The movement of ions across cell membranes is essential for a wide variety of fundamental physiological processes, including secretion, muscle contraction, and neuronal excitation. This movement is possible because of the presence in the cell membrane of a class of integral membrane proteins dubbed ion channels. Ion channels, thanks to the presence of aqueous pores in their structure, catalyze the passage of ions across the otherwise ion-impermeable lipid bilayer. Ion conduction across ion channels is highly regulated, and in the case of voltage-dependent K⁺ channels, the molecular foundations of the voltage-dependent conformational changes leading to the their open (conducting) configuration have provided most of the driving force for research in ion channel biophysics since the pioneering work of Hodgkin and Huxley (Hodgkin, A. L., and Huxley, A. F. (1952) J. Physiol. 117, 500–544). The voltage-dependent K⁺ channels are the prototypical voltage-gated channels and govern the resting membrane potential. They are responsible for returning the membrane potential to its resting state at the termination of each action potential in excitable membranes. The pore-forming subunits (α) of many voltage-dependent K⁺ channels and modulatory β-subunits exist in the membrane as one component of macromolecular complexes, able to integrate a myriad of cellular signals that regulate ion channel behavior. In this review, we have focused on the modulatory effects of β-subunits on the voltage-dependent K⁺ (Kv) channel and on the large conductance Ca²⁺- and voltage-dependent (BK_Ca) channel.

In mammals, calcium influx is required for oocyte maturation and egg activation. The molecular identities of the calcium-permeant channels that underlie the initiation of embryonic development are not established. Here, we describe a transient receptor potential (TRP) ion channel current activated by TRP agonists that is absent in TrpV3(-/-) eggs. TRPV3 current is differentially expressed during oocyte maturation, reaching a peak of maximum density and activity at metaphase of meiosis II (MII), the stage of fertilization. Selective activation of TRPV3 channels provokes egg activation by mediating massive calcium entry. Widely used to activate eggs, strontium application is known to yield normal offspring in combination with somatic cell nuclear transfer. We show that TRPV3 is required for strontium influx, because TrpV3(-/-) eggs failed to conduct Sr(2+) or undergo strontium-induced activation. We propose that TRPV3 is a major mediator of calcium influx in mouse eggs and is a putative target for artificial egg activation.

The internal vestibule of large-conductance Ca2+ voltage-activated K+ (BK) channels contains a ring of eight negative charges not present in K+ channels of lower conductance (Glu386 and Glu389 in hSlo) that modulates channel conductance through an electrostatic mechanism (Brelidze, T.I., X. Niu, and K.L. Magleby. 2003. Proc. Natl. Acad. Sci. USA. 100:9017–9022). In BK channels there are also two acidic amino acid residues in an extracellular loop (Asp326 and Glu329 in hSlo). To determine the electrostatic influence of these charges on channel conductance, we expressed wild-type BK channels and mutants E386N/E389N, D326N, E329Q, and D326N/E329Q channels on Xenopus laevis oocytes, and measured the expressed currents under patch clamp. Contribution of E329 to the conductance is negligible and single channel conductance of D326N/E329Q channels measured at 0 mV in symmetrical 110 mM K+ was 18% lower than the control. Current–voltage curves displayed weak outward rectification for D326N and the double mutant. The conductance differences between the mutants and wild-type BK were caused by an electrostatic effect since they were enhanced at low K+ (30 mM) and vanished at high K+ (1 M K+). We determine the electrostatic potential change, Δφ, caused by the charge neutralization using TEA+ block for the extracellular charges and Ba2+ for intracellular charges. We measured 13 ± 2 mV for Δφ at the TEA+ site when turning off the extracellular charges, and 17 ± 2 mV for the Δφ at the Ba2+ site when the intracellular charges were turned off. To understand the electrostatic effect of charge neutralizations, we determined Δφ using a BK channel molecular model embedded in a lipid bilayer and solving the Poisson-Boltzmann equation. The model explains the experimental results adequately and, in particular, gives an economical explanation to the differential effect on the conductance of the neutralization of charges D326 and E329.

Significance: At fertilization, repetitive calcium signals are induced in mammalian eggs that are essential for activating embryo development. These signals are supported by calcium influx from the surrounding medium. Two ion channels supporting calcium influx into eggs, Ca _V 3.2 and TRPV3, were identified previously. Here we find that a third channel, TRPM7, is essential for this function. In the absence of both TRPM7 and Ca _V 3.2, the calcium signals induced by the fertilizing sperm stop prematurely. Female mice carrying eggs lacking TRPM7 and Ca _V 3.2 are subfertile. Because TRPM7 channel function is modulated by ions in culture medium, these findings highlight the importance of exact culture medium composition used during laboratory procedures for human assisted reproduction therapies.

The Transient Receptor Potential (TRP) channels are a family of cationic ion channels widely distributed in mammalian tissues. In general, the global genetic disruption of individual TRP channels result in phenotypes associated with impairment of a particular tissue and/or organ function. An exception is the genetic ablation of the TRP channel TRPM7, which results in early embryonic lethality. Nevertheless, the function of TRPM7 in oocytes, eggs and pre-implantation embryos remains unknown. Here, we described an outward rectifying non-selective current mediated by a TRP ion channel in immature oocytes (germinal vesicle stage), matured oocytes (metaphase II eggs) and 2-cell stage embryos. The current is activated by specific agonists and inhibited by distinct blockers consistent with the functional expression of TRPM7 channels. We demonstrated that the TRPM7-like channels are homo-tetramers and their activation mediates calcium influx in oocytes and eggs, which is fundamental to support fertilization and egg activation. Lastly, we showed that pharmacological inhibition of the channel function delays pre-implantation embryo development and reduces progression to the blastocyst stage. Our data demonstrate functional expression of TRPM7-like channels in mouse oocytes, eggs and embryos that may play an essential role in the initiation of embryo development.