Building inner ears: recent advances and future challenges for in vitro organoid systems

Wouter H Van Der Valk,Matthew R Steinhart,Karl R Koehler,Jingyuan Zhang

doi:10.1038/s41418-020-00678-8

Abstract

While inner ear disorders are common, our ability to intervene and recover their sensory function is limited. In vitro models of the inner ear, like the organoid system, could aid in identifying new regenerative drugs and gene therapies. Here, we provide a perspective on the status of in vitro inner ear models and guidance on how to improve their applicability in translational research. We highlight the generation of inner ear cell types from pluripotent stem cells as a particularly promising focus of research. Several exciting recent studies have shown how the developmental signaling cues of embryonic and fetal development can be mimicked to differentiate stem cells into “inner ear organoids” containing otic progenitor cells, hair cells, and neurons. However, current differentiation protocols and our knowledge of embryonic and fetal inner ear development in general, have a bias toward the sensory epithelia of the inner ear. We propose that a more holistic view is needed to better model the inner ear in vitro. Moving forward, attention should be made to the broader diversity of neuroglial and mesenchymal cell types of the inner ear, and how they interact in space or time during development. With improved control of epithelial, neuroglial, and mesenchymal cell fate specification, inner ear organoids would have the ability to truly recapitulate neurosensory function and dysfunction. We conclude by discussing how single-cell atlases of the developing inner ear and technical innovations will be critical tools to advance inner ear organoid platforms for future pre-clinical applications.

Highlights

Over 6% of people worldwide suffer from hearing loss [1] and likewise 6% suffer from balance disorders [2]
We have shown that multiple otic placodes and otocyst-like structures can be generated in vitro from a three-dimensional (3D) human pluripotent stem cells (hPSCs) aggregate by modulating transforming growth factor-beta (TGF), bone morphogenetic proteins (BMPs), fibroblast growth factors (FGFs), and WNT signaling and extracellular matrix-related mechanical interactions (Fig. 1a) [21]
Major breakthroughs have been made in inner ear modeling using hPSCs, yet current models are limited in their applications for translational research

Summary

Introduction

Over 6% of people worldwide suffer from hearing loss [1] and likewise 6% suffer from balance disorders [2]. In addition to otic placode derivedcells, lineage tracing experiments in chickens and mice have shown that a limited number of neural crest cells contribute to the otocyst epithelium; it remains unclear what contribution these cells have to non-sensory and sensory epithelia in the inner ear later in development [38, 39] Based on these developmental biology studies, a consensus model of otic induction from PSCs has emerged in recent years (Fig. 1a). Our published data suggest that inner ear organoid epithelia co-develop with a neural crestderived mesenchyme that produces fibrocyte-like cells and cartilage, similar to the POM [21] (Fig. 1a) It is not known whether authentic POM—expressing the transcription factors mentioned above—arise in these cultures or whether the mesoderm-derived components of the POM are present. A parallel-induction and seeding approach of immune cells could be employed to incorporate macrophages into inner ear organoids and set the stage for inflammation studies [125]

Conclusions and future directions

Findings

Compliance with ethical standards