Estradiol increases cortical and trabecular bone accrual and bone strength in an adolescent male-to-female mouse model of gender-affirming hormone therapy

The effects of gender-affirming hormone therapy on the skeletal integrity and fracture risk in transitioning adolescent trans girls are unknown. To address this knowledge gap, we developed a mouse model to simulate male-to-female transition in human adolescents in whom puberty is first arrested by using gonadotrophin-releasing hormone analogs with subsequent estradiol treatment. Puberty was suppressed by orchidectomy in male mice at 5 weeks of age. At 3 weeks post-surgery, male-to-female mice were treated with a high dose of estradiol (~0.85 mg) by intraperitoneal silastic implantation for 12 weeks. Controls included intact and orchidectomized males at 3 weeks post-surgery, vehicle-treated intact males, intact females and orchidectomized males at 12 weeks post-treatment. Compared to male controls, orchidectomized males exhibited decreased peak bone mass accrual and a decreased maximal force the bone could withstand prior to fracture. Estradiol treatment in orchidectomized male-to-female mice compared to mice in all control groups was associated with an increased cortical thickness in the mid-diaphysis, while the periosteal circumference increased to a level that was intermediate between intact male and female controls, resulting in increased maximal force and stiffness. In trabecular bone, estradiol treatment increased newly formed trabeculae arising from the growth plate as well as mineralizing surface/bone surface and bone formation rate, consistent with the anabolic action of estradiol on osteoblast proliferation. These data support the concept that skeletal integrity can be preserved and that long-term fractures may be prevented in trans girls treated with GnRHa and a sufficiently high dose of GAHT. Further study is needed to identify an optimal dose of estradiol that protects the bone without adverse side effects.

Systematic review of computed tomography parameters used for the assessment of subchondral bone in osteoarthritis

OBJECTIVE

To systematically review the published parameters for the assessment of subchondral bone in human osteoarthritis (OA) using computed tomography (CT) and gain an overview of current practices and standards.

DESIGN

A literature search of Medline, Embase and Cochrane Library databases was performed with search strategies tailored to each database (search from 2010 to January 2023). The search results were screened independently by two reviewers against pre-determined inclusion and exclusion criteria. Studies were deemed eligible if conducted in vivo/ex vivo in human adults (>18 years) using any type of CT to assess subchondral bone in OA. Extracted data from eligible studies were compiled in a qualitative summary and formal narrative synthesis.

RESULTS

This analysis included 202 studies. Four groups of CT modalities were identified to have been used for subchondral bone assessment in OA across nine anatomical locations. Subchondral bone parameters measuring similar features of OA were combined in six categories: (i) microstructure, (ii) bone adaptation, (iii) gross morphology (iv) mineralisation, (v) joint space, and (vi) mechanical properties.

CONCLUSIONS

Clinically meaningful parameter categories were identified as well as categories with the potential to become relevant in the clinical field. Furthermore, we stress the importance of quantification of parameters to improve their sensitivity and reliability for the evaluation of OA disease progression and the need for standardised measurement methods to improve their clinical value.

The Utility of Preclinical Models in Understanding the Bone Health of Transgender Individuals Undergoing Gender-Affirming Hormone Therapy

PURPOSE OF REVIEW

To summarise the evidence regarding the effects of gender-affirming hormone therapy (GAHT) on bone health in transgender people, to identify key knowledge gaps and how these gaps can be addressed using preclinical rodent models.

RECENT FINDINGS

Sex hormones play a critical role in bone physiology, yet there is a paucity of research regarding the effects of GAHT on bone microstructure and fracture risk in transgender individuals. The controlled clinical studies required to yield fracture data are unethical to conduct making clinically translatable preclinical research of the utmost importance. Novel genetic and surgical preclinical models have yielded significant mechanistic insight into the roles of sex steroids on skeletal integrity. Preclinical models of GAHT have the potential inform clinical approaches to preserve skeletal integrity and prevent fractures in transgender people undergoing GAHT. This review highlights the key considerations required to ensure the information gained from preclinical models of GAHT are informative.

A20 controls RANK-dependent osteoclast formation and bone physiology

The anti-inflammatory protein A20 serves as a critical brake on NF-κB signaling and NF-κB-dependent inflammation. In humans, polymorphisms in or near the TNFAIP3/A20 gene have been associated with several inflammatory disorders, including rheumatoid arthritis (RA), and experimental studies in mice have demonstrated that myeloid-specific A20 deficiency causes the development of a severe polyarthritis resembling human RA. Myeloid A20 deficiency also promotes osteoclastogenesis in mice, suggesting a role for A20 in the regulation of osteoclast differentiation and bone formation. We show here that osteoclast-specific A20 knockout mice develop severe osteoporosis, but not inflammatory arthritis. In vitro, osteoclast precursor cells from A20 deficient mice are hyper-responsive to RANKL-induced osteoclastogenesis. Mechanistically, we show that A20 is recruited to the RANK receptor complex within minutes of ligand binding, where it restrains NF-κB activation independently of its deubiquitinating activity but through its zinc finger (ZnF) 4 and 7 ubiquitin-binding functions. Together, these data demonstrate that A20 acts as a regulator of RANK-induced NF-κB signaling to control osteoclast differentiation, assuring proper bone development and turnover.

The role of the androgen receptor in the pathogenesis of obesity and its utility as a target for obesity treatments

Obesity is associated with hypothalamic-pituitary-testicular axis dysregulation in males. Here, we summarize recent evidence derived from clinical trials and studies in preclinical animal models regarding the role of androgen receptor (AR) signaling in the pathophysiology of males with obesity. We also discuss therapeutic strategies targeting the AR for the treatment of obesity and their limitations and provide insight into the future research necessary to advance this field.

The AR in bone marrow progenitor cells protects against short-term high-caloric diet-induced weight gain in male mice

We previously identified a novel pathway of testosterone action via the androgen receptor (AR) in bone marrow mesenchymal precursor cells (BM-PCs) to negatively regulate fat mass and improve metabolic function in male mice. This was achieved using our PC-AR Gene Replacement mouse model in which the AR is only expressed in BM-PCs and deleted in all other tissues. We hypothesise that the markedly reduced fat mass and increased insulin sensitivity of PC-AR Gene Replacements will confer protection from diet-induced overweight and obesity. To test this, 6-week-old male PC-AR Gene Replacements and controls (WT, global-AR knockouts (KOs)) were fed a chow or high-caloric diet (HCD) for 8 or 18 weeks. Following 8 weeks (short-term) of HCD, WT and Global-ARKOs had markedly increased subcutaneous white adipose tissue (WAT) and retroperitoneal visceral adipose tissue (VAT) mass compared to chow-fed controls. In contrast, PC-AR Gene Replacements were resistant to WAT and VAT accumulation following short-term HCD feeding accompanied by fewer large adipocytes and upregulation of expression of the metabolic genes Acaca and Pnlpa2. Following long-term HCD feeding for 18 weeks, the PC-AR Gene Replacements were no longer resistant to increased WAT and VAT adiposity, however, maintained their improved whole-body insulin sensitivity with an increased rate of glucose disappearance and increased glucose uptake into subcutaneous WAT. In conclusion, the action of testosterone via the AR in BM-PCs to negatively regulate fat mass and improve metabolism confers resistance from short-term diet-induced weight gain and partial protection from long-term diet-induced obesity in male mice.

Changes in white adipose tissue gene expression in a randomized control trial of dieting obese men with lowered serum testosterone alone or in combination with testosterone treatment

PURPOSE

The aim of this study was to determine early weight loss-associated changes in subcutaneous abdominal white adipose tissue (WAT) gene expression in obese men with lowered serum testosterone by RNA next-generation sequencing.

METHODS

Fourteen men, mean age (IQR) 51.6 years (43.4-54.5), BMI 38.3 kg/m² (34.6-40.8) and total testosterone 8.4 nmol/L (7.5-9.5) provided subcutaneous WAT samples at baseline and after 2 weeks of a very low energy diet.

RESULTS

Body weight loss was similar in participants receiving testosterone (n = 6), -5.27 kg [95% CI -6.17; -4.26], and placebo (n = 8), -4.57 kg [95% CI -6.10; -3.55], p = 0.86. In placebo-treated men, of the 14,410 genes expressed in subcutaneous WAT, four genes, Angiopoietin-like 4, Semaphorin 3 G, Neuropilin 2 and Angiopoietin 4, were upregulated (adjusted false discovery rate P < 0.05). In an exploratory analysis comparing men receiving testosterone and placebo, the most-upregulated gene in the testosterone group (exploratory p < 0.0005) was the neuropeptide y receptor 2.

