HDAC6 deficiency or inhibition blocks FGFR3 accumulation and improves bone growth in a model of achondroplasia

Clinical-genetic analysis of selected genes involved in the development of the human skeleton in 128 Czech patients with suspected congenital skeletal abnormalities

BACKGROUND

Congenital skeletal abnormalities are a heterogeneous group of diseases most commonly associated with small or disproportionate growth, cranial and facial dysmorphisms, delayed bone maturation, etc. Nonetheless, no detailed genotype-phenotype correlation in patients with specific genetic variants is readily available. Ergo, this study focuses on the analysis of patient phenotypes with candidate variants in genes involved in bone growth as detected by molecular genetic analysis.

METHODS

In this study we used molecular genetic methods to analyse the ACAN, COL2A1, FGFR3, IGFALS, IGF1, IGF1R, GHR, NPR2, STAT5B and SHOX genes in 128 Czech children with suspected congenital skeletal abnormalities. Pathogenic variants and variants of unclear clinical significance were identified and we compared their frequency in this study cohort to the European non-Finnish population. Furthermore, a prediction tool was utilised to determine their possible impact on the final protein. All clinical patient data was obtained during pre-test genetic counselling.

RESULTS

Pathogenic variants were identified in the FGFR3, GHR, COL2A1 and SHOX genes in a total of six patients. Furthermore, we identified 23 variants with unclear clinical significance and high allelic frequency in this cohort of patients with skeletal abnormalities. Five of them have not yet been reported in the scientific literature.

CONCLUSION

Congenital skeletal abnormalities may lead to a number of musculoskeletal, neurological, cardiovascular problems. Knowledge of specific pathogenic variants may help us in therapeutic procedures.

[HDAC6, a very specific deacetylase with a potential therapeutic role]

The cytoplasmic histone deacetylase 6 (HDAC6) is defined today as a new key player in the treatment of many diseases. Overexpression of HDAC6 was observed in a variety of diseases. Over the past ten years, plenty of new selective inhibitors of HDAC6 activity have been synthesized and characterized. Many studies have shown the high efficiency and beneficial effects of HDAC6 inhibitors in many diseases such as cancers, neurodegenerative, inflammatory, or neuromuscular diseases. The mechanisms of HDAC6 action that explain the benefit of its inhibition in various pathologies are still unknown. We have recently shown that HDAC6, via the regulation of the microtubule network, plays a role at the level of neuromuscular junctions by controlling acetylcholine receptor delivery.

Pharmacological inhibition of HDAC6 improves muscle phenotypes in dystrophin-deficient mice by downregulating TGF-β via Smad3 acetylation

The absence of dystrophin in Duchenne muscular dystrophy disrupts the dystrophin-associated glycoprotein complex resulting in skeletal muscle fiber fragility and atrophy, associated with fibrosis as well as microtubule and neuromuscular junction disorganization. The specific, non-conventional cytoplasmic histone deacetylase 6 (HDAC6) was recently shown to regulate acetylcholine receptor distribution and muscle atrophy. Here, we report that administration of the HDAC6 selective inhibitor tubastatin A to the Duchenne muscular dystrophy, mdx mouse model increases muscle strength, improves microtubule, neuromuscular junction, and dystrophin-associated glycoprotein complex organization, and reduces muscle atrophy and fibrosis. Interestingly, we found that the beneficial effects of HDAC6 inhibition involve the downregulation of transforming growth factor beta signaling. By increasing Smad3 acetylation in the cytoplasm, HDAC6 inhibition reduces Smad2/3 phosphorylation, nuclear translocation, and transcriptional activity. These findings provide in vivo evidence that Smad3 is a new target of HDAC6 and implicate HDAC6 as a potential therapeutic target in Duchenne muscular dystrophy.

HDAC6 regulates NF-κB signalling to control chondrocyte IL-1-induced MMP and inflammatory gene expression

Elevated pro-inflammatory signalling coupled with catabolic metalloproteinase expression is a common feature of arthritis, leading to cartilage damage, deterioration of the joint architecture and the associated pain and immobility. Countering these processes, histone deacetylase inhibitors (HDACi) have been shown to suppress matrix metalloproteinase (MMP) expression, block cytokine-induced signalling and reduce the cartilage degradation in animal models of the arthritis. In order to establish which specific HDACs account for these chondro-protective effects an HDAC1-11 RNAi screen was performed. HDAC6 was required for both the interleukin (IL)-1 induction of MMP expression and pro-inflammatory interleukin expression in chondrocytes, implicating an effect on NF-κB signalling. Depletion of HDAC6 post-transcriptionally up-regulated inhibitor of κB (IκB), prevented the nuclear translocation of NF-κB subunits and down-regulated NF-κB reporter activation. The pharmacological inhibition of HDAC6 reduced MMP expression in chondrocytes and cartilage collagen release. This work highlights the important role of HDAC6 in pro-inflammatory signalling and metalloproteinase gene expression, and identifies a part for HDAC6 in the NF-κB signalling pathway. By confirming the protection of cartilage this work supports the inhibition of HDAC6 as a possible therapeutic strategy in arthritis.

