Reduction of EphA4 receptor expression after spinal cord injury does not induce axonal regeneration or return of tcMMEP response

Unraveling the Potential of EphA4: A Breakthrough Target and Beacon of Hope for Neurological Diseases

Erythropoietin-producing hepatocellular carcinoma A4 (EphA4) is a transmembrane receptor protein which is a part of the most prominent family of receptor tyrosine kinases (RTKs). It serves a crucial role in both physiological, biological, and functional states binding with their ligand like Ephrins. Its abundance in the majority of the body's systems has been reported. Moreover, it draws much attention in the CNS since it influences axonal and vascular guidance. Also, it has a widespread role at the pathological state of various CNS disorders. Reports suggest it obstructs axonal regeneration in various neurodegenerative diseases and neurological disorders. Although, neuro-regeneration is still an open challenge to the modern drug discovery community.  Hence, in this review, we will provide information about the role of EphA4 in neurological diseases by which it may emerge as a therapeutic target for CNS disease. We will also provide a glance at numerous signaling pathways that activate or inhibit the EphA4-associated biological processes contributing to the course of neurodegenerative diseases. Thus, this work might serve as a basis for futuristic studies that are related to the target-based drug discovery in the field of neuro-regeneration. Pathological and physiological events associated with EphA4 and Ephrin upregulation and interaction.

Regulation of axonal regeneration after mammalian spinal cord injury

One hundred years ago, Ramón y Cajal, considered by many as the founder of modern neuroscience, stated that neurons of the adult central nervous system (CNS) are incapable of regenerating. Yet, recent years have seen a tremendous expansion of knowledge in the molecular control of axon regeneration after CNS injury. We now understand that regeneration in the adult CNS is limited by (1) a failure to form cellular or molecular substrates for axon attachment and elongation through the lesion site; (2) environmental factors, including inhibitors of axon growth associated with myelin and the extracellular matrix; (3) astrocyte responses, which can both limit and support axon growth; and (4) intraneuronal mechanisms controlling the establishment of an active cellular growth programme. We discuss these topics together with newly emerging hypotheses, including the surprising finding from transcriptomic analyses of the corticospinal system in mice that neurons revert to an embryonic state after spinal cord injury, which can be sustained to promote regeneration with neural stem cell transplantation. These gains in knowledge are steadily advancing efforts to develop effective treatment strategies for spinal cord injury in humans.

Unraveling Axon Guidance during Axotomy and Regeneration

During neuronal development and regeneration axons extend a cytoskeletal-rich structure known as the growth cone, which detects and integrates signals to reach its final destination. The guidance cues “signals” bind their receptors, activating signaling cascades that result in the regulation of the growth cone cytoskeleton, defining growth cone advance, pausing, turning, or collapse. Even though much is known about guidance cues and their isolated mechanisms during nervous system development, there is still a gap in the understanding of the crosstalk between them, and about what happens after nervous system injuries. After neuronal injuries in mammals, only axons in the peripheral nervous system are able to regenerate, while the ones from the central nervous system fail to do so. Therefore, untangling the guidance cues mechanisms, as well as their behavior and characterization after axotomy and regeneration, are of special interest for understanding and treating neuronal injuries. In this review, we present findings on growth cone guidance and canonical guidance cues mechanisms, followed by a description and comparison of growth cone pathfinding mechanisms after axotomy, in regenerative and non-regenerative animal models.

Cornel Iridoid Glycoside Improves Locomotor Impairment and Decreases Spinal Cord Damage in Rats

