Rho-GEF trio regulates osteoclast differentiation and function by Rac1/Cdc42

MAGI1 attenuates osteoarthritis by regulating osteoclast fusion in subchondral bone through the RhoA-ROCK1 signaling pathway

Background

Osteoarthritis (OA) is a chronic joint disorder that predominantly affects middle-aged or elderly individuals. Subchondral bone remodeling due to osteoclast hyperactivation is regarded as a major feature of early OA. During osteoclast fusion and multinucleation, the cytoskeleton reorganization leads to the formation of actin belts and ultimately bone resorption. Membrane-associated guanylate kinase with an inverted repeat member 1 (MAGI1) is a scaffolding protein that is crucial for linking the extracellular environment to intracellular signaling pathways and cytoskeleton. However, the role of MAGI1 in subchondral bone osteoclast fusion remains unclear.

Methods

In this study, we collected knee joint samples from OA patients and established the OA mouse model to examine the expression of MAGI1. Furthermore, we established the OA rat model and locally injected rAAV9-mediated shMagi1 into the subchondral bone to knock down MAGI1 expression. Micro-CT, histological staining, and immunofluorescence were employed to assess the effects of MAGI1 knockdown on subchondral bone homeostasis and OA process. We isolated and cultured osteoclasts from femoral and tibial bone marrow. Receptor activator of nuclear factor-κB ligand (RANKL)-stimulated osteoclasts served as an in vitro model for OA and underwent RNA sequencing. We employed gain- and loss-of-function experiments using MAGI1-overexpression plasmids and small interfering RNA to explore the role of MAGI1 in osteoclast differentiation. Further molecular experiments, including RT-qPCR, western blotting, immunofluorescence staining, and LC-MS/MS were performed to investigate underlying mechanisms.

Results

MAGI1 expression was significantly downregulated during RANKL-induced osteoclastogenesis in vitro. Additionally, a progressive decrease in MAGI1 expression was consistently observed in both knee joint samples from OA patients and mouse OA models, correlating with OA progression. Knockdown of MAGI1 in subchondral bone increased osteoclast numbers and worsened subchondral bone microarchitecture and cartilage degeneration; MAGI1 knockdown rats exhibited elevated PDGF-BB, Netrin-1, and CGRP+ sensory innervation. Overexpression and knockdown of MAGI1 suppressed and promoted osteoclast differentiation, respectively. Mechanistically, MAGI1 overexpression decreased the levels of RhoA, ROCK1, and p-p65 in RANKL-treated osteoclasts, which was rescued by the addition of RhoA activator narciclasine.

Conclusion

Our results demonstrate that MAGI1 suppresses osteoclast fusion through the RhoA/ROCK1 signaling pathway, targeting MAGI1 in subchondral bone osteoclasts may be a promising therapeutic strategy mitigate the advancement of OA.

The translational potential of this article

This study reveals that the scaffold protein MAGI1 participates in osteoarthritis progression by regulating osteoclast fusion, providing novel theoretical foundations and potential therapeutic targets for osteoarthritis treatment.

Genetic Basis and Identification of Candidate Genes for Alkalinity Tolerance Trait in Spotted Sea Bass (Lateolabrax maculatus) by Genome-Wide Association Study (GWAS)

Given the challenges of overcrowded coastal aquaculture spaces and insufficient production, utilizing saline-alkaline water areas represents a vital strategy to alleviate these bottlenecks. Spotted sea bass (Lateolabrax maculatus), with its formidable osmoregulatory capabilities, is an ideal candidate to develop a saline-alkaline tolerant strain. In our study, genotypic and phenotypic data from 287 L. maculatus individuals exposed to carbonate alkaline conditions were collected, and a genome-wide association study (GWAS) conducted to elucidate genetic basis related to carbonate alkalinity tolerance trait. Results showed that 14 SNPs and 8 InDels were markedly related to carbonate alkalinity tolerance trait, and 404 candidate genes were pinpointed within a ± 300-kb region surrounding these variants. Notably, the most significant SNP (SNP_05_17240108), along with two adjacent SNPs (SNP_05_17240102 and SNP_05_17240340) and two InDels (InDel_05_17240228 and InDel_05_17240231), was situated in the intron region of trio gene that could play vital roles in cell remodeling, and cell junction and activity of aquaporins to deal with carbonate alkalinity stress. Furthermore, candidate genes were significantly involved in pathways associated with carbohydrate metabolism, cell remodeling, ion transport, and RNA degradation, which were consistent with RNA-Seq analysis results of gills and kidneys in response to alkalinity stress. Our study will contribute to elucidate the genetic basis of alkalinity tolerance and the identified SNPs and InDels could be used for marker-assisted selection (MAS) and genomic selection (GS) for alkalinity tolerance trait in the breeding programs of spotted sea bass.

