Ultrastructural, physiological, and molecular defects in the inner ear of a gene-knockout mouse model for autosomal Alport syndrome

The multifaceted links between hearing loss and chronic kidney disease

Hearing loss affects nearly 1.6 billion people and is the third-leading cause of disability worldwide. Chronic kidney disease (CKD) is also a common condition that is associated with adverse clinical outcomes and high health-care costs. From a developmental perspective, the structures responsible for hearing have a common morphogenetic origin with the kidney, and genetic abnormalities that cause familial forms of hearing loss can also lead to kidney disease. On a cellular level, normal kidney and cochlea function both depend on cilial activities at the apical surface, and kidney tubular cells and sensory epithelial cells of the inner ear use similar transport mechanisms to modify luminal fluid. The two organs also share the same collagen IV basement membrane network. Thus, strong developmental and physiological links exist between hearing and kidney function. These theoretical considerations are supported by epidemiological data demonstrating that CKD is associated with a graded and independent excess risk of sensorineural hearing loss. In addition to developmental and physiological links between kidney and cochlear function, hearing loss in patients with CKD may be driven by specific medications or treatments, including haemodialysis. The associations between these two common conditions are not commonly appreciated, yet have important implications for research and clinical practice.

Molecular differences between neonatal and adult stria vascularis from organotypic explants and transcriptomics

Summary

The stria vascularis (SV), part of the blood-labyrinth barrier, is an essential component of the inner ear that regulates the ionic environment required for hearing. SV degeneration disrupts cochlear homeostasis, leading to irreversible hearing loss, yet a comprehensive understanding of the SV, and consequently therapeutic availability for SV degeneration, is lacking. We developed a whole-tissue explant model from neonatal and adult mice to create a robust platform for SV research. We validated our model by demonstrating that the proliferative behaviour of the SV in vitro mimics SV in vivo, providing a representative model and advancing high-throughput SV research. We also provided evidence for pharmacological intervention in our system by investigating the role of Wnt/β-catenin signaling in SV proliferation. Finally, we performed single-cell RNA sequencing from in vivo neonatal and adult mouse SV and revealed key genes and pathways that may play a role in SV proliferation and maintenance. Together, our results contribute new insights into investigating biological solutions for SV-associated hearing loss.

Significance

Hearing loss impairs our ability to communicate with people and interact with our environment. This can lead to social isolation, depression, cognitive deficits, and dementia. Inner ear degeneration is a primary cause of hearing loss, and our study provides an in depth look at one of the major sites of inner ear degeneration: the stria vascularis. The stria vascularis and associated blood-labyrinth barrier maintain the functional integrity of the auditory system, yet it is relatively understudied. By developing a new in vitro model for the young and adult stria vascularis and using single cell RNA sequencing, our study provides a novel approach to studying this tissue, contributing new insights and widespread implications for auditory neuroscience and regenerative medicine.

Highlights

- We established an organotypic explant system of the neonatal and adult stria vascularis with an intact blood-labyrinth barrier. - Proliferation of the stria vascularis decreases with age in vitro , modelling its proliferative behaviour in vivo . - Pharmacological studies using our in vitro SV model open possibilities for testing injury paradigms and therapeutic interventions. - Inhibition of Wnt signalling decreases proliferation in neonatal stria vascularis.- We identified key genes and transcription factors unique to developing and mature SV cell types using single cell RNA sequencing.

A critical evaluation of "leakage" at the cochlear blood-stria-barrier and its functional significance

The blood-labyrinth-barrier (BLB) is a semipermeable boundary between the vasculature and three separate fluid spaces of the inner ear, the perilymph, the endolymph and the intrastrial space. An important component of the BLB is the blood-stria-barrier, which shepherds the passage of ions and metabolites from strial capillaries into the intrastrial space. Some investigators have reported increased "leakage" from these capillaries following certain experimental interventions, or in the presence of inflammation or genetic variants. This leakage is generally thought to be harmful to cochlear function, principally by lowering the endocochlear potential (EP). Here, we examine evidence for this dogma. We find that strial capillaries are not exclusive, and that the asserted detrimental influence of strial capillary leakage is often confounded by hair cell damage or intrinsic dysfunction of the stria. The vast majority of previous reports speculate about the influence of strial vascular barrier function on the EP without directly measuring the EP. We argue that strial capillary leakage is common across conditions and species, and does not significantly impact the EP or hearing thresholds, either on evidentiary or theoretical grounds. Instead, strial capillary endothelial cells and pericytes are dynamic and allow permeability of varying degrees in response to specific conditions. We present observations from mice and demonstrate that the mechanisms of strial capillary transport are heterogeneous and inconsistent among inbred strains.

The cochlear matrisome: Importance in hearing and deafness

The extracellular matrix (ECM) consists in a complex meshwork of collagens, glycoproteins, and proteoglycans, which serves a scaffolding function and provides viscoelastic properties to the tissues. ECM acts as a biomechanical support, and actively participates in cell signaling to induce tissular changes in response to environmental forces and soluble cues. Given the remarkable complexity of the inner ear architecture, its exquisite structure-function relationship, and the importance of vibration-induced stimulation of its sensory cells, ECM is instrumental to hearing. Many factors of the matrisome are involved in cochlea development, function and maintenance, as evidenced by the variety of ECM proteins associated with hereditary deafness. This review describes the structural and functional ECM components in the auditory organ and how they are modulated over time and following injury.

Sensorineural Hearing Loss in Patients With Chronic Kidney Disease: A Comprehensive Review

This article aims to ascertain the prevalence of loss of hearing in patients with chronic kidney disease (CKD) and also to examine potential causes of sensorineural hearing loss (SNHL) in patients suffering from CKD. It has been discovered in recent years that there is a relationship between the occurrence of SNHL and CKD. Nowadays many people are suffering from CKD. These patients deal with several otorhinolaryngological issues, such as SNHL, candidiasis, epistaxis, halitosis, dysgeusia, xerostomia, and lip and thyroid malignancies. One of the most frequent otorhinolaryngological complications is audiovestibular system impairment. There are various proposed mechanisms for the appearance of loss of hearing in people suffering from CKD. The kidney and the inner ear have multiple functional and structural similarities, which may be the cause of these problems in CKD patients. In addition, changes in the homeostasis of water and electrolytes can affect the endolymphatic fluid and result in endolymphatic hydrops. Finally, some medications, like aminoglycosides and loop diuretics, are well known for their ototoxicity and are utilized to treat patients with CKD. Only a small number of population-based research have so far been able to show a connection between CKD and audiovestibular system impairment. Some investigation has shown that CKD patients are more likely than healthy people to experience vestibular impairment. The quality of life of a patient can be reduced by hearing loss. People with hearing loss experience communication issues in daily life, which negatively affects their cognitive and psychosocial functioning. Social isolation and a poor quality of life in terms of health can all result from hearing loss. In addition, decreased renal function has also been linked to poor quality of life, hospitalization, and cognitive dysfunction.

Research advances in cochlear pericytes and hearing loss

Pericytes are specialized mural cells surrounding endothelial cells in microvascular beds. They play a role in vascular development, blood flow regulation, maintenance of blood-tissue barrier integrity, and control of angiogenesis, tissue fibrosis, and wound healing. In recent decades, understanding of the critical role played by pericytes in retina, brain, lung, and kidney has seen significant progress. The cochlea contains a large population of pericytes. However, the role of cochlear pericytes in auditory pathophysiology is, by contrast, largely unknown. The present review discusses recent progress in identifying cochlear pericytes, mapping their distribution, and defining their role in regulating blood flow, controlling the blood-labyrinth barrier (BLB) and angiogenesis, and involvement in different types of hearing loss.

OCT imaging of endolymphatic hydrops in mice: association with hearing loss

BACKGROUND

The etiology of Ménière's disease (MD) is still not completely clear, but it is believed to be associated with endolymphatic hydrops (EH), which is characterized by auditory functional disorders. Vasopressin injection in C57BL/6J mice can induce EH and serve as a model for MD. Optical Coherence Tomography (OCT) has shown its advantages as a non-invasive imaging method for observing EH.AimInvestigating the relationship between hearing loss and EH to assist clinical hearing assessments and indicate the severity of hydrops.