Purpose. This study was to investigate the effects of cornel iridoid glycoside (CIG) on spinal cord injury (SCI) in rats. Methods. The thoracic cord (at T9) of rats was injured by clip compression for 30 sec. Locomotor function was assessed using the Basso, Beattie, and Bresnahan (BBB) rating scale. Neuroanatomic stereological parameters as well as Nogo-A, p75 neurotrophin receptor (p75NTR), and ROCKII expression were measured by histological processing, immunohistochemistry, and stereological analyses. The axons passing through the lesion site were detected by BDA tracing. Results. Intragastric administration of CIG (60 and 180 mg/kg) improved the locomotor impairment at 10, 17, 24, and 31 days post-injury (dpi) compared with untreated SCI model rats. CIG treatment decreased the volume of the lesion epicenter (LEp) and increased the volume of spared tissue and the number of surviving neurons in the injured spinal cord at 31 dpi. CIG promoted the growth of BDA-positive axons and their passage through the lesion site and decreased the expression of Nogo-A, p75NTR, and ROCKII both in and around the LEp. Conclusion. CIG improved the locomotor impairment, decreased tissue damage, and downregulated the myelin-associated inhibition signaling pathway in SCI rats. The results suggest that CIG may be beneficial for SCI therapy.

Modulation of Vascular ACE by Oxidative Stress in Young Syrian Cardiomyopathic Hamsters: Therapeutic Implications

Increased vascular angiotensin-converting enzyme (ACE) activity and oxidative stress are present in young Syrian cardiomyopathic hamsters (SCH) before the clinical manifestation of heart failure (HF). The developmental time-course of these alterations and their potential interactions, however, are still unknown. We evaluated mRNA and protein levels of ACE, endothelial nitric oxide synthase (eNOS), and inducible nitric oxide synthase (iNOS) in the vasculature of SCH from one to four months of age. Total RNA and proteins were quantified with real-time reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blot, respectively. The role of nitric oxide (NO) on vascular ACE activity was also assessed. ACE mRNA and protein levels were up-regulated in SCH at two months of age compared with controls (CT) (p < 0.05). At this two-month stage, eNOS protein levels were lower in SCH (87%) than in CT (100%) (p < 0.05), although iNOS protein levels increased significantly (482%) compared to CT (100%; p < 0.05). In addition, ACE mRNA expression and activity were modulated by NO at two months of age. Thus, the combination of low eNOS and high iNOS protein levels may underlie vascular renin-angiotensin system (RAS) over-activation. Altogether, these factors may contribute to the development of endothelial dysfunction and vascular hyper-reactivity in the early stages of heart failure, and eventually trigger cardiac deterioration in this animal model of HF.

Fatty Acid Binding Protein 5 Modulates Docosahexaenoic Acid-Induced Recovery in Rats Undergoing Spinal Cord Injury

Omega-3 polyunsaturated fatty acids (n-3 PUFAs) promote functional recovery in rats undergoing spinal cord injury (SCI). However, the precise molecular mechanism coupling n-3 PUFAs to neurorestorative responses is not well understood. The aim of the present study was to determine the spatiotemporal expression of fatty acid binding protein 5 (FABP5) after contusive SCI and to investigate whether this protein plays a role in n-3 PUFA-mediated functional recovery post-SCI. We found that SCI resulted in a robust spinal cord up-regulation in FABP5 mRNA levels (556 ± 187%) and protein expression (518 ± 195%), when compared to sham-operated rats, at 7 days post-injury (dpi). This upregulation coincided with significant alterations in the metabolism of fatty acids in the injured spinal cord, as revealed by metabolomics-based lipid analyses. In particular, we found increased levels of the n-3 series PUFAs, particularly docosahexaenoic acid (DHA; 22:6 n-3) and eicosapentaenoic acid (EPA; 20:5 n-3) at 7 dpi. Animals consuming a diet rich in DHA and EPA exhibited a significant upregulation in FABP5 mRNA levels at 7 dpi. Immunofluorescence showed low basal FABP5 immunoreactivity in spinal cord ventral gray matter NeuN(+) neurons of sham-operated rats. SCI resulted in a robust induction of FABP5 in glial (GFAP(+), APC(+), and NG2(+)) and precursor cells (DCX(+), nestin(+)). We found that continuous intrathecal administration of FABP5 silencing with small interfering RNA (2 μg) impaired spontaneous open-field locomotion post-SCI. Further, FABP5 siRNA administration hindered the beneficial effects of DHA to ameliorate functional recovery at 7 dpi. Altogether, our findings suggest that FABP5 may be an important player in the promotion of cellular uptake, transport, and/or metabolism of DHA post-SCI. Given the beneficial roles of n-3 PUFAs in ameliorating functional recovery, we propose that FABP5 is an important contributor to basic repair mechanisms in the injured spinal cord.

