Rad GTPase deficiency leads to cardiac hypertrophy

Rem2-targeted shRNAs reduce frequency of miniature excitatory postsynaptic currents without altering voltage-gated Ca²⁺ currents

Ca²⁺ influx through voltage-gated Ca²⁺ channels (VGCCs) plays important roles in neuronal cell development and function. Rem2 is a member of the RGK (Rad, Rem, Rem2, Gem/Kir) subfamily of small GTPases that confers potent inhibition upon VGCCs. The physiologic roles of RGK proteins, particularly in the brain, are poorly understood. Rem2 was implicated in synaptogenesis through an RNAi screen and proposed to regulate Ca²⁺ homeostasis in neurons. To test this hypothesis and uncover physiological roles for Rem2 in the brain, we investigated the molecular mechanisms by which Rem2 knockdown affected synaptogenesis and Ca²⁺ homeostasis in cultured rat hippocampal neurons. Expression of a cocktail of shRNAs targeting rat Rem2 (rRem2) reduced the frequency of miniature excitatory postsynaptic currents (mEPSCs) measured 10 d after transfection (14 d in vitro), but did not affect mEPSC amplitude. VGCC current amplitude after rRem2-targeted knockdown was not different from that in control cells, however, at either 4 or 10 d post transfection. Co-expression of a human Rem2 that was insensitive to the shRNAs targeting rRem2 was unable to prevent the reduction in mEPSC frequency after rRem2-targeted knockdown. Over-expression of rRem2 resulted in 50% reduction in VGCC current, but neither the mEPSC frequency nor amplitude was affected. Taken together, the observed effects upon synaptogenesis after shRNA treatment are more likely due to mechanisms other than modulation of VGCCs and Ca²⁺ homeostasis, and may be independent of Rem2. In addition, our results reveal a surprising lack of contribution of VGCCs to synaptogenesis during early development in cultured hippocampal neurons.

Platelet-derived growth factor induces Rad expression through Egr-1 in vascular smooth muscle cells

BACKGROUND

Ras associated with diabetes (Rad) inhibits vascular lesion formation by reducing the attachment and migration of vascular smooth muscle cells (VSMCs). However, the transcriptional regulation of Rad in VSMCs is unclear.

METHODOLOGY AND PRINCIPAL FINDINGS

We found that Platelet-Derived Growth Factor (PDGF)induced Rad expression in a time- and dose-dependent manner in rat aortic smooth muscle cells (RASMCs) using quantitative real-time PCR. By serial deletion analysis of the Rad promoter, we identified that two GC-rich early growth response-1 (Egr-1) binding sites are essential for PDGF-induced Rad promoter activation. Overexpression of Egr-1 in RASMCs strongly stimulated Rad expression while the Egr-1 corepressor, NGFI-A binding protein 2 (NAB2), repressed PDGF-induced Rad up-regulation in a dose-dependent manner. Direct binding of Egr-1 to the Rad promoter region was further confirmed by chromatin immunoprecipitation assays.

CONCLUSIONS

Our results demonstrate that Rad is regulated by PDGF through the transcriptional factor Egr-1 in RASMCs.

Rad GTPase induces cardiomyocyte apoptosis through the activation of p38 mitogen-activated protein kinase

Rad is a member of a subclass of small GTP-binding proteins, the RGK family. In the present study we investigated the role of Rad protein in regulating cardiomyocyte viability. DNA fragmentation and TUNEL assays demonstrated that Rad promoted rat neonatal cardiomyocyte apoptosis. Rad silencing fully blocked serum deprivation induced apoptosis, indicating Rad is necessary for trigger cardiomyocyte apoptosis. Rad overexpression caused a dramatic decrease of the anti-apoptotic molecule Bcl-x(L), whereas Bcl-x(L) overexpression protected cardiomyocytes against Rad-induced apoptosis. Rad-triggered apoptosis was mediated by the activation of p38 MAPK. The p38 blocker SB203580 effectively protected cardiomyocytes against Rad-evoked apoptosis.

Rad GTPase inhibits cardiac fibrosis through connective tissue growth factor

AIMS

Our previous studies documented that Rad (Ras associated with diabetes), a member of the RGK (Rad, Gem, and Kir) family of Ras-related small G protein, is significantly decreased in human failing hearts and plays an important role in attenuating cardiac hypertrophy. The goal of this study is to identify the effect of Rad on cardiac fibrosis and the underlying mechanisms.

