The MEK5/ERK5 mitogen-activated protein kinase cascade is an effector pathway of bone-sustaining bisphosphonates that regulates osteogenic differentiation and mineralization

Cdc42 deletion yielded enamel defects by disrupting mitochondria and producing reactive oxygen species in dental epithelium

Developmental defects of enamel are common due to genetic and environmental factors before and after birth. Cdc42, a Rho family small GTPase, regulates prenatal tooth development in mice. However, its role in postnatal tooth development, especially enamel formation, remains elusive. Here, we investigated Cdc42 functions in mouse enamel development and tooth repair after birth. Cdc42 showed highly dynamic temporospatial patterns in the developing incisors, with robust expression in ameloblast and odontoblast layers. Strikingly, epithelium-specific Cdc42 deletion resulted in enamel defects in incisors. Ameloblast differentiation was inhibited, and hypomineralization of enamel was observed upon epithelial Cdc42 deletion. Proteomic analysis showed that abnormal mitochondrial components, phosphotransferase activity, and ion channel regulator activity occurred in the Cdc42 mutant dental epithelium. Reactive oxygen species accumulation was detected in the mutant mice, suggesting that abnormal oxidative stress occurred after Cdc42 depletion. Moreover, Cdc42 mutant mice showed delayed tooth repair and generated less calcified enamel. Mitochondrial dysfunction and abnormal oxygen consumption were evidenced by reduced Apool and Timm8a1 expression, increased Atp5j2 levels, and reactive oxygen species overproduction in the mutant repair epithelium. Epithelium-specific Cdc42 deletion attenuated ERK1/2 signaling in the labial cervical loop. Aberrant Sox2 expression in the mutant labial cervical loop after clipping might lead to delayed tooth repair. These findings suggested that mitochondrial dysfunction, up-regulated oxidative stress, and abnormal ion channel activity may be among multiple factors responsible for the observed enamel defects in Cdc42 mutant incisors. Overall, Cdc42 exerts multidimensional and pivotal roles in enamel development and is particularly required for ameloblast differentiation and enamel matrix formation.

mPPTMP195 nanoparticles enhance fracture recovery through HDAC4 nuclear translocation inhibition

Delayed repair of fractures seriously impacts patients' health and significantly increases financial burdens. Consequently, there is a growing clinical demand for effective fracture treatment. While current materials used for fracture repair have partially addressed bone integrity issues, they still possess limitations. These challenges include issues associated with autologous material donor sites, intricate preparation procedures for artificial biomaterials, suboptimal biocompatibility, and extended degradation cycles, all of which are detrimental to bone regeneration. Hence, there is an urgent need to design a novel material with a straightforward preparation method that can substantially enhance bone regeneration. In this context, we developed a novel nanoparticle, mPPTMP195, to enhance the bioavailability of TMP195 for fracture treatment. Our results demonstrate that mPPTMP195 effectively promotes the differentiation of bone marrow mesenchymal stem cells into osteoblasts while inhibiting the differentiation of bone marrow mononuclear macrophages into osteoclasts. Moreover, in a mouse femur fracture model, mPPTMP195 nanoparticles exhibited superior therapeutic effects compared to free TMP195. Ultimately, our study highlights that mPPTMP195 accelerates fracture repair by preventing HDAC4 translocation from the cytoplasm to the nucleus, thereby activating the NRF2/HO-1 signaling pathway. In conclusion, our study not only proposes a new strategy for fracture treatment but also provides an efficient nano-delivery system for the widespread application of TMP195 in various other diseases.

Bone and Extracellular Signal-Related Kinase 5 (ERK5)

Bones are vital for anchoring muscles, tendons, and ligaments, serving as a fundamental element of the human skeletal structure. However, our understanding of bone development mechanisms and the maintenance of bone homeostasis is still limited. Extracellular signal-related kinase 5 (ERK5), a recently identified member of the mitogen-activated protein kinase (MAPK) family, plays a critical role in the pathogenesis and progression of various diseases, especially neoplasms. Recent studies have highlighted ERK5's significant role in both bone development and bone-associated pathologies. This review offers a detailed examination of the latest research on ERK5 in different tissues and diseases, with a particular focus on its implications for bone health. It also examines therapeutic strategies and future research avenues targeting ERK5.

Small interfering RNAs in the management of human osteoporosis

BACKGROUND

Osteoporosis results in reduced bone mass and consequent bone fragility. Small interfering RNAs (siRNAs) can be used for therapeutic purposes, as molecular targets or as useful markers to test new therapies.

