LECTURE NOTES ON CONNECTIVE TISSUE PROTEINS
AND DISORDERS
BY
DR UMAR AMINU ABDULLAHI
DEPARTMENT OF CHEMICAL PATHOLOGY AND
IMMUNOLOGY,
COLLEGE OF HEALTH SCIENSES,
USUMANU DANFODIYO UNIVERSITY SOKOTO.
INTRODUCTION
 Types of tissues:
 Nervous tissue: Communication
 Epithelial: Boundaries/barriers
 Connective tissue: Binding and hold things together
 Muscular tissue: Movements
Connective tissue: Tissue that supports, protects, binds/connects,
transports, anchored, and provide immunity to cells, tissues and organs
(cell & ECM).
 Underlined features of connective tissue (CT) structural units
 Cells: Mesenchymal, macrophages, adipocytes, fibroblast etc.
INTRODUCTION CONT’D
 Gels (ground substances): viscous, transparent, varies from fluid to gel that fills
the space between cells and fibres in which CT are embedded.
 Consist of:
o Glycoconjugates (polysaccharides)
• Glycosaminoglycans (GAGs)
• Proteoglycans
• Structural (adhesive) glycoproteins
o Tissue fluid
Fibers: Long, thin extracellular protein polymers secreted by cells in CT
 Collagen
 Elastic
 Reticular
 Classification of CT:
 CT proper: dense or loose
 Dense: regular, irregular and elastic
 Loose: areolar, reticular & adipose
 Supportive: bone & cartilage
 Cartilage: hyaline, elastic & fibrocartilage
 Specialized: adipose tissue, elastic, hematopoietic & mucous tissue.
EXTRACELLULAR MATRIX (ECM)
Extracellular matrix: a dynamic three-dimensional macromolecular
network that provides stability, signaling and structural support to the
organs.
 Cells are embedded in a jelly of proteins and polysaccharides called the
extracellular matrix or the ground substance
 In epithelial tissues, cells are tightly attached to form sheets; ECM is
scanty and forms the basal lamina
 The basal lamina forms a supporting layer underlying the epithelia and
helps prevent the cells from ripping apart
 In connective tissues ECM forms a larger space and carries the
mechanical stress to which the tissue is subjected
 The ECM serves as a reservoir for many extracellular signalling molecules
that control cell growth and differentiation
 In addition, it provides a network through or on which cells can move
during tissue assembly
ECM CONT’D
 Structure: water, proteins & carbohydrates.
 Functions:
 Cell support within a tissue
 Regulation of intercellular adhesion and communication
 Control of cell migration
 ECM components:
 Proteoglycan
 Fibrous structural proteins: collagen & elastin
 Fibrous adhesive proteins: fibronectin & Laminin
ECM CONT’D
 The cells in animal tissues are “glued” together by cell adhesion
molecules (CAMs), proteins embedded in their surface membranes
 CAMs mediate cell-cell and cell-ECM interactions
 CAMs fall into four major families: the cadherins, immunoglobulin (Ig)
superfamily, integrins and selectins
 CAMs mediate, through their extracellular domains, adhesive
interactions between cells of the same type (homotypic adhesion) or
between cells of different types (heterotypic adhesion)
 A CAM on one cell can directly bind to the same kind of CAM on an
adjacent cell (homophilic binding) or to a different class of CAM
(heterophilic binding)
 Clustered CAMs form cell junctions
 The cytosolic domains of CAMs form connections with cytoskeletal
elements
GLYCOSAMINOGLYCAN (GAGs)
 Glycosaminoglycan (GAGs): a long polysaccharide, highly acidic, negatively
charged & unbranched linear polysaccharide. GAGs are located primarily
on the surface of cells or ECM but are also found in secretory vesicles in
some types of cells. Usually attached to protein to form proteoglycan
except in hyaluronan.
 Structurally is made of repeating disaccharide units, the first sugar is Nacetyl glucosamine or N-acetyl galactosamine, and the second sugar is
uronic acid.
 GAGs are degraded by lysosomal enzymes: glycosidases and sulfatases
 Various degrees of deficiency of these enzymes leads to different types
of mucopolysaccharidoses
Rare diseases inherited in an autosomal recessive manner
•e.g. Hurler Syndrome - deficiency of α-L- iduronidase
•
Hunter Syndrome - iduronate sulfatase
Classification
oHyaluronic acid (HA): synovial fluid, vitreous humour, ECM of loose CT,
allows migration of cells during morphogenesis and wound healing.
oChondroitin sulfate & dermatan sulfate: cartilage, bone, and heart valves,
along with HA give compressibility to the bone and cartilage.
oHeparan sulfate and heparin: basement membranes, A hypersulfated
form of heparan sulfate called heparin, produced mostly by mast cells,
plays a key role in allergic reactions.
oKeratan sulfate: cornea, bone, cartilage aggregated with chondroitin
sulfates.
