Sexing Bones: Improving Transparency of Sex Reporting to Address Bias Within Preclinical Studies

Tmem119 deficiency delays bone repair in mice

Tmem119 was identified as a bone anabolic factor in osteoblasts, however the roles of Tmem119 on bone repair have remained unknown. Therefore, we herein investigated the roles of Tmem119 on bone repair by examining the bone repair process after a femoral bone defect using Tmem119-deficient mice. In Tmem119-deficient mice, bone repair after a femoral bone defect was significantly delayed 10 and 14 days after bone injury in female and male mice with 3-dimensional micro-computed tomography analyses, respectively. The number of alkaline phosphatase-positive cells at the damaged sites was significantly decreased 7 days after bone injury in Tmem119-deficient mice, although the number of Osterix-positive cells was not significantly different 4 days after bone injury. The number of tartrate-resistant acid phosphatase-positive multinucleated cells as well as the number and luminal area of CD31-positive vessels at the damaged sites were not significantly different between Tmem119-deficient and wild-type mice. The present study first showed that Tmem119 deficiency delayed bone repair partly through a decrease in the osteoblastic bone formation of differentiated osteoblasts.

Preclinical Rodent Models for Human Bone Disease, Including a Focus on Cortical Bone

Preclinical models (typically ovariectomized rats and genetically altered mice) have underpinned much of what we know about skeletal biology. They have been pivotal for developing therapies for osteoporosis and monogenic skeletal conditions, including osteogenesis imperfecta, achondroplasia, hypophosphatasia, and craniodysplasias. Further therapeutic advances, particularly to improve cortical strength, require improved understanding and more rigorous use and reporting. We describe here how trabecular and cortical bone structure develop, are maintained, and degenerate with aging in mice, rats, and humans, and how cortical bone structure is changed in some preclinical models of endocrine conditions (eg, postmenopausal osteoporosis, chronic kidney disease, hyperparathyroidism, diabetes). We provide examples of preclinical models used to identify and test current therapies for osteoporosis, and discuss common concerns raised when comparing rodent preclinical models to the human skeleton. We focus especially on cortical bone, because it differs between small and larger mammals in its organizational structure. We discuss mechanisms common to mouse and human controlling cortical bone strength and structure, including recent examples revealing genetic contributors to cortical porosity and osteocyte network configurations during growth, maturity, and aging. We conclude with guidelines for clear reporting on mouse models with a goal for better consistency in the use and interpretation of these models.

Influence of Sex and Gender on Musculoskeletal Conditions and How They Are Reported

ABSTRACT

There is increasing evidence that musculoskeletal tissues are differentiallys regulated by sex hormones in males and females. The influence of sex hormones, in addition to other sex-based differences such as in anatomical alignment and immune-system function, impact the prevalence and severity of disease as well as the types of injuries that affect the musculoskeletal system and the outcomes of prevention measures and treatment. Literature specifically addressing sex differences related to the musculoskeletal system is limited, underscoring the imperative for both basic and clinical research on this topic. This review highlights areas of research that have implications for bone and cartilage health, including growth and development, sports injuries, osteoarthritis, osteoporosis, and bone frailty. It is clear that important aspects of the musculoskeletal system have been understudied. Consideration of how sex hormone therapy will affect musculoskeletal tissues in prepuberty, during puberty, and in adults is vital, yet little is known. The purpose of this article is to foster awareness and interest in advancing our understanding of how sex differences influence orthopaedic practice.

Sexually dimorphic effects of prenatal alcohol exposure on the murine skeleton

BACKGROUND

Prenatal alcohol exposure (PAE) can result in lifelong disabilities known as foetal alcohol spectrum disorder (FASD) and is associated with childhood growth deficiencies and increased bone fracture risk. However, the effects of PAE on the adult skeleton remain unclear and any potential sexual dimorphism is undetermined. Therefore, we utilised a murine model to examine sex differences with PAE on in vitro bone formation, and in the juvenile and adult skeleton.

