Innexin Gap Junctions

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Inx2 protein distribution on the surface of the adult brain.

Inx2 protein distribution on the surface of the adult brain.

Image of a section of the posterior surface of a female adult brain. Innexin2 protein (red) was detected using fluorescent antibody staining. Rabbit α-Inx2 and α-rabbit-conjugated rhodamine. The spotty pattern is somewhat similar to that observed when green fluorescent protein is expressed using the inx2-reporter GAL4 line 2GM1 and possibly reflects expression of Inx2 protein in surface glia. This pattern is also seen for Inx3 and Ogre protein. Notably, the most intense fluorescence observed on Innexin-antibody-stained brains is in these putative surface glia. This could be because all brains were stained as wholemounts and the primary or secondary antibodies didn’t penetrate into the interior of brains effectively, or the strong peripheral brain staining could reflect an important role for innexins in the glia ensheathing the brain (the blood brain barrier). This could reflect an evolutionarily conserved role for gap junctions. In vertebrate brains, very high expression of Cx43 and Cx30 is found in the meninges ( Dahl et al. 1995, Theis et al., 2005) and Cx26 is confined to the leptomeninges in adult mice ( Dermeitzel et al., 1989). The importance of connexins at these locations has not been studied but gap junctions potentially play an important role in cells that mediate a barrier function. Oxygen and nutrients are carried through the vertebrate brain by a vascular system and a blood-brain barrier exists to prevent leakage of ions and metabolites that would compromise the integrity of neuronal bathing fluid. The brain itself is surrounded by cerebro-spinal fluid that is prevented from seeping into the brain by the barrier function of the leptomeninges. Insect brains are avascular and brains are surrounded by hemolymph (fly blood) and the fly equivalent of the meninges and blood-brain barrier exists at the periphery of the CNS. Surface glia, cortex glia and perineurial cells all contribute to insect blood-brain barrier formation, isolating brains from hemolymph and thus maintaining the specific ionic milieu around neurons required to allow them to become electrophysiologically competent.

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