CONCLUSIONS

In obese men, dieting is associated with upregulation of WAT-expressed Angiopoietin-like 4, a secreted protein that regulates lipid metabolism, Semaphorin 3 G, a proposed adipocyte differentiation factor and secreted adipokine, and its receptor Neuropilin 2, as well as Angiopoietin 4, a vascular integrity factor. In an exploratory analysis, testosterone was associated with the upregulation of neuropeptide y receptor 2, a receptor involved in appetite regulation. Further studies are needed to confirm these observations and their potential biological implications.

TRIAL REGISTRATION

clinicaltrials.gov, Identifier NCT01616732, Registration date: June 8, 2012.

Genetic Depletion of Amylin/Calcitonin Receptors Improves Memory and Learning in Transgenic Alzheimer's Disease Mouse Models

Based upon its interactions with amyloid β peptide (Aβ), the amylin receptor, a class B G protein-coupled receptor (GPCR), is a potential modulator of Alzheimer's disease (AD) pathogenesis. However, past pharmacological approaches have failed to resolve whether activation or blockade of this receptor would have greater therapeutic benefit. To address this issue, we generated compound mice expressing a human amyloid precursor protein gene with familial AD mutations in combination with deficiency of amylin receptors produced by hemizygosity for the critical calcitonin receptor subunit of this heterodimeric GPCR. These compound transgenic AD mice demonstrated attenuated responses to human amylin- and Aβ-induced depression of hippocampal long-term potentiation (LTP) in keeping with the genetic depletion of amylin receptors. Both the LTP responses and spatial memory (as measured with Morris water maze) in these mice were improved compared to AD mouse controls and, importantly, a reduction in both the amyloid plaque burden and markers of neuroinflammation was observed. Our data support the notion of further development of antagonists of the amylin receptor as AD-modifying therapies.

Distinct roles of androgen receptor, estrogen receptor alpha, and BCL6 in the establishment of sex-biased DNA methylation in mouse liver

Sexual dimorphism in gene regulation, including DNA methylation, is the main driver of sexual dimorphism in phenotypes. However, the questions of how and when sex shapes DNA methylation remain unresolved. Recently, using mice with different combinations of genetic and phenotypic sex, we identified sex-associated differentially methylated regions (sDMRs) that depended on the sex phenotype. Focusing on a panel of validated sex-phenotype dependent male- and female-biased sDMRs, we tested the developmental dynamics of sex bias in liver methylation and the impacts of mutations in the androgen receptor, estrogen receptor alpha, or the transcriptional repressor Bcl6 gene. True hermaphrodites that carry both unilateral ovaries and contralateral testes were also tested. Our data show that sex bias in methylation either coincides with or follows sex bias in the expression of sDMR-proximal genes, suggesting that sex bias in gene expression may be required for demethylation at certain sDMRs. Global ablation of AR, ESR1, or a liver-specific loss of BCL6, all alter sDMR methylation, whereas presence of both an ovary and a testis delays the establishment of male-type methylation levels in hermaphrodites. Moreover, the Bcl6-LKO shows dissociation between expression and methylation, suggesting a distinct role of BCL6 in demethylation of intragenic sDMRs.

Vitamin D receptor expression in mature osteoclasts reduces bone loss due to low dietary calcium intake in male mice

Mature osteoclasts express the vitamin D receptor (VDR) and are able to respond to active vitamin D (1α, 25-dihydroxyvitamin D3; 1,25(OH)2D3) by regulating cell maturation and activity. However, the in vivo consequences of vitamin D signalling directly within functionally mature osteoclasts is only partially understood. To investigate the in vivo role of VDR in mature osteoclasts, conditional deletion of the VDR under control of the cathepsin K promoter (CtskCre/Vdr-/-), was assessed in 6 and 12-week-old mice, either under normal dietary conditions (NormCaP) or when fed a low calcium (0.03 %), low phosphorous (0.08 %) diet (LowCaP). Splenocytes from CtskCre/Vdr-/- mice were co-cultured with MLO-Y4 osteocyte-like cells to assess the effect on osteoclastogenesis. Six-week-old CtskCre/Vdr-/- mice demonstrated a 10 % decrease in vertebral bone volume (p < 0.05), which was associated with increased osteoclast size (p < 0.05) when compared to Vdrfl/fl control mice. Control mice fed a LowCaP diet exhibited extensive trabecular bone loss associated with increased osteoclast surface, number and size (p < 0.0001). Interestingly, CtskCre/Vdr-/- mice fed a LowCaP diet showed exacerbated loss of bone volume fraction (BV/TV%) and trabecular number (Tb.N), by a further 22 % and 21 %, respectively (p < 0.05), suggesting increased osteoclastic bone resorption activity with the loss of VDR in mature osteoclasts under these conditions. Co-culture of CtskCre/Vdr-/- splenocytes with MLO-Y4 cells increased resulting osteoclast numbers 2.5-fold, which were greater in nuclei density and exhibited increased resorption of dentine compared to osteoclasts derived from Vdrfl/fl splenocyte cultures. These data suggest that in addition to RANKL-mediated osteoclastogenesis, intact VDR signalling is required for the direct regulation of the differentiation and activity of osteoclasts in both in vivo and ex vivo settings.

Neuronal androgen receptor is required for activity dependent enhancement of peripheral nerve regeneration

Neuronal activity after nerve injury can enhance axon regeneration and the restoration of function. The mechanism for this enhancement relies in part on hormone receptors, and we previously demonstrated that systemic androgen receptor antagonism blocked the effect of exercise or electrical stimulation on enhancing axon regeneration after nerve injury in both sexes. Here, we tested the hypothesis that the site of this androgen receptor signaling is both neuronal and involves the classical, genomic signaling pathway. In vivo, dorsal root ganglion neurons successfully regenerate in response to activity-dependent neuronal activation, and conditional deletion of the DNA-binding domain of the androgen receptor in adults blocks this effect in males and females. Motoneurons in males and females also respond in this manner, but we also observed a sex difference. In vitro, cultured sensory dorsal root ganglion neurons respond to androgens via traditional androgen receptor signaling mechanisms leading to enhanced neurite growth and did not respond to a testosterone conjugate that is unable to cross the cell membrane. Given our previous observation of a requirement for activity-dependent androgen receptor signaling to promote regeneration in both sexes, we interpret our results to indicate that genomic neuronal androgen receptor signaling is required for activity-dependent axon regeneration in both sexes.

Multidose evaluation of 6,710 drug repurposing library identifies potent SARS-CoV-2 infection inhibitors In Vitro and In Vivo

The SARS-CoV-2 pandemic has caused widespread illness, loss of life, and socioeconomic disruption that is unlikely to resolve until vaccines are widely adopted, and effective therapeutic treatments become established. Here, a well curated and annotated library of 6710 clinical and preclinical molecules, covering diverse chemical scaffolds and known host targets was evaluated for inhibition of SARS-CoV-2 infection in multiple infection models. Multi-concentration, high-content immunocytofluorescence-based screening identified 172 strongly active small molecules, including 52 with submicromolar potencies. The active molecules were extensively triaged by in vitro mechanistic assays, including human primary cell models of infection and the most promising, obatoclax, was tested for in vivo efficacy. Structural and mechanistic classification of compounds revealed known and novel chemotypes and potential host targets involved in each step of the virus replication cycle including BET proteins, microtubule function, mTOR, ER kinases, protein synthesis and ion channel function. In the mouse disease model obatoclax effectively reduced lung virus load by 10-fold. Overall, this work provides an important, publicly accessible, foundation for development of novel treatments for COVID-19, establishes human primary cell-based pharmacological models for evaluation of therapeutics and identifies new insights into SARS-CoV-2 infection mechanisms.

Significance

A bioinformatically rich library of pharmacologically active small molecules with diverse chemical scaffolds and including known host targets were used to identify hundreds of SARS-CoV-2 replication inhibitors using in vitro, ex vivo, and in vivo models. Extending our previous work, unbiased screening demonstrated a propensity for compounds targeting host proteins that interact with virus proteins. Representatives from multiple chemical classes revealed differences in cell susceptibility, suggesting distinct dependencies on host factors and one, Obatoclax, showed 90% reduction of lung virus loads in the mouse disease model. Our findings and integrated analytical approaches will have important implications for future drug screening and how therapies are developed against SARS-CoV-2 and other viruses.

