Expression of Rem2, an RGK family small GTPase, reduces N-type calcium current without affecting channel surface density

The monomeric G proteins AGS1 and Rhes selectively influence Galphai-dependent signaling to modulate N-type (CaV2.2) calcium channels

Activator of G protein Signaling 1 (AGS1) and Ras homologue enriched in striatum (Rhes) define a new group of Ras-like monomeric G proteins whose signaling properties and physiological roles are just beginning to be understood. Previous results suggest that AGS1 and Rhes exhibit distinct preferences for heterotrimeric G proteins, with AGS1 selectively influencing Galphai and Rhes selectively influencing Galphas. Here, we demonstrate that AGS1 and Rhes trigger nearly identical modulation of N-type Ca(2+) channels (Ca(V)2.2) by selectively altering Galphai-dependent signaling. Whole-cell currents were recorded from HEK293 cells expressing Ca(V)2.2 and Galphai- or Galphas-coupled receptors. AGS1 and Rhes reduced basal current densities and triggered tonic voltage-dependent (VD) inhibition of Ca(V)2.2. Additionally, each protein attenuated agonist-initiated channel inhibition through Galphai-coupled receptors without reducing channel inhibition through a Galphas-coupled receptor. The above effects of AGS1 and Rhes were blocked by pertussis toxin (PTX) or by expression of a Gbetagamma-sequestering peptide (masGRK3ct). Transfection with HRas, KRas2, Rap1A-G12V, Rap2B, Rheb2, or Gem failed to duplicate the effects of AGS1 and Rhes on Ca(V)2.2. Our data provide the first demonstration that AGS1 and Rhes exhibit similar if not identical signaling properties since both trigger tonic Gbetagamma signaling and both attenuate receptor-initiated signaling by the Gbetagamma subunits of PTX-sensitive G proteins. These results are consistent with the possibility that AGS1 and Rhes modulate Ca(2+) influx through Ca(V)2.2 channels under more physiological conditions and thereby influence Ca(2+)-dependent events such as neurosecretion.

Transcriptome analysis identifies genes with enriched expression in the mouse central extended amygdala

The central extended amygdala (EAc) is an ensemble of highly interconnected limbic structures of the anterior brain, and forms a cellular continuum including the bed nucleus of the stria terminalis (BNST), the central nucleus of the amygdala (CeA) and the nucleus accumbens shell (AcbSh). This neural network is a key site for interactions between brain reward and stress systems, and has been implicated in several aspects of drug abuse. In order to increase our understanding of EAc function at the molecular level, we undertook a genome-wide screen (Affymetrix) to identify genes whose expression is enriched in the mouse EAc. We focused on the less-well known BNST-CeA areas of the EAc, and identified 121 genes that exhibit more than twofold higher expression level in the EAc compared with whole brain. Among these, 43 genes have never been described to be expressed in the EAc. We mapped these genes throughout the brain, using non-radioactive in situ hybridization, and identified eight genes with a unique and distinct rostro-caudal expression pattern along AcbSh, BNST and CeA. Q-PCR analysis performed in brain and peripheral organ tissues indicated that, with the exception of one (Spata13), all these genes are predominantly expressed in brain. These genes encode signaling proteins (Adora2, GPR88, Arpp21 and Rem2), a transcription factor (Limh6) or proteins of unknown function (Rik130, Spata13 and Wfs1). The identification of genes with enriched expression expands our knowledge of EAc at a molecular level, and provides useful information to toward genetic manipulations within the EAc.

Ion channel regulation by Ras, Rho, and Rab small GTPases

Regulation of ion channels by heterotrimeric guanosine triphosphatases (GTPases), activated by heptathelical membrane receptors, has been the focus of several recent reviews. In comparison, regulation of ion channels by small monomeric G proteins, activated by cytoplasmic guanine nucleotide exchange factors, has been less well reviewed. Small G proteins, molecular switches that control the activity of cellular and membrane proteins, regulate a wide variety of cell functions. Many upstream regulators and downstream effectors of small G proteins now have been isolated. Their modes of activation and action are understood. Recently, ion channels were recognized as physiologically important effectors of small GTPases. Recent advances in understanding how small G proteins regulate the intracellular trafficking and activity of ion channels are discussed here. We aim to provide critical insight into physiological control of ion channel function and the biological consequences of regulation of these important proteins by small, monomeric G proteins.

