Glucagon-like peptide-1 receptor signaling modifies the extent of diabetic kidney disease through dampening the receptor for advanced glycation end products-induced inflammation

Glucagon like peptide-1 (GLP-1) is a hormone produced and released by cells of the gastrointestinal tract following meal ingestion. GLP-1 receptor agonists (GLP-1RA) exhibit kidney-protective actions through poorly understood mechanisms. Here we interrogated whether the receptor for advanced glycation end products (RAGE) plays a role in mediating the actions of GLP-1 on inflammation and diabetic kidney disease. Mice with deletion of the GLP-1 receptor displayed an abnormal kidney phenotype that was accelerated by diabetes and improved with co-deletion of RAGE in vivo. Activation of the GLP-1 receptor pathway with liraglutide, an anti-diabetic treatment, downregulated kidney RAGE, reduced the expansion of bone marrow myeloid progenitors, promoted M2-like macrophage polarization and lessened markers of kidney damage in diabetic mice. Single cell transcriptomics revealed that liraglutide induced distinct transcriptional changes in kidney endothelial, proximal tubular, podocyte and macrophage cells, which were dominated by pathways involved in nutrient transport and utilization, redox sensing and the resolution of inflammation. The kidney-protective action of liraglutide was corroborated in a non-diabetic model of chronic kidney disease, the subtotal nephrectomised rat. Thus, our findings identify a novel glucose-independent kidney-protective action of GLP-1-based therapies in diabetic kidney disease and provide a valuable resource for exploring the cell-specific kidney transcriptional response ensuing from pharmacological GLP-1R agonism.

Structure, aggregation dynamics and crystallization of superfolder green fluorescent protein: Effect of long alkyl chain imidazolium ionic liquids

Green fluorescent protein (GFP) and its variants are widely used in medical and biological research, especially acting as indicators of protein structural integrity, protein-protein interactions and as biosensors. This study employs superfolder GFP (sfGFP) to investigate the impact of varying alkyl chain length of 1-Cn-3-methylimidazolium chloride ionic liquid (IL) series ([Cnmim]Cl, n = 2, 4, 6, 8, 10, 12) on the protein fluorescence, structure, hydration, aggregation dynamics and crystallization behaviour. The results revealed a concentration-dependent decrease in the sfGFP chromophore fluorescence, particularly in long alkyl chain ILs ([C10mim]Cl and [C12mim]Cl). Tryptophan (Trp) fluorescence showed the quenching rate increased with longer alkyl chains indicating a nonpolar interaction between Trp57 and the alkyl chain. Secondary structural changes were observed at the high IL concentration of 1.5 M in [C10mim]Cl and [C12mim]Cl. Small-angle X-ray scattering (SAXS) indicated relatively stable protein sizes, but with IL aggregates present in [C10mim]Cl and [C12mim]Cl solutions. Dynamic light scattering (DLS) data showed increased protein size and aggregation with longer alkyl chain ILs. Notably, ILs and salts, excluding [C2mim]Cl, promoted sfGFP crystallization. This study emphasizes the influence of the cation alkyl chain length and concentration on protein stability and aggregation, providing insights into utilizing IL solvents for protein stabilization and crystallization purposes.

Small angle X-ray scattering investigation of ionic liquid effect on the aggregation behavior of globular proteins

Globular proteins are well-folded model proteins, where ions can substantially influence their structure and aggregation. Ionic liquids (ILs) are salts in the liquid state with versatile ion combinations. Understanding the IL effect on protein behavior remains a major challenge. Here, we employed small angle X-ray scattering to investigate the effect of aqueous ILs on the structure and aggregation of globular proteins, namely, hen egg white lysozyme (Lys), human lysozyme (HLys), myoglobin (Mb), β-lactoglobulin (βLg), trypsin (Tryp) and superfolder green fluorescent protein (sfGFP). The ILs contain ammonium-based cations paired with the mesylate, acetate or nitrate anion. Results showed that only Lys was monomeric, whereas the other proteins formed small or large aggregates in buffer. Solutions with over 17 mol% IL resulted in significant changes in the protein structure and aggregation. The Lys structure was expanded at 1 mol% but compact at 17 mol% with structural changes in loop regions. HLys formed small aggregates, with the IL effect similar to Lys. Mb and βLg mostly had distinct monomer and dimer distributions depending on IL type and IL concentration. Complex aggregation was noted for Tryp and sfGFP. While the anion had the largest ion effect, changing the cation also induced the structural expansion and protein aggregation.

