CREM activator and repressor isoform expression in human male germ cells

The transcription factor cAMP-responsive element modulator (CREM) is known to play a vital role for male fertility as it has been demonstrated that male mice lacking a functional CREM gene are infertile. The CREM gene consists of 14 exons. Owing to alternative exon splicing, CREM gene expression results in the production of functionally different CREM proteins with either activating or repressing potential on target gene expression. Infertile men have been reported to reveal a substantial reduction of CREM activators and additional inaccurately spliced CREM transcripts. In the present study, we analysed the expression of CREM transcripts with the recently reported leader exons theta 1 and theta 2 and identified a new putative CREM repressor, namely theta 1-F-H, in patients with impaired spermatogenesis. In addition, we applied single cell microdissection followed by RT-PCR with leader exons B, theta 1 and theta 2 to assign the expressed CREM activator and repressor isoforms to specific germ cell types within the seminiferous epithelium. Contrary to dogma, we demonstrated CREM activator and repressor isoforms in all germ cell types, but not in Sertoli cells. However, the percentage of germ cell samples that revealed positive RT-PCR signals for these CREM activators was higher in spermatocytes and round spermatids than in spermatogonia and elongated spermatids. It remains unknown whether these activator transcripts are physiologically active. Our data suggest a fine-tuning between CREM activator and repressor isoforms in normal germ cells that might be disturbed during impaired spermatogenesis.