The RGK family of GTP-binding proteins: regulators of voltage-dependent calcium channels and cytoskeleton remodeling

RGK proteins constitute a novel subfamily of small Ras-related proteins that function as potent inhibitors of voltage-dependent (VDCC) Ca(2+) channels and regulators of actin cytoskeletal dynamics. Within the larger Ras superfamily, RGK proteins have distinct regulatory and structural characteristics, including nonconservative amino acid substitutions within regions known to participate in nucleotide binding and hydrolysis and a C-terminal extension that contains conserved regulatory sites which control both subcellular localization and function. RGK GTPases interact with the VDCC beta-subunit (Ca(V)beta) and inhibit Rho/Rho kinase signaling to regulate VDCC activity and the cytoskeleton respectively. Binding of both calmodulin and 14-3-3 to RGK proteins, and regulation by phosphorylation controls cellular trafficking and the downstream signaling of RGK proteins, suggesting that a complex interplay between interacting protein factors and trafficking contribute to their regulation.

Analysis of the Rem2 - voltage dependant calcium channel beta subunit interaction and Rem2 interaction with phosphorylated phosphatidylinositide lipids

Voltage dependant calcium channels (VDCC) play a critical role in coupling electrical excitability to important physiological events such as secretion by neuronal and endocrine cells. Rem2, a GTPase restricted to neuroendocrine cell types, regulates VDCC activity by a mechanism that involves interaction with the VDCC beta subunit (Ca(V)beta). Mapping studies reveal that Rem2 binds to the guanylate kinase domain (GK) of the Ca(V)beta subunit that also contains the high affinity binding site for the pore forming and voltage sensing VDCC alpha subunit (Ca(V)alpha) interaction domain (AID). Moreover, fine mapping indicates that Rem2 binds to the GK domain in a region distinct from the AID interaction site, and competitive inhibition studies reveal that Rem2 does not disrupt Ca(V)alpha - Ca(V)beta binding. Instead, the Ca(V)beta subunit appears to serve a scaffolding function, simultaneously binding both Rem2 and AID. Previous studies have found that in addition to Ca(V)beta binding, Rem2 must be localized to the plasma membrane to inhibit VDCC function. Plasma membrane localization requires the C-terminus of Rem2 and binding studies indicate that this domain directs phosphorylated phosphatidylinositide (PIP) lipids association. Plasma membrane localization may provide a unique point of regulation since the ability of Rem2 to bind PIP lipids is inhibited by the phosphoserine dependant binding of 14-3-3 proteins. Thus, in addition to Ca(V)beta binding, VDCC blockade by Rem2 is likely to be controlled by both the localized concentration of membrane PIP lipids and direct 14-3-3 binding to the Rem2 C-terminus.

Rem inhibits skeletal muscle EC coupling by reducing the number of functional L-type Ca2+ channels

In skeletal muscle, the L-type voltage-gated Ca(2+) channel (1,4-dihydropyridine receptor) serves as the voltage sensor for excitation-contraction (EC) coupling. In this study, we examined the effects of Rem, a member of the RGK (Rem, Rem2, Rad, Gem/Kir) family of Ras-related monomeric GTP-binding proteins, on the function of the skeletal muscle L-type Ca(2+) channel. EC coupling was found to be weakened in myotubes expressing Rem tagged with enhanced yellow fluorescent protein (YFP-Rem), as assayed by electrically evoked contractions and myoplasmic Ca(2+) transients. This impaired EC coupling was not a consequence of altered function of the type 1 ryanodine receptor, or of reduced Ca(2+) stores, since the application of 4-chloro-m-cresol, a direct type 1 ryanodine receptor activator, elicited myoplasmic Ca(2+) release in YFP-Rem-expressing myotubes that was not distinguishable from that in control myotubes. However, YFP-Rem reduced the magnitude of L-type Ca(2+) current by approximately 75% and produced a concomitant reduction in membrane-bound charge movements. Thus, our results indicate that Rem negatively regulates skeletal muscle EC coupling by reducing the number of functional L-type Ca(2+) channels in the plasma membrane.

