But Why Bone? Gerard Karsenty MD, PhD Department of Genetics and Development Columbia University medical Center

Transcription

1 But Why Bone? Gerard Karsenty MD, PhD Department of Genetics and Development Columbia University medical Center

2 The evolution of bone biology Then: a walking and running tool i.e., A SURVIVAL TOOL The tissue affected by osteoporosis, a disease of the end of life

3 A unique cell biological feature of bone Bone is the only tissue that contains a cell type whose only function is to destroy the host tissue

4 Bone (re)modeling The process whereby bone renews itself throughout life Modeling allows growth, the ability to walk, life During adulthood bone remodeling repairs micro and macro damages, i.e. fractures These processes should be energetically expensive

5 Supporting clinical observations Any condition restricting food intake cause of a growth arrest in children They cause osteoporosis in adults Gonadal failure causes bone loss

6 Overarching hypothesis There is a coordinated regulation, endocrine in nature, of bone growth, energy metabolism and reproduction

7 If this hypothesis is correct..hormones affecting energy metabolism should regulate bone mass..bone should be an endocrine organ affecting energy metabolism and reproduction

8 Leptin Adipocyte-derived hormone known to prevent appetite and to favor fertility Appears during evolution with bone

9 High bone mass in the absence of leptin (and of functional gonads) wt ob/ob Bone Volume (%) Bone Formation Rate

10 Leptin is a central link between bone growth and energy intake Brainstem Serotonin Hypothalamus Leptin SNS Bone Adipocytes Melanocortin Appetite

11 Partial gain of function in leptin signaling Leptin Receptor SOCS3 function is inhibited, leptin feedback control should be partially impaired Leu985 This mutation should allow to identify the function(s) of leptin with the lowest threshold of signaling

12 This mutation results in osteoporosis. BV/TV (%)

13 ..without affecting energy metabolism

14 If this hypothesis is correct..hormones affecting energy metabolism should regulate bone mass..bone should be an endocrine organ affecting energy metabolism and reproduction

15 Osteocalcin Osteoblast-specific protein The most abundant non-collagenous protein of the bone extracellular matrix Carboxylated on 3 glutamic residues, i.e., it has high affinity for mineral ions

16 Osteocalcin (Ocn)-/- mice Bone mineralization is absolutely normal The bone ECM looked totally normal Ocn-/- mice were fatty and bred poorly

17 What did we miss about osteocalcin? Abundant in the general circulation (>100ng/mL) Processed as a pre-pro-peptide Cell-specific gene All that are features of an hormone

18 Is osteocalcin fulfilling the presumed endocrine function of bone?

19 A stroke of luck We had at our disposal a loss of function mouse model (Osteocalcin-/-) ( Necessity ) A gain of function mouse model (Esp-/-) ( Sufficiency )

20 Hypoglycemia in the absence of Esp Blood glucose (mg/dl) Age (Month) wt Esp-/- Esp osb -/-

21 Increased insulin sensitivity in the absence of Esp % of initial glucose ITT wt Esp-/- Esp osb -/- Minutes after insulin injection

22 Hyperinsulinemia in the absence of Esp Serum insulin (ng/ml) wt Esp-/- Esp osb -/ Age (Month)

23 Increased insulin secretion in the absence of Esp Serum insulin (ng/ml) GSIS Minutes after glucose injection Blood glucose (mg/dl) Minutes after glucose injection GTT wt Esp-/- Esp osb -/-

24 Hyperglycemia, hypoinsulinemia in the absence of Osteocalcin Blood glucose (mg/dl) Serum Insulin (ng/ml) Age (Month) wt Ocn-/ Age (Month)

25 Decreased insulin sensitivity in the absence of Osteocalcin 120 ITT % of initial glucose wt Ocn-/ Minutes after insulin injection

26 Decreased insulin secretion in the absence of Osteocalcin Serum insulin (ng/ml) GSIS Minutes after glucose injection Blood glucose (mg/dl) GTT Minutes after glucose injection wt Ocn-/-

