Trans-differentiation of outer hair cells into inner hair cells in the absence of INSM1

Abstract

The mammalian cochlea contains two types of mechanosensory hair cells (HCs) that play different and critical roles in hearing. Inner hair cells (IHCs), with an elaborate presynaptic apparatus, signal to cochlear neurons and communicate sound information to the brain. Outer hair cells (OHCs) mechanically amplify sound-induced vibrations, enabling enhanced sensitivity to sound and sharp tuning. Cochlear HCs are solely generated during development and their death, most often of OHCs, is the main cause of deafness. OHCs and IHCs, together with supporting cells, originate embryonically from the prosensory region of the otocyst, but how HCs differentiate into two different types is unknown 1 – 3 . Here we show that Insm1 , which encodes a zinc finger protein transiently expressed in nascent OHCs, consolidates their fate by preventing trans-differentiation into IHCs. In the absence of INSM1 many HCs born embryonically as OHCs switch fates to become mature IHCs. In order to identify the genetic mechanisms by which Insm1 operates, we compared transcriptomes of immature IHCs vs OHCs, as well as OHCs with and without INSM1. OHCs lacking INSM1 upregulate a set of genes, most of which are normally preferentially expressed by IHCs. The homeotic cell transformation of OHCs without INSM1 into IHCs reveals for the first time a mechanism by which these neighboring mechanosensory cells begin to differ: INSM1 represses a core set of early IHC-enriched genes in embryonic OHCs and makes them unresponsive to an IHC-inducing gradient, so that they proceed to mature as OHCs. Without INSM1, some of the OHCs upregulating these few IHC-enriched transcripts trans-differentiate into IHCs, revealing the first candidate genes for IHC-specific differentiation.

Pou4f1 Defines a Subgroup of Type I Spiral Ganglion Neurons and Is Necessary for Normal Inner Hair Cell Presynaptic Ca2+ Signaling

“…In contrast, no puncta were observed in hair cells or supporting cells at either time point. Next, we used the gEAR portal to query four recently published RNAseq datasets (Cai et al, 2015;Elkon et al, 2015;Li et al, 2018;Wiwatpanit et al, 2018) in which hair cells had been isolated at P0 or from adults using fluorescence activated cell sorting (FACS) or visual collection. For comparison, the expression levels of three known hair-cell-specific genes (Pou4f3, Gfi1, Myo7a) were compared with Pou4f1.…”

β-Catenin is required for radial cell patterning and identity in the developing mouse cochlea

“…Last, we performed multiplex in situ hybridization and found that ectopic Fgf8 + IHCs lacked expression of Insm1 (Fig. 3D), which is required for driving an OHC identity (25). Together, these results suggest ectopic formation of IHCs at the expense of OHCs in the lateral compartment before E18.5.…”

The acquisition of positional information across the radial axis of the cochlea

“…The gene is expressed only in nascent OHCs and in its absence, the OHCs transdifferentiate into IHCs. Using Ism1 to sort OHC from IHC populations, the authors identified several additional genes that were specific to IHCs vs OHCs . To differentiate IHCs from OHCs, we note the presence of Insm1 in OHCs in the GRN.…”