Plasma membrane targeting is essential for Rem-mediated Ca2+ channel inhibition

The small GTPase Rem is a potent negative regulator of high voltage-activated Ca(2+) channels and a known interacting partner for Ca(2+) channel accessory beta subunits. The mechanism for Rem-mediated channel inhibition remains controversial, although it has been proposed that Ca(V)beta association is required. Previous work has shown that a C-terminal truncation of Rem (Rem-(1-265)) displays reduced in vivo binding to membrane-localized beta 2a and lacks channel regulatory function. In this paper, we describe a role for the Rem C terminus in plasma membrane localization through association with phosphatidylinositol lipids. Moreover, Rem-(1-265) can associate with beta 2a in vitro and beta 1b in vivo, suggesting that the C terminus does not directly participate in Ca(V)beta association. Despite demonstrated beta 1b binding, Rem-(1-265) was not capable of regulating a Ca(V)1.2-beta 1b channel complex, indicating that beta subunit binding is not sufficient for channel regulation. However, fusion of the CAAX domain from K-Ras4B or H-Ras to the Rem-(1-265) C terminus restored membrane localization and Ca(2+) channel regulation, suggesting that beta binding and membrane localization are independent events required for channel inhibition.

Trafficking and regulation of neuronal voltage-gated calcium channels

The importance of voltage-gated calcium channels is underscored by the multitude of intracellular processes that depend on calcium, notably gene regulation and neurotransmission. Given their pivotal roles in calcium (and hence, cellular) homeostasis, voltage-gated calcium channels have been the subject of intense research, much of which has focused on channel regulation. While ongoing research continues to delineate the myriad of interactions that govern calcium channel regulation, an increasing amount of work has focused on the trafficking of voltage-gated calcium channels. This includes the mechanisms by which calcium channels are targeted to the plasma membrane, and, more specifically, to their appropriate loci within a given cell. In addition, we are beginning to gain some insights into the mechanisms by which calcium channels can be removed from the plasma membrane for recycling and/or degradation. Here we highlight recent advances in our understanding of these fundamentally important mechanisms.

Dominant Negative Suppression of Rad Leads to QT Prolongation and Causes Ventricular Arrhythmias via Modulation of L-type Ca 2+ Channels in the Heart

Disorders of L-type Ca 2+ channels can cause severe cardiac arrhythmias. A subclass of small GTP-binding proteins, the RGK family, regulates L-type Ca 2+ current ( I Ca,L ) in heterologous expression systems. Among these proteins, Rad (Ras associated with diabetes) is highly expressed in the heart, although its role in the heart remains unknown. Here we show that overexpression of dominant negative mutant Rad (S105N) led to an increase in I Ca,L and action potential prolongation via upregulation of L-type Ca 2+ channel expression in the plasma membrane of guinea pig ventricular cardiomyocytes. To verify the in vivo physiological role of Rad in the heart, a mouse model of cardiac-specific Rad suppression was created by overexpressing S105N Rad, using the α-myosin heavy chain promoter. Microelectrode studies revealed that action potential duration was significantly prolonged with visible identification of a small plateau phase in S105N Rad transgenic mice, when compared with wild-type littermate mice. Telemetric electrocardiograms on unrestrained mice revealed that S105N Rad transgenic mice had significant QT prolongation and diverse arrhythmias such as sinus node dysfunction, atrioventricular block, and ventricular extrasystoles, whereas no arrhythmias were observed in wild-type mice. Furthermore, administration of epinephrine induced frequent ventricular extrasystoles and ventricular tachycardia in S105N Rad transgenic mice. This study provides novel evidence that the suppression of Rad activity in the heart can induce ventricular tachycardia, suggesting that the Rad-associated signaling pathway may play a role in arrhythmogenesis in diverse cardiac diseases.

Nuclear localization of endogenous RGK proteins and modulation of cell shape remodeling by regulated nuclear transport

