Mutant Rab24 GTPase is targeted to nuclear inclusions

Rab24 functions in meiotic apparatus assembly and maturational progression in mouse oocyte

Rab GTPases have multiple regulatory functions in intracellular vesicle transport. In recent years, there has been an increasing interest in the roles of Rab proteins in mammalian oocytes. In this paper, we show the specific distribution pattern of Rab24 during mouse oocyte meiosis. Furthermore, we find that Rab24 depletion results in the failure of maturational progression in mouse oocytes. Notably, the frequency of meiotic apparatus abnormality is significantly increased in Rab24-depleted oocytes relative to controls. In addition, lagging chromosomes are readily observed in anaphase/telophase oocytes with Rab24 knockdown. In support of this, the depletion of Rab24 disturbs the kinetochore-microtubule attachments in oocytes, and contributes to the production of aneuploid eggs. Taken together, the results of this study identify Rab24 as a novel factor in the modulation of meiotic apparatus assembly and meiotic progression during mouse oocyte maturation.

Farnesyltransferase Inhibition Exacerbates Eosinophilic Inflammation and Airway Hyperreactivity in Mice with Experimental Asthma: The Complex Roles of Ras GTPase and Farnesylpyrophosphate in Type 2 Allergic Inflammation

Ras, a small GTPase protein, is thought to mediate Th2-dependent eosinophilic inflammation in asthma. Ras requires cell membrane association for its biological activity, and this requires the posttranslational modification of Ras with an isoprenyl group by farnesyltransferase (FTase) or geranylgeranyltransferase (GGTase). We hypothesized that inhibition of FTase using FTase inhibitor (FTI)-277 would attenuate allergic asthma by depleting membrane-associated Ras. We used the OVA mouse model of allergic inflammation and human airway epithelial (HBE1) cells to determine the role of FTase in inflammatory cell recruitment. BALB/c mice were first sensitized then exposed to 1% OVA aerosol or filtered air, and half were injected daily with FTI-277 (20 mg/kg per day). Treatment of mice with FTI-277 had no significant effect on lung membrane-anchored Ras, Ras protein levels, or Ras GTPase activity. In OVA-exposed mice, FTI-277 treatment increased eosinophilic inflammation, goblet cell hyperplasia, and airway hyperreactivity. Human bronchial epithelial (HBE1) cells were pretreated with 5, 10, or 20 μM FTI-277 prior to and during 12 h IL-13 (20 ng/ml) stimulation. In HBE1 cells, FTase inhibition with FTI-277 had no significant effect on IL-13-induced STAT6 phosphorylation, eotaxin-3 peptide secretion, or Ras translocation. However, addition of exogenous FPP unexpectedly augmented IL-13-induced STAT6 phosphorylation and eotaxin-3 secretion from HBE1 cells without affecting Ras translocation. Pharmacological inhibition of FTase exacerbates allergic asthma, suggesting a protective role for FTase or possibly Ras farnesylation. FPP synergistically augments epithelial eotaxin-3 secretion, indicating a novel Ras-independent farnesylation mechanism or direct FPP effect that promotes epithelial eotaxin-3 production in allergic asthma.

KDM4B-mediated epigenetic silencing of miRNA-615-5p augments RAB24 to facilitate malignancy of hepatoma cells

Emerging evidence indicates that dysregulation of microRNAs (miRNAs) contributes to hepatocellular carcinoma (HCC) tumorigenesis and development. Here, we found that miR-615-5p was obviously downregulated in HCC. Furthermore, the deficiency of demethylase KDM4B stimulated the CpG methylation of miR-615-5p promoter and then decreased the miR-615-5p expression. The Ras-related protein RAB24 was found to be downregulated by miR-615-5p. The low level of miR-615-5p increased the expression of RAB24 and facilitated HCC growth and metastasis in vitro and in vivo. Moreover, miR-615-5p suppresses HCC cell growth by influencing cell cycle progression and apoptosis. Downregulation of miR-615-5p and upregulation of RAB24 promotes the epithelial-mesenchymal transition (EMT), adhesion and vasculogenic mimicry (VM) of HCC cells, all of which contribute to cell motility and metastasis. Thus, miR-615-5p, who is downregulated by KDM4B-mediated hypermethylation in its promoter, functions as a tumor suppressor by inhibiting RAB24 expression in HCC. In conclusion, our findings characterize miR-615-5p as an important epigenetically silenced miRNA involved in the Rab-Ras pathway in hepatocellular carcinoma and expand our understanding of the molecular mechanism underlying hepatocarcinogenesis and metastasis.