The calcitonin receptor regulates osteocyte lacunae acidity during lactation in mice

The physiological role of calcitonin, and its receptor, the CTR (or Calcr), has long been debated. We previously provided the first evidence for a physiological role of the CTR to limit maternal bone loss during lactation in mice by a direct action on osteocytes to inhibit osteocytic osteolysis. We now extend these findings to show that CTR gene expression is upregulated two- to three-fold in whole bone of control mice at the end of pregnancy (E18) and lactation (P21) compared to virgin controls. This was associated with an increase in osteoclast activity evidenced by increases in osteoclast surface/bone surface and Dcstamp gene expression. To investigate the mechanism by which the CTR inhibits osteocytic osteolysis, in vivo acidification of the osteocyte lacunae during lactation (P14 days) was assessed using a pH indicator dye. A lower pH was observed in the osteocyte lacunae of lactating Global-CTRKOs compared to controls and was associated with an increase in the gene expression of ATPase H+ transporting V0 subunit D2 (Atp6v0d2) in whole bone of Global-CTRKOs at the end of lacation (P21). To determine whether the CTR is required for the replacement of mineral within the lacunae post-lactation, lacunar area was determined 3 weeks post-weaning. Comparison of the largest 20% of lacunae by area did not differ between Global-CTRKOs and controls post-lactation. These results provide evidence for CTR activation to inhibit osteocytic osteolysis during lactation being mediated by regulating the acidity of the lacunae microenvironment, whilst the CTR is dispensable for replacement of bone mineral within lacunae by osteocytes post-lactation.

Paracrine signalling by cardiac calcitonin controls atrial fibrogenesis and arrhythmia

Atrial fibrillation, the most common cardiac arrhythmia, is an important contributor to mortality and morbidity, and particularly to the risk of stroke in humans¹. Atrial-tissue fibrosis is a central pathophysiological feature of atrial fibrillation that also hampers its treatment; the underlying molecular mechanisms are poorly understood and warrant investigation given the inadequacy of present therapies². Here we show that calcitonin, a hormone product of the thyroid gland involved in bone metabolism³, is also produced by atrial cardiomyocytes in substantial quantities and acts as a paracrine signal that affects neighbouring collagen-producing fibroblasts to control their proliferation and secretion of extracellular matrix proteins. Global disruption of calcitonin receptor signalling in mice causes atrial fibrosis and increases susceptibility to atrial fibrillation. In mice in which liver kinase B1 is knocked down specifically in the atria, atrial-specific knockdown of calcitonin promotes atrial fibrosis and increases and prolongs spontaneous episodes of atrial fibrillation, whereas atrial-specific overexpression of calcitonin prevents both atrial fibrosis and fibrillation. Human patients with persistent atrial fibrillation show sixfold lower levels of myocardial calcitonin compared to control individuals with normal heart rhythm, with loss of calcitonin receptors in the fibroblast membrane. Although transcriptome analysis of human atrial fibroblasts reveals little change after exposure to calcitonin, proteomic analysis shows extensive alterations in extracellular matrix proteins and pathways related to fibrogenesis, infection and immune responses, and transcriptional regulation. Strategies to restore disrupted myocardial calcitonin signalling thus may offer therapeutic avenues for patients with atrial fibrillation.

Androgens stimulate erythropoiesis through the DNA-binding activity of the androgen receptor in non-hematopoietic cells

BACKGROUND

Androgens function through DNA and non-DNA binding-dependent signalling of the androgen receptor (AR). How androgens promote erythropoiesis is not fully understood.

DESIGN AND METHODS

To identify the androgen signalling pathway, we treated male mice lacking the second zinc finger of the DNA-binding domain of the AR (AR^ΔZF2 ) with non-aromatizable 5α-dihydrotestosterone (5α-DHT) or aromatizable testosterone. To distinguish direct hematopoietic and non-hematopoietic mechanisms, we performed bone marrow reconstitution experiments.

RESULTS

In wild-type mice, 5α-DHT had greater erythroid activity than testosterone, which can be aromatized to estradiol. The erythroid response in wild-type mice following 5α-DHT treatment was associated with increased serum erythropoietin (EPO) and its downstream target erythroferrone, and hepcidin suppression. 5α-DHT had no erythroid activity in AR^ΔZF2 mice, proving the importance of DNA binding by the AR. Paradoxically, testosterone, but not 5α-DHT, suppressed EPO levels in AR^ΔZF2 mice, suggesting testosterone following aromatization may oppose the erythroid-stimulating effects of androgens. Female wild-type mice reconstituted with AR^ΔZF2 bone marrow cells remained responsive to 5α-DHT. In contrast, AR^ΔZF2 mice reconstituted with female wild-type bone marrow cells showed no response to 5α-DHT.

CONCLUSION

Erythroid promoting effects of androgens are mediated through DNA binding-dependent actions of the AR in non-hematopoietic cells, including stimulating EPO expression.

Sex-specific adipose tissue imprinting of regulatory T cells

Adipose tissue is an energy store and a dynamic endocrine organ1,2. In particular, visceral adipose tissue (VAT) is critical for the regulation of systemic metabolism3,4. Impaired VAT function-for example, in obesity-is associated with insulin resistance and type 2 diabetes5,6. Regulatory T (Treg) cells that express the transcription factor FOXP3 are critical for limiting immune responses and suppressing tissue inflammation, including in the VAT7-9. Here we uncover pronounced sexual dimorphism in Treg cells in the VAT. Male VAT was enriched for Treg cells compared with female VAT, and Treg cells from male VAT were markedly different from their female counterparts in phenotype, transcriptional landscape and chromatin accessibility. Heightened inflammation in the male VAT facilitated the recruitment of Treg cells via the CCL2-CCR2 axis. Androgen regulated the differentiation of a unique IL-33-producing stromal cell population specific to the male VAT, which paralleled the local expansion of Treg cells. Sex hormones also regulated VAT inflammation, which shaped the transcriptional landscape of VAT-resident Treg cells in a BLIMP1 transcription factor-dependent manner. Overall, we find that sex-specific differences in Treg cells from VAT are determined by the tissue niche in a sex-hormone-dependent manner to limit adipose tissue inflammation.

The androgen receptor in the hypothalamus positively regulates hind-limb muscle mass and voluntary physical activity in adult male mice

We have previously shown that expression of the androgen receptor (AR) in neurons within the brain positively regulates hind-limb muscle mass and physical activity in male mice. To further investigate the region of the brain responsible for mediating these effects of testosterone and to determine whether they are only important for muscle mass accrual during development or whether they are also important for the maintenance of muscle mass in the adult, we deleted the AR specifically in the hypothalamus of adult male mice (Hyp-ARKOs). Hyp-ARKO mice were generated by bilateral stereotaxic microinjection of an adeno-associated virus (AAV) expressing GFP and iCre recombinase under the control of the e-synapsin promoter into the hypothalamus of 10-week-old exon 3-AR floxed male mice. AR mRNA was deleted by 45% in the hypothalamus of Hyp-ARKOs at 5 weeks post-AAV-eSyn-iCre injection. This led to an increase in the mass of the androgen-dependent organs, seminal vesicles and kidneys, by 30% (P < 0.01) and 10% (P < 0.05) respectively, and an increase in serum luteinizing hormone (LH) by 2 fold (P < 0.05). Whilst the mean value for serum testosterone was higher in the Hyp-ARKOs, this did not reach statistical significance. Despite a phenotype consistent with increased androgen bioactivity in Hyp-ARKOs, which would be expected to increase muscle mass, the mass of the hind-limb muscles, gastrocnemius (Gast) (P = 0.001), extensor digitorum longus (EDL) (P < 0.001) and soleus (Sol) (P < 0.01) were paradoxically decreased by 12-19% compared to controls. Voluntary physical activity was reduced by 65% (P < 0.05) in Hyp-ARKO male mice and was associated with a reduction in gene expression of Drd1a and Maob (P ≤ 0.05) in the hypothalamus, suggesting involvement of the brain dopaminergic system. These data provide compelling evidence that androgen signalling via the AR in the hypothalamus acts to positively regulate the maintenance of hind-limb muscle mass and voluntary activity in adult male mice, independent of AR signalling in peripheral tissues.

The androgen receptor in bone marrow progenitor cells negatively regulates fat mass

It is well established that testosterone negatively regulates fat mass in humans and mice; however, the mechanism by which testosterone exerts these effects is poorly understood. We and others have shown that deletion of the androgen receptor (AR) in male mice results in a phenotype that mimics the three key clinical aspects of hypogonadism in human males; increased fat mass and decreased bone and muscle mass. We now show that replacement of the Ar gene specifically in mesenchymal progenitor cells (PCs) residing in the bone marrow of Global-ARKO mice, in the absence of the AR in all other tissues (PC-AR Gene Replacements), completely attenuates their increased fat accumulation. Inguinal subcutaneous white adipose tissue and intra-abdominal retroperitoneal visceral adipose tissue depots in PC-AR Gene Replacement mice were 50-80% lower than wild-type (WT) and 75-90% lower than Global-ARKO controls at 12 weeks of age. The marked decrease in subcutaneous and visceral fat mass in PC-AR Gene Replacements was associated with an increase in the number of small adipocytes and a healthier metabolic profile compared to WT controls, characterised by normal serum leptin and elevated serum adiponectin levels. Euglycaemic/hyperinsulinaemic clamp studies reveal that the PC-AR Gene Replacement mice have improved whole-body insulin sensitivity with higher glucose infusion rates compared to WT mice and increased glucose uptake into subcutaneous and intra-abdominal fat. In conclusion, these data provide the first evidence for an action of androgens via the AR in mesenchymal bone marrow PCs to negatively regulate fat mass and improve metabolic function.

