ATF4 mediation of NF1 functions in osteoblast reveals a nutritional basis for congenital skeletal dysplasiae

ATF4 inhibits TRPV4 function and controls itch perception in rodents and nonhuman primates

ABSTRACT

Acute and chronic itch are prevalent and incapacitating, yet the neural mechanisms underlying both acute and chronic itch are just starting to be unraveled. Activated transcription factor 4 (ATF4) belongs to the ATF/CREB transcription factor family and primarily participates in the regulation of gene transcription. Our previous study has demonstrated that ATF4 is expressed in sensory neurons. Nevertheless, the role of ATF4 in itch sensation remains poorly understood. Here, we demonstrate that ATF4 plays a significant role in regulating itch sensation. The absence of ATF4 in dorsal root ganglion (DRG) neurons enhances the itch sensitivity of mice. Overexpression of ATF4 in sensory neurons significantly alleviates the acute and chronic pruritus in mice. Furthermore, ATF4 interacts with the transient receptor potential cation channel subfamily V member 4 (TRPV4) and inhibits its function without altering the expression or membrane trafficking of TRPV4 in sensory neurons. In addition, interference with ATF4 increases the itch sensitivity in nonhuman primates and enhances TRPV4 currents in nonhuman primates DRG neurons; ATF4 and TRPV4 also co-expresses in human sensory neurons. Our data demonstrate that ATF4 controls pruritus by regulating TRPV4 signaling through a nontranscriptional mechanism and identifies a potential new strategy for the treatment of pathological pruritus.

Cell signaling and transcriptional regulation of osteoblast lineage commitment, differentiation, bone formation, and homeostasis

The initiation of osteogenesis primarily occurs as mesenchymal stem cells undergo differentiation into osteoblasts. This differentiation process plays a crucial role in bone formation and homeostasis and is regulated by two intricate processes: cell signal transduction and transcriptional gene expression. Various essential cell signaling pathways, including Wnt, BMP, TGF-β, Hedgehog, PTH, FGF, Ephrin, Notch, Hippo, and Piezo1/2, play a critical role in facilitating osteoblast differentiation, bone formation, and bone homeostasis. Key transcriptional factors in this differentiation process include Runx2, Cbfβ, Runx1, Osterix, ATF4, SATB2, and TAZ/YAP. Furthermore, a diverse array of epigenetic factors also plays critical roles in osteoblast differentiation, bone formation, and homeostasis at the transcriptional level. This review provides an overview of the latest developments and current comprehension concerning the pathways of cell signaling, regulation of hormones, and transcriptional regulation of genes involved in the commitment and differentiation of osteoblast lineage, as well as in bone formation and maintenance of homeostasis. The paper also reviews epigenetic regulation of osteoblast differentiation via mechanisms, such as histone and DNA modifications. Additionally, we summarize the latest developments in osteoblast biology spurred by recent advancements in various modern technologies and bioinformatics. By synthesizing these insights into a comprehensive understanding of osteoblast differentiation, this review provides further clarification of the mechanisms underlying osteoblast lineage commitment, differentiation, and bone formation, and highlights potential new therapeutic applications for the treatment of bone diseases.

Methionine as a regulator of bone remodeling with fasting

Caloric restriction improves metabolic health but is often complicated by bone loss. We studied bone parameters in humans during a 10-day fast and identified candidate metabolic regulators of bone turnover. Pro-collagen 1 intact N-terminal pro-peptide (P1NP), a bone formation marker, decreased within 3 days of fasting. Whereas dual-energy x-ray absorptiometry measures of bone mineral density were unchanged after 10 days of fasting, high-resolution peripheral quantitative CT demonstrated remodeling of bone microarchitecture. Pathway analysis of longitudinal metabolomics data identified one-carbon metabolism as fasting dependent. In cultured osteoblasts, we tested the functional significance of one-carbon metabolites modulated by fasting, finding that methionine - which surged after 3 days of fasting - affected markers of osteoblast cell state in a concentration-dependent manner, in some instances exhibiting a U-shaped response with both low and high concentrations driving putative antibone responses. Administration of methionine to mice for 5 days recapitulated some fasting effects on bone, including a reduction in serum P1NP. In conclusion, a 10-day fast in humans led to remodeling of bone microarchitecture, potentially mediated by a surge in circulating methionine. These data support an emerging model that points to a window of optimal methionine exposure for bone health.

Advances in the roles of ATF4 in osteoporosis

Osteoporosis (OP) is characterized by reduced bone mass, decreased strength, and enhanced bone fragility fracture risk. Activating transcription factor 4 (ATF4) plays a role in cell differentiation, proliferation, apoptosis, redox balance, amino acid uptake, and glycolipid metabolism. ATF4 induces the differentiation of bone marrow mesenchymal stem cells (BM-MSCs) into osteoblasts, increases osteoblast activity, and inhibits osteoclast formation, promoting bone formation and remodeling. In addition, ATF4 mediates the energy metabolism in osteoblasts and promotes angiogenesis. ATF4 is also involved in the mediation of adipogenesis. ATF4 can selectively accumulate in osteoblasts. ATF4 can directly interact with RUNT-related transcription factor 2 (RUNX2) and up-regulate the expression of osteocalcin (OCN) and osterix (Osx). Several upstream factors, such as Wnt/β-catenin and BMP2/Smad signaling pathways, have been involved in ATF4-mediated osteoblast differentiation. ATF4 promotes osteoclastogenesis by mediating the receptor activator of nuclear factor κ-B (NF-κB) ligand (RANKL) signaling. Several agents, such as parathyroid (PTH), melatonin, and natural compounds, have been reported to regulate ATF4 expression and mediate bone metabolism. In this review, we comprehensively discuss the biological activities of ATF4 in maintaining bone homeostasis and inhibiting OP development. ATF4 has become a therapeutic target for OP treatment.

Abnormal changes of bone metabolism markers with age in children with cerebral palsy

Cerebral palsy (CP) is a broad range of diseases with permanent and nonprogressive motor impairments, carrying a high cost for both the individual and the society. The characteristics of low bone mineral density and high risk of fractures suggest that bone metabolism disorders are present in CP. This study aims to investigate the association between indicators of bone metabolism and children with CP. A total of 139 children (75 children with CP and 64 healthy controls) were included in this cross-sectional study. Participants were divided into three age groups (0-2 years, 2.1-4 years, and 4.1-7 years). All children with CP were diagnosed according to clinical criteria and furtherly divided into clinical subtypes. The levels of total procollagen type I N-terminal propeptide (TPINP), N-MID osteocalcin (OC), beta-crosslaps (β-CTX), 25-hydroxyvitamin D (25-OHD) and parathyroid hormone (PTH) in the serum were measured with corresponding detection kits according to the manufacturer's instructions. Serum levels of TPINP and 25-OHD were lower with older age, whereas β-CTX and PTH were higher with older age. In the CP group, TPINP (age 0-2 years and 2.1-4 years) and OC (age 2.1-4 years) levels were higher, while β-CTX (age 2.1-4 years and 4.1-7 years) and PTH (age 2.1-4 years) values were lower than the control group. In addition, there were no statistically significant differences in the levels of these indicators among the CP subgroups with different clinical characteristics. Our study shows that bone turnover markers, indicators of bone metabolism, in children with CP differ significantly from healthy controls. The indicators we studied changed with age, and they did not correlate with disease severity.

Occipital bone defect caused by neurofibromatosis type I: A case report

RATIONALE

The synergistic effect between nonmalignant lesions can also cause a serious impact on patient survival. This disease involves different organ systems and presents with a variety of clinical manifestations, such as schwannoma, depigmentation, low-grade glioma, and skeletal abnormalities. We report a case of type I neurofibromatosis with an occipital bone defect.

PATIENT CONCERNS

We report a case of a 50-year-old man with type I neurofibromatosis with occipital bone defect.