New insights into mechanisms of nuclear translocation of G-protein coupled receptors

The G-protein coupled receptor (GPCR) signaling was long believed to involve activation of receptor exclusively at the cell surface, followed by its binding to heterotrimeric G-proteins and arrestins to trigger various intracellular signaling cascades, and termination of signaling by internalization of the receptor. It is now accepted that many GPCRs continue to signal after internalization in the endosomes. Since the breakthrough discoveries of nuclear binding sites for their ligands in 1980s, several GPCRs have been detected at cell nuclei. But mechanisms of nuclear localization of GPCRs, many of whom contain putative nuclear localization signals, remain poorly understood to date. Nevertheless, it is known that subcellular trafficking of GPCRs is regulated by members of Ras superfamily of small GTPases, most notably by Rab and Arf GTPases. In this commentary, we highlight several recent studies which suggest novel roles of small GTPases, importins and sorting nexin proteins in the nuclear translocation of GPCRs via vesicular transport pathways. Taken together with increasing evidence for in vivo functionality of the nuclear GPCRs, better understanding of their trafficking will provide valuable clues in cell biology.

Rab24 interacts with the Rab7/Rab interacting lysosomal protein complex to regulate endosomal degradation

Endocytosis is a multistep process engaged in extracellular molecules internalization. Several proteins including the Rab GTPases family coordinate the endocytic pathway. The small GTPase Rab7 is present in late endosome (LE) compartments being a marker of endosome maturation. The Rab interacting lysosomal protein (RILP) is a downstream effector of Rab7 that recruits the functional dynein/dynactin motor complex to late compartments. In the present study, we have found Rab24 as a component of the endosome-lysosome degradative pathway. Rab24 is an atypical protein of the Rab GTPase family, which has been attributed a function in vesicle trafficking and autophagosome maturation. Using a model of transiently expressed proteins in K562 cells, we found that Rab24 co-localizes in vesicular structures labeled with Rab7 and LAMP1. Moreover, using a dominant negative mutant of Rab24 or a siRNA-Rab24 we showed that the distribution of Rab7 in vesicles depends on a functional Rab24 to allow DQ-BSA protein degradation. Additionally, by immunoprecipitation and pull down assays, we have demonstrated that Rab24 interacts with Rab7 and RILP. Interestingly, overexpression of the Vps41 subunit from the homotypic fusion and protein-sorting (HOPS) complex hampered the co-localization of Rab24 with RILP or with the lysosomal GTPase Arl8b, suggesting that Vps41 would affect the Rab24/RILP association. In summary, our data strongly support the hypothesis that Rab24 forms a complex with Rab7 and RILP on the membranes of late compartments. Our work provides new insights into the molecular function of Rab24 in the last steps of the endosomal degradative pathway.

Small RAB GTPases Regulate Multiple Steps of Mitosis

GTPases of the RAB family are key regulators of multiple steps of membrane trafficking. Several members of the RAB GTPase family have been implicated in mitotic progression. In this review, we will first focus on the function of endosome-associated RAB GTPases reported in early steps of mitosis, spindle pole maturation, and during cytokinesis. Second, we will discuss the role of Golgi-associated RAB GTPases at the metaphase/anaphase transition and during cytokinesis.

Nuclear G protein signaling: new tricks for old dogs

According to the standard model of G protein-coupled receptor (GPCR) signaling, GPCRs are localized to the cell membrane where they respond to extracellular signals. Stimulation of GPCRs leads to the activation of heterotrimeric G proteins and their intracellular signaling pathways. However, this model fails to accommodate GPCRs, G proteins, and their downstream effectors that are found on the nuclear membrane or in the nucleus. Evidence from isolated nuclei indicates the presence of GPCRs on the nuclear membrane that can activate similar G protein-dependent signaling pathways in the nucleus as at the cell surface. These pathways also include activation of cyclic adenosine monophosphate, calcium and nitric oxide synthase signaling in cardiomyocytes. In addition, a number of distinct heterotrimeric and monomeric G proteins have been found in the nucleus of various cell types. This review will focus on understanding the function of nuclear G proteins with a focus on cardiac signaling where applicable.

