Proinsulin and the Genetics of Diabetes Mellitus

Insulin plays a central role in the regulation of vertebrate metabolism. The hormone, the post-translational product of a single-chain precursor, is a globular protein containing two chains, A (21 residues) and B (30 residues). Recent advances in human genetics have identified dominant mutations in the insulin gene causing permanent neonatal-onset DM² (1–4). The mutations are predicted to block folding of the precursor in the ER of pancreatic β-cells. Although expression of the wild-type allele would in other circumstances be sufficient to maintain homeostasis, studies of a corresponding mouse model (5–7) suggest that the misfolded variant perturbs wild-type biosynthesis (8, 9). Impaired β-cell secretion is associated with ER stress, distorted organelle architecture, and cell death (10). These findings have renewed interest in insulin biosynthesis (11–13) and the structural basis of disulfide pairing (14–19). Protein evolution is constrained not only by structure and function but also by susceptibility to toxic misfolding.Insulin plays a central role in the regulation of vertebrate metabolism. The hormone, the post-translational product of a single-chain precursor, is a globular protein containing two chains, A (21 residues) and B (30 residues). Recent advances in human genetics have identified dominant mutations in the insulin gene causing permanent neonatal-onset DM² (1–4). The mutations are predicted to block folding of the precursor in the ER of pancreatic β-cells. Although expression of the wild-type allele would in other circumstances be sufficient to maintain homeostasis, studies of a corresponding mouse model (5–7) suggest that the misfolded variant perturbs wild-type biosynthesis (8, 9). Impaired β-cell secretion is associated with ER stress, distorted organelle architecture, and cell death (10). These findings have renewed interest in insulin biosynthesis (11–13) and the structural basis of disulfide pairing (14–19). Protein evolution is constrained not only by structure and function but also by susceptibility to toxic misfolding.     

Printor, a Novel TorsinA-interacting Protein Implicated in Dystonia Pathogenesis

Early onset generalized dystonia (DYT1) is an autosomal dominant neurological disorder caused by deletion of a single glutamate residue (torsinA ΔE) in the C-terminal region of the AAA⁺ (ATPases associated with a variety of cellular activities) protein torsinA. The pathogenic mechanism by which torsinA ΔE mutation leads to dystonia remains unknown. Here we report the identification and characterization of a 628-amino acid novel protein, printor, that interacts with torsinA. Printor co-distributes with torsinA in multiple brain regions and co-localizes with torsinA in the endoplasmic reticulum. Interestingly, printor selectively binds to the ATP-free form but not to the ATP-bound form of torsinA, supporting a role for printor as a cofactor rather than a substrate of torsinA. The interaction of printor with torsinA is completely abolished by the dystonia-associated torsinA ΔE mutation. Our findings suggest that printor is a new component of the DYT1 pathogenic pathway and provide a potential molecular target for therapeutic intervention in dystonia.Early onset generalized torsion dystonia (DYT1) is the most common and severe form of hereditary dystonia, a movement disorder characterized by involuntary movements and sustained muscle spasms (1). This autosomal dominant disease has childhood onset and its dystonic symptoms are thought to result from neuronal dysfunction rather than neurodegeneration (2, 3). Most DYT1 cases are caused by deletion of a single glutamate residue at positions 302 or 303 (torsinA ΔE) of the 332-amino acid protein torsinA (4). In addition, a different torsinA mutation that deletes amino acids Phe³²³–Tyr³²⁸ (torsinA Δ323–328) was identified in a single family with dystonia (5), although the pathogenic significance of this torsinA mutation is unclear because these patients contain a concomitant mutation in another dystonia-related protein, ϵ-sarcoglycan (6). Recently, genetic association studies have implicated polymorphisms in the torsinA gene as a genetic risk factor in the development of adult-onset idiopathic dystonia (7, 8).TorsinA contains an N-terminal endoplasmic reticulum (ER)³ signal sequence and a 20-amino acid hydrophobic region followed by a conserved AAA⁺ (ATPases associated with a variety of cellular activities) domain (9, 10). Because members of the AAA⁺ family are known to facilitate conformational changes in target proteins (11, 12), it has been proposed that torsinA may function as a molecular chaperone (13, 14). TorsinA is widely expressed in brain and multiple other tissues (15) and is primarily associated with the ER and nuclear envelope (NE) compartments in cells (16–20). TorsinA is believed to mainly reside in the lumen of the ER and NE (17–19) and has been shown to bind lamina-associated polypeptide 1 (LAP1) (21), lumenal domain-like LAP1 (LULL1) (21), and nesprins (22). In addition, recent evidence indicates that a significant pool of torsinA exhibits a topology in which the AAA⁺ domain faces the cytoplasm (20). In support of this topology, torsinA is found in the cytoplasm, neuronal processes, and synaptic terminals (2, 3, 15, 23–26) and has been shown to bind cytosolic proteins snapin (27) and kinesin light chain 1 (20). TorsinA has been proposed to play a role in several cellular processes, including dopaminergic neurotransmission (28–31), NE organization and dynamics (17, 22, 32), and protein trafficking (27, 33). However, the precise biological function of torsinA and its regulation remain unknown.To gain insights into torsinA function, we performed yeast two-hybrid screens to search for torsinA-interacting proteins in the brain. We report here the isolation and characterization of a novel protein named printor (protein interactor of torsinA) that interacts selectively with wild-type (WT) torsinA but not the dystonia-associated torsinA ΔE mutant. Our data suggest that printor may serve as a cofactor of torsinA and provide a new molecular target for understanding and treating dystonia.     

