Stage-specific expression of the Atce1/Tisp40alpha isoform of CREB3L4 in mouse spermatids.

The maturation of haploid spermatids into spermatozoa relies on the timely production of proteins required for spermatid differentiation. The mammalian CREB3L4 (cAMP responsive element binding protein 3-like 4) gene encodes a bZIP transcription factor that associates with the membrane of the endoplasmic reticulum. CREB3L4 is presumed to play an important role in protein maturation via its involvement in the cellular response to endoplasmic reticulum stress. In mice, the Creb3l4 gene gives rise to 2 distinct classes of mRNAs through the use of alternate promoters. Transcripts that initiate upstream of the first coding exon encode a 370-amino acid (aa) protein designated Tisp40beta, whereas transcripts that initiate downstream of the first coding exon encode Atce1/Tisp40alpha, a truncated (315-aa) form of Tisp40beta. In the mouse testis, Creb3l4 transcripts are known to be expressed exclusively in postmeiotic spermatids but the presence of CREB3L4 protein in spermatids has not been formally demonstrated. We produced an antibody directed against the carboxy terminus of mouse CREB3L4 and used it in immunostaining experiments to document that CREB3L4 protein accumulates in post-meiotic spermatids in a stage-specific manner. Moreover, we show that Atce1/Tisp40alpha is the major form of CREB3L4 in mouse testis. These findings suggest that testis-specific isoforms of Creb3l4 could play an important role in spermatid differentiation.