SOURCES OF DATA

A systematic search of different databases to May 2023 was performed to define the role of siRNAs in osteoporosis therapy. Fourteen suitable studies were identified.

AREAS OF AGREEMENT

SiRNAs may be useful in studying metabolic processes in osteoporosis and identify possible therapeutic targets for novel drug therapies.

AREAS OF CONTROVERSY

The metabolic processes of osteoporosis are regulated by many genes and cytokines that can be targeted by siRNAs. However, it is not easy to predict whether the in vitro responses of the studied siRNAs and drugs are applicable in vivo.

GROWING POINTS

Metabolic processes can be affected by the effect of gene dysregulation mediated by siRNAs on various growth factors.

AREAS TIMELY FOR DEVELOPING RESEARCH

Despite the predictability of pharmacological response of siRNA in vitro, similar responses cannot be expected in vivo.

Icariin attenuates the calcification of vascular smooth muscle cells through ERα - p38MAPK pathway

Objective

To investigate the relationship between icariin and the osteoblastic differentiation of vascular smooth muscle cells (VSMCs) and the signal pathway involved.

Methods

We applied a universally accepted calcification model of VSMCs induced by β glycerophosphate. Then the VSMCs calcification was observed by treatment with icariin and/or inhibitors of estrogen receptors (ERs) and p38-mitogen-activated protein kinase (MAPK) signaling.

Results

Icariin inhibited osteoblastic differentiation and mineralization of VSMCs due to decreased ALP activity and Runx2 expression. Further study demonstrated that icariin exerted this suppression effect through activating p38-MAPK but not extracellular-regulated kinase, JNK or Akt. An inhibitor of p38-MAPK partially reversed the inhibitory effects of icariin on osteoblastic differentiation. Interestingly, treatment of VSMCs with an ER antagonist ICI182780 and a selective ERα receptor antagonist PPT attenuated icariin-mediated inhibition effect of VSMCs calcification, associated with suppression of p38-MAPK phosphorylation.

Conclusions

Icariin inhibited the osteoblastic differentiation of VSMCs, and that the inhibitory effects were mediated by p38-MAPK pathways through ERα.

Fine-tuning osteoclastogenesis: An insight into the cellular and molecular regulation of osteoclastogenesis

Osteoclasts, the bone-resorbing cells, are essential for the bone remodeling process and are involved in the pathophysiology of several bone-related diseases. The extensive corpus of in vitro research and crucial mouse model studies in the 1990s demonstrated the key roles of monocyte/macrophage colony-stimulating factor, receptor activator of nuclear factor kappa B ligand (RANKL) and integrin αvβ3 in osteoclast biology. Our knowledge of the molecular mechanisms by which these variables control osteoclast differentiation and function has significantly advanced in the first decade of this century. Recent developments have revealed a number of novel insights into the fundamental mechanisms governing the differentiation and functional activity of osteoclasts; however, these mechanisms have not yet been adequately documented. Thus, in the present review, we discuss various regulatory factors including local and hormonal factors, innate as well as adaptive immune cells, noncoding RNAs (ncRNAs), etc., in the molecular regulation of the intricate and tightly regulated process of osteoclastogenesis. ncRNAs have a critical role as epigenetic controllers of osteoclast physiologic activities, including differentiation and bone resorption. The primary ncRNAs, which include micro-RNAs, circular RNAs, and long noncoding RNAs, form a complex network that affects gene transcription activities associated with osteoclast biological activity. Greater knowledge of the involvement of ncRNAs in osteoclast biological activities will contribute to the treatment and management of several skeletal diseases such as osteoporosis, osteoarthritis, rheumatoid arthritis, etc. Moreover, we further outline potential therapies targeting these regulatory pathways of osteoclastogenesis in distinct bone pathologies.

Osteogenic differentiation of human adipose-derived mesenchymal stem cells in a bisphosphonate-functionalized polycaprolactone/gelatin scaffold

