Calcium Homeostasis - BS31019 PDF

Taken from https://www.newcastle-hospitals.nhs.uk/services/endocrine-and-thyroid-surgery/ BS31019 – Calcium homeostasis – Part 2 Dr Claire Y Hepburn  Learning outcomes Describe the roles of Ca2+ -ATPases and Na+/Ca2+ exchangers in the maintenance of basal Ca2+ levels...

Taken from https://www.newcastle-hospitals.nhs.uk/services/endocrine-and-thyroid-surgery/ BS31019 – Calcium homeostasis – Part 2 Dr Claire Y Hepburn  Learning outcomes Describe the roles of Ca2+ -ATPases and Na+/Ca2+ exchangers in the maintenance of basal Ca2+ levels. Present examples of proteins within the cytoplasm and intracellular organelles that buffer free (Ca2+)i, or that have a Ca2+ storage function. Describe the major points of entry of Ca2+ into cells and describe how such portals are regulated; Explain how GPCRs regulate the production of the signalling molecules IP3 and DAG and their effect on Ca2+ release and other signalling proteins. Explain the role of STIM and Orai receptors in membrane Ca2+ flow Presentation name, Your name, Date Recommended reading Medical Physiology. 3rd Edition. Elsevier. Chapter 3 – ? https://www.osmosis.org/learn/Phosphate,_calcium_and_magnesium_home ostasis https://www.osmosis.org/learn/Parathyroid_disorders_and_calcium_imbalan ce:_Pathology_review Nesin, V., Wiley, G., Kousi, M., Ong, E. C., Lehmann, T., Nicholl, D. J., Suri, M., Shahrizaila, N., Katsanis, N., Gaffney, P. M., Wierenga, K. J., & Tsiokas, L. (2014). Activating mutations in STIM1 and ORAI1 cause overlapping syndromes of tubular myopathy and congenital miosis. Proceedings of the National Academy of Sciences of the United States of America, 111(11), 4197–4202. https://doi.org/10.1073/pnas.1312520111 The importance of calcium… The importance of cellular Ca2+ was in part made clearer by Ringer (1883) Maintains excitation and contraction in hearts Further experiments by Locke (1894) followed Neurotransmission in muscle prevented by omission of calcium from bath solution By 1939 and 1940, Mann then Harvey and MacIntosh, respectively, showed Role of calcium in exocytosis of neurotransmitters During WW2, Kamada and Kinoshita (1943) showed Role of calcium in stimulating contraction Intracellular calcium signalling matters Neurotransmitter & hormone release Muscle contraction Cell excitability Voltage-gated calcium channels Calcium-dependent conductance Synaptic plasticity Long-Term potentiation Apoptosis Ischaemic brain damage Cytoskeletal changes Neurite outgrowth Protein synthesis etc. So, what is it about Ca2+ that makes it such an important ion? Such diverse cellular functions must reflect certain unique intrinsic features. It may, for example, be its ionic radius along with the attendant divalent charge. These features may have allowed biological processes to develop discriminating mechanisms. The presence of specific Ca2+ transporters means that its cellular concentrations can be very precisely regulated. Calcium homeostasis Bone Extracellular Ca2+free Protein-Ca2+bound Binding sites on glycocalyx [Ca2+]o = ~ 1 mM Cytoplasmic Ca2+ bound ‘Free’ [Ca2+i] = 0.1 µM binding sites Ca2+ Ca2+ Endoplasmic reticulum or Mitochondria sarcoplasmic reticulum Adapted from Medical Physiology, 3rd Edition, 2017. Dynamics of controlling calcium homeostasis Adapted from Krebs, J. Advances in Experimental Medicine and Biology, 2017; 981. Calcium binding proteins EF-hand motif containing proteins Calmodulin S100 proteins Calcineurin C2-domain containing proteins Protein kinase C (PKC) Phospholipase-A (PLA) Phospholipase-C (PLC) Calcium buffering proteins Calnexin Endoplasmic reticulum 25 calcium binding sites Calsquestrin Sarcoplasmic reticulum 70 calcium binding sites Points of entry for calcium Influx mechanisms Numerous pumps (active), exchangers (secondary active) and channels (passive) are involved in moving Ca2+ into cells or into the different organelles in the cells Ca2+ ATPases ATP2B (plasma membrane), ATP2A (endoplasmic/sarcoplasmic reticulum) and ATP2C (golgi) Voltage-gated Ca2+ channels L-type calcium channels Na+/Ca2+ exchanger NCX1, NCX2 and NCX3 Permeability transition pore Ca2+-ATPases Ca2+-ATPases belongs to the P-Type family of ATPases that transfer calcium across membranes after a muscle has contracted The two