Biology subjects

Bouxsein, M. L.

Publications and source records attributed to Bouxsein, M. L..

7 recordsLinked to original sources

bioRxiv2026

Intermittent parathyroid hormone employs autonomous and non-autonomous mechanisms to drive osteogenesis from Ebf3-expressing skeletal progenitor cells

How systemic hormonal signals coordinate stem cell fate decisions in adult tissues remains incompletely understood. In bone marrow, Cxcl12-abundant reticular (CAR) cells, marked by Early B-cell Factor 3 (Ebf3) expression, are multipotent mesenchymal progenitors that maintain the hematopoietic stem cell niche and serves as a major osteoblast progenitor source during adult bone remodeling. Using inducible lineage tracing coupled with single-cell transcriptomics and conditional genetics in mice, we show that intermittent parathyroid hormone (iPTH; teriparatide) drives osteogenesis from CAR cells by simultaneously engaging cell-intrinsic and cell-extrinsic mechanisms. Directly, iPTH suppresses lineage-enforcing transcription factors Ebf3, Ebf1, and Foxc1, thereby destabilizing progenitor identity and priming CAR cells for osteogenic commitment. Simultaneously, iPTH stimulates osteoclastic bone resorption, releasing TGF{beta} which recruits these primed progenitors to bone surfaces, a process abolished by osteoclast depletion. Preventing CAR cell maturation via Sp7 deletion abrogates iPTH-induced bone gain, establishing these progenitors as essential mediators of bone anabolism. This coupled mechanism, in which intrinsic transcriptional priming converges with extrinsic niche remodeling, is conserved in human CAR cells from teriparatide-treated postmenopausal women, which show concordant suppression of EBF3 and FOXC1 and elevated TGF{beta}-responsive gene signatures. These findings reveal a general principle by which a systemic hormone orchestrates tissue remodeling through simultaneous reprogramming of progenitor identity and remodeling of the niche microenvironment.

physiology↗

bioRxiv2021

Partial prevention of glucocorticoid-induced osteocyte deterioration with osteocrin gene therapy

Glucocorticoid (GC)-induced osteoporosis and subsequent bone fragility are preceded by death and dysfunction at the cellular level. In particular, short-term glucocorticoid excess suppresses osteocyte remodeling of the surrounding bone mineral, causes apoptosis of osteoblasts and osteocytes, and disrupts homeostatic bone remodeling. Preventing apoptosis and preserving osteocyte morphology and function could be effective means of preventing bone loss during glucocorticoid excess. We hypothesized that osteocrin, which preserves osteocyte viability and morphology in other models where osteocyte defects exist, could prevent osteocyte death and dysfunction in a GC excess model. We used a liver-targeted adeno-associated virus (AAV8) to induce osteocrin overexpression in mice one week prior to implantation with prednisolone or placebo pellets. After 28 days, tissues were collected for micro-CT and histological analysis. GC excess caused the expected reduction in cortical bone thickness and osteocyte canalicular length in control AAV8-treated mice, and these effects were blunted in mice overexpressing osteocrin. However, GC-induced changes in cortical porosity, trabecular bone mass, and gene expression were not prevented by osteocrin. While the mechanism of osteocrins effects on osteocyte morphology warrants further investigation, this study does not support a role for this model of osteocrin supplementation to combat the full skeletal effects of GC excess.

physiology↗

bioRxiv2021

Control of osteocyte dendrite formation by Sp7 and its target gene osteocrin

Osteocytes use an elaborate network of dendritic connections to control bone remodeling. Some osteoblasts embed within mineralized bone matrix, change shape, and become osteocytes. The molecular circuitry that drives dendrite formation during "osteocytogenesis" is poorly understood. Here we show that deletion of Sp7, a gene linked to rare and common skeletal disease, in mature osteoblasts and osteocytes causes severe defects in osteocyte dendrites. Unbiased profiling of Sp7 target genes and binding sites reveals unexpected repurposing of this transcription factor to drive dendrite formation. Osteocrin is a Sp7 target gene that promotes osteocyte dendrite formation and rescues phenotypic and molecular defects in Sp7-deficient mice. Single-cell RNA-sequencing demonstrates overt defects in osteocyte maturation in vivo in the absence of Sp7. Sp7-dependent gene networks enriched in developing osteocytes are associated with rare and common human skeletal traits. Moreover, humans homozygous for the osteogenesis imperfecta-causing SP7R316C mutation show dramatic defects in osteocyte morphology. Genes that mark osteocytes in vivo and that are regulated by Sp7 in vitro are highly enriched in neurons, highlighting shared features between osteocytic and neuronal connectivity. Taken together, these findings reveal a crucial role for Sp7 and its target gene Osteocrin in osteocytogenesis, demonstrating that pathways that control osteocyte development influence human bone diseases.

molecular biology↗

bioRxiv2021

Dual targeting of salt inducible kinases and CSF1R uncouples bone formation and bone resorption