Recent trends in bone tissue engineering have focused on the development of biomimetic constructs with appropriate mechanical and physiochemical properties. Here, we report the fabrication of an innovative biomaterial scaffold based on a new bisphosphonate-containing synthetic polymer combined with gelatin. To this end, zoledronate (ZA)-functionalized polycaprolactone (PCL-ZA) was synthesized by a chemical grafting reaction. After adding gelatin to the PCL-ZA polymer solution, the porous PCL-ZA/gelatin scaffold was fabricated by the freeze-casting method. A scaffold with aligned pores and a porosity of 82.04 % was obtained. During in vitro biodegradability test, 49 % of its initial weight lost after 5 weeks. The elastic modulus of the PCL-ZA/gelatin scaffold was 31.4 MPa, and its tensile strength was 4.2 MPa. Based on the results of MTT assay, the scaffold had good cytocompatibility with human Adipose-Derived Mesenchymal Stem Cells (hADMSCs). Furthermore, cells grown in PCL-ZA/gelatin scaffold showed the highest mineralization and ALP activity compared to other test groups. Results of the RT-PCR test revealed that RUNX2, COL 1A1, and OCN genes were expressed in PCL-ZA/gelatin scaffold at the highest level, suggesting its good osteoinductive capacity. These results revealed that PCL-ZA/gelatin scaffold could be considered a proper biomimetic platform for bone tissue engineering.

The role of MEK1/2 and MEK5 in melatonin-mediated actions on osteoblastogenesis, osteoclastogenesis, bone microarchitecture, biomechanics, and bone formation

Melatonin, the primary hormone involved in circadian entrainment, plays a significant role in bone physiology. This study aimed to assess the role of MEK1/2 and MEK5 in melatonin-mediated actions in mouse and human mesenchymal stem cells (MSCs) and on bone using small-molecule inhibitors and CRISPR/Cas9 knockout approaches. Consistent with in vitro studies performed in mMSCs and hMSCs, nightly (25 mg/kg, i.p., 45 days) injections with PD184352 (MEK1/2 inhibitor) or Bix02189 (MEK5 inhibitor) or SC-1-151 (MEK1/2/5 inhibitor) demonstrated that MEK1/2 and MEK5 were the primary drivers underlying melatonin's actions on bone density, microarchitecture (i.e., trabecular number, separation, and connectivity density), and bone mechanical properties (i.e., ultimate stress) through increases in osteogenic (RUNX2, BMP-2, FRA-1, OPG) expression and decreases in PPARγ. Furthermore, CRISPR/Cas9 knockout of MEK1 or MEK5 in mMSCs seeded on PLGA scaffolds and placed into critical-size calvarial defects in Balb(c) mice (male and female) revealed that treatment with melatonin (15 mg/L; p.o., nightly, 90 days) mediates sex-specific actions of MEK1 and MEK5 in new bone formation. This study is the first to demonstrate a role for MEK1/2 and MEK5 in modulating melatonin-mediated actions on bone formation in vivo and in a sex-specific manner.

Krüppel-like factors in bone biology

The krüppel-like factor (KLF) family is a group of zinc finger transcription factors and contributes to different cellular processes such as differentiation, proliferation, migration, and apoptosis. While different studies show the roles of this family in skeletal development-specifically in chondrocyte and osteocyte development and bone homeostasis-there are few reviews summarizing their importance. To fill this gap, this review discusses current knowledge on different functions of the KLF family during skeletal development, including their roles in stem cell maintenance and differentiation, cell apoptosis, and cell cycle. To understand the importance of the KLF family, we also review genotype-phenotype correlations in different animal models. We also discuss how KLF proteins function through different signaling pathways and display their paramount importance in skeletal development. To highlight their roles in cartilage- or bone-related cells, we also use single-cell RNA sequencing publicly available data on mouse hindlimb. We also challenge our knowledge of how the KLF family is epigenetically regulated-e.g., using DNA methylation, histone modifications, and noncoding RNAs-during chondrocyte and osteocyte development.

Efficacy of Zoledronic Acid in the Treatment of Nonmalignant Painful Bone Marrow Lesions: A Triple-Blind, Randomized, Placebo-Controlled Phase III Clinical Trial (ZoMARS)

Bone marrow lesions (BML) represent areas of deteriorated bone structure and metabolism characterized by pronounced water-equivalent signaling within the trabecular bone on magnetic resonance imaging (MRI). BML are associated with repair mechanisms subsequent to various clinical conditions associated with inflammatory and non-inflammatory injury to the bone. There is no approved treatment for this condition. Bisphosphonates are known to improve bone stability in osteoporosis and other bone disorders and have been used off-label to treat BML. A randomized, triple-blind, placebo-controlled phase III trial was conducted to assess efficacy and safety of single-dose zoledronic acid (ZOL) 5 mg iv with vitamin D 1000 IU/d as opposed to placebo with vitamin D 1000 IU/d in 48 patients (randomized 2:1) with BML. Primary efficacy endpoint was reduction of edema volume 6 weeks after treatment as assessed by MRI. After treatment, mean BML volume decreased by 64.53% (±41.92%) in patients receiving zoledronic acid and increased by 14.43% (±150.46%) in the placebo group (p = 0.007). A decrease in BML volume was observed in 76.5% of patients receiving ZOL and in 50% of the patients receiving placebo. Pain level (visual analogue scale [VAS]) and all categories of the pain disability index (PDI) improved with ZOL versus placebo after 6 weeks but reconciled after 6 additional weeks of follow-up. Six serious adverse events occurred in 5 patients, none of which were classified as related to the study drug. No cases of osteonecrosis or fractures occurred. Therefore, single-dose zoledronic acid 5 mg iv together with vitamin D may enhance resolution of bone marrow lesions over 6 weeks along with reduction of pain compared with vitamin D supplementation only. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Cdc42 in osterix-expressing cells alters osteoblast behavior and myeloid lineage commitment

