Direct activation of the proton channel by albumin leads to human sperm capacitation and sustained release of inflammatory mediators by neutrophils

Abstract

Human voltage-gated proton channels (hHv1) extrude protons from cells to compensate for charge and osmotic imbalances due metabolism, normalizing intracellular pH and regulating protein function. Human albumin (Alb), present at various levels throughout the body, regulates oncotic pressure and transports ligands. Here, we report Alb is required to activate hHv1 in sperm and neutrophils. Dose-response studies reveal the concentration of Alb in semen is too low to activate hHv1 in sperm whereas the higher level in uterine fluid yields proton efflux, allowing capacitation, the acrosomal reaction, and oocyte fertilization. Likewise, Alb activation of hHv1 in neutrophils is required to sustain production and release of reactive oxygen species during the immune respiratory burst. One Alb binds to both voltage sensor domains (VSDs) in hHv1, enhancing open probability and increasing proton current. A computational model of the Alb-hHv1 complex, validated by experiments, identifies two sites in Alb domain II that interact with the VSDs, suggesting an electrostatic gating modification mechanism favoring the active “up” sensor conformation. This report shows how sperm are triggered to fertilize, resolving how hHv1 opens at negative membrane potentials in sperm, and describes a role for Alb in physiology that will operate in the many tissues expressing hHv1.

Neutrophil Percentage-to-Albumin Ratio: A Good Parameter for the Evaluation of the Severity of Anti-NMDAR Encephalitis at Admission and Prediction of Short-Term Prognosis

“…It can be seen that the physiological functions of neutrophils and albumin make them play pivotal roles in immune diseases. Albumin stimulates the voltage-gated proton channel in neutrophils and affects the generation of reactive oxygen species (ROS) and the release of elastase ( 17 ). ROS can change the protein structure of host organs and tissues, expose new epitopes, and affect immune tolerance ( 18 ).…”

A Review on the Role of Bicarbonate and Proton Transporters during Sperm Capacitation in Mammals

“…In agreement with this, the expression of HVCN1 and HVCN1Sper is similar between non-capacitated and capacitated sperm cells from the same donor; in contrast, it differs between motile and immotile sperm populations, thus suggesting that they may be relevant for sperm motility [ 137 ]. While the activation kinetics are similar between variants, they differ in their pH sensitivity, with the HVCN1Sper channel being less sensitive to pH i changes than the HVCN1 [ 137 , 161 ]. In humans, such an attenuated sensitivity of HVCN1Sper to pH i changes could be essential for the activation of sperm motility after ejaculation [ 137 ].…”

“…Altogether, the progressive decrease of extracellular Zn 2+ levels and progressive increase of pH o induce HVCN1 gating and H + extrusion [ 40 ]; moreover, the high HVCN1 activity in capacitated sperm could be also related to the phosphorylation of their subunits, presumably by PKC [ 11 , 134 ]. In capacitated sperm of humans and cattle, HVCN1 induces a fast alkalinization at the principal piece, which is crucial for the switch in motility pattern as it drives the activation of CatSper channels and pH-dependent proteins of the axoneme [ 11 , 22 , 133 , 137 , 159 , 161 ]. In both species, HVCN1 channels are also required for acrosomal exocytosis [ 22 , 161 ], although some studies performed in human sperm support that the alkalinization of the sperm head does not only rely upon HVCN1 but also on bicarbonate transporters [ 11 ].…”

“…In capacitated sperm of humans and cattle, HVCN1 induces a fast alkalinization at the principal piece, which is crucial for the switch in motility pattern as it drives the activation of CatSper channels and pH-dependent proteins of the axoneme [ 11 , 22 , 133 , 137 , 159 , 161 ]. In both species, HVCN1 channels are also required for acrosomal exocytosis [ 22 , 161 ], although some studies performed in human sperm support that the alkalinization of the sperm head does not only rely upon HVCN1 but also on bicarbonate transporters [ 11 ]. Interestingly, in pig sperm, HVCN1 is essential for motility hyperactivation during in vitro capacitation but not for progesterone-induced acrosomal exocytosis; moreover, in this species, such a close relationship between HVCN1 activity and Ca 2+ influx has not been observed [ 3 ].…”

State Dependent Anionic Pore Currents Conducted by Single Countercharge Mutants in a Voltage-Sensing Phosphatase

“…Canonical voltage-sensing domains (VSDs) comprise four transmembrane segments S1-S4, forming an antiparallel helix bundle. VSDs undergo defined conformational transitions in response to membrane potential changes to orchestrate the gating of the pore domain of ion channels (1)(2)(3)(4), the catalytic activity of phosphatases (5)(6)(7)(8), the activation of ion exchange of solute carriers (9,10), and the proton-specific conduction of proton channels (11)(12)(13)(14)(15)(16)(17). VSDs share high sequence and structural similarity with each other, particularly the conserved positive gating charge residues on S4 and negative countercharge residues on S1-S3 (18)(19)(20), and a layer of inplane hydrophobic residues on S1-S3 (21)(22)(23)(24), thought to electrically separate the extracellular and intracellular aqueous crevices of the VSDs, preventing passage of water and ions and ensuring a focused electric field (24)(25)(26)(27).…”