a member of the annexin family of proteins that exhibit Ca²⁺-dependent binding to phospholipids. One attractive biological function of ANXA2 is participating in DNA synthesis and cell proliferation. Previous studies have shown that ANXA2 play a role in the development of the central nervous system. However, the biological function of ANXA2 after spinal cord injury (SCI) is still with limited acquaintance. In the present study, we performed a SCI model in adult rats and investigated the dynamic changes of ANXA2 expression in the spinal cord. Western blot analysis indicated a striking expression upregulation of ANXA2 after SCI. Immunohistochemistry further confirmed that ANXA2 immunoactivity was expressed at low levels in normal condition and increased at 5day after SCI. Double immunofluorescence staining prompted that ANXA2 immunoreactivity was found in astrocytes and neurons. Interestingly, ANXA2 expression was increased predominantly in astrocytes. We also examined the expression profiles of proliferating cell nuclear antigen (PCNA), Cyclin D1 and active caspase-3 in the injured spinal cords by western blot. Co-expression of ANXA2/PCNA, ANXA2/Cyclin D1 was detected in glial fibrillary acidic protein. Importantly, double immunofluorescence staining revealed that cell proliferation evaluated by PCNA appeared in many ANXA2-expressing cells and rare caspase-3 was observed in ANXA2-expressing cells after SCI. In addition, ANXA2 knockdown in astrocytes resulted in the increase of PCNA expression after LPS stimulation, showing that ANXA2 inhibited astrocyte proliferation after inflammation. Our data suggested that ANXA2 might play important roles in CNS pathophysiology after SCI.

Down regulation of lncSCIR1 after spinal cord contusion injury in rat

Extensive changes occur at transcriptional level after traumatic spinal cord injury (SCI). In this study, we performed a large scale screening of expression changes of long (>200 nt) RNA transcripts including both coding and non-coding RNA species in a rat contusion SCI model. We validated significant down-regulation of one long non-coding RNA (lncSCIR1) at 1, 4, and 7 days postinjury. lncSCIR1 knockdown promoted astrocyte proliferation and migration in vitro. We further validated the strong association between lncSCIR1 knock down and the expression changes of four mRNAs after injury. Our data indicated that lncSCIR1 down-regulation might play a detrimental role in the pathophysiology of traumatic SCI and thereby provided new insights into the studies of potential therapeutic targets for traumatic central nervous system (CNS) injuries.

MicroRNA dysregulation in spinal cord injury: causes, consequences and therapeutics

Trauma to the spinal cord causes permanent disability to more than 180,000 people every year worldwide. The initial mechanical damage triggers a complex set of secondary events involving the neural, vascular, and immune systems that largely determine the functional outcome of the spinal cord injury (SCI). Cellular and biochemical mechanisms responsible for this secondary injury largely depend on activation and inactivation of specific gene programs. Recent studies indicate that microRNAs function as gene expression switches in key processes of the SCI. Microarray data from rodent contusion models reveal that SCI induces changes in the global microRNA expression patterns. Variations in microRNA abundance largely result from alterations in the expression of the cells at the damaged spinal cord. However, microRNA expression levels after SCI are also influenced by the infiltration of immune cells to the injury site and the death and migration of specific neural cells after injury. Evidences on the role of microRNAs in the SCI pathophysiology have come from different sources. Bioinformatic analysis of microarray data has been used to identify specific variations in microRNA expression underlying transcriptional changes in target genes, which are involved in key processes in the SCI. Direct evidences on the role of microRNAs in SCI are scarcer, although recent studies have identified several microRNAs (miR-21, miR-486, miR-20) involved in key mechanisms of the SCI such as cell death or astrogliosis, among others. From a clinical perspective, different evidences make clear that microRNAs can be potent therapeutic tools to manipulate cell state and molecular processes in order to enhance functional recovery. The present article reviews the actual knowledge on how injury affects microRNA expression and the meaning of these changes in the SCI pathophysiology, to finally explore the clinical potential of microRNAs in the SCI.