Absence of vitamin D receptor in mature osteoclasts results in altered osteoclastic activity and bone loss

Mature osteoclasts express the vitamin D receptor (VDR) and are able to synthesise and respond to 1,25(OH)₂D₃ via CYP27B1 enzyme activity. Whether vitamin D signalling within osteoclasts is necessary for the regulation of osteoclastic bone resorption in an in vivo setting is unclear. To determine the requirement for the VDR- and CYP27B1-mediated activity in mature osteoclasts, conditional deletion mouse models were created whereby either Vdr or Cyp27b1 gene was inactivated by breeding either Vdr^fl/fl or Cyp27b1^fl/fl mice with Cathepsin K-Cre transgenic mice (Cstk^Cre) to generate Ctsk^Cre/Vdr^-/- and Ctsk^Cre/Cyp27b1^-/- mice respectively. To account for potential Ctsk^Cre-meaited off-target deletion of Vdr, Dmp1^Cre were also used determine the effect of Vdr deletion in osteocytes. Furthermore, Ctsk^Cre/Vdr^-/- mice were ovariectomised (OVX) to assess the role of VDR in osteoclasts under bone-loss conditions and bone marrow precursor cells were cultured under osteoclastogenic conditions to assess osteoclast formation. Six-week-old Ctsk^Cre/Vdr^-/- female mice demonstrated a 15% decrease in femoral BV/TV (p<0.05). In contrast, BV/TV remained unchanged in Ctsk^Cre/Cyp27b1^-/- mice as well as in Dmp1^Cre/VDR^-/- mice. When Ctsk^Cre/Vdr^-/- mice were subjected to OVX, the bone loss that occurred in Ctsk^Cre/Vdr^-/- was predominantly due to a diminished volume of thinner trabeculae when compared to control levels. These changes in bone volume in Ctsk^Cre/Vdr^-/- mice occurred without an observable histological change in osteoclast numbers or size. However, while cultured bone marrow-derived osteoclasts from Ctsk^Cre/Vdr^-/- mice were marginally increased when compared to VDR^fl/fl mice, elevated expression of genes such as Cathepsin K, Nfatc1 and VATPase was observed. Collectively, these data indicate that the absence of VDR in mature osteoclasts causes exacerbated bone loss in young mice and during OVX which is associated with enhanced osteoclastic activity and without increased osteoclastogenesis.

Actin alpha cardiac muscle 1 gene expression is upregulated in the skeletal muscle of men undergoing androgen deprivation therapy for prostate cancer

Androgen deprivation therapy (ADT) decreases muscle mass and function but no human studies have investigated the underlying genetic or cellular effects. We tested the hypothesis that ADT will lead to changes in skeletal muscle gene expression, which may explain the adverse muscle phenotype seen clinically. We conducted a prospective cohort study of 9 men with localised prostate cancer who underwent a vastus lateralis biopsy before and after 4 weeks of ADT. Next-generation RNA sequencing was performed and genes differentially expressed following ADT underwent gene ontology mining using Ingenuity Pathway Analysis. Differential expression of genes of interest was confirmed by quantitative PCR (Q-PCR) on gastrocnemius muscle of orchidectomised mice and sham controls (n=11/group). We found that in men, circulating total testosterone decreased from 16.5±4.3nmol/L at baseline to 0.4±0.15nmol/L post-ADT (p<0.001). RNA sequencing identified 19 differentially expressed genes post-ADT (all p<0.05 after adjusting for multiple testing). Gene ontology mining identified 8 genes to be of particular interest due to known roles in androgen-mediated signalling; ABCG1, ACTC1, ANKRD1, DMPK, THY1, DCLK1, CST3 were upregulated and SLC38A3 was downregulated post-ADT. Q-PCR in mouse gastrocnemius muscle confirmed that only one gene, Actc1 was concordantly upregulated (p<0.01) in orchidectomised mice compared with controls. In conclusion, given that ACTC1 upregulation is associated with improved muscle function in certain myopathies, we hypothesise that upregulation of ACTC1 may represent a compensatory response to ADT-induced muscle loss. Further studies will be required to evaluate the role and function of ACTC1.

Androgen Action via the Androgen Receptor in Neurons Within the Brain Positively Regulates Muscle Mass in Male Mice

Although it is well established that exogenous androgens have anabolic effects on skeletal muscle mass in humans and mice, data from muscle-specific androgen receptor (AR) knockout (ARKO) mice indicate that myocytic expression of the AR is dispensable for hind-limb muscle mass accrual in males. To identify possible indirect actions of androgens via the AR in neurons to regulate muscle, we generated neuron-ARKO mice in which the dominant DNA binding-dependent actions of the AR are deleted in neurons of the cortex, forebrain, hypothalamus, and olfactory bulb. Serum testosterone and luteinizing hormone levels were elevated twofold in neuron-ARKO males compared with wild-type littermates due to disruption of negative feedback to the hypothalamic-pituitary-gonadal axis. Despite this increase in serum testosterone levels, which was expected to increase muscle mass, the mass of the mixed-fiber gastrocnemius (Gast) and the fast-twitch fiber extensor digitorum longus hind-limb muscles was decreased by 10% in neuron-ARKOs at 12 weeks of age, whereas muscle strength and fatigue of the Gast were unaffected. The mass of the soleus muscle, however, which consists of a high proportion of slow-twitch fibers, was unaffected in neuron-ARKOs, demonstrating a stimulatory action of androgens via the AR in neurons to increase the mass of fast-twitch hind-limb muscles. Furthermore, neuron-ARKOs displayed reductions in voluntary and involuntary physical activity by up to 60%. These data provide evidence for a role of androgens via the AR in neurons to positively regulate fast-twitch hind-limb muscle mass and physical activity in male mice.

Reduced bone formation markers, and altered trabecular and cortical bone mineral densities of non-paretic femurs observed in rats with ischemic stroke: A randomized controlled pilot study

Background

Immobility and neural damage likely contribute to accelerated bone loss after stroke, and subsequent heightened fracture risk in humans.

Objective

To investigate the skeletal effect of middle cerebral artery occlusion (MCAo) stroke in rats and examine its utility as a model of human post-stroke bone loss.

Methods

Twenty 15-week old spontaneously hypertensive male rats were randomized to MCAo or sham surgery controls. Primary outcome: group differences in trabecular bone volume fraction (BV/TV) measured by Micro-CT (10.5 micron istropic voxel size) at the ultra-distal femur of stroke affected left legs at day 28. Neurological impairments (stroke behavior and foot-faults) and physical activity (cage monitoring) were assessed at baseline, and days 1 and 27. Serum bone turnover markers (formation: N-terminal propeptide of type 1 procollagen, PINP; resorption: C-terminal telopeptide of type 1 collagen, CTX) were assessed at baseline, and days 7 and 27.

Results

No effect of stroke was observed on BV/TV or physical activity, but PINP decreased by -24.5% (IQR -34.1, -10.5, p = 0.046) at day 27. In controls, cortical bone volume (5.2%, IQR 3.2, 6.9) and total volume (6.4%, IQR 1.2, 7.6) were higher in right legs compared to left legs, but these side-to-side differences were not evident in stroke animals.

Conclusion

MCAo may negatively affect bone formation. Further investigation of limb use and physical activity patterns after MCAo is required to determine the utility of this current model as a representation of human post-stroke bone loss.

Mechanisms of Filovirus Entry

Filovirus entry into cells is complex, perhaps as complex as any viral entry mechanism identified to date. However, over the past 10 years, the important events required for filoviruses to enter into the endosomal compartment and fuse with vesicular membranes have been elucidated (Fig. 1). Here, we highlight the important steps that are required for productive entry of filoviruses into mammalian cells.

