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The cytokine receptor activator of nuclear factor kappa B ligand (RANKL), encoded by the Tnfsf11 gene, is essential for osteoclastogenesis and previous studies have shown that deletion of the Tnfsf11 gene using a Dmp1-Cre transgene reduces osteoclast formation in cancellous bone by more than 70%. However, the Dmp1-Cre transgene used in those studies leads to recombination in osteocytes, osteoblasts, and lining cells making it unclear whether one or more of these cell types produce the RANKL required for osteoclast formation in cancellous bone. Because osteoblasts, osteocytes, and lining cells have distinct locations and functions, distinguishing which of these cell types are sources of RANKL is essential for understanding the orchestration of bone remodeling. To distinguish between these possibilities, we have now created transgenic mice expressing the Cre recombinase under the control of regulatory elements of the Sost gene, which is expressed in osteocytes but not osteoblasts or lining cells in murine bone. Activity of the Sost-Cre transgene in osteocytes, but not osteoblast or lining cells, was confirmed by crossing Sost-Cre transgenic mice with tdTomato and R26R Cre-reporter mice, which express tdTomato fluorescent protein or LacZ, respectively, only in cells expressing the Cre recombinase or their descendants. Deletion of the Tnfsf11 gene in Sost-Cre mice led to a threefold decrease in osteoclast number in cancellous bone and increased cancellous bone mass, mimicking the skeletal phenotype of mice in which the Tnfsf11 gene was deleted using the Dmp1-Cre transgene. These results demonstrate that osteocytes, not osteoblasts or lining cells, are the main source of the RANKL required for osteoclast formation in remodeling cancellous bone.

Key Points Estrogen receptor α (ERα)-mediated direct effects of estrogens on osteoclasts, and direct or indirect effects on B lymphocytes, attenuate the resorption of trabecular bone ERα-mediated effects of estrogens on osteoblast progenitors indirectly attenuate resorption at the endocortical surface, via a mechanism initiated outside of the nucleus The ERα of osteoblast progenitors stimulates Wnt signalling and periosteal bone accrual in response to mechanical strain, independently of estrogens The androgen receptor exerts a protective effect on trabecular bone in male mammals, but (unlike estrogen's effects in female mammals), this effect involves actions on osteoblasts and osteocytes Hormonal signals are modified and integrated with various environmental cues in different bone compartments, including mechanical strain, paracrine cytokines and growth factors This Review describes the effects of ERα, AR and RANKL in various cell types involved in bone homeostasis. The authors discuss the implications of findings in animal models for understanding the regulation of trabecular and cortical bone, the integration of hormonal and mechanical signals, as well as the importance of estrogens and androgens in the female versus male skeleton. Mouse models with cell-specific deletion of the estrogen receptor (ER) α, the androgen receptor (AR) or the receptor activator of nuclear factor κB ligand (RANKL), as well as cascade-selective estrogenic compounds have provided novel insights into the function and signalling of ERα and AR. The studies reveal that the effects of estrogens on trabecular versus cortical bone mass are mediated by direct effects on osteoclasts and osteoblasts, respectively. The protection of cortical bone mass by estrogens is mediated via ERα, using a non-nucleus-initiated mechanism. By contrast, the AR of mature osteoblasts is indispensable for the maintenance of trabecular bone mass in male mammals, but not required for the anabolic effects of androgens on cortical bone. Most unexpectedly, and independently of estrogens, ERα in osteoblast progenitors stimulates Wnt signalling and periosteal bone accrual in response to mechanical strain. RANKL expression in B lymphocytes, but not T lymphocytes, contributes to the loss of trabecular bone caused by estrogen deficiency. In this Review, we summarize this evidence and discuss its implications for understanding the regulation of trabecular and cortical bone mass; the integration of hormonal and mechanical signals; the relative importance of estrogens versus androgens in the male skeleton; and, finally, the pathogenesis and treatment of osteoporosis.