METHODS

C57BL/6J mice received 50 μg/100g/day vasopressin injections to induce EH. Auditory function was assessed using auditory brainstem response (ABR) and distortion product otoacoustic emissions (DPOAE). OCT was used to visualize the cochlea.

RESULT

OCT observed accumulation of fluid within the scala media in the cochlear apex. ABR showed significant hearing loss after 4 weeks. DPOAE revealed low-frequency hearing loss at 2 weeks and widespread damage across frequencies at 4 weeks.

CONCLUSION

The development of hearing loss in mouse models of MD is consistent with EH manifestations.SignificanceThis study demonstrates the possibility of indirectly evaluating the extent of EH through auditory assessment and emphasizes the significant value of OCT for imaging cochlear structures.

MiR-29a-deficiency causes thickening of the basilar membrane and age-related hearing loss by upregulating collagen IV and laminin

Age-related hearing loss (ARHL) is the most common sensory degenerative disease and can significantly impact the quality of life in elderly people. A previous study using GeneChip miRNA microarray assays showed that the expression of miR-29a changes with age, however, its role in hearing loss is still unclear. In this study, we characterized the cochlear phenotype of miR-29a knockout (miR-29a^-/-) mice and found that miR-29a-deficient mice had a rapid progressive elevation of the hearing threshold from 2 to 5 months of age compared with littermate controls as measured by the auditory brainstem response. Stereocilia degeneration, hair cell loss and abnormal stria vascularis (SV) were observed in miR-29a^-/- mice at 4 months of age. Transcriptome sequencing results showed elevated extracellular matrix (ECM) gene expression in miR-29a^-/- mice. Both Gene Ontology (GO) annotation and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis revealed that the key differences were closely related to ECM. Further examination with a transmission electron microscope showed thickening of the basilar membrane in the cochlea of miR-29a^-/- mice. Five Col4a genes (Col4a1-a5) and two laminin genes (Lamb2 and Lamc1) were validated as miR-29a direct targets by dual luciferase assays and miR-29a inhibition assays with a miR-29a inhibitor. Consistent with the target gene validation results, the expression of these genes was significantly increased in the cochlea of miR-29a^-/- mice, as shown by RT-PCR and Western blot. These findings suggest that miR-29a plays an important role in maintaining cochlear structure and function by regulating the expression of collagen and laminin and that the disturbance of its expression could be a cause of progressive hearing loss.

High Frequency Hearing Loss in Chronic Renal Disease: A Cross-Sectional Study

Introduction Chronic kidney disease (CKD) patients face multiple complications. One of them is involvement of the auditory system and it deserves more attention than is paid by the current approaches as hearing loss has major repercussions on the quality of life. Early detection can prevent further deterioration of hearing and improve the quality of life of patients suffering from CKD. Material and methods The high frequency (8-18 kHz) audiometry with pure tones was performed in 82 patients with CKD and compared with age and sexes matched healthy control group of 90 patients. Individual ear were used for statistical calculations. Results This study clearly observed that the hearing thresholds for frequencies 8-18 kHz increased in CKD patients when compared to sex and aged matched healthy control group, the thresholds significantly increased in CKD with diabetes mellitus as compared to nondiabetic with CKD. The high frequency thresholds significantly increased in patients on hemodialysis, and were significantly correlated with age, stage of CKD and duration of CKD. Conclusion This study highlights the presence of high frequency (8-18 kHz) hearing loss in patients of CKD. The severity is significantly correlated with age, stage and duration of CKD.

Col4a3-/- Mice on Balb/C Background Have Less Severe Cardiorespiratory Phenotype and SGLT2 Over-Expression Compared to 129x1/SvJ and C57Bl/6 Backgrounds

Alport syndrome (AS) is a hereditary renal disorder with no etiological therapy. In the preclinical Col4a3^-/- model of AS, disease progression and severity vary depending on mouse strain. The sodium-glucose cotransporter 2 (SGLT2) is emerging as an attractive therapeutic target in cardiac/renal pathologies, but its application to AS remains untested. This study investigates cardiorespiratory function and SGLT2 renal expression in Col4a3^-/- mice from three different genetic backgrounds, 129x1/SvJ, C57Bl/6 and Balb/C. male Col4a3^-/- 129x1/SvJ mice displayed alterations consistent with heart failure with preserved ejection fraction (HFpEF). Female, but not male, C57Bl/6 and Balb/C Col4a3^-/- mice exhibited mild changes in systolic and diastolic function of the heart by echocardiography. Male C57Bl/6 Col4a3^-/- mice presented systolic dysfunction by invasive hemodynamic analysis. All strains except Balb/C males demonstrated alterations in respiratory function. SGLT2 expression was significantly increased in AS compared to WT mice from all strains. However, cardiorespiratory abnormalities and SGLT2 over-expression were significantly less in AS Balb/C mice compared to the other two strains. Systolic blood pressure was significantly elevated only in mutant 129x1/SvJ mice. The results provide further evidence for strain-dependent cardiorespiratory and hypertensive phenotype variations in mouse AS models, corroborated by renal SGLT2 expression, and support ongoing initiatives to develop SGLT2 inhibitors for the treatment of AS.

Molecular Basis, Diagnostic Challenges and Therapeutic Approaches of Alport Syndrome: A Primer for Clinicians

Alport syndrome is a genetic and hereditary disease, caused by mutations in the type IV collagen genes COL4A3, COL4A4 and COL4A5, that affects the glomerular basement membrane of the kidney. It is a rare disease with an underestimated prevalence. Genetic analysis of population cohorts has revealed that it is the second most common inherited kidney disease after polycystic kidney disease. Renal involvement is the main manifestation, although it may have associated extrarenal manifestations such as hearing loss or ocular problems. The degree of expression of the disease changes according to the gene affected and other factors, known or yet to be known. The pathophysiology is not yet fully understood, although some receptors, pathways or molecules are known to be linked to the disease. There is also no specific treatment for Alport syndrome; the most commonly used are renin–angiotensin–aldosterone system inhibitors. In recent years, diagnosis has come a long way, thanks to advances in DNA sequencing technologies such as next-generation sequencing (NGS). Further research at the genetic and molecular levels in the future will complete the partial vision of the pathophysiological mechanism that we have, and will allow us to better understand what is happening and how to solve it.

Lack of collagen α6(IV) chain in mice does not cause severe-to-profound hearing loss or cochlear malformation, a distinct phenotype from nonsyndromic hearing loss with COL4A6 missense mutation

Congenital hearing loss affects 1 in every 1000 births, with genetic mutations contributing to more than 50% of all cases. X-linked nonsyndromic hereditary hearing loss is associated with six loci (DFNX1-6) and five genes. Recently, the missense mutation (c.1771G>A, p.Gly591Ser) in COL4A6, encoding the basement membrane (BM) collagen α6(IV) chain, was shown to be associated with X-linked congenital nonsyndromic hearing loss with cochlear malformation. However, the mechanism by which the COL4A6 mutation impacts hereditary hearing loss has not yet been elucidated. Herein, we investigated Col4a6 knockout (KO) effects on hearing function and cochlear formation in mice. Immunohistochemistry showed that the collagen α6(IV) chain was distributed throughout the mouse cochlea within subepithelial BMs underlying the interdental cells, inner sulcus cells, basilar membrane, outer sulcus cells, root cells, Reissner's membrane, and perivascular BMs in the spiral limbus, spiral ligament, and stria vascularis. However, the click-evoked auditory brainstem response analysis did not show significant changes in the hearing threshold of Col4a6 KO mice compared with wild-type (WT) mice with the same genetic background. In addition, the cochlear structures of Col4a6 KO mice did not exhibit morphological alterations, according to the results of high-resolution micro-computed tomography and histology. Hence, loss of Col4a6 gene expression in mice showed normal click ABR thresholds and normal cochlear formation, which differs from humans with the COL4A6 missense mutation c.1771G>A, p.Gly591Ser. Therefore, the deleterious effects in the auditory system caused by the missense mutation in COL4A6 are likely due to the dominant-negative effects of the α6(IV) chain and/or α5α6α5(IV) heterotrimer with an aberrant structure that would not occur in cases with loss of gene expression.

