Claudin 14 knockout mice, a model for autosomal recessive deafness DFNB29, are deaf due to cochlear hair cell degeneration

Sp1 site is crucial for the mouse claudin-19 gene expression in the kidney cells

Members of the claudin family play important roles in the formation of tight junctions in the kidneys, liver and intestine. Claudin-19 (Cldn19), a newly identified member of this family, is highly expressed in the kidney of the mouse. To have a better understanding on mouse claudin-19 gene expression, a 0.9-kb DNA fragment containing the 5'-flanking region of the Cldn19 gene was isolated. DNA sequence comparison between the mouse and human Cldn19 promoter regions exhibited little homology. One transcription initiation site was located at 104 nucleotides upstream of the start codon (ATG) of the Cldn19 gene. The mouse claudin-19 promoter lacked typical CAAT or GC-box. Deletion constructs of the 0.9-kb DNA fragment were generated and fused to a promoterless luciferase (Luc) reporter plasmid. Transfection studies using various kidney cell lines (MDCK, mIMCD3 and HEK293) revealed that the minimal promoter fragment resided in the -39 to -108 region, which contained a number of binding sites for transcription factors including Sp1. Site-directed mutagenesis using specific oligo probes confirmed that Sp1 was crucial for Cldn19 transactivation in the three cell lines studied. Electromobility shift assay confirmed that the nuclear extracts of these cells bound to the Sp1 oligo derived from Cldn19 promoter, but not to the mutated Sp1 oligo probe. Moreover, this DNA-protein complex would be recognized by Sp1 antibody, indicating specific Sp1 binding. Collectively, our data suggest that Sp1 binds to the claudin-19 promoter region and is responsible for its expression in the kidney cell lines in vitro.

The molecular physiology of tight junction pores

Tight junctions form selective barriers that regulate paracellular transport across epithelia. A large family of tetraspanning cell-cell adhesion proteins called claudins create the barrier and regulate electrical resistance, size, and ionic charge selectivity. Study of inherited human claudin diseases and the outcome of the genetic manupulation of claudins in mice, Drosophila, and Caenorhabditis elegans are furthering our understanding of paracellular physiology.

Effects of transdifferentiation and EGF on claudin isoform expression in alveolar epithelial cells

Rat alveolar epithelial type II cells grown on polycarbonate filters form high-resistance monolayers and concurrently acquire many phenotypic properties of type I cells. Treatment with EGF has previously been shown to increase transepithelial resistance across alveolar epithelial cell (AEC) monolayers. We investigated changes in claudin expression in primary cultured AEC during transdifferentiation to the type I cell-like phenotype ( days 0, 1, and 8), and on day 5 in culture ± EGF (10 ng/ml) from day 0 or day 4. Claudins 4 and 7 were increased, whereas claudins 3 and 5 were decreased, on later compared with earlier days in culture. Exposure to EGF led to increases in claudins 4 and 7 and decreases in claudins 3 and 5. Claudin 1 was only faintly detectable in freshly isolated type II cells and remained unchanged over time in culture and after exposure to EGF. These results suggest that increases in transepithelial resistance accompanying AEC transdifferentiation and/or EGF exposure are mediated, at least in part, by changes in the pattern of expression of specific claudin isoforms.

Barriers built on claudins

The fundamental functions of epithelia and endothelia in multicellular organisms are to separate compositionally distinct compartments and regulate the exchange of small solutes and other substances between them. Tight junctions (TJs) between adjacent cells constitute the barrier to the passage of ions and molecules through the paracellular pathway and function as a 'fence' within the plasma membrane to create and maintain apical and basolateral membrane domains. How TJs achieve this is only beginning to be understood. Recently identified components of TJs include the claudins, a family of four-transmembrane-span proteins that are prime candidates for molecules that function in TJ permeability. Their identification and characterization have provided new insight into the diversity of different TJs and heterogeneity of barrier functions in different epithelia and endothelia.

A new locus for nonsyndromic deafness DFNB49 maps to chromosome 5q12.3-q14.1

Cosegregation of markers on chromosome 5q12.3-q14.1 with profound congenital deafness in two Pakistani families (PKDF041 and PKDF141) defines a new recessive deafness locus, DFNB49. A maximum two-point lod score of 4.44 and 5.94 at recombination fraction theta=0 was obtained for markers D5S2055 and D5S424 in families PKDF041 and PKDF141, respectively. Haplotype analysis revealed an 11 cM linkage region flanked by markers D5S647 (74.07 cM) and D5S1501 (85.25 cM). Candidate deafness genes in this region include SLC30A5, OCLN, GTF2H2, and BTF3, encoding solute carrier family 30 (zinc transporter) member 5, occludin, RNA polymerase II transcription initiation factor, and basic transcription factor 3, respectively. Sequence analysis of the coding exons of SLC30A5 in DNA samples from two affected individuals of families PKDF041 and PKDF141 revealed no mutation. The mapping of DFNB49 further confirms the heterogeneity underlying autosomal recessive forms of nonsyndromic deafness.

