Immunological myelin disruption does not alter expression of regeneration-associated genes in intact or axotomized rubrospinal neurons

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

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

Neurotrophins: potential therapeutic tools for the treatment of spinal cord injury

Spinal cord injury permanently disrupts neuroanatomical circuitry and can result in severe functional deficits. These functional deficits, however, are not immutable and spontaneous recovery occurs in some patients. It is highly likely that this recovery is dependent upon spared tissue and the endogenous plasticity of the central nervous system. Neurotrophic factors are mediators of neuronal plasticity throughout development and into adulthood, affecting proliferation of neuronal precursors, neuronal survival, axonal growth, dendritic arborization and synapse formation. Neurotrophic factors are therefore excellent candidates for enhancing axonal plasticity and regeneration after spinal cord injury. Understanding growth factor effects on axonal growth and utilizing them to alter the intrinsic limitations on regenerative growth will provide potent tools for the development of translational therapeutic interventions for spinal cord injury.

Regulation of intrinsic neuronal properties for axon growth and regeneration

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

Setting the stage for functional repair of spinal cord injuries: a cast of thousands

Here we review mechanisms and molecules that necessitate protection and oppose axonal growth in the injured spinal cord, representing not only a cast of villains but also a company of therapeutic targets, many of which have yet to be fully exploited. We next discuss recent progress in the fields of bridging, overcoming conduction block and rehabilitation after spinal cord injury (SCI), where several treatments in each category have entered the spotlight, and some are being tested clinically. Finally, studies that combine treatments targeting different aspects of SCI are reviewed. Although experiments applying some treatments in combination have been completed, auditions for each part in the much-sought combination therapy are ongoing, and performers must demonstrate robust anatomical regeneration and/or significant return of function in animal models before being considered for a lead role.

Treatment of chronically injured spinal cord with neurotrophic factors stimulates betaII-tubulin and GAP-43 expression in rubrospinal tract neurons

Exogenous neurotrophic factors provided at a spinal cord injury site promote regeneration of chronically injured rubrospinal tract (RST) neurons into a peripheral nerve graft. The present study tested whether the response to neurotrophins is associated with changes in the expression of two regeneration-associated genes, betaII-tubulin and growth-associated protein (GAP)-43. Adult female rats were subjected to a right full hemisection lesion via aspiration of the C3 spinal cord. A second aspiration lesion was made 4 weeks later and gel foam saturated in brain-derived neurotrophic factor (BDNF), glial cell-line derived neurotrophic factor (GDNF), or phosphate-buffered saline (PBS) was applied to the lesion site for 60 min. Using in situ hybridization, RST neurons were examined for changes in mRNA levels of betaII-tubulin and GAP-43 at 1, 3, and 7 days after treatment. Based on analysis of gene expression in single cells, there was no effect of BDNF treatment on either betaII-tubulin or GAP-43 mRNA expression at any time point. betaII-Tubulin mRNA levels were enhanced significantly at 1 and 3 days in animals treated with GDNF relative to levels in animals treated with PBS. Treatment with GDNF did not affect GAP-43 mRNA levels at 1 and 3 days, but at 7 days there was a significant increase in mRNA expression. Interestingly, 7 days after GDNF treatment, the mean cell size of chronically injured RST neurons was increased significantly. Although GDNF and BDNF both promote axonal regeneration by chronically injured neurons, only GDNF treatment is associated with upregulation of betaII-tubulin or GAP-43 mRNA. It is not clear from the present study how exogenous BDNF stimulates regrowth of injured axons.

Long-term injured purkinje cells are competent for terminal arbor growth, but remain unable to sustain stem axon regeneration

Long-distance axon regeneration requires the activation of a specific set of neuronal growth-associated genes. Adult Purkinje cells fail to upregulate these molecules in response to axotomy and show extremely weak regenerative properties. Nevertheless, starting from several months after injury, transected Purkinje axons undergo spontaneous sprouting. Here, we asked whether long-term injured Purkinje cells acquire novel intrinsic growth properties that enable them to upregulate growth-associated genes and sustain axon regeneration. To test this hypothesis, we examined axon growth and cell body changes in adult rat Purkinje neurons following axotomy and implantation of embryonic neocortical tissue or Schwann cells into the injury track. Purkinje cells that survived over 6 months after injury/transplantation displayed profuse sprouting in the injured cerebellum and developed extensive networks of terminal branches into embryonic neocortical grafts. In addition, severed Purkinje axons exposed to these transplants 6 months after injury grew faster than their counterparts confronted with the same environment immediately after axotomy. Nevertheless, long-term injured Purkinje cells failed to regenerate stem neurites into Schwann cell grafts, and, under all experimental conditions, they did not upregulate growth-associated molecules, including c-Jun, GAP-43, SNAP-25, and NADPH-diaphorase. These results indicate that the long-term injured Purkinje cells remain unable to activate the gene program required to sustain axon regeneration and their plasticity is restricted to terminal arbor remodeling. We propose that the delayed growth of injured Purkinje cells reflects an adaptive phenomenon by which the severed axon stump develops a new terminal arbor searching for alternative connections with local partners.

Evolution of the Purkinje cell response to injury and regenerative potential during postnatal development of the rat cerebellum

To understand the mechanisms leading to the progressive loss of intrinsic neuronal growth properties during central nervous system development, we have investigated the evolution of the response to injury and regenerative potential of immature Purkinje cells, axotomized at different postnatal ages from postnatal day (P)3 to P12. In adult rodents, these neurons are characterised by a weak cell body response to axotomy, which is associated with a remarkable resistance to injury and a poor regenerative capability. During the first postnatal week, Purkinje cells are strongly sensitive to injury and massively degenerate within a few days. Immature Purkinje cells react to neurite transection by a strong upregulation of c-Jun, accompanied by a moderate, but consistent, expression of the growth-associated protein (GAP)-43. In contrast, nicotinamide adenine dinucleotide monophosphate (NADPH)-diaphorase reactivity, which can be activated by adult Purkinje neurons, is not modified in their juvenile counterparts. The severed Purkinje axons show a vigorous regenerative sprouting both into the lesioned cerebellar environment and into embryonic neocortical tissue transplanted into the injury site. The typical adult features of the response to injury progressively develop during the second postnatal week, when the injured neurons acquire resistance, cell body changes become milder, the regenerative potential declines, and the severed axons undergo characteristic morphological modifications, including torpedoes and the hypertrophy of recurrent collateral branches. This complete reversal of the features and the outcome of the Purkinje cell reaction to axotomy likely results from the profound changes that occur in the maturing Purkinje cells and/or in their microenvironment during this phase of cerebellar development.