METHODS AND RESULTS

Rad knockout (KO) mice showed more severe cardiac fibrosis compared with wild-type littermate controls as detected by Sirius Red staining. Western blot analyses demonstrated that the expression of connective tissue growth factor (CTGF), a key mediator of fibrosis, increased dramatically in Rad KO mice. Overexpression of Rad in cultured neonatal cardiomyocytes suppressed both basal and transforming growth factor-β1-induced CTGF expression. Elevated CTGF expression was observed in cardiomyocytes when Rad was reduced by RNA interference. Moreover, cardiac fibroblasts produced greater extracellular matrix (ECM) when stimulated with conditioned medium from Rad-knockdown cardiomyocytes. ECM production was completely abolished by adding a CTGF-neutralizing antibody into the medium. CCAAT/enhancer-binding protein δ (C/EBP-δ) was demonstrated to activate CTGF in cardiomyocytes. Chromatin immunoprecipitation assay and co-immunoprecipitation further demonstrated that Rad inhibited the binding of C/EBP-δ to the CTGF promoter via direct interaction with C/EBP-δ.

CONCLUSION

Our data reveal that Rad deficiency can lead to cardiac fibrosis. Rad inhibits CTGF expression through binding with C/EBP-δ, thus regulating ECM production in the heart. This study suggests a potential link between decreased Rad levels and increased cardiac fibrosis in human failing hearts.

The ß subunit of voltage-gated Ca2+ channels

Calcium regulates a wide spectrum of physiological processes such as heartbeat, muscle contraction, neuronal communication, hormone release, cell division, and gene transcription. Major entryways for Ca(2+) in excitable cells are high-voltage activated (HVA) Ca(2+) channels. These are plasma membrane proteins composed of several subunits, including α(1), α(2)δ, β, and γ. Although the principal α(1) subunit (Ca(v)α(1)) contains the channel pore, gating machinery and most drug binding sites, the cytosolic auxiliary β subunit (Ca(v)β) plays an essential role in regulating the surface expression and gating properties of HVA Ca(2+) channels. Ca(v)β is also crucial for the modulation of HVA Ca(2+) channels by G proteins, kinases, and the Ras-related RGK GTPases. New proteins have emerged in recent years that modulate HVA Ca(2+) channels by binding to Ca(v)β. There are also indications that Ca(v)β may carry out Ca(2+) channel-independent functions, including directly regulating gene transcription. All four subtypes of Ca(v)β, encoded by different genes, have a modular organization, consisting of three variable regions, a conserved guanylate kinase (GK) domain, and a conserved Src-homology 3 (SH3) domain, placing them into the membrane-associated guanylate kinase (MAGUK) protein family. Crystal structures of Ca(v)βs reveal how they interact with Ca(v)α(1), open new research avenues, and prompt new inquiries. In this article, we review the structure and various biological functions of Ca(v)β, with both a historical perspective as well as an emphasis on recent advances.

Molecular mechanisms, and selective pharmacological rescue, of Rem-inhibited CaV1.2 channels in heart

RATIONALE

In heart, Ca(2+) entering myocytes via Ca(V)1.2 channels controls essential functions, including excitation-contraction coupling, action potential duration, and gene expression. RGK GTPases (Rad/Rem/Rem2/Gem/Kir sub-family of Ras-like GTPases) potently inhibit Ca(V)1.2 channels, an effect that may figure prominently in cardiac Ca(2+) homeostasis under physiological and disease conditions.

OBJECTIVE

To define the mechanisms and molecular determinants underlying Rem GTPase inhibition of Ca(V)1.2 channels in heart and to determine whether such inhibited channels can be pharmacologically rescued.

METHODS AND RESULTS

Overexpressing Rem in adult guinea pig heart cells dramatically depresses L-type calcium current (I(Ca,L)) ( approximately 90% inhibition) and moderately reduces maximum gating charge (Q(max)) (33%), without appreciably diminishing the physical number of channels in the membrane. Rem-inhibited Ca(V)1.2 channels were supramodulated by BAY K 8644 (10-fold increase) compared to control channels (3-fold increase). However, Rem prevented protein kinase A-mediated upregulation of I(Ca,L), an effect achieved without disrupting the sympathetic signaling cascade because protein kinase A modulation of I(KS) (slow component of the delayed rectifier potassium current) remained intact. In accord with its functional impact on I(Ca,L), Rem selectively prevented protein kinase A- but not BAY K 8644-induced prolongation of the cardiac action potential duration. A GTP-binding-deficient Rem[T94N] mutant was functionally inert with respect to I(Ca,L) inhibition. A chimeric construct, Rem(265)-H, featuring a swap of the Rem C-terminal tail for the analogous domain from H-Ras, inhibited I(Ca,L) and Q(max) to the same extent as wild-type Rem, despite lacking the capacity to autonomously localize to the sarcolemma.

