Characterization of truncated testis-specific prolactin receptors in the chicken (Gallus domestics)

We have characterized two unique isoforms of truncated testis-specific (TTS) chicken prolactin receptor (cPRLR) transcripts in the domestic chicken. These two TTS cPRLR isoforms, which correspond to only the intracellular domain of the full-length cPRLR cDNA, are expressed at high levels in the sexually mature chicken. The most predominant TTS cPRLR mRNA contains the Box 1 motif [(+) Box 1 TTS], while the other transcript [(−) Box 1 TTS] contains a unique sequence that replaces the Box 1 motif. Two testes specific exons (TSE 1 and 2) are embedded in introns 8 and 9 of the cPRLR gene. These TSE are alternatively spliced to give rise to four different 5′-untranslated regions (5′-UTR) that are found in (+) Box 1 cPRLR transcripts and to the unique 5′-end of the (−) Box 1 cPRLR mRNA. An alternative promoter was identified in the 5′-flanking region of TSE 1. The levels of (+) Box 1 TTS cPRLR transcripts increase dramatically at the onset of sexual maturity, whereas the full-length cPRLR transcript is depressed. When transiently expressed in 293 cells, the (+) Box 1 cPRLR cDNA constitutively actives a STAT5 responsive reporter gene. The (−) Box 1 cPRLR does not stimulate PRL signaling. Photo-stimulation of pubescent cockerels increases their plasma levels of prolactin, leutinizing hormone, follicle-stimulating hormone and testosterone when compared to that of the photo-inhibited chickens. Photo-stimulation results in higher expression of the TTS cPRLR, while the abundance of full-length cPRLR mRNA is reduced. In contrast, the underdeveloped testes of the photo-inhibited cockerels show predominant expression of the full-length cPRLR mRNA. The expression of these unique truncated [(+) or (−) Box 1] cPRLR transcripts in the testes of sexually-mature chicken suggests a complex mechanism of PRL action on gonadal development and reproductive activity.