Hair cells are the sensory receptors required for proper hearing and balance. In hair cells, function is shaped by summation of local calcium signals that ultimately trigger vesicle release in order to encode a sensory stimulus. We studied calcium signals and vesicle release in neuromast hair cells of the lateral-line system. We observed that calcium signals were extremely heterogeneous within a neuromast cluster, with a subset showing a more robust and persistent calcium influx. It was only in this later subset of hair cells that we observed vesicle release. The purpose of this study is to understand the basis for this functional heterogeneity and to understand why a sensory organ would be organized to function in this manner.
We have developed several transgenic lines that express calcium indicators (GCaMP6s, GCaMP3, RGECO or GGECO) that allow us to stimulate and monitor calcium signals in hair cells or postsynaptic afferents. In addition, we use SypHy, an indicator to monitor hair-cell vesicle release. Overall, this is a very powerful system, where we can continuously record larvae over multiple days to understand how activity of a neuromast or a single hair cell changes over time or during development. This long-term functional imaging, combined with mutant analysis and pharmacology have allowed us to better understand the basis of hair-cell functional heterogeneity.
Our long term imaging experiments indicate that hair cells located around the periphery of the neuromast are most likely to have robust calcium signals and release vesicles. The periphery of the neuromast is where new hair cells are generated. Our data shows that as hair cells mature at the periphery, they have robust calcium signals. Whether they continue to release vesicles ultimately depends on location within the neuromast and whether additional cells are added to the neuromast. Damage to hair cells with robust calcium signal enables other hair cells to release vesicles, indicating all hair cells are primed and competent to encode stimuli. Using genetics, we have found that the functional heterogeneity of hair cells does not result from afferent or efferent modulation. Lastly, our pharmacological results suggest that instead, connections between hair cells or supporting cells may facilitate functional heterogeneity. From these results we hypothesize that not all hair cells may be required to encode sensory signals. In addition, the heterogeneity within the neuromast may play an important role in development and maintaining a healthy balance for encoding sensory information.