Sex-related differences in the skeletal phenotype of aged vitamin D receptor global knockout mice

The role of the vitamin D receptor (VDR) in maintaining skeletal health appears to be complex and dependent on the physiological context. Global Vdr deletion in a mouse model (Vdr^-/-) results in hypocalcemia, secondary hyperparathyroidism and bone features typical of vitamin D-dependent rickets type II. When weanling Vdr^-/- mice are fed a diet containing high levels of calcium, phosphorus and lactose, termed the rescue diet, normalisation of serum calcium, phosphate and parathyroid hormone levels results in prevention of rickets at 10 weeks of age. However, 17 week old male Vdr^-/- mice, fed the rescue diet, have been reported as osteopenic due to a decrease in bone formation when compared to wild type mice. We now report confirmation of this finding with further data on the effect of the rescue diet on appendicular and axial skeletal structures in male and female Vdr^-/- mice at 26 weeks of age compared to Vdr^+/- controls. All Vdr^-/- mice were normocalcemic with no evidence of any mineralization defect. However, male Vdr^-/- mice exhibited significantly reduced mineral in femoral and vertebral bones when compared to control littermate Vdr^+/- mice, consistent with the previously reported data. In contrast, 26-week-old female Vdr^-/- mice demonstrated significantly increased femoral trabecular bone volume although there was decreased vertebral trabecular bone volume, similar to males, and femoral cortical bone volume was unchanged. Thus, the Vdr^-/- mouse model displays sex- and site-specific differences in skeletal structures with long-term feeding of a rescue diet. Although the global Vdr^-/- ablation does not permit the determination of skeletal mechanisms producing these differences, these data confirm skeletal changes even when fed the rescue diet.

Androgen Receptor Structure, Function and Biology: From Bench to Bedside

The actions of androgens such as testosterone and dihydrotestosterone are mediated via the androgen receptor (AR), a ligand-dependent nuclear transcription factor and member of the steroid hormone nuclear receptor family. Given its widespread expression in many cells and tissues, the AR has a diverse range of biological actions including important roles in the development and maintenance of the reproductive, musculoskeletal, cardiovascular, immune, neural and haemopoietic systems. AR signalling may also be involved in the development of tumours in the prostate, bladder, liver, kidney and lung. Androgens can exert their actions via the AR in a DNA binding-dependent manner to regulate target gene transcription, or in a non-DNA binding-dependent manner to initiate rapid, cellular events such as the phosphorylation of 2(nd) messenger signalling cascades. More recently, ligand-independent actions of the AR have also been identified. Given the large volume of studies relating to androgens and the AR, this review is not intended as an extensive review of all studies investigating the AR, but rather as an overview of the structure, function, signalling pathways and biology of the AR as well as its important role in clinical medicine, with emphasis on recent developments in this field.

Wnt Signaling Inhibits Osteoclast Differentiation by Activating Canonical and Noncanonical cAMP/PKA Pathways

Although there has been extensive characterization of the Wnt signaling pathway in the osteoblast lineage, the effects of Wnt proteins on the osteoclast lineage are less well studied. We found that osteoclast lineage cells express canonical Wnt receptors. Wnt3a reduced osteoclast formation when applied to early bone-marrow macrophage (BMM) osteoclast differentiation cultures, whereas late addition did not suppress osteoclast formation. Early Wnt3a treatment inactivated the crucial transcription factor NFATc1 in osteoclast progenitors. Wnt3a led to the accumulation of nuclear β-catenin, confirming activation of canonical Wnt signaling. Reducing low-density lipoprotein receptor-related proteins (Lrp) 5 and Lrp6 protein expression prevented Wnt3a-induced inactivation of NFATc1; however, deletion of β-catenin did not block Wnt3a inactivation of NFATc1, suggesting that this effect was mediated by a noncanonical pathway. Wnt3a rapidly activated the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) pathway and pharmacological stimulation of cAMP/PKA signaling suppressed osteoclast differentiation; Wnt3a-induced NFATc1 phosphorylation was blocked by inhibiting interactions between PKA and A-kinase anchoring proteins (AKAPs). These data indicate that Wnt3a directly suppresses osteoclast differentiation through both canonical (β-catenin) and noncanonical (cAMP/PKA) pathways in osteoclast precursors. In vivo reduction of Lrp5 and Lrp6 expressions in the early osteoclast lineage via Rank promoter Cre recombination reduced trabecular bone mass, whereas disruption of Lrp5/6 expression in late osteoclast precursors via cathepsin K (Ctsk) promoter Cre recombination did not alter the skeletal phenotype. Surprisingly, reduction of Lrp5/6 in the early osteoclast lineage decreased osteoclast numbers, as well as osteoblast numbers. Published studies have previously noted that β-catenin signaling is required for osteoclast progenitor proliferation. Our in vivo data suggest that Rank promoter Cre-mediated deletion of Lrp5/6 may similarly impair osteoclast progenitor proliferation.

Osteoclast TGF-β Receptor Signaling Induces Wnt1 Secretion and Couples Bone Resorption to Bone Formation

Osteoblast-mediated bone formation is coupled to osteoclast-mediated bone resorption. These processes become uncoupled with age, leading to increased risk for debilitating fractures. Therefore, understanding how osteoblasts are recruited to sites of resorption is vital to treating age-related bone loss. Osteoclasts release and activate TGF-β from the bone matrix. Here we show that osteoclast-specific inhibition of TGF-β receptor signaling in mice results in osteopenia due to reduced osteoblast numbers with no significant impact on osteoclast numbers or activity. TGF-β induced osteoclast expression of Wnt1, a protein crucial to normal bone formation, and this response was blocked by impaired TGF-β receptor signaling. Osteoclasts in aged murine bones had lower TGF-β signaling and Wnt1 expression in vivo. Ex vivo stimulation of osteoclasts derived from young or old mouse bone marrow macrophages showed no difference in TGF-β-induced Wnt1 expression. However, young osteoclasts expressed reduced Wnt1 when cultured on aged mouse bone chips compared to young mouse bone chips, consistent with decreased skeletal TGF-β availability with age. Therefore, osteoclast responses to TGF-β are essential for coupling bone resorption to bone formation, and modulating this pathway may provide opportunities to treat age-related bone loss.

A Role for the Calcitonin Receptor to Limit Bone Loss During Lactation in Female Mice by Inhibiting Osteocytic Osteolysis

During lactation, the large transfer of calcium from the mother to the milk is primarily sourced from the maternal skeleton. To determine whether the calcitonin receptor (CTR) plays a physiological role to protect the skeleton from excessive resorption during lactation, we assessed the maternal skeleton of global CTR knockout (CTRKO) and littermate control mice at the end of lactation (postnatal day 21). Micro-computed tomography analyses showed no effect on trabecular or cortical bone in the distal femur and L1 vertebra of maternal global CTR deletion at the end of lactation in global CTRKO mice compared with that in control mice. Bone resorption, as assessed by osteoclast number and activity at the end of lactation, was unaffected by maternal CTR deletion. Cathepsin K, carbonic anhydrase 2, matrix metalloproteinase 13, and receptor activator of nuclear factor-κB ligand mRNA levels, however, were markedly elevated by 3- to 6.5-fold in whole bone of lactating global CTRKO females. Because these genes have been shown to be up-regulated in osteocytes during lactation when osteocytes resorb their surrounding bone matrix, together with their reported expression of the CTR, we determined the osteocyte lacunar area in cortical bone. After lactation, the top 20% of osteocyte lacunar area in global CTRKO mice was 10% larger than the top 20% in control mice. These data are consistent with an increased osteocytic osteolysis in global CTRKO mice during lactation, which is further supported by the increased serum calcium observed in global CTRKO mice after lactation. These results provide evidence for a physiological role for the CTR to protect the maternal skeleton during lactation by a direct action on osteocytes to inhibit osteolysis.

The androgen receptor has no direct antiresorptive actions in mouse osteoclasts

Androgen deficiency or androgen receptor knockout (ARKO) causes high-turnover osteopenia, but the target cells for this effect remain unclear. To examine whether AR in osteoclasts directly suppresses bone resorption, we crossed AR-floxed with cathepsin K-Cre mice. Osteoclast-specific ARKO (ocl-ARKO) mice showed no changes neither in osteoclast surface nor in bone microarchitecture nor in the response to orchidectomy and androgen replacement, indicating that the AR in osteoclasts is not critical for bone maintenance. In line with the lack of a bone phenotype, the levels of AR were very low in osteoclast-enriched cultures derived from bone marrow (BM) and undetectable in osteoclasts generated from spleen precursors. Since tibiae of ubiquitous ARKO mice displayed increased osteoclast counts, the role of AR was further explored using cell cultures from these animals. Osteoclast generation and activity in vitro were similar between ARKO and wildtype control (WT) mice. In co-culture experiments, BM stromal cells (BMSCs) were essential for the suppressive action of AR on osteoclastogenesis and osteoclast activity. Stimulation with 1,25(OH)2 vitamin D3 increased Rankl and decreased Tnfsf11 (osteoprotegerin, Opg) gene expression in BMSCs more than in osteoblasts. This increase in the Rankl/Opg ratio following 1,25(OH)2D3 stimulation was lower, not higher, in ARKO mice. Runx2 expression in BMSCs was however higher in ARKO vs. WT, suggesting that ARKO mice may more readily commit osteoprogenitor cells to osteoblastogenesis. In conclusion, the AR does not seem to suppress bone resorption through direct actions in osteoclasts. BMSCs may however represent an alternative AR target in the BM milieu.