PROTEOGLYCAN
 Proteoglycan: are the products of the covalent linkage of core
proteins with glycosaminoglycans (GAG). a group of glycoproteins
that cushion cells and bind many extracellular molecules
 Highly hydrated
 They contain more carbohydrates than glycoproteins
 the sugars are usually sulfated
 A proteoglycan = 100 proteoglycan monomers
PROTEOGLYCAN CONT’D
 The glycosylation of core proteins begins in the lumen of the
endoplasmic reticulum and continues in the Golgi
 A link trisaccharide containing xylose and two galactose residues can be
the first to attach through O-glycosidic bond with a Serine of the core
proteins and then the other sugars are added
 Or the modified sugars could be directly be attached through Nglycosidic bond with Asparagine residue of the core proteins
 Once chain formation is over, sulfotransferases add sulfate which is
obtained from (PAPS)
• The abundance of negative charges increases polarity
• The proteoglycans bind large amounts of water and fill the gaps
between the other components of the ECM in the form of a
hydrated gel
 Epimerization of glucoronate to iduronate also takes place after the
chain is synthesized
COLLAGEN
Collagen: Are the main insoluble fibrous protein of the connective tissue
accounts for around 25-30 % of the total protein content of the body.
 Major component in ECM
 Found in connective tissue cells of all multicellular animals
Classification
oType I Fibrillar collagen: Found in skin, tendons, bones, most tissues, etc.
oType II Fibrillar collagen: Found in cartilage, vitreous humor, cornea,
intervertebral disc
oType III Fibrillar collagen: skin, muscle, soft tissues, etc., frequently occurs
together with type I
oType IV Non Fibrillar: a sheet-like meshwork, a major part of basal lamina
that assembles into a multi-layered network (All basement membrane)
oType V Fibrillar: Most interstitial tissues associated with type I
oType VI: Most interstitial tissues associated with type I
oType VII: Network forming collagen beneath stratified squamous
epithelia.
oType IX: Fibril-associated, cartilage,
oType X: Hypertrophic cartilage
oType XII: Fibril-associated, tendon, ligaments, some other tissues.
oType XVII: Hemi desmosome
oType XIX: Rhabdomyosarcoma
 All collagens are fibrous proteins made from three polypeptides called
collagen α chains
 Unlike the α helix secondary structure of many proteins, the structure of
collagen is a left-handed helix
 The three chains wrap around each other to form a right-handed triple
helix
 The collagen triple helix can form because of an unusual abundance of
three amino acids: glycine, proline and hydroxy proline
They make up the characteristic repeating pattern Gly-X-Y, where X
and Y can be any amino acid but are often proline and hydroxy
proline and less often lysine and hydroxy lysine
 Glycine is essential because its small side chain, a hydrogen atom, is the
only one that can fit into the crowded center of the three stranded helix
 Although the rigid peptidylproline and peptidyl -hydroxyproline linkages
are not compatible with formation of a classic single-stranded helix, they
stabilize the distinctive three-stranded collagen helix
The hydroxyl group of hydroxyproline (by forming hydrogen
bonding with members of other chains) helps hold its ring in a
conformation that stabilizes the triple helix
 The triple helix of collagen IV is interrupted at several points by nonhelical segments that give flexibility to the structure
 Type IV collagen is a sheet/network forming type
• Triple helices associate with each other through the globular
domains at the C and N terminals
COLLAGEN BIOSYNTHESIS
 Collagen is first synthesized as preprocollagen which contains a signal
sequence
 In the lumen of the endoplasmic reticulum, preprocollagen is modified:
removal of the signal sequence, hydroxylation of proline and lysine
residues and glycosylation of some hydroxylysine residues
 preprocollagen is changed to procollagen
 In the Golgi apparatus procollagen is associated into a triple helix
(initiated through disulfide bonds between C-terminal residues) and
released to the extracellular space
 Extracellular peptidases remove the N- and C-terminal propeptides to
give tropocollagen
 Tropocollagens associate laterally to generate fibrils
 In fibrils, adjacent collagen molecules are displaced from one another by
about one-quarter of their length
Striation is observed in electron micrographs
 The fibrils are strengthened through covalent cross-links
 Lysine and hydroxylysine side chains are modified by extracellular lysyl
oxidases to form aldehydes (allysine and hydoxyallysine) in place of the
amine group at the end of the side chain
 The aldehydes form covalent crosslinks with lysine, hydroxylysine and
histidine residues in adjacent molecules
 The cross-links stabilize the side-by-side packing of collagen molecules
and generate a strong fibril
 Removal of the propeptides and covalent cross-linking take place in the
extracellular space to prevent the potentially catastrophic assembly of
fibrils within the cell
 Type I, II and III collagen fibers are characteristic of the ECM of the (skin,
bone and tendons); cartilage and arteries, respectively
DISORDERS OF COLLAGEN SYNTHESIS
Type I collagen deficient (Bone) : Osteogenesis imperfecta – “brittle bone
syndrome”
 Heterogeneous group of inherited disorders distinguished by bones that easily
bend and fracture
 Abnormally Short chains make up the collagen I triple helix ; or other amino
acids may take the place of glycine, the structurally abnormal pro – α- chains
→ prevent folding of the protein into a triple-helical conformation C/F Blue sclera
o Types
• Type I OI is called Osteogenesis imperfecta tarda: present in early infancy,
fractures secondary to minor trauma.
