Vitamin K2-Dependent GGCX and MGP Are Required for Homeostatic Calcium Regulation of Sperm Maturation

Abstract

Summary A low-calcium microenvironment is essential for spermatozoa to mature in the epididymis; however, it remains unclear how dysregulation of epididymal luminal calcium is associated with male infertility. Using a warfarin-induced vitamin K2 deficiency rat model, we found that vitamin-K-dependent γ-glutamyl carboxylase (GGCX) and matrix Gla protein (MGP) were essential in extracellular calcium signaling of the intercellular communication required for epididymal sperm maturation. We found that GGCX and MGP co-localized in the vesicular structures of epididymal cells and spermatozoa. Calcium-regulated MGP binds to proteins in a biphasic manner; sub-millimolar calcium enhances, whereas excessive calcium inhibits, the binding. Bioinformatic analysis of the calcium-dependent MGP-bound proteome revealed that vesicle-mediated transport and membrane trafficking underlie the intercellular communication networks. We also identified an SNP mutation, rs699664, in the GGCX gene of infertile men with asthenozoospermia. Overall, we revealed that the GGCX-MGP system is integrated with the intercellular calcium signaling to promote sperm maturation.

Generation of a Matrix Gla (Mgp) floxed mouse, followed by conditional knockout, uncovers a new Mgp function in the eye

“…Mgp is secreted in matrix calcifying vesicles. Another interesting possibility would be that of the involvement of Mgp in maintaining the extracellular calcium homeostasis, a recently described action of the gene shown to occur during sperm maturation 33 . In the trabecular meshwork, Mgp is carboxylated, therefore active.…”

“…In the trabecular meshwork, Mgp is carboxylated, therefore active. In the sperm, it was shown that uncarboxylated, inactive MGP which loses its ability to bind calcium, contributes to high Ca concentrations in the epididymal lumen 33 . It would be interesting to determine whether the absence of Mgp in the outflow pathway cells could also disturb the Ca levels on the ECM which could in turn affect extracellular calcium entry and modulate trabecular meshwork function 64 .…”

“…The initial MGP expression profile was later expanded to several other systemic tissues, such as the kidney, lung and eye 28 – 31 and more recently, its expression has been found to be altered in tumors associated with several types of cancers 32 . MGP has also been implicated in the extracellular homeostatic calcium regulation of sperm maturation 33 . Earlier studies from our laboratory identified MGP as one of the most abundant non-housekeeping genes of the human trabecular meshwork of the eye 29 , 34 – 37 , which is the tissue responsible for the maintenance of physiological intraocular pressure (IOP).…”

IP3R Channels in Male Reproduction

“…The distinct distribution or oscillation of calcium levels and the expression of its regulatory proteins during spermatogenesis suggest that calcium signaling may play an essential role in spermatogenesis, in particular in the regulation of the growth and apoptosis of spermatogonia and spermatocytes [7,8]. Calcium signals also participate in the process of epididymal sperm maturation [9]. Studies have shown that some calcium channels maintain calcium homeostasis and regulate calcium signals, in turn affecting spermatogenesis.…”

Establishment of the Variation of Vitamin K Status According to Vkorc1 Point Mutations Using Rat Models

“…Vitamin K is important for many physiological processes as the cofactor of the gamma-glutamyl carboxylase (GGCX) enzyme [2,3]. This enzyme is involved in the activation of many proteins, called vitamin K-dependent proteins (VKDPs), by catalyzing their γ-glutamyl carboxylation.…”