Calcium Metabolism: Regulation and Pathology

• Physiology and Biochemistry of Calcium Metabolism
• Year number: 2
• Week number: 5
• Week name: Back pain from a minor fall

Learning Objectives

• Describe the regulation of serum calcium concentrations including:
  • Effects of regulatory hormones on the GI tract
  • Effects of regulatory hormones on the kidney and bone
  • Parathyroid gland's role in the distribution of calcium and phosphorus in the body
  • Actions of PTH
  • Role of vitamin D in calcium homeostasis
  • Role of calcitonin in calcium homeostasis
• Discuss examples and the underlying mechanisms of pathologies like bone marrow disease, bony invasion, and parathyroid disease that can result in hyper and hypocalcaemia.

Calcium Storage

• 99% of calcium is stored in the bones and teeth (skeleton storage bank with hydroxyapatite) - Ca10(PO4)6(OH)2
• 1% is found in the blood

Calcium Needs

• Bone structure
• Neuronal communication
  • Calcium is implicated to play roles in synaptic plasticity, memory, and neurotransmitter release
• Muscle contraction
• Blood clotting:
  • Activates different factors (e.g., prothrombinase complex (Factors Xa and Va))
  • Converts prothrombin to thrombin
• Cellular functions: exocytosis

Bone Remodelling

• Bones are continuously remodelled through mineral deposition (osteoblasts) and resorption (osteoclasts)
• Osteoblasts build bone using hydroxyapatite crystals (calcium & phosphate)
• Osteoclasts degrade (corrode) bone

Stem Call Lineage

• Stem cells differentiate into osteoblasts or macrophages
• Osteoblasts give rise to: lining cells and osteocytes
• Macrophages give rise to: osteoclasts

Osteoblast Development

• RUNX2 is a key transcription factor associated with osteoblast differentiation

Osteoclast Differentiation Regulation

• Osteoclasts are regulated by osteoblasts
  • RANKL binds to the RANK receptor on osteoclast precursors, inducing osteoclast formation
  • OPG inhibits osteoclastogenesis
• Both M-CSF and RANKL molecules are necessary for osteoclastogenesis and are widely involved in the differentiation of monocyte/macrophage derived cells

Osteoblasts

• Synthesize soft organic matter of bone matrix like collagen (osteoid)
• Lay down collagen fibres
• Fibres harden after being encrusted with calcium and phosphate minerals forming hydroxyapatite crystals

Osteoclasts

• Facilitate mineral resorption by dissolving the crystals and releasing the minerals into the blood
• Use pumping H+ out of the cell along with Cl- ions followed by electrical attraction (Cl-/HCO3- transporter)
• HCl dissolves bone minerals
• Release acid phosphatase and other proteases that digest collagen

Bone Cell Types

• Osteoprogenitor cells (progenitor, ancestor)
  • Stem cells that differentiate into osteoblasts
• Osteoblasts
  • Bone-forming cells (osteogenesis or ossification)
  • Stress and fractures stimulate stem cells to rapidly multiply numbers of osteoblasts
  • Secrete biochemical signals that may regulate bone remodelling, i.e. adjustments to bone shape and density to adapt to stress
• Osteocytes
  • Multiple functions
  • Some osteocytes reabsorb (dissolve) bone matrix while others deposit it (rebuild)
  • Thought to be mechanosensor cells (strain sensors) that control the activity of osteoblasts and osteoclasts within a basic multicellular unit (BMU), a temporary anatomic structure where bone remodelling occurs
• Osteoclasts
  • Giant cells
  • Bone-dissolving cells found on the bone surface
  • Formed by the fusion of stem cells and can contain 50+ nuclei

Bone Remodeling Phases

• Resting stage
• Bone resorption
• Reversal
• Bone formation
• Transition
• Mineralization

Bone Matrix Composition

• Collagen fibres
  • Account for ~1/3 bone weight (organic matter)
  • Provide flexibility
• Calcium phosphate (Ca3(PO4)2)
  • Accounts for ~2/3 bone weight (inorganic matter)
  • Interacts with calcium hydroxide (Ca(OH)2) to form crystals of hydroxyapatite (Ca10 (PO4)6(OH)2) salts
    • Incorporates other salts (calcium carbonate, CaCO3) and ions (Na+, Mg2+, F-)
  • Provides strength

