A comparison of postlesion growth of retinotectal and corticotectal axons after superior colliculus transections in neonatal rats

Rapid plasticity of intact axons following a lesion to the visual pathways during early brain development is triggered by microglial activation

Lesions in the central nervous system (CNS) can often induce structural reorganization within intact circuits of the brain. Several studies show advances in the understanding of mechanisms of brain plasticity and the role of the immune system activation. Microglia, a myeloid derived cell population colonizes the CNS during early phases of embryonic development. In the present study, we evaluated the role of microglial activation in the sprouting of intact axons following lesions of the visual pathways. We evaluated the temporal course of microglial activation in the superior colliculus following a contralateral monocular enucleation (ME) and the possible involvement of microglial cells in the plastic reorganization of the intact, uncrossed, retinotectal pathway from the remaining eye. Lister Hooded rats were enucleated at PND 10 and submitted to systemic treatment with inhibitors of microglial activation: cyclosporine A and minocycline. The use of neuroanatomical tracers allowed us to evaluate the time course of structural axonal plasticity. Immunofluorescence and western blot techniques were used to observe the expression of microglial marker, Iba-1 and the morphology of microglial cells. Following a ME, Iba-1 immunoreactivity showed a progressive increase of microglial activation in the contralateral SC at 24 h, peaking at 72 h after the lesion. Treatment with inhibitors of microglial activation blocked both the structural plasticity of intact uncrossed retinotectal axons and microglial activation as seen by the decrease of Iba-1 immunoreactivity. The local blockade of TNF-α with a neutralizing antibody was also able to block axonal plasticity of the intact eye following a ME. The data support the hypothesis that microglial activation is a necessary step for the regulation of neuroplasticity induced by lesions during early brain development.

Scaffolds to promote spinal cord regeneration

Substantial research effort in the spinal cord injury (SCI) field is directed towards reduction of secondary injury changes and enhancement of tissue sparing. However, pathway repair after complete transections, large lesions, or after chronic injury may require the implantation of some form of oriented bridging structure to restore tissue continuity across a trauma zone. These matrices or scaffolds should be biocompatible and create an environment that facilitates tissue growth and vascularization, and allow axons to regenerate through and beyond the implant in order to reconnect with "normal" tissue distal to the injury. The myelination of regrown axons is another important requirement. In this chapter, we describe recent advances in biomaterial technology designed to provide a terrain for regenerating axons to grow across the site of injury and/or create an environment for endogenous repair. Many different types of scaffold are under investigation; they can be biodegradable or nondegradable, natural or synthetic. Scaffolds can be designed to incorporate immobilized signaling molecules and/or used as devices for controlled release of therapeutic agents, including growth factors. These bridging structures can also be infiltrated with specific cell types deemed suitable for spinal cord repair.

Injury to retinal ganglion cell axons increases polysialylated neural cell adhesion molecule (PSA-NCAM) in the adult rodent superior colliculus

The adult mammalian central nervous system (CNS) exhibits a limited regenerative response to injury. It is well established that polysialylated neural cell adhesion molecule (PSA-NCAM) contributes to nervous system plasticity. In the visual system, PSA-NCAM participates in retinal ganglion cell (RGC) axon growth during development and specifically influences RGC innervation of its principle target tissue, the superior colliculus (SC). The goals of this study were to determine whether PSA-NCAM is expressed in the normal adult mouse SC and to evaluate PSA-NCAM expression following RGC injury. In the normal rostral, but not caudal, SC we find that PSA-NCAM is present in the retinorecipient layers; however, PSA-NCAM and RGC axons do not co-localize. In the deeper collicular layers, PSA-NCAM is observed as several distinct patches that occur at the same depth along the medial-lateral axis throughout the colliculus. RGC axotomy denervates predominantly the contralateral colliculus, where increased PSA-NCAM levels are seen at 7 and 10 days after the injury. Further evaluation of the retinorecipient layers of the partially denervated SC reveals that some intact CTB-traced RGC axons (less than 5%) labeled from the ipsilateral eye do co-localize with PSA-NCAM. This study is the first characterization of PSA-NCAM expression in the normal and partially denervated adult SC and may indicate that PSA-NCAM is involved in attempted visual system remodeling after injury.

Repair of neural pathways by olfactory ensheathing cells

Damage to nerve fibre pathways results in a devastating loss of function, due to the disconnection of nerve fibres from their targets. However, some recovery does occur and this has been correlated with the formation of new (albeit abnormal) connections. The view that an untapped growth potential resides in the adult CNS has led to various attempts to stimulate the repair of disconnectional injuries. A key factor in the failure of axonal regeneration in the CNS after injury is the loss of the aligned glial pathways that nerve fibres require for their elongation. Transplantation of cultured adult olfactory ensheathing cells into lesions is being investigated as a procedure to re-establish glial pathways permissive for the regeneration of severed axons.