Characterization of Sensorineural Hearing Loss in Children with Alport Syndrome

Most adults with Alport syndrome (AS) suffer from progressive sensorineural hearing loss. However, little is known about the early characteristics of hearing loss in children with AS. As a part of the EARLY PRO-TECT Alport trial, this study was the first clinical trial ever to investigate hearing loss in children with AS over a timespan of up to six years Nine of 51 children (18%) had hearing impairment. Audiograms were divided into three age groups: in the 5–9-year-olds, the 4-pure tone average (4PTA) was 8.9 decibel (dB) (n = 15) in those with normal hearing and 43.8 dB (n = 2, 12%) in those with hearing impairment. Among the 10–13-year-olds, 4PTA was 4.8 dB (healthy, n = 12) and 41.4 dB (hearing impaired, n = 6.33%). For the 14–20-year-olds, the 4PTA was 7.0 dB (healthy; n = 9) and 48.2 dB (hearing impaired, n = 3.25%). On average, hearing thresholds of the hearing impaired group increased, especially at frequencies between 1–3 kHz. In conclusion, 18% of children developed hearing loss, with a maximum hearing loss in the audiograms at 1–3 kHz. The percentage of children with hearing impairment increased from 10% at baseline to 18% at end of trial as did the severity of hearing loss.

RNA-seq analysis of gene expression profiles in isolated stria vascularis from wild-type and Alport mice reveals key pathways underling Alport strial pathogenesis

Previous work demonstrates that the hearing loss in Alport mice is caused by defects in the stria vascularis. As the animals age, progressive thickening of strial capillary basement membranes (SCBMs) occurs associated with elevated levels of extracellular matrix expression and hypoxia-related gene and protein expression. These conditions render the animals susceptible to noise-induced hearing loss. In an effort to develop a more comprehensive understanding of how the underlying mutation in the COL4A3 gene influences homeostasis in the stria vascularis, we performed vascular permeability studies combined with RNA-seq analysis using isolated stria vascularis from 7-week old wild-type and Alport mice on the 129 Sv background. Alport SCBMs were found to be less permeable than wild-type littermates. RNA-seq and bioinformatics analysis revealed 68 genes were induced and 61 genes suppressed in the stria from Alport mice relative to wild-type using a cut-off of 2-fold. These included pathways involving transcription factors associated with the regulation of pro-inflammatory responses as well as cytokines, chemokines, and chemokine receptors that are up- or down-regulated. Canonical pathways included modulation of genes associated with glucose and glucose-1-PO4 degradation, NAD biosynthesis, histidine degradation, calcium signaling, and glutamate receptor signaling (among others). In all, the data point to the Alport stria being in an inflammatory state with disruption in numerous metabolic pathways indicative of metabolic stress, a likely cause for the susceptibility of Alport mice to noise-induced hearing loss under conditions that do not cause permanent hearing loss in age/strain-matched wild-type mice. The work lays the foundation for studies aimed at understanding the nature of strial pathology in Alport mice. The modulation of these genes under conditions of therapeutic intervention may provide important pre-clinical data to justify trials in humans afflicted with the disease.

Pericyte abnormalities precede strial capillary basement membrane thickening in Alport mice

In 129 Sv autosomal Alport mice, the strial capillary basement membranes (SCBMs) progressively thicken between 5 and 9 weeks of age resulting in a hypoxic microenvironment with metabolic stress and induction of pro-inflammatory cytokines and chemokines. These events occur concomitant with a drop in endocochlear potential and a susceptibility to noise-induced hearing loss under conditions that do not permanently affect age/strain-matched littermates. Here we aimed to gain an understanding of events that occur before the onset of SCBM thickening. Alport stria has normal thickness and shows levels of extracellular matrix (ECM) molecules in the SCBMs commensurate with wild-type mice. Hearing thresholds in the 3-week Alport mice do not differ from those of wild-type mice. We performed RNAseq analysis using RNA from stria vascularis isolated from 3-week Alport mice and wild type littermates. Data was processed using Ingenuity Pathway Analysis software and further distilled using manual procedures. RNAseq analysis revealed significant dysregulation of genes involved in cell adhesion, cell migration, formation of protrusions, and both actin and tubulin cytoskeletal dynamics. Overall, the data suggested changes in the cellular architecture of the stria might be apparent. To test this notion, we performed dual immunofluorescence analysis on whole mounts of the stria vascularis from these same animals stained with anti-isolectin gs-ib4 (endothelial cell marker) and anti-desmin (pericyte marker) antibodies. The results showed evidence of pericyte detachment and migration as well as the formation of membrane ruffling on pericytes in z-stacked confocal images from Alport mice compared to wild type littermates. This was confirmed by TEM analysis. Earlier work from our lab showed that endothelin A receptor blockade prevents SCBM thickening and ECM accumulation in the SCBMs. Treating cultured pericytes with endothelin-1 induced actin cytoskeletal rearrangement, increasing the ratio of filamentous to globular actin. Collectively, these findings suggest that the change in type IV collagen composition in the Alport SCBMs results in cellular insult to the pericyte compartment, activating detachment and altered cytoskeletal dynamics. These events precede SCBM thickening and hearing loss in Alport mice, and thus constitute the earliest event so far recognized in Alport strial pathology.

X-linked Alport syndrome: pathogenic variant features and further auditory genotype-phenotype correlations in males

OBJECTIVE

To analyze the clinical audiological characteristics of X-Linked Alport syndrome (XLAS) in males and their relationships with genotypes.

METHODS

The clinical data of 87 male patients with AS were reviewed. Hearing levels were evaluated using pure tone audiometry (PTA) testing, acoustic immittance, and otoacoustic emissions (OAE) testing. The genotypes of COL4A5 and the pathogenic variants were analyzed. The relationships between auditory phenotypes and genotypes were analyzed.

RESULTS

Among the 87 patients, the number of patients with normal hearing and hearing loss were 32 and 55, respectively. In all cases, the hearing loss was characterized as bilateral symmetrical sensorineural deafness. Majority of the patients had mild-to-moderate hearing loss. Hearing loss usually started in the middle frequency range and gradually affected high frequencies, at school age and gradually increased with increasing age. However, it maintained a relatively steady level of 50-60 dB HL during the teenage years. The audiometric curves included groove-type in 51 cases (92.73%). Patients were identified to have 60 different COL4A5 pathogenic variants. Of the 49 patients who were followed-up for more than 2 years, 28 cases presented a decreasing trend in the hearing level of about 5 dB per year. The degree of hearing loss was positively correlated with gene mutation type and renal function.

CONCLUSIONS

Hearing loss in males with XLAS is symmetrical sensorineural, and progressive with increasing age. There is a significant correlation between the degree of hearing loss and genotype, renal function, and age.

Hearing loss and renal syndromes

The association between ear and kidney abnormalities has long been recognized; however, the connection between these two disparate organs is not always straightforward. Although Alport syndrome is the most well-known, there are over 20 disorders that need to be considered in the differential diagnosis of patients with both ear and kidney abnormalities. Commonalities are present between the kidney and ear in a number of structural proteins, developmentally important transcription factors, ciliary proteins, and channel proteins, and mutations in these pathways can lead to disease in both organ systems. This manuscript reviews the congenital disorders with both hearing and kidney manifestations.

Temporal Bone Histopathology of X-linked Inherited Alport Syndrome

OBJECTIVE

To describe the histopathologic findings within the human cochlea in X-linked Alport syndrome.