An ice-templated, linearly aligned chitosan-alginate scaffold for neural tissue engineering

Several strategies have been investigated to enhance axonal regeneration after spinal cord injury, however, the resulting growth can be random and disorganized. Bioengineered scaffolds provide a physical substrate for guidance of regenerating axons towards their targets, and can be produced by freeze casting. This technique involves the controlled directional solidification of an aqueous solution or suspension, resulting in a linearly aligned porous structure caused by ice templating. In this study, freeze casting was used to fabricate porous chitosan-alginate (C/A) scaffolds with longitudinally oriented channels. Chick dorsal root ganglia explants adhered to and extended neurites through the scaffold in parallel alignment with the channel direction. Surface adsorption of a polycation and laminin promoted significantly longer neurite growth than the uncoated scaffold (poly-L-ornithine + Laminin = 793.2 ± 187.2 μm; poly-L-lysine + Laminin = 768.7 ± 241.2 μm; uncoated scaffold = 22.52 ± 50.14 μm) (P < 0.001). The elastic modulus of the hydrated scaffold was determined to be 5.08 ± 0.61 kPa, comparable to reported spinal cord values. The present data suggested that this C/A scaffold is a promising candidate for use as a nerve guidance scaffold, because of its ability to support neuronal attachment and the linearly aligned growth of DRG neurites.

Endothelialization of chitosan porous conduits via immobilization of a recombinant fibronectin fragment (rhFNIII7-10)

The present study aimed to develop a pre-endothelialized chitosan (CH) porous hollowed scaffold for application in spinal cord regenerative therapies. CH conduits with different degrees of acetylation (DA; 4% and 15%) were prepared, characterized (microstructure, porosity and water uptake) and functionalized with a recombinant fragment of human fibronectin (rhFNIII(7-10)). Immobilized rhFNIII(7-10) was characterized in terms of amount ((125)I-radiolabelling), exposure of cell-binding domains (immunofluorescence) and ability to mediate endothelial cell (EC) adhesion and cytoskeletal rearrangement. Functionalized conduits revealed a linear increase in immobilized rhFNIII(7-10) with rhFNIII(7-10) concentration, and, for the same concentration, higher amounts of rhFNIII(7-10) on DA 4% compared with DA 15%. Moreover, rhFNIII(7-10) concentrations as low as 5 and 20μg ml(-1) in the coupling reaction were shown to provide DA 4% and 15% scaffolds, respectively, with levels of exposed cell-binding domains exceeding those observed on the control (DA 4% scaffolds incubated in a 20μg ml(-1) human fibronectin solution). These grafting conditions proved to be effective in mediating EC adhesion/cytoskeletal organization on CH with DA 4% and 15%, without affecting the endothelial angiogenic potential. rhFNIII(7-10) grafting to CH could be a strategy of particular interest in tissue engineering applications requiring the use of endothelialized porous matrices with tunable degradation rates.

Characterization of inflammatory gene expression and galectin-3 function after spinal cord injury in mice

Inflammation has long been implicated in secondary tissue damage after spinal cord injury (SCI). Our previous studies of inflammatory gene expression in rats after SCI revealed two temporally correlated clusters: the first was expressed early after injury and the second was up-regulated later, with peak expression at 1-2 weeks and persistent up-regulation through 6 months. To further address the role of inflammation after SCI, we examined inflammatory genes in a second species, mice, through 28 days after SCI. Using anchor gene clustering analysis, we found similar expression patterns for both the acute and chronic gene clusters previously identified after rat SCI. The acute group returned to normal expression levels by 7 days post injury. The chronic group, which included C1qB, p22(phox) and galectin-3, showed peak expression at 7 days and remained up-regulated through 28 days. Immunohistochemistry and western blot analysis showed that the protein expression of these genes was consistent with the mRNA expression. Further exploration of the role of one of these genes, galectin-3, suggests that galectin-3 may contribute to secondary injury. In summary, our findings extend our prior gene profiling data by demonstrating the chronic expression of a cluster of microglial associated inflammatory genes after SCI in mice. Moreover, by demonstrating that inhibition of one such factor improves recovery, the findings suggest that such chronic up-regulation of inflammatory processes may contribute to secondary tissue damage after SCI, and that there may be a broader therapeutic window for neuroprotection than generally accepted.

