The Gene Expression Lab, under the direction of
Dominic Cosgrove, Ph.D., has two main areas of interest:
Both of the above syndromes are currently untreatable hereditary diseases and have the common feature of sensorineural deafness of hearing loss. The shared feature to our approach is to understand the cellular dysfunctions resulting from mutations in the genes that cause the syndromes. This involves the use of modern molecular biology and biochemistry methodologies to probe the normal function of the proteins encoded by these genes, and the use of animal model and cell culture systems to probe the molecular mechanism of disease onset and progression that occurs when the proteins are dysfunctional or absent. Since both diseases have delayed onset and progressive characteristics to their respective pathologies, there is a window of opportunity for targeted therapeutic intervention aiming to delay onset and/or slow the progression of the disease.
Alport syndrome is caused by mutations in any of three type IV collagen genes(COL4A3, COL4A4, or COL4A5). The type IV collagens are found in some (but not all) basement membranes. Debilitating mutations in any of these genes results in the absence of all three proteins in the basement membranes where they are found due to an obligatory association to form heterotrimeric protomers. The disease is characterized by delayed onset progressive glomerulonephritis associated with progressive hearing loss, retinal flecks, and a weak association with aortic aneurisms. We developed a mouse model for the disease here at Boys Town that is freely available to other investigators and used world-wide to study the disease. We have used this model to determine key events that drive the progression of the glomerular disease, which includes the dysregulation of both cytokines and protease systems. We identified the key defect in the inner ear of the model to be thickening of the strial capillary basement membranes, which is again associated with disruption of both cytokine and protease systems.
More recently we have defined a key mechanism underlying the glomerular disease initiation. This mechanism involves biomechanical strain-mediated induction of a cytokine called endothelin-1, which activates receptors on the mesangial compartment of the glomerulus and induces changes in the cytoskeletal dynamics resulting in the formation of invasive filopodia. The filopodia deposit mesangial matrix proteins in the glomerular basement membrane (GBM) which activate expression of the cytokines and protease systems known to impact the progression of the disease. When you block the endothelin A receptor using small molecules, you attenuate the disease onset and progression of the glomerular disease and prevent the thickening of the strial capillary basement membranes. Whether this results in the restoration of normal hearing and strial function is currently under investigation. The pharmacologic endothelin A receptor blocking agents are toxic, and thus unlikely to be used in children. We are currently developing biologics that should overcome the problems with toxicity.
Usher Syndrome is a genetically heterogeneous disorder characterized by congenital deafness associated with delayed onset and progressive retinitis pigmentosa. There are 11 genes encoding a much larger number of protein variants including a non-conventional myosin, scaffold proteins, a g-protein coupled receptor, a calcium integrin binding protein, and cell adhesion proteins. When mutated, cochlear hair cells develop abnormal stereocilia that are splayed, abnormal lengths, and often oriented abnormally within the cuticular plate, indicating a defect in planar cell polarity.These defects are evident by embryonic day 18 in the mouse, long before the onset of hearing. In mature hair cells, a number of Usher proteins are critical components tip links of the stereocilia and of the mechanotransduction channel apparatus which gates the depolarization of the hair cells in response to noise.
All of the Usher proteins are also expressed in the photoreceptors. We have identified a role for these proteins in regulating the light dependent translocation of phototransduction proteins, and find that many of the Usher proteins localize to the transition zone or basal body aspect of the connecting cilia, which infers a possible function in regulating protein trafficking between the inner and outer segments.
There are many interactions that have been demonstrated for the Usher proteins, however the complexity of this system has made progress towards identification of the specific functions of these complexes somewhat elusive. Most Usher proteins localize to the synapses of hair cells and photoreceptors as well, where they may play a role in synaptic maturation.
Functional Genetics LaboratoryDr. Marisa Zallocchi in the
Center for Sensory Neuroscience was created to implement a high throughput forward genetic approach for dissecting the function of Usher protein complexes using the zebrafish model system.