A quantitative analysis of the spatiotemporal pattern of transient receptor potential gene expression in the developing mouse cochlea.

TRP genes encode a diverse family of ion channels which have been implicated in many sensory functions. Because several TRP channels have similar properties to the elusive hair cell transduction channel, recent attention has focused on TRP gene expression in the inner ear. At least four TRP genes are known to be expressed in hair cells: TRPC3, TRPV4, TRPA1, and TRPML3. However, there is little evidence supporting any of these as a component of the transduction complex. Other less well-characterized TRP channels are expressed in the inner ear, in particular, within the organ of Corti. Because of their potential role in sensory function, we investigated the developmental expression of RNA that encodes all 33 TRP subunits as well as several splice variants. We designed a quantitative PCR screen using cochlear samples acquired before, during, and after the time when mechanotransduction is acquired in sensory hair cells (embryonic day 17 to postnatal day 8). Cochleas, which included the organ of Corti, stria vascularis, and Reissner's membrane, were subdivided into four equal quadrants which allowed for regional comparison during development. Expression of RNA transcripts that encoded 33 TRP subunits plus several splice forms and beta-actin were quantified in 28 samples for a total of 1,092 individual measurements, each done in triplicate. We detected RNA that encoded all TRP channels except two: TRPC7 and TRPM8. The largest changes in RNA expression were for TRPA1 (>100-fold), TRPP3 (>50-fold), and TRPC5.2 (>20-fold) which suggested that these subunits may contribute to normal cochlear function. Furthermore, the screen revealed TRPP3 and PKD1L3 RNA expression patterns that were correlated with the acquisition of sensory transduction in outer hair cells (Lelli et al., J Neurophysiol. 101:2961-2973, 2009). Numerous spatiotemporal expression gradients were identified many of which may contribute to the normal functional development of the mouse cochlea.