Peripheral nerve graft with immunosuppression modifies gene expression in axotomized CNS neurons

Adult central nervous system (CNS) neurons do not regenerate severed axons unaided but may regenerate axons into apposed predegenerated peripheral nerve grafts (PNGs). We examined gene expression by using microarray technology in laser-dissected lateral vestibular (LV) neurons whose axons were severed by a lateral hemisection at C3 (HX) and in lateral vestibular nucleus (LVN) neurons that were hemisected at C3 and that received immunosuppression with cyclosporine A (CsA) and a predegenerated PNG (termed I-PNG) into the lesion site. The results provide an expression analysis of temporal changes that occur in LVN neurons in nonregenerative and potentially regenerative states and over a period of 42 days. Axotomy alone resulted in a prolonged change in regulation of probe sets, with more being upregulated than downregulated. Apposition of a PNG with immunosuppression muted gene expression overall. Axotomized neurons (HX) upregulated genes commonly associated with axonal growth, whereas axotomized neurons whose axons were apposed to the PNG showed diminished expression of many of these genes but greater expression of genes related to energy production. The results suggest that axotomized LVN neurons express many genes thought to be associated with regeneration to a greater extent than LVN neurons that are apposed to a PNG. Thus the LVN neurons remain in a regenerative state following axotomy but the conditions provided by the I-PNG appear to be neuroprotective, preserving or enhancing mitochondrial activity, which may provide required energy for regeneration. We speculate that the graft also enables sufficient axonal synthesis of cytoskeletal components to allow axonal growth without marked increase in expression of genes normally associated with regeneration.

Cloning and characterization of SCIRR69: a novel transcriptional factor belonging to the CREB/ATF family

The complete cDNA sequence of a novel gene, SCIRR69 (spinal cord injury and regeneration related no. 69 gene), was obtained by RACE technique. It codes for a protein of 521 amino acid residues homologous to human CREB3l2 (also known as BBF2H7) and mouse CREB3l2. The protein contains a basic DNA binding and leucine zipper dimerization (B-ZIP) motif and a hydrophobic region representing a putative transmembrane domain, similar to the structure of other CREB/ATF transcription factors. Monoclonal antibody against SCIRR69 was developed and could recognize the SCIRR69 protein in both native and denatured forms. Constructing of SCIRR69 fusion proteins with the GAL4 DNA-binding domain disclosed that SCIRR69 functioned as a transcriptional activator and its N-terminal 60 amino acids accounted for the activation ability. SCIRR69 resides in the cytoplasm of primary neurons, whereas neuron damage by incision led to the cleavage and translocation from the cytoplasm to the nucleus. These results suggest that SCIRR69 is activated by proteolytic cleavage at the transmembrane domain in response to neuron damage and its amino-terminal cytoplasmic domain translocates into the nucleus to activate the transcription of target genes.

Molecular target discovery for neural repair in the functional genomics era

A comprehensive understanding of the molecular pathways activated by traumatic neural injury is of major importance for the development of treatments for spinal cord injury (SCI). High-throughput gene expression profiling is a powerful approach to reveal genome-wide changes in gene expression during a specific biological process. Microarray analysis of injured nerves or neurons would ideally generate new hypotheses concerning the progression or deregulation of injury- and repair-related biological processes, such as neural scar formation and axon regeneration. These hypotheses should subsequently be tested experimentally and would eventually provide the molecular substrates for the development of novel therapeutics. Over the last decade, this approach has elucidated numerous extrinsic (mostly neural scar-associated) as well as neuron-intrinsic genes that are regulated following an injury. To date, the main challenge is to translate the observed injury-induced gene expression changes into a mechanistic framework to understand their functional implications. To achieve this, research on neural repair will have to adopt the conceptual advances and analytical tools provided by the functional genomics and systems biology revolution. Based on progress made in bioinformatics, high-throughput and high-content functional cellular screening, and in vivo gene transfer technology, we propose a multistep "roadmap" that provides an integrated strategy for molecular target discovery for repair of the injured spinal cord.

