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The inner ear is derived from the otic placode, one of the numerous cranial sensory placodes that emerges from the pre-placodal ectoderm (PPE) along its anterior-posterior axis. However, the molecular dynamics underlying how the PPE is regionalized are poorly resolved. We used stem cell-derived organoids to investigate the effects of Wnt signaling on early PPE differentiation and found that modulating Wnt signaling significantly increased inner ear organoid induction efficiency and reproducibility. Alongside single-cell RNA sequencing, our data reveal that the canonical Wnt signaling pathway leads to PPE regionalization and, more specifically, medium Wnt levels during the early stage induce (1) expansion of the caudal neural plate border (NPB), which serves as a precursor for the posterior PPE, and (2) a caudal microenvironment that is required for otic specification. Our data further demonstrate Wnt-mediated induction of rostral and caudal cells in organoids and more broadly suggest that Wnt signaling is critical for anterior-posterior patterning in the PPE.

Hearing loss is one of the most common sensory disorders. TMEM43 is expressed in cochlear glia-like supporting cells (GLSs) and is known to be associated with late-onset auditory neuropathy spectrum disorder (ANSD) and progressive hearing loss. Here, we describe the derivation of an induced pluripotent stem cell (iPSC) line from a patient lymphoblastoid cell line (LCL) carrying a single heterozygous nonsense variant (p.Arg372Ter (c.1114C T)) in TMEM43 that leads to a truncated protein lacking the 4th transmembrane domain. This cell line can serve as a tool for disease modelling and development of therapeutic approaches to restore inner ear function.

Summary Mutations in the gene encoding the type II transmembrane protease 3 (TMPRSS3) cause human hearing loss, although the underlying mechanisms that result in TMPRSS3-related hearing loss are still unclear. We combined the use of stem cell-derived inner ear organoids with single-cell RNA sequencing to investigate the role of TMPRSS3. Defective Tmprss3 leads to hair cell apoptosis without altering the development of hair cells and the formation of the mechanotransduction apparatus. Prior to degeneration, Tmprss3-KO hair cells demonstrate reduced numbers of BK channels and lower expressions of genes encoding calcium ion-binding proteins, suggesting a disruption in intracellular homeostasis. A proteolytically active TMPRSS3 was detected on cell membranes in addition to ER of cells in inner ear organoids. Our in vitro model recapitulated salient features of genetically associated inner ear abnormalities and will serve as a powerful tool for studying inner ear disorders., Graphical Abstract, Highlights • Inner ear organoids recapitulated in vivo genetic-associated hair cell degeneration • Tmprss3 mutation leads to apoptosis in hair cells without altering the development • Tmprss3-KO results in reduced BK channels and alters gene expressions in hair cells • TMPRSS3 localized on the cell membrane, The role of TMPRSS3 in auditory hair cell (HC) remains unclear. Nelson and colleagues generated stem cell-derived inner ear organoids with Tmprss3 mutations, which demonstrate that (1) inner ear organoids exhibit comparative effects of the genetic abnormality to its in vivo counterparts, (2) Tmprss3 mutations lead to HC apoptosis and reduced BK channels, and (3) TMPRSS3 localizes to the cell membrane.

The inner ear is derived from the otic placode, one of numerous cranial sensory placodes that emerges from the pre-placodal ectoderm (PPE). However, the molecular dynamics underlying how the PPE is induced and regionalized are poorly resolved. We used stem cell-derived inner ear organoids to investigate the effects of Wnt signaling on otic placode development and found that modulating Wnt signaling to give intermediate activation of its downstream pathway significantly increased inner ear organoid induction efficiency. Single cell RNA-sequencing revealed that the Wnt modulation induces 1) gene signatures of the posterior PPE (pPPE), 2) expansion of the caudal neural plate border (NPB), and 3) a suitable caudal head microenvironment. Our data also suggest that precursors of the pPPE committed to caudal fate during the NPB stage. Taken together, this study improves our understanding of the role of Wnt signaling in the early development of the inner ear.

Sensorineural hearing loss and vestibular dysfunction are caused by damage to neurons and mechanosensitive hair cells, which do not regenerate to any clinically relevant extent in humans. Several protocols have been devised to direct pluripotent stem cells (PSCs) into inner ear hair cells and neurons, which display many properties of their native counterparts. The efficiency, reproducibility, and scalability of these protocols are enhanced by incorporating knowledge of inner ear development. Modeling human diseases in vitro through genetic manipulation of PSCs is already feasible, thereby permitting the elucidation of mechanistic understandings of a wide array of disease etiologies. Early studies on transplantation of PSC-derived otic progenitors have been successful in certain animal models, yet restoration of function and long-term cell survival remain unrealized. Through further research, PSC-based approaches will continue to revolutionize our understanding of inner ear biology and contribute to the development of therapeutic treatments for inner ear disorders., Tang and colleagues reviewed the use of pluripotent stem cells (PSCs) in the treatment and study of inner ear disorders. Precisely timed small-molecule treatments of PSCs generate inner ear hair cells, and support cells and neurons. Stem cell-based transplantation studies have shown successful engraftment in the inner ear and disease mechanisms can be studied in vitro using PSC-derived inner ear tissues.