STUDY DESIGN

Histopathologic analysis of cellular elements within the human cochlea by light microscopy.

MATERIALS AND METHODS

A right and a left cochleae of a man with genetically confirmed X-linked Alport syndrome was studied post-mortem. The temporal bones underwent standard processing for histologic examination. The slides were examined by light microscopy. Graphic reconstruction of the cochlea was performed to quantify hair cells, pathologic changes of the stria vascularis, and loss of cochlear neuronal cells.

RESULTS

There was severe loss of inner hair cells and all three rows of outer hair cells in the apical two turns of the cochlea. The stria vascularis and spiral ligament showed areas of marked loss which became more prominent from base to apex in each ear. The spiral ganglion cell count in the Rosenthal's canal exhibited loss of 20% to 45% compared to matched historical controls. There was a zone of separation between the organ of Corti and the basilar membrane extending along the basal surface of Deiters cells, Hensen cells, Claudius cells and external sulcus cells. The tunnel of Corti and the space of Nuel were filled with cellular elements along the cochlea.

CONCLUSION

The histopathologic findings of cochlear involvement in Alport's syndrome are basement membrane separation from the cells of the organ of Corti, outer and inner hair cell loss, and cellular infilling of the tunnel and extracellular spaces of the organ of Corti. These observations contribute to our understanding of the mechanism of sensorineural hearing loss in these patients.

LEVEL OF EVIDENCE

The triple helix of collagens - an ancient protein structure that enabled animal multicellularity and tissue evolution

The cellular microenvironment, characterized by an extracellular matrix (ECM), played an essential role in the transition from unicellularity to multicellularity in animals (metazoans), and in the subsequent evolution of diverse animal tissues and organs. A major ECM component are members of the collagen superfamily -comprising 28 types in vertebrates - that exist in diverse supramolecular assemblies ranging from networks to fibrils. Each assembly is characterized by a hallmark feature, a protein structure called a triple helix. A current gap in knowledge is understanding the mechanisms of how the triple helix encodes and utilizes information in building scaffolds on the outside of cells. Type IV collagen, recently revealed as the evolutionarily most ancient member of the collagen superfamily, serves as an archetype for a fresh view of fundamental structural features of a triple helix that underlie the diversity of biological activities of collagens. In this Opinion, we argue that the triple helix is a protein structure of fundamental importance in building the extracellular matrix, which enabled animal multicellularity and tissue evolution.

Osteopontin deficiency ameliorates Alport pathology by preventing tubular metabolic deficits

Alport syndrome is a rare hereditary renal disorder with no etiologic therapy. We found that osteopontin (OPN) is highly expressed in the renal tubules of the Alport mouse and plays a causative pathological role. OPN genetic deletion ameliorated albuminuria, hypertension, tubulointerstitial proliferation, renal apoptosis, and hearing and visual deficits in the Alport mouse. In Alport renal tubules we found extensive cholesterol accumulation and increased protein expression of dynamin-3 (DNM3) and LDL receptor (LDLR) in addition to dysmorphic mitochondria with defective bioenergetics. Increased pathological cholesterol influx was confirmed by a remarkably increased uptake of injected DiI-LDL cholesterol by Alport renal tubules, and by the improved lifespan of the Alport mice when crossed with the Ldlr-/- mice with defective cholesterol influx. Moreover, OPN-deficient Alport mice demonstrated significant reduction of DNM3 and LDLR expression. In human renal epithelial cells, overexpressing DNM3 resulted in elevated LDLR protein expression and defective mitochondrial respiration. Our results suggest a potentially new pathway in Alport pathology where tubular OPN causes DNM3- and LDLR-mediated enhanced cholesterol influx and impaired mitochondrial respiration.

Sensorineural hearing loss in patients with chronic renal failure on hemodialysis in Basrah, Iraq

Objectives:

The objective of this study is to determine the effect of hemodialysis on the hearing threshold in patients with chronic renal failure (CRF).

Materials and Methods:

Fifty-nine patients with CRF on regular hemodialysis were followed up for 1 year with a pure-tone audiometric examination every 6 months.

Results:

The mean age of the patients was 41.8 ± 9.2 years (range: 17–50 years). At the beginning of the study, 39 patients (66.1%) had sensorineural hearing loss (SNHL). During the 12-month follow-up, 6 more patients developed SNHL giving a point prevalence rate of 76.3% at the end of the study. The hearing loss was more evident in the higher frequencies. Of the studied patients, 64.4% showed deterioration of the hearing threshold. The mean hearing threshold at the beginning of the study was 29.2 ± 21.1 dB versus 36.9 ± 17.3 dB at the end of the study (P < 0.001). No significant relation was found between age, sex, serum electrolytes, blood urea, and duration of CRF and hearing loss. Multivariate analysis showed that the duration of hemodialysis was the only significant independent predictor of SNHL.

Conclusion:

SNHL is common in patients with CRF on hemodialysis. It was mild to moderate in the majority of patients. Hearing impairment was most obvious at the high frequencies. Most of the patients showed further deterioration in the hearing threshold with the duration of dialysis.

Temporal retinal thinning and the diagnosis of Alport syndrome and Thin basement membrane nephropathy

BACKGROUND AND OBJECTIVES

Alport syndrome is an inherited disease characterized by renal failure, hearing loss, and ocular abnormalities, including temporal retinal thinning. This study compared retinal thinning in Alport syndrome and other renal diseases.

METHODS

Alport syndrome was diagnosed on renal biopsy and genetic testing. Subjects underwent optical coherence tomography (OCT) (Spectralis OCT, Heidelberg Instruments). Retinal thinning was determined from horizontal macular OCT scans through the foveal center using the formula: Temporal thickness index (TTI) = (nasal - temporal thickness) ÷ nasal thickness × 100%, and compared with the normal range for each age group. Statistical analysis was performed using Student's t test, Mann-Whitney U test, and ROC analysis (SPPS, IBM).

RESULTS

The mean temporal retinal thickness index was 12.4 ± 5.2% in men (n = 19) and 7.4 ± 1.4% in women (n = 28) with X-linked Alport syndrome; 13.1 ± 4.5% (n = 4) in recessive disease; 6.4 ± 2.2% (n = 5) in Thin basement membrane nephropathy; and 6.3 ± 3.3% (n = 14) in other renal diseases. Thinning was worse in men than women with X-linked disease (p < 0.01), and worse in men who developed early onset renal failure (R² = 0.75). Temporal retinal thinning was 84% sensitive for men with X-linked Alport syndrome and 67% specific (AUC = 0.83) compared with other renal diseases.

CONCLUSIONS

Retinal temporal thinning is diagnostic for X-linked Alport syndrome in men and distinguishes them this condition from Thin basement membrane nephropathy, but only in men (p = 0.002). Temporal retinal thinning may also identify men and women with the rarer autosomal recessive disease.

Advances and unmet needs in genetic, basic and clinical science in Alport syndrome: report from the 2015 International Workshop on Alport Syndrome

Alport syndrome (AS) is a genetic disease characterized by haematuric glomerulopathy variably associated with hearing loss and anterior lenticonus. It is caused by mutations in the COL4A3, COL4A4 or COL4A5 genes encoding the α3α4α5(IV) collagen heterotrimer. AS is rare, but it accounts for >1% of patients receiving renal replacement therapy. Angiotensin-converting enzyme inhibition slows, but does not stop, the progression to renal failure; therefore, there is an urgent requirement to expand and intensify research towards discovering new therapeutic targets and new therapies. The 2015 International Workshop on Alport Syndrome targeted unmet needs in basic science, genetics and diagnosis, clinical research and current clinical care. In three intensive days, more than 100 international experts including physicians, geneticists, researchers from academia and industry, and patient representatives from all over the world participated in panel discussions and breakout groups. This report summarizes the most important priority areas including (i) understanding the crucial role of podocyte protection and regeneration, (ii) targeting mutations by new molecular techniques for new animal models and potential gene therapy, (iii) creating optimal interaction between nephrologists and geneticists for early diagnosis, (iv) establishing standards for mutation screening and databases, (v) improving widespread accessibility to current standards of clinical care, (vi) improving collaboration with the pharmaceutical/biotech industry to investigate new therapies, (vii) research in hearing loss as a huge unmet need in Alport patients and (viii) the need to evaluate the risk and benefit of novel (including 'repurposing') therapies on an international basis.