Exosomes derived from cyclic mechanical stretch-exposed bone marrow mesenchymal stem cells inhibit RANKL-induced osteoclastogenesis through the NF-κB signaling pathway

Background

Skeletal unloading usually induces severe disuse osteoporosis (DOP), which often occurs in patients subjected to prolonged immobility or in spaceflight astronauts. Increasing evidence suggests that exosomes are important mediators in maintaining the balance between bone formation and resorption. We hypothesized that exosomes play an important role in the maintenance of bone homeostasis through intercellular communication between bone marrow mesenchymal stem cells (BMSCs) and osteoclasts under mechanical loading.

Methods

Cells were divided into cyclic mechanical stretch (CMS)-treated BMSCs and normal static-cultured BMSCs, and exosomes were extracted by ultracentrifugation. After incubation with CMS-treated BMSC-derived exosomes (CMS_Exos) or static-cultured BMSC-derived exosomes (static_Exos), the apoptosis rates of bone marrow macrophages (BMMs) were determined by flow cytometry, and cell viability was detected with a Cell Counting Kit-8 (CCK-8) assay. Osteoclast differentiation was determined with an in vitro osteoclastogenesis assay. Signaling pathway activation was evaluated by western blotting and immunofluorescence staining. Hindlimb unloading (HU)-induced DOP mouse models were prepared to evaluate the function of exosomes in DOP.

Results

Both CMS_Exos and static_Exos could be internalized by BMMs, and CMS_Exos did not affect BMM viability or increase apoptosis. The CMS_Exos effectively suppressed receptor activator of nuclear factor kappa-B ligand (RANKL)-mediated osteoclastogenesis and F-actin ring formation. Further molecular investigation demonstrated that CMS_Exos impaired osteoclast differentiation via inhibition of the RANKL-induced nuclear factor kappa-B (NF-κB) signaling pathway. Both CMS_Exos and static_Exos partly rescued the osteoporosis caused by mechanical unloading; however, the CMS_Exo group showed more obvious rescue. Treatment with CMS_Exos significantly decreased the number of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts. Exosomes derived from CMS-treated BMSCs strongly inhibited osteoclast differentiation by attenuating the NF-κB signaling pathway in vitro and rescued osteoporosis caused by mechanical unloading in an HU mouse model in vivo.

Conclusions

In this research, we demonstrated that Exosomes derived from CMS-treated BMSCs inhibited osteoclastogenesis by attenuating NF-κB signaling pathway activity in vitro and ameliorated bone loss caused by mechanical unloading in an HU mouse model, providing new insights into intercellular communication between osteoblasts and osteoclasts under mechanical loading.

HDAC6 regulates microtubule stability and clustering of AChRs at neuromuscular junctions

Microtubules (MTs) are known to be post-translationally modified at the neuromuscular junction (NMJ), hence increasing their stability. To date however, the function(s) of the dynamic MT network and its relative stability in the formation and maintenance of NMJs remain poorly described. Stabilization of the MT is dependent in part on its acetylation status, and HDAC6 is capable of reversing this post-translational modification. Here, we report that HDAC6 preferentially accumulates at NMJs and that it contributes to the organization and the stability of NMJs. Indeed, pharmacological inhibition of HDAC6 protects against MT disorganization and reduces the size of acetylcholine receptor (AChR) clusters. Moreover, the endogenous HDAC6 inhibitor paxillin interacts with HDAC6 in skeletal muscle cells, colocalizes with AChR aggregates, and regulates the formation of AChR. Our findings indicate that the focal insertion of AChRs into the postsynaptic membrane is regulated by stable MTs and highlight how an MT/HDAC6/paxillin axis participates in the regulation of AChR insertion and removal to control the structure of NMJs.

ACY‑1215, a HDAC6 inhibitor, decreases the dexamethasone‑induced suppression of osteogenesis in MC3T3‑E1 cells

Glucocorticoid-induced osteoporosis is the commonest form of drug-induced osteoporosis. Histone deacetylase 6 (HDAC6) is involved in the differentiation from mesenchymal stem cells to osteoblasts. However, the role of ricolinostat (ACY-1215, HDAC6 inhibitor) in the dexamethasone (Dex)-induced proliferation and differentiation of preosteoblasts remains to be elucidated. The protein expression and mRNA expression levels of HDAC6, osteopontin (OPN), runt-related transcription factor 2 (Runx2), osterix (Osx), collagen I (COL1A1) and glucocorticoid receptor (GR) in MC3T3-E1 cells were analyzed by western blot analysis and reverse transcription-quantitative PCR analysis. The cell viability was detected by CCK-8 assay. The alkaline phosphatase (ALP) activity and capacity of mineralization was determined by ALP assay kit and alizarin red staining. HDAC6 expression was increased in patient serum and Dex-induced MC3T3-E1 cells at a certain concentration range; 1 µM Dex was selected for further experimentation. Cell viability was decreased after Dex induction and restored following ACY-1215 treatment. The ALP activity and capability for mineralization was decreased when MC3T3-E1 cells were induced by 1 µM Dex and was gradually improved by the treatment of ACY-1215 at 1, 5 and 10 mM. The expression of OPN, Runx2, Osx and COL1A1 was similar, with the changes of capability for mineralization. Furthermore, GR expression was increased in Dex-induced MC3T3-E1 cells. ACY-1215 promoted the GR expression in MC3T3-E1 cells from 1–5 mM while GR receptor expression was increased with 10 mM ACY-1215 treatment. In conclusion, ACY-1215 reversed the Dex-induced suppression of proliferation and differentiation of MC3T3-E1 cells.