Severe acute respiratory syndrome coronavirus 2 as a potential cause of type 1 diabetes facilitated by spike protein receptor binding domain attachment to human islet cells: An illustrative case study and experimental data

AIMS

Aim of this study is to report severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, responsible for coronavirus disease 2019 (COVID-19), as a possible cause for type 1 diabetes by providing an illustrative clinical case of a man aged 45 years presenting with antibody-negative diabetic ketoacidosis post-recovery from COVID-19 pneumonia and to explore the potential for SARS-CoV-2 to adhere to human islet cells.

METHODS

Explanted human islet cells from three independent solid organ donors were incubated with the SARS-CoV-2 spike protein receptor biding domain (RBD) fused to a green fluorescent protein (GFP) or a control-GFP, with differential adherence established by flow cytometry.

RESULTS

Flow cytometry revealed dose-dependent specific binding of RBD-GFP to islet cells when compared to control-GFP.

CONCLUSIONS

Although a causal basis remains to be established, our case and in vitro data highlight a potential mechanism by which SARS-CoV-2 infection may result in antibody-negative type 1 diabetes.

Effect of ionic liquids on the fluorescence properties and aggregation of superfolder green fluorescence protein

Proteins generally tend to aggregate with less desirable properties in numerous solvents, which is one of the major challenges in the development of solvents for functional proteins. This work aims to utilize fluorescence spectroscopy and small angle X-ray scattering (SAXS) to understand the effects of ionic liquids (ILs) on the fluorescence and aggregation behavior of superfolder green fluorescent protein (sfGFP). The studied ILs consisted of four different anions coupled with primary, tertiary and quaternary ammonium cations. The results show that the chromophore fluorescence was generally maintained in 1 mol% IL-water mixtures, then decreased with increasing IL concentration. We primarily employed the pseudo-radius of gyration (pseudo-R_g) to evaluate sfGFP aggregation. The sfGFP was less aggregated with nitrate-based ILs compared to in buffer, and more aggregated in the mesylate-based ILs. Further, we show that the polyol additives of glycerol and glucose in IL-water mixtures slightly decreased the sfGFP propensity to aggregate. Size-exclusion chromatography (SEC)-SAXS was used to characterize the monomeric sfGFP in ethylammonium nitrate (EAN) and triethylammonium mesylate (TEAMs)-water mixtures. The presence of 1 mol% TEAMs maintained the sfGFP fluorescence, promoted the compact structure, but slightly increased the amount of large aggregates, which contrasted with that of EAN.

Transactivation of RAGE mediates angiotensin-induced inflammation and atherogenesis

Activation of the type 1 angiotensin II receptor (AT1) triggers proinflammatory signaling through pathways independent of classical Gq signaling that regulate vascular homeostasis. Here, we report that the AT1 receptor preformed a heteromeric complex with the receptor for advanced glycation endproducts (RAGE). Activation of the AT1 receptor by angiotensin II (Ang II) triggered transactivation of the cytosolic tail of RAGE and NF-κB-driven proinflammatory gene expression independently of the liberation of RAGE ligands or the ligand-binding ectodomain of RAGE. The importance of this transactivation pathway was demonstrated by our finding that adverse proinflammatory signaling events induced by AT1 receptor activation were attenuated when RAGE was deleted or transactivation of its cytosolic tail was inhibited. At the same time, classical homeostatic Gq signaling pathways were unaffected by RAGE deletion or inhibition. These data position RAGE transactivation by the AT1 receptor as a target for vasculoprotective interventions. As proof of concept, we showed that treatment with the mutant RAGE peptide S391A-RAGE362-404 was able to inhibit transactivation of RAGE and attenuate Ang II-dependent inflammation and atherogenesis. Furthermore, treatment with WT RAGE362-404 restored Ang II-dependent atherogenesis in Ager/Apoe-KO mice, without restoring ligand-mediated signaling via RAGE, suggesting that the major effector of RAGE activation was its transactivation.