Gene therapy and transplantation in CNS repair: The visual system

Normal visual function in humans is compromised by a range of inherited and acquired degenerative conditions, many of which affect photoreceptors and/or retinal pigment epithelium. As a consequence the majority of experimental gene- and cell-based therapies are aimed at rescuing or replacing these cells. We provide a brief overview of these studies, but the major focus of this review is on the inner retina, in particular how gene therapy and transplantation can improve the viability and regenerative capacity of retinal ganglion cells (RGCs). Such studies are relevant to the development of new treatments for ocular conditions that cause RGC loss or dysfunction, for example glaucoma, diabetes, ischaemia, and various inflammatory and neurodegenerative diseases. However, RGCs and associated central visual pathways also serve as an excellent experimental model of the adult central nervous system (CNS) in which it is possible to study the molecular and cellular mechanisms associated with neuroprotection and axonal regeneration after neurotrauma. In this review we present the current state of knowledge pertaining to RGC responses to injury, neurotrophic and gene therapy strategies aimed at promoting RGC survival, and how best to promote the regeneration of RGC axons after optic nerve or optic tract injury. We also describe transplantation methods being used in attempts to replace lost RGCs or encourage the regrowth of RGC axons back into visual centres in the brain via peripheral nerve bridges. Cooperative approaches including novel combinations of transplantation, gene therapy and pharmacotherapy are discussed. Finally, we consider a number of caveats and future directions, such as problems associated with compensatory sprouting and the reformation of visuotopic maps, the need to develop efficient, regulatable viral vectors, and the need to develop different but sequential strategies that target the cell body and/or the growth cone at appropriate times during the repair process.

Postnatal innervation of the rat superior colliculus by axons of late-born retinal ganglion cells

Rat retinal ganglion cells (RGCs) are generated between embryonic day (E) 13 and E19. Retinal axons first reach the superior colliculus at E16/16.5 but the time of arrival of axons from late-born RGCs is unknown. This study examined (i) whether there is a correlation between RGC genesis and the timing of retinotectal innervation and (ii) when axons of late-born RGCs reach the superior colliculus. Pregnant Wistar rats were injected intraperitoneally with bromodeoxyuridine (BrdU) on E16, E18 or E19. Pups from these litters received unilateral superior colliculus injections of fluorogold (FG) at ages between postnatal (P) day P0 and P6, and were perfused 1-2 days later. RGCs in 3 rats from each BrdU litter were labelled in adulthood by placing FG onto transected optic nerve. Retinas were cryosectioned and the number of FG, BrdU and double-labelled (FG+/BrdU+) RGCs quantified. In the E16 group, the proportion of FG-labelled RGCs that were BrdU+ did not vary with age, indicating that axons from these cells had reached the superior colliculus by P0/P1. In contrast, for the smaller cohorts of RGCs born on E18 or E19, the proportion of BrdU+ cells that were FG+ increased significantly after birth; axons from most RGCs born on E19 were not retrogradely FG-labelled until P4/P5. Thus there is a correlation between birthdate and innervation in rat retinotectal pathways. Furthermore, compared to the earliest born RGCs, axons from late-born RGCs take about three times longer to reach the superior colliculus. Later-arriving axons presumably encounter comparatively different growth terrains en route and eventually innervate more differentiated target structures.

Intravitreal injection of adeno-associated viral vectors results in the transduction of different types of retinal neurons in neonatal and adult rats: a comparison with lentiviral vectors

Replication-deficient viral vectors encoding the marker gene green fluorescent protein (GFP) were injected into the vitreous of newborn, juvenile (P14), and adult rats. We tested two different types of modified virus: adeno-associated viral-2-GFP (AAV-GFP) and lentiviral-GFP vectors (LV-GFP). The extent of retinal cell transduction in different-aged animals was compared 7, 21, and 70 days after eye injections. At all postinjection times, LV-GFP transduction was mostly limited to pigment epithelium and cells in sclera and choroid. In contrast, transduction of large numbers of neural retinal cells was seen 21 and 70 days after AAV-GFP injections. AAV-GFP predominantly transduced neurons, although GFP-positive Müller cells were seen. All neuronal classes were labeled, but the extent of transduction for a given class varied depending on injection age. After P0 injections about 50% of transduced cells were photoreceptors and 30-40% were amacrine or bipolar cells. After adult injections 60-70% of transduced cells were retinal ganglion cells. In adults many GFP-positive retinal axons were traced through the optic nerve/tract and terminal arbors were visualized in central targets.

Reinnervation of the superior colliculus delays down-regulation of ephrin A2 in neonatal rat

Although the adult mammalian optic nerve does not regenerate following lesion, in the neonatal rat, retinal ganglion cell (RGC) axons retain the capacity to grow across lesion sites in the brain. Following a brachial lesion at postnatal day 2 (P2), some RGC axons, together with ingrowing cortico-tectal axons, cross the lesion to reinnervate the superior colliculus (SC). Here we use immunohistochemistry to examine expression of the guidance cue ephrin A2 following a brachial lesion. Normal animals show a steady decrease in ephrin A2 immunoreactivity between P5 and P31, with a low rostral to high caudal gradient being evident only at P5. By contrast, after brachial lesion, values are significantly elevated rostrally at P5 and caudally at P12; moreover, a steep rostro-caudal gradient is present at both ages. By P31 values fall to normal levels. Following unilateral enucleation at P2, levels are not significantly different from normal. Our results show that innervation but not denervation triggers increased ephrin A2 expression after a brachial lesion.