Age-dependent transcriptome and proteome following transection of neonatal spinal cord of Monodelphis domestica (South American grey short-tailed opossum)

This study describes a combined transcriptome and proteome analysis of Monodelphis domestica response to spinal cord injury at two different postnatal ages. Previously we showed that complete transection at postnatal day 7 (P7) is followed by profuse axon growth across the lesion with near-normal locomotion and swimming when adult. In contrast, at P28 there is no axon growth across the lesion, the animals exhibit weight-bearing locomotion, but cannot use hind limbs when swimming. Here we examined changes in gene and protein expression in the segment of spinal cord rostral to the lesion at 24 h after transection at P7 and at P28. Following injury at P7 only forty genes changed (all increased expression); most were immune/inflammatory genes. Following injury at P28 many more genes changed their expression and the magnitude of change for some genes was strikingly greater. Again many were associated with the immune/inflammation response. In functional groups known to be inhibitory to regeneration in adult cords the expression changes were generally muted, in some cases opposite to that required to account for neurite inhibition. For example myelin basic protein expression was reduced following injury at P28 both at the gene and protein levels. Only four genes from families with extracellular matrix functions thought to influence neurite outgrowth in adult injured cords showed substantial changes in expression following injury at P28: Olfactomedin 4 (Olfm4, 480 fold compared to controls), matrix metallopeptidase (Mmp1, 104 fold), papilin (Papln, 152 fold) and integrin α4 (Itga4, 57 fold). These data provide a resource for investigation of a priori hypotheses in future studies of mechanisms of spinal cord regeneration in immature animals compared to lack of regeneration at more mature stages.

Ephrin-B3 decreases the survival of adult rat spinal cord-derived neural stem/progenitor cells in vitro and after transplantation into the injured rat spinal cord

Although transplantation of neural stem/progenitor cells (NSPC) encourages regeneration and repair after spinal cord injury (SCI), the survival of transplanted NSPC is limited. Ephrin-B3 has been shown to reduce the death of endogenous NSPC in the subventricular zone of the mouse brain without inducing uncontrolled proliferation. Due to similarities in the environment of the brain and spinal cord, we hypothesized that ephrin-B3 might reduce the death of both transplanted and endogenous spinal cord-derived NSPC. Both normal and injured (26 g clip compression) spinal cords were examined. Ephrin-B3-Fc was tested, and Fc fragments and phosphate-buffered saline (PBS) were used as controls. We found that EphA4 receptors were expressed by spinal cord-derived NSPC and expressed in the normal and injured rat spinal cord (higher expression in the latter). In vitro, ephrin-B3-Fc did not significantly reduce the survival of NSPC except at 1 μg/mL (P<0.05), but Fc fragments alone reduced NSPC survival at all doses in a dose-dependent fashion. In vivo, intrathecal infusion of ephrin-B3-Fc increased the proliferation of endogenous ependymal cells and the proportion of proliferating cells that expressed the glial fibrillary acidic protein astrocytic marker in the injured spinal cord compared with the infusion of PBS (P<0.05). However, in the injured spinal cord, the infusion of either ephrin-B3-Fc or Fc fragments alone caused a 20-fold reduction in the survival of transplanted NSPC (P<0.001). Thus, after SCI, ephrin-B3-Fc and Fc fragments are toxic to transplanted NSPC.