A novel synthetic small molecule DMFO targets Nrf2 in modulating proinflammatory/antioxidant mediators to ameliorate inflammation

Inflammation is a protective immune response against invading pathogens, however, dysregulated inflammation is detrimental. As the complex inflammatory response involves multiple mediators, including the involvement of reactive oxygen species, concomitantly targeting proinflammatory and antioxidant check-points may be a more rational strategy. We report the synthesis and anti-inflammatory/antioxidant activity of a novel indanedione derivative DMFO. DMFO scavenged reactive oxygen species (ROS) in in-vitro radical scavenging assays and in lipopolysaccharide (LPS)-stimulated RAW 264.7 macrophages. In acute models of inflammation (carrageenan-induced inflammation in rat paw and air pouch), DMFO effectively reduced paw oedema and leucocyte infiltration with an activity comparable to diclofenac. DMFO stabilised mast cells (MCs) in in-vitro A23187 and compound 48/80-induced assays. Additionally, DMFO stabilised MCs in an antigen (ovalbumin)-induced MC degranulation model in-vivo, without affecting serum IgE levels. In a model of chronic immune-mediated inflammation, Freund's adjuvant-induced arthritis, DMFO reduced arthritic score and contralateral paw oedema, and increased the pain threshold with an efficacy comparable to diclofenac but without being ulcerogenic. Additionally, DMFO significantly reduced serum TNFα levels. Mechanistic studies revealed that DMFO reduced proinflammatory genes (IL1β, TNFα, IL6) and protein levels (COX2, MCP1), with a concurrent increase in antioxidant genes (NQO1, haem oxygenase 1 (HO-1), Glo1, Nrf2) and protein (HO-1) in LPS-stimulated macrophages. Importantly, the anti-inflammatory/antioxidant effect on gene expression was absent in primary macrophages isolated from Nrf2 KO mice suggesting an Nrf2-targeted activity, which was subsequently confirmed using siRNA transfection studies in RAW macrophages. Therefore, DMFO is a novel, orally-active, safe (even at 2 g/kg p.o.), a small molecule which targets Nrf2 in ameliorating inflammation.

Recent novel approaches to limit oxidative stress and inflammation in diabetic complications

Diabetes is considered a major burden on the healthcare system of Western and non‐Western societies with the disease reaching epidemic proportions globally. Diabetic patients are highly susceptible to developing micro‐ and macrovascular complications, which contribute significantly to morbidity and mortality rates. Over the past decade, a plethora of research has demonstrated that oxidative stress and inflammation are intricately linked and significant drivers of these diabetic complications. Thus, the focus now has been towards specific mechanism‐based strategies that can target both oxidative stress and inflammatory pathways to improve the outcome of disease burden. This review will focus on the mechanisms that drive these diabetic complications and the feasibility of emerging new therapies to combat oxidative stress and inflammation in the diabetic milieu.

The conjugation protein TcpC from Clostridium perfringens is structurally related to the type IV secretion system protein VirB8 from Gram-negative bacteria

Bacterial conjugation is important for the acquisition of virulence and antibiotic resistance genes. We investigated the mechanism of conjugation in Gram-positive pathogens using a model plasmid pCW3 from Clostridium perfringens. pCW3 encodes tetracycline resistance and contains the tcp locus, which is essential for conjugation. We showed that the unique TcpC protein (359 amino acids, 41 kDa) was required for efficient conjugative transfer, localized to the cell membrane independently of other conjugation proteins, and that membrane localization was important for its function, oligomerization and interaction with the conjugation proteins TcpA, TcpH and TcpG. The crystal structure of the C-terminal component of TcpC (TcpC(99-359)) was determined to 1.8-Å resolution. TcpC(99-359) contained two NTF2-like domains separated by a short linker. Unexpectedly, comparative structural analysis showed that each of these domains was structurally homologous to the periplasmic region of VirB8, a component of the type IV secretion system from Agrobacterium tumefaciens. Bacterial two-hybrid studies revealed that the C-terminal domain was critical for interactions with other conjugation proteins. The N-terminal region of TcpC was required for efficient conjugation, oligomerization and protein-protein interactions. We conclude that by forming oligomeric complexes, TcpC contributes to the stability and integrity of the conjugation apparatus, facilitating efficient pCW3 transfer.

