Ribosomal RNA transcriptional activation and processing in hamster facial motoneurons: Effects of axotomy with or without exposure to testosterone

Abstract

A key step in the ability of neurons to survive injury and successfully regenerate involves ribosomal RNA production. Testosterone propionate (TP), augments facial nerve regeneration in the adult hamster. TP modulates the nucleolar reaction in injured facial motoneurons, such that mature ribosome levels increase more rapidly and in greater magnitude than with injury only. In this study, molecular and electron microscopic stereologic approaches were used to determine the effects of axotomy and steroid treatment on ribosomal transcription and processing in facial motoneurons. Castrated adult male hamsters were subjected to right facial nerve transection at the stylomastoid foramen. Half the animals were subcutaneously implanted with one Silastic TP capsule, with the remainder sham implanted. For the in situ hybridization experiments, postoperative survival times were 0.5, 2, or 6 hours. In situ hybridization with a ribosomal DNA probe specific to the external transcribed spacer region located at the 5' end of the ribosomal gene was accomplished. Transcriptional activation of the rRNA gene occurred rapidly, within 2 hours, after injury only. Unexpectedly, TP treatment did not alter the time course or magnitude of rRNA transcriptional activity. For the electron microscope experiments, the postoperative time of 12 hours was selected. Stereologic analysis of 3 nucleolar subcomponents, fibrillar centers (site of rRNA transcription), nucleolonema (site of rRNA processing), and granular material (site of preribosome storage), was accomplished. TP decreased the nucleolonemal strands and the granular material, relative to injury only. These results suggest that, although rRNA transcription is rapidly activated by axotomy, rRNA processing is temporarily stalled. TP does not affect the early, axotomy-induced transcriptional activation of the ribosomal gene, but may, instead, prevent the subsequent disruption in rRNA processing. An hypothesis for the molecular mechanism by which steroids augment the regenerative capabilities of injured facial motoneurons is presented.

RNA Polymerase 1-driven Transcription as a Mediator of BDNF-induced Neurite Outgrowth

“…We have demonstrated that stimulation of nucleolar transcription in developing forebrain neurons is the major morphogenic effector mechanism of the ERK1/2 pathway. As the nucleolar activation occurs in regenerating neurons (12,13), the morphogenic activity of the nucleolus may also play a role in neuroregeneration. Finally, in severe depression of developmental origin, hippocampal atrophy correlates with reduced nucleolar transcription (45).…”

“…Conversely, increases in nucleolar size and Pol1 activity have been observed during regenerative axonal growth of adult motor neurons (12,13). Although these observations suggest that the nucleolus plays a role in neuronal morphogenesis, direct testing of such a possibility has not been yet reported.…”

Emerging roles of the neuronal nucleolus

“…Some growth abilities are also retained in the mature nervous system such as the growth of axons which occurs during peripheral nerve regeneration after injury (reviewed in [36]). There is a good correlation between regenerative growth of injured neurons and increased nucleolar activity in these cells [39, 40]. Taken together, morphological observations suggest a link between neuronal growth and the dynamic changes of the nucleolus indicative of increased ribosomal biogenesis.…”

Impaired ribosome biogenesis: mechanisms and relevance to cancer and aging

“…Consequently, upregulated ribosome biogenesis supply developing neurites with a sufficient number of ribosomes for the increased local protein synthesis to promote morphogenesis of the neurons [17,302]. Furthermore, it has been also suggested that neurite outgrowth, which is promoted in mature neurons during regeneration of the nerves following injury, depends on the upregulation of ribosome biogenesis [304,305].…”