Philadelphia University + Thomas Jefferson University

Keen, James H.

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James H. Keen, PhD

Contact Dr. Keen

233 S. 10th Street
915 BLSB
Philadelphia, PA 19107

(215) 503-4624
(215) 503-0622 fax

Expertise & Research Interests

The long-term goal of my lab has been an understanding of the structure of clathrin-coated membranes and their role in membrane transport processes within mammalian cells.  It is well established that plasma membrane coated pits mediate endocytosis, and that clathrin coated buds and tubules in the TGN are involved in targeting and recycling of newly synthesized proteins.  But clathrin coats are also present on vacuolar and tubular domains of endosomes where they are involved in sorting to late endosomes, or to retrograde or recycling pathways, respectively, through mechanisms that are only partially understood.  As the early/sorting endosome is a nexus for virtually all extracellular materials entering cells as well as many intracellular trafficking cargoes, understanding sorting mechanisms and structures in the endosomal region remains a key challenge with many implications for human health and disease. In this context, we have identified and characterized distinct clathrin coated structures in the endosomal region that exhibit extremely rapid but localized movement.  Accordingly we termed these structures 'Gyrating'- or 'G-clathrin', and they appear to be clathrin coated buds along or on the ends of membrane tubules.  We have shown that G-clathrin accumulates internalized cargo destined to be recycled to the plasma membrane, including transferrin and c-Met receptor, and that they function in rapid recycling from the endosome which remains a largely uncharacterized pathway.  In recent work, we have shown that the dynamics of G-clathrin requires completion of the Arf6 guanine nucleotide cycle but that Arf1 can also support its formation.  Similarly, both BFA-sensitive and -resistant guanine nucleotide exchange (GEF) proteins modulate G-clathrin levels, together indicating that overlapping mechanisms are operative in G-clathrin regulation.  The complexity of regulation appears to parallel the multiplicity of endosomal sorting pathways.

Current efforts utilize two major approaches. First, we are dissecting the regulation and function of G-clathrin in rapid recycling pathways and identifying the cargos transported, with a focus on the role of GGA proteins and other potential regulators and adaptors.  Second, we are interested in better visualizing the structure and function of G-clathrin in intact cells, using correlative light-electron microscopy (CLEM) in collaborative efforts with the lab of Graça Raposo (Institut Curie, Paris) at the ultrastructural level, and interactions and interconversions of G-clathrin with other endocytic and membrane compartments using high-speed live cell imaging.

Our methodological approaches utilize widefield, TIRF and spinning disk confocal fluorescence microscopy and associated photobleaching/photoactivation and image analysis procedures, the development and application of novel "molecular traps" to probe functions in intact cells in collaboration with John Williams (City of Hope, Duarte CA), and conventional cell and molecular biology techniques such as siRNA-mediated protein depletion and exogenous expression, and others.


Most Recent Peer-Reviewed Publications

  1. Exosome-mediated transfer of αvβ3 integrin from tumorigenic to nontumorigenic cells promotes a migratory phenotype
  2. Integrins influence the size and dynamics of signaling microclusters in a Pyk2-dependent manner
  3. Clathrin light chains are required for the gyrating-clathrin recycling pathway and thereby promote cell migration
  4. Patched-1 proapoptotic activity is downregulated by modification of K1413 by the E3 ubiquitin-protein ligase Itchy homolog
  5. Arf6 regulation of Gyrating-Clathrin
  6. GGA3 functions as a switch to promote met receptor recycling, essential for sustained ERK and cell migration
  7. Development and application of in vivo molecular traps reveals that dynein light chain occupancy differentially affects dynein-mediated processes
  8. Gyrating clathrin: Highly dynamic clathrin structures involved in rapid receptor recycling
  9. Interaction of prostate specific membrane antigen with clathrin and the adaptor protein complex-2
  10. Phosphoinositide 3-kinase C2α links clathrin to microtubule-dependent movement
  11. Dynamics of membrane clathrin-coated structures during cytokinesis
  12. Individual phosphoinositide 3-kinase C2α domain activities independently regulate clathrin function
  13. Membrane targeting of endocytic adaptors: Cargo and lipid do it together
  14. Major histocompatibility complex class I-intercellular adhesion molecule-1 association on the surface of target cells: Implications for antigen presentation to cytotoxic T lymphocytes
  15. Signaling Down the Endocytic Pathway
  16. The E3 ubiquitin ligase AIP4 mediates ubiquitination and sorting of the G protein-coupled receptor CXCR4
  17. A glimpse of coated vesicle creation? Well almost!
  18. G protein-coupled receptor/arrestin3 modulation of the endocytic machinery
  19. The actin-binding protein Hip1R associates with clathrin during early stages of endocytosis and promotes clathrin assembly in vitro
  20. The class II phosphoinositide 3-kinase C2α is activated by clathrin and regulates clathrin-mediated membrane trafficking