The subtilisin-like protease AprV2 is required for virulence and uses a novel disulphide-tethered exosite to bind substrates

Many bacterial pathogens produce extracellular proteases that degrade the extracellular matrix of the host and therefore are involved in disease pathogenesis. Dichelobacter nodosus is the causative agent of ovine footrot, a highly contagious disease that is characterized by the separation of the hoof from the underlying tissue. D. nodosus secretes three subtilisin-like proteases whose analysis forms the basis of diagnostic tests that differentiate between virulent and benign strains and have been postulated to play a role in virulence. We have constructed protease mutants of D. nodosus; their analysis in a sheep virulence model revealed that one of these enzymes, AprV2, was required for virulence. These studies challenge the previous hypothesis that the elastase activity of AprV2 is important for disease progression, since aprV2 mutants were virulent when complemented with aprB2, which encodes a variant that has impaired elastase activity. We have determined the crystal structures of both AprV2 and AprB2 and characterized the biological activity of these enzymes. These data reveal that an unusual extended disulphide-tethered loop functions as an exosite, mediating effective enzyme-substrate interactions. The disulphide bond and Tyr92, which was located at the exposed end of the loop, were functionally important. Bioinformatic analyses suggested that other pathogenic bacteria may have proteases that utilize a similar mechanism. In conclusion, we have used an integrated multidisciplinary combination of bacterial genetics, whole animal virulence trials in the original host, biochemical studies, and comprehensive analysis of crystal structures to provide the first definitive evidence that the extracellular secreted proteases produced by D. nodosus are required for virulence and to elucidate the molecular mechanism by which these proteases bind to their natural substrates. We postulate that this exosite mechanism may be used by proteases produced by other bacterial pathogens of both humans and animals.

Crystallization of the virulent and benign subtilisin-like proteases from the ovine footrot pathogen Dichelobacter nodosus

Dichelobacter nodosus is the principal causative agent of ovine footrot, a disease of significant economic importance to the sheep industry. D. nodosus secretes a number of subtilisin-like serine proteases which mediate tissue damage and presumably contribute to the pathogenesis of footrot. Strains causing virulent footrot secrete the proteases AprV2, AprV5 and BprV and strains causing benign footrot secrete the closely related proteases AprB2, AprB5 and BprB. Here, the cloning, purification and crystallization of AprV2, AprB2, BprV and BprB are reported. Crystals of AprV2 and AprB2 diffracted to 2.0 and 1.7 A resolution, respectively. The crystals of both proteases belonged to space group P1, with unit-cell parameters a = 43.1, b = 46.0, c = 47.2 A, alpha = 97.8, beta = 115.2, gamma = 115.2 degrees for AprV2 and a = 42.7, b = 45.8, c = 45.7 A, alpha = 98.4, beta = 114.0, gamma = 114.6 degrees for AprB2. Crystals of BprV and BprB diffracted to 2.0 and 1.8 A resolution, respectively. The crystals of both proteases belonged to space group P2(1), with unit-cell parameters a = 38.5, b = 89.6, c = 47.7 A, beta = 113.6 degrees for BprV and a = 38.5, b = 90.5, c = 44.1 A, beta = 109.9 degrees for BprB. The crystals of all four proteases contained one molecule in the asymmetric unit, with a solvent content ranging from 36 to 40%.