DIAGNOSIS

The patient was accepted by the local hospital due to sudden right upper limb weakness accompanied by jitter without recognizable cause or inducement. A computerized tomography scan at a local hospital suggested subcutaneous neurofibromatosis with a left occipital cranial defect with thinning bone. On admission physical examination, diffuse multiple masses could be seen throughout the body and head of different sizes and composed of soft and tough textures. The largest one was located in the posterior occipital bone, approximately 8*8 cm in size, with a child tumor and tough texture. Multiple café-au-lait spots could be found on the chest and back, and multiple freckles can be seen in the armpit. The patient underwent surgery. Postoperative pathology showed a spindle cell tumor, which was determined to be neurofibromatosis type I according to immunopathology and clinical data.

INTERVENTIONS

The patient was admitted for surgical treatment. During the operation, the scalp mass was completely abducted and the tumor tissue at the skull defect was sharply separated. Postoperative pathology showed that the peripheral margin and the bottom margin were cleaned.

OUTCOMES

Computerized tomography and magnetic resonance imaging showed that the tumor was completely. There were not any surgical complications. The patient recovered well, was cured and was dismissed from the hospital.

LESSONS

The synergistic effect between nonmalignant lesions can also cause a serious impact on patient survival to encourage early medical intervention. The clinical presentation of neurofibromatosis type I am usually nonmalignant, and in this case, involvement of the skull with bone defect is very rare. Therefore, it is necessary to accumulate relevant cases, reveal the pathogenesis of the disease, predict the development and outcome, and provide more evidence for early therapeutic intervention of similar patients in the future.

An angiogenic approach to osteoanabolic therapy targeting the SHN3-SLIT3 pathway

Often, disorders of impaired bone formation involve not only a cell intrinsic defect in the ability of osteoblasts to form bone, but moreover a broader dysfunction of the skeletal microenvironment that limits osteoblast activity. Developing approaches to osteoanabolic therapy that not only augment osteoblast activity but moreover correct this microenvironmental dysfunction may enable both more effective osteoanabolic therapies and also addressing a broader set of indications where vasculopathy or other forms microenvironment dysfunction feature prominently. We here review evidence that SHN3 acts as a suppressor of not only the cell intrinsic bone formation activity of osteoblasts, but moreover of the creation of a local osteoanabolic microenvironment. Mice lacking Schnurri3 (SHN3, HIVEP3) display a very robust increase in bone formation, that is due to de-repression of ERK pathway signaling in osteoblasts. In addition to loss of SHN3 augmenting the differentiation and bone formation activity of osteoblasts, loss of SHN3 increases secretion of SLIT3 by osteoblasts, which in a skeletal context acts as an angiogenic factor. Through this angiogenic activity, SLIT3 creates an osteoanabolic microenvironment, and accordingly treatment with SLIT3 can increase bone formation and enhance fracture healing. These features both validate vascular endothelial cells as a therapeutic target for disorders of low bone mass alongside the traditionally targeted osteoblasts and osteoclasts and indicate that targeting the SHN3/SLIT3 pathway provides a new mechanism to induce therapeutic osteoanabolic responses.

Icariin Promotes Osteogenic Differentiation in a Cell Model with NF1 Gene Knockout by Activating the cAMP/PKA/CREB Pathway

Neurofibromatosis type 1 is a rare autosomal dominant genetic disorder, with up to 50% of patients clinically displaying skeletal defects. Currently, the pathogenesis of bone disorders in NF1 patients is unclear, and there are no effective preventive and treatment measures. In this study, we found that knockout of the NF1 gene reduced cAMP levels and osteogenic differentiation in an osteoblast model, and icariin activated the cAMP/PKA/CREB pathway to promote osteoblast differentiation of the NF1 gene knockout cell model by increasing intracellular cAMP levels. The PKA selective inhibitor H89 significantly impaired the stimulatory effect of icariin on osteogenesis in the NF1 cell model. In this study, an osteoblast model of NF1 was successfully constructed, and icariin was applied to the cell model for the first time. The results will help to elucidate the molecular mechanism of NF1 bone disease and provide new ideas for the clinical prevention and treatment of NF1 bone disease and drug development in the future.

Specific host metabolite and gut microbiome alterations are associated with bone loss during spaceflight

Understanding the axis of the human microbiome and physiological homeostasis is an essential task in managing deep-space-travel-associated health risks. The NASA-led Rodent Research 5 mission enabled an ancillary investigation of the gut microbiome, varying exposure to microgravity (flight) relative to ground controls in the context of previously shown bone mineral density (BMD) loss that was observed in these flight groups. We demonstrate elevated abundance of Lactobacillus murinus and Dorea sp. during microgravity exposure relative to ground control through whole-genome sequencing and 16S rRNA analyses. Specific functionally assigned gene clusters of L. murinus and Dorea sp. capable of producing metabolites, lactic acid, leucine/isoleucine, and glutathione are enriched. These metabolites are elevated in the microgravity-exposed host serum as shown by liquid chromatography-tandem mass spectrometry (LC-MS/MS) metabolomic analysis. Along with BMD loss, ELISA reveals increases in osteocalcin and reductions in tartrate-resistant acid phosphatase 5b signifying additional loss of bone homeostasis in flight.

Allosteric autoregulation of DNA binding via a DNA-mimicking protein domain: a biophysical study of ZNF410-DNA interaction using small angle X-ray scattering

ZNF410 is a highly-conserved transcription factor, remarkable in that it recognizes a 15-base pair DNA element but has just a single responsive target gene in mammalian erythroid cells. ZNF410 includes a tandem array of five zinc-fingers (ZFs), surrounded by uncharacterized N- and C-terminal regions. Unexpectedly, full-length ZNF410 has reduced DNA binding affinity, compared to that of the isolated DNA binding ZF array, both in vitro and in cells. AlphaFold predicts a partially-folded N-terminal subdomain that includes a 30-residue long helix, preceded by a hairpin loop rich in acidic (aspartate/glutamate) and serine/threonine residues. This hairpin loop is predicted by AlphaFold to lie against the DNA binding interface of the ZF array. In solution, ZNF410 is a monomer and binds to DNA with 1:1 stoichiometry. Surprisingly, the single best-fit model for the experimental small angle X-ray scattering profile, in the absence of DNA, is the original AlphaFold model with the N-terminal long-helix and the hairpin loop occupying the ZF DNA binding surface. For DNA binding, the hairpin loop presumably must be displaced. After combining biophysical, biochemical, bioinformatic and artificial intelligence-based AlphaFold analyses, we suggest that the hairpin loop mimics the structure and electrostatics of DNA, and provides an additional mechanism, supplementary to sequence specificity, of regulating ZNF410 DNA binding.

Amino acid metabolism in skeletal cells

Amino acid metabolism regulates essential cellular functions, not only by fueling protein synthesis, but also by supporting the biogenesis of nucleotides, redox factors and lipids. Amino acids are also involved in tricarboxylic acid cycle anaplerosis, epigenetic modifications, next to synthesis of neurotransmitters and hormones. As such, amino acids contribute to a broad range of cellular processes such as proliferation, matrix synthesis and intercellular communication, which are all critical for skeletal cell functioning. Here we summarize recent work elucidating how amino acid metabolism supports and regulates skeletal cell function during bone growth and homeostasis, as well as during skeletal disease. The most extensively studied amino acid is glutamine, and osteoblasts and chondrocytes rely heavily on this non-essential amino acid during for their functioning and differentiation. Regulated by lineage-specific transcription factors such as SOX9 and osteoanabolic agents such as parathyroid hormone or WNT, glutamine metabolism has a wide range of metabolic roles, as it fuels anabolic processes by producing nucleotides and non-essential amino acids, maintains redox balance by generating the antioxidant glutathione and regulates cell-specific gene expression via epigenetic mechanisms. We also describe how other amino acids affect skeletal cell functions, although further work is needed to fully understand their effect. The increasing number of studies using stable isotope labelling in several skeletal cell types at various stages of differentiation, together with conditional inactivation of amino acid transporters or enzymes in mouse models, will allow us to obtain a more complete picture of amino acid metabolism in skeletal cells.

