Research Overview:
 
 

 

WHY STUDY CELL ADHESION ?

     Cell adhesion plays a crucial role in embryogenesis, differentiation of adult tissues and wound healing.  Our laboratory is interested in the function and regulation of intercellular junction molecules in desmosomes and adherens junctions  that act to mediate cell-cell adhesion and attachment of the cytoskeleton to the cell surface. The transmembrane glycoproteins of these junctions are calcium-dependent adhesion molecules known as cadherins which have been demonstrated to suppress the invasive phenotype when introduced into transformed cells.  In addition, cadherin-associated proteins known as catenins play a critical role in cell signaling during development via the wnt/wingless pathway and in growth control via regulation by the APC (adenomatous polyposis coli) tumor suppressor protein.  Cadherins also engage in bi-directional signaling with growth factor receptors:  on one hand, activated growth factor receptor tyrosine kinases (RTKs) can negatively regulate cadherin function during the acquisition of cell motility in tumor invasion, while on the other hand, cadherins both positively and negatively regulate the activity of RTKs.
 
Desmosomes are prominent adhesive junctions in epithelial cells
 
 
  Desmosomes (see cartoon above) and related adherens junctions are sites at the plasma membrane between adjacent cells where mechanical and chemical signaling pathways converge.  Each type of intercellular junction anchors its own distinct type of cytoskeletal network to the membrane at sites of cadherin-dependent adhesion.  Desmosomes anchor intermediate filaments to the desmosomal cadherins, desmogleins and desmocollins, through a series of adapter proteins whereas adherens junctions anchor microfilaments to cadherins.  In addition to providing tissues with mechanical strength and integrity, these intercellular junctions have also been recognized as sensors that respond to environmental and cellular cues by modulating their assembly state and, possibly, signaling functions.
 
 In the Green lab, the contribution of individual protein building blocks  to adhesion and  junction assembly is being addressed, and the idea that desmosome molecule functions transcend their roles in adhesion is being pursued.   We use a variety of experimental strategies including the ectopic expression of proteins in normally non-adherent cells and expression of dominant negative mutations in cultured cells and transgenic mice.  Protein-protein interactions important for establishing the hierarchy of assembly are being defined by in vitro biochemistry and yeast two hybrid approaches, and mutational analysis based on high resolution structural data is being carried out to elucidate binding interfaces between interacting partners.  We are also carrying out studies to define how desmosome and adherens junction components know how to segregate themselves into distinct plasma membrane domains.  The regulation of junction integrity is being addressed by studying the consequences of growth factor- and oncogene-dependent phosphorylation of individual components on their function and association with other proteins.  Junction dynamics  are being studied using state-of-the-art imaging techniques to track the movements of individual fluorescently-tagged molecules during desmosome assembly and in response to motility-inducing signals. 
 

The role of desmosomal cadherins in adhesion, differentiation and as targets for human autoimmune and infectious disease.

   There are three different desmocollin and four different desmoglein genes, which are are expressed in a highly organized, differentiation-dependent pattern in complex tissues such as the epidermis (see below).  However, why we need so many cadherins in the epidermis is a mystery.  Our recent data suggest that although all of the epidermal cadherins contribute to adhesive strength, desmoglein 1,  which is concentrated in the upper, more differentiated layers of the skin, causes precocious differentiation when introduced by retroviral gene delivery into rapidly growing undifferentiated keratinocytes.  Current work is directed toward defining signaling pathways with which desmosomal cadherins interact to promote differentiation and morphogenesis.  Desmogleins are also targets for autoimmune antibodies in patients with pemphigus and bacterial toxins in patients with Staphylococcus scalded skin syndrome and impetigo.  It is thought that pemphigus antibodies inhibit cell adhesion or impair desmosome structure and assembly state, whereas the toxins proteolytically cleave the extracellular adhesive domain of desmoglein 1; in both cases severe epidermal blisters result.  The exact mechanism leading to blistering is only poorly understood and is the focus of ongoing work.  

Mutations in cell junction genes cause inherited human disease.

   Recent studies have identified mutations in structural proteins that lead to blistering disorders of the epidermis, some of which are fatal.   Mutations in desmoplakin itself (see Armstrong, et al below), leading to a haploinsufficiency which causes a syndrome known as striate palmoplantar keratoderma characterized by lesions on the hands and feet. In addition, mutations in a desmosomal molecule called plakophilin 1 have been shown to lead to aberrations in desmosome structure and epidermal lesions in patients, and truncating mutations in plakoglobin and desmoplakin result in heart, skin and hair defects with early morbidity.  In vitro biochemical and cell biological analysis is being used to understand how specific human gene mutations affect protein function.  Finally, protein interaction screens are being used to identify possible new junction proteins that could be targets for mutation in human disease.
 

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