The members of the RGK small GTP-binding protein family, Kir/Gem, Rad, Rem and Rem2, are multifunctional proteins that regulate voltage-gated calcium channel activity and cell shape remodeling. Calmodulin (CaM) or CaM 14-3-3 are regulators of RGK functions and their association defines the subcellular localization of RGK proteins. Abolition of CaM association results in the accumulation of RGK proteins in the nucleus, whereas 14-3-3 binding maintains them in the cytoplasm. Kir/Gem possesses nuclear localization signals (NLS) that mediate nuclear accumulation through an importin alpha5-dependent pathway (see Mahalakshmi RN, Nagashima K, Ng MY, Inagaki N, Hunziker W, Béguin P. Nuclear transport of Kir/Gem requires specific signals and importin alpha5 and is regulated by Calmodulin and predicted service phosphorylations. Traffic 2007; doi: 10.1111/j.1600-0854.2007.00598.x). Because the extent of nuclear localization depends on the RGK protein and the cell type, the mechanism and regulation of nuclear transport may differ. Here, we extend our analysis to the other RGK members and show that Rem also binds importin alpha5, whereas Rad associates with importins alpha3, alpha5 and beta through three conserved NLS. Predicted phosphorylation of a serine residue within the bipartite NLS affects, as observed for Kir/Gem, nuclear accumulation of Rem, but not that of Rad or Rem2. We also identify an additional regulatory phosphorylation for all RGK proteins that prevents binding of 14-3-3 and thereby interferes with their cytosolic relocalization by 14-3-3. Functionally, nuclear localization of RGK proteins contributes to the suppression of RGK-mediated cell shape remodeling. Importantly, we show that endogenous RGK proteins are localized predominantly in the nucleus of individual cells of the brain cortex 'in situ' as well as in primary hippocampal cells, indicating that transport between the nucleus and their site of action in the cytoplasm (i.e., cytoskeleton, endoplasmic reticulum or plasma membrane) is of physiological relevance for the regulation of RGK protein function.

Nuclear Transport of Kir/Gem Requires Specific Signals and Importin α5 and Is Regulated by Calmodulin and Predicted Serine Phosphorylations

Kir/Gem, together with Rad, Rem and Rem2, is a member of the RGK small GTP-binding protein family. These multifunctional proteins regulate voltage-gated calcium channel (VGCC) activity and cell-shape remodeling. Calmodulin and 14-3-3 binding modulate the functions of RGK proteins. Intriguingly, abolishing the binding of calmodulin or calmodulin and 14-3-3 results in nuclear accumulation of RGK proteins. Under certain conditions, the Ca(v)beta3-subunit of VGCCs can be translocated into the nucleus along with the RGK proteins, resulting in channel inactivation. The mechanism by which nuclear localization of RGK proteins is accomplished and regulated, however, is unknown. Here, we identify specific nuclear localization signals (NLS) in Kir/Gem that are both required and sufficient for nuclear transport. Importin alpha5 binds to Kir/Gem, and its depletion using RNA interference impairs nuclear translocation of this RGK protein. Calmodulin and predicted phosphorylations on serine residues within or in the vicinity of a C-terminal bipartite NLS regulate nuclear translocation by interfering with the association between importinalpha5 and Kir/Gem. These predicted phosphorylations, however, do not affect Kir/Gem-mediated calcium channel downregulation but rather, as shown in the accompanying paper (Mahalakshmi RN, Ng MY, Guo K, Qi Z, Hunziker W, Béguin P. Nuclear localization of endogenous RGK proteins and modulation of cell shape remodeling by regulated nuclear transport. Traffic 2007; doi:10.1111/j.1600-0854.2007.00599.x), interfere with cell-shape remodeling.

An RNAi-based approach identifies molecules required for glutamatergic and GABAergic synapse development

We report the results of a genetic screen to identify molecules important for synapse formation and/or maintenance. siRNAs were used to decrease the expression of candidate genes in neurons, and synapse development was assessed. We surveyed 22 cadherin family members and demonstrated distinct roles for cadherin-11 and cadherin-13 in synapse development. Our screen also revealed roles for the class 4 Semaphorins Sema4B and Sema4D in the development of glutamatergic and/or GABAergic synapses. We found that Sema4D affects the formation of GABAergic, but not glutamatergic, synapses. Our screen also identified the activity-regulated small GTPase Rem2 as a regulator of synapse development. A known calcium channel modulator, Rem2 may function as part of a homeostatic mechanism that controls synapse number. These experiments establish the feasibility of RNAi screens to characterize the mechanisms that control mammalian neuronal development and to identify components of the genetic program that regulate synapse formation and/or maintenance.