Vascular Pathology as a Potential Therapeutic Target in SCI

Acute traumatic spinal cord injury (SCI) is characterized by a progressive secondary degeneration which exacerbates the loss of penumbral tissue and neurological function. Here, we first provide an overview of the known pathophysiological mechanisms involving injured microvasculature and molecular regulators that contribute to the loss and dysfunction of existing and new blood vessels. We also highlight the differences between traumatic and ischemic injuries which may yield clues as to the more devastating nature of traumatic injuries, possibly involving toxicity associated with hemorrhage. We also discuss known species differences with implications for choosing models, their relevance and utility to translate new treatments towards the clinic. Throughout this review, we highlight the potential opportunities and proof-of-concept experimental studies for targeting therapies to endothelial cell-specific responses. Lastly, we comment on the need for vascular mechanisms to be included in drug development and non-invasive diagnostics such as serum and cerebrospinal fluid biomarkers and imaging of spinal cord pathology.

Anti-neuroinflammatory and neurotrophic effects of combined therapy with annexin II and Reg-2 on injured spinal cord

The present study was designed to investigate the neuroprotective effects of Ca(2+)-dependent phospholipid-binding protein annexin II and a secreted protein Reg-2 (regeneration gene protein 2) in spinal cord injury (SCI) model produced by contusion SCI at T(9) using the weight drop method. The agents were delivered intrathecally with Alzet miniosmotic pumps. We found that annexin II and Reg-2 remarkably reduced neuronal death, attenuated tissue damage and alleviated detrimental inflammation in vivo; meanwhile, a significant increase in white matter sparing and myelination area was observed. The propriospinal axons and long-distance supraspinal pathways were protected by the treatments as revealed by retrograde tracing. Basso Beattie Bresnahan locomotor rating scores also revealed a measurable behavioral improvement. However, no evident behavioral improvements in locomotor performance were achieved by the combined treatment with annexin II and Reg-2, compared with the separate treatment with annexin II and Reg-2.

Long descending cervical propriospinal neurons differ from thoracic propriospinal neurons in response to low thoracic spinal injury

BACKGROUND

Propriospinal neurons, with axonal projections intrinsic to the spinal cord, have shown a greater regenerative response than supraspinal neurons after axotomy due to spinal cord injury (SCI). Our previous work focused on the response of axotomized short thoracic propriospinal (TPS) neurons following a low thoracic SCI (T9 spinal transection or moderate spinal contusion injury) in the rat. The present investigation analyzes the intrinsic response of cervical propriospinal neurons having long descending axons which project into the lumbosacral enlargement, long descending propriospinal tract (LDPT) axons. These neurons also were axotomized by T9 spinal injury in the same animals used in our previous study.

RESULTS

Utilizing laser microdissection (LMD), qRT-PCR, and immunohistochemistry, we studied LDPT neurons (located in the C5-C6 spinal segments) between 3-days, and 1-month following a low thoracic (T9) spinal cord injury. We examined the response of 89 genes related to growth factors, cell surface receptors, apoptosis, axonal regeneration, and neuroprotection/cell survival. We found a strong and significant down-regulation of ~25% of the genes analyzed early after injury (3-days post-injury) with a sustained down-regulation in most instances. In the few genes that were up-regulated (Actb, Atf3, Frs2, Hspb1, Nrap, Stat1) post-axotomy, the expression for all but one was down-regulated by 2-weeks post-injury. We also compared the uninjured TPS control neurons to the uninjured LDPT neurons used in this experiment for phenotypic differences between these two subpopulations of propriospinal neurons. We found significant differences in expression in 37 of the 84 genes examined between these two subpopulations of propriospinal neurons with LDPT neurons exhibiting a significantly higher base line expression for all but 3 of these genes compared to TPS neurons.

CONCLUSIONS

Taken collectively these data indicate a broad overall down-regulation in the genes examined, including genes for neurotrophic/growth factor receptors as well as for several growth factors. There was a lack of a significant regenerative response, with the exception of an up-regulation of Atf3 and early up-regulation of Hspb1 (Hsp27), both involved in cell stress/neuroprotection as well as axonal regeneration. There was no indication of a cell death response over the first month post-injury. In addition, there appear to be significant phenotypic differences between uninjured TPS and LDPT neurons, which may partly account for the differences observed in their post-axotomy responses. The findings in this current study stand in stark contrast to the findings from our previous work on TPS neurons. This suggests that different approaches will be needed to enhance the capacity for each population of propriospinal neuron to survive and undergo successful axonal regeneration after SCI.

