Fine structure of extracellular matrix and basal laminae in two types of abnormal collagen production: L-proline analog-treated otocyst cultures and disproportionate micromelia (Dmm/Dmm) mutants

Genetics of hearing and deafness

This article is a review of the genes and genetic disorders that affect hearing in humans and a few selected mouse models of deafness. Genetics is playing an increasingly critical role in the practice of medicine. This is not only in part to the importance that genetic knowledge has on traditional genetic diseases but also in part to the fact that genetic knowledge provides an understanding of the fundamental biological process of most diseases. The proteins coded by the genes related to hearing loss (HL) are involved in many functions in the ear, such as cochlear fluid homeostasis, ionic channels, stereocilia morphology and function, synaptic transmission, gene regulation, and others. Mouse models play a crucial role in understanding of the pathogenesis associated with these genes. Different types of familial HL have been recognized for years; however, in the last two decades, there has been tremendous progress in the discovery of gene mutations that cause deafness. Most of the cases of genetic deafness recognized today are monogenic disorders that can be broadly classified by the mode of inheritance (i.e., autosomal dominant, autosomal recessive, X-linked, and mitochondrial inheritance) and by the presence of associated phenotypic features (i.e., syndromic; and nonsyndromic). In terms of nonsyndromic HL, the chromosomal locations are currently known for ∼ 125 loci (54 for dominant and 71 for recessive deafness), 64 genes have been identified (24 for dominant and 40 for recessive deafness), and there are many more loci for syndromic deafness and X-linked and mitochondrial DNA disorders (http://hereditaryhearingloss.org). Thus, today's clinician must understand the science of medical genetics as this knowledge can lead to more effective disease diagnosis, counseling, treatment, and prevention.

MicroRNAs and epigenetic regulation in the mammalian inner ear: implications for deafness

Sensorineural hearing loss is the most common sensory disorder in humans and derives, in most cases, from inner-ear defects or degeneration of the cochlear sensory neuroepithelial hair cells. Genetic factors make a significant contribution to hearing impairment. While mutations in 51 genes have been associated with hereditary sensorineural nonsyndromic hearing loss (NSHL) in humans, the responsible mutations in many other chromosomal loci linked with NSHL have not been identified yet. Recently, mutations in a noncoding microRNA (miRNA) gene, MIR96, which is expressed specifically in the inner-ear hair cells, were linked with progressive hearing loss in humans and mice. Furthermore, additional miRNAs were found to have essential roles in the development and survival of inner-ear hair cells. Epigenetic mechanisms, in particular, DNA methylation and histone modifications, have also been implicated in human deafness, suggesting that several layers of noncoding genes that have never been studied systematically in the inner-ear sensory epithelia are required for normal hearing. This review aims to summarize the current knowledge about the roles of miRNAs and epigenetic regulatory mechanisms in the development, survival, and function of the inner ear, specifically in the sensory epithelia, tectorial membrane, and innervation, and their contribution to hearing.

Azetidine-2-carboxylic acid in garden beets (Beta vulgaris)

Azetidine-2-carboxylic acid (L-Aze) is a toxic and teratogenic non-protein amino acid. In many species, including man, L-Aze is misincorporated into protein in place of proline, altering collagen, keratin, hemoglobin, and protein folding. In animal models of teratogenesis, it causes a wide range of malformations. The role of L-Aze in human disease has been unexplored, probably because the compound has not been associated with foods consumed by humans. Herein we report the presence of L-Aze in the garden or table beet (Beta vulgaris).

Analysis of genes from inner ear developmental-stage cDNA subtraction reveals molecular regionalization of the otic capsule

Although the gross embryology of inner ear development has been documented for several different vertebrate species at a descriptive level, our understanding of the molecular mechanisms involved remains rudimentary. Therefore, we have used cDNA subtraction and normalization procedures to define genes upregulated in the 13.5dpc mouse inner ear, a developmental stage where inner ear morphogenesis and tissue remodeling is active and differentiation of future hair cells is being initiated. We recovered 33 different genes from this subtraction and using gene-specific primers have confirmed the transcriptional upregulation of 26 of these in the 13.5dpc inner ear. Northern analyses were used to investigate splicing differences between the inner ear and the whole embryo at 13.5dpc. Spatial localization of expression was determined through whole-ear in situ hybridization analysis, and selected genes were analyzed in more detail through in situ hybridization of tissue sections. These data illustrate that the genes isolated in this study are expressed in the developing otic capsule and/or neuroepithelium. Furthermore, the expression patterns also reveal molecular heterogeneity in the developing capsule and indicate that for some genes, the chondrogenic otic capsule is composed of distinct domains of gene expression.

Transport of pharmacologically active proline derivatives by the human proton-coupled amino acid transporter hPAT1

Several proline derivatives such as L-azetidine-2-carboxylic acid, cis-4-hydroxy-L-proline, and 3,4-dehydro-DL-proline prevent procollagen from folding into a stable triple-helical conformation, thereby reducing excessive deposition of collagen in fibrotic processes and the growth of tumors. This study was performed to investigate whether the recently discovered human proton-coupled amino acid transporter 1 (hPAT1) is capable of transporting such pharmacologically relevant proline derivatives and also GABA analogs. Uptake of L-[3H]proline and [3H]glycine in Caco-2 cells was Na+-independent but strongly H+-dependent. The L-proline uptake was saturable and mediated by a single transport system (hPAT1) with an affinity constant of 2.0 +/- 0.2 mM. The uptake of L-[3H]proline was inhibited by D-proline, trans-4-hydroxy-L-proline, cis-4-hydroxy-L-proline, cis-4-hydroxy-D-proline, 3,4-dehydro-DL-proline, L-azetidine-2-carboxylic acid, 3-amino-1-propanesulfonic acid, D- and L-pipecolic acid, l-thiaproline, and many others. Apical uptake and transepithelial flux of L-[3H]proline across Caco-2 cell monolayers were strongly inhibited by proline derivatives in proportions corresponding to their respective affinity constants at hPAT1. The basolateral to apical flux of L-[3H]proline was only 8% of that in the opposite direction. Apical uptake of unlabeled L-proline, cis-4-hydroxy-L-proline, and L-azetidine-2-carboxylic acid was stimulated by an inside directed H+ gradient 2- to 3-fold. Total apical to basolateral flux of proline derivatives was moderately correlated with their inhibitory potency for L-[3H]proline uptake and flux inhibition. We conclude that 1) the substrate specificity of hPAT1 is very much broader than so far reported and 2) the system accepts therapeutically relevant proline and GABA derivatives. hPAT1 is a promising candidate for new ways of oral drug delivery.

Biologic effects of and clinical disorders caused by nonprotein amino acids

Despite the fact that nonprotein amino acids are present in many commonly eaten foods, the biologic and clinical significance of this class of molecules has been largely overlooked. This is owing in part to their relatively low concentrations and their negligible nutritive value. Many of these compounds have the ability to interfere with a wide range of fundamental biochemical processes and cause disease. It is likely that the clinical effects of the ingestion of some nonprotein amino acids are yet to be described. Serious disorders in humans have followed the ingestion of these compounds as the result of food faddism, prodded by the commercial promotion of inadequately tested products. In view of the current popularity of herbal remedies and alternative medicine, these facts serve as another reminder to health care providers and the public at large about the need for critical analysis of the alleged benefits and the risks of exotic remedies and nutritional supplements. Beyond the public health issues they raise, non-protein amino acids take on significance because their misincorporation into proteins can trigger vigorous autoimmune attacks. To what extent this mechanism is responsible for highly prevalent diseases of autoimmunity remains to be determined.