The Chemical Chaperone, PBA, Reduces ER Stress and Autophagy and Increases Collagen IV α5 Expression in Cultured Fibroblasts From Men With X-Linked Alport Syndrome and Missense Mutations

Introduction

X-linked Alport syndrome (OMIM 301050) is caused by COL4A5 missense variants in 40% of families. This study examined the effects of chemical chaperone treatment (sodium 4-phenylbutyrate) on fibroblast cell lines derived from men with missense mutations.

Methods

Dermal fibroblast cultures were established from 2 affected men and 3 normals. Proliferation rates were examined, the collagen IV α5 chain localized with immunostaining, and levels of the intra- and extracellular chains quantitated with an in-house enzyme-linked immunosorbent assay. COL4A5 mRNA was measured using quantitative reverse transcriptase polymerase chain reaction. Endoplasmic reticulum (ER) size was measured on electron micrographs and after HSP47 immunostaining. Markers of ER stress (ATF6, HSPA5, DDIT3), autophagy (ATG5, BECN1, ATG7), and apoptosis (CASP3, BAD, BCL₂) were also quantitated by quantitative reverse transcriptase polymerase chain reaction. Measurements were repeated after 48 hours of incubation with 10 mM sodium 4-phenylbutyrate acid.

Results

Both COL4A5 missense variants were associated with reduced proliferation rates on day 6 (P = 0.01 and P = 0.03), ER enlargement, and increased mRNA for ER stress and autophagy (all P values < 0.05) when compared with normal. Sodium 4-phenylbutyrate treatment increased COL4A5 transcript levels (P < 0.01), and reduced ER size (P < 0.01 by EM and P < 0.001 by immunostaining), ER stress (p HSPA5 and DDIT3, all P values < 0.01) and autophagy (ATG7, P < 0.01). Extracellular collagen IV α5 chain was increased in the M1 line only (P = 0.06).

Discussion

Sodium 4-phenylbutyrate increases collagen IV α5 mRNA levels, reduces ER stress and autophagy, and possibly facilitates collagen IV α5 extracellular transport. Whether these actions delay end-stage renal failure in men with X-linked Alport syndrome and missense mutations will only be determined with clinical trials.

Alport syndrome: facts and opinions

In this commentary, I review recent advances in Alport syndrome genetics, diagnostics, and therapeutics. I also offer some opinions regarding strategies to optimize the early identification of affected individuals to promote early therapeutic intervention.

Endothelin-1 mediated induction of extracellular matrix genes in strial marginal cells underlies strial pathology in Alport mice

Alport syndrome, a type IV collagen disorder, manifests as glomerular disease associated with hearing loss with thickening of the glomerular and strial capillary basement membranes (SCBMs). We have identified a role for endothelin-1 (ET-1) activation of endothelin A receptors (ET_ARs) in glomerular pathogenesis. Here we explore whether ET-1 plays a role in strial pathology. Wild type (WT) and Alport mice were treated with the ET_AR antagonist, sitaxentan. The stria vascularis was analyzed for SCBM thickness and for extracellular matrix (ECM) proteins. Additional WT and Alport mice were exposed to noise or hypoxia and the stria analyzed for hypoxia-related and ECM genes. A strial marginal cell line cultured under hypoxic conditions, or stimulated with ET-1 was analyzed for expression of hypoxia-related and ECM transcripts. Noise exposure resulted in significantly elevated ABR thresholds in Alport mice relative to wild type littermates. Alport stria showed elevated expression of collagen α1(IV), laminin α2, and laminin α5 proteins relative to WT. SCBM thickening and elevated ECM protein expression was ameliorated by ET_AR blockade. Stria from normoxic Alport mice and hypoxic WT mice showed upregulation of hypoxia-related, ECM, and ET-1 transcripts. Both ET-1 stimulation and hypoxia up-regulated ECM transcripts in cultured marginal cells. We conclude that ET-1 mediated activation of ET_ARs on strial marginal cells results in elevated expression of ECM genes and thickening of the SCBMs in Alport mice. SCBM thickening results in hypoxic stress further elevating ECM and ET-1 gene expression, exacerbating strial pathology.

Bull's eye and pigment maculopathy are further retinal manifestations of an abnormal Bruch's membrane in Alport syndrome

BACKGROUND AND OBJECTIVES

The retinal features of Alport syndrome include a central and peripheral fleck retinopathy, temporal retinal thinning, and a macular hole. Here we describe further retinal abnormalities.

METHODS

We identified a case of bull's eye maculopathy 20 years previously in a 68-year-old female, and reviewed archived retinal images from our cohort of X-linked (28 males, 28 females) or autosomal recessive (n = 13) Alport syndrome. All individuals had Alport syndrome confirmed on genetic testing or renal biopsy, were examined by an ophthalmologist, and underwent retinal imaging (KOWA non-mydriatic camera, Japan).

RESULTS

The index case had the p.Q379X variant in COL4A5 and currently had renal impairment, (eGFR = 45 ml/min/1.73 m²), bilateral hearing loss, and central and peripheral retinopathies. Her maculopathy had deteriorated, and she had a bilateral central visual field loss. Optical coherence tomography (Heidelberg Spectralis) demonstrated a disrupted retinal pigment epithelium and retinal atrophy. We identified a further early bull's eye maculopathy (1/69, 1.4%) from a female with autosomal recessive disease and normal renal function. We also noted a subtle pigment maculopathy associated with an abnormal retinal pigment epithelium in 27 (27/69, 39%) subjects with Alport syndrome, in both males (8/28, 29%) and females (13/28, 46%) with X-linked disease, and in autosomal recessive disease (6/13, 38%).

CONCLUSIONS

The bull's eye and pigment maculopathies in Alport syndrome result mainly from the damaged Bruch's membrane and overlying retinal pigment epithelium. Bull's eye maculopathy affects vision and patients should undergo regular monitoring for retinal complications.

Glomerular Filtration Rate and Urine Albumin to Creatinine Ratio Associated With Hearing Impairment Among Korean Adults With Diabetes: A Nationwide Population-Based Study

The objective of this study was to examine the association of estimated glomerular filtration rate (eGFR) and urine albumin to creatinine ratio (ACR) with hearing impairment among diabetic adults in Korea. The study was based on data from Korea National Health and Nutrition Examination Survey 2011 to 2012. Participants were 1206 diabetic adults, aged over 19 years, who completed audiometric testing supervised by nationally certified clinicians. Hearing impairment was defined in three grades: no hearing impairment (pure-tone average 0-25 dB), slight hearing impairment (26-40 dB), and disabling hearing impairment (>40 dB) in the better ear at frequencies 0.5, 1, 2, 3, 4 and 6 kHz. Using logistic regression, risk of hearing impairment was assessed after having controlled for confounding factors. Higher levels of ACR and lower levels of eGFR correlated with an increase in percentage of disabling hearing impairment both unilaterally and bilaterally (P < 0.001). Controlling for possible confounding covariates, odds ratios for hearing impairment showed tendency to increase in higher ACR groups (P for trend = 0.029). Similar pattern was examined between eGFR and hearing impairment (P for trend = 0.006). Odds ratios were 1.981 (1.146, 3.424) for ACR Q4 and 2.773 (1.286, 5.983) for eGFR < 60 mL/min. Fall in eGFR and rise in ACR correlated with severity of hearing impairment. The association existed independently of age, sex, body mass index (BMI), smoking, drinking, exercise, new onset of diabetes, education, income, mental stress, noise exposure, and metabolic syndrome.