Role of microglia in neurotrauma

Microglia are the primary mediators of the immune defense system of the CNS and are integral to the subsequent inflammatory response. The role of microglia in the injured CNS is under scrutiny, as research has begun to fully explore how postinjury inflammation contributes to secondary damage and recovery of function. Whether microglia are good or bad is under debate, with strong support for a dual role or differential activation of microglia. Microglia release a number of factors that modulate secondary injury and recovery after injury, including pro- and anti-inflammatory cytokines, chemokines, nitric oxide, prostaglandins, growth factors, and superoxide species. Here we review experimental work on the complex and varied responses of microglia in terms of both detrimental and beneficial effects. Addressed in addition are the effects of microglial activation in two examples of CNS injury: spinal cord and traumatic brain injury. Microglial activation is integral to the response of CNS tissue to injury. In that light, future research is needed to focus on clarifying the signals and mechanisms by which microglia can be guided to promote optimal functional recovery.

Intrinsic response of thoracic propriospinal neurons to axotomy

Background

Central nervous system axons lack a robust regenerative response following spinal cord injury (SCI) and regeneration is usually abortive. Supraspinal pathways, which are the most commonly studied for their regenerative potential, demonstrate a limited regenerative ability. On the other hand, propriospinal (PS) neurons, with axons intrinsic to the spinal cord, have shown a greater regenerative response than their supraspinal counterparts, but remain relatively understudied in regards to spinal cord injury.

Results

Utilizing laser microdissection, gene-microarray, qRT-PCR, and immunohistochemistry, we focused on the intrinsic post-axotomy response of specifically labelled thoracic propriospinal neurons at periods from 3-days to 1-month following T9 spinal cord injury. We found a strong and early (3-days post injury, p.i) upregulation in the expression of genes involved in the immune/inflammatory response that returned towards normal by 1-week p.i. In addition, several regeneration associated and cell survival/neuroprotective genes were significantly up-regulated at the earliest p.i. period studied. Significant upregulation of several growth factor receptor genes (GFRa1, Ret, Lifr) also occurred only during the initial period examined. The expression of a number of pro-apoptotic genes up-regulated at 3-days p.i. suggest that changes in gene expression after this period may have resulted from analyzing surviving TPS neurons after the cell death of the remainder of the axotomized TPS neuronal population.

Conclusions

Taken collectively these data demonstrate that thoracic propriospinal (TPS) neurons mount a very dynamic response following low thoracic axotomy that includes a strong regenerative response, but also results in the cell death of many axotomized TPS neurons in the first week after spinal cord injury. These data also suggest that the immune/inflammatory response may have an important role in mediating the early strong regenerative response, as well as the apoptotic response, since expression of all of three classes of gene are up-regulated only during the initial period examined, 3-days post-SCI. The up-regulation in the expression of genes for several growth factor receptors during the first week post-SCI also suggest that administration of these factors may protect TPS neurons from cell death and maintain a regenerative response, but only if given during the early period after injury.

Effect of cervical spinal cord hemisection on the expression of axon growth markers

To evaluate the plasticity processes occurring in the spared and injured tissue after partial spinal cord injury, we have compared the level of axon growth markers after a C2 cervical hemisection in rats between the contralateral (spared) and ipsilateral (injured) cervical cord using western blotting and immunohistochemical techniques. In the ipsilateral spinal cord 7 days after injury, although GAP-43 levels were increased in the ventral horn caudal to the injury, they were globally decreased in the whole structure (C1-C6). By contrast, in the contralateral intact side 7 days and 1 month after injury, we have found an increase of GAP-43 and betaIII tubulin levels, suggesting that processes of axonal sprouting may occur in the spinal region contralateral to the injury. This increase of GAP-43 in the contralateral spinal cord after cervical hemisection may account, at least partially, to the spontaneous ipsilateral recovery observed after a cervical hemisection.

