Rab44, a novel large Rab GTPase, negatively regulates osteoclast differentiation by modulating intracellular calcium levels followed by NFATc1 activation

Rab GTPases, Active Members in Antigen-Presenting Cells, and T Lymphocytes

Processes such as cell migration, phagocytosis, endocytosis, and exocytosis refer to the intense exchange of information between the internal and external environment in the cells, known as vesicular trafficking. In eukaryotic cells, these essential cellular crosstalks are controlled by Rab GTPases proteins through diverse adaptor proteins like SNAREs complex, coat proteins, phospholipids, kinases, phosphatases, molecular motors, actin, or tubulin cytoskeleton, among others, all necessary for appropriate mobilization of vesicles and distribution of molecules. Considering these molecular events, Rab GTPases are critical components in specific biological processes of immune cells, and many reports refer primarily to macrophages; therefore, in this review, we address specific functions in immune cells, concretely in the mechanism by which the GTPase contributes in dendritic cells (DCs) and, T/B lymphocytes.

The Role of Rab GTPases in the development of genetic and malignant diseases

Small GTPases have been shown to play an important role in several cellular functions, including cytoskeletal remodeling, cell polarity, intracellular trafficking, cell-cycle, progression and lipid transformation. The Ras-associated binding (Rab) family of GTPases constitutes the largest family of GTPases and consists of almost 70 known members of small GTPases in humans, which are known to play an important role in the regulation of intracellular membrane trafficking, membrane identity, vesicle budding, uncoating, motility and fusion of membranes. Mutations in Rab genes can cause a wide range of inherited genetic diseases, ranging from neurodegenerative diseases, such as Parkinson's disease (PD) and Alzheimer's disease (AD) to immune dysregulation/deficiency syndromes, like Griscelli Syndrome Type II (GS-II) and hemophagocytic lymphohistiocytosis (HLH), as well as a variety of cancers. Here, we provide an extended overview of human Rabs, discussing their function and diseases related to Rabs and Rab effectors, as well as focusing on effects of (aberrant) Rab expression. We aim to underline their importance in health and the development of genetic and malignant diseases by assessing their role in cellular structure, regulation, function and biology and discuss the possible use of stem cell gene therapy, as well as targeting of Rabs in order to treat malignancies, but also to monitor recurrence of cancer and metastasis through the use of Rabs as biomarkers. Future research should shed further light on the roles of Rabs in the development of multifactorial diseases, such as diabetes and assess Rabs as a possible treatment target.

Characterization of Rab32- and Rab38-positive lysosome-related organelles in osteoclasts and macrophages

Both the biogenesis and functions of osteoclasts and macrophages involves dynamic membrane traffic. We screened transcript levels for Rab family small GTPases related to osteoclasts and identified Rab38. Rab38 expression is upregulated during osteoclast differentiation and maturation. In osteoclasts, both Rab38 and its paralog, Rab32, colocalize to lysosome-related organelles (LROs). In macrophages, Rab32 is also found in LROs. LROs are part of the endocytic pathway but are distinct from lysosomes. After receptor activator of NF-κB ligand stimulation, LROs contain cathepsin K and tartrate-resistant acid phosphatase inside and help both proteins to accumulate around bone resorption pits. After osteoclast maturation, these enzymes are hardly found within LROs. In macrophages derived from Rab32 and Rab38 double knockout mice, both acidification and V-ATPase a3 localization were severely compromised. Both the double knockout macrophage and bafilomycin-treated wildtype macrophage show an increase in Lamp1-positive organelles, implying that biogenesis of lysosomes and LROs are related. These results indicate that Rab32 and Rab38 both play a crucial role in LRO biogenesis in macrophages and in osteoclasts.

Rab44 negatively regulates myoblast differentiation by controlling fusogenic protein transport and mTORC1 signaling

Skeletal muscle is composed of multinucleated myotubes formed by the fusion of mononucleated myoblasts. Skeletal muscle differentiation, termed as myogenesis, have been investigated using the mouse skeletal myoblast cell line C2C12. It has been reported that several "small" Rab proteins, major membrane-trafficking regulators, possibly regulate membrane protein transport in C2C12 cells; however, the role of Rab proteins in myogenesis remains unexplored. Rab44, a member of "large" Rab GTPases, has recently been identified as a negative regulator of osteoclast differentiation. In this study, using C2C12 cells, we found that Rab44 expression was upregulated during myoblast differentiation into myotubes. Knockdown of Rab44 enhanced myoblast differentiation and myotube formation. Consistent with these results, Rab44 knockdown in myoblasts increased expression levels of several myogenic marker genes. Rab44 knockdown increased the surface accumulation of myomaker and myomixer, two fusogenic proteins required for multinucleation, implying enhanced cell fusion. Conversely, Rab44 overexpression inhibited myoblast differentiation and tube formation, accompanied by decreased expression of some myogenic markers. Furthermore, Rab44 was found to be predominantly localized in lysosomes, and Rab44 overexpression altered the number and size of lysosomes. Considering the underlying molecular mechanism, Rab44 overexpression impaired the signaling pathway of the mechanistic target of rapamycin complex1 (mTORC1) in C2C12 cells. Namely, phosphorylation levels of mTORC1 and downstream mTORC1 substrates, such as S6 and P70-S6K, were notably lower in Rab44 overexpressing cells than those in control cells. These results indicate that Rab44 negatively regulates myoblast differentiation into myotubes by controlling fusogenic protein transport and mTORC1 signaling.

