Kynurenine 3-monooxygenase limits de novo NAD+ synthesis through dietary tryptophan in renal proximal tubule epithelial cell models.

Nicotinamide adenine dinucleotide (NAD+) is a pivotal coenzyme, essential for cellular reactions, metabolism, and mitochondrial function. Depletion of kidney NAD+ levels and reduced de novo NAD+ synthesis through the tryptophan-kynurenine pathway are linked to acute kidney injury (AKI), whereas augmenting NAD+ shows promise in reducing AKI. We investigated de novo NAD+ biosynthesis using in vitro, ex vivo, and in vivo models to understand its role in AKI. Two-dimensional (2-D) cultures of human primary renal proximal tubule epithelial cells (RPTECs) and HK-2 cells showed limited de novo NAD+ synthesis, likely due to low pathway enzyme gene expression. Using three-dimensional (3-D) spheroid culture model improved the expression of tubular-specific markers and enzymes involved in de novo NAD+ synthesis. However, de novo NAD+ synthesis remained elusive in the 3-D spheroid culture, regardless of injury conditions. Further investigation revealed that 3-D cultured cells could not metabolize tryptophan (Trp) beyond kynurenine (KYN). Intriguingly, supplementation of 3-hydroxyanthranilic acid into RPTEC spheroids was readily incorporated into NAD+. In a human precision-cut kidney slice (PCKS) ex vivo model, de novo NAD+ synthesis was limited due to substantially downregulated kynurenine 3-monooxygenase (KMO), which is responsible for KYN to 3-hydroxykynurenine conversion. KMO overexpression in RPTEC 3-D spheroids successfully reinstated de novo NAD+ synthesis from Trp. In addition, in vivo study demonstrated that de novo NAD+ synthesis is intact in the kidney of the healthy adult mice. Our findings highlight disrupted tryptophan-kynurenine NAD+ synthesis in in vitro cellular models and an ex vivo kidney model, primarily attributed to KMO downregulation.NEW & NOTEWORTHY Nicotinamide adenine dinucleotide (NAD+) is essential in regulating mitochondrial function. Reduced NAD+ synthesis through the de novo pathway is associated with acute kidney injury (AKI). Our study reveals a disruption in de novo NAD+ synthesis in proximal tubular models, but not in vivo, attributed to downregulation of enzyme kynurenine 3-monooxygenase (KMO). These findings highlight a crucial role of KMO in governing de novo NAD+ biosynthesis within the kidney, shedding light on potential AKI interventions.

Conjugation of a peptide to an antibody engineered with free cysteines dramatically improves half-life and activity.

The long circulating half-life and inherently bivalent architecture of IgGs provide an ideal vehicle for presenting otherwise short-lived G-protein-coupled receptor agonists in a format that enables avidity-driven enhancement of potency. Here, we describe the site-specific conjugation of a dual agonist peptide (an oxyntomodulin variant engineered for potency and in vivo stability) to the complementarity-determining regions (CDRs) of an immunologically silent IgG4. A cysteine-containing heavy chain CDR3 variant was identified that provided clean conjugation to a bromoacetylated peptide without interference from any of the endogenous mAb cysteine residues. The resulting mAb-peptide homodimer has high potency at both target receptors (glucagon receptor, GCGR, and glucagon-like peptide 1 receptor, GLP-1R) driven by an increase in receptor avidity provided by the spatially defined presentation of the peptides. Interestingly, the avidity effects are different at the two target receptors. A single dose of the long-acting peptide conjugate robustly inhibited food intake and decreased body weight in insulin resistant diet-induced obese mice, in addition to ameliorating glucose intolerance. Inhibition of food intake and decrease in body weight was also seen in overweight cynomolgus monkeys. The weight loss resulting from dosing with the bivalently conjugated dual agonist was significantly greater than for the monomeric analog, clearly demonstrating translation of the measured in vitro avidity to in vivo pharmacology.

Insulin receptor substrate-2 in beta-cells decreases diabetes in nonobese diabetic mice.