RGK small GTP-binding proteins interact with the nucleotide kinase domain of Ca2+-channel beta-subunits via an uncommon effector binding domain

RGK proteins (Kir/Gem, Rad, Rem, and Rem2) form a small subfamily of the Ras superfamily. Despite a conserved GTP binding core domain, several differences suggest that structure, mechanism of action, and functional regulation differ from Ras. RGK proteins down-regulate voltage-gated calcium channel activity by binding in a GTP-dependent fashion to the Cavbeta subunits. Mutational analysis combined with homology modeling reveal a novel effector binding mechanism distinct from that of other Ras GTPases. In this model the Switch 1 region acts as an allosteric activator that facilitates electrostatic interactions between Arg-196 in Kir/Gem and Asp-194, -270, and -272 in the nucleotide-kinase (NK) domain of Cavbeta3 and wedging Val-223 and His-225 of Kir/Gem into a hydrophobic pocket in the NK domain. Kir/Gem interacts with a surface on the NK domain that is distinct from the groove where the voltage-gated calcium channel Cavalpha1 subunit binds. A complex composed of the RGK protein and the Cavbeta3 and Cav1.2 subunits could be revealed in vivo using coimmunoprecipitation experiments. Intriguingly, docking of the RGK protein to the NK domain of the Cavbeta subunit is reminiscent of the binding of GMP to guanylate kinase.

Dose-dependent and isoform-specific modulation of Ca2+ channels by RGK GTPases

Although inhibition of voltage-gated calcium channels by RGK GTPases (RGKs) represents an important mode of regulation to control Ca²⁺ influx in excitable cells, their exact mechanism of inhibition remains controversial. This has prevented an understanding of how RGK regulation can be significant in a physiological context. Here we show that RGKs—Gem, Rem, and Rem2—decreased Ca_V1.2 Ca²⁺ current amplitude in a dose-dependent manner. Moreover, Rem2, but not Rem or Gem, produced dose-dependent alterations on gating kinetics, uncovering a new mode by which certain RGKs can precisely modulate Ca²⁺ currents and affect Ca²⁺ influx during action potentials. To explore how RGKs influence gating kinetics, we separated the roles mediated by the Ca²⁺ channel accessory β subunit's interaction with its high affinity binding site in the pore-forming α_1C subunit (AID) from its other putative contact sites by utilizing an α_1C•β3 concatemer in which the AID was mutated to prevent β subunit interaction. This mutant concatemer generated currents with all the hallmarks of β subunit modulation, demonstrating that AID-β–independent interactions are sufficient for β subunit modulation. Using this construct we found that although inhibition of current amplitude was still partially sensitive to RGKs, Rem2 no longer altered gating kinetics, implicating different determinants for this specific mode of Rem2-mediated regulation. Together, these results offer new insights into the molecular mechanism of RGK-mediated Ca²⁺ channel current modulation.

Presynaptic Ca2+ channels--integration centers for neuronal signaling pathways

Calcium influx into presynaptic nerve terminals via voltage-gated Ca2+ channels is an essential step in neurotransmitter release. The predominant Ca2+ channel species in synaptic nerve terminals are P/Q-type and N-type channels, with their relative levels of expression varying across the nervous system. The different distributions of these two channel subtypes are reflected in their distinct physiological and pathological roles, yet their activity is regulated by common mechanisms and both function as part of larger signaling complexes that enable their precise regulation and subcellular targeting. Here, we provide a broad overview of molecular and cellular mechanisms that regulate Ca2+ channels, and how these cellular signaling pathways are integrated at the level of the channel protein.

Analysis of the complex between Ca2+ channel beta-subunit and the Rem GTPase

Voltage-gated calcium channels are multiprotein complexes that regulate calcium influx and are important contributors to cardiac excitability and contractility. The auxiliary beta-subunit (CaV beta) binds a conserved domain (the alpha-interaction domain (AID)) of the pore-forming CaV alpha1 subunit to modulate channel gating properties and promote cell surface trafficking. Recently, members of the RGK family of small GTPases (Rem, Rem2, Rad, Gem/Kir) have been identified as novel contributors to the regulation of L-type calcium channel activity. Here, we describe the Rem-association domain within CaV beta2a. The Rem interaction module is located in a approximately 130-residue region within the highly conserved guanylate kinase domain that also directs AID binding. Importantly, CaV beta mutants were identified that lost the ability to bind AID but retained their association with Rem, indicating that the AID and Rem association sites of CaV beta2a are structurally distinct. In vitro binding studies indicate that the affinity of Rem for CaV beta2a interaction is lower than that of AID for CaV beta2a. Furthermore, in vitro binding studies indicate that Rem association does not inhibit the interaction of CaV beta2a with AID. Instead, CaV beta can simultaneously associate with both Rem and CaV alpha1-AID. Previous studies had suggested that RGK proteins may regulate Ca2+ channel activity by blocking the association of CaV beta subunits with CaV alpha1 to inhibit plasma membrane trafficking. However, surface biotinylation studies in HIT-T15 cells indicate that Rem can acutely modulate channel function without decreasing the density of L-type channels at the plasma membrane. Together these data suggest that Rem-dependent Ca2+ channel modulation involves formation of a Rem x CaV beta x AID regulatory complex without the need to disrupt CaV alpha1 x CaV beta association or alter CaV alpha1 expression at the plasma membrane.