Rab44 deficiency accelerates recovery from muscle damage by regulating mTORC1 signaling and transport of fusogenic regulators

The skeletal muscle is a tissue that shows remarkable plasticity to adapt to various stimuli. The development and regeneration of skeletal muscles are regulated by numerous molecules. Among these, we focused on Rab44, a large Rab GTPase, that has been recently identified in immune cells and osteoclasts. Recently, bioinformatics data has revealed that Rab44 is upregulated during the myogenic differentiation of myoblasts into myotubes in C2C12 cells. Thus, Rab44 may be involved in myogenesis. Here, we have investigated the effects of Rab44 deficiency on the development and regeneration of skeletal muscle in Rab44 knockout (KO) mice. Although KO mice exhibited body and muscle weights similar to those of wild-type (WT) mice, the histochemical analysis showed that the myofiber cross-sectional area (CSA) of KO mice was significantly smaller than that of WT mice. Importantly, the results of muscle regeneration experiments using cardiotoxin revealed that the CSA of KO mice was significantly larger than that of WT mice, suggesting that Rab44 deficiency promotes muscle regeneration. Consistent with the in vivo results, in vitro experiments indicated that satellite cells derived from KO mice displayed enhanced proliferation and differentiation. Mechanistically, KO satellite cells exhibited an increased mechanistic target of rapamycin complex 1 (mTORC1) signaling compared to WT cells. Additionally, enhanced cell surface transport of myomaker and myomixer, which are essential membrane proteins for myoblast fusion, was observed in KO satellite cells compared to WT cells. Therefore, Rab44 deficiency enhances muscle regeneration by modulating the mTORC1 signaling pathway and transport of fusogenic regulators.

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.

Effect of mitophagy in the formation of osteomorphs derived from osteoclasts

Osteoclasts are specialized multinucleated giant cells with unique bone-destroying capacities. A recent study revealed that osteoclasts undergo an alternative cell fate by dividing into daughter cells called osteomorphs. To date, no studies have focused on the mechanisms of osteoclast fission. In this study, we analyzed the alternative cell fate process in vitro and, herein, reported the high expression of mitophagy-related proteins during osteoclast fission. Mitophagy was further confirmed by the colocalization of mitochondria with lysosomes, as observed in fluorescence images and transmission electron microscopy. We investigated the role played by mitophagy in osteoclast fission via drug stimulation experiments. The results showed that mitophagy promoted osteoclast division, and inhibition of mitophagy induced osteoclast apoptosis. In summary, this study reveals the role played by mitophagy as the decisive link in osteoclasts' fate, providing a new therapeutic target and perspective for the clinical treatment of osteoclast-related diseases.

Rab44 Deficiency Induces Impaired Immune Responses to Nickel Allergy

Rab44 was recently identified as an atypical Rab GTPase that possesses EF-hand and coiled-coil domains at the N-terminus, and a Rab-GTPase domain at the C-terminus. Rab44 is highly expressed in immune-related cells such as mast cells, macrophages, osteoclasts, and granulocyte-lineage cells in the bone marrow. Therefore, it is speculated that Rab44 is involved in the inflammation and differentiation of immune cells. However, little is known about the role of Rab44 in inflammation. In this study, we showed that Rab44 was upregulated during the early phase of differentiation of M1- and M2-type macrophages. Rab44-deficient mice exhibited impaired tumor necrosis factor alpha and interleukin-10 production after lipopolysaccharide (LPS) stimulation. The number of granulocytes in Rab44-deficient mice was lower, but the lymphocyte count in Rab44-deficient mice was significantly higher than that in wild-type mice after LPS stimulation. Moreover, Rab44-deficient macrophages showed impaired nickel-induced toxicity, and Rab44-deficient mice showed impaired nickel-induced hypersensitivity. Upon nickel hypersensitivity induction, Rab44-deficient mice showed different frequencies of immune cells in the blood and ears. Thus, it is likely that Rab44 is implicated in immune cell differentiation and inflammation, and Rab44 deficiency induces impaired immune responses to nickel allergies.

HSP90 drives the Rab11a-mediated vesicular transport of the cell surface receptors in osteoclasts

Rab11a, which ubiquitously localizes to early and recycling endosomes, is required for regulating the vesicular transport of cellular cargos. Interestingly, our previous study revealed that Rab11a served as a negative regulator of osteoclastogenesis by facilitating the lysosomal proteolysis of (1) colony-stimulating factor-1 (c-fms) receptor and (2) receptor activator of nuclear factor-κB (RANK) receptor, thereby resulting in inhibition of osteoclast (OC) differentiation, maturation, and bone-resorbing activity. However, the molecular mechanisms of how Rab11a negatively affected osteoclastogenesis were largely unknown. Heat shock protein (HSP90), including two isoforms HSP90α and HSP90β, necessitates the stability, maturation, and activity of a broad range of its clients, and is essentially required for a vast array of signal transduction pathways in nonstressful conditions. Furthermore, cumulative evidence suggests that HSP90 is a vital element of the vesicular transport network. Indeed, our recent study revealed that HSP90, a novel effector protein of Rab11b, modulated Rab11b-mediated osteoclastogenesis. In this study, we also found that Rab11a interacted with both HSP90α and HSP90β in OCs. Upon blockade of HSP90 ATPase activity by a specific inhibitor(17-allylamino-demethoxygeldanamycin), we showed that (1) the ATPase domain of HSP90 was a prerequisite for the interaction between HSP90 and Rab11a, and (2) the interaction of HSP90 to Rab11a sufficiently maintained the inhibitory effects of Rab11a on osteoclastogenesis. Altogether, our findings undoubtedly indicate a novel role of HSP90 in regulating Rab11a-mediated osteoclastogenesis.