PTIP Regulates 53BP1 and SMC1 at the DNA Damage Sites

Although PTIP is implicated in the DNA damage response, through interactions with 53BP1, the function of PTIP in the DNA damage response remain elusive. Here, we show that RNF8 controls DNA damage-induced nuclear foci formation of PTIP, which in turn regulates 53BP1 localization to the DNA damage sites. In addition, SMC1, a substrate of ATM, could not be phosphorylated at the DNA damage sites in the absence of PTIP. The PTIP-dependent pathway is important for DNA double strand breaks repair and DNA damage-induced intra-S phase checkpoint activation. Taken together, these results suggest that the role of PTIP in the DNA damage response is downstream of RNF8 and upstream of 53BP1. Thus, PTIP regulates 53BP1-dependent signaling pathway following DNA damage.The DNA damage response pathways are signal transduction pathways with DNA damage sensors, mediators, and effectors, which are essential for maintaining genomic stability (1–3). Following DNA double strand breaks, histone H2AX at the DNA damage sites is rapidly phosphorylated by ATM/ATR/DNAPK (4–10), a family homologous to phosphoinositide 3-kinases (11, 12). Subsequently, phospho-H2AX (γH2AX) provides the platform for accumulation of a larger group of DNA damage response factors, such as MDC1, BRCA1, 53BP1, and the MRE11·RAD50·NBS1 complex (13, 14), at the DNA damage sites. Translocalization of these proteins to the DNA double strand breaks (DSBs)³ facilitates DNA damage checkpoint activation and enhances the efficiency of DNA damage repair (14, 15).Recently, PTIP (Pax2 transactivation domain-interacting protein, or Paxip) has been identified as a DNA damage response protein and is required for cell survival when exposed to ionizing radiation (IR) (1, 16–18). PTIP is a 1069-amino acid nuclear protein and has been originally identified in a yeast two-hybrid screening as a partner of Pax2 (19). Genetic deletion of the PTIP gene in mice leads to early embryonic lethality at embryonic day 8.5, suggesting that PTIP is essential for early embryonic development (20). Structurally, PTIP contains six tandem BRCT (BRCA1 carboxyl-terminal) domains (16–18, 21). The BRCT domain is a phospho-group binding domain that mediates protein-protein interactions (17, 22, 23). Interestingly, the BRCT domain has been found in a large number of proteins involved in the cellular response to DNA damages, such as BRCA1, MDC1, and 53BP1 (7, 24–29). Like other BRCT domain-containing proteins, upon exposure to IR, PTIP forms nuclear foci at the DSBs, which is dependent on its BRCT domains (16–18). By protein affinity purification, PTIP has been found in two large complexes. One includes the histone H3K4 methyltransferase ALR and its associated cofactors, the other contains DNA damage response proteins, including 53BP1 and SMC1 (30, 31). Further experiments have revealed that DNA damage enhances the interaction between PTIP and 53BP1 (18, 31).To elucidate the DNA damage response pathways, we have examined the upstream and downstream partners of PTIP. Here, we report that PTIP is downstream of RNF8 and upstream of 53BP1 in response to DNA damage. Moreover, PTIP and 53BP1 are required for the phospho-ATM association with the chromatin, which phosphorylates SMC1 at the DSBs. This PTIP-dependent pathway is involved in DSBs repair.     

Loop 2 Structure in Glycine and GABAA Receptors Plays a Key Role in Determining Ethanol Sensitivity