CONCLUSIONS

Rem predominantly inhibits I(Ca,L) in heart by arresting surface Ca(V)1.2 channels in a low open probability gating mode, rather than by interfering with channel trafficking. Moreover, Rem-inhibited Ca(V)1.2 channels can be selectively rescued by BAY K 8644 but not protein kinase A-dependent phosphorylation. Contrary to findings in reconstituted systems, Rem-induced ablation of cardiac I(Ca,L) requires GTP-binding, but not membrane-targeting of the nucleotide binding domain. These findings provide a different perspective on the molecular mechanisms and structural determinants underlying RGK GTPase inhibition of Ca(V)1.2 channels in heart, and suggest new (patho)physiological dimensions of this crosstalk.

Identification of genes preferentially methylated in hepatitis C virus-related hepatocellular carcinoma

Chronic infections by hepatitis B virus (HBV) and hepatitis C virus (HCV) appear to be the most significant causes of hepatocellular carcinoma (HCC). Aberrant promoter methylation is known to be deeply involved in cancer, including in HCC. In this study, we analyzed aberrant promoter methylation by methylated DNA immunoprecipitation-on-chip analysis on a genome-wide scale in six HCCs including three HBV-related and three HCV-related HCCs, six matched noncancerous liver tissues, and three normal liver tissues. Candidate genes with promoter methylation were detected more frequently in HCV-related HCC. Candidate genes methylated preferentially to HBV-related or HCV-related HCCs were detected and selected, and methylation levels of the selected genes were validated by quantitative methylation analysis using MALDI-TOF mass spectrometry using 125 liver tissue samples, including 61 HCCs (28 HBV-related HCCs and 33 HCV-related HCCs) and 59 matched noncancerous livers, and five normal livers. Among analyzed genes, preferential methylation in HBV-related HCC was validated in one gene only. However, 15 genes were found to be methylated preferentially in HCV-related HCC, which was independent from age. Hierarchical clustering of HCC using these genes stratified HCV-related HCC as a cluster of frequently methylated samples. The 15 genes included genes inhibitory to cancer-related signaling such as RAS/RAF/ERK and Wnt/beta-catenin pathways. Methylation of dual specificity phosphatase 4 (DUSP4), cytochrome P450, family 24, subfamily A, polypeptide 1 (CYP24A1), and natriuretic peptide receptor A (NPR1) significantly correlated with recurrence-free survival. It was indicated that genes methylated preferentially in HCV-related HCC exist, and that DNA methylation might play an important role in HCV-related HCC by silencing cancer-related pathway inhibitors, and might perhaps be useful as a prognostic marker.

Genomics of cardiac remodeling in angiotensin II-treated wild-type and LOX-1-deficient mice

We studied the gene expression profile during cardiac hypertrophy induced by angiotensin (ANG) II in wild-type mice and the influence of LOX-1 deletion on the gene expression profile. Wild-type and LOX-1 knockout mice were given saline or ANG II infusion for 4 wk. The saline-treated LOX-1 knockout mice showed upregulation of several genes including Ddx3y and Eif2s3y. ANG II infusion enhanced expression of genes known to be associated with cardiac remodeling, such as Agt, Ace, Timp4, Fstl, and Tnfrst12a, as well as oxidant stress-related genes Gnaq, Sos1, and Rac1. Some other strongly upregulated genes identified in this study have not been previously associated with LOX-1 deletion and/or hypertension. To confirm these observations with ANG II infusion and LOX-1 deletion, cultured HL-1 mouse cardiomyocytes were exposed to ANG II or transfected with pCI-neo/LOX-1, which resulted in severalfold increase in reactive oxygen species generation, upregulation of ANG II type 1 (AT1) receptor, and cardiomyocyte growth. Quantitative PCR analysis of these treated cardiomyocytes confirmed upregulation of many of the genes identified in the in vivo study. This study provides the first set of data on the gene expression profiling of cardiac tissue treated with ANG II and expands on the important role of LOX-1 in cardiac response to ANG II.