Partial change in EphA4 knockout mouse phenotype: loss of diminished GFAP upregulation following spinal cord injury

In a previous study we found that the EphA4 receptor inhibits regeneration following spinal cord injury by blocking regrowth of axons and regulation of astrocyte reactivity. In our original studies using EphA4 null mice [Goldshmit et al., J. Neurosci., 2004] we found attenuated astrocyte reactivity following spinal cord injury. Several other studies have now supported the role of EphA4 in regulating neural regeneration but a recent study [Herrmann et al., Exp. Neurol., 2010] did not find an effect of EphA4 on astrocyte reactivity. Re-examination of astrocytic gliosis following injury in our current cohort of EphA4 null mice revealed that they no longer showed attenuation of astrocyte reactivity, however other EphA4 null mouse phenotypes, such as decreased size of the dorsal funiculus were unaltered. We hypothesised that long-term breeding on the C57Bl/6 background may influence the EphA4-mediated astrocyte phenotype and compared astrocytic gliosis at 4 days following spinal cord injury in wildtype and EphA4 null mice on the C57Bl/6 background and backcrossed C57Bl/6×129Sv(F2) mice, as well as wildtype 129Sv mice. 129Sv mice had increased GFAP expression and increased numbers of reactive GFAP astrocytes compared to C57Bl/6 mice. There was no significant effect of EphA4 deletion on GFAP expression in C57Bl/6 mice or the F2 crosses other than a moderately decreased number of EphA4 null astrocytes in C57Bl/6 mice using one of two antibodies. Therefore, there has been an apparent change in EphA4-mediated astroglial phenotype associated with long term breeding of the EphA4 colony but it does not appear to be influenced by background mouse strain.

Translational neuromodulation: approximating human transcranial magnetic stimulation protocols in rats

OBJECTIVE

Transcranial magnetic stimulation (TMS) is a well-established clinical protocol with numerous potential therapeutic and diagnostic applications. Yet, much work remains in the elucidation of TMS mechanisms, optimization of protocols, and in development of novel therapeutic applications. As with many technologies, the key to these issues lies in the proper experimentation and translation of TMS methods to animal models, among which rat models have proven popular. A significant increase in the number of rat TMS publications has necessitated analysis of their relevance to human work. We therefore review the essential principles for the approximation of human TMS protocols in rats as well as specific methods that addressed these issues in published studies.

MATERIALS AND METHODS

We performed an English language literature search combined with our own experience and data. We address issues that we see as important in the translation of human TMS methods to rat models and provide a summary of key accomplishments in these areas.

RESULTS

  An extensive literature review illustrated the growth of rodent TMS studies in recent years. Current advances in the translation of single, paired-pulse, and repetitive stimulation paradigms to rodent models are presented. The importance of TMS in the generation of data for preclinical trials is also highlighted.

CONCLUSIONS

Rat TMS has several limitations when considering parallels between animal and human stimulation. However, it has proven to be a useful tool in the field of translational brain stimulation and will likely continue to aid in the design and implementation of stimulation protocols for therapeutic and diagnostic applications.

Neural regenerative strategies incorporating biomolecular axon guidance signals

There are currently no acceptable cures for central nervous system injuries, and damage induced large gaps in the peripheral nervous system have been challenging to bridge to restore neural functionality. Innervation by neurons is made possible by the growth cone. This dynamic structure is unique to neurons, and can directly sense physical and chemical activity in its environment, utilizing these cues to propel axons to precisely reach their targets. Guidance can occur through chemoattractive factors such as neurotrophins and netrins, chemorepulsive agents like semaphorins and slits, or contact-mediated molecules such as ephrins and those located in the extracellular matrix. The understanding of biomolecular activity during nervous system development and injury has generated new techniques and tactics for improving and restoring function to the nervous system after injury. This review will focus on the major neuronal guidance molecules and their utility in current tissue engineering and neural regenerative strategies.

