Regulation of prothrombin, thrombin receptor, and protease nexin-1 expression during development and after denervation in muscle.
Prothrombin, thrombin receptor (ThR), and protease nexin-1 (PN-1) mRNA levels in mouse muscle were quantified using competitive reverse transcriptase-polymerase chain reaction during development and after denervation to examine the possible role of thrombin in activity-dependent synapse elimination at the neuromuscular junction. The results showed that the levels of prothrombin and ThR were maximal at birth and decreased by two orders of magnitude by postnatal day 20 (P20). The level of PN-1 mRNA was fairly constant during development except for a 4-fold to 5-fold downregulation at P10 and P15, the periods of maximal synapse elimination at the rodent neuromuscular junction. The expression of prothrombin mRNA in muscle at birth was 41-fold and 22-fold lower than those of ThR and PN-1, respectively, and the level of difference between prothrombin and PN-1 reached almost three orders of magnitude at adulthood. Denervation of adult muscle resulted in a reversal of the relative expression levels of the three genes. There were rapid 8-fold and 10-fold increases in prothrombin and ThR mRNA, respectively, and a 2-fold decrease in PN-1 mRNA. The changes in mRNA levels of the three genes after denervation indicated that these genes were regulated in a innervation-dependent manner and that nerve activity may play an important regulatory role in the expression of prothrombin, ThR, and PN-1. The concurrent regulation of prothrombin and ThR suggests that thrombin-mediated cellular activities in muscle may be affected via the activation of ThR. An elevated level of local thrombin or thrombin-like activity might result from the decreased inhibitory activity of PN-1 during the period of peak synapse elimination in muscle development.