Achondroplasia: a comprehensive clinical review

Achondroplasia is the most common of the skeletal dysplasias that result in marked short stature (dwarfism). Although its clinical and radiologic phenotype has been described for more than 50 years, there is still a great deal to be learned about the medical issues that arise secondary to this diagnosis, the manner in which these are best diagnosed and addressed, and whether preventive strategies can ameliorate the problems that can compromise the health and well being of affected individuals. This review provides both an updated discussion of the care needs of those with achondroplasia and an exploration of the limits of evidence that is available regarding care recommendations, controversies that are currently present, and the many areas of ignorance that remain.

HDAC6 regulates dental mesenchymal stem cells and osteoclast differentiation

Background

Dental and periodontal tissue development is a complicated process involving a finely regulated network of communication among various cell types. Understanding the mechanisms involved in regulating dental mesenchymal stem cells (MSCs) and osteoclast cell differentiation is critical. However, it is still unclear whether histone deacetylase HDAC6 is involved in dental MSCs fate determination and osteoclast differentiation.

Methods

We used shRNA and siRNA knockdown to explore the role of HDAC6 in dental MSCs odontogenic differentiation and osteoclasts maturation.

Results

Based on HDAC6 knockdown dental MSCs, our data suggest that HDAC6 knockdown significantly increases alkaline phosphate activity and mineralized nodules formation. Additionally, mRNA expression of odontogenic marker genes (OSX, OCN, and OPN) was induced by HDAC6 knockdown. By using HDAC6 siRNA, we knocked down HDAC6 in osteoclast precursor RAW 264.7 cells. Our data suggests that HDAC6 knockdown significantly inhibited osteoclasts differentiation. Additionally, mRNA expression of osteoclast marker genes Trap, Mmp9, and Ctsk was decreased by HDAC6 knockdown.

Conclusions

Our study demonstrated that HDAC6 plays an important role in regulating dental MSCs and osteoclasts differentiation.

Suppression of FGFR3- and MYC-dependent oncogenesis by tubacin: association with HDAC6-dependent and independent activities

Fibroblast growth factor receptor 3 (FGFR3) is amplified, translocated or mutated in a number of different human cancer types, but most commonly in bladder cancers. We previously found that the accumulation of FGFR3 is dependent on histone deacetylase 6 (HDAC6). Here we show that HDAC6 loss or inhibition reduces FGFR3 accumulation in cells made tumorigenic by ectopic expression of a mutant activated version of FGFR3 together with the MYC oncoprotein and in a bladder cancer cell line whose tumorigenicity is dependent on expression of a translocated version of FGFR3. In tumor xenoplant assays, HDAC6 deficiency or small molecule inhibition by the selective HDAC6 inhibitors tubacin or tubastatin A was found to significantly impede tumor growth. However, tubacin was more effective at inhibiting tumor growth than tubastatin A or HDAC6 deficiency. The superior anti-tumor activity of tubacin was linked to its ability to not only inhibit accumulation of mutant FGFR3, but also to cause robust downregulation of MYC and cyclin D1, and to induce a DNA damage response and apoptosis. Neither HDAC6 deficiency nor treatment with tubastatin A altered MYC or cyclin D1 levels, and neither induced a DNA damage response or apoptosis. Thus while tubacin and tubastatin A inhibit HDAC6 with similar selectivity and potency, our results reveal unique HDAC6-independent activities of tubacin that likely contribute to its potent anti-tumor activity.

Molecular therapeutic strategies for FGFR3 gene-related skeletal dysplasia

The FGFR3 gene encodes fibroblast growth factor receptor 3 protein, a negative regulator of chondrogenesis. Gain-of-function mutations result in constitutively activated FGFR3, leading to aberrant signal transduction, and accounting for inhibition of chondrocyte proliferation and differentiation. Generally, these pathogenic mutations maintain FGFR3 in an active state and cause diverse phenotypes in patients with skeletal dysplasia. For decades, studies have revealed the molecular mechanisms of constitutively activated FGFR3 and relevant therapeutic strategies. By modulating the FGFR3-induced signalling pathway with methods such as blocking binding between ligands and receptors, blocking tyrosine kinase activities, or antagonising the FGFR3 downstream signalling pathway, these strategies offer the possibility to ameliorate FGFR3 gene-related skeletal dysplasia phenotypes. In this review, we describe the mechanisms of potential therapeutic targets and underlying regulators and then systematically review molecular therapeutic strategies for FGFR3 gene-related skeletal dysplasia based on current knowledge.