Maintenance of taste receptor cell presynaptic sites requires gustatory nerve fibers

Abstract

The turnover and re-establishment of peripheral taste synapses is vital to maintain connectivity between the primary taste receptor cells and the gustatory neurons which relay taste information from the tongue to the brain. Despite the importance of neuron-taste cell reconnection, mechanisms governing synapse assembly and the specificity of synaptic connections is largely unknown. Here we use the expression of presynaptic proteins, CALHM1 and Bassoon, to probe whether nerve fiber connectivity is an initiating factor for the recruitment of presynaptic machinery in different populations of taste cells. Under homeostatic conditions, the vast majority (>90%) of presynaptic sites are directly adjacent to nerve fibers. In the days immediately following gustatory nerve transection and complete denervation, Bassoon and CALHM1 puncta are markedly reduced. This suggests that nerve fiber innervation is crucial for the recruitment and maintenance of presynaptic sites. In support of this, we find that expression of Bassoon and Calhm1 mRNA transcripts are significantly reduced after denervation. During nerve fiber regeneration into the taste bud, presynaptic sites begin to replenish, but are not as frequently connected to nerve fibers as intact controls ([~]50% compared to >90%). This suggests that gustatory neuron proximity, rather than direct contact, likely drives taste receptor cells to express and aggregate presynaptic proteins at the cell membrane. Together, these data support the idea that trophic factors secreted by gustatory nerve fibers prompt taste receptor cells to produce presynaptic specializations at the cell membrane, which in turn may guide neurons to form mature synapses. These findings provide new insights into the mechanisms driving synaptogenesis and synaptic plasticity within the rapidly changing taste bud environment.