Tricellulin is a tight-junction protein necessary for hearing

Abstract

The inner ear has fluid-filled compartments of different ionic compositions, including the endolymphatic and perilymphatic spaces of the organ of Corti; the separation from one another by epithelial barriers is required for normal hearing. TRIC encodes tricellulin, a recently discovered tight-junction (TJ) protein that contributes to the structure and function of tricellular contacts of neighboring cells in many epithelial tissues. We show that, in humans, four different recessive mutations of TRIC cause nonsyndromic deafness (DFNB49), a surprisingly limited phenotype, given the widespread tissue distribution of tricellulin in epithelial cells. In the inner ear, tricellulin is concentrated at the tricellular TJs in cochlear and vestibular epithelia, including the structurally complex and extensive junctions between supporting and hair cells. We also demonstrate that there are multiple alternatively spliced isoforms of TRIC in various tissues and that mutations of TRIC associated with hearing loss remove all or most of a conserved region in the cytosolic domain that binds to the cytosolic scaffolding protein ZO-1. A wild-type isoform of tricellulin, which lacks this conserved region, is unaffected by the mutant alleles and is hypothesized to be sufficient for structural and functional integrity of epithelial barriers outside the inner ear.

Video 1.

The 3D reconstruction of the series of optical z-sections through the cuticular plates of organ of Corti outer hair cells. The video (a .MOV file [online only] that can be viewed with QuickTime [available at the Apple Web site]) shows three rows of outer hair cells surrounded by supporting cells. Tricellulin (red) is concentrated at contact points where three bTJs outlined by ZO-1 staining (green) converge in epithelial cells from the organ of Corti. ZO-1 (green) and occludin (shown in fig. 6) staining intersect tricellulin at the apical and basal end of the tricellular junction, which extends downward from the apical surface and spans the entire depth of the hair-cell cuticular plate. The 3D reconstruction was created using LSM510 software (Carl Zeiss MicroImaging).

Figure 1.

DFNB49-affected families segregating hearing loss associated with mutations of TRIC. A, Human pedigrees cosegregating hearing loss linked to markers in the DFNB49 interval. B, Haplotype of five STR markers linked to TRIC on chromosome 5 for families PKDF141 and PKDF340. Family PKDF141 has a maximum 2-point LOD score of 5.82 (θ=0) for marker D5S589. Individual IV:10 in family PKDF141 has the same disease haplotype and carries the same mutant allele of TRIC as does her spouse, suggesting common ancestry. Family PKDF340 has a maximum 2-point LOD score of 6.1 (θ=0) for GATA141B10. Meiotic recombinations define a linkage interval for DFNB49 between D5S629 and D5S637, which, on a physical map, is from 68,311,260 to 71,336,684 bp (UCSC browser; May 2004 assembly). The proximal and distal breakpoints were identified in deaf individuals V:6 (PKDF141) and VI:2 (PKDF340), respectively. C, Representative sequence chromatograms of four mutations of TRIC.

Figure 2.

Pure-tone air-conduction averages (0.5, 1, 2, and 4 kHz) for the better-hearing ears of seven of the eight DFNB49-affected families. The X-axis is age, and the Y-axis is the average decibel (dBHL) at which the threshold of hearing is detected. There is inter- and intrafamilial variability in the severity of hearing loss due to recessive mutations of TRIC, which may be caused by genetic modifiers and/or environmental factors. For control subjects and normal-hearing members of families, hearing thresholds measured in ambient-noise conditions were 25–30 dBHL.

Figure 3.

Wild-type isoforms of tricellulin, western-blot analysis, and aberrant RNA splice products that are due to mutant alleles of TRIC. A, Wild-type alternative splice isoforms of human TRIC and mouse Tric. Black bars under exon 2 show the position of the four predicted transmembrane helices. The red underline (also in panel C) indicates the occludin-ELL domain. The gray rectangles of exons 1 and 2 and of exon 7 are 5′ and 3′ UTRs, respectively. B, Western-blot analyses—through use of anti-tricellulin antibody HL5573 and protein extracts from adult C57BL/6J mouse pancreas, inner ear, retina, and lung—show a 64-kDa band in all lanes corresponding to a protein of the predicted size for Tric-a. In the retina, there is an abundant isoform of TRIC that is ∼80 kDa, which is also present at a lower concentration in lung and inner ear. In protein from lung, there is a 64-kDa band, whereas there are two bands at ∼64 kDa in protein from the cochlea. The right panel shows a western-blot analysis with a reduced amount of protein loaded on the gel for lung and a shorter exposure for the lane with protein from the cochlea. C, Aberrant splice products from exon-trapping assays. IVS3-1G→A causes the use of a cryptic splice-acceptor site within exon 4 of TRIC, which results in the deletion of the first 17 nt of exon 4 and a nonsense mutation (p.C395fsX396). IVS3-1G→A and IVS4+2T→C cause skipping of exon 4, which results in a frameshift and premature termination of translation (p.C395fsX403). IVS4+2delTGAG mRNA uses a cryptic splice-donor site in intron 4 that introduces 23 bp of intron 4, which results in a premature stop codon (p.K445fsX461). Locations of the mutant alleles are indicated by an asterisk (*). SDv and SAv are the splice-donor and splice-acceptor sites, respectively, encoded in vector sequence of pSPL3. D, Summary of RT-PCR data from lymphoblastoid cell lines of normal and (DFNB49) deaf individuals who have splice-site mutations of TRIC. The wild-type control showed the predicted splicing of exon 4 to exon 5 and alternative splicing of exon 3. mRNA isolated from lymphoblastoid cells from one deaf subject (each) of families PKDF399, PKDF443, PKDF058, and PKDF340 yielded aberrant splicing, and the locations of premature stop codons are indicated. Arrowheads indicate the locations of RT-PCR primers.