The present study tests the hypothesis that the structure of extracellular domain Loop 2 can markedly affect ethanol sensitivity in glycine receptors (GlyRs) and γ-aminobutyric acid type A receptors (GABA_ARs). To test this, we mutated Loop 2 in the α1 subunit of GlyRs and in the γ subunit of α1β2γ2GABA_ARs and measured the sensitivity of wild type and mutant receptors expressed in Xenopus oocytes to agonist, ethanol, and other agents using two-electrode voltage clamp. Replacing Loop 2 of α1GlyR subunits with Loop 2 from the δGABA_AR (δL2), but not the γGABA_AR subunit, reduced ethanol threshold and increased the degree of ethanol potentiation without altering general receptor function. Similarly, replacing Loop 2 of the γ subunit of GABA_ARs with δL2 shifted the ethanol threshold from 50 mm in WT to 1 mm in the GABA_A γ-δL2 mutant. These findings indicate that the structure of Loop 2 can profoundly affect ethanol sensitivity in GlyRs and GABA_ARs. The δL2 mutations did not affect GlyR or GABA_AR sensitivity, respectively, to Zn²⁺ or diazepam, which suggests that these δL2-induced changes in ethanol sensitivity do not extend to all allosteric modulators and may be specific for ethanol or ethanol-like agents. To explore molecular mechanisms underlying these results, we threaded the WT and δL2 GlyR sequences onto the x-ray structure of the bacterial Gloeobacter violaceus pentameric ligand-gated ion channel homologue (GLIC). In addition to being the first GlyR model threaded on GLIC, the juxtaposition of the two structures led to a possible mechanistic explanation for the effects of ethanol on GlyR-based on changes in Loop 2 structure.Alcohol abuse and dependence are significant problems in our society, with ∼14 million people in the United States being affected (1, 2). Alcohol causes over 100,000 deaths in the United States, and alcohol-related issues are estimated to cost nearly 200 billion dollars annually (2). To address this, considerable attention has focused on the development of medications to prevent and treat alcohol-related problems (3–5). The development of such medications would be aided by a clear understanding of the molecular structures on which ethanol acts and how these structures influence receptor sensitivity to ethanol.Ligand-gated ion channels (LGICs)² have received substantial attention as putative sites of ethanol action that cause its behavioral effects (6–12). Research in this area has focused on investigating the effects of ethanol on two large superfamilies of LGICs: 1) the Cys-loop superfamily of LGICs (13, 14), whose members include nicotinic acetylcholine, 5-hydroxytryptamine₃, γ-aminobutyric acid type A (GABA_A), γ-aminobutyric acid type C, and glycine receptors (GlyRs) (10, 11, 15–20) and 2) the glutamate superfamily, including N-methyl d-aspartate, α-amino-3-hydroxyisoxazolepropionic acid, and kainate receptors (21, 22). Recent studies have also begun investigating ethanol action in the ATP-gated P2X superfamily of LGICs (23–25).A series of studies that employed chimeric and mutagenic strategies combined with sulfhydryl-specific labeling identified key regions within Cys-loop receptors that appear to be initial targets for ethanol action that also can determine the sensitivity of the receptors to ethanol (7–12, 18, 19, 26–30). This work provides several lines of evidence that position 267 and possibly other sites in the transmembrane (TM) domain of GlyRs and homologous sites in GABA_ARs are targets for ethanol action and that mutations at these sites can influence ethanol sensitivity (8, 9, 26, 31).Growing evidence from GlyRs indicates that ethanol also acts on the extracellular domain. The initial findings came from studies demonstrating that α1GlyRs are more sensitive to ethanol than are α2GlyRs despite the high (∼78%) sequence homology between α1GlyRs and α2GlyRs (32). Further work found that an alanine to serine exchange at position 52 (A52S) in Loop 2 can eliminate the difference in ethanol sensitivity between α1GlyRs and α2GlyRs (18, 20, 33). These studies also demonstrated that mutations at position 52 in α1GlyRS and the homologous position 59 in α2GlyRs controlled the sensitivity of these receptors to a novel mechanistic ethanol antagonist (20). Collectively, these studies suggest that there are multiple sites of ethanol action in α1GlyRs, with one site located in the TM domain (e.g. position 267) and another in the extracellular domain (e.g. position 52).Subsequent studies revealed that the polarity of the residue at position 52 plays a key role in determining the sensitivity of GlyRs to ethanol (20). The findings with polarity in the extracellular domain contrast with the findings at position 267 in the TM domain, where molecular volume, but not polarity, significantly affected ethanol sensitivity (9). Taken together, these findings indicate that the physical-chemical parameters of residues at positions in the extracellular and TM domains that modulate ethanol effects and/or initiate ethanol action in GlyRs are not uniform. Thus, knowledge regarding the physical-chemical properties that control agonist and ethanol sensitivity is key for understanding the relationship between the structure and the actions of ethanol in LGICs (19, 31, 34–40).GlyRs and GABA_ARs, which differ significantly in their sensitivities to ethanol, offer a potential method for identifying the structures that control ethanol sensitivity. For example, α1GlyRs do not reliably respond to ethanol concentrations less than 10 mm (32, 33, 41). Similarly, γ subunit-containing GABA_ARs (e.g. α1β2γ2), the most predominantly expressed GABA_ARs in the central nervous system, are insensitive to ethanol concentrations less than 50 mm (42, 43). In contrast, δ subunit-containing GABA_ARs (e.g. α4β3δ) have been shown to be sensitive to ethanol concentrations as low as 1–3 mm (44–51). Sequence alignment of α1GlyR, γGABA_AR, and δGABA_AR revealed differences between the Loop 2 regions of these receptor subunits. Since prior studies found that mutations of Loop 2 residues can affect ethanol sensitivity (19, 20, 39), the non-conserved residues in Loop 2 of GlyR and GABA_AR subunits could provide the physical-chemical and structural bases underlying the differences in ethanol sensitivity between these receptors.The present study tested the hypothesis that the structure of Loop 2 can markedly affect the ethanol sensitivity of GlyRs and GABA_ARs. To accomplish this, we performed multiple mutations that replaced the Loop 2 region of the α1 subunit in α1GlyRs and the Loop 2 region of the γ subunit of α1β2γ2 GABA_ARs with corresponding non-conserved residues from the δ subunit of GABA_AR and tested the sensitivity of these receptors to ethanol. As predicted, replacing Loop 2 of WT α1GlyRs with the homologous residues from the δGABA_AR subunit (δL2), but not the γGABA_AR subunit (γL2), markedly increased the sensitivity of the receptor to ethanol. Similarly, replacing the non-conserved residues of the γ subunit of α1β2γ2 GABA_ARs with δL2 also markedly increased ethanol sensitivity of GABA_ARs. These findings support the hypothesis and suggest that Loop 2 may play a role in controlling ethanol sensitivity across the Cys-loop superfamily of receptors. The findings also provide the basis for suggesting structure-function relationships in a new molecular model of the GlyR based on the bacterial Gloeobacter violaceus pentameric LGIC homologue (GLIC).     

