Dr. A. Mobasheri   Seminar 29 March 2010
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Dr. A. Mobasheri Seminar 29 March 2010

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Invited seminar at the Heart and Lung Institute, Imperial College London, 29 March 2010

Invited seminar at the Heart and Lung Institute, Imperial College London, 29 March 2010

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Dr. A. Mobasheri   Seminar 29 March 2010 Dr. A. Mobasheri Seminar 29 March 2010 Presentation Transcript

  • Linking Potassium Channels to Mechanical and Chemical Transduction in Chondrocytes
    Ali Mobasheri
    Imperial College London
    Harefield Heart Science Centre
    29 March 2010
  • Current Research Projects
    Developingin vitro models of osteoarthritis using cartilage and synovium
    Mesenchymal stem cells and cartilage tissue engineering
    Cartilage proteomics (identification of biomarkers in the cartilage secretome)
    Plant derived phytochemicals as anti-inflammatory agents for arthritis
    Comparative physiology of aquaporin water channels
    Exploring the chondrocyte “channelome”
  • Articular Cartilage
    Mechanically unique connective tissue designed to:
    withstand and distribute load
    act as an elastic shock absorber
    provide a wear resistant surface to articulating joints
  • Avascular, aneural and alymphatic
    Contains a single cell type: the chondrocyte
    Derived from mesenchymal progenitor cells
    Articular Cartilage
  • The Chondrocyte
    Nucleus
    Cytoplasm
    ECM
    Synthesizes a mechanically resilient extracellular matrix of collagens and aggregating proteoglycans
  • Major Constituentsof Cartilage
    Water (interstitial fluid)
    Type II collagen and other collagens (collagens IX & XI)
    Proteoglycans (aggrecan)
    Non-collagenous proteins
    Chondrocytes
    Ions, growth factors etc.
    Interactions between water & cations substantially influences load bearing performance of cartilagematrix
  • Major Constituents of Articular Cartilage Matrix
    COMP
    Aggrecan
    Chondrocyte
    Fibronectin
    Hyaluronan
    Collagen IX
    Decorin
    Collagen II
    Fibromodulin
    Biglycan
    Thrombospondin
  • Typical Cell
  • Ionic Composition of Cartilage
  • Resting Cartilage
    Loaded Cartilage
    Pressure = 1 atm
    [Na+] = 240-300 mM
    350 mOsm
    Normal cell volume
    Load
    Pressure = 50-200 atm
    [Na+] = 250-350 mM
    380-480 mOsm
    Cell shrinkage leading to the elevation of local cation concentrations (Na+, K+ and Ca2+) and activation of volume regulatory ion and osmolyte transport systems
    Possible changes to the cell membrane potential and activity of ion channels.
  • Ca2+-activated
    K+ channels
    (BK, MaxiK)
    AQP1, AQP3
    H2O, Glycerol,
    Urea
    K+
    Chondrocyte
    VGCC
    Ca2+
    Other K+ channels
    (including KATP channels)
    Na+
    VGSC
    Na+
    ENaC
  • Stretch / Voltage Activated
    Sodium Channels (ENaC,
    VASC)
    Na+/H+ Exchange
    NHE1, NHE2, NHE3
    NHE4
    Na+
    HCO3- / Sulphate
    240-350 mM Na+ : 5 mM K+
    Anion Exchange
    AE2
    H+
    Steep concentration gradient
    Maxi K+ Channels Calcium activated K channels
    Na+
    Cl-
    K+
    Na+ : K+
    Passive diffusion or
    non-specific leakage
    Na+
    3Na+
    CHONDROCYTE
    K+
    ATP
    Na, K-ATPase
    a1b1, a1b2, a1b3,
    a2b1, a2b2, a2b3,
    a3b1, a3b2 & a3b3
    2Cl-
    2K+
    Cotransporter
    NKCC1
    H2O
    ATP
    AQP Water Channels
    Stretch / Voltage Activated Ca2+ Channels
    Ca2+ ATPase
    PMCA1
    Ca2+
    Ca2+
  • Potassium Channels in Chondrocytes
    Quantitative analysis of voltage-gated potassium currents in chondrocytes – 2005
    Evidence for functional ATP-sensitive (K(ATP)) potassium channels in chondrocytes – 2007
    Characterization of a stretch-activated potassium channel in chondrocytes -2010
    Transient receptor potential channels in chondrocytes (new project)
  • 6TM Potassium Channel Structure
    The a subunit is formed from 6 transmembrane segments and is associated with a regulatory b subunit
    B) Four a subunits form the pore
  • 2TM Potassium Channel Structure
    Four of these subunits cluster to form the active channel. Each subunit is composed of two membrane-spanning helices connected by a P loop
  • BK (MaxiK) Channels
    Channels potentially involved in mechanotransduction and chemotransduction
  • BK (MaxiK) Channels in Chondrocytes
  • Ion channels are activated by membrane stretch
  • Stretch activates a high conductance potassium channel in chondrocytes
  • TEA inhibits stretch induced hyperpolarizationin chondrocytes
  • Distribution of the BK channel (KCNMB1 and KCNMNB1) in cartilage
  • Putative role for BK channels in chondrocyte volume regulation
  • KATP Channels
    Channels potentially involved in glucose sensing
  • NH2
    NH2
    COOH
    COOH
    Kir6.2
    KATP = SURx + Kir6.x
    SUR
    Four Kir6.x subunits + four SUR subunits combine to form the functional channel
    KATP channel
  • Glucose Sensing in Pancreas
  • KATP Channels in Chondrocytes
    Chondrocytes are highly sensitive to variations in extracellular glucose levels in the extracellular matrix
    In other pancreas, heart and brain glucose sensing is partly mediated by KATP channels
    We have investigated whether chondrocytes too express functional KATP channels, which might serve to couple metabolic state with cell activity
  • Chondrocytes Express Functional ATP-sensitive Potassium Channels(KATP)
  • KATP Channel Sensitivity to Glibenclamide
  • Kir6.1 Expression in Chondrocytes
    Kir6.1 expressed in
    chondrocytes in the
    same isoform present
    in pancreatic β cells
  • Glucose Sensing in Chondrocytes
  • Acknowledgements
    Funding:
  • Acknowledgements