MLX phosphorylation stabilizes the ChREBP-MLX heterotetramer on tandem E-boxes to control carbohydrate and lipid metabolism.

The heterodimeric ChREBP-MLX transcription factor complex is a key mediator that couples intracellular sugar levels to carbohydrate and lipid metabolism. To promote the expression of target genes, two ChREBP-MLX heterodimers form a heterotetramer to bind a tandem element with two adjacent E-boxes, called Carbohydrate Responsive Element (ChoRE). How the ChREBP-MLX hetero-tetramerization is achieved and regulated, remains poorly understood.

Hypoxia rewires glucose and glutamine metabolism in different sources of skeletal stem and progenitor cells similarly, except for pyruvate.

Skeletal stem and progenitor cells (SSPCs) are crucial for bone development, homeostasis, and repair. SSPCs are considered to reside in a rather hypoxic niche in the bone, but distinct SSPC niches have been described in different skeletal regions, and they likely differ in oxygen and nutrient availability. Currently it remains unknown whether the different SSPC sources have a comparable metabolic profile and respond in a similar manner to hypoxia.

The serine synthesis pathway drives osteoclast differentiation through epigenetic regulation of NFATc1 expression.

Bone-resorbing osteoclasts are vital for postnatal bone health, as increased differentiation or activity results in skeletal pathologies such as osteoporosis. The metabolism of mature osteoclasts differs from their progenitor cells, but whether the observed metabolic changes are secondary to the altered cell state or actively drive the process of cell differentiation is unknown. Here, we show that transient activation of the serine synthesis pathway (SSP) is essential for osteoclastogenesis, as deletion of the rate-limiting enzyme phosphoglycerate dehydrogenase in osteoclast progenitors impairs their differentiation and results in increased bone mass.

Isolation and characterization of murine young-adult long bone skeletal progenitors.

Skeletal stem and progenitor cells (SSPCs) constitute a reservoir of bone-forming cells necessary for bone development, modeling and remodeling, as well as for fracture healing. Recent advances in tools to identify and isolate SSPCs have revealed that cells with multipotent properties are present not only in neonatal bone, but also in adult bone marrow and periosteum. The long bone metaphysis and endosteum have been proposed as an additional SSPC niche, although approaches to study their cellular and molecular characteristics are still limited.

De novo serine synthesis regulates chondrocyte proliferation during bone development and repair.

The majority of the mammalian skeleton is formed through endochondral ossification starting from a cartilaginous template. Cartilage cells, or chondrocytes, survive, proliferate and synthesize extracellular matrix in an avascular environment, but the metabolic requirements for these anabolic processes are not fully understood. Here, using metabolomics analysis and genetic in vivo models, we show that maintaining intracellular serine homeostasis is essential for chondrocyte function.

Nestin-GFP transgene labels skeletal progenitors in the periosteum.

The periosteum is critical for bone repair and contains skeletal stem cells (SSCs), but these cells are still poorly characterized. In the bone marrow, cells expressing the Nes-GFP transgene have been described to be SSCs. Here, we investigated whether Nes-GFP expression also typifies SSCs in the periosteum.

An Ectopic Imaging Window for Intravital Imaging of Engineered Bone Tissue.

Tissue engineering is a promising branch of regenerative medicine, but its clinical application remains limited because thorough knowledge of the in vivo repair processes in these engineered implants is limited. Common techniques to study the different phases of bone repair in mice are destructive and thus not optimal to gain insight into the dynamics of this process. Instead, multiphoton-intravital microscopy (MP-IVM) allows visualization of (sub)cellular processes at high resolution and frequency over extended periods of time when combined with an imaging window that permits optical access to implants in vivo.

Osteocytic oxygen sensing controls bone mass through epigenetic regulation of sclerostin.

Preservation of bone mass is crucial for healthy ageing and largely depends on adequate responses of matrix-embedded osteocytes. These cells control bone formation and resorption concurrently by secreting the WNT/β-catenin antagonist sclerostin (SOST). Osteocytes reside within a low oxygen microenvironment, but whether and how oxygen sensing regulates their function remains elusive.

Regulatory elements driving the expression of skeletal lineage reporters differ during bone development and adulthood.

To improve bone healing or regeneration more insight in the fate and role of the different skeletal cell types is required. Mouse models for fate mapping and lineage tracing of skeletal cells, using stage-specific promoters, have advanced our understanding of bone development, a process that is largely recapitulated during bone repair. However, validation of these models is often only performed during development, whereas proof of the activity and specificity of the used promoters during the bone regenerative process is limited.

Expansion of murine periosteal progenitor cells with fibroblast growth factor 2 reveals an intrinsic endochondral ossification program mediated by bone morphogenetic protein 2.