Bone Mineralization

• Organic matter (Osteoid)
  • Synthesized by osteoblasts
  • Collagen, carbohydrate-protein complexes (glycosaminoglycans, proteoglycans, and glycoproteins)
• Inorganic matter
  • 85% hydroxyapatite (crystallized calcium phosphate salt) [Ca10(PO4)6(OH)2]
  • 10% calcium carbonate
  • Other minerals (fluoride, sodium, potassium, and magnesium)
• Role of certain proteins: help regulate ordered deposition of minerals by regulating the amount and size of hydroxyapatite crystals
  • Osteocalcin protein
    • Secreted by osteoblasts
    • Attaches to hydroxyapatite and binds calcium to bone allowing bone to be built in the right pattern which gives bone its strength
  • Oseteonectin protein
    • Secreted by osteoblasts during bone formation
    • Initiates mineralization and promotes mineral crystal formation
    • High affinity for collagen and calcium
  • Osteopontin (bone sialoprotein I BSP-1)
  • Secreted by osteoblasts, pre-osteoblasts, osteocytes, cardiac fibroblasts, and myoblasts
  • Synthesis is stimulated by calcitriol (Vitamin D)
  • High affinity for calcium
  • Forms an integral part of the mineralized matrix and regulates crystal growth
  • Contributes to bone remodeling by promoting osteoclastogenesis and osteoclast activity through CD44- and av3-mediated cell signalling.
  • OPN serves to initiate the process by which osteoclasts develop their riffled borders to begin bone resorption- OPN contains and RGD integrin-binding motif
  • Osteopontin takes a role in bone tissue destruction by two basic mechanisms
  • Providing identification and adherence of osteoclastic cells by integrin av3-mediated OPN
  • Modulation of osteoclastic function via integrin av3

Demineralization of the Clavicle

• Bone: 50-70% minerals, 20-40% organic matrix, 5-10% water
• Dry weight matrix: 25% organic, 75% inorganic
  • Organic component resists tension
  • Inorganic component resists compression
• Removal of inorganic component makes bone flexible
• Removal of organic component makes bone brittle

Rickets

• Soft bones due to deficiency of calcium salts (Vitamin D deficiency)

Osteogenesis Imperfecta

• Brittle bone disease
• Genetic
• Excessively brittle bones due to lack of protein, collagen
• Mutations in COL1A1 and COL1A2 genes provide instructions for making type I collagen

Regulation

• Calcium homeostasis is regulated by calcitriol, calcitonin, and parathyroid hormone (PTH)
• Plasma Calcium: 2.2 - 2.6 mmol/l
• Ionised Calcium: 1.1 - 1.4 mmol/l

Calcitriol (Activated Vitamin D)

• Results in bone resorption in Kidney
• Results in the reduced excretion of Ca2+
• Results in the absorption of Ca2+ and phosphate

Parathyroid Hormone

• Secreted by the parathyroid gland on the posterior aspect of the thyroid gland in the neck
  • Humans have 4 parathyroid glands
• Released when calcium levels decrease
  • 1% drop in blood calcium levels doubles the secretion of PTH
• Stimulates osteoclast activity & inhibits osteoblast activity

Correction for Hypocalcemia (Parathyroid Hormone Dependent)

• Increased osteoclast activity leads to more bone resorption
• Reduced osteoblast activity leads to less bone deposition
• More urinary phosphate excretion
• Prevention of hydroxyapatite formation
• Less urinary calcium excretion & conservation of calcium

What increases blood Calcium

• Low calcium levels in plasma with parathyroid glands secreting parathyroid hormone (PTH)
• PTH stimulates osteoclasts to accelerate their erosion of bone matrix & release stored calcium ions into the body
• PTH enhances the calcium absorbing effects of calcitriol in the intestines
• Kidneys increasing renal production of calcitriol stimulates reabsorption in the kidney & intestines

Calcitonin

• Secreted by C cells in the thyroid gland
• Released when calcium levels increase
• Inhibits osteoclast activity and stimulates osteoblast activity

Correction for Hypercalcemia (Calcitonin Dependent)

• Reduced osteoclast activity & less bone resorption
• Increased osteoblast activity & more bone deposition

What decreases blood calcium?

• High calcium ion levels in plasma with C cells secreting calcitonin
• Calcitonin inhibits osteoclasts, deposit calcium ions within matrix of bones
• Decreasing PTH or calcitriol, there will be a decrease in Calcium ions in the intestines.
• Calcitonin inhibits excretory effect on the kidneys & suppressed Calcium ions reabsorption.

