Bone Marrow Oxytocin Mediates the Anabolic Action of Estrogen on the Skeleton

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Notch controls skeletogenesis, but its role in the remodeling of adult bone remains conflicting. In mature mice, the skeleton can become osteopenic or osteosclerotic depending on the time point at which Notch is activated or inactivated. Using adult EGFP reporter mice, we find that Notch expression is localized to osteocytes embedded within bone matrix. Conditional activation of Notch signaling in osteocytes triggers profound bone formation, mainly due to increased mineralization, which rescues both age-associated and ovariectomy-induced bone loss and promotes bone healing following osteotomy. In parallel, mice rendered haploinsufficient in γ-secretase presenilin-1 (Psen1), which inhibits downstream Notch activation, display almost-absent terminal osteoblast differentiation. Consistent with this finding, pharmacologic or genetic disruption of Notch or its ligand Jagged1 inhibits mineralization. We suggest that stimulation of Notch signaling in osteocytes initiates a profound, therapeutically relevant, anabolic response.

Section: Discussionmentioning

confidence: 99%

Anabolic actions of Notch on mature bone

Liu

Ping

et al. 2016

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“…Finally, to examine potential antianabolic actions of Oxt via the Avpr1α, we used a murine lactation model of bone loss in which the Oxtr was deleted specifically in osteoblasts (10,34). Mice lose bone maximally at weeks 2 and 3 of lactation, after which there is prompt skeletal recovery at weaning.…”

Section: Discussionmentioning

confidence: 99%

“…We find high levels of Oxtr expression on both osteoclasts and osteoblasts (2,10), in addition to their abundant expression in breast and uterine tissue, where they regulate lactation and parturition, respectively (11). Avpr1αs, in contrast, are distributed more ubiquitously, whereas Avpr2s are localized mainly in the kidney, where they regulate free water excretion (12).…”

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confidence: 92%

Functions of vasopressin and oxytocin in bone mass regulation

Sun

Tamma

Yuen

et al. 2015

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Prior studies show that oxytocin (Oxt) and vasopressin (Avp) have opposing actions on the skeleton exerted through high-affinity G protein-coupled receptors. We explored whether Avp and Oxtr can share their receptors in the regulation of bone formation by osteoblasts. We show that the Avp receptor 1α (Avpr1α) and the Oxt receptor (Oxtr) have opposing effects on bone mass: Oxtr −/− mice have osteopenia, and Avpr1α −/− mice display a high bone mass phenotype. More notably, this high bone mass phenotype is reversed by the deletion of Oxtr in Oxtr −/− :Avpr1α −/− doublemutant mice. However, although Oxtr is not indispensable for Avp action in inhibiting osteoblastogenesis and gene expression, Avpstimulated gene expression is inhibited when the Oxtr is deleted in Avpr1α −/− cells. In contrast, Oxt does not interact with Avprs in vivo in a model of lactation-induced bone loss in which Oxt levels are high. Immunofluorescence microscopy of isolated nucleoplasts and Western blotting and MALDI-TOF of nuclear extracts show that Avp triggers Avpr1α localization to the nucleus. Finally, a specific Avpr2 inhibitor, tolvaptan, does not affect bone formation or bone mass, suggesting that Avpr2, which primarily functions in the kidney, does not have a significant role in bone remodeling.O ver the past decade, we have described direct actions of anterior and posterior pituitary hormones on the skeleton (1-8). We have shown that these actions are exerted via G protein-coupled receptors resident on both osteoblasts and osteoclasts. We also find that the skeleton is highly sensitive to the action of posterior pituitary hormones; for example, mice haploinsufficient in oxytocin (Oxt) have osteopenic bones, but lactation is normal; lactation is impaired only in Oxt −/− mice (2). Likewise, Tshr haploinsufficient mice are completely euthyroid with normal thyroid follicles but display significant osteopenia (4). The exquisite sensitivity of the skeleton to pituitary hormones comes as no surprise, considering that the pituitary gland and the skeleton are both evolutionarily more primitive than target endocrine organs (7).Apart from the known actions of growth hormone on the skeleton, Tsh, Fsh, Acth, Oxt, and vasopressin (Avp) have all been shown to regulate the formation and/or function of both osteoblasts and osteoclasts and thus to control bone remodeling in vivo (2-4, 6-8). The two neurohypophyseal hormones Oxt and Avp have opposing functions (2, 3). Oxt stimulates and Avp inhibits osteoblast formation. Consequently, the genetic deletion of the Oxt receptor (Oxtr) and Avp receptor 1α (Avpr1α) yields opposing phenotypes, notably osteopenia in Oxtr −/− mice and high bone mass in Avpr1α −/− mice (2, 3). These findings may explain the rapid recovery of bone loss at weaning when plasma Oxt levels are high (9) and also the profound loss of bone noted in chronic hyponatremic states, such as the syndrome of inappropriate antidiuretic hormone secretion (SIADH), in which serum Avp levels are elevated (3).We find high levels of Oxtr expression on ...