Expression and activation of ephexin is altered after spinal cord injury

Failure of axon regeneration after traumatic spinal cord injury (SCI) is attributable in part to the presence of inhibitory molecular interactions. Recent evidence demonstrates that activation of Eph signaling pathways leads to modulation of growth cone dynamics and repulsion through the activation of ephexin, a novel guanine nucleotide exchange factor (GEF). However, little is known about the expression and modulation of Eph molecular targets in the injured spinal cord. In this study, we determined the expression profile of ephexin after a moderate spinal cord contusion at thoracic level (T10) in young adult rats. Western-blot studies showed increased protein expression in injured rats at 4 and 7 days postinjury (DPI) when compared with control animals. The protein levels returned to normal at 14 DPI and remained steady until 28 DPI. However, immunoprecipitation studies of the phosphorylated ephexin demonstrated that this protein is activated by day 2 until 14 DPI. Expression of ephexin was noticeable in neurons, axons, microglia/macrophages, and reactive astrocytes, and co-localized with EphA3, A4, and A7. These results demonstrate the presence of ephexin in the adult spinal cord and its activation after SCI. Therefore, we show, for the first time, the spatiotemporal pattern of ephexin expression and activation after contusive SCI. Collectively, our data support our previous findings on the putative nonpermissive roles of Eph receptors after SCI and the possible involvement of ephexin in the intracellular cascade of events.

EphA4 blockers promote axonal regeneration and functional recovery following spinal cord injury in mice

Upregulation and activation of developmental axon guidance molecules, such as semaphorins and members of the Eph receptor tyrosine kinase family and their ligands, the ephrins, play a role in the inhibition of axonal regeneration following injury to the central nervous system. Previously we have demonstrated in a knockout model that axonal regeneration following spinal cord injury is promoted in the absence of the axon guidance protein EphA4. Antagonism of EphA4 was therefore proposed as a potential therapy to promote recovery from spinal cord injury. To further assess this potential, two soluble recombinant blockers of EphA4, unclustered ephrin-A5-Fc and EphA4-Fc, were examined for their ability to promote axonal regeneration and to improve functional outcome following spinal cord hemisection in wildtype mice. A 2-week administration of either of these blockers following spinal cord injury was sufficient to promote substantial axonal regeneration and functional recovery by 5 weeks following injury. Both inhibitors produced a moderate reduction in astrocytic gliosis, indicating that much of the effect of the blockers may be due to promotion of axon growth. These studies provide definitive evidence that soluble inhibitors of EphA4 function offer considerable therapeutic potential for the treatment of spinal cord injury and may have broader potential for the treatment of other central nervous system injuries.

Expression profile and role of EphrinA1 ligand after spinal cord injury

Spinal cord injury (SCI) triggers the re-expression of inhibitory molecules present in early stages of development, contributing to prevention of axonal regeneration. Upregulation of EphA receptor tyrosine kinases after injury suggest their involvement in the nervous system's response to damage. However, the expression profile of their ephrinA ligands after SCI is unclear. In this study, we determined the expression of ephrinA ligands after contusive SCI. Adult Sprague-Dawley female rats were injured using the MASCIS impactor device at the T10 vertebrae, and levels of ephrinA mRNA and protein determined at different time points. Identification of the cell phenotype expressing the ephrin ligand and colocalization with Eph receptors was performed with immunohistochemistry and confocal microscopy. Behavioral studies were made, after blocking ephrinA1 expression with antisense (AS) oligonucleotides, to assess hindlimb locomotor activity. Real-time PCR demonstrated basal mRNA levels of ephrin (A1, A2, A3, and A5) in the adult spinal cord. Interestingly, ephrinA1 was the only ligand whose mRNA levels were significantly altered after SCI. Although ephrinA1 mRNA levels increased after 2 weeks and remain elevated, we did not observe this pattern at the protein level as revealed by western blot analysis. Immunohistochemical studies showed ephrinA1 expression in reactive astrocytes, axons, and neurons and also their colocalization with EphA4 and A7 receptors. Behavioral studies revealed worsening of locomotor activity when ephrinA1 expression was reduced. This study suggests that ephrinA1 ligands play a role in the pathophysiology of SCI.