METHODS

Pregnant C57BL/6J female mice received 5% ethanol in their drinking water during gestation. Primary calvarial osteoblasts were isolated from neonatal offspring and mineralised bone nodule formation and gene expression assessed. Skeletal phenotyping of 4- and 12-week-old male and female offspring was conducted by micro-computed tomography (µCT), 3-point bending, growth plate analyses, and histology.

RESULTS

Osteoblasts from male and female PAE mice displayed reduced bone formation, compared to control (≤ 30%). Vegfa, Vegfb, Bmp6, Tgfbr1, Flt1 and Ahsg were downregulated in PAE male osteoblasts only, whilst Ahsg was upregulated in PAE females. In 12-week-old mice, µCT analysis revealed a sex and exposure interaction across several trabecular bone parameters. PAE was detrimental to the trabecular compartment in male mice compared to control, yet PAE females were unaffected. Both male and female mice had significant reductions in cortical parameters with PAE. Whilst male mice were negatively affected along the tibial length, females were only distally affected. Posterior cortical porosity was increased in PAE females only. Mechanical testing revealed PAE males had significantly reduced bone stiffness compared to controls; maximum load and yield were reduced in both sexes. PAE had no effect on total body weight or tibial bone length in either sex. However, total growth plate width in male PAE mice compared to control was reduced, whilst female PAE mice were unaffected. 4-week-old mice did not display the altered skeletal phenotype with PAE observed in 12-week-old animals.

CONCLUSIONS

Evidence herein suggests, for the first time, that PAE exerts divergent sex effects on the skeleton, possibly influenced by underlying sex-specific transcriptional mechanisms of osteoblasts. Establishing these sex differences will support future policies and clinical management of FASD.

Deletion of FNDC5/irisin modifies murine osteocyte function in a sex-specific manner

Irisin, released from exercised muscle, has been shown to have beneficial effects on numerous tissues but its effects on bone are unclear. We found significant sex and genotype differences in bone from wildtype (WT) mice compared to mice lacking Fndc5 (knockout [KO]), with and without calcium deficiency. Despite their bone being indistinguishable from WT females, KO female mice were partially protected from osteocytic osteolysis and osteoclastic bone resorption when allowed to lactate or when placed on a low-calcium diet. Male KO mice have more but weaker bone compared to WT males, and when challenged with a low-calcium diet lost more bone than WT males. To begin to understand responsible molecular mechanisms, osteocyte transcriptomics was performed. Osteocytes from WT females had greater expression of genes associated with osteocytic osteolysis and osteoclastic bone resorption compared to WT males which had greater expression of genes associated with steroid and fatty acid metabolism. Few differences were observed between female KO and WT osteocytes, but with a low-calcium diet, the KO females had lower expression of genes responsible for osteocytic osteolysis and osteoclastic resorption than the WT females. Male KO osteocytes had lower expression of genes associated with steroid and fatty acid metabolism, but higher expression of genes associated with bone resorption compared to male WT. In conclusion, irisin plays a critical role in the development of the male but not the female skeleton and protects male but not female bone from calcium deficiency. We propose irisin ensures the survival of offspring by targeting the osteocyte to provide calcium in lactating females, a novel function for this myokine.

Deletion of FNDC5/Irisin modifies murine osteocyte function in a sex-specific manner

Irisin, released from exercised muscle, has been shown to have beneficial effects on numerous tissues but its effects on bone are unclear. We found significant sex and genotype differences in bone from wildtype (WT) mice compared to mice lacking Fndc5 (KO), with and without calcium deficiency. Despite their bone being indistinguishable from WT females, KO female mice were partially protected from osteocytic osteolysis and osteoclastic bone resorption when allowed to lactate or when placed on a low-calcium diet. Male KO mice have more but weaker bone compared to WT males, and when challenged with a low-calcium diet lost more bone than WT males. To begin to understand responsible molecular mechanisms, osteocyte transcriptomics was performed. Osteocytes from WT females had greater expression of genes associated with osteocytic osteolysis and osteoclastic bone resorption compared to WT males which had greater expression of genes associated with steroid and fatty acid metabolism. Few differences were observed between female KO and WT osteocytes, but with a low calcium diet, the KO females had lower expression of genes responsible for osteocytic osteolysis and osteoclastic resorption than the WT females. Male KO osteocytes had lower expression of genes associated with steroid and fatty acid metabolism, but higher expression of genes associated with bone resorption compared to male WT. In conclusion, irisin plays a critical role in the development of the male but not the female skeleton and protects male but not female bone from calcium deficiency. We propose irisin ensures the survival of offspring by targeting the osteocyte to provide calcium in lactating females, a novel function for this myokine.

