Stria vascularis morphogenesis in vitro

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.

Transgenic Tg(Kcnj10-ZsGreen) fluorescent reporter mice allow visualization of intermediate cells in the stria vascularis

The stria vascularis (SV) is a stratified epithelium in the lateral wall of the mammalian cochlea, responsible for both endolymphatic ion homeostasis and generation of the endocochlear potential (EP) critical for normal hearing. The SV has three layers consisting predominantly of basal, intermediate, and marginal cells. Intermediate and marginal cells form an intricate interdigitated network of cell projections making discrimination of the cells challenging. To enable intermediate cell visualization, we engineered by BAC transgenesis, reporter mouse lines expressing ZsGreen fluorescent protein under the control of Kcnj10 promoter and regulatory sequences. Kcnj10 encodes KCNJ10 protein (also known as Kir4.1 or Kir1.2), an ATP-sensitive inwardly-rectifying potassium channel critical to EP generation, highly expressed in SV intermediate cells. In these transgenic mice, ZsGreen fluorescence mimics Kcnj10 endogenous expression in the cochlea and was detected in the intermediate cells of the SV, in the inner phalangeal cells, Hensen's, Deiters' and pillar cells, in a subset of spiral ganglion neurons, and in glial cells. We show that expression of the transgene in hemizygous mice does not alter auditory function, nor EP. These transgenic Tg(Kcnj10-ZsGreen) mice allow live and fixed tissue visualization of ZsGreen-expressing intermediate cells and will facilitate future studies of stria vascularis cell function.

Transgenic Tg(Kcnj10-ZsGreen) Fluorescent Reporter Mice Allow Visualization of Intermediate Cells in the Stria Vascularis

The stria vascularis (SV) is a stratified epithelium in the lateral wall of the mammalian cochlea, responsible for both endolymphatic ion homeostasis and generation of the endocochlear potential (EP) critical for normal hearing. The SV has three layers consisting predominantly of basal, intermediate, and marginal cells. Intermediate and marginal cells form an intricate interdigitated network of cell projections making discrimination of the cells challenging. To enable intermediate cell visualization, we engineered by BAC transgenesis, reporter mouse lines expressing ZsGreen fluorescent protein under the control of Kcnj10 promoter and regulatory sequences. Kcnj10 encodes KCNJ10 protein (also known as Kir4.1 or Kir1.2), an ATP-sensitive inwardly-rectifying potassium channel critical to EP generation, highly expressed in SV intermediate cells. In these transgenic mice, ZsGreen fluorescence mimics Kcnj10 endogenous expression in the cochlea and was detected in the intermediate cells of the SV, in the inner phalangeal cells, Hensen's, Deiters' and pillar cells, in a subset of spiral ganglion neurons, and in glial cells. We show that expression of the transgene in hemizygous mice does not alter auditory function, nor EP These transgenic Tg(Kcnj10-ZsGreen) mice allow live and fixed tissue visualization of ZsGreen-expressing intermediate cells and will facilitate future studies of stria vascularis cell function.

Expression of apoptosis-related genes in the organ of Corti, modiolus and stria vascularis of newborn rats

Cell death in the inner ear tissues is an important mechanism leading to hearing impairment. Here, using microarrays and real-time RT-PCR we analyzed expression of selected apoptosis-related genes in rat's inner ear. We determined the gene expression in tissues freshly isolated from neonatal rats (3-5 days old) and compared it to that of explants cultured for 24 h under normoxic or hypoxic conditions. For the analyses, we used pooled samples of the organ of Corti (OC), modiolus (MOD) and stria vascularis (SV), respectively. We observed region-specific changes in gene expression between the fresh tissues and the normoxic culture. In the OC, expression of the proapoptotic genes caspase-2, caspase-3, caspase-6 and calpain-1 was downregulated. In the MOD, the antioxidative defense SOD-2 and SOD-3 were upregulated. In the SV, caspase-2, caspase-6, calpain-1 and SOD-3 were downregulated and SOD-2 upregulated. We speculate that these changes could reflect survival shift in transcriptome of inner ear explants tissues under in vitro conditions. With the exception of SOD-2, hypoxic culture conditions induced the same changes in gene expression as the normoxic conditions indicating that culture preparation is likely the dominating factor, which modifies the gene expression pattern. We conclude that various culture conditions induce different expression pattern of apoptosis-related genes in the organotypic cochlear cultures, as compared to fresh tissues. This transcriptional pattern may reflect the survival ability of specific tissues and could become a tempting target for a pharmacological intervention in inner ear diseases.

