Differential regulation of S100β and mRNAS coding for S100‐like proteins (42A and 42C) during development and after lesion of rat sciatic nerve

The changes in the levels of S1007beta; (a protein that stimulates neurite extension and neuronal survival) and 42A and 42C (S100‐like proteins whose mRNAs are induced in PC 12 cells by nerve growth factor) during development and after rat sciatic nerve lesions were analyzed. S100β, 42A, and 42C mRNAs showed differential regulation during development. S100β mRNA was present both in sciatic nerve and brain, and increased more than 11‐fold during the first 3 wk of nerve postnatal development. 42A and 42C mRNAs were essentially restricted to sciatic nerve, with little found in either embryonic or adult brain. The levels of 42C and 42A mRNAs in sciatic nerve increased 4 ‐and 14 ‐fold, respectively, by postnatal day 23 compared to postnatal day 2. 42A, 42C, and S100β mRNAs also showed a differential regulation during sciatic nerve degeneration and regeneration. Axotomized and control sciatic nerves were examined by Northern blots at various times after a crush or cut injury. 42A and 42C mRNA levels increased rapidly in the distal segment of axotomized nerve, remained two‐ to five‐fold higher than controls at day 14 after injury but returned to control levels by 40 days. In contrast, S100β mRNA showed a three‐fold decrease in the axotomized nerve between days 1 and 3 after injury, and slowly returned towards control levels over the next few weeks. The decrease in S100β mRNA was reflected by a corresponding decrease in S100β protein levels. The induction of 42A and 42C mRNAs and repression of S100β mRNA remained if nerve regeneration was prevented. These data suggest that, in spite of the high degree of sequence homology, S100β and its two homologues 42A and 42C play different roles in neuronal development and regeneration.