Fibrillar Proteins

Describe what the extracellular matrix is and the main components?

The “ground substance” in connective tissues between cells

Components:

• Collagens: provide the tensile strength of the tissue

• Elastic Fibers: provide the resilience of the tissue

• Proteoglycans: provide the resilience of the tissue

• Hyaluronan: provide the resilience of the tissue

• Glycoprotein: mediate adhesive interactions between other matrix molecules or between cells and matrix molecules

What synthesize the ECM? What does the ECM do? What are cell surface receptors and what do they do? What happens when the tissue is damaged?

• Resident cells

• provide an optimal environment for the cells.
  

• Cells bind to extracellular matrix molecules through cell surface receptors.

  • These receptors allow the cells to sense changes in ECM and respond accordingly.
    

• If tissue = damaged, primary response of cells = restore integrity of tissue by re-synthesizing ECM

When does the ECM get remodeled by cells? How are ECM remodeled? How are these molecules regulated?

Remodeling and Degradation when:

• tissue differentiation (matrix composition changes)

• cell migration (cells have to move through the matrix)

Remodeling Mechanism:

• the cells secrete matrix metalloproteinases (MMPs, Zn2+) that degrade the matrix proteins.

Regulation:

• Cells also secrete TIMPs (tissue inhibitors of metalloproteinases) that inhibits MMPs

extent of tissue remodeling dependent on ratio of MMPs and TIMPs

How do cancer cells invade surrounding tissues?

Cancer cells secrete large amounts of MMPs to invade the surrounding tissue.

What is collagen and describe its structure

• The main fibrillar proteins in connective tissues
  

• triple helical structure
  

• heavily cross-linked → very stable

What are the three major groups of collagen? Describe where they are?

• Fibril forming (I-III)

• Network forming (forms meshes) (IV)

• Fibril-associated (associates with Type I, II, III) (V, VI)

When is the propeptide of collagen removed extracellularly?

removed extracellularly by proteolytic digestion after three procollagen chains are assembled into a triple helical structure.

Describe Collagen’s chain

• 3 α chains (can be identical or different)

• The long helical part of the chains is composed of Gly- Proline - Hydroxypoline;

  • favor triple helix formation

Describe the synthesis of collagen

1. Synthesis of procollagen chains occurs in RER

   • Certain proline and lysine residues → hydroxyproline and hydroxylysine (co-translational modification!)

     • Some hydroxylysine residues are also glycosylated.

2. 3 pro-alpha chains assemble

   • Intra/Inter disulfide bonds form at C-term.

3. The triple helix forms in the rough ER

   • formation proceeds from C to N terminus.

     • held together by hydrogen bonds

     • Hydroxyproline stabilizes triple helix formation.

4. Propeptides are cleaved off after procollagen is secreted into the extracellular space.

   • enzyme = N and C procollagen peptidases

   • Then, collagen monomers assemble into fibrils.

How are collagen fibrils formed? Stabilized? How are they viewed under the electronic microscope

• formed from collagen’s triple helix units that associate in a staggered manner.
  

• stabilized by covalent crosslinking between collagen triple helices.
  

• staggered assembly produces characteristic striated pattern of collagen when viewed under electron microscope

What is the triple helix stabilized by? Describe the synthesis of this

• stabilized by hydrogen bonds and hydroxyproline.

• produced by prolyl hydroxylase.

  • requires Fe2+ and vitamin C (ascorbate) as cofactors.

What participates in the cross linking of collagen chains? How is this produced?

• Hydroxylysine

  • produced by lysyl hydroxylase

  • requires Fe2+ and vitamin C (ascorbate) as cofactors.

Describe how the covalen crosslinking within collagen fibers are created?

• formation of oxidized lysine and hydroxylysine

• Oxidation of lysine and hydroxylysine catalyzed by lysyl oxidase.

  • Cu2+ as cofactor

• Takes place in ECM

• The allysine (or hydroxyallysine) residue will react with
  lysine/hydroxylysine/allysine/hydroxyallysine residues to
  create covalent crosslinks between collagen chains

Describe the assembly of Type IV Collagen Network

• main basement membrane collagen

• In the extracellular space, two type IV triple helices attach at their C- terminal, forming a dimer

• Dimers associate through their N- terminal ends to form tetramers and eventually a large network

Describe the pathology of Scruvy and the symptoms

• Caused by lack of fruits and vegetables in the diet.
  