Abr, a Rho-regulating protein, modulates osteoclastogenesis by enhancing lamellipodia formation by interacting with poly(ADP-ribose) glycohydrolase

BACKGROUND

Osteoclasts are multinucleated bone-resorbing cells formed by the fusion of monocyte/macrophage lineage. During osteoclast differentiation, Rho GTPases are involved in various processes, including cell migration, adhesion, and polarity. However, the role of Rho-regulatory molecules in the regulation of osteoclast differentiation remains unclear. In this study, among these genes, we focused on active breakpoint cluster region-related (Abr) protein that is a multifunctional regulator of Rho GTPases.

METHODS AND RESULTS

We examined using knockdown and overexpression experiments in RANKL-stimulated RAW-D macrophages whether Abr regulates osteoclast differentiation and cell morphology. We observed an increase in Abr expression during osteoclast differentiation and identified expression of a variant of the Abr gene in osteoclasts. Knockdown of Abr suppressed osteoclast differentiation and resorption. Abr knockdown markedly inhibited the expression of osteoclast markers, such as Nfatc1, c-fos, Src, and Ctsk in osteoclasts. Conversely, overexpression of Abr enhanced the formation of multinucleated osteoclasts, bone resorption activity, and osteoclast marker gene expression. Moreover, Abr overexpression accelerated lamellipodia formation and induced the formation of well-developed actin in osteoclasts. Importantly, the Abr protein interacted with poly(ADP-ribose) glycohydrolase (PARG) and Rho GTPases, including RhoA, Rac1/2/3, and Cdc42 in osteoclasts.

CONCLUSIONS

Taken together, these results indicate that Abr modulates osteoclastogenesis by enhancing lamellipodia formation via its interaction with PARG.

RhoA promotes osteoclastogenesis and regulates bone remodeling through mTOR-NFATc1 signaling

BACKGROUND

The cytoskeletal architecture of osteoclasts (OCs) and bone resorption activity must be appropriately controlled for proper bone remodeling, which is associated with osteoporosis. The RhoA protein of GTPase plays a regulatory role in cytoskeletal components and contributes to osteoclast adhesion, podosome positioning, and differentiation. Although osteoclast investigations have traditionally been performed by in vitro analysis, however, the results have been inconsistent, and the significance of RhoA in bone physiology and pathology is still unknown.

METHODS

We generated RhoA knockout mice by specifically deleting RhoA in the osteoclast lineage to understand more about RhoA's involvement in bone remodeling. The function of RhoA in osteoclast differentiation and bone resorption and the mechanisms were assessed using bone marrow macrophages (BMMs) in vitro. The ovariectomized (OVX) mouse model was adopted to examine the pathological effect of RhoA in bone loss.

RESULTS

Conditional deletion of RhoA in the osteoclast lineage causes a severe osteopetrosis phenotype, which is attributable to a bone resorption suppression. Further mechanistic studies suggest that RhoA deficiency suppresses Akt-mTOR-NFATc1 signaling during osteoclast differentiation. Additionally, RhoA activation is consistently related to the significant enhancement the osteoclast activity, which culminates in the development of an osteoporotic bone phenotype. Furthermore, in mice, the absence of RhoA in osteoclast precursors prevented occurring OVX-induced bone loss.

CONCLUSION

RhoA promoted osteoclast development via the Akt-mTOR-NFATc1 signaling pathway, resulting a osteoporosis phenotype, and that manipulating RhoA activity might be a therapeutic strategy for osteoporotic bone loss.

Effect of cadmium on Rho GTPases signal transduction during osteoclast differentiation

Osteoclasts are the key target cells for cadmium (Cd)-induced bone metabolism diseases, while Rho GTPases play an important role in osteoclast differentiation and bone resorption. To identify new therapeutic targets of Cd-induced bone diseases; we evaluated signal transduction through Rho GTPases during osteoclast differentiation under the influence of Cd. In osteoclastic precursor cells, 10 nM Cd induced pseudopodia stretching, promoted cell migration, upregulated the levels of Cdc42, and RhoQ mRNAs and downstream Rho-associated coiled-coil kinase 1 (ROCK1) and ROCK2 proteins, and downregulated the actin-related protein 2/3 (ARP2/3) levels. Cd at 2 and 5 μM shortened the pseudopodia, inhibited cell migration, and decreased ROCK1, ROCK2, and ARP2/3 protein levels; Cd at 5 μM also reduced the mRNA expression levels of Rac1, Rac2, and RhoU mRNAs and decreased the level of phosphorylated (p)-cofilin. In osteoclasts, 10 nM Cd induced the formation of sealing zones, slightly upregulated Cdc42 mRNA levels and ROCK2 and ARP2/3 protein levels and significantly reduced p-cofilin levels. Cd at 2 μM and 5 μM Cd blocked the fusion of precursor cells; and 5 μM Cd downregulated the expression levels of RhoB, Rac1, Rac3, and RhoU mRNAs, and ROCK1, p-cofilin and ARP2/3 protein levels, significantly. In vivo, Cd (at 5 or 25 mg/L) increased the levels of key proteins RhoA, Rac1/2/3, Cdc42, and RhoU and their mRNAs in bone marrow cells. In summary, the results suggested that Cd affected the differentiation process of osteoclast and altered the expression of several Rho GTPases, which might be crucial targets of Cd during the differentiation of osteoclasts.