Osteoblasts are not only responsible for bone formation. They also support hematopoiesis. This requires responding to cues originating from several signaling pathways, a task performed by Rho GTPases. We therefore examined several transgenic mouse models and used inhibitors of Cdc42 in vitro. Deletion of Cdc42 in vivo using the Osterix promoter suppressed osteoblast function, while its deletion in differentiating osteoblasts using the Collagen-α1(I) promoter decreased osteoblast numbers. In both cases, bone mineral density diminished confirming the importance of Cdc42. Evaluation of hematopoiesis revealed that deletion of Cdc42 using the Osterix, but not the Collagen-α1(I) promoter increased the common myeloid progenitors (CMPs) in the bone marrow as well as the erythrocytes and the thrombocytes/platelets in peripheral blood. Causality between Cdc42 loss in early osteoblasts and increased myelopoiesis was confirmed in vitro. Work in vitro supported the conclusion that interleukin-4 mediated the increase in myelopoiesis. Thus, Cdc42 is required for healthy bone through regulation of bone formation in Osterix-expressing osteoblasts and the number of osteoblasts in differentiating osteoblasts. In addition, its expression in early osteoblasts/stromal cells modulates myelopoiesis. This highlights the importance of osteoblasts in regulating hematopoiesis.

Enzymes in the Cholesterol Synthesis Pathway: Interactomics in the Cancer Context

A global protein interactome ensures the maintenance of regulatory, signaling and structural processes in cells, but at the same time, aberrations in the repertoire of protein-protein interactions usually cause a disease onset. Many metabolic enzymes catalyze multistage transformation of cholesterol precursors in the cholesterol biosynthesis pathway. Cancer-associated deregulation of these enzymes through various molecular mechanisms results in pathological cholesterol accumulation (its precursors) which can be disease risk factors. This work is aimed at systematization and bioinformatic analysis of the available interactomics data on seventeen enzymes in the cholesterol pathway, encoded by HMGCR, MVK, PMVK, MVD, FDPS, FDFT1, SQLE, LSS, DHCR24, CYP51A1, TM7SF2, MSMO1, NSDHL, HSD17B7, EBP, SC5D, DHCR7 genes. The spectrum of 165 unique and 21 common protein partners that physically interact with target enzymes was selected from several interatomic resources. Among them there were 47 modifying proteins from different protein kinases/phosphatases and ubiquitin-protein ligases/deubiquitinases families. A literature search, enrichment and gene co-expression analysis showed that about a quarter of the identified protein partners was associated with cancer hallmarks and over-represented in cancer pathways. Our results allow to update the current fundamental view on protein-protein interactions and regulatory aspects of the cholesterol synthesis enzymes and annotate of their sub-interactomes in term of possible involvement in cancers that will contribute to prioritization of protein targets for future drug development.

The MEK5/ERK5 Pathway in Health and Disease

The MEK5/ERK5 mitogen-activated protein kinases (MAPK) cascade is a unique signaling module activated by both mitogens and stress stimuli, including cytokines, fluid shear stress, high osmolarity, and oxidative stress. Physiologically, it is mainly known as a mechanoreceptive pathway in the endothelium, where it transduces the various vasoprotective effects of laminar blood flow. However, it also maintains integrity in other tissues exposed to mechanical stress, including bone, cartilage, and muscle, where it exerts a key function as a survival and differentiation pathway. Beyond its diverse physiological roles, the MEK5/ERK5 pathway has also been implicated in various diseases, including cancer, where it has recently emerged as a major escape route, sustaining tumor cell survival and proliferation under drug stress. In addition, MEK5/ERK5 dysfunction may foster cardiovascular diseases such as atherosclerosis. Here, we highlight the importance of the MEK5/ERK5 pathway in health and disease, focusing on its role as a protective cascade in mechanical stress-exposed healthy tissues and its function as a therapy resistance pathway in cancers. We discuss the perspective of targeting this cascade for cancer treatment and weigh its chances and potential risks when considering its emerging role as a protective stress response pathway.