Tissue profiling MALDI mass spectrometry reveals prominent calcium-binding proteins in the proteome of regenerative MRL mouse wounds

MRL/MpJ-Fas(lpr) mice exhibit the ability to regenerate ear tissue excised by dermal punches. This is an exceptional model to identify candidate proteins that may regulate regeneration in typically nonregenerative tissues. Identification of key molecules involved in regeneration can broaden our understanding of the wound-healing process and generate novel therapeutic approaches. Tissue profiling by matrix-assisted laser desorption ionization mass spectrometry is a rapid, powerful proteomic tool that allows hundreds of proteins to be detected from specific regions of intact tissue specimens. To identify these candidate molecules, protein expression in ear punches was examined after 4 and 7 days using tissue profiling of MRL/MpJ-Fas(lpr) mice and the nonregenerative mouse strain C57BL/6J. Spectral analysis revealed distinct proteomic differences between the regenerative and nonregenerative phenotypes, including the calcium-binding proteins calgranulin A and B, calgizzarin, and calmodulin. Spatial distributions for these differentially expressed proteins within the injured regions were confirmed by immunohistochemistry.

Is the modulation of retinoid and retinoid-associated signaling a future therapeutic strategy in neurological trauma and neurodegeneration?

The complex molecular pathways that mediate the effects of vitamin A and its derivatives, are increasingly recognized as a component of the repair capacity that could be activated to induce protection and regeneration in the mature nervous tissue. Retinoid and retinoid-associated signaling plays an essential role in normal neurodevelopment and appears to remain active in the adult CNS. In this paper, we review evidence which supports the hypothesis of an activation of retinoid-associated signaling molecular pathways in the mature nervous tissue and its significance in the context of neurodegenerative, trauma-induced and psychiatric disorders, at spinal and supra-spinal levels. Finally, we summarize the potential therapeutic avenues based on the modulation of retinoid targets undergoing reactivation under conditions of acute injury and chronic degeneration in the central nervous system, and discuss some of the unresolved issues linked to this treatment strategy.

Chronic erythropoietin-mediated effects on the expression of astrocyte markers in a rat model of contusive spinal cord injury

Using a standardized rat model of contusive spinal cord injury (SCI; [Gorio A, Gokmen N, Erbayraktar S, Yilmaz O, Madaschi L, Cichetti C, Di Giulio AM, Vardar E, Cerami A, Brines M (2002) Recombinant human erythropoietin counteracts secondary injury and markedly enhances neurological recovery from experimental spinal cord trauma. Proc Natl Acad Sci U S A 99:9450-9455]), we previously showed that the administration of recombinant human erythropoietin (rhEPO) improves both tissue sparing and locomotory outcome. In the present study, to better understand rhEPO-mediated effects on chronic astrocyte response to SCI in rat, we have used immunocytochemical methods combined with confocal and electron microscopy to investigate, 1 month after injury, the effects of a single rhEPO administration on the expression of a) aquaporin 4 (AQP4), the main astrocytic water channel implicated in edema development and resolution, and two molecules (dystrophin and syntrophin) involved in its membrane anchoring; b) glial fibrillary acidic protein (GFAP) and vimentin as markers of astrogliosis; c) chondroitin sulfate proteoglycans of the extracellular matrix which are upregulated after SCI and can inhibit axonal regeneration and influence neuronal and glial properties. Our results show that rhEPO administration after SCI modifies astrocytic response to injury by increasing AQP4 immunoreactivity in the spinal cord, but not in the brain, without apparent modifications of dystrophin and syntrophin distribution. Attenuation of astrogliosis, demonstrated by the semiquantitative analysis of GFAP labeling, was associated with a reduction of phosphacan/RPTP zeta/beta, whereas the levels of lecticans remained unchanged. Finally, the relative volume of a microvessel fraction was significantly increased, indicating a pro-angiogenetic or a vasodilatory effect of rhEPO. These changes were consistently associated with remarkable reduction of lesion size and with improvement in tissue preservation and locomotor recovery, confirming previous observations and underscoring the potentiality of rhEPO for the therapeutic management of SCI.