Androgen receptor action in osteoblasts in male mice is dependent on their stage of maturation

Androgen action via the androgen receptor (AR) is essential for normal skeletal growth and bone maintenance post-puberty in males; however, the molecular and cellular mechanisms by which androgens exert their actions in osteoblasts remains relatively unexplored in vivo. To identify autonomous AR actions in osteoblasts independent of AR signaling in other tissues, we compared the extent to which the bone phenotype of the Global-ARKO mouse was restored by replacing the AR in osteoblasts commencing at either the (1) proliferative or (2) mineralization stage of their maturation. In trabecular bone, androgens stimulated trabecular bone accrual during growth via the AR in proliferating osteoblasts and maintained trabecular bone post-puberty via the AR in mineralizing osteoblasts, with its predominant action being to inhibit bone resorption by decreasing the ratio of receptor activator of NF-κB ligand (RANKL) to osteoprotegerin (OPG) gene expression. During growth, replacement of the AR in proliferating but not mineralizing osteoblasts of Global-ARKOs was able to partially restore periosteal circumference, supporting the concept that androgen action in cortical bone to increase bone size during growth is mediated via the AR in proliferating osteoblasts. This study provides further significant insight into the mechanism of androgen action via the AR in osteoblasts, demonstrating that it is dependent on the stage of osteoblast maturation.

Human androgen deficiency: insights gained from androgen receptor knockout mouse models

The mechanism of androgen action is complex. Recently, significant advances have been made into our understanding of how androgens act via the androgen receptor (AR) through the use of genetically modified mouse models. A number of global and tissue-specific AR knockout (ARKO) models have been generated using the Cre-loxP system which allows tissue- and/or cell-specific deletion. These ARKO models have examined a number of sites of androgen action including the cardiovascular system, the immune and hemopoetic system, bone, muscle, adipose tissue, the prostate and the brain. This review focuses on the insights that have been gained into human androgen deficiency through the use of ARKO mouse models at each of these sites of action, and highlights the strengths and limitations of these Cre-loxP mouse models that should be considered to ensure accurate interpretation of the phenotype.

Blunted sympathoinhibitory responses in obesity-related hypertension are due to aberrant central but not peripheral signalling mechanisms

The gut hormone cholecystokinin (CCK) acts at subdiaphragmatic vagal afferents to induce renal and splanchnic sympathoinhibition and vasodilatation, via reflex inhibition of a subclass of cardiovascular-controlling neurons in the rostroventrolateral medulla (RVLM). These sympathoinhibitory and vasodilator responses are blunted in obese, hypertensive rats and our aim in the present study was to determine whether this is attributable to (i) altered sensitivity of presympathetic vasomotor RVLM neurons, and (ii) aberrant peripheral or central signalling mechanisms. Using a diet-induced obesity model, male Sprague-Dawley rats exhibited either an obesity-prone (OP) or obesity-resistant (OR) phenotype when placed on a medium high fat diet for 13-15 weeks; control animals were placed on a low fat diet. OP animals had elevated resting arterial pressure compared to OR/control animals (P < 0.05). Barosensitivity of RVLM neurons was significantly attenuated in OP animals (P < 0.05), suggesting altered baroreflex gain. CCK induced inhibitory responses in RVLM neurons of OR/control animals but not OP animals. Subdiaphragmatic vagal nerve responsiveness to CCK and CCK1 receptor mRNA expression in nodose ganglia did not differ between the groups, but CCK induced significantly less Fos-like immunoreactivity in both the nucleus of the solitary tract and the caudal ventrolateral medulla of OP animals compared to controls (P < 0.05). These results suggest that blunted sympathoinhibitory and vasodilator responses in obesity-related hypertension are due to alterations in RVLM neuronal responses, resulting from aberrant central but not peripheral signalling mechanisms. In obesity, blunted sympathoinhibitory mechanisms may lead to increased regional vascular resistance and contribute to the development of hypertension.

Normal phenotype in conditional androgen receptor (AR) exon 3-floxed neomycin-negative male mice

Androgens (testosterone and dihydrotestosterone) acting via the androgen receptor (AR) are required for male sexual differentiation, and also regulate the development of many other tissues including muscle, fat and bone. We previously generated an AR(lox) mouse line with exon 3 of the AR gene targeted by loxP sites. The deletion of exon 3 is in-frame, so only the DNA binding-dependent actions of the AR are deleted, but non-DNA binding-dependent actions are retained. This line also contained an antibiotic resistance selection cassette, neomycin (neo) in intron 3, which was also flanked by loxP sites. Hemizygous AR(lox) male mice demonstrated a phenotype of hyperandrogenization, with increased mass of androgen-dependent tissues. We hypothesized that this hyperandrogenization was likely to be due to the presence of the neo cassette. In this study, we have generated an AR(lox) neo-negative mouse line, using the EIIa-cre deleter mouse line to remove the neo cassette. Hemizygous AR(lox) neo-negative male mice have a normal phenotype, with normal body mass and normal mass of androgen-dependent tissues including the testis, seminal vesicles, kidney, spleen, heart and retroperitoneal fat. This neo-negative exon 3-targeted mouse line is the only floxed AR mouse line available to study the DNA binding-dependent actions of the AR in a tissue-specific manner, and is suitable for investigation in all tissues. This study demonstrates the importance of removing the selection cassette, which can potentially alter the phenotype of floxed mouse lines even in the absence of detectable effects on target gene expression.

The pleiotropic effects of vitamin D in bone

A current controversial question related to vitamin D supplementation is what level of serum 25-hydroxyvitamin D3 (25(OH)D3) is required to reduce the incidence of osteoporotic fractures. The reasoning behind vitamin D supplementation has been mostly derived from the role of vitamin D to promote intestinal calcium absorption and reduce bone resorption. While minimum 25(OH)D3 levels of 20nmol/L are required for sufficient intestinal calcium absorption to prevent osteomalacia, the mechanistic details of how higher 25(OH)D3 levels, well beyond that required for optimal calcium absorption, are able to prevent fractures and increase bone mineral density is unclear. Substantial evidence has arisen over the past decade that conversion of 25(OH)D3 to 1,25(OH)2D3via the 1-alpha hydroxylase (CYP27B1) enzyme in osteoblasts, osteocytes, chondrocytes and osteoclasts regulates processes such as cell proliferation, maturation and mineralization as well as bone resorption, which are all dependent on the presence the of the vitamin D receptor (VDR). We and others have also shown that increased vitamin D activity in mature osteoblasts by increasing levels of VDR or CYP27B1 leads to improved bone mineral volume using two separate transgenic mouse models. While questions remain regarding activities of vitamin D in bone to influence the anabolic and catabolic processes, the biological importance of vitamin D activity within the bone is unquestioned. However, a clearer understanding of the varied mechanisms by which vitamin D directly and indirectly influences mineral bone status are required to support evidence-based recommendations for vitamin D supplementation to reduce the risk of fractures. This article is part of a Special Issue entitled 'Vitamin D workshop'.

Calcitonin: physiology or fantasy?

Calcitonin, a potent hypocalcemic hormone produced by the C-cells of the thyroid, was first discovered by Harold Copp in 1962. The physiological significance of calcitonin has been questioned, but recent studies using genetically modified mouse models have uncovered additional actions of calcitonin acting through its receptor (CTR) that are of particular significance to the regulation of bone and calcium homeostasis. Mice in which the CTR is deleted in osteoclasts are more susceptible to induced hypercalcemia and exogenous calcitonin is able to lower serum calcium in younger animals. These data are consistent with the hypothesis that calcitonin can regulate serum calcium by inhibiting the efflux of calcium from bone, and that this action is most important when bone turnover is high. Calcitonin has also been implicated in protecting the skeleton from excessive loss of bone mineral during times of high calcium demand, such as lactation. This action may be linked to an intriguing and as yet unexplained observation that calcitonin inhibits bone formation, because deletion of the CTR leads to increased bone formation. We propose several mechanisms by which calcitonin could protect the skeleton by regulating bone turnover, acting within the bone and/or centrally. A new more holistic notion of the physiological role of calcitonin in bone and calcium homeostasis is required and we have highlighted some important knowledge gaps so that future calcitonin research will help to achieve such an understanding.