“…More recently, we and others have shown that Oxt also stimulates bone formation directly by interacting with an osteoblastic Oxtr (3,4). The genetic deletion of Oxt or Oxtr in mice thus decreases osteoblast differentiation and bone formation, causing osteopenia (5,6). We believe that this action supports maternal skeletal remineralization after the intergenerational transfer of calcium during pregnancy and lactation (5).…”

mentioning

confidence: 66%

“…We have shown previously that osteoblasts express abundant plasma membrane Oxtrs that drive cell differentiation to a mature, mineralizing phenotype in response to pituitary-and bone marrow-derived Oxt (4,6). However, the mechanism of this effect has remained uncertain.…”

Section: Discussionmentioning

confidence: 99%

Osteoblast regulation via ligand-activated nuclear trafficking of the oxytocin receptor

Benedetto

Sun

Zambonin

et al. 2014

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We report that oxytocin (Oxt) receptors (Oxtrs), on stimulation by the ligand Oxt, translocate into the nucleus of osteoblasts, implicating this process in the action of Oxt on osteoblast maturation. Sequential immunocytochemistry of intact cells or isolated nucleoplasts stripped of the outer nuclear membrane showed progressive nuclear localization of the Oxtr; this nuclear translocation was confirmed by monitoring the movement of Oxtr-EGFP as well as by immunogold labeling. Nuclear Oxtr localization was conclusively shown by Western immunoblotting and MS of nuclear lysate proteins. We found that the passage of Oxtrs into the nucleus was facilitated by successive interactions with β-arrestins (Arrbs), the small GTPase Rab5, importin-β (Kpnb1), and transportin-1 (Tnpo1). siRNA-mediated knockdown of Arrb1, Arrb2, or Tnpo1 abrogated Oxt-induced expression of the osteoblast differentiation genes osterix (Sp7), Atf4, bone sialoprotein (Ibsp), and osteocalcin (Bglap) without affecting Erk phosphorylation. Likewise and again, without affecting pErk, inhibiting Arrb recruitment by mutating Ser rich clusters of the nuclear localization signal to Ala abolished nuclear import and Oxtr-induced gene expression. These studies define a previously unidentified mechanism for Oxtr action on bone and open possibilities for direct transcriptional modulation by nuclear G protein-coupled receptors. T he oxytocin (Oxt) receptor (Oxtr), a member of the rhodopsin-type (class I) family of G protein-coupled receptors (GPCRs), responds to the neurohypophyseal hormone Oxt to stimulate lactation and social bonding (1, 2). More recently, we and others have shown that Oxt also stimulates bone formation directly by interacting with an osteoblastic Oxtr (3, 4). The genetic deletion of Oxt or Oxtr in mice thus decreases osteoblast differentiation and bone formation, causing osteopenia (5, 6). We believe that this action supports maternal skeletal remineralization after the intergenerational transfer of calcium during pregnancy and lactation (5). However, the precise mechanism through which Oxtr activation translates into increased bone formation remains unclear.Oxt binding to Oxtrs is known to activate both G protein G αi -and G αq/11 -mediated phospholipase C pathways (7). However, in common with other GPCRs, prolonged or repetitive agonist stimulation by Oxt results in Oxtr internalization through β-arrestin (Arrb). GPCR internalization can activate signaling pathways quite distinct from those activated by the same receptors resident at the cell surface. Hence, desensitization of primary G protein-dependent signaling can be followed by a second wave of Arrb-mediated signaling (8). β-Arrestins can also recruit signaling proteins that connect GPCRs to various cytoplasmic effector pathways, such as mitogen-activated protein kinase (MAPK) and protein kinase B/phosphatidylinositol-4,5-bisphosphate 3-kinase (Akt/PI3K) (9, 10). Such mechanisms can elicit delayed genomic responses.After being internalized, GPCRs are either recycled back to the plasma m...