S2 W5 - Interactions b/w Cell + Environment

Extracellular matrix

Network that provides structural support to surrounding cells

Where are epithelial cells found

Epithelial cell sheets form epithelium; cover external surfaces, organs, line internal body cavities

What are the five different junctions of epithelial cells in order from the apical to basal side (top to bottom)

1. Tight junctions

2. Adherens junction

3. Desmosomes

4. Gap junctions

5. Hemidesmosomes

Which of the junctions are cell-cell anchoring

Adhereins junctions and desmosomes - link cytoskeletons of neighbouring cells

Which of the junctions are cell-ECM anchoring

Hemidesmosomes - link cytoskeleton to basal lamina

Tight junction function

1. Prevents apical and basal membrane protein mixing

   1. Recall:

      1. Apical: Na+ - glucose symp

      2. Basal: GLUT uniporter, Na+ K+ pump

2. Creates tight seal b/w cells

3. Forms sealing strands to prevent molecule movement

Tight junction structure

• Two transmembrane proteins: Claudin + Occludin

• Extracellular domain of one binds to extracellular domain of other

• Homophilic binding (Cla-Cla, Occ-Occ)

Anchoring junctions

Provide mechanical strength to epithelium, includes adherents, desmosomes, hemidesomosomes

What forms anchoring junctions

Adhesion proteins and linker proteins linked to the cytoskeleton

What are the two parts of transmembrane adhesion proteins

1. Extracellular domain

• interact w/ adhesion proteins of neighbouring cell side or ECM

  • adheren junctions, desmosomes, hemidesmosomes

2. Intracellular domain

• interact with linker proteins

Intracellular linker proteins

Proteins in cytosol that connect cytoskeleton and membrane proteins

Explain the general cell-cell anchoring junction pathway

Cell 1 cytoskeleton — linker protein — adhesion protein — adhesion protein of neighbouring cell — linker proteins — cell 2 cytoskeleton

Explain the general cell-ECM anchoring junction pathway

Cytoskeleton — linker protein — adhesion protein — ECM (basal lamina)

Anchoring junction protein specificity

Each anchoring junction has specific transmembrane adhesion proteins and intracellular linker proteins

What transmembrane adhesion proteins are involved at adherens junctions

Cadherens with hemophilic binding

What type of cytoskeletal filaments link to cadheren adhesion proteins through linker proteins

Actin filaments

What cytoskeletal filaments do desmosomes and hemidesmosomes link to

Intermediate filaments e.g. Keratin filaments

• Desmosomes: KF w/ neighbouring cells

• Hemidesmosomes: anchor KF to basal laminate

What transmembrane adhesion proteins are involved at desmosomes

Nonclassical cadherens proteins

• desmogelin, democollin

• thru homophilic and heterophilic binding

What transmembrane adhesion proteins are present at hemidesmosomes

Integrins

Gap function structure

Gap between cells with cytosolic channel running through

• 1 subunit = connexin

• 6 connexINS = connexON

• 2 connexons = channel

Function of gapjunctions

Couple cells tgt electrically and metabolically

• allow ions + metabolites to pass thru

  • not very selective (small; <1000 daltons = pass)

Characteristic of gap junctions

Are gated; can be open or closed thru intra/extracellular signals

What are some examples of gap junctions opening / closing

• Increase of cytosolic Ca2+ = close gap junction

• Membrane damage = Ca2+ leak in = close gap junction so neighbour cell doesn’t lose metabolites too

Do plant cells have cell junctions

No, cell wall holds cells together and provides mechanical strength

Plasmodesmata

Intercellular junctions that allow for communication between cells in plants by crossing cell wall

What allows communication between cells in plants

Cytoplasmic channels in plasmodesmata that create continuous PM and ER across plasmodesmata

What is allowed through the cytoplasmic channels of the plasmodesmata

• Small soluble molecules (< 1000 daltons)

• Larger soluble molecules are trafficked by gates, proteins

What is an example of gating of the plasmodesmata

Callose is polysaccharide in plant that controls what passes thru by closing off plasmodesmata

What are animal tissues composed of?

1. Epithelial tissue cells

2. Connective tissue cells

3. Basal lamina

Examples of epithelial tissue

Intestinal lining, skin etc.

Characteristic of epithelial tissue cells

• Closely attached to e/o

• thin basal lamina (limited ECM)

• Cytoskeletal filaments provide resistance to mechanical stress

Examples of connective tissue

Skin, bone, tendon, cartilage etc.

Characteristics of connective tissue cells

• rarely connected

• attached to matrix (plentiful ECM)

• ECM provides resistance to mechanical stress

What are the 3 major classes of macromolecules in the ECM

1. Glycosaminoglycans (GAGs) + proteoglycans

2. Fibrous proteins

3. Glycoproteins

Structure of GAGs in connective tissue ECM

• long, linear

• repeating disaccharide

• highly neg chg (attracts water)

What do GAGs form and why is it good

Forms hydrated gels, able to resist compression and fill space

What are proteoglycans (in connective tissue ECM)

• subclass of glycoprotein

• has one sugar side chain that is GAG

Collagen within connective tissue ECM

Fibrous protein that provides tensile strength and resists stretching

What is an example of a typical collagen found in connective tissue ECM

Fibril-forming collagen

—> 3 chains = triple helix = assemble into collagen fibril = pack into collagen fibre

What form is collagen secreted as?

Procollagen

Where is procollagen secreted from

Fibroblasts (skin, tendon, other connective tissue)

Osteoblasts (bone)

What binds to collagen in the ECM to organize?

Fibronectin and integral

Explain the binding of Fibronectin, integral, and collagen

Fibronectin binds to collagen and integrin — Integrin binds to adaptor proteins — Adaptor proteins connect to actin filaments

Function of elastin in connective tissue ECM

Give tissue elasticity, resilience, allow it to stretch

Function of basal lamina of epithelial tissue ECM

• Separates epithelia from tissue underneath

  • Prevents fibroblasts from connective tissue from interacting with epithelial cells

  • allows passage of macrophages / lymphocytes

Characteristics of basal laminate of epithelia tissue ECM

• Specialized type of ECM

• Thin

• Influences polarity

What contributes to basal laminate linkage

• Where epithelia attatch

• Basal laminate anchored by hemidesmosomes

• Organized by laminin

Function of laminin

(blue) glycoprotein that organizes basal lamina

• Links integrin (green) to type IV collagen (red)

What transmembrane adhesion proteins are involved at hemidesmosomes

Integrins bind laminin (glycoprotein that organizes basal laminate) to the basal lamina

Main components of the plant cell wall

Cellulose, pectin

Function of cellulose in cell wall

Provide tensile strength

Function of pectin in cell wall

Fills space to provide resistance to compression

Where are cell wall components synthesized

• Plasma membrane (e.g. cellulose synthase complex)

• Golgi apparatus —> exported by exocytosis