Coronin1C Is a GDP-Specific Rab44 Effector That Controls Osteoclast Formation by Regulating Cell Motility in Macrophages

Osteoclasts are multinucleated bone-resorbing cells that are formed by the fusion of macrophages. Recently, we identified Rab44, a large Rab GTPase, as an upregulated gene during osteoclast differentiation that negatively regulates osteoclast differentiation. However, the molecular mechanisms by which Rab44 negatively regulates osteoclast differentiation remain unknown. Here, we found that the GDP form of Rab44 interacted with the actin-binding protein, Coronin1C, in murine macrophages. Immunoprecipitation experiments revealed that the interaction of Rab44 and Coronin1C occurred in wild-type and a dominant-negative (DN) mutant of Rab44, but not in a constitutively active (CA) mutant of Rab44. Consistent with these findings, the expression of the CA mutant inhibited osteoclast differentiation, whereas that of the DN mutant enhanced this differentiation. Using a phase-contrast microscope, Coronin1C-knockdown osteoclasts apparently impaired multinuclear formation. Moreover, Coronin1C knockdown impaired the migration and chemotaxis of RAW-D macrophages. An in vivo experimental system demonstrated that Coronin1C knockdown suppresses osteoclastogenesis. Therefore, the decreased cell formation and fusion of Coronin1C-depleted osteoclasts might be due to the decreased migration of Coronin1C-knockdown macrophages. These results indicate that Coronin1C is a GDP-specific Rab44 effector that controls osteoclast formation by regulating cell motility in macrophages.

Rab34 plays a critical role as a bidirectional regulator of osteoclastogenesis

Accumulating evidence suggests that Rab GTPases representing the largest branch of Ras superfamily have recently emerged as the core factors for the regulation of osteoclastogenesis through modulating vesicular transport amongst specific subcellular compartments. Among these, Rab34 GTPase has been identified to be important for the post-Golgi secretory pathway and for phagocytosis; nevertheless, its specific role in osteoclastogenesis has been completely obscure. Here, upon the in vitro model of osteoclast formation derived from murine macrophages like RAW-D cells or bone marrow-derived macrophages, we reveal that Rab34 regulates osteoclastogenesis bidirectionally. More specifically, Rab34 serves as a negative regulator of osteoclast differentiation by promoting the lysosome-induced proteolysis of two osteoclastogenic surface receptors, c-fms and RANK, via the axis of early endosomes-late endosomes-lysosomes, leading to alleviate the transcriptional activity of two of the master regulator of osteoclast differentiation, c-fos and NFATc-1, eventually attenuating osteoclast differentiation and bone resorption. Besides, Rab34 plays a crucial role in modulating the secretory network of lysosome-related proteases including matrix metalloprotease 9 and Cathepsin K across the ruffled borders of osteoclasts, contributing to the regulation of bone resorption.

Genetic Influence on Frequencies of Myeloid-Derived Cell Subpopulations in Mouse

Differences in frequencies of blood cell subpopulations were reported to influence the course of infections, atopic and autoimmune diseases, and cancer. We have discovered a unique mouse strain B10.O20 containing extremely high frequency of myeloid-derived cells (MDC) in spleen. B10.O20 carries 3.6% of genes of the strain O20 on the C57BL/10 genetic background. It contains much higher frequency of CD11b⁺Gr1⁺ cells in spleen than both its parents. B10.O20 carries O20-derived segments on chromosomes 1, 15, 17, and 18. Their linkage with frequencies of blood cell subpopulations in spleen was tested in F₂ hybrids between B10.O20 and C57BL/10. We found 3 novel loci controlling MDC frequencies: Mydc1, 2, and 3 on chromosomes 1, 15, and 17, respectively, and a locus controlling relative spleen weight (Rsw1) that co-localizes with Mydc3 and also influences proportion of white and red pulp in spleen. Mydc1 controls numbers of CD11b⁺Gr1⁺ cells. Interaction of Mydc2 and Mydc3 regulates frequency of CD11b⁺Gr1⁺ cells and neutrophils (Gr1⁺Siglec-F^- cells from CD11b⁺ cells). Interestingly, Mydc3/Rsw1 is orthologous with human segment 6q21 that was shown previously to determine counts of white blood cells. Bioinformatics analysis of genomic sequence of the chromosomal segments bearing these loci revealed polymorphisms between O20 and C57BL/10 that change RNA stability and genes’ functions, and we examined expression of relevant genes. This identified potential candidate genes Smap1, Vps52, Tnxb, and Rab44. Definition of genetic control of MDC can help to personalize therapy of diseases influenced by these cells.