Increased osteoclastogenesis contributes to bone loss in the Costello syndrome Hras G12V mouse model

RAS GTPases are ubiquitous GDP/GTP-binding proteins that function as molecular switches in cellular signalling and control numerous signalling pathways and biological processes. Pathogenic mutations in RAS genes severely affect cellular homeostasis, leading to cancer when occurring in somatic cells and developmental disorders when the germline is affected. These disorders are generally termed as RASopathies and among them Costello syndrome (CS) is a distinctive entity that is caused by specific HRAS germline mutations. The majority of these mutations affect residues 12 and 13, the same sites as somatic oncogenic HRAS mutations. The hallmarks of the disease include congenital cardiac anomalies, impaired thriving and growth, neurocognitive impairments, distinctive craniofacial anomalies, and susceptibility to cancer. Adult patients often present signs of premature aging including reduced bone mineral density and osteoporosis. Using a CS mouse model harbouring a Hras G12V germline mutation, we aimed at determining whether this model recapitulates the patients’ bone phenotype and which bone cells are driving the phenotype when mutated. Our data revealed that Hras G12V mutation induces bone loss in mice at certain ages. In addition, we identified that bone loss correlated with an increased number of osteoclasts in vivo and Hras G12V mutations increased osteoclastogenesis in vitro. Last, but not least, mutant osteoclast differentiation was reduced by treatment in vitro with MEK and PI3K inhibitors, respectively. These results indicate that Hras is a novel regulator of bone homeostasis and an increased osteoclastogenesis due to Hras G12V mutation contributes to bone loss in the Costello syndrome.

Amino acid metabolism in primary bone sarcomas

Primary bone sarcomas, including osteosarcoma (OS) and Ewing sarcoma (ES), are aggressive tumors with peak incidence in childhood and adolescence. The intense standard treatment for these patients consists of combined surgery and/or radiation and maximal doses of chemotherapy; a regimen that has not seen improvement in decades. Like other tumor types, ES and OS are characterized by dysregulated cellular metabolism and a rewiring of metabolic pathways to support the biosynthetic demands of malignant growth. Not only are cancer cells characterized by Warburg metabolism, or aerobic glycolysis, but emerging work has revealed a dependence on amino acid metabolism. Aside from incorporation into proteins, amino acids serve critical functions in redox balance, energy homeostasis, and epigenetic maintenance. In this review, we summarize current studies describing the amino acid metabolic requirements of primary bone sarcomas, focusing on OS and ES, and compare these dependencies in the normal bone and malignant tumor contexts. We also examine insights that can be gleaned from other cancers to better understand differential metabolic susceptibilities between primary and metastatic tumor microenvironments. Lastly, we discuss potential metabolic vulnerabilities that may be exploited therapeutically and provide better-targeted treatments to improve the current standard of care.

SLC38A2 provides proline and alanine to regulate postnatal bone mass accrual in mice

Amino acids have recently emerged as important regulators of osteoblast differentiation and bone formation. Osteoblasts require a continuous supply of amino acids to sustain biomass production to fuel cell proliferation, osteoblast differentiation and bone matrix production. We recently identified proline as an essential amino acid for bone development by fulfilling unique synthetic demands that are associated with osteoblast differentiation. Osteoblasts rely on the amino acid transporter SLC38A2 to provide proline to fuel endochondral ossification. Despite this, very little is known about the function or substrates of SLC38A2 during bone homeostasis. Here we demonstrate that the neutral amino acid transporter SLC38A2 is expressed in osteoblast lineage cells and provides proline and alanine to osteoblast lineage cells. Genetic ablation of SLC38A2 using Prrx1Cre results in decreased bone mass in both male and female mice due to a reduction in osteoblast numbers and bone forming activity. Decreased osteoblast numbers are attributed to impaired proliferation and osteogenic differentiation of skeletal stem and progenitor cells. Collectively, these data highlight the necessity of SLC38A2-mediated proline and alanine uptake during postnatal bone formation and bone homeostasis.

Identifying Bone Matrix Impairments in a Mouse Model of Neurofibromatosis Type 1 (NF1) by Clinically Translatable Techniques

Three-to-four percent of children with neurofibromatosis type 1 (NF1) present with unilateral tibia bowing, fracture, and recalcitrant healing. Alkaline phosphatase (ALP) enzyme therapy prevented poor bone mineralization and poor mechanical properties in mouse models of NF1 skeletal dysplasia; but transition to clinical trials is hampered by the lack of a technique that (i) identifies NF1 patients at risk of tibia bowing and fracture making them eligible for trial enrollment and (ii) monitors treatment effects on matrix characteristics related to bone strength. Therefore, we assessed the ability of matrix-sensitive techniques to provide characteristics that differentiate between cortical bone from mice characterized by postnatal loss of Nf1 in Osx-cre^Tet-Off ;Nf1^flox/flox osteoprogenitors (cKO) and from wild-type (WT) mice. Following euthanasia at two time points of bone disease progression, femur and tibia were harvested from both genotypes (n ≥ 8/age/sex/genotype). A reduction in the mid-diaphysis ultimate force during three-point bending at 20 weeks confirmed deleterious changes in bone induced by Nf1 deficiency, regardless of sex. Pooling females and males, low bound water (BW), and low cortical volumetric bone mineral density (Ct.vBMD) were the most accurate outcomes in distinguishing cKO from WT femurs with accuracy improving with age. Ct.vBMD and the average unloading slope (Avg-US) from cyclic reference point indentation tests were the most sensitive in differentiating WT from cKO tibias. Mineral-to-matrix ratio and carbonate substitution from Raman spectroscopy were not good classifiers. However, when combined with Ct.vBMD and BW (femur), they helped predict bending strength. Nf1 deficiency in osteoprogenitors negatively affected bone microstructure and matrix quality with deficits in properties becoming more pronounced with duration of Nf1 deficiency. Clinically measurable without ionizing radiation, BW and Avg-US are sensitive to deleterious changes in bone matrix in a preclinical model of NF1 bone dysplasia and require further clinical investigation as potential indicators of an onset of bone weakness in children with NF1. © 2022 American Society for Bone and Mineral Research (ASBMR).

Physical and Chemical Properties, Biosafety Evaluation, and Effects of Nano Natural Deer Bone Meal on Bone Marrow Mesenchymal Stem Cells

At present, bone-based products are abundant, and the main sources are bovine bone and pig bone, but there are few studies on the development of deer bone as a bone repair material. Deer bone has important osteogenic effects in the theory of traditional Chinese medicine. It is rich in protein, ossein, and a variety of trace elements, with the effect of strengthening tendons and bones. Nanomaterials and their application in the repair of bone defects have become a research hotspot in bone tissue engineering. In this study, nano-deer bone meal (nBM), nano-calcined deer bone meal, and nano-demineralized bone matrix were successfully prepared. It was found that the Ca/P ratio in deer bone was significantly higher than that in cow bone and human bone tissue, and deer bone contained beneficial trace elements, such as potassium, iron, selenium, and zinc, which were not found in cow bone. The three kinds of deer bone powders prepared in this study had good biocompatibility and met the implantation standards of medical biomaterials. Cell function studies showed that compared with other bone powders, due to the presence of organic active ingredients and inorganic calcium and phosphate salts, nBM had excellent performance in the proliferation, adhesion, migration, and differentiation of bone marrow mesenchymal stem cells. These findings indicate that nBM can be used as a potential osteoinductive active nanomaterial to enhance bone tissue engineering scaffolds with certain application prospects.