A PH Domain in the Arf GTPase-activating Protein (GAP) ARAP1 Binds Phosphatidylinositol 3,4,5-Trisphosphate and Regulates Arf GAP Activity Independently of Recruitment to the Plasma Membranes

ARAP1 is a phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P₃)-dependent Arf GTPase-activating protein (GAP) with five PH domains that regulates endocytic trafficking of the epidermal growth factor receptor (EGFR). Two tandem PH domains are immediately N-terminal of the Arf GAP domain, and one of these fits the consensus sequence for PtdIns(3,4,5)P₃ binding. Here, we tested the hypothesis that PtdIns(3,4,5)P₃-dependent recruitment mediated by the first PH domain of ARAP1 regulates the in vivo and in vitro function of ARAP1. We found that PH1 of ARAP1 specifically bound to PtdIns(3,4,5)P₃, but with relatively low affinity (≈1.6 μm), and the PH domains did not mediate PtdIns(3,4,5)P₃-dependent recruitment to membranes in cells. However, PtdIns(3,4,5)P₃ binding to the PH domain stimulated GAP activity and was required for in vivo function of ARAP1 as a regulator of endocytic trafficking of the EGFR. Based on these results, we propose a variation on the model for the function of phosphoinositide-binding PH domains. In our model, ARAP1 is recruited to membranes independently of PtdIns(3,4,5)P₃, the subsequent production of which triggers enzymatic activity.Pleckstrin homology (PH)² domains are a common structural motif encoded by the human genome (1, 2). Approximately 10% of PH domains bind to phosphoinositides. These PH domains are thought to mediate phosphoinositide-dependent recruitment to membranes (1–3). Most PH domains likely have functions other than or in addition to phosphoinositide binding. For example, PH domains have been found to bind to protein and DNA (4–12). In addition, some PH domains have been found to be structurally and functionally integrated with adjacent domains (13, 14). A small fraction of PH domain-containing proteins (about 9% of the human proteins) have multiple PH domains arranged in tandem, which have been proposed to function as adaptors but have only been examined in one protein (15, 16). Arf GTPase-activating proteins (GAPs) of the ARAP family are phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P₃)-dependent Arf GAPs with tandem PH domains (17, 18). The function of specific PH domains in regulating Arf GAP activity and for biologic activity has not been described.Arf GAPs are proteins that induce the hydrolysis of GTP bound to Arfs (19–23). The Arf proteins are members of the Ras superfamily of GTP-binding proteins (24–27). The six Arf proteins in mammals (five in humans) are divided into three classes based on primary sequence: Arf1, -2, and -3 are class 1, Arf4 and -5 are class 2, and Arf6 is class 3 (23, 24, 27–29). Class 1 and class 3 Arf proteins have been studied more extensively than class 2. They have been found to regulate membrane traffic and the actin cytoskeleton.The Arf GAPs are a family of proteins with diverse domain structures (20, 21, 23, 30). ARAPs, the most structurally complex of the Arf GAPs, contain, in addition to an Arf GAP domain, the sterile α motif (SAM), five PH, Rho GAP, and Ras association domains (17, 18, 31, 32). The first and second and the third and fourth PH domains are tandem (Fig. 1). The first and third PH domains of the ARAPs fit the consensus for PtdIns(3,4,5)P₃ binding (33–35). ARAPs have been found to affect actin and membrane traffic (21, 23). ARAP3 regulates growth factor-induced ruffling of porcine aortic endothelial cells (31, 36, 37). The function is dependent on the Arf GAP and Rho GAP domains. ARAP2 regulates focal adhesions, an actin cytoskeletal structure (17). ARAP2 function requires Arf GAP activity and a Rho GAP domain capable of binding RhoA·GTP. ARAP1 has been found to have a role in membrane traffic (18). The protein associates with pre-early endosomes involved in the attenuation of EGFR signals. The function of the tandem PH domains in the ARAPs has not been examined.Open in a separate windowFIGURE 1.ARAP1 binding to phospholipids. A, schematic of the recombinant proteins used in this study. Domain abbreviations: Ank, ankyrin repeat; PLCδ-PH, PH domain of phospholipase C δ; RA, Ras association motif; RhoGAP, Rho GTPase-activating domain. B, ARAP1 phosphoinositide binding specificity. 500 nm PH1-Ank recombinant protein was incubated with sucrose-loaded LUVs formed by extrusion through a 1-μm pore filter. LUVs contained PtdIns alone or PtdIns with 2.5 μm PtdIns(3,4,5)P₃, 2.5 μm PtdIns(3)P, 2.5 μm PtdIns(4)P, 2.5 μm PtdIns(5)P, 2.5 μm PtdIns(3,4)P₂, 2.5 μm PtdIns(3,5)P₂, or 2.5 μm PtdIns(4,5)P₂ with a total phosphoinositide concentration of 50 μm and a total phospholipid concentration of 500 μm. Vesicles were precipitated by ultracentrifugation, and associated proteins were separated by SDS-PAGE. The amount of precipitated protein was determined by densitometry of the Coomassie Blue-stained gels with standards on each gel. C, PtdIns(3,4,5)P₃-dependent binding of ARAP1 to LUVs. 1 μm PH1-Ank or ArfGAP-Ank recombinant protein was incubated with 1 mm sucrose-loaded LUVs formed by extrusion through a 1-μm pore size filter containing varying concentration of PtdIns(3,4,5)P₃. Precipitation of LUVs and analysis of associated proteins were performed as described in B. The average ± S.E. of three independent experiments is presented.Here we investigated the role of the first two PH domains of ARAP1 for catalysis and in vivo function. The first PH domain fits the consensus sequence for PtdIns(3,4,5)P₃ binding (33–35). The second does not fit a phosphoinositide binding consensus but is immediately N-terminal to the GAP domain. We have previously reported that the PH domain that occurs immediately N-terminal of the Arf GAP domain of ASAP1 is critical for the catalytic function of the protein (38, 39). We tested the hypothesis that the two PH domains of ARAP1 function independently; one recruits ARAP1 to PtdIns(3,4,5)P₃-rich membranes, and the other functions with the catalytic domain. As predicted, PH1 interacted specifically with PtdIns(3,4,5)P₃, and PH2 did not. However, both PH domains contributed to catalysis independently of recruitment to membranes. None of the PH domains in ARAP1 mediated PtdIns(3,4,5)P₃-dependent targeting to plasma membranes (PM). PtdIns(3,4,5)P₃ stimulated GAP activity, and the ability to bind PtdIns(3,4,5)P₃ was required for ARAP1 to regulate membrane traffic. We propose that ARAP1 is recruited independently of PtdIns(3,4,5)P₃ to the PM where PtdIns(3,4,5)P₃ subsequently regulates its GAP activity to control endocytic events.     

