Laboratory/Faculty

Laboratory of Biological Systems
Group of Cell Biology

LecturerYoshiaki Hirako
Analysis of epithelial cell adhesion to the basement membrane
LecturerKingo Takiguchi
Molecular physiology of biomembrane using liposomes
▶Laboratory HP
▶Japanese

Analysis of regulatory mechanism of adhesion protein complexes connecting epithelia to the basement membrane

The emergence of multicellular organisms was an important evolutionary event that has led to the development of higher organisms.  With this evolutionary jump, it became possible for cells to specialize functionally.  Multicellular organisms are partially composed of organs, which are formed from different combinations of tissues, each of which comprises a group(s) of specialized/differentiated cells.  Thus, an essential function of eukaryotic cells is cell adhesion, by which cells can combine to form tissues, and tissues in turn can combine to form organs.  In general, there are two types of cell adhesion: cell-cell adhesion and cell-to-ECM (extracellular matrix) adhesion.  Cells have developed special adhesion apparatuses to attach firmly to the ECM. One such apparatus is the hemidesmosome, which anchors the cytoskeleton on its cytoplasmic side and binds to the basement membrane on its extracellular side.  The basement membrane is a sheet of 50-100 nm thickness, and is composed of various ECM proteins.  Basement membranes exist at the borders of different tissues, compartmentalizing the tissues and providing cells with a foothold for adhesion.  It has been suggested that basement membranes play important roles in cell differentiation, proliferation, and tumor metastasis.

Our group has been working to clarify the molecular architecture of the hemidesmosome and basement membrane.

The figure below shows immunofluorescence micrographs of cultured keratinocytes.  Keratinocytes (a) secrete laminin-332 (b), a component of basement membranes, thereby forming an extracellular structure. Immunofluorescence microscopy reveals that type XVII collagen (c), a hemidesmosomal transmembrane protein, is often found alongside laminin-332.  Our electron microscopic studies have shown an interesting molecular configuration of type XVII collagen, shown in the second figure: the globular head in the cytoplasm indirectly connects to intermediate filaments, and the remaining rope-like portion attaches externally to the basement membrane.  In addition to these molecular and cellular studies, we have begun to investigate the regulation and function of the hemidesmosomal adhesion system at the tissue level, using organotypic three-dimensional culture systems.

Study of dynamics of biological membranes

Liposome (artificial lipid membrane vesicle) is the most simplified model of biological membranes, and has been used in many studies. The dynamics of cell-sized giant liposomes in an aqueous solution can be directly observed by various optical microscopes. Using the real-time imaging, we aim to elucidate the molecular mechanisms that control the dynamics of biological membranes.

We will reveal the dynamic mechanism, through the observation of the phenomenon, which would be induced into liposomes by various cytoskeletal proteins including actin, molecular motors, membrane-interacting peptide, or surfactants. Fig. 3 is an example of these research results, showing membrane tubulation induced by interaction between liposome and F-BAR domain proteins. For endocytosis and exocytosis, or intracellular membrane vesicle transport, etc., it is important to deform the membrane in a tubular shape. Proteins belonging to the F-BAR domain protein family are thought to be involved in these deformations. Photographs are real-time dark field images showing membrane tubulation processes induced by FBP17 or PSTPIP1, both are member of the family. The observation revealed that, even proteins belonging to the same family, there is a case where tubulation processes are different.

Recently, we are also conducting study focusing on the characteristics of lipid membranes and cytoskeletons as natural soft matter, such as the nematic liquid crystal formation by actin filaments, and the layer formation by lipids or the actin dynamics in liquid-liquid phase separation. Fig. 4 shows that the liposomes deformed into a spindle shape by the nematic liquid crystal formation of the actin filaments encapsulated inside, and that the repeated deformation of the spindle-shaped liposome that was successfully induced by the reversible reaction between the severing by strong excitation light irradiation against fluorescently labeled actin filaments and the spontaneous annealing after the irradiation.

Fig.1

Fig.2

Fig.3

Fig.4

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