Scanning Electron Microscopic Examination of the Extracellular Matrix in the Decellularized Mouse and Human Cochlea

Decellularized tissues have been used to investigate the extracellular matrix (ECM) in a number of different tissues and species. Santi and Johnson JARO 14:3-15 (2013) first described the decellularized inner ear in the mouse, rat, and human using scanning thin-sheet laser imaging microscopy (sTSLIM). The purpose of the present investigation is to examine decellularized cochleas in the mouse and human at higher resolution using scanning electron microscopy (SEM). Fresh cochleas were harvested and decellularized using detergent extraction methods. Following decellularization, the ECM of the bone, basilar membrane, spiral limbus, and ligament remained, and all of the cells were removed from the cochlea. A number of similarities and differences in the ECM of the mouse and human were observed. A novel, spirally directed structure was present on the basilar membrane and is located at the border between Hensen and Boettcher cells. These septa-like structures formed a single row in the mouse and multiple rows in the human. The basal lamina of the stria vascularis capillaries was present and appeared thicker in the human compared with the mouse. In the mouse, numerous openings beneath the spiral prominence that previously housed the root processes of the external sulcus cells were observed but in the human there was only a single row of openings. These and other anatomical differences in the ECM between the mouse and human may reflect functional differences and/or be due to aging; however, decellularized cochleas provide a new way to examine the cochlear ECM and reveal new observations.

Gene expression profiles of the cochlea and vestibular endorgans: localization and function of genes causing deafness

OBJECTIVES

We sought to elucidate the gene expression profiles of the causative genes as well as the localization of the encoded proteins involved in hereditary hearing loss.

METHODS

Relevant articles (as of September 2014) were searched in PubMed databases, and the gene symbols of the genes reported to be associated with deafness were located on the Hereditary Hearing Loss Homepage using localization, expression, and distribution as keywords.

RESULTS

Our review of the literature allowed us to systematize the gene expression profiles for genetic deafness in the inner ear, clarifying the unique functions and specific expression patterns of these genes in the cochlea and vestibular endorgans.

CONCLUSIONS

The coordinated actions of various encoded molecules are essential for the normal development and maintenance of auditory and vestibular function.

Prevalence and patterns of hearing loss among chronic kidney disease patients undergoing haemodialysis

BACKGROUND

The prevalence, degree, and patterns of hearing loss associated with chronic kidney disease (CKD) reported by various studies differ significantly. The effects of haemodialysis and duration of disease on hearing loss remain unclear.

AIMS

The aim of this study was to determine the prevalence and degree of hearing loss in CKD patients on haemodialysis.

METHODS

This study included 120 CKD patients on haemodialysis. Information regarding age, gender, duration of disease, subjective hearing loss, exposure to ototoxic drugs, comorbidities like diabetes, hypertension, and hypothyroidism, renal functions, electrolytes and number of haemodialysis sessions received were obtained. An equal number of age and sex matched controls were used to determine prevalence of hearing loss in CKD patients after subjecting both groups to pure tone audiometry. We compared CKD patients with and without hearing loss for association of hearing loss with disease duration, number of haemodialysis, and blood parameters.

RESULTS

Hearing loss was present in 41.7 per cent of CKD patients, significantly higher than controls (p=0.001), and was mild in the majority of patients. Impairment was noted across high and low frequencies of audiometric testing. Median duration of disease was the same (18 months) among CKD patients with and without hearing loss (p=0.62). CKD patients with hearing loss received 72 haemodialysis compared to 122 sessions by those without hearing loss (p=0.04).

CONCLUSION

Mild sensorineural hearing loss is common in CKD. Hearing loss has no specific pattern as it prevails at high and low frequencies. Hearing loss may be inversely associated with the number of haemodialysis sessions but not with duration of disease.

Alport syndrome--insights from basic and clinical research

In 1927, Arthur C. Alport first published his description of a triad of symptoms in a family with hereditary congenital haemorrhagic nephritis, deafness and ocular changes. A few years after his death, this group of symptoms was renamed Alport syndrome. To this day, Alport syndrome still inevitably leads to end-stage renal disease and the need for renal replacement therapy, starting in young adulthood. During the past two decades, research into this rare disease has focused on the effects of mutations in collagen type IV and the role of changes in podocytes and the glomerular basement membrane that lead to early kidney fibrosis. Animal models of Alport syndrome also demonstrate the pathogenetic importance of interactions between podocytes and the extracellular matrix. Such models might also help researchers to answer basic questions about podocyte function and the development of fibrosis, and to develop new therapeutic approaches that might be of use in other kidney diseases. In this Review, we discuss the latest basic and clinical research on Alport syndrome, focusing on the roles of podocyte pathology and the extracellular matrix. We also highlight early diagnosis and treatment options for young patients with this disorder.

Cochlin expression in vestibular endorgans obtained from patients with Meniere's disease

The distribution of cochlin and its associated basement membrane proteins (collagen IV, collagen II, laminin-β2, and nidogen-1) were evaluated in the vestibular endorgans of subjects with Meniere's disease and compared with normal specimens. Cochlin mRNA expression in vestibular endorgans from Meniere's disease specimens was also investigated. Specimens were obtained from patients who had Meniere's disease and who were undergoing ablative labyrinthectomy. Control specimens were obtained both from autopsy specimens with documented normal audiovestibular function and from patients undergoing labyrinthectomy for acoustic neuroma excision. In the normal control specimens, cochlin immunoreactivity was found evenly distributed in the stroma of the cristae ampullaris and maculae of the utricle. In Meniere's specimens, cochlin immunoreactivity was markedly increased; this was associated with an increase in cochlin mRNA expression as shown by real-time reverse transcription with the polymerase chain reaction. Collagen IV and laminin-β2 immunoreactivity was significantly decreased in Meniere's specimens. Nidogen-1 and collagen II immunoreactivity was unchanged in Meniere's specimens when compared with normal samples. Cochlin upregulation has been implicated in the hereditary audiovestibulopathy, DFNA9. The increased expression of cochlin and decreased expression of collagen IV and laminin in Meniere's disease are suggestive that the overexpression of cochlin contributes to the dysfunctional inner ear homeostasis seen in this disease.

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.

Rapid identification of a disease allele in mouse through whole genome sequencing and bulk segregation analysis

In a pedigree of C57BL/6J mice homozygous for germline mutations induced by the mutagen N-ethyl-N-nitrosourea (ENU), numerous animals died under specific pathogen-free (SPF) conditions between 6 and 7 months of age. Death was caused by nephritic syndrome, which progressed to renal failure associated with focal segmental glomerulosclerosis. To identify the mutation responsible for renal disease, we sequenced genomic DNA from an affected animal using the Applied Biosystems SOLiD sequencing platform. Approximately 74% of the nucleotides comprising coding sequences and splice junctions in the mouse genome were covered at least three times. Within this portion of the genome, 64 discrepancies were flagged as potential homozygous mutations and 82 were flagged as potential heterozygous mutations. A total of 10 of these calls, all homozygous, were validated by capillary sequencing. One of the validated mutations disrupted splicing of the Col4a4 transcript. Genetic mapping by bulk segregation analysis excluded all mutations but this one as the cause of renal disease in Aoba mice. Col4a4 has not been targeted in the mouse, and this strain, named Aoba, represents the first functionally null allele in this species. Our study demonstrates the speed and utility of whole genome sequencing coupled with low resolution meiotic mapping as a means of identifying causative mutations induced by ENU.