EphA4 deficient mice maintain astroglial-fibrotic scar formation after spinal cord injury

One important aspect of recovery and repair after spinal cord injury (SCI) lies in the complex cellular interactions at the injury site that leads to the formation of a lesion scar. EphA4, a promiscuous member of the EphA family of repulsive axon guidance receptors, is expressed by multiple cell types in the injured spinal cord, including astrocytes and neurons. We hypothesized that EphA4 contributes to aspects of cell-cell interactions at the injury site after SCI, thus modulating the formation of the astroglial-fibrotic scar. To test this hypothesis, we studied tissue responses to a thoracic dorsal hemisection SCI in an EphA4 mutant mouse line. We found that EphA4 expression, as assessed by beta-galactosidase reporter gene activity, is associated primarily with astrocytes in the spinal cord, neurons in the cerebral cortex and, to a lesser extent, spinal neurons, before and after SCI. However, we did not observe any overt reduction of glial fibrillary acidic protein (GFAP) expression in the injured area of EphA4 mutants in comparison with controls following SCI. Furthermore, there was no evident disruption of the fibrotic scar, and the boundary between reactive astrocytes and meningeal fibroblasts appeared unaltered in the mutants, as were lesion size, neuronal survival and inflammation marker expression. Thus, genetic deletion of EphA4 does not significantly alter the astroglial response or the formation of the astroglial-fibrotic scar following a dorsal hemisection SCI in mice. In contrast to what has been proposed, these data do not support a major role for EphA4 in reactive astrogliosis following SCI.

Molecular, anatomical, physiological, and behavioral studies of rats treated with buprenorphine after spinal cord injury

Acute pain is a common symptom experienced after spinal cord injury (SCI). The presence of this pain calls for treatment with analgesics, such as buprenorphine. However, there are concerns that the drug may exert other effects besides alleviation of pain. Among those reported are in vitro changes in gene expression, apoptosis, and necrosis. In this investigation, the effect of buprenorphine was assessed at the molecular, behavioral, electrophysiological, and histological levels after SCI. Rats were injured at the T10 thoracic level using the NYU impactor device. Half of the animals received buprenorphine (0.05 mg/kg) for 3 consecutive days immediately after SCI, and the other half were untreated. Microarray analysis (n = 5) was performed and analyzed using the Array Assist software. The genes under study were grouped in four categories according to function: regeneration, apoptosis, second messengers, and nociceptive related genes. Microarray analysis demonstrated no significant difference in gene expression between rats treated with buprenorphine and the control group at 2 and 4 days post-injury (DPI). Experiments performed to determine the effect of buprenorphine at the electrophysiological (tcMMEP), behavioral (BBB, grid walking and beam crossing), and histological (luxol staining) levels revealed no significant difference at 7 and 14 DPI in the return of nerve conduction, functional recovery, or white matter sparing between control and experimental groups (p > 0.05, n = 6). These results show that buprenorphine (0.05 mg/kg) can be used as part of the postoperative care to reduce pain after SCI without affecting behavioral, physiological, or anatomical parameters.

Key roles of Ephs and ephrins in retinotectal topographic map formation

Cellular and molecular mechanisms involved in the development of topographic ordered connections in the central nervous system (CNS) constitute a key issue in neurobiology because neural connectivities are the base of the CNS normal function. We discuss the roles of the Eph/ephrin system in the establishment of retinotopic projections onto the tectum/colliculus, the most detailed studied model of topographic mapping. The expression patterns of Ephs and ephrins in opposing gradients both in the retina and the tectum/colliculus, label the local addresses on the target and give specific sensitivities to growth cones according to their topographic origin in the retina. We postulate that the highest levels of these gradients could signal both the entry as well as the limiting boundaries of the target. Since Ephs and ephrins are membrane-bound molecules, they may function as both receptors and ligands producing repulsive or attractant responses according to their microenvironment and play central roles in a variety of developmental events such as axon guidance, synapse formation and remodeling. Due to different experimental approaches and the inherent species-specific differences, some results appear contradictory and should be reanalyzed. Nevertheless, these studies about the roles of the Eph/ephrin system in retinotectal/collicular mapping support general principles in order to understand CNS development and could be useful to design regeneration therapies.