Morphogens are molecules that operate as gradients in developing embryos to endow cells with different fates depending upon the duration and/or concentration of morphogen exposure. Several secreted ligands known to function as morphogens in other systems are present in the developing inner ears of birds and mammals, including molecules in the Wnt gene family. Wnt transcripts are asymmetrically distributed across the radial axis of the embryonic cochlea. Current research reveals that disruption of this localized expression in vivo by ectopic gene transfer can transform cell fates in the developing cochlea of the chicken embryo. This emerging evidence that a Wnt morphogen may be active in the cochlea leads to 3 lines of inquiry.
In Aim 1, deep-sequencing of transcripts will be undertaken to compare control ears to those in which the abneural-side of the hearing organ has been converted to a neural-side fate by ectopic Wnt delivery. This is expected to reveal genes that are differentially employed on the two sides of the sensory organ, some of which will have the ability to directly influence aspects of radial identity, such as the temporal regulation f cell division, the choice of cell types, and the distribution and class of axonal innervation. Accumulating new evidence suggests that microRNAs, known for their ability to transcriptionally repress target transcripts, can form interacting networks with molecules involved in the Wnt signaling pathway in cancer cells and in embryos.
In Aim 2, changes in the microRNA profile will be linked to the transformation of cell types across the radial axis of the chicken hearing organ that are induced by ectopic Wnt delivery. This should provide specific candidate microRNAs that interact with the Wnt pathway to form either feedback or feedforward networks and thereby influence cellular phenotypes across the radial axis of the hearing organ.
Aim 3 will switch to studying the developing mouse cochlea to identify which Wnt signaling molecules are distributed in a manner consistent with a morphogen function in this species. Then, available mouse mutants that block the secretion of all Wnt ligands will provide a valuable resource to reveal whether specific Wnt ligands form sub-networks with specific microRNAs. To accomplish this, embryonic cochleas lacking Wnt secretion will be cultured in the presence of different Wnt ligands and the downstream microRNAs affected by these treatments will be compared. The overall significance of these studies is that they may offer a better understanding of the gene networks that contribute to forming and patterning cochlear cell types, with a long-term goal of informing the design and implementation of biological-based treatments of hearing loss in human patients.
Our sense of hearing depends upon the exquisite patterning of cells in the cochlea of the inner ear. Two types of hair cells with very different functional connections to the brain are separated across the radial dimension of the hearing organ. We will study the molecular mechanisms directing embryonic cochlear cells to adopt different fates in animal models, with the hope that this information will inform the design of future biologically-based therapeutic treatments of hearing loss in human patients.
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