Faculty, Graduate Program
Our research program aims to elucidate how temporal and spatial control of actin dynamics creates the diverse cytoskeletal structures that influence cell and tissue architecture and physiology. We focus on the tropomodulin (Tmod) family of actin filament pointed-end capping proteins, which bind tropomyosins (TMs), regulate actin dynamics and turnover, and stabilize actin cytoskeletal architecture. Four canonical, ~40 kD Tmod isoforms (Tmods 1-4) and three larger variants (leiomodins 1-3) exhibit tissue-specific and developmentally regulated patterns of expression in vertebrates. Our studies use biochemical approaches with purified proteins, in vitro approaches with cultured cells, in vivo approaches with transgenic mouse models, and translational studies with human tissues. We primarily focus on three experimental model systems: red blood cells (RBCs), skeletal muscle, and the ocular lens.
In RBCs, Tmod1 caps the pointed ends of short, TM-coated actin filaments, which are interconnected by spectrin tetramer strands to form an isotropic lattice with quasi-hexagonal symmetry on the inner face of the RBC membrane. Key unsolved questions are how the short actin filaments are assembled during RBC membrane biogenesis, and how actin filament remodeling contributes to RBC function. We examine RBC membrane structure in transgenic mouse models using biochemistry, fluorescence and electron microscopy, and RBC mechanics by osmotic fragility and microfluidic shearing assays. Currently, we are examining RBC production via studies of erythroblast differentiation and enucleation in hematopoietic organs (fetal liver, bone marrow, spleen), and novel roles of Tmod3 in regulating cytoskeletal reorganization during these processes. These experiments will elucidate how actin regulation contributes to the pathogenesis of human hemolytic anemias due to RBC instability and altered lifespan, and to congenital anemias due to defects in RBC biogenesis and production.
In skeletal muscle, Tmod1 and Tmod4 control thin filament lengths and actin-myosin crossbridge formation in sarcomeres, thereby regulating muscle-specific contractile function. Key unsolved questions are how Tmods coordinate thin filament lengths within each sarcomere and muscle, and how Tmods’ interactions with TMs influence thin filament activation and actin-myosin interactions. We take in vitro and in vivo approaches, utilizing actin polymerization assays, muscle physiology, fluorescence microscopy, and quantitative imaging, to measure thin filament lengths, actin exchange at pointed ends, and contractile dynamics. These experiments are relevant to understanding a group of congenital human muscle diseases termed nemaline myopathies, which are characterized by thin filament length dysregulation and aberrant extrasarcomeric protein aggregates. Current studies are testing the hypothesis that proteolytic cleavage of Tmods may also contribute to the pathogenesis of congenital muscular dystrophies.
In the ocular lens, Tmod1 stabilizes TM-coated actin filaments in the spectrin-actin network on fiber cell membranes, controlling fiber cell morphology and hexagonal packing while contributing to lens mechanical resilience. The goal of our work is to define the molecular basis for spectrin-actin network control of membrane subdomain assembly, mechanical stability, and cell interactions during lens development and aging. Our work utilizes lenses from mice with targeted mutations or deletions in components of the spectrin-actin network, with confocal fluorescence microscopy, image analysis, and proteomics approaches to define interactions and structural relationships. For functional analysis, optical, electrophysiological, and biomechanical experiments are performed. Our studies provide insight into how membrane domain dysfunction leads to loss of lens transparency, cataracts, and blindness in humans as well as loss of lens resilience in presbyopia.
Overarching questions are: (1) To what extent are common molecular mechanisms of actin dynamics utilized to specify actin filament architecture and function in diverse cell and tissue contexts? (2) To what extent can Tmod perturbation experiments provide insights into normal tissue development and physiology, and into pathological mechanisms of human disease? When answering these questions, our multidisciplinary and multi-scale approach allows us to connect the dots -- from actin dynamics regulation, to assembly and organization of cytoskeletal structures in cells, to morphogenetic differentiation during development, to cell and tissue physiology in both health and disease.
