Sperm Chromatin Protamination: An Endocrine Perspective

Abstract

During spermiogenesis, the elongating rat spermatid chromatin undergoes a gradual process of condensation which is initiated in the round spermatids at "step 7" of cytodifferentation in stage VII and extending to elongated spermatids at "step 19" of cytodifferentiation in stage VIII. The mechanism of chromatin condensation in the elongating spermatids is an elaborate process that encompasses several biochemical and biological aspects culminating in the deposition of protamine in DNA grooves. The protamination of sperm chromatin involves expression and storage of proteins involved in condensation, removal and degradation of nuclear histones and their replacement by transition proteins and protamine 1, transcriptional silencing and DNA repair, reduction of nuclear volume, repackaging of protaminated chromatin in torroids and development of a characteristic head shape and perforatorium. A study was undertaken in my laboratory to delineate the role of follicle stimulating hormone (FSH) and testosterone in the condensation of nuclear chromatin in the elongating spermatids of sexually competent species of rat. Towards this end, sexually competent male Holtzmann rats were treated with 20 mg/Kg/d per os (oral supplementum) of cyproterone acetate and 3 mg/Kg/d i.p (intra peritoneal) of fluphenazine decanoate to induce a functional deficiency in either testosterone or FSH. In both rat models, membrane-impermeable CMA3 fluorescent dye uptake assay for GC-rich prospective sites of DNA protamination, was indicative of insufficiency of protamine 1 in spermatozoa taken from caput epididymides of treated rats whereas a fluorescent TUNEL assay indicated the presence of nicked chromatin strands only in protamine-deficient spermatozoa derived from caput epididymides of fluphenazine-treated rats with functional deficiency of FSH. Western blotting of acid-soluble sperm basic proteins had confirmed the near absence of protamine 1 in treated rat spermatozoa in both models. Electron Microscopic evaluation too revealed fine ultrastructural changes in the nuclear membrane of cyproterone acetate as well as fluphenazine decanoate treated spermatozoa derived from caput epididymides. Electrophoretic analysis of caput sperm nuclear basic proteins substantiated the observations at cellular level and revealed a pattern of abnormal persistence of acid-soluble nuclear basic proteins in both rat models, the levels being more prominent in fluphenazine treated rats. Our studies suggest that adequate levels of both FSH and testosterone could be essential during the stages of spermatidal condensation and led us to hypothesize the existence of an endocrine-regulated molecular mechanism for histone to protamine transition and maintenance of chromatin integrity during chromatin condensation in the testis during spermiogenesis.

Intercellular communication in Arabidopsis thaliana pollen discovered via AHG3 transcript movement from the vegetative cell to sperm

“…During animal spermiogenesis, mRNA localization and translation are regulated in a stage-specific manner (22)(23)(24). During spermiogenesis, DNA-binding histones are replaced by protamines, resulting in chromatin condensation and cessation of transcription in elongating spermatids (25,26). Hence, in haploid spermatids, transcription and translation are temporally uncoupled to ensure protein synthesis in transcriptionally silent germ cells.…”

Low 17beta-Estradiol Levels in Cnr1 Knock-Out Mice Affect Spermatid Chromatin Remodeling by Interfering with Chromatin Reorganization

“…Moreover, estrogens, directly or indirectly, affected SPZ chromatin acting on the testis rather than epididymis, since histone-to-protamine exchange takes place during spermiogenesis Overall, our results demonstrated that estrogens, indirectly via stimulatory effects on FSH secretion and/or directly via paracrine actions within the testis, play a key role in spermiogenesis. Interestingly, it is still a matter of debate whether FSH and/or estrogen affect chromatin remodeling in SPT in either a synergistic or an independent way with androgens [42,49,58,71,72]. However, the loss of Cnr1 did not impair fertility [15]: this likely occurs because of mechanisms of SPZ selection (Cnr1 À/À mice produce a sperm with heterogeneous histone content and chromatin quality) and/ or DNA repair operating in the female reproductive tract [73].…”

“…Gonadotropins (LH and FSH) and sex steroids, including androgens and estrogens, were both considered because of their role in male reproduction of vertebrates [34][35][36][37][38][39][40][41][42][43]. LH-stimulated androgen production and action are both necessary for germ cell progression in males.…”

Estrogens and Spermiogenesis: New Insights from Type 1 Cannabinoid Receptor Knockout Mice

“…Accordingly, core-histones are displaced in their acetylated state [66]. Available reports suggest that the process of histone displacement requires (i) DNA nick/repair induced by topoisomerase which relieves torsional stress associated with histone-to-PRM exchange; (ii) hyperacetylation of histone tails by histone acetyl transferase with a concerted down regulation of histone deacetylase; (iii) histone removal mediated by the recruiting protein BRDT (testis-specific bromodomain protein) able to bind histone acetylated lysines, and (iv) acetylated histone degradation through polyubiquitylation of N-terminal lysines [31]. Histone displacement ends at step 13 SPT [47].…”

“…The main future endpoint will be the characterization of E 2 mechanisms to better understand whether its action is direct and/or mediated. Indeed, it is still a matter of debate whether E 2 and/or FSH affect chromatin remodeling in SPT in either a synergistic or an independent way with androgens [31]. Gene deletion animal models have revealed that both FSH and testosterone levels are implicated in the regulation of chromatin condensation during spermiogenesis [31].…”