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.

Unilateral Hypofunction of the Masseter Leads to Molecular and 3D Morphometric Signs of Atrophy in Ipsilateral Agonist Masticatory Muscles in Adult Mice

Mice are commonly used to study mandibular dynamics due to their similarity in chewing cycle patterns with humans. Adult mice treated unilaterally with botulinum toxin type A (BoNTA) in the masseter exhibit atrophy of this muscle characterized by an increase in the gene expression of atrophy-related molecular markers, and a reduction in both muscle fiber diameter and muscle mass at 14d. However, the impact of this muscle imbalance on the non-treated masticatory muscles remains unexplored. Here, we hypothesize that the unilateral masseter hypofunction leads to molecular and 3D morphometric signs of atrophy of the masseter and its agonist masticatory muscles in adult mice. Twenty-three 8-week-old male BALB/c mice received a single injection of BoNTA in the right masseter, whereas the left masseter received the same volume of saline solution (control side). Animals were euthanized at 2d, 7d, and 14d, and the masticatory muscles were analyzed for mRNA expression. Five heads were harvested at 14d, fixed, stained with a contrast-enhanced agent, and scanned using X-ray microtomography. The three-dimensional morphometric parameters (the volume and thickness) from muscles in situ were obtained. Atrogin-1/MAFbx, MuRF-1, and Myogenin mRNA gene expression were significantly increased at 2 and 7d for both the masseter and temporalis from the BoNTA side. For medial pterygoid, increased mRNA gene expression was found at 7d for Atrogin-1/MAFbx and at 2d-7d for Myogenin. Both the volume and thickness of the masseter, temporalis, and medial pterygoid muscles from the BoNTA side were significantly reduced at 14d. In contrast, the lateral pterygoid from the BoNTA side showed a significant increase in volume at 14d. Therefore, the unilateral hypofunction of the masseter leads to molecular and morphological signs of atrophy in both the BoNTA-injected muscle and its agonistic non-injected masticatory muscles. The generalized effect on the mouse masticatory apparatus when one of its components is intervened suggests the need for more clinical studies to determine the safety of BoNTA usage in clinical dentistry.

Fragile X Messenger Ribonucleoprotein 1 (FMR1), a novel inhibitor of osteoblast/osteocyte differentiation, regulates bone formation, mass, and strength in young and aged male and female mice

Fragile X Messenger Ribonucleoprotein 1 (FMR1) gene mutations lead to fragile X syndrome, cognitive disorders, and, in some individuals, scoliosis and craniofacial abnormalities. Four-month-old (mo) male mice with deletion of the FMR1 gene exhibit a mild increase in cortical and cancellous femoral bone mass. However, consequences of absence of FMR1 in bone of young/aged male/female mice and the cellular basis of the skeletal phenotype remain unknown. We found that absence of FMR1 results in improved bone properties with higher bone mineral density in both sexes and in 2- and 9-mo mice. The cancellous bone mass is higher only in females, whereas, cortical bone mass is higher in 2- and 9-mo males, but higher in 2- and lower in 9-mo female FMR1-knockout mice. Furthermore, male bones show higher biomechanical properties at 2mo, and females at both ages. Absence of FMR1 increases osteoblast/mineralization/bone formation and osteocyte dendricity/gene expression in vivo/ex vivo/in vitro, without affecting osteoclasts in vivo/ex vivo. Thus, FMR1 is a novel osteoblast/osteocyte differentiation inhibitor, and its absence leads to age-, site- and sex-dependent higher bone mass/strength.