B.A., Oberlin College, 1974
Ph.D., Harvard University, 1980
1980-1982 Jane Coffin Childs Postdoctoral Fellow NIADDK, NIH, and Dept Cell Biology and Anatomy, Johns Hopkins University School of Medicine; 1983-1984 Research Associate, Dept Cell Biology and Anatomy, Johns Hopkins University School of Medicine; 1984-1987 Assistant Professor, Dept Anatomy and Cell Biology, Harvard Medical School; 1987-1993 Assistant Professor, Depts Molecular and Cell Biology, The Scripps Research Institute (TSRI); 1993-2000 Associate Professor, Dept Cell Biology, TSRI; 2000-2013 Professor, Dept Cell Biology, TSRI; 2013 Professor, Dept Cell and Molecular Biology, TSRI; 2013- Associate Dean for Graduate Studies, TSRI.
1975-1978 National Science Foundation Predoctoral Fellowship Award; 1980-1982 Jane Coffin Childs Foundation Postdoctoral Fellowship Award; 1983-1984 NIH New Investigator Research Grant Award; 1990-1995 American Heart Association Established Investigator Award; 2001-2003 Chair, Cell and Developmental Function 6 (CDF6) Study Section for Postdoctoral Fellowships and AREA grants, NIH; 2003 Chair, “Motile and Contractile Systems” Gordon Research Conference, Colby-Sawyer College, NH; 2009-2010 Erythrocyte and Leukocyte Biology (ELB) NIH Study Section; 2010-2013 Molecular and Cellular Hematology (MCH) NIH Study Section; 2010 ASCB Program Committee, 2011 Annual Meeting; 2011 Chair, “Red Cells” Gordon Research Conference, Proctor Academy, Andover, NH; 2011 Lens and Cataract Program Planning Panel, National Eye Institute, NIH; 2011-pres. Associate Program Director & Imaging Core Director, San Diego Skeletal Muscle Research Center (NIAMS/NIH), P30 Core for UCSD, TSRI, Sanford-Burnham, Salk and San Diego State University; 2014 Program Organizer Lens Section, International Society for Eye Research XX1st Biennial Conference. Editorial Boards and Consulting: 2005- Editorial Board, Cytoskeleton; 2012-2013, Editorial Board Member, Journal of Biological Chemistry; 2013-pres. Associate Editor, Journal of Biological Chemistry.
For a complete list of publications: http://www.scripps.edu/fowler/publications.html
Yamashiro S, Gokhin DS, Kimura S, Nowak RB, Fowler VM. Tropomodulins: Pointed-end capping proteins that regulate actin filament architecture in diverse cell types. Cytoskeleton (Hoboken). 2012 Apr 4. doi: 10.1002/cm.21031 [Epub ahead of print]
Nowak RB, Fowler VM. Tropomodulin1 Constrains Fiber Cell Geometry During Elongation and Maturation in the Lens Cortex. J Histochem Cytochem. 2012 Apr 3. [Epub ahead of print]
Damani S, Bacconi A, Libiger O, Chourasia AH, Serry R, Gollapudi R, Goldberg R. Rapeport K, Haaser S,K, Kuhn P, Wood M, Carragher B, Schork NJ, Jiang J, Rao C, Connelly M, Fowler VM, Topol EJ. Characterization of Circulating Endothelial Cells in Acute Myocardial Infarction. Science Translational Medicine. 2012;4(126):126ra33.
Gokhin DS, Kim NE, Lewis SA, Hoenecke HR, D'Lima DD, Fowler VM. Thin-filament length correlates with fiber type in human skeletal muscle. Am J Physiol Cell Physiol. 2012;302(3):C555-65.
Gokhin DS, Fowler VM. The sarcoplasmic reticulum: actin and tropomodulin hit the links. BioArchitecture. 2011;1(4):175-9.
Gokhin DS, Fowler VM. Tropomodulin Capping of Actin Filaments in Striated Muscle Development and Physiology. Journal of Biomedicine and Biotechnology. 2011;2011:103069.
Gokhin DS, Fowler VM. Cytoplasmic γ-actin and tropomodulin isoforms link to the sarcoplasmic reticulum in skeletal muscle fibers. J Cell Biol. 2011 July 11; 194(1):105-20.
Fath T, Fischer RS, Dehmelt L, Halpain S, Fowler VM. Tropomodulins are negative regulators of neurite outgrowth. Eur J Cell Biol. 2011 Apr; 90(4):291-300.