IL-13 Controls IL-33 Activity through Modulation of ST2

IL-33 is a multifunctional cytokine that mediates local inflammation upon tissue damage. IL-33 is known to act on multiple cell types including group 2 innate lymphoid cells (ILC2s), Th2 cells, and mast cells to drive production of Th2 cytokines including IL-5 and IL-13. IL-33 signaling activity through transmembrane ST2L can be inhibited by soluble ST2 (sST2), which acts as a decoy receptor. Previous findings suggested that modulation of IL-13 levels in mice lacking decoy IL-13Rα2, or mice lacking IL-13, impacted responsiveness to IL-33. In this study, we used Il13 ^-/- mice to investigate whether IL-13 regulates IL-33 activity by modulating the transmembrane and soluble forms of ST2. In Il13 ^-/- mice, the effects of IL-33 administration were exacerbated relative to wild type (WT). Il13 ^-/- mice administered IL-33 i.p. had heightened splenomegaly, more immune cells in the peritoneum including an expanded ST2L⁺ ILC2 population, increased eosinophilia in the spleen and peritoneum, and reduced sST2 in the circulation and peritoneum. In the spleen, lung, and liver of mice given IL-33, gene expression of both isoforms of ST2 was increased in Il13 ^-/- mice relative to WT. We confirmed fibroblasts to be an IL-13-responsive cell type that can regulate IL-33 activity through production of sST2. This study elucidates the important regulatory activity that IL-13 exerts on IL-33 through induction of IL-33 decoy receptor sST2 and through modulation of ST2L⁺ ILC2s.

Large Rab GTPases: Novel Membrane Trafficking Regulators with a Calcium Sensor and Functional Domains

Rab GTPases are major coordinators of intracellular membrane trafficking, including vesicle transport, membrane fission, tethering, docking, and fusion events. Rab GTPases are roughly divided into two groups: conventional “small” Rab GTPases and atypical “large” Rab GTPases that have been recently reported. Some members of large Rab GTPases in mammals include Rab44, Rab45/RASEF, and Rab46. The genes of these large Rab GTPases commonly encode an amino-terminal EF-hand domain, coiled-coil domain, and the carboxyl-terminal Rab GTPase domain. A common feature of large Rab GTPases is that they express several isoforms in cells. For instance, Rab44’s two isoforms have similar functions, but exhibit differential localization. The long form of Rab45 (Rab45-L) is abundantly distributed in epithelial cells. The short form of Rab45 (Rab45-S) is predominantly present in the testes. Both Rab46 (CRACR2A-L) and the short isoform lacking the Rab domain (CRACR2A-S) are expressed in T cells, whereas Rab46 is only distributed in endothelial cells. Although evidence regarding the function of large Rab GTPases has been accumulating recently, there are only a limited number of studies. Here, we report the recent findings on the large Rab GTPase family concerning their function in membrane trafficking, cell differentiation, related diseases, and knockout mouse phenotypes.

A novel role of HSP90 in regulating osteoclastogenesis by abrogating Rab11b-driven transport

Heat shock protein 90 (HSP90) is a highly conserved molecular chaperone that plays a pivotal role in folding, activating and assembling a variety of client proteins. In addition, HSP90 has recently emerged as a crucial regulator of vesicular transport of cellular proteins. In our previous study, we revealed Rab11b negatively regulated osteoclastogenesis by promoting the lysosomal proteolysis of c-fms and RANK surface receptors via the axis of early endosome-late endosome-lysosomes. In this study, using an in vitro model of osteoclasts differentiated from murine macrophage-like RAW-D cells, we revealed that Rab11b interacted with both HSP90 isoforms, HSP90 alpha (HSP90α) and HSP90 beta (HSP90β), suggesting that Rab11b is an HSP90 client. Using at specific blocker for HSP90 ATPase activity, 17-allylamino-demethoxygeldanamycin (17-AAG), we found that the HSP90 ATPase domain is indispensable for maintaining the interaction between HSP90 and Rab11b in osteoclasts. Nonetheless, its ATPase activity is not required for regulating the turnover of endogenous Rab11b. Interestingly, blocking the interaction between HSP90 and Rab11b by either HSP90-targeting small interfering RNA (siHSP90) or 17-AAG abrogated the inhibitory effects of Rab11b on osteoclastogenesis by suppressing the Rab11b-mediated transport of c-fms and RANK surface receptors to lysosomes via the axis of early endosome-late endosome-lysosomes, alleviating the Rab11b-mediated proteolysis of these surface receptors in osteoclasts. Based on our observations, we propose a HSP90/Rab11b-mediated regulatory mechanism for osteoclastogenesis by directly modulating the c-fms and RANK surface receptors in osteoclasts, thereby contributing to the maintenance of bone homeostasis.

