cached image
Bechara Kachar, M.D. - US grants
Affiliations: | Section on Structural Cell Biology | National Institute on Deafness and Other Communication Disorders, Bethesda, MD, United States |
Area:
Structural Cell BiologyWebsite:
http://www.nidcd.nih.gov/research/scientists/kacharb.aspWe are testing a new system for linking grants to scientists.
The funding information displayed below comes from the NIH Research Portfolio Online Reporting Tools and the NSF Award Database.The grant data on this page is limited to grants awarded in the United States and is thus partial. It can nonetheless be used to understand how funding patterns influence mentorship networks and vice-versa, which has deep implications on how research is done.
You can help! If you notice any innacuracies, please sign in and mark grants as correct or incorrect matches.
High-probability grants
According to our matching algorithm, Bechara Kachar is the likely recipient of the following grants.Years | Recipients | Code | Title / Keywords | Matching score |
---|---|---|---|---|
2012 — 2018 | Kachar, Bechara | ZIAActivity Code Description: Undocumented code - click on the grant title for more information. |
Molecular Organization of Intercellular Junctions in the Inner Ear @ Deafness & Other Communication Disorders Tight junctions (TJ) consist of networks of linear contacts or sealing strands between adjoining cells, forming a permeability barrier to control the flow of molecules in the intercellular space. TJ strands are formed by a chain of transmembrane proteins on either side of the contacting membranes, maintained by both cis (intra-membrane) and trans (inter-membrane) interactions. The molecular architecture and mechanism of TJ strand assembly is just beginning to be elucidated. Claudins, a large family of transmembrane proteins, are the main components of the TJs. Multiple claudins (including 9, 11 and 14) are expressed in the inner ear and are required for normal hearing. We have shown that in OHCs claudin-14 and claudin-9/6 segregate into morphologically distinguishable TJ subdomains. This contrasts with the common view that most claudins co-assemble into heteromeric strands. A recent report on the crystal structure of claudin-15 has provided new insights into the architecture of TJs, revealing that the two extracellular loops arrange into a characteristic β-sheet fold allowing for specific cis-interactions between adjacent claudins. No trans-interactions, supporting the transcellular strand architecture, were identified. Our long term goal is to attain a molecular-level description of TJ assembly and a complete ultrastructural model of the TJ strand backbone in the inner ear. These findings will collectively provide the basis to understand how various claudins, and TJ accessory proteins expressed in hair cells and supporting cells, assemble to form the homotypic and heterotypic TJs of the inner ear. To study TJ molecular architecture, we have been using in silico structures and homology modeling/molecular simulation to build plausible 3D models as well as a combination of Molecular Dynamics simulations and mutagenesis approach to assess the formation of TJ strands from heterologously expressed, fluorescently tagged claudin in COS7 cells by fluorescence and freeze-fracture imaging. Based on the claudin-15 crystal structure and homology modeling/molecular simulation, we performed atomic-level conformation screen of over 2,500 claudin strand candidate structures and identified by energy minimization an optimal octameric unitary model (below) consisting of a double row of tightly associated claudins forming a single strand in each membrane. The model has several key supporting features including: tightly packed hydrophobic residues form a hydrophobic wall leaving most charged residues distributed on the loop region facing the aqueous environment; the conserved amino acid W137 stabilizes the interactions between claudins across from each other by π-π stacking (attractive noncovalent interactions between aromatic rings); the inter-molecular π-π stacking of conserved F146F147 result in concerted cis- and trans- interactions. To validate this model, several amino acid substitutions were designed and tested in molecular dynamics simulations before introducing them in a GFP-tagged claudin-15 cDNA vector for expression in COS7 cells to verify their implications for TJ strand formation. As predicted by our model, the replacement F146F147/LL abolished TJ strand formation, while F146F147/WW retained the ability to form TJ strands. Mutations in NMII have been linked to hearing loss in humans (Seri et al., 2000). We recently showed that bipolar NMII filaments integrate with peri-junctional actin to form a belt of muscle-like sarcomeres encircling the apex of hair- and supportingcells (Ebrahim et al., 2013). The sarcomeres appear to be linked across adjacent cells, forming a transcellular, contractile network. This novel sarcomeric network, which