Immunocytochemical distribution of WARP (von Willebrand A domain-related protein) in the inner ear

The basic components of the epithelial, perineural, and perivascular basement membranes in the inner ear have been well-documented in several animal models and in the human inner ear. The von Willebrand A domain-related protein (WARP) is an extracellular matrix molecule with restricted expression in cartilage, and a subset of basement membranes in peripheral nerves, muscle, and central nervous system vasculature. It has been suggested that WARP has an important role in maintaining the blood-brain barrier. To date no studies on WARP distribution have been performed in the inner ear, which is equipped with an intricate vasculature network. In the present study, we determined the distribution of WARP by immunocytochemistry in the human inner ear using auditory and vestibular endorgans microdissected from human temporal bones obtained at autopsy. All subjects (n=5, aged 55-87years old) had documented normal auditory and vestibular function. We also determined the WARP immunolocalization in the mouse inner ear. WARP immunoreactivity localized to the vasculature throughout the stroma of the cristae ampullaris, the maculae utricle, and saccule in the human and mouse. In the human and mouse inner ear, WARP immunoreactivity delineated blood vessels located in the stria vascularis, spiral ligament, sub-basilar region, stromal tissue, and the spiral and vestibular ganglia. The distinct localization of WARP in the inner ear vasculature suggests an important role in maintaining its integrity. In addition, WARP allows delineation of microvessels in the inner ear allowing the study of vascular pathology in the development of otological diseases.

Immunohistochemical distribution of basement membrane proteins in the human inner ear from older subjects

The immunolocalization of several basement membrane (BM) proteins was investigated in vestibular endorgans microdissected from temporal bones obtained from subjects with a documented normal auditory and vestibular function (n=5, average age=88 years old). Fluorescent immunostaining using antibodies directed at collagen IV alpha 2, nidogen-1, laminin-beta2, alpha-dystroglycan, and tenascin-C was applied to cryosections from human cochlea, cristae ampullares, utricular and saccular maculae. Collagen IV alpha 2, nidogen-1, and laminin-beta2 localized to all subepithelial cochlear BMs, Reissner's membrane, strial and spiral ligamental perineural and perivascular BMs, and the spiral limbus. Tenascin-C localized to the basilar membrane and the osseous spiral lamina. alpha-Dystroglycan localized to most cochlear BMs except those in the spiral ligament, basilar membrane and spiral limbus. Collagen IV, nidogen-1, and laminin-beta2 localized to the subepithelial BMs of the maculae and cristae ampullares, and the perineural and perivascular BMs within the underlying stroma. The BM underlying the transitional and dark cell region of the cristae ampullares also expressed collagen IV, nidogen-1, and laminin-beta2. Tenascin-C localized to the subepithelial BMs of the utricular maculae and cristae ampullares, and to calyx-like profiles throughout the vestibular epithelium, but not to the perineural and perivascular BMs. alpha-Dystroglycan colocalized with aquaporin-4 in the basal vestibular supporting cell, and was also expressed in the subepithelial BMs, as well as perivascular and perineural BMs. This study provides the first comprehensive immunolocalization of these ECM proteins in the human inner ear. The validity of the rodent models for inner ear disorders secondary to BM pathologies was confirmed as there is a high degree of conservation of expression of these proteins in the human inner ear. This information is critical to begin to unravel the role that BMs may play in human inner ear physiology and audiovestibular pathologies.

Inner ear defects and hearing loss in mice lacking the collagen receptor DDR1

Discoidin domain receptor 1 (DDR1) is a tyrosine kinase receptor that is activated by native collagen. The physiological functions of DDR1 include matrix homeostasis and cell growth, adhesion, branching, and migration, but the specific role of DDR1 in the development and function of the inner ear has not been analyzed. Here, we show that deletion of the DDR1 gene in mouse is associated with a severe decrease in auditory function and substantial structural alterations in the inner ear. Immunohistochemical analysis demonstrated DDR1 expression in several locations in the cochlea, mostly associated with basement membrane and fibrillar collagens; in particular in basal cells of the stria vascularis, type III fibrocytes, and cells lining the basilar membrane of the organ of Corti. In the stria vascularis, loss of DDR1 function resulted in altered morphology of the basal cells and accumulation of electron-dense matrix within the strial epithelial layer in conjunction with a focal and progressive deterioration of strial cells. Cell types in proximity to the basilar membrane, such as Claudius', inner and outer sulcus cells, also showed marked ultrastructural alterations. Changes in the organ of Corti, such as deterioration of the supporting cells, specifically the outer hair cells, Deiters', Hensen's and bordering cells, are likely to interfere with mechanical properties of the organ and may be responsible for the hearing loss observed in DDR1-null mice. These findings may also have relevance to the role of DDR1 in other disease processes, for example, those affecting the kidney.

Alport syndrome and thin basement membrane nephropathy

Both Alport syndrome and thin basement membrane nephropathy (TBMN) can be considered as genetic diseases of the GBM involving the alpha3/alpha4/alpha5 network of type IV collagen. Mutations in any of the COL4A3, COL4A4 or COL4A5 genes can lead to total or partial loss of this network. Males with mutations in the X-linked COL4A5 gene develop Alport syndrome with progressive renal disease and sometimes extra-renal disease. Females who are heterozygous for a COL4A5 mutation are considered to be carriers for X-linked Alport syndrome. Although their clinical course and GBM ultrastructural changes can sometimes mimic TBMN, more often it tends to be more progressive than usually seen in TBMN. Males or females who are heterozygous for COL4A3 or COL4A4 mutations usually manifest as TBMN, with nonprogressive hematuria, while those who are homozygous or combined heterozygotes develop autosomal-recessive Alport syndrome. Thus, individuals with TBMN can be considered to be carriers for autosomal-recessive Alport syndrome, but there remain some exceptions in which patients heterozygous for COL4A3 or COL4A4 mutations develop autosomal-dominant Alport syndrome. Distinguishing between all these groups of patients requires a combination of family history and a renal biopsy for electron microscopic examination of the GBM and immunohistochemical staining of the GBM for the alpha3, alpha4 and alpha5 chains of type IV collagen. Mutational analysis of the COL4A3, COL4A4, and COL4A5 genes, whenever it becomes available, will be a valuable adjunct to the diagnostic workup in these patients. Novel therapeutic approaches may one day provide a treatment or cure for these patients, avoiding the need for transplantation and dialysis.

Role for macrophage metalloelastase in glomerular basement membrane damage associated with alport syndrome

Alport syndrome is a glomerular basement membrane (GBM) disease caused by mutations in type IV collagen genes. A unique irregular thickening and thinning of the GBM characterizes the progressive glomerular pathology. The metabolic imbalances responsible for these GBM irregularities are not known. Here we show that macrophage metalloelastase (MMP-12) expression is >40-fold induced in glomeruli from Alport mice and is markedly induced in glomeruli of both humans and dogs with Alport syndrome. Treatment of Alport mice with MMI270 (CGS27023A), a broad spectrum MMP inhibitor that blocks MMP-12 activity, results in largely restored GBM ultrastructure and function. Treatment with BAY-129566, a broad spectrum MMP inhibitor that does not inhibit MMP-12, had no effect. We show that inhibition of CC chemokine receptor 2 (CCR2) receptor signaling with propagermanium blocks induction of MMP-12 mRNA and prevents GBM damage. CCR2 receptor is expressed in glomerular podocytes of Alport mice, suggesting MCP-1 activation of CCR2 on podocytes may underlie induction of MMP-12. These data indicate that the irregular GBM that characterizes Alport syndrome may be mediated, in part, by focal degradation of the GBM due to MMP dysregulation, in particular, MMP-12. Thus, MMP-12/CCR2 inhibitors may provide a novel and effective therapeutic stra-tegy for Alport glomerular disease.