Canine hereditary ataxia in old english sheepdogs and gordon setters is associated with a defect in the autophagy gene encoding RAB24

Old English Sheepdogs and Gordon Setters suffer from a juvenile onset, autosomal recessive form of canine hereditary ataxia primarily affecting the Purkinje neuron of the cerebellar cortex. The clinical and histological characteristics are analogous to hereditary ataxias in humans. Linkage and genome-wide association studies on a cohort of related Old English Sheepdogs identified a region on CFA4 strongly associated with the disease phenotype. Targeted sequence capture and next generation sequencing of the region identified an A to C single nucleotide polymorphism (SNP) located at position 113 in exon 1 of an autophagy gene, RAB24, that segregated with the phenotype. Genotyping of six additional breeds of dogs affected with hereditary ataxia identified the same polymorphism in affected Gordon Setters that segregated perfectly with phenotype. The other breeds tested did not have the polymorphism. Genome-wide SNP genotyping of Gordon Setters identified a 1.9 MB region with an identical haplotype to affected Old English Sheepdogs. Histopathology, immunohistochemistry and ultrastructural evaluation of the brains of affected dogs from both breeds identified dramatic Purkinje neuron loss with axonal spheroids, accumulation of autophagosomes, ubiquitin positive inclusions and a diffuse increase in cytoplasmic neuronal ubiquitin staining. These findings recapitulate the changes reported in mice with induced neuron-specific autophagy defects. Taken together, our results suggest that a defect in RAB24, a gene associated with autophagy, is highly associated with and may contribute to canine hereditary ataxia in Old English Sheepdogs and Gordon Setters. This finding suggests that detailed investigation of autophagy pathways should be undertaken in human hereditary ataxia.

Neurodegenerative diseases are one of the most important causes of decline in an aging population. An important subset of these diseases are known as the hereditary ataxias, familial neurodegenerative diseases that affect the cerebellum causing progressive gait disturbance in both humans and dogs. We identified a mutation in RAB24, a gene associated with autophagy, in Old English Sheepdogs and Gordon Setters with hereditary ataxia. Autophagy is a process by which cell proteins and organelles are removed and recycled and its critical role in maintenance of the continued health of cells is becoming clear. We evaluated the brains of affected dogs and identified accumulations of autophagosomes within the cerebellum, suggesting a defect in the autophagy pathway. Our results suggest that a defect in the autophagy pathway results in neuronal death in a naturally occurring disease in dogs. The autophagy pathway should be investigated in human hereditary ataxia and may represent a therapeutic target in neurodegenerative diseases.

Rab24 is required for normal cell division

Rab24 is an atypical member of the Rab GTPase family whose distribution in interphase cells has been characterized; however, its function remains largely unknown. In this study, we have analyzed the distribution of Rab24 throughout cell division. We have observed that Rab24 was located at the mitotic spindle in metaphase, at the midbody during telophase and in the furrow during cytokinesis. We have also observed partial co-localization of Rab24 and tubulin and demonstrated its association to microtubules. Interestingly, more than 90% of transiently transfected HeLa cells with Rab24 presented abnormal nuclear connections (i.e., chromatin bridges). Furthermore, in CHO cells stably transfected with GFP-Rab24wt, we observed a large percentage of binucleated and multinucleated cells. In addition, these cells presented an extremely large size and multiple failures in mitosis, as aberrant spindle formation (metaphase), delayed chromosomes (telophase) and multiple cytokinesis. A marked increase in binucleated, multinucleated and multilobulated nucleus formation was observed in HeLa cells depleted of Rab24. We also present evidence that a fraction of Rab24 associates with microtubules. In addition, Rab24 knock down resulted in misalignment of chromosomes and abnormal spindle formation in metaphase leading to the appearance of delayed chromosomes during late telophase and failures in cytokinesis. Our findings suggest that an adequate level of Rab24 is necessary for normal cell division. In summary, Rab24 modulates several mitotic events, including chromosome segregation and cytokinesis, perhaps through the interaction with microtubules.