Repeated Domains of Leptospira Immunoglobulin-like Proteins Interact with Elastin and Tropoelastin

Leptospira spp., the causative agents of leptospirosis, adhere to components of the extracellular matrix, a pivotal role for colonization of host tissues during infection. Previously, we and others have shown that Leptospira immunoglobulin-like proteins (Lig) of Leptospira spp. bind to fibronectin, laminin, collagen, and fibrinogen. In this study, we report that Leptospira can be immobilized by human tropoelastin (HTE) or elastin from different tissues, including lung, skin, and blood vessels, and that Lig proteins can bind to HTE or elastin. Moreover, both elastin and HTE bind to the same LigB immunoglobulin-like domains, including LigBCon4, LigBCen7′–8, LigBCen9, and LigBCen12 as demonstrated by enzyme-linked immunosorbent assay (ELISA) and competition ELISAs. The LigB immunoglobulin-like domain binds to the 17th to 27th exons of HTE (17–27HTE) as determined by ELISA (LigBCon4, K_D = 0.50 μm; LigBCen7′–8, K_D = 0.82 μm; LigBCen9, K_D = 1.54 μm; and LigBCen12, K_D = 0.73 μm). The interaction of LigBCon4 and 17–27HTE was further confirmed by steady state fluorescence spectroscopy (K_D = 0.49 μm) and ITC (K_D = 0.54 μm). Furthermore, the binding was enthalpy-driven and affected by environmental pH, indicating it is a charge-charge interaction. The binding affinity of LigBCon4D341N to 17–27HTE was 4.6-fold less than that of wild type LigBCon4. In summary, we show that Lig proteins of Leptospira spp. interact with elastin and HTE, and we conclude this interaction may contribute to Leptospira adhesion to host tissues during infection.Pathogenic Leptospira spp. are spirochetes that cause leptospirosis, a serious infectious disease of people and animals (1, 2). Weil syndrome, the severe form of leptospiral infection, leads to multiorgan damage, including liver failure (jaundice), renal failure (nephritis), pulmonary hemorrhage, meningitis, abortion, and uveitis (3, 4). Furthermore, this disease is not only prevalent in many developing countries, it is reemerging in the United States (3). Although leptospirosis is a serious worldwide zoonotic disease, the pathogenic mechanisms of Leptospira infection remain enigmatic. Recent breakthroughs in applying genetic tools to Leptospira may facilitate studies on the molecular pathogenesis of leptospirosis (5–8).The attachment of pathogenic Leptospira spp. to host tissues is critical in the early phase of Leptospira infection. Leptospira spp. adhere to host tissues to overcome mechanical defense systems at tissue surfaces and to initiate colonization of specific tissues, such as the lung, kidney, and liver. Leptospira invade hosts tissues through mucous membranes or injured epidermis, coming in contact with subepithelial tissues. Here, certain bacterial outer surface proteins serve as microbial surface components recognizing adhesive matrix molecules (MSCRAMMs)² to mediate the binding of bacteria to different extracellular matrices (ECMs) of host cells (9). Several leptospiral MSCRAMMs have been identified (10–18), and we speculate that more will be identified in the near future.Lig proteins are distributed on the outer surface of pathogenic Leptospira, and the expression of Lig protein is only found in low passage strains (14, 16, 17), probably induced by environmental cues such as osmotic or temperature changes (19). Lig proteins can bind to fibrinogen and a variety of ECMs, including fibronectin (Fn), laminin, and collagen, thereby mediating adhesion to host cells (20–23). Lig proteins also constitute good vaccine candidates (24–26).Elastin is a component of ECM critical to tissue elasticity and resilience and is abundant in skin, lung, blood vessels, placenta, uterus, and other tissues (27–29). Tropoelastin is the soluble precursor of elastin (28). During the major phase of elastogenesis, multiple tropoelastin molecules associate through coacervation (30–32). Because of the abundance of elastin or tropoelastin on the surface of host cells, several bacterial MSCRAMMs use elastin and/or tropoelastin to mediate adhesion during the infection process (33–35).Because leptospiral infection is known to cause severe pulmonary hemorrhage (36, 37) and abortion (38), we hypothesize that some leptospiral MSCRAMMs may interact with elastin and/or tropoelastin in these elastin-rich tissues. This is the first report that Lig proteins of Leptospira interact with elastin and tropoelastin, and the interactions are mediated by several specific immunoglobulin-like domains of Lig proteins, including LigBCon4, LigBCen7′–8, LigBCen9, and LigBCen12, which bind to the 17th to 27th exons of human tropoelastin (HTE).     