The multifaceted role of ATF4 in regulating glucose-stimulated insulin secretion

Stress-inducible transcription factor ATF4 is essential for survival and identity of β-cell during stress conditions. However, the physiological role of ATF4 in β-cell function is not yet completely understood. To understand the role of ATF4 in glucose-stimulated insulin secretion (GSIS), β-cell-specific Atf4 knockout (βAtf4KO) mice were phenotypically characterized. Insulin secretion and mechanistic analyses were performed using islets from control Atf4f/f and βAtf4KO mice to assess key regulators for triggering and amplifying signals for GSIS. βAtf4KO mice displayed glucose intolerance due to reduced insulin secretion. Moreover, βAtf4KO islets exhibited a decrease in both the insulin content and first-phase insulin secretion. The analysis of βAtf4KO islets showed that ATF4 is required for insulin production and glucose-stimulated ATP and cAMP production. The results demonstrate that ATF4 contributes to the multifaceted regulatory process in GSIS even under stress-free conditions.

The crosstalk between bone remodeling and energy metabolism: A translational perspective

Genetics in model organisms has progressively broken down walls that previously separated different disciplines of biology. One example of this holistic evolution is the recognition of the complex relationship that exists between the control of bone mass (bone remodeling) and energy metabolism in mammals. Numerous hormones orchestrate this crosstalk. In particular, the study of the leptin-mediated regulation of bone mass has not only revealed the existence of a central control of bone mass but has also led to the realization that sympathetic innervation is a major regulator of bone remodeling. This happened at a time when the use of drugs aiming at treating osteoporosis, the most frequent bone disease, has dwindled. This review will highlight the main aspects of the leptin-mediated regulation of bone mass and how this led to the realization that β-blockers, which block the effects of the sympathetic nervous system, may be a viable option to prevent osteoporosis.

SLC38A2 provides proline to fulfil unique synthetic demands arising during osteoblast differentiation and bone formation

Cellular differentiation is associated with the acquisition of a unique protein signature which is essential to attain the ultimate cellular function and activity of the differentiated cell. This is predicted to result in unique biosynthetic demands that arise during differentiation. Using a bioinformatic approach, we discovered osteoblast differentiation is associated with increased demand for the amino acid proline. When compared to other differentiated cells, osteoblast-associated proteins including RUNX2, OSX, OCN and COL1A1 are significantly enriched in proline. Using a genetic and metabolomic approach, we demonstrate that the neutral amino acid transporter SLC38A2 acts cell autonomously to provide proline to facilitate the efficient synthesis of proline-rich osteoblast proteins. Genetic ablation of SLC38A2 in osteoblasts limits both osteoblast differentiation and bone formation in mice. Mechanistically, proline is primarily incorporated into nascent protein with little metabolism observed. Collectively, these data highlight a requirement for proline in fulfilling the unique biosynthetic requirements that arise during osteoblast differentiation and bone formation.

The Extracellular Signal-Regulated Kinase Mitogen-Activated Protein Kinase Pathway in Osteoblasts

Extracellular signal-regulated kinases (ERKs) are evolutionarily ancient signal transducers of the mitogen-activated protein kinase (MAPK) family that have long been linked to the regulation of osteoblast differentiation and bone formation. Here, we review the physiological functions, biochemistry, upstream activators, and downstream substrates of the ERK pathway. ERK is activated in skeletal progenitors and regulates osteoblast differentiation and skeletal mineralization, with ERK serving as a key regulator of Runt-related transcription factor 2, a critical transcription factor for osteoblast differentiation. However, new evidence highlights context-dependent changes in ERK MAPK pathway wiring and function, indicating a broader set of physiological roles associated with changes in ERK pathway components or substrates. Consistent with this importance, several human skeletal dysplasias are associated with dysregulation of the ERK MAPK pathway, including neurofibromatosis type 1 and Noonan syndrome. The continually broadening array of drugs targeting the ERK pathway for the treatment of cancer and other disorders makes it increasingly important to understand how interference with this pathway impacts bone metabolism, highlighting the importance of mouse studies to model the role of the ERK MAPK pathway in bone formation.

Bioenergetic Metabolism In Osteoblast Differentiation

PURPOSE OF REVIEW

Osteoblasts are responsible for bone matrix production during bone development and homeostasis. Much is known about the transcriptional regulation and signaling pathways governing osteoblast differentiation. However, less is known about how osteoblasts obtain or utilize nutrients to fulfill the energetic demands associated with osteoblast differentiation and bone matrix synthesis. The goal of this review is to highlight and discuss what is known about the role and regulation of bioenergetic metabolism in osteoblasts with a focus on more recent studies.

RECENT FINDINGS

Bioenergetic metabolism has emerged as an important regulatory node in osteoblasts. Recent studies have begun to identify the major nutrients and bioenergetic pathways favored by osteoblasts as well as their regulation during differentiation. Here, we highlight how osteoblasts obtain and metabolize glucose, amino acids, and fatty acids to provide energy and other metabolic intermediates. In addition, we highlight the signals that regulate nutrient uptake and metabolism and focus on how energetic metabolism promotes osteoblast differentiation. Bioenergetic metabolism provides energy and other metabolites that are critical for osteoblast differentiation and activity. This knowledge contributes to a more comprehensive understanding of osteoblast biology and may inform novel strategies to modulate osteoblast differentiation and bone anabolism in patients with bone disorders.

Bone tissue homeostasis and risk of fractures in Costello syndrome: A 4-year follow-up study

Costello syndrome (CS) is a neurodevelopmental disorder with a distinctive musculoskeletal phenotype and reduced bone mineral density (BMD) caused by activating de novo mutations in the HRAS gene. Herein, we report the results of a prospective study evaluating the efficacy of a 4-year vitamin D supplementation on BMD and bone health. A cohort of 16 individuals ranging from pediatric to adult age with molecularly confirmed CS underwent dosages of bone metabolism biomarkers (serum/urine) and dual-energy X-ray absorptiometry (DXA) scans to assess bone and body composition parameters. Results were compared to age-matched control groups. At baseline evaluation, BMD was significantly reduced (p ≤ 0.05) compared to controls, as were the 25(OH)vitD levels. Following the 4-year time interval, despite vitamin D supplementation therapy at adequate dosages, no significant improvement in BMD was observed. The present data confirm that 25(OH)vitD and BMD parameters are reduced in CS, and vitamin D supplementation is not sufficient to restore proper BMD values. Based on this evidence, routine monitoring of bone homeostasis to prevent bone deterioration and possible fractures in adult patients with CS is highly recommended.

Characterization of bone homeostasis in individuals affected by cardio-facio-cutaneous syndrome

Cardio-facio-cutaneous syndrome (CFCS) is a rare disorder characterized by distinctive craniofacial appearance, cardiac, neurologic, cutaneous, and musculoskeletal abnormalities. It is due to heterozygous mutations in BRAF, MAP2K1, MAP2K2, and KRAS genes, belonging to the RAS/MAPK pathway. The role of RAS signaling in bone homeostasis is highly recognized, but data on bone mineral density (BMD) in CFCS are lacking. In the present study we evaluated bone parameters, serum and urinary bone metabolites in 14 individuals with a molecularly confirmed diagnosis of CFCS. Bone assessment was performed through dual X-ray absorptiometry (DXA); height-adjusted results were compared to age- and sex-matched controls. Blood and urinary bone metabolites were also analyzed and compared to the reference range. Despite vitamin D supplementation and almost normal bone metabolism biomarkers, CFCS patients showed significantly decreased absolute values of DXA-assessed subtotal and lumbar BMD (p ≤ 0.05), compared to controls. BMD z-scores and t-scores (respectively collected for children and adults) were below the reference range in CFCS, while normal in healthy controls. These findings confirmed a reduction in BMD in CFCS and highlighted the importance of monitoring bone health in these affected individuals.

