Xenopus TRPN1 (NOMPC) localizes to microtubule-based cilia in epithelial cells, including inner-ear hair cells

Abstract

In vertebrates, the senses of hearing and balance depend on hair cells, which transduce sounds with their hair bundles, containing actin-based stereocilia and microtubule-based kinocilia. A longstanding question in auditory science is the identity of the mechanically sensitive transduction channel of hair cells, thought to be localized at the tips of their stereocilia. Experiments in zebrafish implicated the transient receptor potential (TRP) channel NOMPC (drTRPN1) in this role; TRPN1 is absent from the genomes of higher vertebrates, however, and has not been localized in hair cells. Another candidate for the transduction channel, TRPA1, apparently is required for transduction in mammalian and nonmammalian vertebrates. This discrepancy raises the question of the relative contribution of TRPN1 and TRPA1 to transduction in nonmammalian vertebrates. To address this question, we cloned the TRPN1 ortholog from the amphibian Xenopus laevis, generated an antibody against the protein, and determined the protein's cellular and subcellular localization. We found that TRPN1 is prominently located in lateral-line hair cells, auditory hair cells, and ciliated epidermal cells of developing Xenopus embryos. In ciliated epidermal cells TRPN1 staining was enriched at the tips and bases of the cilia. In saccular hair cells, TRPN1 was located prominently in the kinocilial bulb, a component of the mechanosensory hair bundles. Moreover, we observed redistribution of TRPN1 upon treatment of hair cells with calcium chelators, which disrupts the transduction apparatus. This result suggests that although TRPN1 is unlikely to be the transduction channel of stereocilia, it plays an essential role, functionally related to transduction, in the kinocilium.ciliary ͉ mechanosensory ͉ transduction ͉ transient receptor potential channel T he senses of hearing and balance depend on the operation of hair cells in the inner ear and, in aquatic vertebrates, the lateral line (for review see ref. 1). Hair cells bear microscopically fine projections, the stereocilia, on their apical surfaces; these stereocilia contain actin filaments and are grouped in hair bundles. Individual stereocilia within a bundle are connected by different types of links, most notably the tip links, which interconnect the tips of the stereocilia. Most hair cells, with the exception of those from the hearing organs of mammals and birds, contain on their apical surface in addition to the stereocilia one kinocilium, which is coupled to the hair bundle by kinocilial links. The kinocilium is a true cilium containing a (9 ϩ 2) arrangement of microtubules, similar to motile cilia. The displacement of the hair bundle by mechanical forces increases tension in the tip links, opening the mechanotransduction channels and depolarizing the hair cell. In frog hair cells, the mechanical forces are delivered through the kinocilium to the stereocilia (2).The transduction channels of vertebrate hair cells are nonselective cation channels with a high permeability to Ca 2ϩ (3-5). They are blocked ...

Direct gating and mechanical integrity of Drosophila auditory transducers require TRPN1

“…One candidate for an auditory transduction channel and its gating spring is TRPN1, which acts as a mechanotransduction channel in nematode touch receptors 5 and bears an N-terminal ankyrin spring 6 . TRPN1 has been implicated in the function of zebrafish auditory hair cells 7 and the Drosophila ear 8 , but its importance for auditory transduction is uncertain: In hair cells, TRPN1 localizes to kinocilia that are dispensable for transduction 9 ; and in the Drosophila ear insensitive sound-responses persist when TRPN1 is lost 10 . The latter sound-responses have been traced back to TRPN1-independent gravity/wind-sensitive cells in the ear of the fly that occur along with sensitive auditory sensory cells whose sensory function is abolished by the loss of TRPN1 10 .…”

Whirlin complexes with p55 at the stereocilia tip during hair cell development

“…There is a variety of evidence to indicate that cadherin 23 (CDH23) and a transient receptor potential (TRP) channel 1 (TRPA1) are components of the tip link and the mechanotransduction channel, respectively, the key machinery of the mechanotransduction apparatus (23)(24)(25). Interestingly, CDH23 and transient receptor potential channel 1 (TRPN1), another TRP family member, are also localized at the kinocilia of hair cells in rodents and Xenopus, respectively (26,27). It will be important to explore whether there are any functional interactions among p55, 4.1R, and these other groups of stereocilia proteins.…”

Cell biology of polycystin-2

“…With the exception of TRPV5 and TRPV6, which are highly Ca 2+ selective and TRPM4 which is practically impermeable to Ca 2+ , all other members of the TRP superfamily form nonselective cation channels in the plasma membrane, which permeate divalent cations to various degrees [15]. In terms of their subcellular localizations, TRP channels have been localized in various subcellular sites such as plasma membrane [12], endoplasmic reticulum [16], lysosomes [17][18][19], and primary cilium [20][21][22]. Consistently, PKD2 expression has been reported in the plasma membrane [23,24], ER [25], primary cilium [26], centrosome [27], and mitotic spindles in dividing cells [28].…”