Stage-specific expression of RESP18 in the testes.

RESP18 (regulated endocrine-specific protein of 18 KD) is an endoplasmic reticulum (ER) protein that was identified by coordinate dopaminergic regulation with pro-opiomelanocortin in the rat neurointermediate pituitary. Many attributes of RESP18 suggest an important function in neuroendocrine cells. Several neuropeptides, growth factors, and enzymes involved in biosynthesis of classical chemical neurotransmitters, have been identified in germ cells, Sertoli cells, and spermatozoa. In this study, screening of reproductive tissues revealed high levels of RESP18 protein and mRNA in the testes but not in ovaries or epididymis. The testes and sperm expressed 18-KD RESP18 and a unique 19-KD isoform. To better understand RESP18 expression in the testes, we have examined the stages of the cycle of the seminiferous epithelium by immunohistochemistry and Western blot analyses. Immunohistochemical analysis showed that RESP18 protein was expressed exclusively in spermatocytes and maturing spermatids. RESP18 protein was expressed at high levels in Step 1-8 round spermatids, in which the PC4 prohormone convertase, nerve growth factor, and proenkephalin are also expressed. Western blots, Northern blots, and indirect immunofluorescence staining demonstrated RESP18 expression in sperm.