Research ArticleCell Biology

Chemical synapses without synaptic vesicles: Purinergic neurotransmission through a CALHM1 channel-mitochondrial signaling complex

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Sci. Signal.  08 May 2018:
Vol. 11, Issue 529, eaao1815
DOI: 10.1126/scisignal.aao1815

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Tasting with ATP

The savory taste of umami, the sweetness of sugar, and the bitterness of quinine are transduced by type II taste cells. Unlike most other receptor cells, type II taste cells release their neurotransmitter, ATP, through voltage-gated CALHM1 channels instead of neurotransmitter-containing vesicles. Romanov et al. found that the source of ATP was unusual, large mitochondria, closely opposed to clusters of CALHM1 channels within the plasma membrane of type II taste cells. This arrangement enables an alternate method of chemical neurotransmission that does not rely on vesicles.

Abstract

Conventional chemical synapses in the nervous system involve a presynaptic accumulation of neurotransmitter-containing vesicles, which fuse with the plasma membrane to release neurotransmitters that activate postsynaptic receptors. In taste buds, type II receptor cells do not have conventional synaptic features but nonetheless show regulated release of their afferent neurotransmitter, ATP, through a large-pore, voltage-gated channel, CALHM1. Immunohistochemistry revealed that CALHM1 was localized to points of contact between the receptor cells and sensory nerve fibers. Ultrastructural and super-resolution light microscopy showed that the CALHM1 channels were consistently associated with distinctive, large (1- to 2-μm) mitochondria spaced 20 to 40 nm from the presynaptic membrane. Pharmacological disruption of the mitochondrial respiratory chain limited the ability of taste cells to release ATP, suggesting that the immediate source of released ATP was the mitochondrion rather than a cytoplasmic pool of ATP. These large mitochondria may serve as both a reservoir of releasable ATP and the site of synthesis. The juxtaposition of the large mitochondria to areas of membrane displaying CALHM1 also defines a restricted compartment that limits the influx of Ca2+ upon opening of the nonselective CALHM1 channels. These findings reveal a distinctive organelle signature and functional organization for regulated, focal release of purinergic signals in the absence of synaptic vesicles.

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