kinds of Ca2+-ATPase are: Plasma membrane Ca2+ ATPase (PMCA) 1:1 H+ and Ca2+ Sarcoplasmic reticulum Ca2+ ATPase (SERCA) 2:2 H+ and Ca2+ Adapted from Medical Physiology, 3rd Edition, 2017. Ca2+-ATPases - PMCA PMCA moves 1 Ca2+ out of cell for each ATP hydrolysed Activity can be increased by binding of calmodulin to the c-terminal domain Encoded by the 4 genes (ATP2B1-4) of which you get 4 splice variants (PMCA1-4) Adapted from Olesen, et al.,, Nature., 2007; 450, 1036-1042. Ca2+-ATPases - SERCA SERCA moves 2 Ca2+ into the sarcoplasmic reticulum for each ATP hydrolysed Two main states: E1 – high calcium affinity (μM) E2 – low calcium affinity (mM) Activity can be regulated by ATP and phospholamban (PLN) Encoded by the 3 genes (ATP2A1-3) from which you get multiple splice variants Adapted from Olesen, et al.,, Nature., 2007; 450, 1036-1042. Metabolites of vitamin D…1, 25-Dihydroxyvitamin D NCX belongs to the Ca2+/cation antiporter superfamily and is responsible for moving Ca2+ out of the cell 3:1 Na+ and Ca2+ The activity of this exchangers can be changed by Na+ and Ca2+ - in the calcium binding domains (CBD) Encoded by the SLC8 gene and produces 3 NCX isoforms NCX1 – ubiquitously expressed NCX2 and NCX3 - brain and skeletal muscle Adapted from Giladi, Tal and Khananshivili, Front. Physiol., 2016; 7, 30. Voltage-gated Ca2+ channels (VGCC/VOC) α1 subunit – largest subunit. Contains the ion selective residues (glutamine) and the voltage sensor (lysine or arginine). β subunit – intracellular subunit. Expression, membrane trafficking and regulation i.e amplitude of Ca2+ current and activation/inactivation kinetics. α2δ subunit – extracellular subunit. Two proteins joined by disulphide bridges. Smaller effect on kinetics and Ca2+ current amplitude. γ subunit – variable effects on channel function. Adapted from Cold Spring Harb Perspect Biol 2011;3:a003947.. Voltage-gated Ca2+ channels Voltage-gated Ca2+ channels α are encoded by CACNA1 genes Three subfamilies: Cav1 L-type Ca2+ channels (skeletal muscle, smooth muscle, cardiac muscle, secretory tissues and nervous system) Cav2 P/Q-type Ca2+ channels N-type Ca2+ channels R-type Ca2+ channels (nervous system) Cav3 T-type Ca2+ channels (neurons and the heart for pacemaker activity) Inositol 1,4, 5-trisphosphate (IP3) Ca2+ release from intracellular stores (like the endoplasmic reticulum; ER) rely on activation of inositol 1,4, 5- trisphosphate receptors (IP3R) in the membrane of the ER. IP3R are tetramers of 4 transmembrane spanning helices Adapted from Medical Physiology, 3rd Edition, 2017. Adapted from Medical Physiology, 3rd Edition, 2017. Adapted from Foskett, J.K., et al.,, Physiol. Rev., 2007; 87, 3593-658. IP3 and calmodulin Store-operated calcium entry Intracellular stores of Ca2+ are finite in supply and tend to produce short peaks in cytosolic Ca2+ when opened Some cellular processes require more prolonged increases in cytosolic Ca2+ (transcription) Enter store-operated calcium entry (SOCE)! Store-operated calcium entry SOCE relies on two key proteins, STIM1 and Orai. They operate in this process as follows: STIMs are single-pass transmembrane protein that sense the changes of ER luminal Ca2+ levels. Upon Ca2+ store-depletion, STIM1 proteins undergo oligomerisation and translocation to ER/SR-PM (known as puncta formation), This attracts the plasma membrane Orai1 to the same site = interaction Channel activation and opening of these channels facilitating movement of Ca2+ into the cell Store-operated calcium entry mutations Very rare! But… For those with gain of function mutations in STIM1 - increased SOCE Adapted from Nesin, V et al.,, PNAS., 2014, 111, 3197-4202. Summary Homeostasis of calcium occurs extracellularly but also intracellularly Intracellular calcium is primarily bound by calcium-binding proteins or calcium- buffering proteins, either in the cytoplasm but importantly in cellular organelles like the sarcoplasmic reticulum The entry and egress of calcium from the cell and cellular organelles by a selection of proteins: Ca2+ ATPases ATP2B (plasma membrane), ATP2A (endoplasmic/sarcoplasmic reticulum) and ATP2C (golgi) Voltage-gated Ca2+ channels L-type calcium channels Na+/Ca2+ exchanger NCX1, NCX2 and NCX3 Permeability transition pore Thank you. Questions?