Insulin receptor substrate-2 (Irs2) integrates insulin-like signals with glucose and cAMP agonists to regulate beta-cell growth, function, and survival. This study investigated whether increased Irs2 concentration in beta-cells could reduce beta-cell destruction and the incidence of type 1 diabetes in nonobese diabetic (NOD) mice. NOD mice were intercrossed with C57BL/6 mice overexpressing Irs2 specifically in beta-cells to create NOD(Irs2) mice. After backcrossing NOD(Irs2) mice for 12 generations, glucose homeostasis and diabetes incidence were compared against NOD littermates. Compared with 12-wk-old NOD mice, the progression of severe insulitis was reduced and islet mass was increased in NOD(Irs2) mice. Moreover, the risk of diabetes decreased 50% in NOD(Irs2) mice until the experiment was terminated at 40 wk of age. Nondiabetic NOD(Irs2) mice displayed better glucose tolerance than nondiabetic NOD mice throughout the duration of the study and up to the age of 18 months. The effect of Irs2 to increase islet mass and improve glucose tolerance raised the possibility that NOD(Irs2) mice might have an increased capacity to respond to anti-CD3 antibody, which can induce remission of overt diabetes in some NOD mice. Anti-CD3 antibody injections restored glucose tolerance in newly diabetic NOD and NOD(Irs2) mice; however, anti-CD3-treated NOD(Irs2) mice were less likely than NOD mice to relapse during the experimental period because they displayed 10-fold greater beta-cell mass and mitogenesis. In conclusion, increased Irs2 attenuated the progression of beta-cell destruction, promoted beta-cell mitogenesis, and reduced diabetes incidence in NOD(Irs2) mice.

Decreased sensitivity to changes in the concentration of metal ions as the basis for the hyperactivity of DtxR(E175K).

The metal-ion-activated diphtheria toxin repressor (DtxR) is responsible for the regulation of virulence and other genes in Corynebacterium diphtheriae. A single point mutation in DtxR, DtxR(E175K), causes this mutant repressor to have a hyperactive phenotype. Mice infected with Mycobacterium tuberculosis transformed with plasmids carrying this mutant gene show reduced signs of the tuberculosis infection. Corynebacterial DtxR is able to complement mycobacterial IdeR and vice versa. To date, an explanation for the hyperactivity of DtxR(E175K) has remained elusive. In an attempt to address this issue, we have solved the first crystal structure of DtxR(E175K) and characterized this mutant using circular dichroism, isothermal titration calorimetry, and other biochemical techniques. The results show that although DtxR(E175K) and the wild type have similar secondary structures, DtxR(E175K) gains additional thermostability upon activation with metal ions, which may lead to this mutant requiring a lower concentration of metal ions to reach the same levels of thermostability as the wild-type protein. The E175K mutation causes binding site 1 to retain metal ion bound at all times, which can only be removed by incubation with an ion chelator. The crystal structure of DtxR(E175K) shows an empty binding site 2 without evidence of oxidation of Cys102. The association constant for this low-affinity binding site of DtxR(E175K) obtained from calorimetric titration with Ni(II) is K(a)=7.6+/-0.5x10(4), which is very similar to the reported value for the wild-type repressor, K(a)=6.3x10(4). Both the wild type and DtxR(E175K) require the same amount of metal ion to produce a shift in the electrophoretic mobility shift assay, but unlike the wild type, DtxR(E175K) binding to its cognate DNA [tox promoter-operator (toxPO)] does not require metal-ion supplementation in the running buffer. In the timescale of these experiments, the Mn(II)-DtxR(E175K)-toxPO complex is insensitive to changes in the environmental cation concentrations. In addition to Mn(II), Ni(II), Co(II), Cd(II), and Zn(II) are able to sustain the hyperactive phenotype. These results demonstrate a prominent role of binding site 1 in the activation of DtxR and support the hypothesis that DtxR(E175K) attenuates the expression of virulence due to the decreased ability of the Me(II)-DtxR(E175K)-toxPO complex to dissociate at low concentrations of metal ions.

Role of the N-terminal helix in the metal ion-induced activation of the diphtheria toxin repressor DtxR.