Identification of gene pathways altered by deletion of the androgen receptor specifically in mineralizing osteoblasts and osteocytes in mice

Androgens play a key role in skeletal growth and maintenance in males and can mediate their actions, at least in part, via the androgen receptor (AR) in osteoblasts. To investigate the mechanisms by which androgens exert their effects via the AR in mineralizing osteoblasts and osteocytes, we identified gene targets/pathways regulated by the AR using targeted gene expression and microarray approaches on bone isolated from mice in which the AR is specifically deleted in mineralizing osteoblasts and osteocytes (mOBL-ARKOs). Gene ontology mining indicated a number of biological processes to be affected in the bones of mOBL-ARKOs including skeletal and muscular system development and carbohydrate metabolism. All genes identified to have altered expression in the bones of mOBL-ARKOs were confirmed by Q-PCR for their androgen responsiveness in an androgen deprivation and replacement mouse model. The osteoblast genes Col1a1 and Bglap and the osteoclast genes Ctsk and RANKL (Tnfs11) were upregulated in the bones of mOBL-ARKOs, consistent with the increased matrix synthesis, mineralization, and bone resorption observed previously in these mice. Of significant interest, we identified genes involved in carbohydrate metabolism (adiponectin and Dpp4) and in growth and development (GH, Tgfb (Tgfb2), Wnt4) as potential targets of androgen action via the AR in mineralizing osteoblasts.

A physiological role for androgen actions in the absence of androgen receptor DNA binding activity

We tested the hypothesis that androgens have physiological actions via non-DNA binding-dependent androgen receptor (AR) signaling pathways in males, using our genetically modified mice that express a mutant AR with deletion of the 2nd zinc finger of the DNA binding domain (AR(ΔZF2)) that cannot bind DNA. In cultured genital skin fibroblasts, the mutant AR(ΔZF2) has normal ligand binding ability, phosphorylates ERK-1/2 in response to 1 min DHT treatment (blocked by the AR antagonist bicalutamide), but has reduced androgen-dependent nuclear localization compared to wildtype (WT). AR(ΔZF2) males have normal baseline ERK-1/2 phosphorylation, with a 1.5-fold increase in Akt phosphorylation in AR(ΔZF2) muscle vs WT. To identify physiological actions of non-DNA binding-dependent AR signaling, AR(ΔZF2) males were treated for 6 weeks with dihydrotestosterone (DHT). Cortical bone growth was suppressed by DHT in AR(ΔZF2) mice (6% decrease in periosteal and 7% decrease in medullary circumference vs untreated AR(ΔZF2) males). In conclusion, these data suggest that non-DNA binding dependent AR actions suppress cortical bone growth, which may provide a mechanism to fine-tune the response to androgens in bone.

Decreased body weight in young Osterix-Cre transgenic mice results in delayed cortical bone expansion and accrual

Conditional gene inactivation using the Cre/loxP system has lead to significant advances in our understanding of the function of genes in a wide range of disciplines. It is becoming increasingly apparent in the literature, that Cre transgenic mice may themselves have a phenotype. In the following study we describe the bone phenotype of a commonly used Cre transgenic mouse line to study osteoblasts, the Osx-GFP::Cre (Osx-Cre) mice. Cortical and trabecular bone parameters were determined in the femurs of Osx-Cre mice at 6 and 12 weeks of age by microtomography (μCT). At 6 weeks of age, Osx-Cre mice had reduced body weight by 22% (P < 0.0001) and delayed cortical bone expansion and accrual, characterized by decreases in periosteal circumference by 7% (P < 0.05) and cortical thickness by 11% (P < 0.01), compared to wild type controls. Importantly, the cortical bone phenotype of the skeletally immature Osx-Cre mice at 6 weeks of age could be accounted for by their low body weight. The delayed weight gain and cortical growth of Osx-Cre mice was overcome by 12 weeks of age, with no differences observed between Osx-Cre and wild type controls. In conclusion, Osx-Cre expressing mice display a delayed growth phenotype in the absence of doxycycline treatment, evidenced by decreased cortical bone expansion and accrual at 6 weeks of age, as an indirect result of decreased body weight. While this delay in growth is overcome by adulthood at 12 weeks of age, caution together with appropriate data analysis must be considered when assessing the experimental data from skeletally immature Cre/loxP knockout mice generated using the Osx-Cre mouse line to avoid misinterpretation.

The role of the calcitonin receptor in protecting against induced hypercalcemia is mediated via its actions in osteoclasts to inhibit bone resorption

Despite the therapeutic value of calcitonin in treating bone disease, a biological role of endogenous calcitonin is controversial. Having previously demonstrated that the CTR has a biological role in protecting against calcium stress using a global CTRKO mouse model, the purpose of this study was to determine whether the protection conferred by the CTR during induced hypercalcemia is mediated via CTR expression on osteoclasts. Mice were generated, in which the CTR was deleted specifically within osteoclasts (OCL-CTRKOs) and compared with mice in which the CTR was deleted globally (global CTRKOs). Significantly, peak serum calcium levels following induced hypercalcemia were >18% higher in global-CTRKOs and OCL-CTRKOs than controls (P<0.01) due to increased bone resorption (P<0.05). Peak serum calcium levels relative to controls were greater in global-CTRKO males than OCL-CTRKO males (P<0.001), which may, at least in part, be due to increased reabsorption of calcium in the kidney (P<0.01). Controls for all analyses were sex-matched littermates with normal CTR expression. In conclusion, the CTR protects against hypercalcemia predominantly via its inhibitory action on osteoclasts.

DNA-binding-dependent androgen receptor signaling contributes to gender differences and has physiological actions in males and females

We used our genomic androgen receptor (AR) knockout (ARKO) mouse model, in which the AR is unable to bind DNA to: 1) document gender differences between males and females; 2) identify the genomic (DNA-binding-dependent) AR-mediated actions in males; 3) determine the contribution of genomic AR-mediated actions to these gender differences; and 4) identify physiological genomic AR-mediated actions in females. At 9 weeks of age, control males had higher body, heart and kidney mass, lower spleen mass, and longer and larger bones compared to control females. Compared to control males, ARKO males had lower body and kidney mass, higher splenic mass, and reductions in cortical and trabecular bone. Deletion of the AR in ARKO males abolished the gender differences in heart and cortical bone. Compared with control females, ARKO females had normal body weight, but 14% lower heart mass and heart weight/body weight ratio. Relative kidney mass was also reduced, and relative spleen mass was increased. ARKO females had a significant reduction in cortical bone growth and changes in trabecular architecture, although with no net change in trabecular bone volume. In conclusion, we have shown that androgens acting via the genomic AR signaling pathway mediate, at least in part, the gender differences in body mass, heart, kidney, spleen, and bone, and play a physiological role in the regulation of cardiac, kidney and splenic size, cortical bone growth, and trabecular bone architecture in females.

Severe subfertility in mice with androgen receptor inactivation in sex accessory organs but not in testis

Androgen action on sex accessory organs influences rodent fertility, but the mechanisms remain unclear and investigation is difficult without the ability to restrict androgen action in specific tissues. We used Cre-LoxP technology to generate male mice with prostate epithelial-specific androgen receptor deficiency (denoted PEARKO). In addition to prostate, these males have reduced androgen action due to tissue-selective androgen receptor inactivation in seminal vesicle, epididymis, and vas deferens, whereas the testis is unaffected. We find that fertility of PEARKO males was severely reduced, compared with littermates with prominent defects in copulatory plug formation, which were smaller, softer, and more friable than controls. Despite normal testis sperm production, sperm numbers were reduced in caput but increased in cauda epididymis, suggesting alterations in sperm epididymal transit kinetics associated with increased rate of spontaneous acrosome reaction and abnormal flagellar morphology in PEARKO cauda epididymal sperm. Whereas the quantitative in vitro fertilizing ability of PEARKO epididymal sperm was normal, fewer fertilized oocytes were flushed from the oviducts of females after natural mating with PEARKO males. These data show that sperm formed in mice with impaired androgen action restricted to accessory glands and epididymis are quantitatively normal in number and in vitro fertilizing function but that severe in vivo subfertility reflects other functions related to sperm transport and survival in female reproductive tract that determine fertility in vivo.