A small GTPase, human Rab32, is required for the formation of autophagic vacuoles under basal conditions

Here we show that a small GTPase, Rab32, is a novel protein required for the formation of autophagic vacuoles. We found that the wild-type or GTP-bound form of human Rab32 expressed in HeLa and COS cells is predominantly localized to the endoplasmic reticulum (ER), and overexpression induces the formation of autophagic vacuoles containing an autophagosome marker protein LC3, the ER-resident protein calnexin and endosomal/lysosomal membrane protein LAMP-2, even under nutrient-rich conditions. The recruitment of Rab32 to the ER membrane was necessary for autophagic vacuole formation, suggesting involvement of the ER as a source of autophagosome membranes. In contrast, the expression of the inactive form of, or siRNA-specific for, Rab32 caused the formation of p62/SQSTM1 and ubiquitinated protein-accumulating aggresome-like structures and significantly prevented constitutive autophagy. We postulate that Rab32 facilitates the formation of autophagic vacuoles whose membranes are derived from the ER and regulates the clearance of aggregated proteins by autophagy.

Induced expressions of Rab24 GTPase and LC3 in nerve-injured motor neurons

Rab24 is a member of the Rab GTPase family, but its function is unclear. Here, we demonstrated increase in Rab24 mRNA in nerve-injured hypoglossal motor neurons of rats. Expression of Rab24 mRNA was also induced in differentiated PC12 cells following proteasome inhibitor (MG132) treatment. MG132 treatment further induced expression of microtubule-associated protein light chain 3 (LC3), and accumulation of LC3-II, a processed form of LC3 and the most reliable marker for autophagy. Induction of LC3 mRNA and accumulation of LC3-II were also observed in nerve-injured hypoglossal motor neurons, and partial co-localization of Rab24 and LC3 was demonstrated by immunohistochemistry. The present data suggest that nerve injury promotes autophagy-like events, and this may be an important response for degradation of unnecessary and misfolded proteins to recycle limited amino acids, and synthesize new proteins that are necessary for survival and nerve regeneration responses.

Tyrosine phosphorylation of the Rab24 GTPase in cultured mammalian cells

Several members of the large family of Rab GTPases have been shown to function in vesicular trafficking in mammalian cells. However, the exact role of Rab24 remains poorly defined. Rab24 differs from other Rab proteins in that it has a low intrinsic GTPase activity and is not efficiently prenylated. Here we report an additional unique property of Rab24; i.e., the protein can undergo tyrosine phosphorylation when overexpressed in cultured cells. Immunoblot analyses with specific anti-phosphotyrosine monoclonal antibodies revealed the presence of phosphotyrosine (pTyr) on myc-Rab24 in whole cell lysates and immunoprecipitated samples. No pTyr was detected on other overexpressed myc-tagged GTPases (H-Ras, Rab1b, Rab6, Rab11 or Rab13). Comparisons of myc-Rab24 in the soluble and particulate fractions from HEK293 and HEp-2 cells indicated that the cytosolic pool of Rab24 was more heavily phosphorylated than the membrane pool. Treatment of transfected cells with the broad-spectrum tyrosine kinase inhibitor, genistein, as well as the specific Src-family kinase inhibitor, PP2, eliminated the pTyr signal from Rab24. In contrast the receptor tyrosine kinase inhibitor, tyrphostin A25, had no effect. Tyrosine phosphorylation of Rab24 was reduced by alanine substitution of two unique tyrosines, one found in a strong consensus phosphorylation motif (Y [Formula: see text] ) in the hypervariable domain (Y172) and the other falling within the GXXXGK(S/T) motif known as the P-loop (Y17). The latter region is known to influence GTP hydrolysis in Rab proteins, so the phosphorylation of Y17 could contribute to the low intrinsic GTPase activity of Rab24. This is the first report of tyrosine phosphorylation in any member of the Ras superfamily and it raises the possibility that this type of modification could influence Rab24 targeting and interactions with effector protein complexes.