Background and Hypothesis: The carefully timed treatment of mouse embryonic stem cell (mESC) cultures with small molecules results in mESC differentiation into 3D organoids containing all components of the inner ear such as hair cells (HCs), support cells and neurons. Loss of TMPRSS3 function, which is a transmembrane extracellular protease, has been previously shown to lead to rapid HC degeneration between culture days 36 (D36) and D38 in 3D organoids. Mosaic organoids would allow for analysis of developmental dynamics, cell-cell interactions and even therapeutic rescue efficiency. We hypothesized that we could develop an inner ear mosaic organoid containing cells with and without TMPRSS3 and that the Tmprsss3KO mosaic organoids would have greater hair cell survival than Tmprss3KO-only organoids due to the compensatory effect of intact TMPRSS3 on control hair cells within the same vesicle. Experimental Design or Project Methods: Two mESC cell lines (R1E background) were previously generated using CRISPR-Cas9n. Control reporter line (tdTomato) expresses a tdTomato reporter gene in all cells under the CAG promoter at the ROSA26 locus. Tmprss3 knockout line (Tmprss3KO) contains a 2A-nGFP cassette with a premature stop codon into exon 2 of the Tmprss3 gene. We generated Tmprss3KO mosaic (Tmprss3KO:tdTomato) and control mosaic (R1E:tdTomato) organoids. The aggregates were analyzed on days 25, 33 and 38 after being fixed, sectioned and stained for tdTomato, SOX2, MYO7A, cleaved CASPASE3, and BK channels using immunohistochemistry. Results: We have successfully used mESCs to generate Tmprss3KO mosaic inner ear organoids with similar efficiency to that of control mosaic organoids. Preliminary data suggests that the hair cell survival rates were similar across all vesicle types in the Tmprss3KO mosaic organoids. Additionally, the Tmprss3KO mosaic organoids had a significantly decreased overall BK channel expression by D38. Conclusion and Potential Impact: The successful generation of mosaic organoids achieved here under both conditions sets the stage for future studies of intercellular interactions and therapeutics in this domain. More replicates are necessary to make a definitive conclusion about the effectiveness of control cells on Tmprss3KO rescue.

Sea anemones have an extraordinary capability to repair damaged hair bundles, even after severe trauma. A group of secreted proteins, named repair proteins (RPs), found in mucus covering sea anemones significantly assists the repair of damaged hair bundle mechanoreceptors both in the sea anemone Haliplanella luciae and the blind cavefish Astyanax hubbsi. The polypeptide constituents of RPs must be identified in order to gain insight into the molecular mechanisms by which repair of hair bundles is accomplished. In this study, several polypeptides of RPs were isolated from mucus using blue native PAGE and then sequenced using LC-MS/MS. Thirty-seven known polypeptides were identified, including Hsp70s, as well as many polypeptide subunits of the 20S proteasome. Other identified polypeptides included those involved in cellular stress responses, protein folding, and protein degradation. Specific inhibitors of Hsp70s and the 20S proteasome were employed in experiments to test their involvement in hair bundle repair. The results of those experiments suggested that repair requires biologically active Hsp70s and 20S proteasomes. A model is proposed that considers the function of extracellular Hsp70s and 20S proteasomes in the repair of damaged hair cells.

Mammalian hair cells possess only a limited ability to repair damage after trauma. In contrast, sea anemones show a marked capability to repair damaged hair bundles by means of secreted repair proteins (RPs). Previously, it was found that recovery of traumatized hair cells in blind cavefish was enhanced by anemone-derived RPs; therefore, the ability of anemone RPs to assist recovery of damaged hair cells in mammals was tested here. After a 1 h incubation in RP-enriched culture media, uptake of FM1-43 by experimentally traumatized murine cochlear hair cells was restored to levels comparable to those exhibited by healthy controls. In addition, RP-treated explants had significantly more normally structured hair bundles than time-matched traumatized control explants. Collectively, these results indicate that anemone-derived RPs assist in restoring normal function and structure of experimentally traumatized hair cells of the mouse cochlea.

Cadherin 23 (CDH23), a component of tip links in hair cells of vertebrate animals, is essential to mechanotransduction by hair cells in the inner ear. A homolog of CDH23 occurs in hair bundles of sea anemones. Anemone hair bundles are located on the tentacles where they detect the swimming movements of nearby prey. The anemone CDH23 is predicted to be a large polypeptide featuring a short exoplasmic C-terminal domain that is unique to sea anemones. Experimentally masking this domain with antibodies or mimicking this domain with free peptide rapidly disrupts mechanotransduction and morphology of anemone hair bundles. The loss of normal morphology is accompanied, or followed by a decrease in F-actin in stereocilia of the hair bundles. These effects were observed at very low concentrations of the reagents, 0.1–10 nM, and within minutes of exposure. The results presented herein suggest that: (1) the interaction between CDH23 and molecular partners on stereocilia of hair bundles is dynamic and; (2) the interaction is crucial for normal mechanotransduction and morphology of hair bundles.