Rem GTPase interacts with the proximal CaV1.2 C-terminus and modulates calcium-dependent channel inactivation

The Rem, Rem2, Rad, and Gem/Kir (RGK) GTPases, comprise a subfamily of small Ras-related GTP-binding proteins, and have been shown to potently inhibit high voltage-activated Ca(2+) channel current following overexpression. Although the molecular mechanisms underlying RGK-mediated Ca(2+) channel regulation remains controversial, recent studies suggest that RGK proteins inhibit Ca(2+) channel currents at the plasma membrane in part by interactions with accessory channel β subunits. In this paper, we extend our understanding of the molecular determinants required for RGK-mediated channel regulation by demonstrating a direct interaction between Rem and the proximal C-terminus of Ca(V)1.2 (PCT), including the CB/IQ domain known to contribute to Ca(2+)/calmodulin (CaM)-mediated channel regulation. The Rem2 and Rad GTPases display similar patterns of PCT binding, suggesting that the Ca(V)1.2 C-terminus represents a common binding partner for all RGK proteins. In vitro Rem:PCT binding is disrupted by Ca(2+)/CaM, and this effect is not due to Ca(2+)/CaM binding to the Rem C-terminus. In addition, co-overexpression of CaM partially relieves Rem-mediated L-type Ca(2+) channel inhibition and slows the kinetics of Ca(2+)-dependent channel inactivation. Taken together, these results suggest that the association of Rem with the PCT represents a crucial molecular determinant in RGK-mediated Ca(2+) channel regulation and that the physiological function of the RGK GTPases must be re-evaluated. Rather than serving as endogenous inhibitors of Ca(2+) channel activity, these studies indicate that RGK proteins may play a more nuanced role, regulating Ca(2+) currents via modulation of Ca(2+)/CaM-mediated channel inactivation kinetics.

Rem, a member of the RGK GTPases, inhibits recombinant CaV1.2 channels using multiple mechanisms that require distinct conformations of the GTPase

Rad/Rem/Gem/Kir (RGK) GTPases potently inhibit Ca(V)1 and Ca(V)2 (Ca(V)1-2) channels, a paradigm of ion channel regulation by monomeric G-proteins with significant physiological ramifications and potential biotechnology applications. The mechanism(s) underlying how RGK proteins inhibit I(Ca) is unknown, and it is unclear how key structural and regulatory properties of these GTPases (such as the role of GTP binding to the nucleotide binding domain (NBD), and the C-terminus which contains a membrane-targeting motif) feature in this effect. Here, we show that Rem inhibits Ca(V)1.2 channels by three independent mechanisms that rely on distinct configurations of the GTPase: (1) a reduction in surface density of channels is accomplished by enhancing dynamin-dependent endocytosis, (2) a diminution of channel open probability (P(o)) that occurs without impacting on voltage sensor movement, and (3) an immobilization of Ca(V) channel voltage sensors. The presence of both the Rem NBD and C-terminus (whether membrane-targeted or not) in one molecule is sufficient to reconstitute all three mechanisms. However, membrane localization of the NBD by a generic membrane-targeting module reconstitutes only the decreased P(o) function (mechanism 2). A point mutation that prevents GTP binding to the NBD selectively eliminates the capacity to immobilize voltage sensors (mechanism 3). The results reveal an uncommon multiplicity in the mechanisms Rem uses to inhibit I(Ca), predict new physiological dimensions of the RGK GTPase-Ca(V) channel crosstalk, and suggest original approaches for developing novel Ca(V) channel blockers.

Rad As a Novel Regulator of Excitation–Contraction Coupling and β-Adrenergic Signaling in Heart

Rationale : Rad (Ras associated with diabetes) GTPase, a monomeric small G protein, binds to Ca v β subunit of the L-type Ca 2+ channel (LCC) and thereby regulates LCC trafficking and activity. Emerging evidence suggests that Rad is an important player in cardiac arrhythmogenesis and hypertrophic remodeling. However, whether and how Rad involves in the regulation of excitation–contraction (EC) coupling is unknown.

Objective : This study aimed to investigate possible role of Rad in cardiac EC coupling and β-adrenergic receptor (βAR) inotropic mechanism.