Receptor activator of NFkB ligand (RANKL) is a TNF-family cytokine required for osteoclast formation, as well as immune cell and mammary gland development. It is produced as a membrane-bound protein that can be shed to form a soluble protein. We created mice harboring a sheddase-resistant form of RANKL, in which soluble RANKL is undetectable in the circulation. Lack of soluble RANKL does not affect bone mass or structure in growing mice but reduces osteoclast number and increases cancellous bone mass in adult mice. Nonetheless, the bone loss caused by estrogen deficiency is unaffected by the lack of soluble RANKL. Lymphocyte number, lymph node development, and mammary gland development are also unaffected by the absence of soluble RANKL. These results demonstrate that the membrane-bound form of RANKL is sufficient for most functions of this protein but that the soluble form does contribute to physiological bone remodeling in adult mice. RANKL is a cytokine produced as a membrane-bound and a secreted protein. Here, using mice lacking soluble RANKL, the authors show that the secreted protein is important for osteoclast function, but not for mammary gland and lymphocyte development.

In this trial involving adult ex-prisoners who had a history of opioid dependence, extended-release naltrexone resulted in a lower rate of opioid relapse than did usual treatment (brief counseling and referrals). The drug did not reduce rates of reincarceration or unsafe sex. Opioid-use disorder is a chronic relapsing condition that has serious public health consequences. Opioid dependence disproportionately affects U.S. criminal justice system populations, and relapse and overdose deaths occur at high rates after release from incarceration. 1 Evidence-based opioid-agonist maintenance therapies for opioid dependence (methadone and buprenorphine) are effective in prison, jail, and community reentry (i.e., parole) settings 2 – 5 but have historically been unavailable or discouraged among criminal justice clients. 6 – 8 Extended-release naltrexone (Vivitrol, Alkermes), a sustained-release monthly injectable formulation of the full mu-opioid receptor antagonist, was approved by the Food and Drug Administration in 2010 for the prevention of relapse to . . .

To date, the dogma in the field has been that RANKL, an essential cytokine in osteoclast maturation, is released by osteoblasts as a way to coordinate bone growth and bone loss during adult bone remodeling. Now, Hiroshi Takayanagi and colleagues, as well as Charles O'Brien and colleagues, have independently found that osteocytes are the predominant source of RANKL in the adult mouse. As RANKL signaling is a key target in treating osteoporosis, these results have potentially important implications for disease management. Osteoclasts resorb the mineralized matrices formed by chondrocytes or osteoblasts. The cytokine receptor activator of nuclear factor-κB ligand (RANKL) is essential for osteoclast formation and thought to be supplied by osteoblasts or their precursors, thereby linking bone formation to resorption. However, RANKL is expressed by a variety of cell types, and it is unclear which of them are essential sources for osteoclast formation. Here we have used a mouse strain in which RANKL can be conditionally deleted and a series of Cre-deleter strains to demonstrate that hypertrophic chondrocytes and osteocytes, both of which are embedded in matrix, are essential sources of the RANKL that controls mineralized cartilage resorption and bone remodeling, respectively. Moreover, osteocyte RANKL is responsible for the bone loss associated with unloading. Contrary to the current paradigm, RANKL produced by osteoblasts or their progenitors does not contribute to adult bone remodeling. These results suggest that the rate-limiting step of matrix resorption is controlled by cells embedded within the matrix itself.