The metal ion-regulated transcriptional repressor DtxR has been shown to repress the transcription of the diphtheria toxin and other genes associated with ferrous ion homeostasis in Corynebacterium diphtheriae. In vivo studies of single-alanine mutations located in the N-terminal helix of DtxR show that the activity of the mutants is reduced compared to that of the wild type. The three-dimensional structures of the apo and activated forms of DtxR show conformational changes in the N-terminal helix resulting from metal ion activation. We have studied the N-terminal helix mutants DtxR(D6A,C102D), DtxR(E9A,C102D), and DtxR(M10A,C102D) using crystallographic and calorimetric techniques to gain insight into the possible reasons for such behavior at a molecular level. The binding affinities for metal ion extracted from the calorimetric titrations of the mutants DtxR(D6A,C102D) and DtxR(E9A,C102D) are very similar to those found for DtxR(C102D), while the same experiments performed with the mutant DtxR(M10A,C102D), bearing the M10A mutation located in binding site 2, show a decreased binding affinity in a predictable fashion. These results suggest that the decreased activity observed in these mutants cannot be explained exclusively by changes in the binding affinity of the repressor. The crystal structures of Ni-DtxR(M10A,C102D), Ni-DtxR(E9A,C102D), and Ni-DtxR(D6A,C102D) clearly show the presence of two metal ions bound. In the structure of Ni-DtxR(M10A,C102D), a water replaces Met10 in binding site 2. In the structure of Ni-DtxR(D6A,C102D), the nonhelical conformation of the N-terminal region characteristic of the activated form is absent. The side chain of Asp6 is critical in stabilization of the nonhelical conformation. This conformation is identical in all high-resolution structures of activated DtxR with an intact N-terminal helix, suggesting relevance in DtxR's regulatory function.

The stress-activated mitogen-activated protein kinase signaling cascade promotes exit from mitosis.

In budding yeast, a signaling network known as the mitotic exit network (MEN) triggers exit from mitosis. We find that hypertonic stress allows MEN mutants to exit from mitosis in a manner dependent on the high osmolarity glycerol (HOG) mitogen-activated protein (MAP) kinase cascade. The HOG pathway drives exit from mitosis in MEN mutants by promoting the activation of the MEN effector, the protein phosphatase Cdc14. Activation of Cdc14 depends on the Cdc14 early anaphase release network, a group of proteins that functions in parallel to the MEN to promote Cdc14 function. Notably, exit from mitosis is promoted by the signaling branch defined by the Sho1 osmosensing system, but not by the Sln1 osmosensor of the HOG pathway. Our results suggest that the stress MAP kinase pathway mobilizes programs to promote completion of the cell cycle and entry into G1 under unfavorable conditions.

The protein kinase Kin4 inhibits exit from mitosis in response to spindle position defects.

Accurate nuclear position is essential for each daughter cell to receive one DNA complement. In budding yeast, a surveillance mechanism known as the spindle position checkpoint ensures that exit from mitosis only occurs when the anaphase nucleus is positioned along the mother-bud axis. We identified the protein kinase Kin4 as a component of the spindle position checkpoint. KIN4 prevents exit from mitosis in cells with mispositioned nuclei by inhibiting the mitotic exit network (MEN), a GTPase signaling cascade that promotes exit from mitosis. Kin4 is active in cells with mispositioned nuclei and predominantly localizes to mother cells, where it is ideally situated to inhibit MEN signaling at spindle pole bodies (SPBs) when anaphase spindle elongation occurs within the mother cell.

The Sla2p talin domain plays a role in endocytosis in Saccharomyces cerevisiae.

Clathrin-binding adaptors play critical roles for endocytosis in multicellular organisms, but their roles in budding yeast have remained unclear. To address this question, we created a quadruple mutant yeast strain lacking the genes encoding the candidate clathrin adaptors Yap1801p, Yap1802p, and Ent2p and containing a truncated version of Ent1p, Ent1DeltaCBMp, missing its clathrin-binding motif. This strain was viable and competent for endocytosis, suggesting the existence of other redundant adaptor-like factors. To identify these factors, we mutagenized the quadruple clathrin adaptor mutant strain and selected cells that were viable in the presence of full-length Ent1p, but inviable with only Ent1DeltaCBMp; these strains were named Rcb (requires clathrin binding). One mutant strain, rcb432, contained a mutation in SLA2 that resulted in lower levels of a truncated protein lacking the F-actin binding talin homology domain. Analyses of this sla2 mutant showed that the talin homology domain is required for endocytosis at elevated temperature, that SLA2 exhibits genetic interactions with both ENT1 and ENT2, and that the clathrin adaptors and Sla2p together regulate the actin cytoskeleton and revealed conditions under which Yap1801p and Yap1802p contribute to viability. Together, our data support the view that Sla2p is an adaptor that links actin to clathrin and endocytosis.