Methods and Results : Adenoviral overexpression of Rad by 3-fold in rat cardiomyocytes suppressed LCC current ( I Ca ), [Ca 2+ ] i transients, and contractility by 60%, 42%, and 38%, respectively, whereas the “gain” function of EC coupling was significantly increased, due perhaps to reduced “redundancy” of LCC in triggering sarcoplasmic reticulum release. Conversely, ≈70% Rad knockdown by RNA interference increased I Ca (50%), [Ca 2+ ] i transients (52%) and contractility (58%) without altering EC coupling efficiency; and the dominant negative mutant RadS105N exerted a similar effect on I Ca . Rad upregulation caused depolarizing shift of LCC activation and hastened time-dependent LCC inactivation; Rad downregulation, however, failed to alter these attributes. The Na + /Ca 2+ exchange activity, sarcoplasmic reticulum Ca 2+ content, properties of Ca 2+ sparks and propensity for Ca 2+ waves all remained unperturbed regardless of Rad manipulation. Rad overexpression, but not knockdown, negated βAR effects on I Ca and Ca 2+ transients.

Conclusion : These results establish Rad as a novel endogenous regulator of cardiac EC coupling and βAR signaling and support a parsimonious model in which Rad buffers Ca v β to modulate LCC activity, EC coupling, and βAR responsiveness.

Inappropriate left ventricular mass in children and young adults with chronic renal insufficiency

Increased left ventricular (LV) mass (M) in children with chronic renal insufficiency (CRI) might represent an adaptive mechanism to compensate for increased workload. We hypothesized that in children with CRI, pre-dialysis, values of left ventricular mass (LVM) exceed compensatory values for individual cardiac load. Complete anthropometric characteristics, biochemical profile and echocardiograms were obtained for 33 children with CRI, pre-dialysis (age 1-23 years, mean 12.2 +/- 5.0 years), and 33 age- and gender-matched healthy controls. LV dimensions, wall thicknesses and volume were measured. Endocardial and midwall shortening, ejection fraction, LVM, LVM index, relative wall thickness, circumferential wall stress and excess LVM (as ratio of observed LVM to value predicted from body size, gender and cardiac workload) were analysed. Patients with CRI showed higher values of LVM index, resulting in higher prevalence of LV hypertrophy (36.3% vs 9%, P < 0.05). The ratio of excess LVM was greater in patients with CRI than in healthy controls (126 +/- 19% and 103 +/- 13%, respectively, P < 0.001). LV ejection fraction, midwall fractional shortening and stress-corrected midwall shortening were lower in patients with CRI than in controls. We concluded that, in children with CRI, the values of LVM are higher than those needed to sustain individual cardiac load than in healthy controls, a condition associated with LV hypertrophy and reduced systolic performance.

Engineering proteins for custom inhibition of Ca(V) channels

The influx of Ca(2+) ions through voltage-dependent calcium (Ca(V)) channels links electrical signals to physiological responses in all excitable cells. Not surprisingly, blocking Ca(V) channel activity is a powerful method to regulate the function of excitable cells, and this is exploited for both physiological and therapeutic benefit. Nevertheless, the full potential for Ca(V) channel inhibition is not being realized by currently available small-molecule blockers or second-messenger modulators due to limitations in targeting them either to defined groups of cells in an organism or to distinct subcellular regions within a single cell. Here, we review early efforts to engineer protein molecule blockers of Ca(V) channels to fill this crucial niche. This technology would greatly expand the toolbox available to physiologists studying the biology of excitable cells at the cellular and systems level.

Skeletal muscle gene expression after myostatin knockout in mature mice

There is much interest in developing anti-myostatin agents to reverse or prevent muscle atrophy in adults, so it is important to characterize the effects of reducing myostatin activity after normal muscle development. For assessment of the effect of loss of myostatin signaling on gene expression in muscle, RNA from mice with postdevelopmental myostatin knockout was analyzed with oligonucleotide microarrays. Myostatin was undetectable in muscle within 2 wk after Cre recombinase activation in 4-month-old male mice with floxed myostatin genes. Three months after myostatin depletion, muscle mass had increased 26% (vs. 2% after induction of Cre activity in mice with normal myostatin genes), at which time the expression of several hundred genes differed in knockout and control mice at nominal P < 0.01. In contrast to previously reported effects of constitutive myostatin knockout, postdevelopmental knockout did not downregulate expression of genes encoding slow isoforms of contractile proteins or genes encoding proteins involved in energy metabolism. Several collagen genes were expressed at 20-50% lower levels in the myostatin-deficient muscles, which had approximately 25% less collagen than normal muscles as reflected by hydroxyproline content. Most of the other genes affected by myostatin depletion have not been previously linked to myostatin signaling. Gene set enrichment analysis suggested that Smads are not the only transcription factors with reduced activity after myostatin depletion. These data reinforce other evidence that myostatin regulates collagen production in muscle and demonstrate that many of the previously reported effects of constitutive myostatin deficiency do not occur when myostatin is knocked out in mature muscles.