Parallel kinase pathways stimulate actin polymerization at depolarized mitochondria

Mitochondrial damage (MtD) represents a dramatic change in cellular homeostasis, necessitating metabolic changes and stimulating mitophagy. One rapid response to MtD is a rapid peri-mitochondrial actin polymerization termed ADA (acute damage-induced actin). The activation mechanism for ADA is unknown. Here, we use mitochondrial depolarization or the complex I inhibitor metformin to induce ADA. We show that two parallel signaling pathways are required for ADA. In one pathway, increased cytosolic calcium in turn activates PKC-β, Rac, WAVE regulatory complex, and Arp2/3 complex. In the other pathway, a drop in cellular ATP in turn activates AMPK (through LKB1), Cdc42, and FMNL formins. We also identify putative guanine nucleotide exchange factors for Rac and Cdc42, Trio and Fgd1, respectively, whose phosphorylation states increase upon mitochondrial depolarization and whose suppression inhibits ADA. The depolarization-induced calcium increase is dependent on the mitochondrial sodium-calcium exchanger NCLX, suggesting initial mitochondrial calcium efflux. We also show that ADA inhibition results in enhanced mitochondrial shape changes upon mitochondrial depolarization, suggesting that ADA inhibits these shape changes. These depolarization-induced shape changes are not fragmentation but a circularization of the inner mitochondrial membrane, which is dependent on the inner mitochondrial membrane protease Oma1. ADA inhibition increases the proteolytic processing of an Oma1 substrate, the dynamin GTPase Opa1. These results show that ADA requires the combined action of the Arp2/3 complex and formin proteins to polymerize a network of actin filaments around mitochondria and that the ADA network inhibits the rapid mitochondrial shape changes that occur upon mitochondrial depolarization.

Rho-GEF Trio regulates osteosarcoma progression and osteogenic differentiation through Rac1 and RhoA

Osteosarcoma (OS) is the most common primary bone tumor. Its high mortality rate and metastasis rate seriously threaten human health. Currently, the treatment has reached a plateau, hence we urgently need to explore new therapeutic directions. In this paper, we found that Trio was highly expressed in osteosarcoma than normal tissues and promoted the proliferation, migration, and invasion of osteosarcoma cells. Furthermore, Trio inhibited osteosarcoma cells' osteogenic differentiation in vitro and accelerated the growth of osteosarcoma in vivo. Given Trio contains two GEF domains, which have been reported as the regulators of RhoGTPases, we further discovered that Trio could regulate osteosarcoma progression and osteogenic differentiation through activating RhoGTPases. In summary, all our preliminary results showed that Trio could be a potential target and prognostic marker of osteosarcoma.

ELMO1 Regulates RANKL-Stimulated Differentiation and Bone Resorption of Osteoclasts

Bone homeostasis is a metabolic balance between the new bone formation by osteoblasts and old bone resorption by osteoclasts. Excessive osteoclastic bone resorption results in low bone mass, which is the major cause of bone diseases such as rheumatoid arthritis. Small GTPases Rac1 is a key regulator of osteoclast differentiation, but its exact mechanism is not fully understood. ELMO and DOCK proteins form complexes that function as guanine nucleotide exchange factors for Rac activation. Here, we report that ELMO1 plays an important role in differentiation and bone resorption of osteoclasts. Osteoclast precursors derived from bone marrow monocytes (BMMs) of Elmo1^–/– mice display defective adhesion and migration during differentiation. The cells also have a reduced activation of Rac1, p38, JNK, and AKT in response to RANKL stimulation. Importantly, we show that bone erosion is alleviated in Elmo1^–/– mice in a rheumatoid arthritis mouse model. Taken together, our results suggest that ELMO1, as a regulator of Rac1, regulates osteoclast differentiation and bone resorption both in vitro and in vivo.