Structure of Insoluble Rat Sperm Glyceraldehyde-3-phosphate Dehydrogenase (GAPDH) via Heterotetramer Formation with Escherichia coli GAPDH Reveals Target for Contraceptive Design

Sperm glyceraldehyde-3-phosphate dehydrogenase has been shown to be a successful target for a non-hormonal contraceptive approach, but the agents tested to date have had unacceptable side effects. Obtaining the structure of the sperm-specific isoform to allow rational inhibitor design has therefore been a goal for a number of years but has proved intractable because of the insoluble nature of both native and recombinant protein. We have obtained soluble recombinant sperm glyceraldehyde-3-phosphate dehydrogenase as a heterotetramer with the Escherichia coli glyceraldehyde-3-phosphate dehydrogenase in a ratio of 1:3 and have solved the structure of the heterotetramer which we believe represents a novel strategy for structure determination of an insoluble protein. A structure was also obtained where glyceraldehyde 3-phosphate binds in the P_s pocket in the active site of the sperm enzyme subunit in the presence of NAD. Modeling and comparison of the structures of human somatic and sperm-specific glyceraldehyde-3-phosphate dehydrogenase revealed few differences at the active site and hence rebut the long presumed structural specificity of 3-chlorolactaldehyde for the sperm isoform. The contraceptive activity of α-chlorohydrin and its apparent specificity for the sperm isoform in vivo are likely to be due to differences in metabolism to 3-chlorolactaldehyde in spermatozoa and somatic cells. However, further detailed analysis of the sperm glyceraldehyde-3-phosphate dehydrogenase structure revealed sites in the enzyme that do show significant difference compared with published somatic glyceraldehyde-3-phosphate dehydrogenase structures that could be exploited by structure-based drug design to identify leads for novel male contraceptives.Glyceraldehyde-3-phosphate dehydrogenase-S (GAPDS³ in rat; GAPDH2 in human) is the sperm-specific isoform of GAPDH (1–3) and the sole GAPDH enzyme in sperm. GAPDS is highly conserved between species showing 94% identity between rat and mouse and 87% identity between rat and human. Within a particular species, GAPDS also shows significant sequence identity to its GAPDH paralogue, 70, 70, and 68% for rat, mouse, and human, respectively. The most striking difference between GAPDS and GAPDH is the presence of an N-terminal polyproline region in GAPDS, which is 97 residues in rat (accession number {"type":"entrez-nucleotide","attrs":{"text":"AJ297631","term_id":"9931190","term_text":"AJ297631"}}AJ297631), 105 in mouse (3), and 72 in human (2). GAPDS is restricted to the principal piece of the sperm flagellum (1, 2, 4) where it is localized to the fibrous sheath (5), an association proposed to be mediated via the N-terminal polyproline extension.GAPDS first came to prominence as a contraceptive target during the 1970s (6–8). Investigations showed that treatment of sperm with α-chlorohydrin or a number of related compounds could inhibit GAPDS activity (9–11), sperm motility (9–13), and the fertilization of oocytes in vitro (14). The metabolite of these compounds, 3-chlorolactaldehyde (15–17), selectively inhibited GAPDS, having no effect on the activity of somatic cell GAPDH (18, 19), providing the specificity required for a potential contraceptive. Questions surrounding these particular compounds were raised when a number of side effects were evident from in vivo trials (7, 20–22); however, the design of small molecule inhibitors of GAPDS may provide a viable alternative. Its potential as a contraceptive target was supported by data from mice where GAPDS^−/− males (23) were infertile because of defects in sperm motility.Glyceraldehyde-3-phosphate dehydrogenases are tetrameric enzymes that catalyze the oxidative phosphorylation of d-glyceraldehyde 3-phosphate (Glc-3-P) into 1,3-diphosphoglycerate in the presence of an NAD cofactor via a two-step chemical mechanism (24). The first models of substrate binding were proposed on the basis of crystal structures of the holoenzyme from lobster (25) and Bacillus stearothermophilus (26), and Moras and co-workers (25) identified two anion-binding sites postulated to correspond to those binding the C-3 phosphate group of d-Glc-3-P (P_s site) and the inorganic phosphate ion (P_i site).Structure-based design of small molecules to inhibit GAPDH is not unprecedented. GAPDH has been targeted from protozoan parasites (27–30), as the bloodstream forms rely solely on glycolysis for energy production (31, 32). A number of mammalian GAPDH structures have also been solved, including rabbit muscle (33, 34), human liver (35), and human placenta (36); however, no structures are available for sperm-specific isoforms of this enzyme.Active heterotetramers of GAPDH between different species have been reported and biochemically characterized previously, both in ratios of 2:2 and 3:1 (37–40). In this study we have successfully obtained crystals of rat recombinant GAPDS as a heterotetramer with Escherichia coli GAPDH in a 1:3 ratio. To understand the basis of inhibition of the sperm isoform by substrate analogue 3-chlorolactaldehyde, a metabolite of α-chlorohydrin, a structure was also determined in the presence of the substrate glyceraldehyde 3-phosphate. The sperm-specific structure was compared with the human placental GAPDH structure (PDB entry 1U8F; Ref. 36) to identify differences that may provide a target for the design of inhibitors specific to the GAPDS protein. The unique structural features identified offer potential candidates for further investigation as inhibitor targets.     

