Early Wnt Signaling Activation Promotes Inner Ear Differentiation via Cell Caudalization in Mouse Stem Cell-Derived Organoids

Harmony in the Molecular Orchestra of Hearing: Developmental Mechanisms from the Ear to the Brain

Auditory processing in mammals begins in the peripheral inner ear and extends to the auditory cortex. Sound is transduced from mechanical stimuli into electrochemical signals of hair cells, which relay auditory information via the primary auditory neurons to cochlear nuclei. Information is subsequently processed in the superior olivary complex, lateral lemniscus, and inferior colliculus and projects to the auditory cortex via the medial geniculate body in the thalamus. Recent advances have provided valuable insights into the development and functioning of auditory structures, complementing our understanding of the physiological mechanisms underlying auditory processing. This comprehensive review explores the genetic mechanisms required for auditory system development from the peripheral cochlea to the auditory cortex. We highlight transcription factors and other genes with key recurring and interacting roles in guiding auditory system development and organization. Understanding these gene regulatory networks holds promise for developing novel therapeutic strategies for hearing disorders, benefiting millions globally.

Advance and Application of Single-cell Transcriptomics in Auditory Research

Hearing loss and deafness, as a worldwide disability disease, have been troubling human beings. However, the auditory organ of the inner ear is highly heterogeneous and has a very limited number of cells, which are largely uncharacterized in depth. Recently, with the development and utilization of single-cell RNA sequencing (scRNA-seq), researchers have been able to unveil the complex and sophisticated biological mechanisms of various types of cells in the auditory organ at the single-cell level and address the challenges of cellular heterogeneity that are not resolved through by conventional bulk RNA sequencing (bulk RNA-seq). Herein, we reviewed the application of scRNA-seq technology in auditory research, with the aim of providing a reference for the development of auditory organs, the pathogenesis of hearing loss, and regenerative therapy. Prospects about spatial transcriptomic scRNA-seq, single-cell based genome, and Live-seq technology will also be discussed.

Mapping the developmental potential of mouse inner ear organoids at single-cell resolution

Inner ear organoids recapitulate development and are intended to generate cell types of the otic lineage for applications such as basic science research and cell replacement strategies. Here, we use single-cell sequencing to study the cellular heterogeneity of late-stage mouse inner ear organoid sensory epithelia, which we validated by comparison with datasets of the mouse cochlea and vestibular epithelia. We resolved supporting cell sub-types, cochlear-like hair cells, and vestibular type I and type II-like hair cells. While cochlear-like hair cells aligned best with an outer hair cell trajectory, vestibular-like hair cells followed developmental trajectories similar to in vivo programs branching into type II and then type I extrastriolar hair cells. These results highlight the transcriptional accuracy of the organoid developmental program but will also inform future strategies to improve synaptic connectivity and regional specification.

[Organoids-the key to novel therapies for the inner ear? German version]

The sensitivity and the complexity of the human inner ear in conjunction with the lack of regenerative capacity are the main reasons for hearing loss and tinnitus. Progress in the development of protective and regenerative therapies for the inner ear often failed in the past not least due to the fact that no suitable model systems for cell biological and pharmacological in vitro studies were available. A novel technology for creating "mini-organs", so-called organoids, could solve this problem and has now also reached inner ear research. It makes it possible to produce inner ear organoids from cochlear stem/progenitor cells, embryonic and induced pluripotent stem cells that mimic the structural characteristics and functional properties of the natural inner ear. This review focuses on the biological basis of these inner ear organoids, the current state of research and the promising prospects that are now opening up for basic and translational inner ear research.

Genetic and pharmacologic alterations of claudin9 levels suffice to induce functional and mature inner hair cells

Hearing loss is the most common form of sensory deficit. It occurs predominantly due to hair cell (HC) loss. Mammalian HCs are terminally differentiated by birth, making HC loss incurable. Here, we show the pharmacogenetic downregulation of Cldn9, a tight junction protein, generates robust supernumerary inner HCs (IHCs) in mice. The putative ectopic IHCs have functional and synaptic features akin to typical IHCs and were surprisingly and remarkably preserved for at least fifteen months >50% of the mouse's life cycle. In vivo, Cldn9 knockdown using shRNA on postnatal days (P) P1-7 yielded analogous functional putative ectopic IHCs that were equally durably conserved. The findings suggest that Cldn9 levels coordinate embryonic and postnatal HC differentiation, making it a viable target for altering IHC development pre- and post-terminal differentiation.