The Protease Degrading Sperm Histones Post-Fertilization in Sea Urchin Eggs Is a Nuclear Cathepsin L That Is Further Required for Embryo Development

Abstract

Proteolysis of sperm histones in the sea urchin male pronucleus is the consequence of the activation at fertilization of a maternal cysteine protease. We previously showed that this protein is required for male chromatin remodelling and for cell-cycle progression in the newly formed embryos. This enzyme is present in the nucleus of unfertilized eggs and is rapidly recruited to the male pronucleus after insemination. Interestingly, this cysteine-protease remains co-localized with chromatin during S phase of the first cell cycle, migrates to the mitotic spindle in M-phase and is re-located to the nuclei of daughter cells after cytokinesis. Here we identified the protease encoding cDNA and found a high sequence identity to cathepsin proteases of various organisms. A phylogenetical analysis clearly demonstrates that this sperm histone protease (SpHp) belongs to the cathepsin L sub-type. After an initial phase of ubiquitous expression throughout cleavage stages, SpHp gene transcripts become restricted to endomesodermic territories during the blastula stage. The transcripts are localized in the invaginating endoderm during gastrulation and a gut specific pattern continues through the prism and early pluteus stages. In addition, a concomitant expression of SpHp transcripts is detected in cells of the skeletogenic lineage and in accordance a pharmacological disruption of SpHp activity prevents growth of skeletal rods. These results further document the role of this nuclear cathepsin L during development.

Histone proteolysis: A proposal for categorization into ‘clipping’ and ‘degradation’

“…This was first attributed to a cystein protease named SpH in sea urchin, where this process has been studied most extensively 36 . This SpH enzyme was recently found to be a nuclear Cathepsin L (CATL) 39 . Histone degradation in earlier Eukaryotes has also been described during specific stages of development, such as macronucleus degradation in, for example, Tetrahymena 43 .…”

“…This was first attributed to a cystein protease named SpH in sea urchin, where this process has been studied most extensively 36 . This SpH enzyme was recently found to be a nuclear Cathepsin L (CATL) 39 .…”

Evolutionary History of Cathepsin L (L-like) Family Genes in Vertebrates

“…Among endopeptidase cysteine proteases, cathepsin L is an important family because of its multifunctional role in many biochemical pathways, including intracellular protein degradation, antigen presentation, and cellular development 20 - 22 . Although relatively detailed information has been accumulated regarding the structure and function of this enzyme, only fragmentary data are presently available with regard to the evolutionary relationship among vertebrate species.…”

Sequestration to lipid droplets promotes histone availability by preventing turnover of excess histones

“…If correct, inhibition of histone degradation should be able to reverse the effects of lack of Jabba on histone levels. Since in many systems excess histones are turned over by proteasome-mediated degradation (Imschenetzky et al, 1997;Lin et al, 1991;Morin et al, 2012;Singh et al, 2009), this pathway is an obvious candidate for mediating histone degradation in oocytes. We therefore opted to pharmacologically inhibit the proteasome using in vitro egg chamber maturation (IVEM) (Spracklen & Tootle, 2013), to avoid secondary effects on whole-organism viability.…”