27 Osteocalcin is an insulin secretagogue 8 Osteocalcin Insulin (ng/100 islets/min) Acetylcholine+Neostigmine KCl Duration of Perifusion (min)

28 Overarching hypothesis There is a coordinated regulation, endocrine in nature, of bone growth, energy metabolism and reproduction

29 Co-culture mesenchymal cells-gonads Primary: -Osteoblasts -Myoblasts -Fibroblats -Adipocytes Ovary explants or Testis explants or Primary Cells

30 Osteoblasts do not affect sex steroid production by ovaries

31 Osteoblasts enhance testosterone production by testis

32 Osteocalcin favors male fertility in vivo

33 The search for an osteocalcin receptor Gprc6a

34 Peripheral testicular failure Patients with idiopathic oligospermia, Low testosterone, high LH circulating levels, normal kariotype Sequence in them and controls all exons of Osteocalcin and GPRC6A

35 GPRC6A mutation in male patients with sterility Mutation in GPRC6A Highly conserved region SIFT Prediction

36 Clin Endocrinol (Oxf) The rs polymorphism in GPRC6A gene is associated with insulin resistance in normal weight and obese subjects A. Di Nisio, M.Santa Rocca, G. P. Fadini, L. De Toni, G. Marcuzzo, M.C. Marescotti, M. Sanna, M. Plebani, R. Vettor, A. Avogaro, C.Foresta

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38 Osteocalcin also signals in the brain

39 Osteocalcin decreases anxiety ( open field test) Control Ocn-/-

40 Osteocalcin is necessary for learning and memory Control Ocn-/-

41 Brain development Cognition Insulin Glucose homeostasis Testosterone Male fertility

42 Can we identify features shared by all known functions of osteocalcin? Can use them as tools to identify novel functions of osteocalcin? Can we then infer from all these functions a rationale for why bone would be an endocrine organ?

43 Physiological functions regulated by osteocalcin are severely affected by aging Brain development Cognition Insulin Glucose homeostasis Testosterone Male fertility

44 Circulating osteocalcin levels decrease early in life Mice Total Uncarb OCN Total Uncarb OCN Osteocalcin (ng/ml) Females Osteocalcin (ng/ml) Males Months Months

45 Circulating osteocalcin levels decrease early in life Women Men Osteocalcin (ng/ml) Osteocalcin (ng/ml) Years Years

46 Two of these functions are needed when living in an hostile environment Brain development Cognition Insulin Glucose homeostasis Testosterone Male fertility

47 Does osteocalcin promote other physiological functions that are needed when living in an hostile environment?

48 Adaptation to Exercise A survival function that has been conserved during evolution Exercise requires that muscle uptakes and catabolizes its two main nutrients: glucose and fatty acids Insulin cannot explain the increase in muscle function during exercise because insulin is an anabolic hormone Muscle function during exercise must be regulated by other systemic cues (myokines, hormones)

49 Circulating osteocalcin levels markedly increase during exercise Rest Run Osteocalcin (ng/ml) Rest Insulin (ng/ml) Hours after run

50 Osteocalcin signaling in myofibers favors muscle function during exercise Distance (m) Time (min) Gprc6a f/f Gprc6a Mck -/ Distance (m) Time (min) Gprc6a Mck +/- Ocn+/- Ocn+/-;Gprc6a Mck +/

51 Exogenous osteocalcin restores muscle function in old mice Distance (m) Time (min) m 12 m 15 m 3 m 12 m 15 m Vehicle Ocn Ocn

52 Bone as an endocrine organ Brain development Cognition Insulin synthesis Testosterone synthesis Male fertility Adaptation to exercise

53 Acknowledgements NK. Lee M. Ferron F. Oury L. Khrimian P. Mera A. Obri J. Shimazu J. Wei C. Confavreux R. De Cabo E. Kandel I. Kurland T. McGraw M. Puchowicz