Role of the molybdoflavoenzyme aldehyde oxidase homolog 2 in the biosynthesis of retinoic acid: generation and characterization of a knockout mouse

The mouse aldehyde oxidase AOH2 (aldehyde oxidase homolog 2) is a molybdoflavoenzyme. Harderian glands are the richest source of AOH2, although the protein is detectable also in sebaceous glands, epidermis, and other keratinized epithelia. The levels of AOH2 in the Harderian gland and skin are controlled by genetic background, being maximal in CD1 and C57BL/6 and minimal in DBA/2, CBA, and 129/Sv strains. Testosterone is a negative regulator of AOH2 in Harderian glands. Purified AOH2 oxidizes retinaldehyde into retinoic acid, while it is devoid of pyridoxal-oxidizing activity. Aoh2(-/-) mice, the first aldehyde oxidase knockout animals ever generated, are viable and fertile. The data obtained for this knockout model indicate a significant role of AOH2 in the local synthesis and biodisposition of endogenous retinoids in the Harderian gland and skin. The Harderian gland's transcriptome of knockout mice demonstrates overall downregulation of direct retinoid-dependent genes as well as perturbations in pathways controlling lipid homeostasis and cellular secretion, particularly in sexually immature animals. The skin of knockout mice is characterized by thickening of the epidermis in basal conditions and after UV light exposure. This has correlates in the corresponding transcriptome, which shows enrichment and overall upregulation of genes involved in hypertrophic responses.

Epac Mediates β-Adrenergic Receptor–Induced Cardiomyocyte Hypertrophy

Cardiac hypertrophy is promoted by adrenergic overactivation and can progress to heart failure, a leading cause of mortality worldwide. Although cAMP is among the most well-known signaling molecules produced by β-adrenergic receptor stimulation, its mechanism of action in cardiac hypertrophy is not fully understood. The identification of Epac (exchange protein directly activated by cAMP) proteins as novel sensors for cAMP has broken the dogma surrounding cAMP and protein kinase A. However, their role and regulation in the mature heart remain to be defined. Here, we show that cardiac hypertrophy induced by thoracic aortic constriction increases Epac1 expression in rat myocardium. Adult ventricular myocytes isolated from banded animals display an exaggerated cellular growth in response to Epac activation. At the molecular level, Epac1 hypertrophic effects are independent of its classic effector, Rap1, but rather involve the small GTPase Ras, the phosphatase calcineurin, and Ca 2+ /calmodulin-dependent protein kinase II. Importantly, we find that in response to β-adrenergic receptor stimulation, Epac1 activates Ras and induces adult cardiomyocyte hypertrophy in a cAMP-dependent but protein kinase A–independent manner. Knockdown of Epac1 strongly reduces β-adrenergic receptor–induced hypertrophic program. Finally, we report for the first time that Epac1 is mainly expressed in human heart as compared with Epac2 isoform and is increased in heart failure. Taken together, our data demonstrate that the guanine nucleotide exchange factor Epac1 contributes to the hypertrophic effect of β-adrenergic receptor in a protein kinase A–independent fashion and may, therefore, represent a novel therapeutic target for the treatment of cardiac disorders.

Small GTP-binding proteins and their regulators in cardiac hypertrophy

Small GTP-binding proteins (small G proteins) act as GDP-GTP-regulated molecular switches and are activated by guanine nucleotide exchange factors (GEFs) in response to diverse extracellular stimuli. During this last decade, numerous molecular and cellular studies, as well as genetically-modified animal models, have highlighted the role of small G proteins in the regulation of cardiac hypertrophy. The growing interest in small G protein signalling comes from the fact that chronic hypertrophic response is considered maladaptive and predisposes individuals to heart failure. Although some of the hypertrophic signalling pathways involving small G proteins have now been identified, a central question deals with the identity of the GEFs that modulate small G protein activation in the context of cardiac hypertrophy. Here, we discuss the precise regulation of Ras and Rho subfamilies of GTPases by GEFs and other regulatory proteins during cardiac hypertrophy. In addition, we summarize recent published data, mainly those describing the role of small G proteins in the development of myocardial hypertrophy and we further present the importance of their downstream effectors in myocardial remodelling.