• Prolyl and lysyl hydroxylases are not efficient (require Vitamin C).
  

• The collagen triple helix is less stable and crosslinking is also reduced.
  

• General connective tissue disease.

• Symptoms

  • Bruises on skin (especially on legs)

  • Bleeding gums, loose teeth

  • Delayed wound healing

  • Bone and joint abnormalities (especially in infants)

Describe the pathology of Osteogenesis Imperfecta (OI) and the symptoms

Types I, II, III, IV = Type I collagen mutations.

• Type I : Reduced number of collagen fibrils

• Types II, III and IV OI: Defective collagen fibrils
  

Severity can range from perinatal lethal to mild predisposition to fractures

Characteristic symptoms:

• Increased incidence of fractures

• Short stature

• Grey or brown teeth that wear down easily (dentinogenesis imperfecta)

• Blue sclera

Describe the pathology of Achondrogenesis Type 2 and the symptoms

• Mutation in type II collagen

• Dwarfism, short stature

• Defective cartilage formation and bone ossification

• Death before birth or shortly after birth

Describe the pathology of Kyphoscoliotic Ehlers-Danlos Syndrome (EDS)and the symptoms; other types?

• Loss-of-function mutations in lysyl hydroxylase

  • progressive scoliosis (abnormal curving of the spine)

  • hyperextensive skin

  • delayed wound healing, easy bruising, thin scars

  • joint hypermobility

  • increased risk of vascular rupture

• Other types:

  • vascular type EDS: mutation in type III

  • classic type EDS: mutation in Type V collagen

Describe the pathology of Alport Syndrome and the symptoms

• mutations in Type IV collagen genes

• Affects glomerular basement membranes → renal failure

• X-linked or autosomal recessive

• Main symptoms

  • Hematuria (appearance of red blood cells in urine)

  • Proteinuria (increased protein in urine)

  • Renal insufficiency

  • Anterior lenticonus (conical shape of the lens)

  • Sensorineural hearing loss (mutation affects the basement membrane of the organ of Corti in the cochlea of the inner ear)

NOTE: Memorizing tech: “The AIRPORT (sounds like alport) was giving out KIDNEY BEANS (renal failure) in the BASEMENT (basement membrane)

Describe the pathology of Collagen Related Myopathies and the symptoms

• Type VI collagen connects the skeletal muscle basement
  membrane to the fibrillar collagen network.

• Mutations in type VI collagen lead to muscle weakness.

• Severity of the disease depends on the type of mutation.

What is Elastin? What does it allow? How is it synthesized? What happens in extracellular space? What does correct assembly require?

• major component of elastic fibers

• allows tissues to stretch and contract

• Widespread localization

• synthesized in the ER and secreted as monomers by the cells

• In the extracellular space, lysine residues are oxidized by lysyl oxidase, then form crosslinks between elastin monomers

• Correct assembly of elastic fibers requires microfibrils formed by fibrillins

What is Marfan Syndrome? Symptoms?

• Defective microfibril formation of elastic fibers

  • Via mutations in fibrillin-1

Skeletal

• Very tall stature,

• Disproportionally long limbs, fingers, and toes

• Hyperflexible joints

• kyphoscoliosis

Oculars

• Disclocation of the lens of the eyes (ectopia lentis)

Cardiovascular

• Dilation of the aorta

• Heart valve problem
  
  

Describe the receptors cells use to interact with the ECM

(function? Binds what? Why this is important?)

• integrins (αβ dimer)

  • anchors cells to ECM

  • bind to collagens and glycoproteins, such as fibronectin or laminin.

    • Integrin-laminin interactions important for formation of basement membranes.

What are laminins? Structure? Synthesis? Interaction?

• Basement membrane proteins (characteristic cross-shape)

• Heterotrimers (α, β and γ chains); stabilized by disulfide bond

• Multiple variants of the individual chains exist.

• synthesis: ER; assembly: Golgi

• Interact with basement membrane proteins and integrins (cell receptor)

What is Junctional Epidermolysis Bullosa caused by? What does it affect? and the symptoms?

• mutations in certain laminins and integrins.

• Affects basement membrane of epidermis and mucosal membranes.

• Main symptom:

  • fragile skin → extreme blistering

What Laminin 2 (Merosin)-Related Muscular Dystrophy caused by and the symptoms?

• Mutations in the α2 chain of laminin impair interaction between basement membrane and skeletal muscle cells.

• It leads to moderate to severe muscle weakness depending on type of mutation.