Biphasic role of calcium in mouse sperm capacitation signaling pathways

The tyrosine kinase FER is responsible for the capacitation-associated increase in tyrosine phosphorylation in murine sperm

Sperm capacitation is required for fertilization. At the molecular level, this process is associated with fast activation of protein kinase A. Downstream of this event, capacitating conditions lead to an increase in tyrosine phosphorylation. The identity of the tyrosine kinase(s) mediating this process has not been conclusively demonstrated. Recent experiments using stallion and human sperm have suggested a role for PYK2 based on the use of small molecule inhibitors directed against this kinase. However, crucially, loss-of-function experiments have not been reported. Here, we used both pharmacological inhibitors and genetically modified mice models to investigate the identity of the tyrosine kinase(s) mediating the increase in tyrosine phosphorylation in mouse sperm. Similar to stallion and human, PF431396 blocks the capacitation-associated increase in tyrosine phosphorylation. Yet, sperm from Pyk2(-/-) mice displayed a normal increase in tyrosine phosphorylation, implying that PYK2 is not responsible for this phosphorylation process. Here, we show that PF431396 can also inhibit FER, a tyrosine kinase known to be present in sperm. Sperm from mice targeted with a kinase-inactivating mutation in Fer failed to undergo capacitation-associated increases in tyrosine phosphorylation. Although these mice are fertile, their sperm displayed a reduced ability to fertilize metaphase II-arrested eggs in vitro.

A novel biosensor to study cAMP dynamics in cilia and flagella

The cellular messenger cAMP regulates multiple cellular functions, including signaling in cilia and flagella. The cAMP dynamics in these subcellular compartments are ill-defined. We introduce a novel FRET-based cAMP biosensor with nanomolar sensitivity that is out of reach for other sensors. To measure cAMP dynamics in the sperm flagellum, we generated transgenic mice and reveal that the hitherto methods determining total cAMP levels do not reflect changes in free cAMP levels. Moreover, cAMP dynamics in the midpiece and principal piece of the flagellum are distinctively different. The sole cAMP source in the flagellum is the soluble adenylate cyclase (SACY). Although bicarbonate-dependent SACY activity requires Ca²⁺, basal SACY activity is suppressed by Ca²⁺. Finally, we also applied the sensor to primary cilia. Our new cAMP biosensor features unique characteristics that allow gaining new insights into cAMP signaling and unravel the molecular mechanisms underlying ciliary function in vitro and in vivo.

DOI:http://dx.doi.org/10.7554/eLife.14052.001

Cells can change the way they grow, move or develop in response to information from their environment. This information is first detected at the surface of the cell and then the information is relayed around the interior of the cell by signaling molecules known as “second messengers”. A molecule called cAMP is a well-known second messenger that is involved in many different signaling pathways. Therefore, the levels of cAMP in specific areas of the cell need to be precisely regulated to enable different signaling pathways to be activated at specific times and locations.

Some cells have hair-like structures called cilia or flagella on their surface. Cilia and flagella are able to move the fluid that surrounds the cells or even move the cells themselves. The second messenger cAMP plays an essential role in making cilia move, but it is challenging to analyze the dynamics of cAMP – that this, how the levels of this molecule change over time – in these structures. The levels of cAMP in live cells can only be measured using fluorescent biosensors. Introducing these biosensors into specific cell structures is difficult and they are not sensitive enough to respond to low levels of cAMP. Furthermore, it is difficult to measure cAMP activity inside such tiny structures using these biosensors.

Mukherjee, Jansen, Jikeli et al. now address some of these challenges by creating a new cAMP biosensor that has several unique features. Most importantly, it can respond to very low levels of cAMP, making it more sensitive than previous biosensors. Mukherjee et al. test this new biosensor in the flagella of sperm cells from mice, which reveals how the production of cAMP is regulated in the flagellum. The new biosensor also shows that different parts of the flagellum can have different cAMP dynamics. In the future, this new biosensor could be used to study cAMP in other structures and compartments within cells.

DOI:http://dx.doi.org/10.7554/eLife.14052.002

Carbonic anhydrases and their functional differences in human and mouse sperm physiology

Fertilization is a key reproductive event in which sperm and egg fuse to generate a new individual. Proper regulation of certain parameters (such as intracellular pH) is crucial for this process. Carbonic anhydrases (CAs) are among the molecular entities that control intracellular pH dynamics in most cells. Unfortunately, little is known about the function of CAs in mammalian sperm physiology. For this reason, we re-explored the expression of CAI, II, IV and XIII in human and mouse sperm. We also measured the level of CA activity, determined by mass spectrometry, and found that it is similar in non-capacitated and capacitated mouse sperm. Importantly, we found that CAII activity accounts for half of the total CA activity in capacitated mouse sperm. Using the general CA inhibitor ethoxyzolamide, we studied how CAs participate in fundamental sperm physiological processes such as motility and acrosome reaction in both species. We found that capacitated human sperm depend strongly on CA activity to support normal motility, while capacitated mouse sperm do not. Finally, we found that CA inhibition increases the acrosome reaction in capacitated human sperm, but not in capacitated mouse sperm.