occurs universally across different epithelial tissues, provides a well-defined target to investigate the role of NMII in junctional homeostasis. We now plan to investigate the ultrastructural relationship of the sarcomeric belt with other components of the AJC, its mechanisms of assembly and turnover, isoform-specific properties, and cellular functions. We have preliminary data to suggest that in the organ of Corti both the expression level and the characteristic sarcomeric pattern of the predominantly expressed NMIIC appear to diminish from early (P0-P4) to later postnatal (>P7) stages, correlating with early postnatal development of the organ. Additionally, we have data showing a significant redistribution and/or increase in expression levels of NMIIA at the AJC of NMIIC-/- mice, suggesting that the absence of an auditory phenotype in these mice may be due to a degree of compensation by NMIIA. However, epithelia from NMIIC-/- mice show significant differences in cell packing and geometry indicative of reduced junctional tension (in the organ of Corti these present as planar cell polarity defects). Collectively these data suggest that compensation by NMIIA is incomplete, likely due to differences in isoform-specific kinetic and biochemical properties. Other functional data from model cell lines and intestinal epithelia show the specific recruitment of NMIIA, the most kinetically active isoform, to sites of epithelial rearrangement during apoptosis. We are also currently investigating differential inter-cellular isoform distribution patterns that we hypothesize confer differential dynamic properties across heterotypic junctions. Finally, we have the first evidence that we know of, for the existence of NMII heterofilaments (bipolar filaments comprising multiple isoforms) in vivo. We postulate that such heterofilaments provide a regulatory mechanism to further modulate the resultant kinetic properties of the sarcomeric belt in different functional contexts. |
1 |
2012 — 2018 | Kachar, Bechara | ZIAActivity Code Description: Undocumented code - click on the grant title for more information. |
Structural and Molecular Basis of Transduction in Auditory Sensory Organs @ Deafness & Other Communication Disorders Myosin III (MYO3) proteins are associated with the auditory and visual sensory systems in both invertebrates and vertebrates. In Drosophila, MYO3 (NINAC) is responsible for the transport and localization of the photo-transduction machinery. In humans, mutations in MYO3A cause non-syndromic progressive hearing loss. Our lab focuses on studying the interrelated functions of two MYO3 paralogs, MYO3A and MYO3B and their associated cargoes in regulating structure and mechanosensitivity of the actin-based stereocilia. MYO3A differs from MYO3B in that it contains an extended tail domain with an additional actin binding motif. In collaboration with Chris Yengo's lab (PSU) we examined how the motor and tail domains of these two myosins influence the formation and elongation of actin protrusions. MYO3A is a two-fold faster motor with enhanced ATPase activity and actin affinity. A chimera in which the MYO3A tail was fused to the MYO3B motor demonstrated that motor activity correlates with formation and elongation of actin protrusions. Removal of MYO3A tail abolishes the ability to enhance actin protrusion formation and elongation. MYO3A also slows filopodia dynamics and enhances filopodia lifetime. We conclude that the unique enhanced motor activity in combination with its extended tail actin-binding motif favors MYO3A in its role in elongation and maintenance of actin protrusions. Whole-exome sequencing of samples from affected members of a Brazilian family with bilateral late-onset non-syndromic hearing-loss performed by the laboratory of Regina C. M. Neto (USP, Brazil), revealed a novel mutation (T2090G) in MYO3A. Segregation studies confirmed the presence of the mutation in 25 affected individuals from seven-generations of the same family, showing autosomal dominant inheritance. The mutation alters a conserved residue (L697W) in MYO3A motor domain. Biochemical studies from the Yengo's lab shows that the mutation causes a reduction in motor ATPase and motility of MYO3A. We further demonstrate that the mutant MYO3A causes reduced actin protrusion formation in COS7 cells. Coexpression of L697W and wild-type MYO3A in hair cells and COS7 cells in the presence of the MYO3A cargo ESPN1, revealed a predominant tipward accumulation of L697W at the stereocilia and filopodia. We argue that the higher duty ratio of L697W may mediate a dominant negative effect caused by the enhanced stability of a slower motor (L697W) at actin protrusion tips. We propose that L697W with reduced motor activity may impact stereocilia length regulation, cargo transport, and mechanotransduction. Our results shed light on the dominant phenotype of a novel, late onset-deafness causing, MYO3A mutation. Hair cells tightly control the dimensions of