Chromatin control of sperm nuclear compaction and shaping – ORTHOSPERM

During spermiogenesis, the differentiation of male gametes, histones are replaced with sperm-specific chromosomal proteins known as protamines. This unique chromatin reorganization process is associated with extreme DNA compaction and the acquisition of a highly controlled, species-specific nuclear shape that can be amazingly diverse and complex in animals. These cellular mechanisms remain poorly understood and rarely studied. The team headed by B. Loppin (LBMS – ENS Lyon) studies spermiogenesis in the fruit fly Drosophila. In this model insect, sperm chromatin is organized with a cocktail of several partially redundant protamine-like proteins of the MST-HMG Box family, which entirely replace histones in maturing spermatid nuclei. This relative complexity hinders functional studies as well as modeling experiments. With its intrinsic features (needle-shape sperm nuclei, a unique protamine gene, massive sperm production) and its genomic (genome sequence, transcriptomes) and genetic (CRISPR/Cas9, transgenesis) resources, the cricket Gryllus bimaculatus is an excellent insect model to study spermiogenesis. Through transcriptomic approaches, we have been able to identify a unique candidate cricket protamine gene of the MST HMG Box type that we named ProtGb1. Based on a rich set of preliminary observations, we formulate the hypothesis that ProtGb1 plays a central role in the organization and compaction of sperm chromatin, which is expected to influence the final shape of the mature sperm nucleus. The ORTHOSPERM project, supported by a multidisciplinary consortium, will combine various techniques in microscopy (confocal, electronic, Super Resolution), in vivo and in vitro functional approaches, proteomics and biophysic modeling to study the influence of chromatin organization on the poorly understood process of sperm nuclear shaping.