Bone formation and resorption are typically coupled, such that the efficacy of anabolic osteoporosis treatments may be limited by bone destruction. The multi-kinase inhibitor YKL-05-099 potently inhibits salt inducible kinases (SIKs) and may represent a promising new class of bone anabolic agents. Here we report that YKL-05-099 increases bone formation in hypogonadal female mice without increasing bone resorption. Postnatal mice with inducible, global deletion of SIK2 and SIK3 show increased bone mass, increased bone formation, and, distinct from the effects of YKL-05-099, increased bone resorption. No cell-intrinsic role of SIKs in osteoclasts was noted. In addition to blocking SIKs, YKL-05-099 also binds and inhibits CSF1R, the receptor for the osteoclastogenic cytokine M-CSF. Modeling reveals that YKL-05-099 binds to SIK2 and CSF1R in a similar manner. Dual targeting of SIK2/3 and CSF1R induces bone formation without concomitantly increasing bone resorption and thereby may overcome limitations of most current anabolic osteoporosis therapies.

physiology↗

bioRxiv2020

Systems genetics analyses in Diversity Outbred mice inform human bone mineral density GWAS and identify Qsox1 as a novel determinant of bone strength

Genome-wide association studies (GWASs) for osteoporotic traits have identified over 1000 associations; however, their impact has been limited by the difficulties of causal gene identification and a strict focus on bone mineral density (BMD). Here, we used Diversity Outbred (DO) mice to directly address these limitations by performing the first systems genetics analysis of 55 complex skeletal phenotypes. We applied a network approach to cortical bone RNA-seq data to discover 72 genes likely to be causal for human BMD GWAS associations, including the novel genes SERTAD4 and GLT8D2. We also performed GWAS in the DO for a wide-range of bone traits and identified Qsox1 as a novel gene influencing cortical bone accrual and bone strength. Our results provide a new perspective on the genetics of osteoporosis and highlight the ability of the mouse to inform human genetics.

genetics↗

bioRxiv2020

Trpm8 modulates thermogenesis in a sex-dependent manner without influencing cold-induced bone loss

Trpm8 (transient receptor potential cation channel, subfamily M, member 8) is expressed by sensory neurons, and it is involved in the detection of cold temperatures. Increased TRPM8 activity triggers an increase in uncoupling protein 1 (Ucp1)-dependent brown adipose tissue (BAT) thermogenesis. Bone density and marrow adipose tissue are regulated by rodent housing temperature and brown adipose tissue, suggesting one mediator of this effect may be TRPM8. In order to test whether TRPM8 was involved in the co-regulation of thermogenesis and bone homeostasis, we examined the bone phenotypes of one-year-old Trpm8 knockout mice (Trpm8-KO) and then exposed them to a 4-week cold temperature challenge. Male Trpm8-KO mice had lower bone mineral density than WT, with smaller bone size (femur length and cross-sectional area) being the most striking finding, and exhibited a delayed cold acclimation with increased BAT expression of Ucp1, Dio2 and Cidea at the end of the study. In contrast to males, female Trpm8-KO mice had low vertebral bone microarchitectural parameters, but no genotype-specific alterations in body temperature. Interestingly, Trpm8 was not required for cold-induced trabecular bone loss in either sex, but may be required for cold-induced changes in marrow adiposity in males. Furthermore, bone marrow adipose tissue accumulation in females was significantly blunted by Trpm8 deletion. In summary, we identified sex differences in the role of TRPM8 in maintaining body temperature, bone density and marrow adipose tissue. Identifying mechanisms through which cold temperature and brown adipose tissue influence bone could help to ameliorate potential bone side effects of obesity treatments designed to stimulate thermogenesis.

physiology↗

bioRxiv2020

Pharmacological inhibition of longevity regulator PAPP-A restrains mesenchymal stromal cell activity

Reducing insulin-like growth factor (IGF) signaling is one of the best conserved and characterized mechanisms to extend longevity. Pregnancy associated plasma protein A (PAPP-A) is a secreted metalloprotease that increases IGF availability by cleaving IGF binding proteins. PAPP-A inhibition reduces local IGF signaling, limits the progression of multiple age-related diseases, and extends lifespan, but the mechanisms behind these pleiotropic effects remains unknown. Here, we developed and utilized a PAPP-A neutralizing antibody to discover that adulthood inhibition of this protease reduced collagen and extracellular matrix (ECM) gene expression in multiple tissues in mice. Using bone marrow to explore this effect, we identified mesenchymal stromal cells (MSCs) as the source of PAPP-A and primary responders to PAPP-A inhibition. Short-term treatment with anti-PAPP-A reduced IGF signaling in MSCs, altered MSC expression of collagen/ECM, and decreased MSC number. This affected MSC-dependent functions, decreasing myelopoiesis and osteogenesis. Our data demonstrate that PAPP-A inhibition reduces the activity and number of IGF-dependent mesenchymal progenitor cells and their differentiated progeny, and that this reduction leads to functional changes at the tissue level. MSC-like cells are present in virtually all tissues, and aberrant collagen and ECM production from mesenchymal cells drives aspects of aging and age-related diseases, thus this may be a mechanism by which PAPP-A deficiency enhances lifespan and healthspan. SummaryInhibition of PAPP-A, a regulator of IGF signaling, decreases multi-tissue collagen and extracellular matrix gene expression and modulates mesenchymal stromal cell activity in murine bone marrow.

physiology↗