PIK3CA mutation affects the proliferation of colorectal cancer cells through the PI3K-MEK/PDK1-GPT2 pathway

The phosphatidylinositol‑3‑kinase catalytic subunit α (PIK3CA) gene is mutated in numerous human cancers. This mutation promotes the proliferation of tumor cells; however, the underlying mechanism is still not clear. In the present study, it was revealed that the PIK3CA mutation in colorectal cancer (CRC) HCT116 (MUT) rendered the cells more dependent on glutamine by regulating the glutamic‑pyruvate transaminase 2 (GPT2). The dependence of glutamine increased the proliferation of cells in a normal environment and resistance to a suboptimal environment. Further study revealed that the mutated PIK3CA could regulate GPT2 expression not only through signal transduction molecule 3‑phosphoinositide‑dependent kinase (PDK1) but also through mitogen‑activated protein kinase (MEK) molecules. In HCT116 cells, MEK inhibitor treatment could reduce the expression of GPT2 signaling molecules, thereby inhibiting the proliferation of CRC cells. A new signal transduction pathway, the PI3K/MEK/GPT2 pathway was identified. Based on these findings, MEK and PDK1 inhibitors were combined to inhibit the aforementioned pathway. It was revealed that the combined application of MEK and PDK1 inhibitors could promisingly inhibit the proliferation of MUT compared with the application of PI3K inhibitors, PDK1 inhibitors, or MEK inhibitors alone. In vivo, MEK inhibitors alone and combined inhibitors had stronger tumor‑suppressing effects. There was no significant difference between the PDK1‑inhibitor group and normal group in vivo. Thus, these results indicated that mutated PI3K affected GPT2 mediated by the MEK/PDK1 dual pathway, and that the PI3K/MEK/GPT2 pathway was more important in vivo. Inhibiting MEK and PDK1 concurrently could effectively inhibit the proliferation of CRC cells. Targeting the MEK and PDK1 signaling pathway may provide a novel strategy for the treatment of PIK3CA‑mutated CRC.

RASopathies: The musculoskeletal consequences and their etiology and pathogenesis

The RASopathies comprise an ever-growing number of clinical syndromes resulting from germline mutations in components of the RAS/MAPK signaling pathway. While multiple organs and tissues may be affected by these mutations, this review will focus on how these mutations specifically impact the musculoskeletal system. Herein, we review the genetics and musculoskeletal phenotypes of these syndromes in humans. We discuss how mutations in the RASopathy syndromes have been studied in translational mouse models. Finally, we discuss how signaling molecules within the RAS/MAPK pathway are involved in normal and abnormal bone biology in the context of osteoblasts, osteoclasts and chondrocytes.

SLC1A5 provides glutamine and asparagine necessary for bone development in mice

Osteoblast differentiation is sequentially characterized by high rates of proliferation followed by increased protein and matrix synthesis, processes that require substantial amino acid acquisition and production. How osteoblasts obtain or maintain intracellular amino acid production is poorly understood. Here, we identify SLC1A5 as a critical amino acid transporter during bone development. Using a genetic and metabolomic approach, we show SLC1A5 acts cell autonomously to regulate protein synthesis and osteoblast differentiation. SLC1A5 provides both glutamine and asparagine which are essential for osteoblast differentiation. Mechanistically, glutamine and to a lesser extent asparagine support amino acid biosynthesis. Thus, osteoblasts depend on Slc1a5 to provide glutamine and asparagine, which are subsequently used to produce non-essential amino acids and support osteoblast differentiation and bone development.

The investigation of energy metabolism in osteoblasts and osteoclasts

The maintenance of bone homeostasis is critical for bone health. It is vulnerable to cause bone loss, even severely osteoporosis when the balance between bone formation and absorption is interrupted. Growing evidence has shown that energy metabolism disorders, such as abnormal glucose metabolism, irregular amino acid metabolism, and aberrant lipid metabolism, can damage bone homeostasis, causing or exacerbating bone mass loss and osteoporosis-related fractures. Here, we summarize the studies of energy metabolism in osteoblasts and osteoclasts and provide a better appreciation of how energy metabolism, especially glucose metabolism maintains bone homeostasis. With this knowledge, new avenues will be unraveled to understand and cue bone-related diseases such as osteoporosis.

Impacts of NF1 Gene Mutations and Genetic Modifiers in Neurofibromatosis Type 1

Neurofibromatosis type 1 (NF1) is a tumor predisposition genetic disorder that directly affects more than 1 in 3,000 individuals worldwide. It results from mutations of the NF1 gene and shows almost complete penetrance. NF1 patients show high phenotypic variabilities, including cafe-au-lait macules, freckling, or other neoplastic or non-neoplastic features. Understanding the underlying mechanisms of the diversities of clinical symptoms might contribute to the development of personalized healthcare for NF1 patients. Currently, studies have shown that the different types of mutations in the NF1 gene might correlate with this phenomenon. In addition, genetic modifiers are responsible for the different clinical features. In this review, we summarize different genetic mutations of the NF1 gene and related genetic modifiers. More importantly, we focus on the genotype–phenotype correlation. This review suggests a novel aspect to explain the underlying mechanisms of phenotypic heterogeneity of NF1 and provides suggestions for possible novel therapeutic targets to prevent or delay the onset and development of different manifestations of NF1.

Sites of Cre-recombinase activity in mouse lines targeting skeletal cells

The Cre/Lox system is a powerful tool in the biologist's toolbox, allowing loss-of-function and gain-of-function studies, as well as lineage tracing, through gene recombination in a tissue-specific and inducible manner. Evidence indicates, however, that Cre transgenic lines have a far more nuanced and broader pattern of Cre activity than initially thought, exhibiting "off-target" activity in tissues/cells other than the ones they were originally designed to target. With the goal of facilitating the comparison and selection of optimal Cre lines to be used for the study of gene function, we have summarized in a single manuscript the major sites and timing of Cre activity of the main Cre lines available to target bone mesenchymal stem cells, chondrocytes, osteoblasts, osteocytes, tenocytes, and osteoclasts, along with their reported sites of "off-target" Cre activity. We also discuss characteristics, advantages, and limitations of these Cre lines for users to avoid common risks related to overinterpretation or misinterpretation based on the assumption of strict cell-type specificity or unaccounted effect of the Cre transgene or Cre inducers. © 2021 American Society for Bone and Mineral Research (ASBMR).

Regulation and Role of Transcription Factors in Osteogenesis

Bone is a dynamic tissue constantly responding to environmental changes such as nutritional and mechanical stress. Bone homeostasis in adult life is maintained through bone remodeling, a controlled and balanced process between bone-resorbing osteoclasts and bone-forming osteoblasts. Osteoblasts secrete matrix, with some being buried within the newly formed bone, and differentiate to osteocytes. During embryogenesis, bones are formed through intramembraneous or endochondral ossification. The former involves a direct differentiation of mesenchymal progenitor to osteoblasts, and the latter is through a cartilage template that is subsequently converted to bone. Advances in lineage tracing, cell sorting, and single-cell transcriptome studies have enabled new discoveries of gene regulation, and new populations of skeletal stem cells in multiple niches, including the cartilage growth plate, chondro-osseous junction, bone, and bone marrow, in embryonic development and postnatal life. Osteoblast differentiation is regulated by a master transcription factor RUNX2 and other factors such as OSX/SP7 and ATF4. Developmental and environmental cues affect the transcriptional activities of osteoblasts from lineage commitment to differentiation at multiple levels, fine-tuned with the involvement of co-factors, microRNAs, epigenetics, systemic factors, circadian rhythm, and the microenvironments. In this review, we will discuss these topics in relation to transcriptional controls in osteogenesis.

Amino acid metabolism and autophagy in skeletal development and homeostasis

Bone is an active organ that is continuously remodeled throughout life via formation and resorption; therefore, a fine-tuned bone (re)modeling is crucial for bone homeostasis and is closely connected with energy metabolism. Amino acids are essential for various cellular functions as well as an energy source, and their synthesis and catabolism (e.g., metabolism of carbohydrates and fatty acids) are regulated through numerous enzymatic cascades. In addition, the intracellular levels of amino acids are maintained by autophagy, a cellular recycling system for proteins and organelles; under nutrient deprivation conditions, autophagy is strongly induced to compensate for cellular demands and to restore the amino acid pool. Metabolites derived from amino acids are known to be precursors of bioactive molecules such as second messengers and neurotransmitters, which control various cellular processes, including cell proliferation, differentiation, and homeostasis. Thus, amino acid metabolism and autophagy are tightly and reciprocally regulated in our bodies. This review discusses the current knowledge and potential links between bone diseases and deficiencies in amino acid metabolism and autophagy.