• Type II OI is called Osteogenesis imperfecta congenita: more severe, patients
die in utero or in the neonatal period of pulmonary hypoplasia, most patients
with severe OI have mutations in the gene for either the pro 1- or pro 2- αchain of Type I collagen.
 Type II (cartilage): Chondrodysplasia
 Type III (major blood vessels): Ehlers-Danlos syndrome (Rubber man syndrome): is a
heterogeneous group of CT disorders
 lack of peptidases; resistance to peptidases; insufficient synthesis of collagen; rapid
degradation;…
 recurrent dislocation of joints (hypermobility), hyper extensible skin.
 NB: All types of collagen are defective (type III most commonly).
o Types of EDS
• Classica EDS: defect in pro α1 or pro α2 chains of collagen Type V in many, but not all, families;
autosomal dominant
• Hypermobility EDS: defects unknown; autosomal dominant
• Vascular EDS: structural defects in pro α1 chain of collagen Type III encoded by the COL3A1
gene; autosomal dominant.
• Kyphoscoliosis: deficiency of lysyl hydroxylase, a collagen modifying enzyme; autosomal
recessive
• Arthrochalasia: deficient processing of N-terminal end of pro α1 or pro α2 chains of collagen
type I ; autosomal dominant
• Dermatosporaxis: deficiency of procollagen I N-terminal peptidase; autosomal recessive
 Type IV collagen (basement membrane): Alport’s syndrome
 C/F haematuria, sensory neural hearing loss (SNHL), anterior lenticornus
(excessive curvature of cornea)
 Autoimmune collagen disorders
 Good pasture syndrome: Auto-antibodies against basement membrane.
o C/F: haematuria, haemoptysis
 Bullo-vesical disorders
 Bullous Pemphigus: auto-ab against hemi desmosome
 Pemphigus Vulgaris: auto-ab against desmosome + acantolysis
 Menke’s disease (deficiency of lysyl oxidase)
 Other abnormalities in collagen
 Homocystinuria
• Accumulated homocysteine reacts with lysyl aldehydes to block cross linking
• Skeletal deformities, vascular and ocular defects
 Deficiency of Ascorbic acid
• Defective hydroxylation of collagen
• Poor wound healing and increase fragility of blood vessels
 Lathyrism
• Ingestion of lathyrus sativa
• Contain toxic agent beta oxalyl amino alanine that inhibit lysyl oxidase
 Certain arterial aneurysms (collagen III)
 Ulrich muscular dystrophy (collagen VI)
 Certain chondrodysplasia (collagen IX and XI)
 Kniest dysplasia (collagen II)
FIBRONECTIN
 Is a large molecular weight (about 440,000 Da) adhesive dimeric glycoprotein
found in body fluids and connective tissues, one of the major component of ECM.
 Synthesized by a variety of cells including liver hepatocytes, lung fibroblasts,
endothelial cells, macrophages and type 2 alveolar epithelial cells.
 The protein is composed of similar but not identical polypeptides
 Distributed widely in human body, found on cell surfaces, extracellular matrix,
and basal laminae.
 Fibronectins are dimers of two similar polypeptides linked at their C-termini
by two disulfide bonds
 Each chain comprises six functional regions with different ligand-binding
specificities
 Fibrin binding domains, Heparin binding domains, RGD sequences, Cell
surface receptor binding domains, Collagen binding domains, Heparin and
Fibrin binding domains.
 Integrins are usually bound by repeats of Arg-Gly-Asp (RGD) sequences of
fibronectins
 Fibronectin has a wide variety of biological functions including cell migration, cell
differentiation and apoptosis, these biological activities are mediated through
interaction with various members of integrin family and cell surface
proteoglycans.
 Types of Fibronectin
 Circulating plasma fibronectin (soluble protomeric form): synthesized by
hepatocytes.
o Most of fibronectin in Adults is plasma fibronectin.
o A soluble dimeric form, found in the blood and other body fluids, enhancing
blood clotting, wound healing and phagocytosis.
o Good indicator of pathological conditions associated with injury of the RES,
radiation-induced lung injury, and in patients with Broncho-pulmonary carcinoma
o Tumour metastasis, thereby making circulating fibronectin or its isoforms
potential Tumour markers, similarly, increased plasma concentrations of
fibronectin is associated with various carcinomas e.g. breast, colonic, lung and
ovarian carcinomas.
 Locally synthesized cellular fibronectin (insoluble multimeric form)
o Synthesized by several cells including epithelial and connective tissue
cells and is deposited into the extra cellular matrix.
oLocated on the cell surfaces it helps cell-cell attachment
oDerived from endothelium which play an important role in
assembling of sub endothelial matrix
oPlays key roles in various cellular events that include cell migration
and proliferation of fibroblasts and endothelial cells
o Physiologically, plays an important role in cell adhesion, motility and
tissue repair.