Cell Adhesion, Polarity, and Epithelia in the Dawn of Metazoans

Transporting epithelia posed formidable conundrums right from the moment that Du Bois Raymond discovered their asymmetric behavior, a century and a half ago. It took a century and a half to start unraveling the mechanisms of occluding junctions and polarity, but we now face another puzzle: lest its cells died in minutes, the first high metazoa (i.e., higher than a sponge) needed a transporting epithelium, but a transporting epithelium is an incredibly improbable combination of occluding junctions and cell polarity. How could these coincide in the same individual organism and within minutes? We review occluding junctions (tight and septate) as well as the polarized distribution of Na+-K+-ATPase both at the molecular and the cell level. Junctions and polarity depend on hosts of molecular species and cellular processes, which are briefly reviewed whenever they are suspected to have played a role in the dawn of epithelia and metazoan. We come to the conclusion that most of the molecules needed were already present in early protozoan and discuss a few plausible alternatives to solve the riddle described above.

Radixin deficiency causes deafness associated with progressive degeneration of cochlear stereocilia

Ezrin/radixin/moesin (ERM) proteins cross-link actin filaments to plasma membranes to integrate the function of cortical layers, especially microvilli. We found that in cochlear and vestibular sensory hair cells of adult wild-type mice, radixin was specifically enriched in stereocilia, specially developed giant microvilli, and that radixin-deficient (Rdx^−/−) adult mice exhibited deafness but no obvious vestibular dysfunction. Before the age of hearing onset (∼2 wk), in the cochlea and vestibule of Rdx^−/− mice, stereocilia developed normally in which ezrin was concentrated. As these Rdx^−/− mice grew, ezrin-based cochlear stereocilia progressively degenerated, causing deafness, whereas ezrin-based vestibular stereocilia were maintained normally in adult Rdx^−/− mice. Thus, we concluded that radixin is indispensable for the hearing ability in mice through the maintenance of cochlear stereocilia, once developed. In Rdx^−/− mice, ezrin appeared to compensate for radixin deficiency in terms of the development of cochlear stereocilia and the development/maintenance of vestibular stereocilia. These findings indicated the existence of complicate functional redundancy in situ among ERM proteins.

Disruption of the cingulin gene does not prevent tight junction formation but alters gene expression

Cingulin, a component of vertebrate tight junctions, contains a head domain that controls its junctional recruitment and protein interactions. To determine whether lack of junctional cingulin affects tight-junction organization and function, we examined the phenotype of embryoid bodies derived from embryonic stem cells carrying one or two alleles of cingulin with a targeted deletion of the exon coding for most of the predicted head domain. In homozygous (-/-) embryoid bodies, no full-length cingulin was detected by immunoblotting and no junctional labeling was detected by immunofluorescence. In hetero- and homozygous (+/- and -/-) embryoid bodies, immunoblotting revealed a Triton-soluble, truncated form of cingulin, increased levels of the tight junction proteins ZO-2, occludin, claudin-6 and Lfc, and decreased levels of ZO-1. The +/- and -/- embryoid bodies contained epithelial cells with normal tight junctions, as determined by freeze-fracture and transmission electron microscopy, and a biotin permeability assay. The localization of ZO-1, occludin and claudin-6 appeared normal in mutant epithelial cells, indicating that cingulin is not required for their junctional recruitment. Real-time quantitative reverse-transcription PCR (real-time qRT-PCR) showed that differentiation of embryonic stem cells into embryoid bodies was associated with up-regulation of mRNAs for several tight junction proteins. Microarray analysis and real-time qRT-PCR showed that cingulin mutation caused a further increase in the transcript levels of occludin, claudin-2, claudin-6 and claudin-7, which were probably due to an increase in expression of GATA-6, GATA-4 and HNF-4alpha, transcription factors implicated in endodermal differentiation. Thus, lack of junctional cingulin does not prevent tight-junction formation, but gene expression and tight junction protein levels are altered by the cingulin mutation.

Identification of unique transcripts from a mouse full-length, subtracted inner ear cDNA library

A small-scale full-length library construction approach was developed to facilitate production of a mouse full-length cDNA encyclopedia representing approximately 250 enriched, normalized, and/or subtracted cDNA libraries. One library produced using this approach was a subtracted adult mouse inner ear cDNA library (sIEa). The average size of the inserts was approximately 2.5 kb, with the majority ranging from 0.5 to 7.0 kb. From this library 22,574 sequence reads were obtained from 15,958 independent clones. Sequencing and chromosomal localization established 5240 clusters, with 1302 clusters being unique and 359 representing new ESTs. Our sIEa library contributed 56.1% of the 7773 nonredundant Unigene clusters associated with the four mouse inner ear libraries in the NCBI dbEST. Based on homologous chromosomal regions between human and mouse, we identified 1018 UniGene clusters associated with the deafness locus critical regions. Of these, 59 clusters were found only in our sIEa library and represented approximately 50% of the identified critical regions.

HUMANNONSYNDROMICSENSORINEURALDEAFNESS

Given the unique biological requirements of sound transduction and the selective advantage conferred upon a species capable of sensitive sound detection, it is not surprising that up to 1% of the approximately 30,000 or more human genes are necessary for hearing. There are hundreds of monogenic disorders for which hearing loss is one manifestation of a syndrome or the only disorder and therefore is nonsyndromic. Herein we review the supporting evidence for identifying over 30 genes for dominantly and recessively inherited, nonsyndromic, sensorineural deafness. The state of knowledge concerning their biological roles is discussed in the context of the controversies within an evolving understanding of the intricate molecular machinery of the inner ear.