Development of the rat corticospinal tract through an altered glial environment

Gfap-positive radial glial cells are an essential progenitor population for later-born neurons and glia in the zebrafish spinal cord

Radial glial cells are presumptive neural stem cells (NSCs) in the developing nervous system. The direct requirement of radial glia for the generation of a diverse array of neuronal and glial subtypes, however, has not been tested. We employed two novel transgenic zebrafish lines and endogenous markers of NSCs and radial glia to show for the first time that radial glia are essential for neurogenesis during development. By using the gfap promoter to drive expression of nuclear localized mCherry we discerned two distinct radial glial-derived cell types: a major nestin+/Sox2+ subtype with strong gfap promoter activity and a minor Sox2+ subtype lacking this activity. Fate mapping studies in this line indicate that gfap+ radial glia generate later-born CoSA interneurons, secondary motorneurons, and oligodendroglia. In another transgenic line using the gfap promoter-driven expression of the nitroreductase enzyme, we induced cell autonomous ablation of gfap+ radial glia and observed a reduction in their specific derived lineages, but not Blbp+ and Sox2+/gfap-negative NSCs, which were retained and expanded at later larval stages. Moreover, we provide evidence supporting classical roles of radial glial in axon patterning, blood-brain barrier formation, and locomotion. Our results suggest that gfap+ radial glia represent the major NSC during late neurogenesis for specific lineages, and possess diverse roles to sustain the structure and function of the spinal cord. These new tools will both corroborate the predicted roles of astroglia and reveal novel roles related to development, physiology, and regeneration in the vertebrate nervous system. GLIA 2016;64:1170-1189.

The unique histopathological responses of the injured spinal cord. Implications for neuroprotective therapy

Tissue destruction at the primary site of a spinal cord injury leads to persistent necrosis that progressively enlarges the lesion. Steroids attenuate this necrotizing process and promote tissue repair even though such anti-inflammatory drugs interfere with wound healing in non-CNS organs. To address this paradox, the spinal cord of rats and mice was crushed extradurally and the effects of the following anti-inflammatory agents studied by light microscopical image analysis: allopurinol, aminoguanidine, indomethacin, a bacterial lipopolysaccharide, naproxen, and pregnenolone. The contribution of Wallerian degeneration to progressive necrosis was studied in a mutant mouse strain (WldS) that is characterized by delayed Wallerian degeneration. In rats, the anti-inflammatory agents selectively attenuated progressive necrosis and encouraged wound healing. In mice, considerable tissue repair occurred without pharmacological intervention; this wound-healing process was delayed in the mutant WldS strain. Since spinal cord injury results in concomitant tissue necrosis and wound healing, a goal of neuroprotective therapy is to regulate the dynamic balance between these destructive and reparative processes.

Effects of spinal cord X-irradiation on the recovery of paraplegic rats

Axonal regrowth is limited in the adult CNS, especially in the spinal cord, one of the major sites of traumatic lesions. Pathophysiological changes occurring after spinal cord injury include complex acute, subacute, and late processes. In this study, we assessed whether X-irradiation interferes with the acute/subacute phases, thereby improving the functional recovery of paraplegic animals. Two days after acute compression of adult rat spinal cords, various doses (0, 2, 5, 10, 20 Gy) of X-rays were administered as one single dose to the compression site. The animals were functionally evaluated over the course of 1 month after injury, using the Tarlov scale and the Rivlin and Tator scale. We also designed a "physiological" scale, including an assessment of urinary function and infection, appropriate for the evaluation of spinal-cord-lesioned animals. Behavioral analysis suggested that the high doses, 20 Gy and, to a lesser extent, 5 and 10 Gy, were toxic, as shown by morbidity rate and "physiological" score. The 2-Gy group showed better motor performances than the lesioned nonirradiated (LNI) animals and the 5- and 20-Gy groups. Motor performance in the 5-, 10-, and 20-Gy groups was poorer than that seen in the LNI group. Gliosis was reduced in the 2-Gy group compared to LNI animals, and there was high levels of gliosis in the highly (>/=5 Gy) irradiated animals. There was a 23% less lesion-induced syringomyelia in the 2-Gy group than in the other groups (LNI and 5-20 Gy). Thus, low doses of X-rays may interfere with the formation of syringomyelia and glial scar, thereby facilitating the recovery of paraplegic animals. These findings suggest that low-dose irradiation of the lesion site, in association with other therapies, is a potentially promising treatment for improving recovery after spinal cord injury.

