Imaging molecular structure and physiological function of gap junctions and hemijunctions by multimodal atomic force microscopy

Gap junctions are specialized plasma membrane structures that join neighboring cells via specialized intercellular ion channels (hemichannels) and provide a direct pathway for cell-cell communication. They presumably mediate regulation of growth, transmission of developmental signals, coordination of muscle contraction, and maintenance of metabolic homeostasis. Hemichannels are also present in the non-junctional regions of the cell plasma membrane and they provide a direct pathway for communication between the cytoplasm and the extracellular region. Recent studies suggest that gap junctional communication is much more complex than previously anticipated, in terms of both its structure as well as its activity. While the mechanism of gap junction activity is being studied extensively, their quaternary structure, assembly, and conformational changes underlying gating of their activity as well as their physiological role are poorly understood because, due to their complex structure, these junctions are less amenable to existing techniques for high-resolution three-dimensional structure-function analyses. Atomic Force Microscopy (AFM) images molecular structure of biological specimens in an aqueous environment, allows on-line perturbations, and can be coupled with electrophysiological, biochemical, and other microscopic techniques. The present review examines the potential of AFM application for the study of the molecular structure of hydrated, native gap junctions and hemijunctions as well as their physiological functions. Special attention is paid to new, complementary, or provocative findings from AFM studies of both vertebrate and invertebrate gap junctions and hemijunctions.