The scavenger receptor class B member 1 (SCARB1), encoded by Scarb1 , is a cell surface receptor for high density lipoproteins, low density lipoproteins (LDL), oxidized LDL (OxLDL), and phosphocholine-containing oxidized phospholipids (PC-OxPLs). Scarb1 is expressed in multiple cell types, including osteoblasts and macrophages. PC-OxPLs, present on OxLDL and apoptotic cells, adversely affect bone metabolism. Overexpression of E06 IgM – a natural antibody that recognizes PC-OxPLs– increases cancellous and cortical bone at 6 months of age in both sexes and protects against age- and high fat diet- induced bone loss, by increasing bone formation. We have reported that SCARB1 is the most abundant scavenger receptor for OxPLs in osteoblastic cells, and osteoblasts derived from Scarb1 knockout mice ( Scarb1 KO) are protected from the pro-apoptotic and anti-differentiating effects of OxLDL. Skeletal analysis of Scarb1 KO mice produced contradictory results, with some studies reporting elevated bone mass and others reporting low bone mass. To clarify if Scarb1 mediates the negative effects of PC-OxPLs in bone, we deleted it in osteoblast lineage cells using Osx1-Cre transgenic mice. Bone mineral density (BMD) measurements and micro-CT analysis of cancellous and cortical bone at 6 months of age did not reveal any differences between Scarb1 ΔOSX-l mice and their wild-type (WT), Osx1-Cre, or Scarb1 fl/fl littermate controls. We then investigated whether PC-OxPLs could exert their anti-osteogenic effects via activation of SCARB1 in myeloid cells by deleting Scarb1 in LysM-Cre expressing cells. BMD measurements and micro-CT analysis at 6 months of age did not show any differences between Scarb1 ΔLysM mice and their WT, LysM-Cre, or Scarb1 fl/fl controls. Based on this evidence, we conclude that the adverse skeletal effects of PC-OxPLs in adult mice are not mediated by Scarb1 expressed in osteoblast lineage cells or myeloid cells.

NAD is an essential co-factor for cellular energy metabolism and multiple other processes. Systemic NAD + deficiency has been implicated in skeletal deformities during development in both humans and mice. NAD levels are maintained by multiple synthetic pathways but which ones are important in bone forming cells is unknown. Here, we generate mice with deletion of Nicotinamide Phosphoribosyltransferase ( Nampt ), a critical enzyme in the NAD salvage pathway, in all mesenchymal lineage cells of the limbs. At birth, Nampt ΔPrx1 exhibit dramatic limb shortening due to death of growth plate chondrocytes. Administration of the NAD precursor nicotinamide riboside during pregnancy prevents the majority of in utero defects. Depletion of NAD post-birth also promotes chondrocyte death, preventing further endochondral ossification and joint development. In contrast, osteoblast formation still occurs in knockout mice, in line with distinctly different microenvironments and reliance on redox reactions between chondrocytes and osteoblasts. These findings define a critical role for cell-autonomous NAD homeostasis during endochondral bone formation. Deficiency in NAD+ has been implicated in skeletal deformities during development in both humans and mice. Here, the authors use mice that lack the critical enzyme of the NAD+ salvage pathway Nampt in mesenchymal lineage cells to show that the NAD salvage pathway is indispensable for endochondral but not intramembranous bone development.

The scavenger receptor class B member 1 (SR-B1 or Scarb1) is a cell surface receptor for high density lipoproteins. It also binds oxidized low density lipoproteins and phosphocholine-containing oxidized phospholipids (PC-OxPL), which adversely affect bone homeostasis. Overexpression of a single chain form of the antigen-binding domain of E06 IgM–a natural antibody that recognizes PC-OxPL–increases trabecular and cortical bone mass in female and male mice by stimulating bone formation. We have previously reported that Scarb1 is the most abundant scavenger receptor for PC-OxPL in calvaria-derived osteoblastic cells. Additionally, bone marrow- and calvaria-derived osteoblasts from Scarb1 knockout mice (Scarb1 KO) are protected from the pro-apoptotic and anti-differentiating effects of OxPL. Previous skeletal analysis of Scarb1 KO mice has produced contradictory results, with some studies reporting elevated bone mass but another study reporting low bone mass. To clarify the role of Scarb1 in osteoblasts, we deleted Scarb1 specifically in cells of the osteoblast lineage using Osx1-Cre transgenic mice. We observed no difference in bone mineral density measured by DXA in either female or male Osx1-Cre;Scarb1 fl/fl mice compared to wild type (WT), Osx1-Cre, or Scarb1 fl/fl littermate controls. Additionally, microCT analysis of 6-month-old females and 7-month-old males did not detect any difference in trabecular or cortical bone mass between genotypes. These results indicate that expression of Scarb1 in cells of the osteoblast lineage does not play an important role in bone homeostasis and, therefore, it is not essential for the effects of PC-OxPL on these cells.

