Hyperactivation, a swimming pattern of mammalian sperm in the oviduct, is essential for fertilization. It is characterized by asymmetrical flagellar beating and an increase of cytoplasmic Ca2+. We observed that some mouse sperm swimming in the oviduct produce high-amplitude pro-hook bends (bends in the direction of the hook on the head), whereas other sperm produce high-amplitude anti-hook bends. Switching direction of the major bends could serve to redirect sperm toward oocytes. We hypothesized that different Ca2+ signaling pathways produce high-amplitude pro-hook and anti-hook bends. In vitro, sperm that hyperactivated during capacitation (because of activation of CATSPER plasma membrane Ca2+ channels) developed high-amplitude pro-hook bends. The CATSPER activators procaine and 4-aminopyridine (4-AP) also induced high-amplitude pro-hook bends. Thimerosal, which triggers a Ca2+ release from internal stores, induced high-amplitude anti-hook bends. Activation of CATSPER channels is facilitated by a pH rise, so both Ca2+ and pH responses to treatments with 4-AP and thimerosal were monitored. Thimerosal triggered a Ca2+ increase that initiated at the base of the flagellum, whereas 4-AP initiated a rise in the proximal principal piece. Only 4-AP triggered a flagellar pH rise. Proteins were extracted from sperm for examination of phosphorylation patterns induced by Ca2+ signaling. Procaine and 4-AP induced phosphorylation of proteins on threonine and serine, whereas thimerosal primarily induced dephosphorylation of proteins. Tyrosine phosphorylation was unaffected. We concluded that hyperactivation, which is associated with capacitation, can be modulated by release of Ca2+ from intracellular stores to reverse the direction of the dominant flagellar bend and, thus, redirect sperm.

Two distinct Ca2+ signaling pathways alter the symmetry of mouse sperm flagellar beating.

Although the endocrine capacity of bone is widely recognized, interactions between bone and the reproductive system have until now focused on the gonads as a regulator of bone remodeling. We now show that in males, bone acts as a regulator of fertility. Using co-culture assays, we demonstrate that osteoblasts are able to induce testosterone production by the testes, while they fail to influence estrogen production by the ovaries. Analyses of cell-specific loss- and gain-of-function models reveal that the osteoblast-derived hormone osteocalcin performs this endocrine function. By binding to a G-protein coupled receptor expressed in the Leydig cells of the testes, osteocalcin regulates in a CREB-dependent manner the expression of enzymes required for testosterone synthesis, promoting germ cell survival. This study expands the physiological repertoire of osteocalcin, and provides the first evidence that the skeleton is an endocrine regulator of reproduction.

Hyperactivation, a motility pattern of mammalian sperm in the oviduct, is essential to fertilization. Hyperactivation helps sperm to swim effectively through oviductal mucus, to escape from the sperm reservoir, and to penetrate the cumulus matrix and zona pellucida of the oocyte. There is some evidence that mammalian sperm can undergo chemotaxis; however, the relationship of chemotaxis to hyperactivation is unknown. Ca2+ signaling is involved in hyperactivation and implicated in chemotaxis as well. In vivo, sperm hyperactivate in the lower oviduct, far from the cumulus-oocyte complex and possibly beyond the influence of chemotactic gradients emanating from the oocyte or cumulus. Thus, sperm are likely to be hyperactivated before sensing chemotactic gradients. Chemotactic signals might modulate hyperactivation to direct sperm toward oocytes as they reach a region of influence. Ca2+-directed modulation of hyperactivation is a potential mechanism of this process.

A model for the relationship of hyperactivation and chemotaxis is proposed, hyperactivated sperm modulate the pattern of flagellar beating in response to chemotactic signals.