Response of facial and rubrospinal neurons to axotomy: Changes in mRNA expression for cytoskeletal proteins and GAP-43

Abstract

Neurons confined within the mammalian CNS usually do not regenerate after axonal injury, while axonal regeneration is the rule in the PNS. It has been hypothesized that this may be related to differences in the microenvironment of the PNS versus CNS and to differences in the neuronal response to injury. In order to test the latter hypothesis, we compared changes in gene expression after axotomy in two populations of neurons: rat facial motoneurons and rat rubrospinal neurons. In situ hybridization with cDNA probes for the medium and light neurofilament protein revealed a reduced mRNA content in both facial and rubrospinal neurons at all times investigated (i.e., 1, 2, and 3 weeks after axotomy). On the other hand, mRNAs for actin and tubulin were increased in both neuronal populations during the first week after axotomy. While this increase was sustained in facial motoneurons for several weeks, total tubulin mRNA and actin mRNA were decreased in rubrospinal neurons at 2 and 3 weeks after axotomy, coincident with their atrophy. The developmentally regulated Tα1 tubulin mRNA, which was previously shown to be reexpressed in facial motoneurons after axotomy, was elevated severalfold in axotomized rubrospinal neurons, and increased levels persisted in some rubrospinal neurons as late as 7 weeks after axotomy. Similarly, the developmentally regulated GAP-43 mRNA increased in both axotomized facial and rubrospinal neurons, and increased levels were sustained in some axotomized rubrospinal neurons for at least 7 weeks. The response of rubrospinal neurons to axotomy in the cervical spinal cord is, in the first week, qualitatively similar to the response of facial motoneurons. However, by 2 weeks after axotomy there is a generalized reduction in mRNA levels for all three cytoskeletal proteins that is associated with neuronal atrophy. During this period, mRNA levels for the two specific markers of the growth state, Tα1 tubulin and GAP-43, remain elevated. Thus, axotomy of rubrospinal neurons appears to set in motion two independent events. First, an axotomy signal initiates a cell-body reaction similar to that of PNS neurons, including increased mRNA levels for Tα1 tubulin and GAP-43. Later, a generalized cellular atrophy and decrease in mRNA levels occur without reversing the specific responses of Tα1 and GAP-43 to axotomy. We conclude that the failure of rubrospinal neurons to regenerate is not due to a failure to initiate gene-expression changes characteristic of regenerating peripheral neurons. However, the inability to sustain increased tubulin and actin expression may be related to the abortion of initial sprouting responses of the injured axons.