Deletion of the Chloride Transporter Slc26a7 Causes Distal Renal Tubular Acidosis and Impairs Gastric Acid Secretion

SLC26A7 (human)/Slc26a7 (mouse) is a recently identified chloride-base exchanger and/or chloride transporter that is expressed on the basolateral membrane of acid-secreting cells in the renal outer medullary collecting duct (OMCD) and in gastric parietal cells. Here, we show that mice with genetic deletion of Slc26a7 expression develop distal renal tubular acidosis, as manifested by metabolic acidosis and alkaline urine pH. In the kidney, basolateral Cl⁻/HCO₃⁻ exchange activity in acid-secreting intercalated cells in the OMCD was significantly decreased in hypertonic medium (a normal milieu for the medulla) but was reduced only mildly in isotonic medium. Changing from a hypertonic to isotonic medium (relative hypotonicity) decreased the membrane abundance of Slc26a7 in kidney cells in vivo and in vitro. In the stomach, stimulated acid secretion was significantly impaired in isolated gastric mucosa and in the intact organ. We propose that SLC26A7 dysfunction should be investigated as a potential cause of unexplained distal renal tubular acidosis or decreased gastric acid secretion in humans.The collecting duct segment of the distal kidney nephron plays a major role in systemic acid base homeostasis by acid secretion and bicarbonate absorption. The acid secretion occurs via H⁺-ATPase and H-K-ATPase into the lumen and bicarbonate is absorbed via basolateral Cl⁻/HCO₃⁻ exchangers (1–4). The tubules, which are located within the outer medullary region of the kidney collecting duct (OMCD),² have the highest rate of acid secretion among the distal tubule segments and are therefore essential to the maintenance of acid base balance (2).The gastric parietal cell is the site of generation of acid and bicarbonate through the action of cytosolic carbonic anhydrase II (5, 6). The intracellular acid is secreted into the lumen via gastric H-K-ATPase, which works in conjunction with a chloride channel and a K⁺ recycling pathway (7–10). The intracellular bicarbonate is transported to the blood via basolateral Cl⁻/HCO₃⁻ exchangers (11–14).SLC26 (human)/Slc26 (mouse) isoforms are members of a conserved family of anion transporters that display tissue-specific patterns of expression in epithelial cells (15–24). Several SLC26 members can function as chloride/bicarbonate exchangers. These include SLC26A3 (DRA), SLC26A4 (pendrin), SLC26A6 (PAT1 or CFEX), SLC26A7, and SLC26A9 (25–31). SLC26A7 and SLC26A9 can also function as chloride channels (32–34).SLC26A7/Slc26a7 is predominantly expressed in the kidney and stomach (28, 29). In the kidney, Slc26a7 co-localizes with AE1, a well-known Cl⁻/HCO₃⁻ exchanger, on the basolateral membrane of (acid-secreting) A-intercalated cells in OMCD cells (29, 35, 36) (supplemental Fig. 1). In the stomach, Slc26a7 co-localizes with AE2, a major Cl⁻/HCO₃⁻ exchanger, on the basolateral membrane of acid secreting parietal cells (28). To address the physiological function of Slc26a7 in the intact mouse, we have generated Slc26a7 ko mice. We report here that Slc26a7 ko mice exhibit distal renal tubular acidosis and impaired gastric acidification in the absence of morphological abnormalities in kidney or stomach.     

Lysophosphatidic Acid Enhances Pulmonary Epithelial Barrier Integrity and Protects Endotoxin-induced Epithelial Barrier Disruption and Lung Injury