Rab44 isoforms similarly promote lysosomal exocytosis, but exhibit differential localization in mast cells

Rab44 is a large Rab GTPase containing a Rab GTPase domain and some additional N-terminal domains. We recently used Rab44-deficient mice to demonstrate that Rab44 regulates granule exocytosis in mast cells and IgE-mediated anaphylaxis. In mouse mast cells, Rab44 is expressed as two isoforms, namely, the long and short forms; however, the characteristics of these two isoforms remain unknown. Here, we investigated secretion and localization of the human long Rab44 isoform and the two mouse isoforms and their mutants expressed in rat basophilic leukemia (RBL)-2H3 cells. Expression of the human long isoform and both mouse isoforms caused an increase in β-hexosaminidase secretion. Confocal and quantitative analyses showed that both human and mouse long isoforms localized mainly to lysosomes while the mouse short isoform localized mainly to the ER. Live imaging with LysoTracker indicated that the size and number of LysoTracker-positive vesicles were altered by the various mutants. Ionomycin treatment partially altered localization of both long isoforms to the plasma membrane and cytosol, whereas it had little effect on colocalization of the short isoform with lysosomes. Mechanistically, both human and mouse Rab44 proteins interacted with vesicle-associated membrane protein 8 (VAMP8), a v-SNARE protein. Therefore, Rab44 isoforms similarly promote lysosomal exocytosis, but exhibit differential localization in mast cells.

Affinity-based proteomics reveals novel binding partners for Rab46 in endothelial cells

Rab46 is a novel Ca²⁺-sensing Rab GTPase shown to have important functions in endothelial and immune cells. The presence of functional Ca²⁺-binding, coiled-coil and Rab domains suggest that Rab46 will be important for coupling rapid responses to signalling in many cell types. The molecular mechanisms underlying Rab46 function are currently unknown. Here we provide the first resource for studying Rab46 interacting proteins. Using liquid chromatography tandem mass spectrometry (LC–MS/MS) to identify affinity purified proteins that bind to constitutively active GFP-Rab46 or inactive GFP-Rab46 expressed in endothelial cells, we have revealed 922 peptides that interact with either the GTP-bound Rab46 or GDP-bound Rab46. To identify proteins that could be potential Rab46 effectors we performed further comparative analyses between nucleotide-locked Rab46 proteins and identified 29 candidate effector proteins. Importantly, through biochemical and imaging approaches we have validated two potential effector proteins; dynein and the Na²⁺/ K⁺ ATPase subunit alpha 1 (ATP1α1). Hence, our use of affinity purification and LC–MS/MS to identify Rab46 neighbouring proteins provides a valuable resource for detecting Rab46 effector proteins and analysing Rab46 functions.

Rab family of small GTPases: an updated view on their regulation and functions

The Rab family of small GTPases regulates intracellular membrane trafficking by orchestrating the biogenesis, transport, tethering, and fusion of membrane-bound organelles and vesicles. Like other small GTPases, Rabs cycle between two states, an active (GTP-loaded) state and an inactive (GDP-loaded) state, and their cycling is catalyzed by guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Because an active form of each Rab localizes on a specific organelle (or vesicle) and recruits various effector proteins to facilitate each step of membrane trafficking, knowing when and where Rabs are activated and what effectors Rabs recruit is crucial to understand their functions. Since the discovery of Rabs, they have been regarded as one of the central hubs for membrane trafficking, and numerous biochemical and genetic studies have revealed the mechanisms of Rab functions in recent years. The results of these studies have included the identification and characterization of novel GEFs, GAPs, and effectors, as well as post-translational modifications, for example, phosphorylation, of Rabs. Rab functions beyond the simple effector-recruiting model are also emerging. Furthermore, the recently developed CRISPR/Cas technology has enabled acceleration of knockout analyses in both animals and cultured cells and revealed previously unknown physiological roles of many Rabs. In this review article, we provide the most up-to-date and comprehensive lists of GEFs, GAPs, effectors, and knockout phenotypes of mammalian Rabs and discuss recent findings in regard to their regulation and functions.

The Inhibitory Role of Rab11b in Osteoclastogenesis through Triggering Lysosome-Induced Degradation of c-Fms and RANK Surface Receptors

Rab11b, abundantly enriched in endocytic recycling compartments, is required for the establishment of the machinery of vesicle trafficking. Yet, no report has so far characterized the biological function of Rab11b in osteoclastogenesis. Using in vitro model of osteoclasts differentiated from murine macrophages like RAW-D cells or bone marrow-derived macrophages, we elucidated that Rab11b served as an inhibitory regulator of osteoclast differentiation sequentially via (i) abolishing surface abundance of RANK and c-Fms receptors; and (ii) attenuating nuclear factor of activated T-cells c1 (NFATc-1) upstream signaling cascades, following RANKL stimulation. Rab11b was localized in early and late endosomes, Golgi complex, and endoplasmic reticulum; moreover, its overexpression enlarged early and late endosomes. Upon inhibition of lysosomal function by a specific blocker, chloroquine (CLQ), we comprehensively clarified a novel function of lysosomes on mediating proteolytic degradation of c-Fms and RANK surface receptors, drastically ameliorated by Rab11b overexpression in RAW-D cell-derived osteoclasts. These findings highlight the key role of Rab11b as an inhibitor of osteoclastogenesis by directing the transport of c-Fms and RANK surface receptors to lysosomes for degradation via the axis of early endosomes-late endosomes-lysosomes, thereby contributing towards the systemic equilibrium of the bone resorption phase.