Spinal cord injury-induced up-regulation of AHNAK, expressed in cells delineating cystic cavities, and associated with neoangiogenesis

To investigate the molecular basis for the poor regenerative capacity of the mammalian central nervous system (CNS) after injury, we searched for genes whose expression was affected by an adult rat spinal cord hemi-section. Differential screening of a rat spinal cord expression library was performed using polyclonal antibodies raised against lesioned spinal cord tissue. A striking overexpression was found for ahnak, encoding a 700-kDa protein, in normal CNS present only in the blood-brain barrier (BBB) forming vascular endothelial cells. Indeed, very early after spinal cord injury (SCI), high levels of membrane-associated AHNAK are observed on non-neuronal cells invading the lesion site. With time, AHNAK distribution spreads rostrally and caudally concomitant with the process of tissue inflammation and axon degeneration, delineating the interior surface of cystic cavities, mainly in front of barrier-forming astrocytes. Strong overexpression is also observed on vascular endothelial cells reacting to BBB breakdown. Based on our detailed analysis of its spatiotemporal and cellular expression, and its previously described function in BBB, we suggest that AHNAK expression is associated with cell types displaying tissue-protective barrier properties. Our study may thus contribute to the elucidation of the precise molecular and cellular events that eventually render traumatic spinal cord tissue non-permissive for regeneration.

Degenerative and spontaneous regenerative processes after spinal cord injury

Spinal cord injury results in acute as well as progressive secondary destruction of local and distant nervous tissue through a number of degenerative mechanisms. Spinal cord injury also initiates a number of endogenous neuroprotective and regenerative responses. Understanding of these mechanisms might identify potential targets for treatments after spinal cord injury in humans. Here, we first discuss recent developments in our understanding of the immediate traumatic and subsequent secondary degeneration of local tissue and long projecting pathways in animal models. These include the inflammatory and vascular responses during the acute phase, as well as cell death, demyelination and scar formation in the subacute and chronic phases. Secondly, we discuss the spontaneous axonal regeneration of injured and plasticity of uninjured systems, and other repair-related responses in animals, including the upregulation of regeneration-associated genes in some neurons, increases in neurotrophic factors in the spinal cord and remyelination by oligodendrocyte precursors and invading Schwann cells. Lastly, we comment on the still limited understanding of the neuropathology in humans, which is largely similar to that in rodents. However, there also are potentially important differences, including the reduced glial scarring, inflammation and demyelination, the increased Schwannosis and the protracted Wallerian degeneration in humans. The validity of current rodent models for human spinal cord injury is also discussed. The emphasis of this review is on the literature from 2002 to early 2005.

Identification of up-regulated genes after complete spinal cord transection in adult rats

Spinal cord injury (SCI) initiates a cascade of events and these responses to injury are likely to be mediated and reflected by changes in mRNA concentrations. As a step towards understanding the complex mechanisms underlying repair and regeneration after SCI, the gene expression pattern was examined 4.5 days after complete transection at T8-9 level of rat spinal cord. Improved subtractive hybridization was used to establish a subtracted cDNA library using cDNAs from normal rat spinal cord as driver and cDNAs from injured spinal cord as tester. By expressed sequence tag (EST) sequencing, we obtained 73 EST fragments from this library, representing 40 differentially expressed genes. Among them, 32 were known genes and 8 were novel genes. Functions of all annotated genes were scattered in almost every important field of cell life such as DNA repair, detoxification, mRNA quality control, cell cycle control, and signaling, which reflected the complexity of SCI and regeneration. Then we verified subtraction results with semiquantitative RT-PCR for eight genes. These analyses confirmed, to a large extent, that the subtraction results accurately reflected the molecular changes occurring at 4.5 days post-SCI. The current study identified a number of genes that may shed new light on SCI-related inflammation, neuroprotection, neurite-outgrowth, synaptogenesis, and astrogliosis. In conclusion, the identification of molecular changes using improved subtractive hybridization may lead to a better understanding of molecular mechanisms responsible for repair and regeneration after SCI.