Key Points Addictive disorders are common, account for a tremendous disease burden and are in need of improved medical treatments. Alcohol use accounts for more disease burden than any other addictive drug with the exception of nicotine. The discovery of naltrexone as a medication for alcoholism was conceptually groundbreaking, because it demonstrated the feasibility of pharmacotherapy for an addictive disorder using a mechanism other than replacement therapy. Overall, however, the effect size of naltrexone turned out to be small, and despite its evidence base, this medication has not gained widespread clinical use. Clinical experience and meta-analyses have long indicated that clinical response to naltrexone is remarkably variable. Over a decade ago, functional genetic variation was discovered at the locus encoding the target for naltrexone, the mu-opioid receptor (MOR), and this was shortly followed by the suggestion that efficacy of naltrexone may be restricted to carriers of the minor allele at this locus. Recently, a series of translational studies in humans, non-human primates and humanized mice has provided consistent support for the notion that alcohol reward is in part mediated by an alcohol–endogenous opioid–dopamine cascade, that this cascade is more vigorously activated by alcohol in carriers of the minor allele at the OPRM1 gene locus that encodes the MOR, and that these subjects are thereby rendered particularly or maybe selectively sensitive to naltrexone. Alcohol reinforcement is mediated by multiple systems, among which opioids and dopamine are but two, and are mostly involved in pleasurable, positively reinforcing alcohol effects experienced mostly in earlier stages of the addictive process. As patients continue heavy alcohol use, a pathological activation of brain stress systems occurs, and sets the scene for negatively reinforced alcohol use — that is, alcohol use aimed at eliminating anxiety and dysphoria that emerges during abstinence. Corticotropin-releasing factor (CRF), the hypothalamic release factor for pituitary adrenocorticotropic hormone (ACTH), is also widely expressed in extrahypothalamic networks that mediate behavioural and emotional stress responses. Recent work has shown that the CRF system becomes activated following a prolonged history of brain alcohol exposure, and its activity is key to negatively reinforced alcohol seeking and use. Genetic variation that influences the functional activity of the CRF system has been found in rats, non-human primates and humans, and has been shown to be associated with various alcohol use phenotypes in all three species. This suggests that pharmacogenetic effects may need to be considered when CRF receptor 1 (CRF 1 ) antagonists are developed for the treatment of alcoholism. GABAergic and serotonergic transmission are also involved in the pathophysiology of alcoholism, and pharmacogenetic effects of variants within both these systems have also been suggested. A potential implication of these findings is that pharmacogenetic effects may turn out to be the rule rather than the exception, and that much more attention will have to be paid to personalizing pharmacotherapy of addictive disorders. Current addiction pharmacotherapies have limited success. Focusing on alcohol addiction, Heilig and colleagues review the evidence that genetic heterogeneity in the opioid, corticotropin-releasing factor, GABA and serotonin systems may underlie differential treatment responses, and that personalized therapies tailored to patient genotype may lead to more successful treatment for alcohol addiction. Addictive disorders are partly heritable, chronic, relapsing conditions that account for a tremendous disease burden. Currently available addiction pharmacotherapies are only moderately successful, continue to be viewed with considerable scepticism outside the scientific community and have not become widely adopted as treatments. More effective medical treatments are needed to transform addiction treatment and address currently unmet medical needs. Emerging evidence from alcoholism research suggests that no single advance can be expected to fundamentally change treatment outcomes. Rather, studies of opioid, corticotropin-releasing factor, GABA and serotonin systems suggest that incremental advances in treatment outcomes will result from an improved understanding of the genetic heterogeneity among patients with alcohol addiction, and the development of personalized treatments.