Role of the EF-hand and coiled-coil domains of human Rab44 in localisation and organelle formation

Rab44 is a large Rab GTPase that contains an amino-terminal EF-hand domain, a coiled-coil domain, and a carboxyl-terminal Rab GTPase domain. However, the roles of the EF-hand and coiled-coil domains remain unclear. Here, we constructed various deletion and point mutants of human Rab44. When overexpressed in HeLa cells, the wild-type Rab44 (hWT) formed ring-like structures, and partially localised to lysosomes. The dominant negative mutant, hT847N, localised to lysosomes and the cytosol, while the constitutively active mutant, hQ892L, formed ring-like structures, and partially localised to the plasma membrane and nuclei. The hΔEF, hΔcoil, and h826-1021 mutants also formed ring-like structures; however, their localisation patterns differed from hWT. Analysis of live imaging with LysoTracker revealed that the size of LysoTracker-positive vesicles was altered by all other mutations than the hC1019A and hΔEF. Treatment with ionomycin, a Ca²⁺ ionophore, induced the translocation of hWT and hΔcoil into the plasma membrane and cytosol, but had no effect on the localisation of the hΔEF and h826-1021 mutants. Thus, the EF- hand domain is likely required for the partial translocation of Rab44 to the plasma membrane and cytosol following transient Ca²⁺ influx, and the coiled-coil domain appears to be important for localisation and organelle formation.

Implications of Altered Endosome and Lysosome Biology in Space Environments

Space exploration poses multiple challenges for mankind, not only on a technical level but also to the entire physiology of the space traveller. The human system must adapt to several environmental stressors, microgravity being one of them. Lysosomes are ubiquitous to every cell and essential for their homeostasis, playing significant roles in the regulation of autophagy, immunity, and adaptation of the organism to changes in their environment, to name a few. Dysfunction of the lysosomal system leads to age-related diseases, for example bone loss, reduced immune response or cancer. As these conditions have been shown to be accelerated following exposure to microgravity, this review elucidates the lysosomal response to real and simulated microgravity. Microgravity activates the endo-lysosomal system, with resulting impacts on bone loss, muscle atrophy and stem cell differentiation. The investigation of lysosomal adaptation to microgravity can be beneficial in the search for new biomarkers or therapeutic approaches to several disease pathologies on earth as well as the potential to mitigate pathophysiology during spaceflight.

Rab11A Functions as a Negative Regulator of Osteoclastogenesis through Dictating Lysosome-Induced Proteolysis of c-fms and RANK Surface Receptors

Osteoclast differentiation and activity are controlled by two essential cytokines, macrophage colony-stimulating factor (M-CSF) and the receptor activator of nuclear factor-κB ligand (RANKL). Rab11A GTPase, belonging to Rab11 subfamily representing the largest branch of Ras superfamily of small GTPases, has been identified as one of the crucial regulators of cell surface receptor recycling. Nevertheless, the regulatory role of Rab11A in osteoclast differentiation has been completely unknown. In this study, we found that Rab11A was strongly upregulated at a late stage of osteoclast differentiation derived from bone marrow-derived macrophages (BMMs) or RAW-D murine osteoclast precursor cells. Rab11A silencing promoted osteoclast formation and significantly increased the surface levels of c-fms and receptor activator of nuclear factor-κB (RANK) while its overexpression attenuated osteoclast formation and the surface levels of c-fms and RANK. Using immunocytochemical staining for tracking Rab11A vesicular localization, we observed that Rab11A was localized in early and late endosomes, but not lysosomes. Intriguingly, Rab11A overexpression caused the enhancement of fluorescent intensity and size-based enlargement of early endosomes. Besides, Rab11A overexpression promoted lysosomal activity via elevating the endogenous levels of a specific lysosomal protein, LAMP1, and two key lysosomal enzymes, cathepsins B and D in osteoclasts. More importantly, inhibition of the lysosomal activity by chloroquine, we found that the endogenous levels of c-fms and RANK proteins were enhanced in osteoclasts. From these observations, we suggest a novel function of Rab11A as a negative regulator of osteoclastogenesis mainly through (i) abolishing the surface abundance of c-fms and RANK receptors, and (ii) upregulating lysosomal activity, subsequently augmenting the degradation of c-fms and RANK receptors, probably via the axis of early endosomes-late endosomes-lysosomes in osteoclasts.

Rab GTPases in Osteoclastic Bone Resorption and Autophagy

Small guanosine triphosphate hydrolases (GTPases) of the Rab family are involved in plasma membrane delivery, fusion events, and lysosomal and autophagic degradation pathways, thereby regulating signaling pathways and cell differentiation and function. Osteoclasts are bone-resorbing cells that maintain bone homeostasis. Polarized vesicular trafficking pathways result in the formation of the ruffled border, the osteoclast’s resorptive organelle, which also assists in transcytosis. Here, we reviewed the different roles of Rab GTPases in the endomembrane machinery of osteoclasts and in bone diseases caused by the dysfunction of these proteins, with a particular focus on autophagy and bone resorption. Understanding the molecular mechanisms underlying osteoclast-related bone disease development is critical for developing and improving therapies.

Expression and localisation of Rab44 in immune-related cells change during cell differentiation and stimulation

Rab44 is a large Rab GTPase that contains a Rab-GTPase domain and some additional domains, such as EF-hand and coiled-coil domains at the N-terminus. Our previous study showed that Rab44 negatively regulates osteoclast differentiation by modulating intracellular calcium levels; however, aside from those findings, there is little information concerning Rab44 on other cells or tissues. In this study, we showed that Rab44 was highly expressed in bone marrow cells among various mouse tissues. Immunohistochemical studies indicated that Rab44 was detectable by only a small number of cells in the immune-related tissues and that Rab44 was partially detected in CD117-positive cells, but not in Stem cell antigen 1-positive cells in the bone marrow. Rab44 expression levels were decreased during differentiation of immune-related cells, such as neutrophils, macrophages, and dendritic cells compared with bone marrow cells. Although endogenous Rab44 in macrophages was localised in lysosomes, lipopolysaccharide (LPS) stimulation led to partial translocation to early endosomes and the plasma membrane. Moreover, Rab44 expression levels were altered by treatment with various immunomodulators, including LPS. These results indicate that Rab44 expression and localisation in bone marrow cells and macrophages alters with cell differentiation and stimulation.

