Poster Sessions

CB -12

Aurea Sousa

A. D Sousa, F. T Salles, A. M. Pillai, M. A Bhat, B. Kachar

Septate junction proteins play a role in localizing KCNQ4 at the calyceal synapses of vestibular hair cell

Septate junctions (SJs) are intercellular junctions responsible for paracellular barrier formation and segregation of membrane domains in invertebrate epithelial cells. During evolution, tight junctions have replaced most SJs and the only known vertebrate permeability barrier to use SJ proteins is the axon-glial junction. The cell adhesion molecules CASPR and NF155 are key components of vertebrate SJs, are homologous to those of invertebrate SJs, and the comparative function of these proteins has been confirmed by studies of both Drosophila and mouse mutants. Axon-glial junctions promote segregation of sodium and potassium channels on the nodal and juxtaparanodal membranes, respectively. In this work, we analyzed rat, mouse and guinea pig inner ear tissue and found strong immunoreactivity for SJ proteins along the calyceal synaptic contact between type 1 vestibular hair cells and the afferent innervation. High-resolution fluorescence images show that CASPR and NF155 are also colocalized with the potassium channels KCNQ4. The immunoreactivity for CASPR and NF155 appears at around P3 and is fully developed at around P21, preceding by a few days the development of KCNQ4 immunoreactivity. Importantly, KCNQ4 localization is disrupted in adult CASPR knockout mice. We conclude that CASPR has a role in the scaffolding of proteins at the synaptic contacts between hair cells and the calyx. Intriguingly, as in the case of myelinated axons, synapses between calyces and type I hair cells appeared during vertebrate evolution. This report of a previously unappreciated presence and function of SJ proteins in a vertebrate system other than myelinated axons suggests a possible evolutionary significance for selection of SJs for these vertebrate-specific systems.