The MACPF/CDC family of pore-forming toxins

Pore-forming toxins (PFTs) are commonly associated with bacterial pathogenesis. In eukaryotes, however, PFTs operate in the immune system or are deployed for attacking prey (e.g. venoms). This review focuses upon two families of globular protein PFTs: the cholesterol-dependent cytolysins (CDCs) and the membrane attack complex/perforin superfamily (MACPF). CDCs are produced by Gram-positive bacteria and lyse or permeabilize host cells or intracellular organelles during infection. In eukaryotes, MACPF proteins have both lytic and non-lytic roles and function in immunity, invasion and development. The structure and molecular mechanism of several CDCs are relatively well characterized. Pore formation involves oligomerization and assembly of soluble monomers into a ring-shaped pre-pore which undergoes conformational change to insert into membranes, forming a large amphipathic transmembrane β-barrel. In contrast, the structure and mechanism of MACPF proteins has remained obscure. Recent crystallographic studies now reveal that although MACPF and CDCs are extremely divergent at the sequence level, they share a common fold. Together with biochemical studies, these structural data suggest that lytic MACPF proteins use a CDC-like mechanism of membrane disruption, and will help understand the roles these proteins play in immunity and development.

Rosella: a fluorescent pH-biosensor for reporting vacuolar turnover of cytosol and organelles in yeast

We have developed a method for monitoring autophagy using Rosella, a biosensor comprised of a fast-maturing pH-stable red fluorescent protein fused to a pH-sensitive green fluorescent protein variant. Its mode of action relies upon differences in pH between different cellular compartments and the vacuole. Here we demonstrate its utility in yeast (Saccharomyces cerevisiae) by expression in the cytosol, and targeting to mitochondria or to the nucleus. When cells were cultured in nitrogen depleted medium, uptake of the compartment labelled with the biosensor (i.e., cytosol, mitochondria, or nucleus) into the vacuole was observed. We showed that this vacuolar uptake was, for cytosol and mitochondria, an ATG8-dependent process while the uptake of the nucleus was significantly reduced in the absence of Atg8p and can be said to be partially ATG8-dependent. We further demonstrated the value of the biosensor as a reporter of autophagy by employing fluorescence-activated cell sorting of discrete populations of cells undergoing autophagy.

A common fold mediates vertebrate defense and bacterial attack

Proteins containing membrane attack complex/perforin (MACPF) domains play important roles in vertebrate immunity, embryonic development, and neural-cell migration. In vertebrates, the ninth component of complement and perforin form oligomeric pores that lyse bacteria and kill virus-infected cells, respectively. However, the mechanism of MACPF function is unknown. We determined the crystal structure of a bacterial MACPF protein, Plu-MACPF from Photorhabdus luminescens, to 2.0 angstrom resolution. The MACPF domain reveals structural similarity with poreforming cholesterol-dependent cytolysins (CDCs) from Gram-positive bacteria. This suggests that lytic MACPF proteins may use a CDC-like mechanism to form pores and disrupt cell membranes. Sequence similarity between bacterial and vertebrate MACPF domains suggests that the fold of the CDCs, a family of proteins important for bacterial pathogenesis, is probably used by vertebrates for defense against infection.

GABA production by glutamic acid decarboxylase is regulated by a dynamic catalytic loop

Gamma-aminobutyric acid (GABA) is synthesized by two isoforms of the pyridoxal 5'-phosphate-dependent enzyme glutamic acid decarboxylase (GAD65 and GAD67). GAD67 is constitutively active and is responsible for basal GABA production. In contrast, GAD65, an autoantigen in type I diabetes, is transiently activated in response to the demand for extra GABA in neurotransmission, and cycles between an active holo form and an inactive apo form. We have determined the crystal structures of N-terminal truncations of both GAD isoforms. The structure of GAD67 shows a tethered loop covering the active site, providing a catalytic environment that sustains GABA production. In contrast, the same catalytic loop is inherently mobile in GAD65. Kinetic studies suggest that mobility in the catalytic loop promotes a side reaction that results in cofactor release and GAD65 autoinactivation. These data reveal the molecular basis for regulation of GABA homeostasis.

An overview of the serpin superfamily

Serpins are a broadly distributed family of protease inhibitors that use a conformational change to inhibit target enzymes. They are central in controlling many important proteolytic cascades, including the mammalian coagulation pathways. Serpins are conformationally labile and many of the disease-linked mutations of serpins result in misfolding or in pathogenic, inactive polymers.