Biochemistry of Bone and Cartilage PDF

Biochemistry of Bone and Cartilage Prof.Dr. Gülden Burçak 2023-2024 1 Bone  A mineralized connective tissue a dynamic structure that undergoes continuous remodeling cycles consisting of resorption followed by deposition of new bone tissue  Type I collagen is the major protein  Type V collagen an...

Biochemistry of Bone and Cartilage Prof.Dr. Gülden Burçak 2023-2024 1 Bone  A mineralized connective tissue a dynamic structure that undergoes continuous remodeling cycles consisting of resorption followed by deposition of new bone tissue  Type I collagen is the major protein  Type V collagen and a number of noncollageneous proteins  Bone-specific proteins: Chondroitin sulfate proteoglycan, osteocalcin and bone sialoprotein  Crystalline hydroxyapatite: Ca10(PO4)6(OH)2 confers on bone the strength and resilience along with Na+, Mg2+, CO32¯, and F¯ 2 Noncollagenous Proteins of Bone  These proteins account for only 10 -15% of bone protein mass but as they are very small molecules on a molar basis they are as common as collagen in bone  Most are synthesized by osteoblasts, but a number arrive in bone via the circulation (albumin and PDGF being usually negatively charged are bound to hydroxyapatite)  A dermatan sulphate proteoglycan (versican) may act with hyaluronan in the early stages of bone formation to delineate areas that are destined to become bone  Chondroitin sulfate proteoglycans (decorin and biglycan) are expressed in the newly forming bone and may influence cellular proliferation and differentiation 3  Matrix Gla-protein, bone Gla-protein (osteocalcin) and protein S are proteins modified by the post-translational γ-carboxylation, a process that enhances Ca binding  The major glycoproteins are alkaline phosphatase (ALP) and osteonectin (a phosphorylated glycoprotein) which may regulate both cellular activity and matrix mineralization Osteonectin is also present in several other tissues that undergo rapid remodelling  The most abundant of the bone-derived non-collagenous proteins are osteocalcin and osteonectin, both having a high affinity for bone mineral 4  Some non-collagenous glycoproteins that contain the RGD tripeptide may serve to promote the attachment of bone cells to bone matrix and may regulate their function Some are specific to bone, e.g., bone sialoprotein Most are more ubiquitously expressed, e.g., fibronectin, osteopontin, thrombospondins, vitronectin 5 Bone Remodeling  Bone is a dynamic structure that undergoes resorption followed by deposition of new bone tissue: Remodeling  ~ 4% of compact bone and 20% of trabecular bone is renewed annually in the typical healthy adult  Both physical and hormonal signals (PTH, calcitriol, calcitonin, corticosteroids, estrogens) affect remodeling  During each remodeling cycle, osteoblasts are formed from undifferentiated mesenchymal cells and pass through the phases of proliferation, matrix maturation and mineralization 6 Calcitropic Hormones and Calcium Homeostasis 7 Osteoblasts  Mononuclear cells derived from pluripotent mesenchymal progenitor cells  Responsible for the synthesis and maintenance of bone matrix (osteoid) and its subsequent mineralization  Have high levels of ALP activity necessary for mineralization  Synthesize various growth factors and cytokines  Have receptors for PTH and 1,25(OH)2D, a number of cytokines, growth factors and sex hormones  Play a key role in regulating osteoclastic bone resorption via the RANK – RANKL – OPG system RANK (receptor activator of nuclear factor-κB) is expressed in osteoclasts RANKL (ligand of RANK) and OPG (osteoprotegerin) are expressed in osteoblasts 8 Alkaline phosphatase (ALP)  ALP is an enzyme in the cell membrane (ectoenzyme) that catalyzes the hydrolysis of monophosphates under alkaline conditions (pH 8 to 10)  ALP of liver, bone and kidney are isoforms of the same gene product, they differ in their carbohydrate side chains the tissue-nonspecific gene (TNALP)  ALP generates Pi by using nucleoside triphosphates and PPi as substrates other major sources of Pi, such as intestinal absorption, contribute to the bulk of the phosphate needed for hydroxyapatite deposition 13  Small matrix vesicles containing Ca2+ and phosphate are formed by budding from the osteoblast membrane mineralization begins in the gaps between successive collagen (type I) molecules  Acidic phosphoproteins (bone sialoprotein and osteopontin) may provide an initial scaffold for mineralization  PPi inhibits the formation of hydroxyapatite; Pi/PPi is critical for mineralization and the main function of ALP appears to be the hydrolsis of PPi  Certain proteoglycans and glycoproteins inhibit nucleation 10 Osteocytes play a key role in regulation of bone remodelling  Osteocytes and bone lining cells are terminally differentiated osteoblasts that are no longer involved in active bone formation