Cancer-associated fibroblasts-mediated ATF4 expression promotes malignancy and gemcitabine resistance in pancreatic cancer via the TGF-β1/SMAD2/3 pathway and ABCC1 transactivation

Cancer-associated fibroblasts (CAFs) contribute to malignant progression and chemoresistance in pancreatic ductal adenocarcinoma (PDAC). However, little is known about the underlying mechanism. In this study, we investigated the potential role and mechanisms of activating transcription factor 4 (ATF4) in CAFs-induced malignancy and gemcitabine resistance. We demonstrated that ATF4 is overexpressed in PDAC and associated with a poor prognosis. Silencing ATF4 expression decreased proliferation, colony formation, migration, gemcitabine sensitivity, and sphere formation. Subsequently, we revealed that CAFs secrete TGF-β1 to upregulate the expression of ATF4 in PDAC cells via the SMAD2/3 pathway and induce cancer progression, cancer stemness, and gemcitabine resistance. Furthermore, we demonstrated that ATF4 directly binds to the ABCC1 promoter region to activate transcription. In summary, these data demonstrate that CAFs contribute to malignancy and gemcitabine resistance in PDAC by upregulating the expression of ATF4 via the TGF-β1/SMAD2/3 axis and highlight that ATF4 is an attractive therapeutic target for combating gemcitabine resistance in PDAC.

ATF4 selectively regulates heat nociception and contributes to kinesin-mediated TRPM3 trafficking

Effective treatments for patients suffering from heat hypersensitivity are lacking, mostly due to our limited understanding of the pathogenic mechanisms underlying this disorder. In the nervous system, activating transcription factor 4 (ATF4) is involved in the regulation of synaptic plasticity and memory formation. Here, we show that ATF4 plays an important role in heat nociception. Indeed, loss of ATF4 in mouse dorsal root ganglion (DRG) neurons selectively impairs heat sensitivity. Mechanistically, we show that ATF4 interacts with transient receptor potential cation channel subfamily M member-3 (TRPM3) and mediates the membrane trafficking of TRPM3 in DRG neurons in response to heat. Loss of ATF4 also significantly decreases the current and KIF17-mediated trafficking of TRPM3, suggesting that the KIF17/ATF4/TRPM3 complex is required for the neuronal response to heat stimuli. Our findings unveil the non-transcriptional role of ATF4 in the response to heat stimuli in DRG neurons.

The enzymatic hydrolysates from deer sinew promote MC3T3-E1 cell proliferation and extracellular matrix synthesis by regulating multiple functional genes

Background

Deer Sinew serves as a medicinal food, and has been used for treating skeletal diseases, especially bone diseases in a long history. Thus, it could become an alternative option for the prevention and therapeutic remedy of bone-related diseases. In our previous study, we established an optimal extraction process of the enzymatic hydrolysates from Chinese Sika deer sinews (DSEH), and we demonstrated that DSEH significantly promoted the proliferation of MC3T3-E1 cells (an osteoblast-like cell line) with a certain dose-effect relationship. However, the precise molecular mechanism of deer sinew in regulating bone strength is still largely unknown. The aim of this study was to explore the underlying molecular mechanism of DSEH on MC3T3-E1 cells proliferation and extracellular matrix synthesis.

Methods

Preparation and quality control were performed as previously described. The effect of DSEH at different administrated concentrations on cell proliferation was measured using both CCK-8 and MTT assays, and the capacity of DSEH on extracellular matrix synthesis was detected by Alizarin red staining and quantification. The gene expression pattern change of MC3T3-E1 cells under the treatment of DSEH was investigated by RNA-seq analysis accompanied with validation methods.

Results

We demonstrated that DSEH promoted MC3T3-E1 cell proliferation and extracellular matrix synthesis by regulating multiple functional genes. DSEH significantly increased the expression levels of genes that promoted cell proliferation such as Gstp1, Timp1, Serpine1, Cyr61, Crlf1, Thbs1, Ctgf, P4ha2, Sod3 and Nqo1. However, DSEH significantly decreased the expression levels of genes that inhibited cell proliferation such as Mt1, Cdc20, Gas1, Nrp2, Cmtm3, Dlk2, Sema3a, Rbm25 and Hspb6. Furthermore, DSEH mildly increased the expression levels of osteoblast gene markers.

Conclusions

Our findings suggest that DSEH facilitate MC3T3-E1 cell proliferation and extracellular matrix synthesis to consolidate bone formation and stability, but prevent MC3T3-E1 cells from oxidative stress-induced damage, apoptosis and further differentiation. These findings deepened the current understanding of DSEH on regulating bone development, and provided theoretical support for the discovery of optional prevention and treatment for bone-related diseases.

Biphasic regulation of glutamine consumption by WNT during osteoblast differentiation

Osteoblasts are the principal bone-forming cells. As such, osteoblasts have enhanced demand for amino acids to sustain high rates of matrix synthesis associated with bone formation. The precise systems utilized by osteoblasts to meet these synthetic demands are not well understood. WNT signaling is known to rapidly stimulate glutamine uptake during osteoblast differentiation. Using a cell biology approach, we identified two amino acid transporters, γ(+)-LAT1 and ASCT2 (encoded by Slc7a7 and Slc1a5, respectively), as the primary transporters of glutamine in response to WNT. ASCT2 mediates the majority of glutamine uptake, whereas γ(+)-LAT1 mediates the rapid increase in glutamine uptake in response to WNT. Mechanistically, WNT signals through the canonical β-catenin (CTNNB1)-dependent pathway to rapidly induce Slc7a7 expression. Conversely, Slc1a5 expression is regulated by the transcription factor ATF4 downstream of the mTORC1 pathway. Targeting either Slc1a5 or Slc7a7 using shRNA reduced WNT-induced glutamine uptake and prevented osteoblast differentiation. Collectively, these data highlight the critical nature of glutamine transport for WNT-induced osteoblast differentiation.This article has an associated First Person interview with the joint first authors of the paper.

Biophysical phenotyping of mesenchymal stem cells along the osteogenic differentiation pathway

Mesenchymal stem cells represent an important resource, for bone regenerative medicine and therapeutic applications. This review focuses on new advancements and biophysical tools which exploit different physical and chemical markers of mesenchymal stem cell populations, to finely characterize phenotype changes along their osteogenic differentiation process. Special attention is paid to recently developed label-free methods, which allow monitoring cell populations with minimal invasiveness. Among them, quantitative phase imaging, suitable for single-cell morphometric analysis, and nanoindentation, functional to cellular biomechanics investigation. Moreover, the pool of ion channels expressed in cells during differentiation is discussed, with particular interest for calcium homoeostasis.Altogether, a biophysical perspective of osteogenesis is proposed, offering a valuable tool for the assessment of the cell stage, but also suggesting potential physiological links between apparently independent phenomena.

Energy Metabolism and Ketogenic Diets: What about the Skeletal Health? A Narrative Review and a Prospective Vision for Planning Clinical Trials on this Issue

The existence of a common mesenchymal cell progenitor shared by bone, skeletal muscle, and adipocytes cell progenitors, makes the role of the skeleton in energy metabolism no longer surprising. Thus, bone fragility could also be seen as a consequence of a “poor” quality in nutrition. Ketogenic diet was originally proven to be effective in epilepsy, and long-term follow-up studies on epileptic children undergoing a ketogenic diet reported an increased incidence of bone fractures and decreased bone mineral density. However, the causes of such negative impacts on bone health have to be better defined. In these subjects, the concomitant use of antiepileptic drugs and the reduced mobilization may partly explain the negative effects on bone health, but little is known about the effects of diet itself, and/or generic alterations in vitamin D and/or impaired growth factor production. Despite these remarks, clinical studies were adequately designed to investigate bone health are scarce and bone health related aspects are not included among the various metabolic pathologies positively influenced by ketogenic diets. Here, we provide not only a narrative review on this issue, but also practical advice to design and implement clinical studies on ketogenic nutritional regimens and bone health outcomes. Perspectives on ketogenic regimens, microbiota, microRNAs, and bone health are also included.

