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BSC 300, Week 9

by: Ashley Bartolomeo

BSC 300, Week 9 BSC 300

Ashley Bartolomeo
GPA 3.9

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Notes on second half of chapter 7
Cell biology
Dr. Jenny
Class Notes
Cell, Biology
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This 4 page Class Notes was uploaded by Ashley Bartolomeo on Thursday October 13, 2016. The Class Notes belongs to BSC 300 at University of Alabama - Tuscaloosa taught by Dr. Jenny in Fall. Since its upload, it has received 7 views. For similar materials see Cell biology in Biology at University of Alabama - Tuscaloosa.


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Date Created: 10/13/16
Chapter 7 Part 2 Interactions Between Cells and Their Environment Interactions of Cells with Extracellular Materials  Integrins: animal specific transmembrane heterodimers with distinct  and  subunits generate diverse heterodimers with different ligand specificity o Anchoring: critical for adhesion of cells to their substratum or to other cells o Signaling: major role in integrating extracellular and intracellular environments. Can signal both from extracellular to cytoplasm and from cell to ECM Model of Integrin Activation  An inactive integrin dimer sits at the cell surface in a bent conformation incapable o interacting with ECM proteins  Inside out signaling: activation of a number of cytoplasmic ligands binds the cytoplasmic tails and separates the  and  chains. This induces a conformational change that allows integrin to bind ECM ligands  Such regulation promotes strong interactions between cells and the ECM  Integrin proteins also participate in outside in signaling: extracellular ECM ligands (collagen and fibronectin, primarily) can bind sites in some integrin dimers inducing conformational change  Such signaling leads to cytoplasmic interactions with diverse proteins that may affect: o Cytoskeletal dynamics: promoting formation of focal adhesions and hemidesmosomes: actin cytoskeleton supported docking sites with the ECM and other cells o Change in cell shape, motility o Cytoplasmic signaling cascades that lead to gene activation or inactivation o Secretion of signaling molecules from other cells o Previously mentioned that cells survival is ECM contact dependent. This is mediated via integrin signaling o Most cancer cells have lost this anchorage dependency and can exist without connection to the ECM Focal Adhesions  Focal adhesions: docking sites where cells adhere to their substratum and send signals to the cell interior  Cytoplasmic domains of integrins contain binding sites for a variety of cytoplasmic proteins  These proteins relay both mechanical and chemical information to the cell interior altering cytoskeleton dynamics and gene expression  Can be rapidly assembled/disassembled in order to promote cellular migration Hemidesmosomes  Hemidesmosomes: rivets that promote basal attachment of epithelial cells to the basement membrane o Contain a dense plaque with intermediate filaments consisting of the protein keratin projecting into the cytoplasm o Keratin filaments are linked to the ECM by membrane spanning integrins that anchor the hemidesmosome to the ECM via interactions with laminin Interactions of Cells with Other Cells  Cells have cell type-specific surface-recognition proteins that maintain tissue organization  Selectins, integrins, cadherins and immunoglobins  Selectins: family of integral membrane glycoproteins that recognize and bind specific glycosylation patterns on cell surface proteins o Contain a small cytoplasmic domain, a single membrane spanning domain, and a large extracellular segment o Mediate transient interactions between migrating white blood cells (leukocytes) and blood vessel walls at sites of inflammation o Three types, all calcium dependent:  E selectin- on endothelial cells  P selectin- on platelets and endothelial cells  L selectin- on white blood cells  Immunoglobulin superfamily (IgSF)- most are involved in immune function, however some mediate calcium independent cell-cell interactions o VCAM (vascular cell-adhesion molecule) o NCAM (neural cell adhesion molecule) o L1 (neural development) o Promote axonal guidance during neural development, and defects in their function can lead to severe developmental challenges involving failure of the spinal column to properly close (spina bifida and hydrocephaly)  Cadherins: large family of glycoproteins that mediate Ca2+ dependent cell-cell adhesion o Cadherins join cells of similar types to one another: preferentially bind the same cadherin on neighboring cells o Family of at least 30 related glycoproteins  E cadherin (epithelial)  N cadherin (neural)  P cadherin (placental) o Also involved in transmitting signals from the ECM to the cytoplasm Cadherins and the EMT  Mediate many of the changes in adhesive contacts during embryonic development by driving epithelial mesenchymal transition (EMT)  During embryogenesis, cells gain and lose specific adhesive properties allowing cells to dissociate from an epithelium and migrate independently: mesenchymal cell  By regulating expression of cadherins, cells during embryogenesis can undergo EMT and migrate to new positions, or partition a developing epithelium into distinct tissues Cadherins  Development of the neural tube: E cadherin expression in epithelium. N cadherin expression in neural tube cells The Human Perspective: The Role of Cell Adhesion in Inflammation and Metastasis  Cancer is the result of abnormal cell proliferation, but it’s generally not the primary tumor, but secondary tumors following metastasis that are the killers  Metastatic cells have unique cell adhesion properties compared to normal cells: o Are less adhesive (cadherins) o No longer anchorage dependent (integrins) o Are able to penetrate several barriers (MMPs) o Are able to invade normal tissues (MMPs, EMT) Molecular Architecture of Adherens Junctions  Adherens junctions – form “belts” near apical surface: junctional complexes that promote tight cell-cell contact  Cytoplasmic domains of cadherin molecules are connected to the actin filaments of the cytoskeleton by linking proteins, including -catenin  -catenin is also a key element in a signaling pathway leading from the cell surface to the cell nucleus Structure of a Desmosome  Desmosomes- disk shaped adhesive junctions between cells  Found in a variety of tissues subject to mechanical stress, e.g. skin, heart, cervix  Contain cadherins that link the two cells across a narrow gap  And linked to distinct cytoskeletal proteins – intermediate filaments Tight Junctions: Sealing the Extracellular Space  Epithelial cells must protect the tissues they surround and prevent inappropriate diffusion or passage of material between cells  Tight junctions (TJs) – specialized contacts between epithelial cells  Located apical to junctional complex between adjacent cells  Serve as barrier to free diffusion of water and solutes both into and out of the tissue that epithelia surround  Composed of the two TM proteins occludins and claudins  Some are permeable to specific ions or solutes Gap Junctions and Plasmodesmata: Connections that Mediating Intercellular Communication  Gap junction – open channels between animal cells that allow cytoplasmic sharing and intercellular communication  Each channel is composed of six copies of the TM protein connexin  Connexins are organized into a complex called connexon  Connexons from adjacent cells create a single GAP junction  Molecules up to 1000 Da can pass through GAP junctions (very small proteins)  Play an important role in ion passage allowing connected muscle cells to contract synchronously Mediating Intercellular Communication  A new type of communication has recently been discovered – tunneling nanotubes aka cytonemes  Protrusions that extend from the plasma membrane that enable different animal cells to touch over long distance  These processes are only about 100 nm in diameter and are supported by an internal actin “skeleton”  Research suggests these long narrow membrane enclosed tubes likely deliver some signaling molecules between cells (as opposed to the signals being released into the extracellular space and diffusing to the target cell)  Plasmodesmata: cytoplasmic channels passing through cell walls of adjacent plant cells  Plant cells are separate by impermeable cellulose rich cell wall  Plasmodesmata are lined by plasma membrane  Serve as sites of cell-cell communication, similar to animal GAP junctions


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