ERK5 negatively regulates Kruppel-like factor 4 and promotes osteogenic lineage cell proliferation in response to MEK5 overexpression or fluid shear stress

Aim of the study: Fluid shear stress (FSS) plays a critical role in osteoblast proliferation via extracellular signal-regulated kinase 5 (ERK5). Kruppel-like factor 4 (KLF4) knockout robustly enhances bone formation due to increased osteoblast differentiation and mineralization. However, the effect of KLF4 on osteoblast proliferation is unresolved. Therefore, the aim of our study was to investigate the effect of KLF4 on osteogenic lineage cell proliferation and the relationship between KLF4 and ERK5. Materials and methods: MC3T3-E1 cells were treated with FSS and/or KLF4 siRNA, cell viability was accessed by Edu labeling and CCK-8 assay, and proliferative gene expression were assessed by PCR array. Bone marrow stromal cells (BMSCs) were infected with adenovirus expressing KLF4 and/or constitutively active MEK5, cell viability was evaluated using crystal violet staining, colony formation assay, and cell WST1 assay. The levels of KLF4 and ERK5 phosphorylation were identified through qRT-PCR and western blot, respectively. Results: KLF4 expression was significantly down-regulated by FSS exposure, however, this was reversed by ERK5 siRNA. KLF4 overexpression inhibited colony formation efficiency and cell viability in BMSCs. Adenoviruses expressing constitutively active MEK5 increased ERK5 phosphorylation, which inhibited KLF4 expression, and promoted BMSC proliferation. FSS-induced osteoblast proliferation also involved elevation of Cyclin B2 and Cdc14b as well as repressed expression of P27. Conclusions: KLF4 negatively regulates osteogenic lineage cell proliferation, and ERK5 negatively regulates KLF4 expression and promotes osteogenic lineage cell proliferation.

Nitrogen Containing Bisphosphonates Impair the Release of Bone Homeostasis Mediators and Matrix Production by Human Primary Pre-Osteoblasts

Bisphosphonates (BPs) represent the first-line treatment for a wide array of bone disorders. Despite their well-known action on osteoclasts, the effects they induce on osteoblasts are still unclear. In order to shed light on this aspect we evaluated the impact of two nitrogen containing bisphosphonates, Alendronate (ALN) and Zoledronate (ZOL), on human primary pre-osteoblasts. At first, we showed an inhibitory effect on cell viability and alkaline phosphatase activity starting from µM concentrations of both drugs. In addition, an inhibitory trend on mineralized nodules deposition was observed. Then low doses of both ALN and ZOL rapidly increased the release of the pro-inflammatory mediators TNFα and IL-1β, while increased DKK-1 and Sclerostin, both inhibitors of osteoblastogenesis. Finally, ALN and 10^-7M ZOL decreased the expression of type I Collagen and Osteopontin, while both drugs slightly stimulated SPARC production. With these results, we would like to suggest a direct inhibitory action on bone-forming cells by nitrogen containing bisphosphonates.

Roles of Mitogen-Activated Protein Kinases in Osteoclast Biology

Bone undergoes continuous remodeling, which is homeostatically regulated by concerted communication between bone-forming osteoblasts and bone-degrading osteoclasts. Multinucleated giant osteoclasts are the only specialized cells that degrade or resorb the organic and inorganic bone components. They secrete proteases (e.g., cathepsin K) that degrade the organic collagenous matrix and establish localized acidosis at the bone-resorbing site through proton-pumping to facilitate the dissolution of inorganic mineral. Osteoporosis, the most common bone disease, is caused by excessive bone resorption, highlighting the crucial role of osteoclasts in intact bone remodeling. Signaling mediated by mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38, has been recognized to be critical for normal osteoclast differentiation and activation. Various exogenous (e.g., toll-like receptor agonists) and endogenous (e.g., growth factors and inflammatory cytokines) stimuli contribute to determining whether MAPKs positively or negatively regulate osteoclast adhesion, migration, fusion and survival, and osteoclastic bone resorption. In this review, we delineate the unique roles of MAPKs in osteoclast metabolism and provide an overview of the upstream regulators that activate or inhibit MAPKs and their downstream targets. Furthermore, we discuss the current knowledge about the differential kinetics of ERK, JNK, and p38, and the crosstalk between MAPKs in osteoclast metabolism.