Rabs in Signaling and Embryonic Development

Rab GTPases play key roles in various cellular processes. They are essential, among other roles, to membrane trafficking and intracellular signaling events. Both trafficking and signaling events are crucial for proper embryonic development. Indeed, embryogenesis is a complex process in which cells respond to various signals and undergo dramatic changes in their shape, position, and function. Over the last few decades, cellular studies have highlighted the novel signaling roles played by Rab GTPases, while numerous studies have shed light on the important requirements of Rab proteins at various steps of embryonic development. In this review, we aimed to generate an overview of Rab contributions during animal embryogenesis. We first briefly summarize the involvement of Rabs in signaling events. We then extensively highlight the contribution of Rabs in shaping metazoan development and conclude with new approaches that will allow investigation of Rab functions in vivo.

Membrane trafficking in osteoclasts and implications for osteoporosis

Osteoclasts are large multinucleated cells exquisitely adapted to resorb bone matrix. Like other eukaryotes, osteoclasts possess an elaborate ensemble of intracellular organelles through which solutes, proteins and other macromolecules are trafficked to their target destinations via membrane-bound intermediaries. During bone resorption, membrane trafficking must be tightly regulated to sustain the structural and functional polarity of the osteoclasts’ membrane domains. Of these, the ruffled border (RB) is most characteristic, functioning as the osteoclasts' secretory apparatus. This highly convoluted organelle is classically considered to be formed by the targeted fusion of acidic vesicles with the bone-facing plasma membrane. Emerging findings disclose new evidence that the RB is far more complex than previously envisaged, possessing discrete subdomains that are serviced by several intersecting endocytic, secretory, transcytotic and autophagic pathways. Bone-resorbing osteoclasts therefore serve as a unique model system for studying polarized membrane trafficking. Recent advances in high-resolution microscopy together with the convergence of genetic and cell biological studies in humans and in mice have helped illuminate the major membrane trafficking pathways in osteoclasts and unmask the core molecular machinery that governs these distinct vesicle transport routes. Among these, small Rab GTPases, their binding partners and members of the endocytic sorting nexin family have emerged as critical regulators. This mini review summarizes our current understanding of membrane trafficking in osteoclasts, the key molecular participants, and discusses how these transport machinery may be exploited for the development of new therapies for metabolic disorders of bone-like osteoporosis.

KBTBD11, a novel BTB-Kelch protein, is a negative regulator of osteoclastogenesis through controlling Cullin3-mediated ubiquitination of NFATc1

Kelch repeat and BTB domain-containing protein 11 (KBTBD11) is a member of the KBTBD subfamily of proteins that possess a BTB domain and Kelch repeats. Despite the presence of the Kbtbd11 gene in mammalian genomes, there are few reports about KBTBD11 at present. In this study, we identified the novel protein KBTBD11 as a negative regulator of osteoclast differentiation. We found that expression of KBTBD11 increased during osteoclastogenesis. Small-interfering-RNA-mediated knockdown of KBTBD11 enhanced osteoclast formation, and markedly increased the expression of several osteoclast marker genes compared with control cells. Conversely, KBTBD11 overexpression impaired osteoclast differentiation, and decreased the expression of osteoclast marker genes. Among six major signaling pathways regulating osteoclast differentiation, KBTBD11 predominantly influenced the nuclear factor of activated T cell cytoplasmic-1 (NFATc1) pathway. Mechanistically, KBTBD11 was found to interact with an E3 ubiquitin ligase, Cullin3. Further experiments involving immunoprecipitation and treatment with MG132, a proteasome inhibitor, showed that the KBTBD11–Cullin3 promotes ubiquitination and degradation of NFATc1 by the proteasome. Considering that NFATc1 is an essential factor for osteoclast differentiation, the KBTBD11 and Cullin3 probably regulate the levels of NFATc1 through the ubiquitin-proteasome degradation system. Thus, KBTBD11 negatively modulates osteoclast differentiation by controlling Cullin3-mediated ubiquitination of NFATc1.

Rab GTPases in Osteoclastic Endomembrane Systems

Osteoclasts (OCs) are bone-resorbing cells that maintain bone homeostasis. OC differentiation, survival, and activity are regulated by numerous small GTPases, including those of the Rab family, which are involved in plasma membrane delivery and lysosomal and autophagic degradation pathways. In resorbing OCs, polarized vesicular trafficking pathways also result in formation of the ruffled membrane, the resorbing organelle, and in transcytosis.

