Stability and transcriptional regulation of gamma-glutamyl transpeptidase mRNA expression in the initial segment of the rat epididymis.

Gamma-glutamyl transpeptidase (GGT) is an enzyme believed to play a role in the protection of maturing spermatozoa in the epididymis. Our previous studies have shown that four GGT mRNAs (I-IV) transcribed from the single-copy rat GGT gene are differentially expressed and regulated in the rat epididymis. In particular, the normal expression of GGT mRNA(IV) in the epididymal initial segment is dependent upon the presence of testicular factors. The objective of this study was to test the hypothesis that the decreased expression of GGT mRNA(IV) in the initial segment following the in vivo removal of testicular factors by efferent duct ligation (EDL) is due to a decrease in stability and/or transcription rate. The stability of the GGT mRNAs was evaluated by measuring the rate of mRNA decay. These stability studies showed that GGT mRNA(IV) exhibited a rapid initial decay that slowed at later times to a decay rate similar to that of GGT mRNAs(II,III). The decay rates were not different following sham-operation or EDL, and thus the stability of GGT mRNAs were not influenced by the in vivo loss of testicular factors. Results of transcription analysis revealed that the transcription rate of GGT mRNA(IV) in the initial segment fell by approximately 68% following a 12-hour EDL. Additionally, secondary-structure models indicate two families of folding patterns for GGT mRNA(IV), which could be the reason for the two decay regimes detected in the stability study. Thus, the decreased expression level of GGT mRNA(IV) in the initial segment following the in vivo loss of testicular factors is a function of a decreased transcription rate and intricate decay kinetics.