Separate populations of receptor cells and presynaptic cells in mouse taste buds

Taste buds are aggregates of 50-100 cells, only a fraction of which express genes for taste receptors and intracellular signaling proteins. We combined functional calcium imaging with single-cell molecular profiling to demonstrate the existence of two distinct cell types in mouse taste buds. Calcium imaging revealed that isolated taste cells responded with a transient elevation of cytoplasmic Ca2+ to either tastants or depolarization with KCl, but never both. Using single-cell reverse transcription (RT)-PCR, we show that individual taste cells express either phospholipase C beta 2 (PLC beta 2) ( an essential taste transduction effector) or synaptosomal-associated protein 25 (SNAP25) ( a key component of calcium-triggered transmitter exocytosis). The two functional classes revealed by calcium imaging mapped onto the two gene expression classes determined by single-cell RT-PCR. Specifically, cells responding to tastants expressed PLC beta 2, whereas cells responding to KCl depolarization expressed SNAP25. We demonstrate this by two methods: first, through sequential calcium imaging and single-cell RT-PCR; second, by performing calcium imaging on taste buds in slices from transgenic mice in which PLC beta 2-expressing taste cells are labeled with green fluorescent protein. To evaluate the significance of the SNAP25-expressing cells, we used RNA amplification from single cells, followed by RT-PCR. We show that SNAP25-positive cells also express typical presynaptic proteins, including a voltage-gated calcium channel (alpha 1A), neural cell adhesion molecule, synapsin-II, and the neurotransmitter-synthesizing enzymes glutamic acid decarboxylase and aromatic amino acid decarboxylase. No synaptic markers were detected in PLC beta 2 cells by either amplified RNA profiling or by immunocytochemistry. These data demonstrate the existence of at least two molecularly distinct functional classes of taste cells: receptor cells and synapse-forming cells.