Radiolabeled Amino Acid Uptake Assays in Primary Bone Cells and Bone Explants

Radiolabeled amino acid uptake assays are a highly sensitive method used to characterize the uptake of amino acids by cells or tissues in culture. This method is an excellent tool to quantify changes in amino acid consumption that are associated with states of cellular differentiation and/or disease. The methods presented here can be adapted to measure the transport of all amino acids and can be applied to cultured cells and bone explants.

TAOK3 is a MAP3K contributing to osteoblast differentiation and skeletal mineralization

Current anabolic drugs to treat osteoporosis and other disorders of low bone mass all have important limitations in terms of toxicity, contraindications, or poor efficacy in certain contexts. Addressing these limitations will require a better understanding of the molecular pathways, such as the mitogen activated protein kinase (MAPK) pathways, that govern osteoblast differentiation and, thereby, skeletal mineralization. Whereas MAP3Ks functioning in the extracellular signal-regulated kinases (ERK) and p38 pathways have been identified in osteoblasts, MAP3Ks mediating proximal activation of the c-Jun N-terminal kinase (JNK) pathway have yet to be identified. Here, we demonstrate that thousand-and-one kinase 3 (TAOK3, MAP3K18) functions as an upstream activator of the JNK pathway in osteoblasts both in vitro and in vivo. Taok3-deficient osteoblasts displayed defective JNK pathway activation and a marked decrease in osteoblast differentiation markers and defective mineralization, which was also confirmed using TAOK3 deficient osteoblasts derived from human MSCs. Additionally, reduced expression of Taok3 in a murine model resulted in osteopenia that phenocopies aspects of the Jnk1-associated skeletal phenotype such as occipital hypomineralization. Thus, in vitro and in vivo evidence supports TAOK3 as a proximal activator of the JNK pathway in osteoblasts that plays a critical role in skeletal mineralization.

Anacardic acid-mediated regulation of osteoblast differentiation involves mitigation of inflammasome activation pathways

Disruption of the finely tuned osteoblast-osteoclast balance is the underlying basis of several inflammatory bone diseases, such as osteomyelitis, osteoporosis, and septic arthritis. Prolonged and unrestrained exposure to inflammatory environment results in reduction of bone mineral density by downregulating osteoblast differentiation. Earlier studies from our laboratory have identified that Anacardic acid (AA), a constituent of Cashew nut shell liquid that is used widely in traditional medicine, has potential inhibitory effect on gelatinases (MMP2 and MMP9) which are over-expressed in numerous inflammatory conditions (Omanakuttan et al. in Mol Pharmacol, 2012 and Nambiar et al. in Exp Cell Res, 2016). The study demonstrated for the first time that AA promotes osteoblast differentiation in lipopolysaccharide-treated osteosarcoma cells (MG63) by upregulating specific markers, like osteocalcin, receptor activator of NF-κB ligand, and alkaline phosphatase. Furthermore, expression of the negative regulators, such as nuclear factor-κB, matrix metalloproteinases (MMPs), namely MMP13, and MMP1, along with several inflammatory markers, such as Interleukin-1β and Nod-like receptor protein 3 were downregulated by AA. Taken together, AA expounds as a novel template for development of potential pharmacological therapeutics for inflammatory bone diseases.

miR-214 Attenuates Aortic Valve Calcification by Regulating Osteogenic Differentiation of Valvular Interstitial Cells

Calcific aortic valve disease (CAVD) is a common heart valve disease in aging populations, and aberrant osteogenic differentiation of valvular interstitial cells (VICs) plays a critical role in the pathogenesis of ectopic ossification of the aortic valve. miR-214 has been validated to be involved in the osteogenesis process. Here, we aim to investigate the role and mechanism of miR-214 in CAVD progression. miR-214 expression was significantly downregulated in CAVD aortic valve leaflets, accompanied by upregulation of osteogenic markers. Overexpression of miR-214 suppressed osteogenic differentiation of VICs, while silencing the expression of miR-214 promoted this function. miR-214 directly targeted ATF4 and Sp7 to modulate osteoblastic differentiation of VICs, which was proved by dual luciferase reporter assay and rescue experiment. miR-214 knockout rats exhibited higher mean transvalvular velocity and gradient. The expression of osteogenic markers in aortic valve leaflets of miR-214 knockout rats was upregulated compared to that of the wild-type group. Taken together, our study showed that miR-214 inhibited aortic valve calcification via regulating osteogenic differentiation of VICs by directly targeting ATF4 and Sp7, indicating that miR-214 may act as a profound candidate of targeting therapy for CAVD.

The Amino Acid Sensor Eif2ak4/GCN2 Is Required for Proliferation of Osteoblast Progenitors in Mice

Skeletal stem/progenitor cells (SSPC) are critical regulators of bone homeostasis by providing a continuous supply of osteoblasts throughout life. In response to inductive signals, SSPC proliferate before osteoblast differentiation. Proliferation requires the duplication of all cellular components before cell division. This imposes a unique biosynthetic requirement for amino acids that can be used for biomass production. Thus, the ability to sense and respond to amino acid availability is likely a major determinant for proliferation. Using a cellular and genetic approach, we demonstrate the amino acid sensor GCN2 is required to support the robust proliferative capacity of SSPC during bone homeostasis. GCN2 ablation results in decreased postnatal bone mass due primarily to reduced osteoblast numbers. Decreased osteoblast numbers is likely attributed to reduced SSPC proliferation as loss of GCN2 specifically affected proliferation in cultured bone marrow stromal cells (BMSCs) without impacting osteoblast differentiation in vitro. Mechanistically, GCN2 regulates proliferation by increasing amino acid uptake downstream of the transcriptional effector ATF4. Collectively, these data suggest amino acid sensing through the GCN2/ATF4 pathway is indispensable for robust SSPC proliferation necessary for bone homeostasis. © 2020 American Society for Bone and Mineral Research.

Energy metabolism: A newly emerging target of BMP signaling in bone homeostasis

Energy metabolism is the process of generating energy (i.e. ATP) from nutrients. This process is indispensable for cell homeostasis maintenance and responses to varying conditions. Cells require energy for growth and maintenance and have evolved to have multiple pathways to produce energy. Both genetic and functional studies have demonstrated that energy metabolism, such as glucose, fatty acid, and amino acid metabolism, plays important roles in the formation and function of bone cells including osteoblasts, osteocytes, and osteoclasts. Dysregulation of energy metabolism in bone cells consequently disturbs the balance between bone formation and bone resorption. Metabolic diseases have also been reported to affect bone homeostasis. Bone morphogenic protein (BMP) signaling plays critical roles in regulating the formation and function of bone cells, thus affecting bone development and homeostasis. Mutations of BMP signaling-related genes in mice have been reported to show abnormalities in energy metabolism in many tissues, including bone. In addition, BMP signaling correlates with critical signaling pathways such as mTOR, HIF, Wnt, and self-degradative process autophagy to coordinate energy metabolism and bone homeostasis. These findings will provide a newly emerging target of BMP signaling and potential therapeutic strategies and the improved management of bone diseases. This review summarizes the recent advances in our understanding of (1) energy metabolism in regulating the formation and function of bone cells, (2) function of BMP signaling in whole body energy metabolism, and (3) mechanistic interaction of BMP signaling with other signaling pathways and biological processes critical for energy metabolism and bone homeostasis.