Figure 4.

Alignment of human tricellulin encoded by TRIC-a and TRIC-b and tricellulin orthologs from five vertebrates. Black and red bars indicate the predicted transmembrane helices and occludin-ELL domain, respectively. There is no significant amino acid–sequence similarity between TRIC (formerly referred to as “MARVELD2”) and MARVELD1 or MARVELD3 on chromosomes 10 and 16, respectively.

Figure 5.

The predicted protein structure of TRIC-a, the longest isoform encoded by TRIC. Topology of the tricellulin was predicted by TMpred software. Of the 37 residues, 15 (40.5%) in the first extracellular loop are glycine and tyrosine. Red and blue amino acids indicate negatively and positively charged residues, respectively. The N- and C-termini consist of 31.6% and 41.4% charged residues, respectively. The position of an arginine codon mutated in family PKDF340 (p.R500X) is shown as a green R. Orange daggers indicate boundaries of the six coding exons of TRIC. Arrows indicate the locations of translation frameshifts due to splice-acceptor mutation (IVS3-1G→A) and splice-donor mutations (IVS4+2T→C and IVS4+2delTGAG), which occur at codons for residues C395 and K445, respectively. The red bar indicates the occludin-ELL domain.

Figure 6.

Immunofluorescence confocal images from P16 mouse inner-ear epithelia stained with anti-tricellulin pAb PB705 (red) and anti-occludin FITC-conjugated antibody (green) or mouse anti-Z0-1 mAb (shown in the insets [green]). The left panels represent a flattened stack of optical z-sections through the cuticular plates of organ-of-Corti hair cells. The middle panels represent an optical section at the top of round-shaped cuticular plates of vestibular hair cells (white dots) that are surrounded by polygonally shaped apical portions of supporting cells. The right panels represent an optical section through the apical plasma membrane of the polygonally shaped marginal cells of the stria vascularis. In the organ of Corti, there are three rows of outer hair cells and one row of inner hair cells (indicated by an asterisk [*] [left panel]) surrounded by supporting cells. Tricellulin is concentrated at contact points where three occludin-positive TJs converge in epithelial cells from the organ of Corti (left panels), in vestibular epithelia of the utricle (middle panels), and in marginal cells of the stria vascularis (right panels). Fainter staining for tricellulin is seen at bTJs (arrow [left panel]). In hair cells of the organ of Corti, occludin (left panel) and ZO-1 (insets [left panel] and video 1 [online only]) staining intersect tricellulin at the apical and basal ends of the tricellular junction, which extends downward from the apical surface and spans the entire depth of the hair-cell cuticular plate (double-headed arrow). In the vestibular epithelia and stria vascularis, tricellulin appears to be colocalized also with Z0-1 at the apical end of the tricellular junctions (insets [middle and left panels]). Scale bars=5 μm.

Figure 7.

Freeze-fracture images of inner ear tTJs. A, At the apical end of an outer hair cell (OHC) in the organ of Corti of an adult guinea pig, the fracture has run across the apical membrane, cross-fracturing the bases of the stereocilia in the W-shaped bundle. At the border of the outer hair cell, where it would have contacted supporting cells, the fracture runs at 90° to the apical surface. The fracture exposes the whole depth of the hair cell tTJ down the lateral plasma membrane (arrow). The drawing shows TJ strands at the level of the most apical part of the lateral membrane (vertical red bars indicate the length of the tTJ; green lines represent ZO-1 staining outlining the cuticular plate at the top and bottom) (see fig. 6 [left panels]). B, The tTJ (arrow) at the site of contact of an outer hair cell with two supporting cells (1 and 2) is a distinct ridge from which short strands extend zipperlike to either side. C, For the mouse utricular macula, tTJs are less complex than those of an outer hair cell but also appear zipperlike. D, For the marginal cells of the stria vascularis, the tTJs appear similar to those of conventional epithelial cells such as Eph4 cells. Scale bars=5 μm.

Figure 8.

The binding of tricellulin to ZO-1, disrupted in p.R500X-truncated tricellulin. Proteins were resolved by SDS-PAGE and were stained with Coomassie Brilliant Blue. Top panel, lane 1, Control has only the N-terminal half of ZO-1 immobilized on agarose beads (ZO1-N; amino acids 1–888). Lanes 2 and 3, GST-fusion proteins encoding the corresponding domains of hDOC (amino acids 371–522) and a mutant occludin (hDOC K433D) that cannot bind to ZO1-N, used as positive and negative controls, respectively. Lanes 4–7, GST-fusion proteins encoding TRIC-a and TRIC-a1 bind ZO1-N, whereas truncated tricellulin, because of p.R500X, has reduced binding to ZO1-N. Compare lanes 4 and 5 with lanes 6 and 7. The purified ZO1-N fusion protein (arrowhead) was also resolved for comparison; there is little, if any, degradation of fusion proteins detected after binding. Middle panel, Purified GST-fusion proteins carried through the binding assay without ZO1-N. Bottom panel, Little or no binding of fusion proteins to agarose alone. The molecular weight markers are shown in kilodaltons.