A heterozygous mutation of GALNTL5 affects male infertility with impairment of sperm motility

Abstract

For normal fertilization in mammals, it is important that functionally mature sperm are motile and have a fully formed acrosome. The glycosyltransferase-like gene, human polypeptide N-acetylgalactosaminyltransferase-like protein 5 (GALNTL5), belongs to the polypeptide N-acetylgalactosamine-transferase (pp-GalNAc-T) gene family because of its conserved glycosyltransferase domains, but it uniquely truncates the C-terminal domain and is expressed exclusively in human testis. However, glycosyltransferase activity of the human GALNTL5 protein has not been identified by in vitro assay thus far. Using mouse Galntl5 ortholog, we have examined whether GALNTL5 is a functional molecule in spermatogenesis. It was observed that mouse GALNTL5 localizes in the cytoplasm of round spermatids in the region around the acrosome of elongating spermatids, and finally in the neck region of spermatozoa. We attempted to establish Galntl5-deficient mutant mice to investigate the role of Galntl5 in spermiogenesis and found that the heterozygous mutation affected male fertility due to immotile sperm, which is diagnosed as asthenozoospermia, an infertility syndrome in humans. Furthermore, the heterozygous mutation of Galntl5 attenuated glycolytic enzymes required for motility, disrupted protein loading into acrosomes, and caused aberrant localization of the ubiquitin-proteasome system. By comparing the protein compositions of sperm from infertile males, we found a deletion mutation of the exon of human GALNTL5 gene in a patient with asthenozoospermia. This strongly suggests that the genetic mutation of human GALNTL5 results in male infertility with the reduction of sperm motility and that GALNTL5 is a functional molecule essential for mammalian sperm formation.

A novel splice variant of the bovine GALNTL5 gene identified in Chinese Holstein bull testis tissue and its mRNA expression

“…Notably, GALNTL5 also temporarily localizes in the acroplaxome and manchette, which is a transient microtubular/actin structure, during spermatid elongation and finally accumulates in the neck region around the head-tail coupling apparatus of mature spermatozoa. These findings lend strong evidence for GALNTL5 being involved in spermiogenesis (Takasaki et al, 2014).…”

“…In the spermatozoa of heterozygous mutant mice, some researchers observed impairment of protein loadings into acrosomes and aberrant localization of the ubiquitin-proteasome system. Moreover, colocalization of GALNTL5 signals with UBC3B, an E2 ubiquitin conjugating enzyme similar to UBC3/CDC34, and ubiquitin signals was observed in the ubiquitin-proteasome system in the neck region of spermatozoa from wild-type mice (Takasaki et al, 2014).…”

“…GALNTL5 possesses the highly conserved catalytic domains of pp-GalNAc-T but uniquely truncates the conserved lectin domain at the C-terminus. The GALNTL5 mRNA is expressed exclusively in the testes, mainly in the cytoplasm of round and elongated spermatids during spermiogenesis and not in the outermost cells of the seminiferous tubules, which contain spermatogonia and somatic Sertoli cells (Takasaki et al, 2014). GALNTL5 is indispensable for the formation of mature sperm and may have a unique role in mammalian spermiogenesis.…”

Human male infertility and its genetic causes

“…In order to investigate the biological role of human GALNTL5 (7q36.1) in the testis, the localization of the mouse orthologous protein, GALNTL5, was observed in spermatogenesis and Galntl5 ‐deficient mice were established 87. The heterozygous mutant male mice were infertile because of their immotile spermatozoa, a feature that corresponded to human azoospermia 87.…”

“…It also was reported that the heterozygous mutation of Galntl5 in mice impaired the amounts of glycolytic enzymes that are required for motility, protein loading into the acrosomes, and localization of the ubiquitin–proteasome system in the neck region in the spermatozoa, which consists of common cytoplasmic constituents that are shared among the haploid spermatids through intracellular bridges 88, 89. Screening for male patients with infertility that was caused by a mutation of the GALNTL5 gene was conducted and one patient was identified among 200 whose asthenozoospermia was attributable to a heterozygous single nucleotide deletion of maternal inheritance in the exon of GALNTL5 87. This strongly suggests that the genetic mutation of human GALNTL5 results in male infertility, with a reduction of sperm motility, and that GALNTL5 is a protein that is essential for mammalian sperm formation.…”