Dihydroartemisinin represses osteoclastogenesis of bone marrow macrophages through reduced NFATc1 expression and impaired phosphorylation of IκBα

Osteoclasts are multinucleated bone resorbing cells whose differentiation is regulated by several important signaling pathways. Several lines of evidence indicate that dihydroartemisinin (DHA), an anti-malarial drug, inhibits osteoclast differentiation with little cytotoxicity. However, the detailed inhibitory mechanisms of DHA on osteoclastogenesis from native cells remain to be elucidated. In this study, we investigated the effects of DHA on the differentiation of bone marrow-derived macrophages into osteoclasts. DHA inhibited receptor activator of nuclear factor κ-B ligand (RANKL)-induced osteoclast formation and its bone resorbing activity. Mechanistically, DHA treatment markedly abolished phosphorylation of IκBα, and slightly affected a p38 MAPK dependent pathway. Moreover, DHA treatment induced down-regulation of nuclear factor of activated T cells cytoplasmic-1 (NFATc1), a master regulator for osteoclast differentiation and its target proteins, such as Src and cathepsin K. These results indicate that DHA represses RANKL-induced osteoclastogenesis of bone marrow macrophages through reduced NFATc1 expression and impaired phosphorylation of IκBα.

Modulating calcium-mediated NFATc1 and mitogen-activated protein kinase deactivation underlies the inhibitory effects of kavain on osteoclastogenesis and bone resorption

Osteoclasts are responsible for bone resorption during the process of bone remodeling. Increased osteoclast numbers and bone resorption activity are the main factors contributing to bone loss-related diseases such as osteoporosis. Therefore, modulating the formation and function of osteoclasts is critical for the effective treatment of osteolysis and osteoporosis. Kavain is the active ingredient extracted from the root of the kava plant, which possesses known anti-inflammatory properties. However, the effects of kavain on osteoclastogenesis and bone resorption remain unclear. In this study, we found that kavain inhibits receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclast differentiation and fusion using tartrate-resistant acid phosphatase staining and immunofluorescence. Furthermore, kavain inhibited bone resorption performed by osteoclasts. Using reverse transcription-polymerase chain reaction and western blot analysis, we found that kavain downregulates the expression of osteoclast marker genes, such as nuclear factor of activated T cells, cytoplasmic 1 (Nfatc1), v-atpase d2 (Atp6v0d2), dendrocyte expressed seven transmembrane protein (Dcstamp), matrix metallopeptidase 9 (Mmp9), cathepsin K (Ctsk), and Acp5. Additionally, kavain repressed RANKL-induced calcium oscillations, nuclear factor of activated T cells activation, and mitogen-activated protein kinase phosphorylation, while leaving NF-κB unaffected. We found no effects of kavain on either osteoblast proliferation or differentiation. Besides, kavain inhibited bone loss in ovariectomized mice by suppressing osteoclastogenesis. Collectively, these data suggest a potential use for kavain as a candidate drug for the treatment of osteolytic diseases.

Transcription factor EB influences invasion and migration in oral squamous cell carcinomas

OBJECTIVE

Transcription factor EB (TFEB) is a master regulator of lysosomal biogenesis and plays an important role in various cancers. However, the function of TFEB in oral squamous cell carcinomas has not been examined. The aim of this study was to elucidate the role of TFEB in oral squamous cell carcinomas.

MATERIALS AND METHODS

Expression levels of TFEB were examined in six different human oral squamous carcinoma cells: HSC2, HSC3, HSC4, SAS, OSC20, and SCC25. Knockdown of TFEB using small interfering RNA in HSC2 and HSC4 cells was performed. Cell morphology was observed by immunofluorescence microscopy. Cell proliferation, invasion, and adhesion were analyzed.

RESULTS

Expression levels of TFEB were high in HSC2, moderate in HSC4 and SCC25, and low in HSC3 and OSC20 cells. Knockdown of TFEB did not affect proliferation of HSC2 and HSC4 cells, but did induced enlargement of lysosomes and endosomes in HSC4 cells. TFEB silencing reduced invasion and migration of these HSC cell squamous carcinoma cells; however, increased cell adhesion was also observed.

CONCLUSION

TFEB knockdown reduces invasion and migration of cancer cells, likely through lysosomal regulation. Taken together, TFEB influences cell invasion and migration of oral squamous cell carcinomas.

Actin binding LIM 1 (abLIM1) negatively controls osteoclastogenesis by regulating cell migration and fusion

Actin binding LIM 1 (abLIM1) is a cytoskeletal actin-binding protein that has been implicated in interactions between actin filaments and cytoplasmic targets. Previous biochemical and cytochemical studies have shown that abLIM1 interacts and co-localizes with F-actin in the retina and muscle. However, whether abLIM1 regulates osteoclast differentiation has not yet been elucidated. In this study, we examined the role of abLIM1 in osteoclast differentiation and function. We found that abLIM1 expression was upregulated during receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclast differentiation, and that a novel transcript of abLIM1 was exclusively expressed in osteoclasts. Overexpression of abLIM1 in the murine monocytic cell line, RAW-D suppressed osteoclast differentiation and decreased expression of several osteoclast-marker genes. By contrast, small interfering RNA-induced knockdown of abLIM1 enhanced the formation of multinucleated osteoclasts and markedly increased the expression of the osteoclast-marker genes. Mechanistically, abLIM1 regulated the localization of tubulin, migration, and fusion in osteoclasts. Thus, these results indicate that abLIM1 negatively controls osteoclast differentiation by regulating cell migration and fusion mediated via actin formation.