Research in the laboratory is focused on basic developmental mechanisms guiding cellular polarization, morphogenesis, and axon guidance within the cochlea and vestibular maculae.
We anticipate that healing the damaged ear will include a reiteration of those developmental events that first built it. Therefore, understanding those processes is an important prerequisite for the success of hearing and balance therapies.
vestibular system of the inner ear, motion is detected via the mechanical
deflection of a bundle of stereocilia located at the top of sensory receptor
hair cells. The bundle is anatomically and
functionally polarized because only deflections towards a lone kinocilium
positioned at one side of the hair cell surface produce excitatory responses to
acceleration or gravity. Thus, the range
of motion that can be detected by any individual hair cell is determined by the
polarized orientation of its stereociliary bundle.
In order to generate a complete sensory representation of motion in space, vestibular hair cells of the utricle and saccule are found in arrays spanning the full 360° range of bundle orientations. This is achieved in part through Planar Bipolarity. Thus, while the stereociliary bundles of neighboring hair cells are similarly oriented, the hair cells are also divided between two groups with oppositely oriented bundles that respond to motion in opposite directions. These groups are separated by a single intercellular junction often referred to as the Line of Polarity Reversal (LPR).
Our goal is to identify the cellular and molecular mechanisms that direct the development of Planar Bipolarity through formation of the LPR, and thereby establish this sensory representation of gravity and acceleration. We are pursuing this using a combination of knockout and transgenic mouse models.
cochlea is innervated by spiral ganglion neurons, which relay sound information
from sensory receptor hair cells to central auditory targets. A common
pathology in deafness models is the loss of synapses between the hair cells and
their spiral ganglion neurons. Thus, any
successful hearing restoration therapy will include reinnervation of the
damaged cochlea, a process which is likely to mimic the innervation that occurs
during development. As a result, understanding early developmental events is an
important and essential prerequisite for many therapeutic strategies.
A subset of spiral ganglion neurons has nociceptive characteristics and are thus equipped to detect acoustic trauma, which may be important for preserving function. These are the type II spiral ganglion neurons, which constitute a minority of cochlear afferents but innervate all three rows of outer hair cells. The development of type II neurons is unique because their peripheral axons project beyond the inner hair cells and subsequently make a distinct 90° turn towards the cochlear base to synapse with multiple outer hair cells. While many aspects of outer hair cell innervation are unknown, we have found that planar cell polarity (PCP) signaling is required for the 90° turn that directs the peripheral axon towards the cochlear base.
The goal of this research is to understand how a cell polarity signaling pathway influences growth cone behavior. We have found that the PCP proteins are not required in the growth cone itself but rather in the environment that the growth cone is navigating. An exciting possibility is that PCP signals emanating from organ of Corti supporting cells provides guidance cues. This hypothesis is being directly tested using a combination of knockout and transgenic mouse models.
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In this experiment Evan Ratzan demonstrated that the identity of Type I vestibular hair cells is determined shortly after mitosis in the developing mouse utricle. He did this by simultaneously labeling mitotic cells with EdU while genetically labeling the differentiating Type I hair cells in FGF8 mcm transgenic mice (panels C-E). This was remarkable because prior to this finding, it was thought that vestibular hair cell types could only be distinguished from each other after birth.
See Ratzan, Moon, Deans, Development, 2020
Using a detailed series of conditional knockout mice, Satish Ghimire demonstrated that the Core PCP proteins function non-cell autonomously from supporting cells of the cochlear duct to guide axon turning. In this example, removing Frizzled3 and Frizzled6 from the organ of Corti results in the random turning of Type II Spiral Ganglion Neuron peripheral axons (panels F-H). This is significant because during axon guidance in other systems, the PCP proteins function within the growth cone itself.
SeeGhimire and Deans, J. Neuroscience, 2019 and Ghimire, Ratzan, and Deans, Development, 2018
Michelle Stoller was able to show domineering non-autonomy using Cre recombinase to generate ‘clonal’ boundaries between wild type and mutants cells in the mouse utricle (panels A,B). This was significant because it demonstrated that the supporting cells contributed to PCP signaling and coordinated the orientation of neighboring hair cells. Not only was this our favorite figure from her postdoc, it provided inspiration for the journal cover.
See Stoller et al., Developmental Biology, 2018
Using an image captured by Satish Ghimire we provided an
exciting cover for Developmental Biology. Our review in this edition highlights
Satish’s dissertation work and his contributions to our understanding of
See Deans MR. Dev Biol. March 2022
A long effort initiated by Evan Ratzan culminated in this exciting collaboration with Basile Tarchini from Jackson Labs. Together we demonstrated that STK32A (a kinase with no known prior function) acts within the inner ear to regulate GPR156 distribution in hair cells and thereby position the Line of Polarity Reversal in the developing utricle and saccule. See Jia and Ratzan et al. 2023