L-type calcium channel alpha-subunit and protein kinase inhibitors modulate Rem-mediated regulation of current

Cardiac voltage-gated L-type Ca channels (Ca(V)) are multiprotein complexes, including accessory subunits such as Ca(V)beta2 that increase current expression. Recently, members of the Rad and Gem/Kir-related family of small GTPases have been shown to decrease current, although the mechanism remains poorly defined. In this study, we evaluated the contribution of the L-type Ca channel alpha-subunit (Ca(V)1.2) to Ca(V)beta2-Rem inhibition of Ca channel current. Specifically, we addressed whether protein kinase A (PKA) modulation of the Ca channel modifies Ca(V)beta2-Rem inhibition of Ca channel current. We first tested the effect of Rem on Ca(V)1.2 in human embryonic kidney 293 (HEK-293) cells using the whole cell patch-clamp configuration. Rem coexpression with Ca(V)1.2 reduces Ba current expression under basal conditions, and Ca(V)beta2a coexpression enhances Rem block of Ca(V)1.2 current. Surprisingly, PKA inhibition by 133 nM H-89 or 50 microM Rp-cAMP-S partially relieved the Rem-mediated inhibition of current activity both with and without Ca(V)beta2a. To test whether the H-89 action was a consequence of the phosphorylation status of Ca(V)1.2, we examined Rem regulation of the PKA-insensitive Ca(V)1.2 serine 1928 (S1928) to alanine mutation (Ca(V)1.2-S1928A). Ca(V)1.2-S1928A current was not inhibited by Rem and when coexpression with Ca(V)beta2a was not completely blocked by Rem coexpression, suggesting that the phosphorylation of S1928 contributes to Rem-mediated Ca channel modulation. As a model for native Ca channel complexes, we tested the ability of Rem overexpression in HIT-T15 cells and embryonic ventricular myocytes to interfere with native current. We find that native current is also sensitive to Rem block and that H-89 pretreatment relieves the ability of Rem to regulate Ca current. We conclude that Rem is capable of regulating L-type current, that release of Rem block is modulated by cellular kinase pathways, and that the Ca(V)1.2 COOH terminus contributes to Rem-dependent channel inhibition.

Overexpression of Rad in muscle worsens diet-induced insulin resistance and glucose intolerance and lowers plasma triglyceride level

Rad is a low molecular weight GTPase that is overexpressed in skeletal muscle of some patients with type 2 diabetes mellitus and/or obesity. Overexpression of Rad in adipocytes and muscle cells in culture results in diminished insulin-stimulated glucose uptake. To further elucidate the potential role of Rad in vivo, we have generated transgenic (tg) mice that overexpress Rad in muscle using the muscle creatine kinase (MCK) promoter-enhancer. Rad tg mice have a 6- to 12-fold increase in Rad expression in muscle as compared to wild-type littermates. Rad tg mice grow normally and have normal glucose tolerance and insulin sensitivity, but have reduced plasma triglyceride levels. On a high-fat diet, Rad tg mice develop more severe glucose intolerance than the wild-type mice; this is due to increased insulin resistance in muscle, as exemplified by a rightward shift in the dose-response curve for insulin stimulated 2-deoxyglucose uptake. There is also a unexpected further reduction of the plasma triglyceride levels that is associated with increased levels of lipoprotein lipase in the Rad tg mice. These results demonstrate a potential synergistic interaction between increased expression of Rad and high-fat diet in creation of insulin resistance and altered lipid metabolism present in type 2 diabetes.