Axon guidance of outgrowing corticospinal fibres in the rat

This review is concerned with the development of the rat corticospinal tract (CST). The CST is a long descending central pathway, restricted to mammals, which is involved both in motor and sensory control. The rat CST is a very useful model in experimental research on the development of fibre systems in mammals because of its postnatal outgrowth throughout the spinal cord as well as its experimental accessibility. Hence mechanisms underlying axon outgrowth and subsequent target cell finding can be studied relatively easily. In this respect the corticospinal tract forms an important example and model system for the better understanding of central nervous system development in general.

Corticospinal tract regrowth

The natural ability of the adult central nervous system of higher vertebrates to recover from injury is highly limited. This limitation is most likely due to an inhospitable environment and/or intrinsic incapacities of the neurons to re-extend their neurites after injury or axotomy. The rat corticospinal tract is the largest tract leading from brain to spinal cord and is often used as a model in developmental and regeneration studies. The extensive know-how of factors involved in the development of the corticospinal tract did provide the foundation for many studies on corticospinal tract regrowth after injury in the adult spinal cord. The results of these experiments, as discussed in this review, have led to important contributions to the further understanding of central nervous system regeneration.

Experimental spinal cord injury: Wallerian degeneration in the dorsal column is followed by revascularization, glial proliferation, and nerve regeneration

The presence of adequate blood supply is a critical factor in recovery from traumatic injuries. We have examined whether the revascularization of the injured tissues is as crucial a precondition for wound healing in the spinal cord as in other organs. The development of the initial primary lesion (PL) after spinal crush injury in rats is followed by the formation of a unique tunnel-like dorsal column lesion (DCL) that extends rostrocaudally for many millimeters from the primary injury site. The DCL has been shown to result from Wallerian degeneration of the long spinal tracts in the dorsal column. In this study, we compared the processes of revascularization, wound healing, and nerve regeneration in the PL and the DCL by light microscopy after a crush injury of the cord. The spinal cord of 54 adults rats was crushed at T8 with jewelers forceps. The rats were allowed to survive from 3 h up to 8 weeks after spinal cord injury. The PL appeared immediately after injury and the DCL began to develop 6 h later. Infiltration of neutrophils, which is the first sign of the inflammatory responses to injury, began several hours later in the DCL than in the PL. Secondary vascular injury then occurred which resulted in hemorrhage around the DCL and rapid enlargement of the lesion during the remainder of the first week. Subsequent changes in the PL and DCL were entirely different. The PL underwent progressive enlargement and cavitation such that by 8 weeks, the lesion contained only very few cells, vessels, and axons scattered between huge fluid-filled cavities. The DCL, on the other hand, was maximal in size at 1 week and declined significantly in size and cavitation thereafter. By 8 weeks it was highly vascularized, contained abundant nerve fibers, and lacked any trace of cavitation. These findings amplify the current view that ischemia plays a critical role in spinal cord trauma by showing that revascularization precedes tissue repair and nerve regeneration in the dorsal columns. We conclude (a) that a well-vascularized lesion permits the ingrowth of glial and other cells which give rise to a supportive matrix for the nerve regeneration and (b) that procedures which induce revascularization or angiogenesis will ameliorate the cascade of progressive tissue necrosis.

Conditional ablation of cerebellar astrocytes in postnatal transgenic mice

Astrocytes have been proposed to have multiple roles in the development and maintenance of the vertebrate CNS. To facilitate documentation of these roles, we designed a transgene to enable their ablation at selectable times. The transgene consists of the coding region for the herpes simplex virus-thymidine kinase (HSV-TK) under the control of the human glial fibrillary acidic protein gene promoter. The HSV-TK is innocuous but converts the antiherpetic agent ganciclovir (GCV) to a toxic product that interferes with DNA replication in proliferating cells. In a developmental study, transgenic mice were treated with GCV during the first postnatal week, with evaluation at P19. Treated mice displayed severe ataxia. Histological examination revealed disrupted astrocyte development, particularly in the cerebellum, with marked secondary effects on other cell types. Cerebellar defects included a loss in the numbers of astrocytes and an overall reduction in cerebellar size and disruption of the normally well defined cellular layers. Radial glia were disordered, Purkinje cells were ectopically distributed and displayed abnormal dendritic trees, and granule cells were markedly depleted. These effects were more severe in animals treated on postnatal day 1 versus treatment at day 5. A major factor causing granule cell death was excitotoxicity attributable to activation of NMDA receptors. These results suggest a critical role for astrocytes in cerebellar development.

