Expression of proteins involved in calcium homeostasis during crista ampullaris regeneration

Most of the events related to the mechano-transduction process in hair cell are regulated by cytoplasmic Ca++ concentrations. Ca++ influx trough the mechano-transduction channels regulates the adaptation process in the stereocilia, while Ca++ entering into the cell through the voltage gated channels regulates the neurotransmitter release. Moreover Ca++ activates the K+ channels that dominate the inhibitory postsynaptic potential at efferent synapses. Cytoplasmic Ca++ concentration is regulated by a complex system of proteins, including the Na+/Ca++ exchanger, Ca++ pumps (PMCAs, SERCAs) and a copious supply of diffusible proteins that buffers free Ca++ in specific cell compartments. These proteins intercept Ca++ near its sites of entry and restrict the spread of the free ion. Moreover this buffering limits the period during which Ca++ local concentration are high enough to trigger its physiological effects. In frog crista ampullaris PMCAs seems to be the most relevant mechanism of Ca++ extrusion since the lack of a functional Na+/Ca++ exchanger. We investigated the distribution of different subtypes of PMCAs finding that each isoform has its specific expression domain along the crista. We also demonstrate a similar compartmentalization for the IP3R, which is involved in Ca++ release from the IP3-sensitive intracellular stores, and for some buffering proteins. These data and the morphological identification of diverse hair cell types differentially distributed along the crista ampullaris, suggest the presence in the sensory epithelium of distinct functional domains where the mechanisms of Ca++ homeostasis regulation are not completely overlapping. This hypothesis is confirmed by the observation that the different regions of the crista ampullaris show a different sensibility to the ototoxic damage induced by gentamicin treatment: although the mechanism of actions of this drug has not jet completely understood, aminoglycosides are known to interfere with Ca++ homeostasis. Our studies on frog crista ampullaris regeneration showed that the complete crista functional activity is restored well afterwards its morphological recovery. Indeed, while the appearance of a stereociliary apparatus expressing a correct pattern of Ca++ pumps seems to be sufficient to restore hair cell basal activity, the recovery of the evoked sensory discharge seems to need also the expression of Ca++ buffering proteins. This study try to correlate the appearance of the different proteins involved in Ca++ homeostasis with the complete functional recovery of the sensory epithelium.