Found in mature bone, very long-lived, with a half life of 25 years, involved in the maintenance of the bone matrix  At sites of skeletal microdamage, osteocytes undergo apoptosis and release apoptotic bodies which express RANKL  RANKL promotes the recruitment of osteoclasts to the area to initiate remodelling to allow repair of the damaged bone  Osteocytes produce sclerostin, an endogenous inhibitor of osteoblast differentiation and function  Osteocytes produce the phosphaturic hormone fibroblast growth factor 23 (FGF23) which plays a role in regulation of phosphate metabolism Osteoclasts  Terminally differentiated cells that arise from hematopoietic precursors in the monocyte lineage and do not divide  Multinucleated giant cells that are specialized for resorption of bone, have large numbers of mitochondria and lysosomes  Osteoclast development and function are regulated by osteoblast and osteocytederived cytokines, OPG and RANKL  Macrophage colony stimulating factor-1 (M-CSF) secreted from osteoblasts potentiates the effects of RANKL  RANK is expressed on the surface of osteoclast precursors  RANKL and RANK interaction promotes osteoclastogenesis Osteoclast function is regulated mainly by the RANK-RANKL-OPG system NF-κB is a transcription factor that regulates the expression of the genes encoding many cytokines, chemokines, growth factors, and cell adhesion molecules RANK ligands are NF-κB activators which are proinflammatory cytokines,bacteria and viruses, reactive oxygen species and mitogens 13  RANKL shed from the osteoblast surface (soluble form of the protein) may confer osteoclastogenic activity distant from the cell of origin, in particular the osteocyte  Osteoprotegerin acts as a decoy receptor for RANKL, and functions as an endogenous inhibitor of osteoclastogenesis  The relative levels of expression of RANKL and OPG determine the rate at which osteoclastogenesis occurs  The function of osteoclasts is increased by PTH, 1,25(OH)2D, PGE2 and IL-11 and decreased by sex hormones the production of RANKL and/or OPG are changed  Calcitonin acts on osteoclasts and directly inhibits bone resorption 14 Osteoclast mediated bone resorption  Multinucleated cells recruited by the action of CSF-1 and activated by RANKL attach to the bone surface via integrin- mediated binding to bone matrix proteins  When enough binding has occured, the osteoclast is anchored, a sealed space is formed and a ruffled border is created  Acid and enzymes are secreted into the sealed space between the ruffled border and the bone surface, the resorption area  In the acid environment (pH ≤ 4.0) the solubility of hydroxyapatite and its breakdown increases tartrate-resistant acid phosphatase (TRACP) and proteolytic enzymes (lysosomal acid proteases, cathepsins and matrix metalloproteinases) are activated 15 Schematic illustration of the role of the osteoclast in bone Carbonic anhydrase functions in the production of H+s and H+K+ATPase in the pumping of H+s into the resorption area 16 Metabolic bone diseases  Partial uncoupling or imbalance between bone resorption and formation result in metabolic bone diseases  Decreased bone mass (osteopenia) is more common than abnormal increases of bone mass Rickets and osteomalacia: A mineralization defect usually due to vitamin D deficiency resulting in an increase in osteoid (unmineralized organic matrix of bone) Osteoporosis: A pathophysiological process that impairs osteoblastic bone formation or increases bone resorption Renal osteodystrophy: Disorders of bone and mineral metabolism associated with chronic kidney disease 17 Cartilage  The collagen forms a network of fine fibrils that give shape to the cartilage hyaline cartilage (the major type) has type II collagen elastic cartilage has elastin fibroelastic cartilage has type I collagen  The proteoglycans are responsible for compressibility and deformability  The chondrocytes, embedded in the matrix produce and degrade collagen and the proteoglycans  Cartilage is an avascular tissue and obtains most of its nutrients from synovial fluid 18 Aggrecan The major, most complex proteoglycan, ~ 2 × 103 kDa contains hyaluronic acid, chondroitin sulfate and keratan sulfate, link and core proteins contributes to the development, tensile strength and resilience of cartilage 19  Cartilage exhibits slow but continuous turnover  TGF-β and IGF-I generally exert an anabolic effect on cartilage  Various proteases (collagenases and stromelysin) are catabolic  An inflammatory response causes a loss in cartilage organization; IL-1 and TNF-α stimulate the production of proteases  An autoimmune attack on articular proteins alters the balance between cartilage degradation and formation  Achondroplasia: Replacement of Gly by Arg in FGFR3 causes the defective functioning of FGFR3; abnormal development and growth of cartilage leading to short-limbed dwarfism 20