Targeted Overexpression of the Neurite Growth-Associated Protein B-50/GAP-43 in Cerebellar Purkinje Cells Induces Sprouting after Axotomy But Not Axon Regeneration into Growth-Permissive Transplants

Abstract

B-50/GAP-43 is a nervous tissue-specific protein, the expression of which is associated with axon growth and regeneration. Its overexpression in transgenic mice produces spontaneous axonal sprouting and enhances induced remodeling in several neuron populations (Aigner et al., 1995;Holtmaat et al., 1995). We examined the capacity of this protein to increase the regenerative potential of injured adult central axons, by inducing targeted B-50/GAP-43 overexpression in Purkinje cells, which normally show poor regenerative capabilities. Thus, transgenic mice were produced in which B-50/GAP-43 overexpression was driven by the Purkinje cell-specific L7 promoter. Uninjured transgenic Purkinje cells displayed normal morphology, indicating that transgene expression does not modify the normal phenotype of these neurons. By contrast, after axotomy numerous transgenic Purkinje cells exhibited profuse sprouting along the axon and at its severed end. Nevertheless, despite these growth phenomena, which never occurred in wild-type mice, the severed transgenic axons were not able to regenerate, either spontaneously or into embryonic neural or Schwann cell grafts placed into the lesion site. Finally, although only a moderate Purkinje cell loss occurred in wild-type cerebella after axotomy, a considerable number of injured transgenic neurons degenerated, but they could be partially rescued by the different transplants placed into the lesion site. Thus, B-50/GAP-43 overexpression substantially modifies Purkinje cell response to axotomy, by inducing growth processes and decreasing their resistance to injury. However, the presence of this protein is not sufficient to enable these neurons to accomplish a full program of axon regeneration.

Degradation of Chondroitin Sulfate Proteoglycans Induces Sprouting of Intact Purkinje Axons in the Cerebellum of the Adult Rat

“…Injection of anti-Nogo-A antibodies in the cerebellar cortex induces sprouting from Purkinje axons in the granular layer (Buffo et al, 2000), and a similar phenomenon is observed after injury in GAP-43 (growthassociated protein-43)-overexpressing Purkinje cells (Buffo et al, 1997;Gianola and Rossi, 2004). In both conditions, in addition to the outgrowth of terminal fibers from the infraganglionic plexus, sprouting also occurs along the corticofugal neurite, and this phenomenon may be accompanied by disruption of the normal axon-myelin relationship (Gianola and Rossi, 2004).…”

AAV-mediated expression of CNTF promotes long-term survival and regeneration of adult rat retinal ganglion cells

“…GAP43 has an important role in promoting neurite outgrowth 23,24,55 but it is not a survival factor per se, indeed prolonged over-expression of GAP43 in other CNS neurons can sometimes induce cell death. 60 …”

In vivo single branch axotomy induces GAP-43–dependent sprouting and synaptic remodeling in cerebellar cortex

“…The involvement of GAP-43 in neuronal structural plasticity was demonstrated in many studies based on gene depletion or overexpression of its wild-type or mutated variants in cultured cells and transgenic mice (21,22). GAP-43 overexpression is sufficient to induce neurite formation and axonal sprouting in different regions of adult CNS (21,23,24).…”