A Quantitative Analysis of the Spatiotemporal Patt

Authors: Yukako Asai Jeffrey R Holt Gwenaëlle S G Géléoc

Publish Date: 2009/10/16

Volume: 11, Issue: 1, Pages: 27-37

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Abstract

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 wellcharacterized 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 betaactin were quantified in 28 samples for a total of 1092 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 100fold TRPP3 50fold and TRPC52 20fold 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 1012961–2973 2009 Numerous spatiotemporal expression gradients were identified many of which may contribute to the normal functional development of the mouse cochleaThe Transient Receptor Potential TRP superfamily encodes over 33 distinct proteins which have been implicated in numerous cellular and sensory functions including detection of chemical thermal and mechanical stimuli Montell 2005 Damann et al 2008 The mammalian family is divided into six subfamilies TRPC Canonical TRPM Melastatin TRPV Vanilloid TRPA Ankyrin TRPP Polycystin and TRPML Mucolipin Over the past decade interest in TRP expression in the ear has risen in part due to the ongoing search for the elusive hair cell transduction channel which has biophysical properties consistent with those of several TRP channels The transduction channel is a nonselective cation channel with a conductance of about 100 pS permeable to calcium and sensitive to inhibition by TRP channel antagonists ruthenium red and lanthanum in turtle Farris et al 2004 and mouse Géléoc and Holt unpublished Several TRP channel subunits have been presented as hair cell transduction channel candidates including TRPN1 Walker et al 2000 TRPV4 Liedtke et al 2000 TRPA1 Corey et al 2004 and recently TRPML3 Grimm et al 2007 Nagata et al 2008 van Aken et al 2008 Yet targeted gene deletion of single TRP genes has not revealed deficits in hair cell mechanotransduction However since TRP channels may consist of heteromultimers the hair cell transduction channel may be composed of more than one type of TRP subunit hence upregulation of subunits that remain may compensate for absent subunits Furthermore it is possible that the identity of the hair cell transduction channel remains hidden within the pool of TRP genes not yet examined Alternatively the hair cell transduction channel may be composed of ion channels that do not belong to the TRP familyDespite the lack of evidence supporting a role for TRP channels in hair cell transduction it is clear that TRP gene expression is critical for normal auditory function Analysis of two TRP genes TRPV4 and TRPML3 expressed in the ear revealed important nonsensory roles for TRPs in hearing and balance TRPV4 is expressed in hair cells stria vascularis and vestibular dark cells Liedtke et al 2000 Takumida et al 2005 and disruption of TRPV4 causes delayedonset hearing loss and increases susceptibility to acoustic injury Tabuchi et al 2005 TRPML3 is expressed in sensory hair cells and in the marginal cells of the stria vascularis Di Palma et al 2002 Nagata et al 2008 Van Aken et al 2008 Mutations in TRPML3 lead to deafness in varitintwaddler Va mice due to defects in stria vascularis and hair cells The Va mutation induces constitutive activation of inward currents which causes cellular degeneration Nagata et al 2008 Van Aken et al 2008

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