Hypercalcemia

• Excessive amounts of Ca2+ bind to the cell surface, increasing the charge difference across the cell membrane, which makes the Na+ channels less responsive
• Calcium blocks sodium channels and inhibits depolarization of nerve and muscle fibres,
• Nerve and muscle cells less responsive and excitable
• depression of the nervous system, emotional disturbances, muscle weakness, sluggish reflexes, sometimes cardiac arrest, at levels greater than 12 mg/dL

Hypercalcemia Causes

• Overactive parathyroid gland- small noncancerous tumors on one or more of the four parathyroid glands
• Bone cancer- metastases can increase osteoclast activity
• Multiple Myeloma increase osteoclast activity
• Other diseases such as tuberculosis and sarcoidosis that may raise levels of Vitamins D
• Digestive tract absorbing too much calcium

Hypocalcemia

• Leads to increased excitability in the nervous system
• Decreasing the charge differences across the cell membrane makes Na+ channels more responsive
• Results in muscle tremors, spasms, or cramps (tetany)
• Muscle of the larynx contracts tightly (laryngospasm), shutting airflow and cause suffocation, when Calcium plasma concentrations fall to 4 mg/dL or 1 mmol/l

Hypocalcemia Causes

• Vitamin D deficiency
• Diarrhea
• Thyroid Tumors
  • May accidentally remove parathyroid gland
• Pregnancy & lactation
  • Calcium is demanded by ossification of Fetal Skeleton & synthesis of Milk.
• Underactive Parathyroid Glands
• Treatment: IV/oral calcium supplements, & increase intake of Vitamins D Supplements.

Hypoparathyroidism

• Commonly caused by accidental damage during thyroid surgery
• Autoimmune disease of PTH
• Extensive Cancers radiations that affect radiation functions
• Low level of Magnesium prevents PTH releases
• Rapid decline leads to rapid calcium blood level decline, requiring therapies
• A fatal tetany occurs in 3-4 days

Hyperparathyroidism

• Typically caused by PTH primary secretions
• Gut failing and calcium deficiency leads to secondary hyperparathyroidism
• Secondary Vitamin D deficiency
• As the Kidney doesn't convert vitamins, this makes Bones deformed or Fragile, leading to more blood levels of calcium-phosphate; promotes more calcium phosphate

Defined Bone-Related Conditions

• Osteoarthritis is the degeneration of joint cartilage and the underlying bone
• Osteoporosis is decreased bone density
• Rheumatoid arthritis is an autoimmune inflammatory disorder of the hands and feet

Osteoporosis

• Common bone disease: loss of bone density
• Typical fracture is the head of the radius
• Focal & sub-trochanteric stress fracture

Multiple Myeloma

• Plasma Cells are the fully differentiated B-lymphocytes, that create specific antibodies
• Immature B cells mature into plasma, return to fight future infections
• When DNA plasma cells are damaged in lymphs, it creates tumours
• These tumours mainly accumulate in bone marrows, osteoclast activity surfaces
• 1500 new Australians are diagnosed each year, usually at 60+-year-old- or rarely under 40 years old
• M-proteins, myelomas, para-proteins & immunoglobulin chains are causes as renal failure factors.
• VDJ recombination is a process in the bone marrow where B cells assemble gene segments in B cells
• B cells receive high-affinity responses w Plasma from lymph & circulate to receptors
• There is somatic mutations for germinal centres for high affinity B cells
• Hypermutation in the lymph node diversifies antibody generation
• Clonal Selection produces more anti-bodies using immune systems

Symptoms of Multiple Myeloma

• Bone Pains
• Shortness of Breath
• Kidney Problems
• Confused Drowsiness
• Lower count or infections

Chronic Myeloid Leukaemia

• Increased myeloid in bone marrows (WBC)
• Characteristic chromosomal translocation called Philadelphia (22)
• Uncontrolled Kinase w Gleevec treatment
• Philadelphia chromosome & protein are genomic instability & abnormalities
• BCR-ABL protein drives to cell cycle to speedup & outgrow to control leading to death in the nucleus
• Increase in WBC, decrease in RBW with tear drop cells
• Immature Neutrophils increases with segmenting
• Stem Cells disorder, where is inability to function properly to fight & mature of the Granulocytes
• Granulocytes & eosinophils increases
• Anemia & Breathlessness