Genetic influences on cellular reactions to spinal cord injury: a wound-healing response present in normal mice is impaired in mice carrying a mutation (WldS) that causes delayed Wallerian degeneration

Progressive tissue necrosis is a process unique to the injured mammalian spinal cord which often leads to gradually increasing cavitation and enlargement of the lesion. To evaluate the role of neuronal degeneration in initiating this response, histopathological changes were compared in C57BL and WldS (delayed Wallerian degeneration mutation) mice. The spinal cord was crushed at T8, producing a primary lesion at the site of the trauma and a secondary lesion extending rostrocaudally in the dorsal columns (where long ascending and descending fiber tracts undergo Wallerian degeneration). Cavitation was relatively mild at both sites and developed mainly at the margins of the lesions. In striking contrast to spinal cord injury in rats, progressive necrosis did not occur in mice; instead, the primary and secondary lesion sites became filled in by macrophages and fibroblasts embedded in a well-vascularized collagenous stroma. Quantitative image analysis revealed that the primary lesion decreased dramatically in size and cavitation between 2 and 3 weeks in C57BL, whereas in WldS the reduction in size and cavitation began later (at 4 weeks) and was less complete. The initial development of the secondary lesion began later and its healing was less complete in WldS than C57BL. These results are consistent with the hypothesis that neuronal damage, including Wallerian degeneration, triggers inflammatory responses leading to tissue repair. For this reason, any delay in neuronal degeneration, as in the WldS mutation, results in deficient tissue repair as reflected in the larger size of both primary and dorsal column lesions.

Anatomy, development and lesion-induced plasticity of rodent corticospinal tract

In this review the current knowledge of the anatomy, development and plasticity of the rodent corticospinal tract is summarised. Recent technical advancements, especially in neuronal tracing methods, have provided much new data concerning the anatomy of the corticospinal tract. The rodent corticospinal axons project to the subcortical nuclei via collateral branches. These collateral branches of corticospinal axons are formed by delayed interstitial budding during early postnatal periods. Corticospinal neurons are generated in the ventricular zone during a short time lag, migrate into the cortical plate, and settle in layer V of the cerebral cortex. The migration of corticospinal neurons is experimentally deranged by prenatal exposure to alcohol or genetically affected by the reeler genetic locus (rl), resulting in generation of ectopic corticospinal neurons. Such experimentally or genetically induced ectopic corticospinal neurons are a good model for examining whether target recognition and path finding are affected by the intracortical position of corticospinal neurons. Some chemical molecules (e.g. L1 and B-50/GAP43) are transiently expressed in the corticospinal tract during the perinatal period, while others (e.g. protein kinase C gamma subspecies and alpha CaM kinase II) are permanently expressed in the adult corticospinal tract. The only chemical marker specific for layer V corticofugal neurons is an antibody to a soluble protein, protein 35. Since the corticospinal tract in the rodent is an easily identified group of fibers situated in the most ventral portion of the dorsal funiculus of the spinal cord and exhibits considerable postnatal development, it has often been utilized in the neurological studies on plasticity and regenerative capacity of the lesioned central nervous system. Recently, it has been clarified that growing corticospinal fibers have the ability to penetrate and traverse across the lesion sites under certain special conditions.

Patterns of Schwann cell myelination of axons within the spinal cord

Patterns of Schwann cell myelination of long-projecting axons in the spinal cord were studied. The goal was to determine if such axons arising from neurons whose somata and processes are normally confined to the central nervous system can interact effectively with Schwann cells, the myelinating cells of the peripheral nervous system. In one paradigm Schwann cells develop in the dorsal funiculi of the lumbar spinal cord subsequent to radiation-induced alterations in development of the glial populations. Light and electron microscopic evaluations were made in the region of the corticospinal tracts (CSTs), which in the rat occupy the base of the dorsal funiculi. At 90 days following irradiation, larger axons of these tracts (> 1.5 microns in diameter) were myelinated by Schwann cells, and smaller axons were ensheathed by them. In the second paradigm cultured Schwann cells were injected into the medial portions of the ventral funiculi at 13 days post-irradiation when the glial population was markedly reduced. Earlier investigations from this laboratory demonstrated that Schwann cells do not develop in the irradiated ventral funiculi, as they do dorsally. When placed in proximity to long-projecting axons in the medial portion of the ventral funiculi, the Schwann cells either formed compact myelin sheaths or ensheathed axons, depending upon their diameter. Fasciculation and presence of collagen were characteristic of this paradigm but were absent from the Schwann cell-occupied regions of the CSTs. This probably relates to the presence of fibroblasts in the injected cultures.(ABSTRACT TRUNCATED AT 250 WORDS)

