Intracellular pH in sperm physiology

Abstract

Intracellular pH (pHi) regulation is essential for cell function. Notably, several unique sperm ion transporters and enzymes whose elimination causes infertility are either pHi dependent or somehow related to pHi regulation. Amongst them are: CatSper, a Ca(2+) channel; Slo3, a K(+) channel; the sperm-specific Na(+)/H(+) exchanger and the soluble adenylyl cyclase. It is thus clear that pHi regulation is of the utmost importance for sperm physiology. This review briefly summarizes the key components involved in pHi regulation, their characteristics and participation in fundamental sperm functions such as motility, maturation and the acrosome reaction.

Keywords: Intracellular pH; Sperm; Sperm chemotaxis; Sperm motility.

Figure 1. Model of mammalian sperm pHi regulation during capacitation/hyperactivation

The CO₂ / HCO₃⁻ are equilibrated inside the female tract by membrane associated carbonic anhydrases (CAs). The influx of HCO₃ into the sperm is mediated by Na⁺/HCO₃ cotransporters (NBC) and Cl/HCO₃ exchangers (SLC26). Probably the cytoplasmic CAs also contribute to the HCO₃ increase by conversion of cytosolic CO₂. Specifically, SLC26A3 and A6 isoforms appear to have a physical interaction with CFTR, which may permeate HCO₃ besides Cl. Aside, HCO₃ influx hyperpolarizes mouse spermatozoa in a [Na⁺]e-dependent manner via NBC. HCO₃activates (along with Ca²⁺) a soluble adenylyl cyclase (sAC) leading to a cAMP increase, which activates a PKA (which may also depend on [Ca²⁺]i) and also may bind to the cyclic nucleotide binding domain of the sperm specific Na⁺/H⁺ exchanger (sNHE) inducing a pHi increase in mouse. sNHE may also be stimulated by the hyperpolarization that occurs upon sperm capacitation. Unlike mouse, a voltage-dependent H⁺ channel (H_v) may be involved in human sperm pHi increase. In both species, the pHi increase (among other factors) activates a sperm specific Ca²⁺ permeable channel known as CatSper, which allows an intracellular Ca²⁺ increase essential for hyperactivation. We include an addition Ca²⁺ transporter to indicate that other systems participate in [Ca²⁺]i regulation. On the other hand, K⁺ channels Slo3 and Slo1 (or their heterotetramer) are activated by intracellular alkalinization and/or by Ca²⁺ (human sperm), contributing to the hyperpolarization that occurs during capacitation. CO₂ is also generated inside mitochondria through the tricarboxylic acid cycle (TCA) and it may serve as substrate for intracellular or intramitochondrial CAs. The HCO₃ may also activate intramitochondrial sAC which in turn would trigger protein phosphorylation. Although mammalian sperm possess many mitochondria inside the midpiece, for didactic purposes only one mitochondrion is shown here.

Figure 2. The speract signaling cascade in sea urchin sperm

After binding to its receptor (1), speract stimulates a membrane guanylyl cyclase (GC), which elevates cGMP (2) that activates tetrameric cGMP-regulated K⁺ channels (tetraKCNG), causing a membrane potential (Em) hyperpolarization (4) due to K⁺ efflux (3). This Em change may activate a Na⁺/H⁺ exchanger (sNHE) (5a), remove inactivation from voltage-activated Ca²⁺ channels (Ca_V), enhance hyperpolarization-activated and cyclic nucleotide-gated channels (HCN), and facilitate Ca²⁺ extrusion (5d) by K⁺-dependent Na⁺/Ca²⁺ exchangers (NCKX). sNHE activation increases pHi (5a). HCN opening causes Na⁺ influx (5c) and contributes to depolarize Em activating Ca_Vs, which increase [Ca²⁺]i (6) and enhance asymmetric flagellar bending allowing sperm to turn. Possibly, the rise of [Ca²⁺]i also opens Ca²⁺-regulated K⁺ channels (CaKC) and Ca²⁺-regulated Cl channels (CaCC), which then contribute to again hyperpolarize Em (7), removing inactivation from Ca_V channels and opening HCN channels. During this period sperm swim in a straighter trajectory whose regulation is ill defined but critical for chemotaxis. This succession of events occurs cyclically, generating orchestrated transient [Ca²⁺]i increases (6) that produce a sequence of turns until one or more of the second messengers in the pathway are downregulated. The pHi (5a) and cAMP increases (5b) stimulate an undetermined Ca²⁺ influx pathway that contributes to the sustained [Ca²⁺]i increase (6), and possibly to the depolarization that accompanies the speract response. It is now worth exploring if CatSper participates in this process. Inset: A sperm drifting circular swimming trajectory in a chemoattractant gradient (orange shadow surrounding the egg) is stimulated periodically due to changes in the rate of chemoattractant capture. When swimming towards the egg in an ascending gradient, the onset of [Ca²⁺]i fluctuations is suppressed until the cell detects an ascending to descending gradient inversion. After a ∼200 ms delay, the spermatozoon undergoes a transient [Ca²⁺]i increase just before reaching the gradient minima (yellow circles). The rate of change in [Ca²⁺]i (d[Ca²⁺]i/dt) controls sperm trajectory. As a result, sperm turn (yellow) when they are swimming away from the egg and swim in a straighter trajectory (blue) while coming closer to the chemoattractant source (i. e., the egg jelly).