Lysophosphatidic acid (LPA), a bioactive phospholipid, induces a wide range of cellular effects, including gene expression, cytoskeletal rearrangement, and cell survival. We have previously shown that LPA stimulates secretion of pro- and anti-inflammatory cytokines in bronchial epithelial cells. This study provides evidence that LPA enhances pulmonary epithelial barrier integrity through protein kinase C (PKC) δ- and ζ-mediated E-cadherin accumulation at cell-cell junctions. Treatment of human bronchial epithelial cells (HBEpCs) with LPA increased transepithelial electrical resistance (TER) by ∼2.0-fold and enhanced accumulation of E-cadherin to the cell-cell junctions through Gα_i-coupled LPA receptors. Knockdown of E-cadherin with E-cadherin small interfering RNA or pretreatment with EGTA (0.1 mm) prior to LPA (1 μm) treatment attenuated LPA-induced increases in TER in HBEpCs. Furthermore, LPA induced tyrosine phosphorylation of focal adhesion kinase (FAK) and overexpression of the FAK inhibitor, and FAK-related non-kinase-attenuated LPA induced increases in TER and E-cadherin accumulation at cell-cell junctions. Overexpression of dominant negative protein kinase δ and ζ attenuated LPA-induced phosphorylation of FAK, accumulation of E-cadherin at cell-cell junctions, and an increase in TER. Additionally, lipopolysaccharide decreased TER and induced E-cadherin relocalization from cell-cell junctions to cytoplasm in a dose-dependent fashion, which was restored by LPA post-treatment in HBEpCs. Intratracheal post-treatment with LPA (5 μm) reduced LPS-induced neutrophil influx, protein leak, and E-cadherin shedding in bronchoalveolar lavage fluids in a murine model of acute lung injury. These data suggest a protective role of LPA in airway inflammation and remodeling.The airway epithelium is the site of first contact for inhaled environmental stimuli, functions as a physical barrier to environmental insult, and is an essential part of innate immunity. Epithelial barrier disruption is caused by inhaled allergens, dust, and irritants, resulting in inflammation, bronchoconstriction, and edema as seen in asthma and other respiratory diseases (1–4). Furthermore, increased epithelial permeability also results in para-cellular leakage of large proteins, such as albumin, immunoglobulin G, and polymeric immunoglobulin A, into the airway lumen (5, 6). The epithelial cell-cell junctional complex is composed of tight junctions, adherens junctions, and desmosomes. These adherens junctions play a pivotal role in regulating the activity of the entire junctional complex because the formation of adherens junctions subsequently leads to the formation of other cell-cell junctions (7–9). The major adhesion molecules in the adherens junctions are the cadherins. E-cadherin is a member of the cadherin family that mediates calcium-dependent cell-cell adhesion. The N-terminal ectodomain of E-cadherin contains homophilic interaction specificity, and the cytoplasmic domain binds to catenins, which interact with actin (10–13). Plasma membrane localization of E-cadherin is critical for the maintenance of epithelial cell-cell junctions and airway epithelium integrity (7, 10, 14). A decrease of adhesive properties of E-cadherin is related to the loss of differentiation and the subsequent acquisition of a higher motility and invasiveness of epithelial cells (10, 14, 15). Dislocation or shedding of E-cadherin in the airway epithelium induces epithelial shedding and increases airway permeability in lung airway diseases (10, 14, 16). In an ovalbumin-challenged guinea pig model of asthma, it has been demonstrated that E-cadherin is dislocated from the lateral margins of epithelial cells (10). Histamine increases airway para-cellular permeability and results in an increased susceptibility of airway epithelial cells to adenovirus infection by interrupting E-cadherin adhesion (14). Serine phosphorylation of E-cadherin by casein kinase II, GSK-3β, and PKD1/PKC² μ enhanced E-cadherin-mediated cell-cell adhesion in NIH3T3 fibroblasts and LNCaP prostate cancer cells (11, 17). However, the regulation and mechanism by which E-cadherin is localized within the pulmonary epithelium is not fully known, particularly during airway remodeling.LPA, a naturally occurring bioactive lipid, is present in body fluids, such as plasma, saliva, follicular fluid, malignant effusions, and bronchoalveolar lavage (BAL) fluids (18–20). Six distinct high affinity cell-surface LPA receptors, LPA-R_1–6, have been cloned and described in mammals (21–26). Extracellular activities of LPA include cell proliferation, motility, and cell survival (27–30). LPA exhibits a wide range of effects on differing cell types, including pulmonary epithelial, smooth muscle, fibroblasts, and T cells (31–35). LPA augments migration and cytokine synthesis in lymphocytes and induces chemotaxis of Jurkat T cells through Matrigel membranes (34). LPA induces airway smooth muscle cell contractility, proliferation, and airway repair and remodeling (35, 36). LPA also potently stimulates IL-8 (31, 37–39), IL-13 receptor α2 (IL-13Rα2) (40), and COX-2 gene expression and prostaglandin E₂ release (41) in HBEpCs. Prostaglandin E₂ and IL-13Rα2 have anti-inflammatory properties in pulmonary inflammation (42, 43). These results suggest that LPA may play a protective role in lung disease by stimulating an innate immune response while simultaneously attenuating the adaptive immune response. Furthermore, intravenous injection with LPA attenuated bacterial endotoxin-induced plasma tumor necrosis factor-α production and myeloperoxidase activity in the lungs of mice (44), suggesting an anti-inflammatory role for LPA in a murine model of sepsis.We have reported that LPA induces E-cadherin/c-Met accumulation in cell-cell contacts and increases TER in HBEpCs (45). Here, for the first time, we report that LPA-induced increases in TER are dependent on PKCδ, PKCζ, and FAK-mediated E-cadherin accumulation at cell-cell junctions. Furthermore, we demonstrate that post-treatment of LPA rescues LPS-induced airway epithelial disruption in vitro and reduces E-cadherin shedding in a murine model of ALI. This study identifies the molecular mechanisms linking the LPA and LPA receptors to maintaining normal pulmonary epithelium barrier function, which is critical in developing novel therapies directed at ameliorating pulmonary diseases.