Vytautas K. Verselis

Connexin Channel Stucture Function

Gap junctions are intercellular channels that mediate direct signaling between cells. Encoded by the gene family of connexins, gap junction channels are formed by the docking of two hemichannels or connexons, one contributed by each of two cells in apposition.  Hemichannels, while recognized as precursors to cell-cell channels, are now also known to constitute functional membrane channels outside of regions of cell contact.

Connexins, both in cell-cell and undocked-hemichannel configurations, constitute ion channels with characteristically large aqueous pores. In addition to mediating fluxes of inorganic ions, connexin channels have been shown to be permeable to a variety of metabolites and signaling molecules such as amino acids, nucleotides, small peptides and sugars. Recently, Cx43 gap junction channels were shown to mediate antigen cross-presentation through peptide transfer and to be permeable to some siRNAs. These channels, however, are not simply conduits for any molecule up to a certain cut-off size, often estimated to be in the ~1kDa range, and the pore properties of different connexin channels are not all the same. Some connexin channels prefer cations while others prefer anions, some restrict fluxes of molecules above ~400Da and others, specifically heteromers, can even discriminate among cyclic nucleotides and isomers of inositol triphosphate. Unitary conductances range from less than 10pS to greater than 300pS.

It is becoming increasingly evident that the different channel properties play important roles in tissue-specific function. An important function appears to be in selecting which biological signals are to be transmitted between cells and/or across the plasma membrane. There are also pathogenic functions that include contributions to cellular toxicity after ischemia, particularly for hemichannels that participate in promoting cell death by mediating loss of cellular metabolites and/or excessive entry of Ca 2+. Using a combination of molecular genetic, biophysical and imaging approaches, we study how connexin channels and hemichannels are assembled, how they are regulated and what signals go through them. Using the substituted cysteine accessibility method, we are mapping residues to the pore in order to define to the permeation pathway in molecular terms. Studies aimed at understanding how hemichannels are regulated indicate that membrane voltage, extracellular Ca 2+ and pH, alone or in combination, are effective at controlling hemichannel opening. We are exploring mechanisms of action of these agents as well as additional regulatory elements involving intracellular cascades. We have identified an important region of the connexin located at the border of the first transmembrane and extracellular loop domains (TM1/E1) that plays a central role in governing permeation as well as gating. A number of naturally occurring mutations in this region are linked to human disorders and we are examining how altered channel properties contribute to the expression of disease phenotypes. We have initiated studies of mutations that map to this region of the human Cx26 protein, which are linked to sensineural deafness and keratoderma.

Selected Publications