ABSTRACT
Connexins are a large family of proteins involved in formation of gap junctions that allow the direct transfer of small molecules and ions between neighboring cells (1). Transmission at gap junction synapses is very fast allowing the production of almost instantaneously acting potentials. Gap junctions are rare between mammalian neurons, but are common in nonneural cells, such as glia, epithelial cells, and smooth and cardiac muscle cells. Three groups of connexins exist, named alpha, beta, and non-alpha-nonbeta, as shown in Figure 1. Each connexin is identified by a number directly related to its molecular weight. Connexins form hexameric hemichannels (termed “connexons”) in the endoplasmic reticulum, which are then translocated into the plasma membrane. The connexon then “docks” with a connexon of an adjacent cell to form a functional channel termed a “gap junction.” Connexons can form either homotypic, heterotypic, or heteromeric channels. Connexins are expressed in many different tissues, including skin and inner ear. In the epidermis, gap junctions appear to play a role in the coordination of keratinocyte growth and differentiation (2), whereas several arguments support the idea that gap junctions have an important role in auditory transduction. The auditory organ has gap junctions between the outer hair cells and supporting cells (including melanocytes), providing a morphological basis for the occurrence of intracellular responses to sound in supporting cells and for electric coupling of receptor cells. The endothelium of the scala media of the cochlea is involved in the production of a receptor response to the auditory stimulus and is separated
from the endolymphatic space by tight junctions in the marginal cell layer, which is coupled to gap junctions. Immunohistochemical and ultrastructural analysis of some members of this protein family (connexin 26) in the rat cochlea showed that gap junctions in both epithelial and connective tissue cells are involved in recycling endolymphatic potassium ions (3).