Intermittent Fugu parathyroid hormone 1 (1-34) is an anabolic bone agent in young male rats and osteopenic ovariectomized rats

Human parathyroid hormone (hPTH) is currently the only treatment for osteoporosis that forms new bone. Previously we described a fish equivalent, Fugu parathyroid hormone 1 (fPth1) which has hPTH-like biological activity in vitro despite fPth1(1-34) sharing only 53% identity with hPTH(1-34). Here we demonstrate the in vivo actions of fPth1(1-34) on bone. In study 1, young male rats were injected intermittently for 30 days with fPth1 [30 microg-1,000 microg/kg body weight (b.w.), (30fPth1-1,000fPth1)] or hPTH [30 microg-100 microg/kg b.w. (30hPTH-100hPTH)]. In proximal tibiae at low doses, the fPth1 was positively correlated with trabecular bone volume/total volume (TbBV/TV) while hPTH increased TbBV/TV, trabecular thickness (TbTh) and trabecular number (TbN). 500fPth1 and 1000fPth1 increased TbBV/TV, TbTh, TbN, mineral apposition rate (MAR) and bone formation rate/bone surface (BFR/BS) with a concomitant decrease in osteoclast surface and number. In study 2 ovariectomized (OVX), osteopenic rats and sham operated (SHAM) rats were injected intermittently with 500 microg/kg b.w. of fPth1 (500fPth1) for 11 weeks. 500fPth1 treatment resulted in increased TbBV/TV (151%) and TbTh (96%) in the proximal tibiae due to increased bone formation as assessed by BFR/BS (490%) and MAR (131%). The effect was restoration of TbBV/TV to SHAM levels without any effect on bone resorption. 500fPth1 also increased TbBV/TV and TbTh in the vertebrae (L6) and cortical thickness in the mid-femora increasing bone strength at these sites. fPth1 was similarly effective in SHAM rats. Notwithstanding the low amino acid sequence homology with hPTH (1-34), we have clearly established the efficacy of fPth1 (1-34) as an anabolic bone agent.

A floxed allele of the androgen receptor gene causes hyperandrogenization in male mice

We previously generated a conditional floxed mouse line to study androgen action, in which exon 3 of the androgen receptor (AR) gene is flanked by loxP sites, with the neomycin resistance gene present in intron 3. Deletion of exon 3 in global AR knockout mice causes androgen insensitivity syndrome, characterized by genotypic males lacking normal masculinization. We now report that male mice carrying the floxed allele (AR(lox)) have the reverse phenotype, termed hyperandrogenization. AR(lox) mice have increased mass of androgen-dependent tissues, including kidney, (P < 0.001), seminal vesicle (P < 0.001), levator ani muscle (P = 0.001), and heart (P < 0.05). Serum testosterone is not significantly different. Testis mass is normal, histology shows normal spermatogenesis, and AR(lox) males are fertile. AR(lox) males also have normal AR mRNA levels in kidney, brain, levator ani, liver, and testis. This study reaffirms the need to investigate the potential phenotypic effects of floxed alleles in the absence of cre in tissue-specific knockout studies. In addition, this androgen hypersensitivity model may be useful to further investigate the effects of subtle perturbations of androgen action in a range of androgen-responsive systems in the male.

Female mice haploinsufficient for an inactivated androgen receptor (AR) exhibit age-dependent defects that resemble the AR null phenotype of dysfunctional late follicle development, ovulation, and fertility

The role of classical genomic androgen receptor (AR) mediated actions in female reproductive physiology remains unclear. Female mice homozygous for an in-frame deletion of exon 3 of the Ar (AR(-/-)) were subfertile, exhibiting delayed production of their first litter (AR(+/+) = 22 d vs. AR(-/-) = 61 d, P < 0.05) and producing 60% fewer pups/litter (AR(+/+): 8.1 +/- 0.4 vs. AR(-/-): 3.2 +/- 0.9, P < 0.01). Heterozygous females (AR(+/-)) exhibited an age-dependent 55% reduction (P < 0.01) in pups per litter, evident from 6 months of age (P < 0.05), compared with AR(+/+), indicating a significant gene dosage effect on female fertility. Ovulation was defective with a significant reduction in corpora lutea numbers (48-79%, P < 0.01) in 10- to 12- and 26-wk-old AR(+/-) and AR(-/-) females and a 57% reduction in oocytes recovered from naturally mated AR(-/-) females (AR(+/+): 9.8 +/- 1.0 vs. AR(-/-): 4.2 +/- 1.2, P < 0.01); however, early embryo development to the two-cell stage was unaltered. The delay in first litter, reduction in natural ovulation rate, and aromatase expression in AR(+/-) and AR(-/-) ovaries, coupled with the restored ovulation rate by gonadotropin hyperstimulation in AR(-/-) females, suggest aberrant gonadotropin regulation. A 2.7-fold increase (AR(+/+): 35.4 +/- 13.4 vs. AR(-/-): 93.9 +/- 6.1, P < 0.01) in morphologically unhealthy antral follicles demonstrated deficiencies in late follicular development, although growing follicle populations and growth rates were unaltered. This novel model reveals that classical genomic AR action is critical for normal ovarian function, although not for follicle depletion and that haploinsufficiency for an inactivated AR may contribute to a premature reduction in female fecundity.

Disruption of prostate epithelial androgen receptor impedes prostate lobe-specific growth and function

Prostate development and maturation requires stromal-epithelial interactions and androgen action via the androgen receptor (AR) within these compartments. However, the specific roles of epithelial and stromal AR in postnatal prostate differentiation are unclear. We used Cre-LoxP technology to determine the prostate phenotype in mice with epithelial-selective genetic inactivation of the AR leaving the stromal AR functionally intact. We find that prostate development abolished in mice globally lacking a functional AR can be rescued by restricting the AR knockout to the postnatal prostate epithelium. We show that, at 8 wk of age, prostate epithelial AR knockout (PEARKO) mice exhibit prostate development with normal branching morphogenesis but lobe-specific decrease in prostate weight and hindered structural and functional differentiation of the mature prostate epithelium. No change was observed in PEARKO testis weight or serum testosterone compared with littermate controls. The most striking change was increased proliferation and abnormal lesions of epithelial cells predominantly in the anterior lobe of PEARKO mice. These findings highlight the vital role of stromal AR in postnatal prostate growth and structural differentiation and emphasize the requirement of epithelial AR in maintaining functional differentiation and restraining proliferation of epithelial cells in a lobe-specific manner. This unique PEARKO mouse provides a new paradigm with which to define the molecular mechanisms of the androgen signaling in mature prostate lobes in vivo and provides insight into the identification of better targets for treatment of prostate cancer and hyperplasia.

Osteoblast deletion of exon 3 of the androgen receptor gene results in trabecular bone loss in adult male mice

The mechanism of androgen action on bone was studied in male mice with the AR deleted in mature osteoblasts. These mice had decreased trabecular bone volume associated with a decrease in trabecular number, suggesting that androgens may act directly on osteoblasts to maintain trabecular bone.

INTRODUCTION

Androgens modulate bone cell activity and are important for the maintenance of bone mass. However, the mechanisms by which they exert these actions on bone remain poorly defined. The aim of this study was to investigate the role of androgens acting through the classical androgen receptor (AR) signaling pathways (i.e., DNA-binding dependent pathways) in osteoblasts using male mice in which exon 3 of the AR gene was deleted specifically in mature osteoblasts.

MATERIALS AND METHODS

Mice with a floxed exon 3 of the AR gene were bred with Col 2.3-cre transgenic mice, in which Cre recombinase is expressed in mineralizing osteoblasts. The skeletal phenotype of mutant mice was assessed by histomorphometry and quantitative microCT at 6, 12, and 32 weeks of age (n=8 per group). Wildtype, hemizygous exon 3 floxed and hemizygous Col 2.3-cre male littermates were used as controls. Data were analyzed by one-way ANOVA and Tukey's posthoc test.

RESULTS

microCT analysis of the fifth lumbar vertebral body showed that these mice had reduced trabecular bone volume (p<0.05) at 32 weeks of age compared with controls. This was associated with a decrease in trabecular number (p<0.01) at 12 and 32 weeks of age, suggesting increased bone resorption. These effects were accompanied by a reduction in connectivity density (p<0.01) and an increase in trabecular separation (p<0.01). A similar pattern of trabecular bone loss was observed in the distal femoral metaphysis at 32 weeks of age.

CONCLUSIONS

These findings show that inactivation of the DNA binding-dependent functions of the AR, specifically in mature osteoblasts in male mice, results in increased bone resorption and decreased structural integrity of the bone, leading to a reduction in trabecular bone volume at 32 weeks of age. These data provide evidence of a role for androgens in the maintenance of trabecular bone volume directly through DNA binding-dependent actions of the AR in mature osteoblasts.