Malformation of stria vascularis in the developing inner ear of the German waltzing guinea pig

Auditory function and cochlear morphology have previously been described in the postnatal German waltzing guinea pig, a strain with recessive deafness. In the present study, cochlear histopathology was further investigated in the inner ear of the developing German waltzing guinea pig (gw/gw). The lumen of the cochlear duct diminished progressively from embryonic day (E) 35 to E45 and was absent at E50 because of the complete collapse of Reissner's membrane onto the hearing organ. The embryonic stria vascularis, consisting of a simple epithelium, failed to transform into the complex trilaminar tissue seen in normal animals and displayed signs of degeneration. Subsequent degeneration of the sensory epithelium was observed from E50 and onwards. Defective and insufficient numbers of melanocytes were observed in the developing gw/gw stria vascularis. A gene involved in cochlear melanocyte development, Pax3, was markedly reduced in lateral wall tissue of the cochlea of both E40 and adult gw/gw individuals, whereas its expression was normal in the skin and diaphragm muscle of adult gw/gw animals. The Pax3 gene may thus be involved in the pathological process but is unlikely to be the primary mutated gene in the German waltzing guinea pig. TUNEL assay showed no signs of apoptotic cell death in the developing stria vascularis of this type of guinea pig. Thus, malformation of the stria vascularis appears to be the primary defect in the inner ear of the German waltzing guinea pig. Defective and insufficient numbers of melanocytes might migrate to the developing stria vascularis but fail to provide the proper support for the subsequent development of marginal and basal cells, thereby leading to stria vascularis malformation and dysfunction in the inner ear of the German waltzing guinea pig.

The development of the stria vascularis in the human foetus

The development of the stria vascularis in the human cochlea was studied in step sections of 81 human foetal temporal bones. The stria vascularis primordium can be identified as a ridge of epithelial cells on the lateral wall of the cochlear duct. The first signs of differentiation appear at the 11th week, but it is not until the 17th-18th week that the typical trilaminar structure is observed. The appearance of similar cells with notched nuclei in both marginal and mesenchymal layers at this stage suggests the possibility that some of the intermediate cells may be of epithelial origin. By the 21st week, the overall appearance resembles that of the adult structure. This occurs 1 week after the opening of the tunnel of Corti, and possibly marks the onset of cochlear function.

Differentiation of cyst-forming stria vascularis tissues in vitro

The marginal cells of the stria vascularis possess distinctive morphological characteristics associated with their role in endolymph production. Interestingly, when stria-derived epithelial cells are grown in association with the underlying mesenchyme, the final differentiation of these cell types does not occur. Beyond the rudimentary polarity that is established, similar to that shown in epithelial monolayers, cells in culture bear only a slight resemblance to their marginal cell counterparts in vivo. The ultrastructural features that typify these epithelia, extensive cytoplasmic invaginations, with an abundance of mitochondria, and darkly stained cytoplasm, are not evident under standard culture conditions. In order to determine whether fluid transport, a key function of the stria vascularis, has an effect on the ultrastructural morphology, we examined de novo stria vascularis tissues that formed a fluid-filled cyst in vitro. We found that only cells associated with the luminal structure demonstrated dark cytoplasmic staining and amplification of the basolateral membrane of the marginal cells. Additionally, other epithelial features, such as mitochondria-rich and microvilli-rich cells, were observed in cyst-forming tissues. The enhancement of the marginal cell specializations was not as robust as that observed in vivo; however, they were clearly more extensive when compared to cells in the same culture that were not associated with a fluid-filled lumen. Thus it appears that fluid transport may be necessary to maximize differentiation of stria vascularis tissues in vitro.