Neurofibromatosis type 1: New developments in genetics and treatment

Neurofibromatosis type 1 is the most common neurocutaneous syndrome, with a frequency of 1 in 2500 persons. Diagnosis is paramount in the pretumor stage to provide proper anticipatory guidance for a number of neoplasms, both benign and malignant. Loss-of-function mutations in the NF1 gene result in truncated and nonfunctional production of neurofibromin, a tumor suppressor protein involved in downregulating the RAS signaling pathway. New therapeutic and preventive options include tyrosine kinase inhibitors, mTOR inhibitors, interferons, and radiofrequency therapy. This review summarizes recent updates in genetics, mutation analysis assays, and treatment options targeting aberrant genetic pathways. We also propose modified diagnostic criteria and provide an algorithm for surveillance of patients with neurofibromatosis type 1.

Low protein intake during reproduction compromises the recovery of lactation-induced bone loss in female mouse dams without affecting skeletal muscles

Lactation-induced bone loss occurs due to high calcium requirements for fetal growth but skeletal recovery is normally achieved promptly postweaning. Dietary protein is vital for fetus and mother but the effects of protein undernutrition on the maternal skeleton and skeletal muscles are largely unknown. We used mouse dams fed with normal (N, 20%) or low (L, 8%) protein diet during gestation and lactation and maintained on the same diets (NN, LL) or switched from low to normal (LN) during a 28 d skeletal restoration period post lactation. Skeletal muscle morphology and neuromuscular junction integrity was not different between any of the groups. However, dams fed the low protein diet showed extensive bone loss by the end of lactation, followed by full skeletal recovery in NN dams, partial recovery in LN and poor bone recovery in LL dams. Primary osteoblasts from low protein diet fed mice showed decreased in vitro bone formation and decreased osteogenic marker gene expression; promoter methylation analysis by pyrosequencing showed no differences in Bmpr1a, Ptch1, Sirt1, Osx, and Igf1r osteoregulators, while miR-26a, -34a, and -125b expression was found altered in low protein fed mice. Therefore, normal protein diet is indispensable for maternal musculoskeletal health during the reproductive period.

Bioinformatics analysis of the biological changes involved in the osteogenic differentiation of human mesenchymal stem cells

The mechanisms underlying the osteogenic differentiation of human bone marrow mesenchymal stem cells (hBMSCs) remain unclear. In the present study, we aimed to identify the key biological processes during osteogenic differentiation. To this end, we downloaded three microarray data sets from the Gene Expression Omnibus (GEO) database: GSE12266, GSE18043 and GSE37558. Differentially expressed genes (DEGs) were screened using the limma package, and enrichment analysis was performed. Protein-protein interaction network (PPI) analysis and visualization analysis were performed with STRING and Cytoscape. A total of 240 DEGs were identified, including 147 up-regulated genes and 93 down-regulated genes. Functional enrichment and pathways of the present DEGs include extracellular matrix organization, ossification, cell division, spindle and microtubule. Functional enrichment analysis of 10 hub genes showed that these genes are mainly enriched in microtubule-related biological changes, that is sister chromatid segregation, microtubule cytoskeleton organization involved in mitosis, and spindle microtubule. Moreover, immunofluorescence and Western blotting revealed dramatic quantitative and morphological changes in the microtubules during the osteogenic differentiation of human adipose-derived stem cells. In summary, the present results provide novel insights into the microtubule- and cytoskeleton-related biological process changes, identifying candidates for the further study of osteogenic differentiation of the mesenchymal stem cells.

Autophagy in bone homeostasis and the onset of osteoporosis

Autophagy is an evolutionarily conserved intracellular process, in which domestic cellular components are selectively digested for the recycling of nutrients and energy. This process is indispensable for cell homeostasis maintenance and stress responses. Both genetic and functional studies have demonstrated that multiple proteins involved in autophagic activities are critical to the survival, differentiation, and functioning of bone cells, including osteoblasts, osteocytes, and osteoclasts. Dysregulation at the level of autophagic activity consequently disturbs the balance between bone formation and bone resorption and mediates the onset and progression of multiple bone diseases, including osteoporosis. This review aims to introduce the topic of autophagy, summarize the understanding of its relevance in bone physiology, and discuss its role in the onset of osteoporosis and therapeutic potential.

Identification and characterization of NF1 and non-NF1 congenital pseudarthrosis of the tibia based on germline NF1 variants: genetic and clinical analysis of 75 patients

Background

Congenital pseudarthrosis of the tibia (CPT) is a rare disease. Some patients present neurofibromatosis type 1 (NF1), while some others do not manifest NF1 (non-NF1). The etiology of CPT, particularly non-NF1 CPT, is not well understood. Here we screened germline variants of 75 CPT cases, including 55 NF1 and 20 non-NF1. Clinical data were classified and analyzed based on NF1 gene variations to investigate the genotype-phenotype relations of the two types of patients.

Results

Using whole-exome sequencing and Multiplex Ligation-Dependent Probe Amplification, 44 out of 55 NF1 CPT patients (80.0%) were identified as carrying pathogenic variants of the NF1 gene. Twenty-five variants were novel; 53.5% of variants were de novo, and a higher proportion of their carriers presented bone fractures compared to inherited variant carriers. No NF1 pathogenic variants were found in all 20 non-NF1 patients. Clinical features comparing NF1 CPT to non-NF1 CPT did not show significant differences in bowing or fracture onset, lateralization, tissue pathogenical results, abnormality of the proximal tibial epiphysis, and follow-up tibial union after surgery. A considerably higher proportion of non-NF1 patients have cystic lesion (Crawford type III) and used braces after surgery.

Conclusions

We analyzed a large cohort of non-NF1 and NF1 CPT patients and provided a new perspective for genotype-phenotype features related to germline NF1 variants. Non-NF1 CPT in general had similar clinical features of the tibia as NF1 CPT. Germline NF1 pathogenic variants could differentiate NF1 from non-NF1 CPT but could not explain the CPT heterogeneity of NF1 patients. Our results suggested that non-NF1 CPT was probably not caused by germline NF1 pathogenic variants. In addition to NF1, other genetic variants could also contribute to CPT pathogenesis. Our findings would facilitate the interpretation of NF1 pathogenic variants in CPT genetic counseling.

Supplementary information

The online version of this article (10.1186/s13023-019-1196-0) contains supplementary material, which is available to authorized users.

The L-type amino acid transporter LAT1 inhibits osteoclastogenesis and maintains bone homeostasis through the mTORC1 pathway

L-type amino acid transporter 1 (LAT1), which is encoded by solute carrier transporter 7a5 (Slc7a5), plays a crucial role in amino acid sensing and signaling in specific cell types, contributing to the pathogenesis of cancer and neurological disorders. Amino acid substrates of LAT1 have a beneficial effect on bone health directly and indirectly, suggesting a potential role for LAT1 in bone homeostasis. Here, we identified LAT1 in osteoclasts as important for bone homeostasis. Slc7a5 expression was substantially reduced in osteoclasts in a mouse model of ovariectomy-induced osteoporosis. The osteoclast-specific deletion of Slc7a5 in mice led to osteoclast activation and bone loss in vivo, and Slc7a5 deficiency increased osteoclastogenesis in vitro. Loss of Slc7a5 impaired activation of the mechanistic target of rapamycin complex 1 (mTORC1) pathway in osteoclasts, whereas genetic activation of mTORC1 corrected the enhanced osteoclastogenesis and bone loss in Slc7a5-deficient mice. Last, Slc7a5 deficiency increased the expression of nuclear factor of activated T cells, cytoplasmic 1 (Nfatc1) and the nuclear accumulation of NFATc1, a master regulator of osteoclast function, possibly through the canonical nuclear factor κB pathway and the Akt-glycogen synthase kinase 3β signaling axis, respectively. These findings suggest that the LAT1-mTORC1 axis plays a pivotal role in bone resorption and bone homeostasis by modulating NFATc1 in osteoclasts, thereby providing a molecular connection between amino acid intake and skeletal integrity.