stereocilia to form a precisely regulated staircase pattern that is essential for normal mechanotransduction. We observed that eliminating espin-1 (ESPN-1), an isoform of ESPN and a myosin-III cargo, dramatically alters the slope of the stereocilia staircase in a subset of hair cells. Furthermore, in collaboration with Peter Barr-Gillespie's lab (OHSU) we performed a series of experiments that show that espin-like (ESPNL), primarily present in developing stereocilia, is also a myosin-III cargo and is essential for normal hearing. ESPN-1 and ESPNL each bind MYO3A and MYO3B, but differentially influence how the two motors function. Consequently, functional properties of different motor-cargo combinations differentially affect molecular transport and the length of actin protrusions. This mechanism is used by hair cells to establish the required range of stereocilia lengths within a single cell. To determine the compensation between ESPN-1 and ESPNL on stereocilia length regulation, we made an ESPN-1/ ESPNL double knockout mouse. In this mouse the stereocilia bundles in the utricle are normal, but those in the organ of Corti have very short lower two rows of stereocilia. This phenotype is most severe in outer hair cells. These shortened stereocilia are present at P6 or before and persist into adulthood. ESPN-1 knockouts have similar mechanotransduction (MET) currents to wildtype while the ESPNL knockout had 40% and double knockout had 12% of control MET. While the stereocilia length regulation in the double knockout is compromised, the MET machinery appears intact. The differences in phenotype between the utricle, outer hair cells and inner hair cells suggest that stereocilia length regulation is a complex process that differs between type of hair cell and includes but is not limited to the MYO3/ESPN/ESPNL system. We continue to investigate the localization and molecular nature of the mechanosensitive channel complexes in hair cells. Previous reports and results from our laboratory have shown that there are multiple proteins expressed in hair cells that are essential for mechanotransduction (MET). It has been shown that deletion of TMIE and TMC proteins completely eliminates conventional MET in hair cells. However, even in the absence of conventional MET, hair cells are still capable of reverse polarity mechanotransduction. The molecular nature of the MET channel has long been elusive and it is still not clear whether TMIE or TMCs can form the actual channel pore. Therefore, we developed a stimulation and recording electrophysiology protocol to detect mechanically activated currents in HEK293T cells transfected with the candidate proteins. As positive control, we used cells transfected with Piezo 1, a protein that has been reported to be important in mechanosensitivity. We show that cells transfected with Piezo 1 respond with large, mechanically activated currents upon mechanical perturbation/stimulation of the cell membrane. However, we were unable to evoke mechanically activated currents in cells transfected with TMIE, TMC1, TMC2, TMC4, or combinations of these proteins. Further examining the role of TMIE in MET, we have shown that full deletion of this protein completely eliminates conventional MET from hair cells. However, all tested hair cells exhibited reverse polarity MET upon sine wave stimulation or square pulse stimulation with fluid jet. These results confirm that, like TMC1, TMIE is absolutely necessary for conventional MET, but it is not clear if it is part of the pore forming element of the MET channel, or if it interacts with TMC and/or other proteins to form the MET complex as previously suggested. Investigating the developmental regulation of the reverse polarity MET in TMIE knockout animals, we discovered that these reverse polarity responses can be evoked in hair cells before the onset of hearing (P2-P9). However, they significantly decline and are almost entirely absent or very difficult to evoke in animals older than P7. We are currently examining the spatiotemporal localization of TMIE, P2X2, Piezo 1, Piezo 2, TMC4 and TMC5 using transgenic mice expressing GFP and mCherry -tagged versions of these candidate MET channel complex proteins. The MET channel is a large conductance, non-specific cation channel highly permeable to calcium. Using high spatiotemporal resolution calcium imaging in cultured inner ear tissue from mice expressing the calcium indicator GCaMP3 we observe widespread spontaneous calcium transients in stereocilia in both pre-hearing (P3-P10) and mature (P17-P21) hair cells. Application of MET channel blocker dihydrostreptomycin eliminated all calcium transients observed in both auditory and vestibular stereocilia. More importantly, in the organ of Corti, this treatment also eliminated calcium transients from the tallest rows of stereocilia, confirming our earlier findings that MET-like channels are present in previously unrecognized locations within the hair bundles. |
1 |