Revisão sobre a perda auditiva na Síndrome de Alport, analisando os aspectos clínicos, genéticos e biomoleculares

A Síndrome de Alport é uma desordem hereditária, caracterizada por hematúria, freqüentemente levando à falência renal. Pode ser acompanhada de alterações extra-renais, tais como: perda auditiva (PA) sensório-neural e alterações oculares. São descritas formas dominantes ligadas ao X, devidas às mutações no lócus COL4A5 e uma forma autossômica recessiva resultando de mutações no lócus COL4A3 ou COL4A4. Ainda foi sugerido um tipo autossômico dominante de SA. A doença decorre de alterações nas cadeias de colágeno tipo IV e os sintomas refletem o comprometimento da membrana basal de vários órgãos. As redes alfa3.alfa4.alfa5(IV) ocorrem no rim, na cóclea e no olho. O objetivo foi caracterizar a PA neste grupo de pacientes. Quando o quadro progride para o estágio final de falência renal, o melhor método de tratamento é o transplante, que tem contribuído para o aumento da sobrevida. Nesta revisão bibliográfica, observamos que: 1. A SA caracteriza-se por hematúria, que evolui para falência renal e pode ser acompanhada de manifestações extra-renais. A PA é um achado extra-renal freqüente e um dos primeiros sintomas na SA, sendo um fator relevante para o prognóstico da evolução da doença renal; 2. A SA é genética e decorre da alteração das cadeias do colágeno tipo IV nas membranas basais; 3. A perda auditiva na SA é sensório-neural, de intensidade variável, progressiva e simétrica. Acomete as freqüências médias e altas; 4. Na investigação das perdas auditivas, o otorrinolaringologista deve incluir um exame de urina. É fundamental que o otologista atue no acompanhamento deste grupo de pacientes.

Revision about hearing loss in the Alport's syndrome, analyzing the clinical, genetic and bio-molecular aspects

Alport Syndrome is a genetic disorder characterized by hematuria, which often leads to renal failure. It may also be accompanied by extra-renal alterations, such as: sensorineural hearing loss, and ocular abnormalities. Dominant forms related to the X chromosome and caused by mutations in the locus COL4A5 have been described, as well as an autossomic recessive form resulting from mutations in the locus COL4A3 or COL4A4. An autossomic dominant type of AS has also been reported. The disease is caused by changes in the collagen type IV chains, where symptoms reflect the damage to the basal membrane of several organs. The α3.α4.α5(IV) networks are found in the kidneys, cochlea and eyes. The objective was to characterize AS in this group of patients. In the current literature review it was found that: 1. AS is characterized by hematuria that may develop into renal failure and can also be accompanied by extra-renal manifestations. Hearing loss is a frequent extra-renal finding and one of the first symptoms of AS, therefore representing a relevant factor in the prognosis of the renal disease; 2. It is a genetic disorder resulting from abnormalities in the chains of collagen type IV in the basal membranes; 3. The hearing loss in AS is typically sensorineural with variable intensities, progressive and symmetrical, affecting middle and high frequencies; 4. Otolaryngologists should include a urine test in the SNHL work-up. It is essential to have an otologist involved in the treatment of these patients.

Abnormal basement membrane in the inner ear and the kidney of the Mpv17-/- mouse strain: ultrastructural and immunohistochemical investigations

The loss of the function of the peroxisomal Mpv17-protein and associated imbalanced radical oxygen species (ROS) homeostasis leads to an early onset of focal segmental glomerulosclerosis and sensorineural deafness associated with severe degeneration of cochlear structures. An excessive enlargement of basal laminae of the stria vascularis capillaries and glomeruli indicates numerous changes in their molecular composition. The basement membrane (BM) of the glomeruli and the stria vascularis are simultaneously affected in early stages of the disease and the lamination, splitting of the membrane and formation of the "basket weaving" seen at the onset of the disease in the kidney are similar to the ultrastructural alterations characteristic for Alporta9s syndrome. The progressive alteration of the BMs is accompanied by irregularity in the distribution of the collagen IV subunits and by an accumulation of the laminin B2(gamma1) in the inner ear and B(beta1) in the kidney. Since Mpv17 protein contributes to ROS homeostasis, further studies are necessary to elucidate downstream signaling molecules activated by ROS. These studies explain the cellular responses to missing Mpv17-protein, such as accumulation of the extracellular matrix, degeneration, and apoptosis in the inner ear.

Matrix metalloproteinase dysregulation in the stria vascularis of mice with Alport syndrome: implications for capillary basement membrane pathology

Alport syndrome results from mutations in genes encoding collagen alpha3(IV), alpha4(IV), or alpha5(IV) and is characterized by progressive glomerular disease associated with a high-frequency sensorineural hearing loss. Earlier studies of a gene knockout mouse model for Alport syndrome noted thickening of strial capillary basement membranes in the cochlea, suggesting that the stria vascularis is the primary site of cochlear pathogenesis. Here we combine a novel cochlear microdissection technique with molecular analyses to illustrate significant quantitative alterations in strial expression of mRNAs encoding matrix metalloproteinases-2, -9, -12, and -14. Gelatin zymography of extracts from the stria vascularis confirmed these findings. Treatment of Alport mice with a small molecule inhibitor of these matrix metalloproteinases exacerbated strial capillary basement membrane thickening, demonstrating that alterations in basement membrane metabolism result in matrix accumulation in the strial capillary basement membranes. This is the first demonstration of true quantitative analysis of specific mRNAs for matrix metalloproteinases in a cochlear microcompartment. Further, these data suggest that the altered basement membrane composition in Alport stria influences the expression of genes involved in basement membrane metabolism.

Temporal Bone Histopathology in Alport Syndrome

OBJECTIVE

To determine the histopathologic abnormalities within the cochlea in Alport syndrome.

BACKGROUND

Alport syndrome, which manifests as hereditary nephritis and sensorineural hearing loss (SNHL), is caused by mutations in genes that code for the proportional, variant3, proportional, variant4, and proportional, variant5 chains of type IV collagen. The proportional, variant3, proportional, variant4, and proportional, variant5 chains of type IV collagen are present in the basement membrane of the organ of Corti. Previous temporal bone studies have failed to identify histopathologic correlates for the SNHL.

METHODS

We examined temporal bones from nine individuals with a clinical diagnosis of Alport syndrome. One of our cases also had genetic testing that showed a mutation in the type IV collagen proportional, variant5 chain gene.

RESULTS

By light microscopy, eight of nine cases demonstrated two unique pathologic changes: 1) a "zone of separation" between the basilar membrane and overlying cells of the organ of Corti and 2) presence of cells filling the tunnel of Corti and extracellular spaces of Nuel. The cytologic losses of hair cells, stria vascularis, and cochlear neuronal cells were insufficient to account for the observed SNHL in our cases. Electron microscopy was performed in four cases; all four demonstrated the following: 1) the zone of separation that was observed at light microscopy occurred between the basement membrane and the basilar membrane, 2) the cells within the tunnel of Corti and spaces of Nuel were morphologically similar to supporting cells, and 3) the basement membrane of strial capillaries and the spiral vessel (under the basilar membrane) were normal.

CONCLUSIONS

The histopathologic correlates of cochlear involvement in Alport syndrome are abnormalities of the basement membrane of cells of the organ of Corti and dysmorphogenesis (cellular infilling of the tunnel and extracellular spaces) of the organ of Corti. We hypothesize that these abnormalities result in SNHL by altering cochlear micromechanics.

A domain-specific usherin/collagen IV interaction may be required for stable integration into the basement membrane superstructure

Usherin is a basement membrane protein encoded by the gene associated with Usher syndrome type IIa, the most common deaf/blind disorder. This report demonstrates a specific interaction between type IV collagen and usherin in the basement membrane, with a 1:1 stoichiometry for binding. Genetic and biochemical approaches were used to explore the role of type IV collagen binding in usherin function. We demonstrate binding occurs between the LE domain of usherin and the 7S domain of type IV collagen. A purified fusion peptide comprising the first four LE modules was shown to compete with full-length recombinant usherin for type IV collagen binding. However, synonymous fusion peptides with single amino acid substitutions resulting from missense mutations that were known to cause Usher syndrome type IIa in humans, failed to compete. Only mutations in loop b of the LE domain abolished binding activity. Co-immunoprecipitation and western blot analysis of testicular basement membranes from the Alport mouse model show a 70% reduction in type IV collagen is associated with a similar reduction in usherin, suggesting the usherin/collagen (IV) interaction stabilizes usherin in the basement membrane. Thus, the domain-specific interaction between usherin and type IV collagen appears essential to usherin stability in vivo, and loss of this interaction may result in Usher pathology in humans.