Expression of two temporally distinct microglia-related gene clusters after spinal cord injury

The dual role of microglia in cytotoxicity and neuroprotection is believed to depend on the specific, temporal expression of microglial-related genes. To better clarify this issue, we used high-density oligonucleotide microarrays to examine microglial gene expression after spinal cord injury (SCI) in rats. We compared expression changes at the lesion site, as well as in rostral and caudal regions after mild, moderate, or severe SCI. Using microglial-associated anchor genes, we identified two clusters with different temporal profiles. The first, induced by 4 h postinjury to peak between 4 and 24 h, included interleukin-1beta, interleukin-6, osteopontin, and calgranulin, among others. The second was induced 24 h after SCI, and peaked between 72 h and 7 days; it included C1qB, Galectin-3, and p22(phox). These two clusters showed similar expression profiles regardless of injury severity, albeit with slight decreases in expression in mild or severe injury vs. moderate injury. Expression was also decreased rostral and caudal to the lesion site. We validated the expression of selected cluster members at the mRNA and protein levels. In addition, we demonstrated that stimulation of purified microglia in culture induces expression of C1qB, Galectin-3, and p22(phox). Finally, inhibition of p22(phox) activity within microglial cultures significantly suppressed proliferation in response to stimulation, confirming that this gene is involved in microglial activation. Because microglial-related factors have been implicated both in secondary injury and recovery, identification of temporally distinct clusters of genes related to microglial activation may suggest distinct roles for these groups of factors.

Genetic approaches to autonomic dysreflexia

Autonomic dysreflexia is a potentially life-threatening condition in which episodic hypertension occurs after injuries above the mid-thoracic segments of the spinal cord. Despite the seriousness of this condition, little is known of the molecular mechanisms that lead to its development. The completed sequencing of the mouse genome, its dense genetic map, and the large repository of engineered and spontaneous mouse mutants, make the mouse an ideal model organism in which to study the molecular mechanisms underlying autonomic dysreflexia. We subjected two wild-type strains of mice, 129Sv and C57BL/6, and one spontaneous mouse mutant, Wallerian degeneration slow (Wld s), to spinal cord transection and clip-compression injury. We found that the incidence of autonomic dysreflexia is greatly reduced, compared to spinal cord-transected wild-type mice, in Wld s mice after both injury paradigms and in 129Sv and C57BL/6 that have undergone the clip-compression injury. We also found that the amplitude of the dysreflexic response was greater in cord-compressed 129Sv than in C57BL/6 mice. These results implicate axonal degeneration as an important source of signals that trigger the development of autonomic dysreflexia and are discussed in the context of mouse genetics, interstrain differences and possible molecular mechanisms underlying autonomic dysreflexia after spinal cord injury.

Gene expression profiling of experimental traumatic spinal cord injury as a function of distance from impact site and injury severity

Changes in gene expression contribute to pathophysiological alterations following spinal cord injury (SCI). We examined gene expression over time (4 h, 24 h, 7 days) at the impact site, as well as rostral and caudal regions, following mild, moderate, or severe contusion SCI in rats. High-density oligonucleotide microarrays were used that included approximately 27,000 genes/ESTs (Affymetrix RG-U34; A, B and C arrays), together with multiple analyses (MAS 5.0, dChip). Alterations after mild injury were relatively rapid (4 and 24 h), whereas they were delayed and prolonged after severe injury (24 h and 7 days). The number and magnitude of gene expression changes were greatest at the injury site after moderate injury and increased in rostral and caudal regions as a function of injury severity. Sham surgery resulted in expression changes that were similar to mild injury, suggesting the importance of using time-linked surgical controls as well as naive animals for these kinds of studies. Expression of many genes and ESTs was altered; these were classified functionally based on ontology. Overall representation of these functional classes varied with distance from the site of injury and injury severity, as did the individual genes that contributed to each functional class. Different clustering approaches were used to identify changes in neuronal-specific genes and several transcription factors that have not previously been associated with SCI. This study represents the most comprehensive evaluation of gene expression changes after SCI to date. The results underscore the power of microarray approaches to reveal global genomic responses as well as changes in particular gene clusters and/or families that may be important in the secondary injury cascade.