Evidence That the Loss of Homeostatic Signals Induces Regeneration-Associated Alterations in Neuronal Gene Expression

Abstract

Axotomy of motoneurons leads to upregulation of Tα1 α-tubulin and p75 NGF receptor mRNAs. To distinguish whether these increases are due to interruption of ongoing homeostatic signals or to positive factors derived from non-neuronal cells of the injured nerve, we developed an experimental paradigm that allowed us to selectively block some facets of axonal signaling without initiating a peripheral nerve injury response. More specifically, the marginal mandibular and buccal branches of the facial nerve were locally cooled to 4-8°C (a cold block) for 36-60 hr. This treatment effectively and reversibly blocked fast axonal transport, as monitored by the ability of facial motor neurons to retrogradely transport fluorogold from their terminals to their cell bodies. Light microscopy of semithin epoxy sections demonstrated that the cold block treatment did not lead to macrophage invasion or to morphological alterations in the nerve either proximal or distal to the cold block region. In situ hybridization and image analysis of retrogradely labeled facial motor neurons indicated that such a cold block induced Tα1 α-tubulin and p75 NGF receptor mRNAs to the same level as did a corresponding nerve transection. In contrast, T26 α-tubulin mRNA, which does not increase following axotomy, was not affected by the cold block treatment. These results suggest that neurons sense their status by a constant flow of information carried, circulated, or maintained by fast axonal transport and that the axotomy-induced increases in Tα1 and p75 NGF receptor mRNAs in motor neurons are, to a great extent, due to loss of such homeostatic signals.