K+ and pH homeostasis in the developing rat spinal cord is impaired by early postnatal X-irradiation

Activity-related transient changes in extracellular K+ concentration ([K+]e) and pH (pHe) were studied by means of ion-selective microelectrodes in neonatal rat spinal cords isolated from pups 2-14 days of age. Pups 1 to 2 days old were X-irradiated to impair gliogenesis and spinal cords were isolated 2-13 days postirradiation (PI). In 2- to 14-day-old pups PI stimulation produced ionic changes that were the same as those in 3- to 6-day-old control (non-irradiated) pups; e.g. the [K+]e increased by 4.03 +/- 0.24 mM (mean +/- S.E.M., n = 30) at a stimulation frequency of 10 Hz and this was accompanied by an alkaline shift of 0.048 +/- 0.004 pH units (mean +/- S.E.M., n = 32) pH units. By contrast, stimulation in non-irradiated 10- to 14-day-old pups produced smaller [K+]e changes, of 1.95 +/- 0.12 mM (mean +/- S.E.M., n = 30), and an acid shift of 0.035 +/- 0.003 pH units which was usually preceded by a scarcely discernible initial alkaline shift, as is also the case in adult rats. Our results show that the decrease in [K+]e ceiling level and the development of the acid shift in pHe are blocked by X-irradiation. Concomitantly, typical continuous development of GFAP-positive reaction was disrupted and densely stained astrocytes in gray matter of 10- to 14-day-old pups PI revealed astrogliosis. In control 3- to 6-day-old pups and in pups PI the stimulation-evoked alkaline, but not the acid, shift was blocked by Mg2+ and picrotoxin (10(-6) M). The acid shift was blocked, and the alkaline shift enhanced, by acetazolamide, Ba2+, amiloride and SITS. Application of GABA evoked an alkaline shift in the pHe baseline which was blocked by picrotoxin and in HEPES-buffered solution. By contrast, the stimulus-evoked alkaline shifts were enhanced in HEPES-buffered solutions. The results suggest a dual mechanism of the stimulus-evoked alkaline shifts. Firstly, the activation of GABA-gated anion (Cl-) channels induces a passive net efflux of bicarbonate, which may lead to a fall in neuronal intracellular pH and to a rise in the pHe. Secondly, bicarbonate independent alkaline shifts may arise from synaptic activity resulting in a flux of acid equivalents.

Glial response to dorsal root lesion in the irradiated spinal cord

Exposure of the rat lumbar spinal cord to X-rays during the early postnatal period results in a marked reduction in the glial populations within the irradiated region. The present study was undertaken to determine what effects this reduction of glia, particularly astrocytes, has on the pattern and characteristics of the scar formation that follows root injury in the normal spinal cord. Morphological assessments 60 days following injury of the right L4 dorsal root revealed a distinct difference in the extent of the astrocyte response between the irradiated and the nonirradiated rats. In the nonirradiated animals, a thick astrocytic scar composed of multiple layers of astrocyte processes formed over the dorsal horn and adjacent portions of the dorsal surface of the cord. This astrocytic response was not confined to the surface of the spinal cord but extended also into the root, i.e., into regions normally considered as PNS. In irradiated rats, the astrocytes did not form a thick scar nor did they extend into the injured root. Instead, they formed a glia limitans and were at most only one or two layers thick over the region of cord comparable to that occupied by the thick astrocytic scar in the nonirradiated rats. Mechanisms involved in the glial response of the irradiated spinal cord to dorsal root injury are discussed, particularly with regard to the possible positive effect that this reduction in scar formation may have on regrowth of injured dorsal root axons into the spinal cord environment.

The corticospinal tract attains a normal configuration in the absence of myelin: observations in jimpy mutant mice

The corticospinal projection was examined in dysmyelinated, jimpy mice and in unaffected littermates following cortical injections of either wheat germ agglutinin conjugated to horseradish peroxidase or biocytin. Corticospinal axons in both phenotypes traverse the medulla within a well-defined pyramidal tract, decussate within several fascicles at the spinomedullary junction, and extend down the spinal cord in a compact bundle in the ventral-most part of the dorsal funiculus. Very few labeled fibers are seen separated from the main bundle. This normal configuration of the corticospinal tract is attained despite the virtual absence of CNS myelin in jimpy mice. It seems unlikely then that the myelin normally present in fiber bundles adjacent to this relatively late emerging projection can significantly influence pathway selection during its development.