The preservation of the bone-forming potential of skeletal progenitor cells during their ex vivo expansion remains one of the major challenges for cell-based bone regeneration strategies. We report that expansion of murine periosteal cells in the presence of FGF2, a signal present during the early stages of fracture healing, is necessary and sufficient to maintain their ability to organize in vivo into a cartilage template which gives rise to mature bone. Implantation of FGF2-primed cells in a large bone defect in mice resulted in complete healing, demonstrating the feasibility of using this approach for bone tissue engineering purposes.

Increased skeletal VEGF enhances beta-catenin activity and results in excessively ossified bones.

Vascular endothelial growth factor (VEGF) and beta-catenin both act broadly in embryogenesis and adulthood, including in the skeletal and vascular systems. Increased or deregulated activity of these molecules has been linked to cancer and bone-related pathologies. By using novel mouse models to locally increase VEGF levels in the skeleton, we found that embryonic VEGF over-expression in osteo-chondroprogenitors and their progeny largely pheno-copied constitutive beta-catenin activation.

Duodenal calcium absorption in dexamethasone-treated mice: functional and molecular aspects.

Reduced intestinal calcium absorption may be part of the pathogenesis of glucocorticoid-induced osteoporosis. 1,25(OH)2D3 is the major regulator of the expression of the active duodenal calcium absorption genes: TRPV6 (influx), calbindin-D9K (intracellular transfer) and PMCA1b (extrusion). We investigated the influence of dexamethasone (5 days: 2 mg/kg bw) on calcium absorption in vivo and on the expression of intestinal and renal calcium transporters in calcium-deprived mice.

Vitamin D receptor in chondrocytes promotes osteoclastogenesis and regulates FGF23 production in osteoblasts.

Genomic actions induced by 1alpha25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] are crucial for normal bone metabolism, mainly because they regulate active intestinal calcium transport. To evaluate whether the vitamin D receptor (VDR) has a specific role in growth-plate development and endochondral bone formation, we investigated mice with conditional inactivation of VDR in chondrocytes. Growth-plate chondrocyte development was not affected by the lack of VDR.

Placental growth factor mediates mesenchymal cell development, cartilage turnover, and bone remodeling during fracture repair.

Current therapies for delayed- or nonunion bone fractures are still largely ineffective. Previous studies indicated that the VEGF homolog placental growth factor (PlGF) has a more significant role in disease than in health. Therefore we investigated the role of PlGF in a model of semi-stabilized bone fracture healing.

Soluble VEGF isoforms are essential for establishing epiphyseal vascularization and regulating chondrocyte development and survival.

VEGF is crucial for metaphyseal bone vascularization. In contrast, the angiogenic factors required for vascularization of epiphyseal cartilage are unknown, although this represents a developmentally and clinically important aspect of bone growth. The VEGF gene is alternatively transcribed into VEGF(120), VEGF(164), and VEGF(188) isoforms that differ in matrix association and receptor binding.

NuSAP, a novel microtubule-associated protein involved in mitotic spindle organization.

Here, we report on the identification of nucleolar spindle-associated protein (NuSAP), a novel 55-kD vertebrate protein with selective expression in proliferating cells. Its mRNA and protein levels peak at the transition of G2 to mitosis and abruptly decline after cell division. Microscopic analysis of both fixed and live mammalian cells showed that NuSAP is primarily nucleolar in interphase, and localizes prominently to central spindle microtubules during mitosis.

Pregnancy in mice lacking the vitamin D receptor: normal maternal skeletal response, but fetal hypomineralization rescued by maternal calcium supplementation.

Fetal mineralization appears to be driven by the pregnancy-induced stimulation of intestinal Ca absorption. We thus hypothesized that mineralization would be impaired in fetuses of mice that lack the vitamin D receptor (VDR). Here we report on the maternal response to pregnancy, and the fetal mineralization, in mice with a homozygous disruption of the VDR gene (VDR-/-) mated with wild-type (wt) males.

Impaired angiogenesis and endochondral bone formation in mice lacking the vascular endothelial growth factor isoforms VEGF164 and VEGF188.

Vascular endothelial growth factor (VEGF)-mediated angiogenesis is an important part of bone formation. To clarify the role of VEGF isoforms in endochondral bone formation, we examined long bone development in mice expressing exclusively the VEGF120 isoform (VEGF120/120 mice). Neonatal VEGF120/120 long bones showed a completely disturbed vascular pattern, concomitant with a 35% decrease in trabecular bone volume, reduced bone growth and a 34% enlargement of the hypertrophic chondrocyte zone of the growth plate.