Interleukin (IL)-1beta maturation is accomplished by caspase-1-mediated proteolysis, an essential element of innate immunity. NLRs constitute a recently recognized family of caspase-1-activating proteins, which contain a nucleotide-binding oligomerization domain and leucine-rich repeat (LRR) domains and which assemble into multiprotein complexes to create caspase-1-activating platforms called "inflammasomes." Using purified recombinant proteins, we have reconstituted the NALP1 inflammasome and have characterized the requirements for inflammasome assembly and caspase-1 activation. Oligomerization of NALP1 and activation of caspase-1 occur via a two-step mechanism, requiring microbial product, muramyl-dipeptide, a component of peptidoglycan, followed by ribonucleoside triphosphates. Caspase-1 activation by NALP1 does not require but is enhanced by adaptor protein ASC. The findings provide the biochemical basis for understanding how inflammasome assembly and function are regulated, and shed light on NALP1 as a direct sensor of bacterial components in host defense against pathogens.

Medial vascular calcification (MVC) is a pathological phenomenon common to a variety of conditions, including aging, chronic kidney disease, diabetes, obesity, and a variety of rare genetic diseases, that causes vascular stiffening and can lead to heart failure. These conditions share the common feature of tissue-nonspecific alkaline phosphatase (TNAP) upregulation in the vasculature. To evaluate the role of TNAP in MVC, we developed a mouse model that overexpresses human TNAP in vascular smooth muscle cells in an X-linked manner. Hemizygous overexpressor male mice (Tagln-Cre+/-; HprtALPL/Y, or TNAP-OE) show extensive vascular calcification, high blood pressure, cardiac hypertrophy and have a median age of death of 44 days, whereas the cardiovascular phenotype is much less pronounced and life expectancy is longer in heterozygous (Tagln-Cre+/-; HprtALPL/-) female TNAP-OE mice. Gene expression analysis showed upregulation of osteoblast and chondrocyte markers and decreased expression of vascular smooth muscle markers in the aortas of TNAP-OE mice. Through medicinal chemistry efforts, we developed inhibitors of TNAP with drug-like pharmacokinetic characteristics. TNAP-OE mice were treated with the prototypical TNAP inhibitor SBI-425 or vehicle to evaluate the feasibility of TNAP inhibition in vivo. Treatment with this inhibitor significantly reduced aortic calcification and cardiac hypertrophy, and extended lifespan over vehicle-treated controls, in the absence of secondary effects on the skeleton. This study shows that TNAP in the vasculature contributes to the pathology of MVC and that it is a druggable target. This article is protected by copyright. All rights reserved

Benzoylformate decarboxylase is a member of the family of enzymes that are dependent on the cofactor thiamin diphosphate. A structure of this enzyme binding (R)-mandelate, a competitive inhibitor, suggests that at least two hydrogen bonds are formed between the substrate, benzoylformate, and active site side chains. The first is between the carboxylate group of benzoylformate and the hydroxyl group of S26, and the second is between carbonyl group of the substrate and an imidazole nitrogen of H70. Steady-state kinetic studies indicate that the catalytic parameters are strongly affected in three active site mutants, S26A, H70A, and H281A. The K(m) of S26A was increased most dramatically, 25-fold more than that of the wild-type enzyme, while the K(i) of (R)-mandelate was increased 100-fold, suggesting that the serine hydroxyl is important for substrate binding. The k(cat) of H70A is reduced more than 3 orders of magnitude, strongly implicating this residue in catalysis, and H281 showed significant, but smaller magnitude, effects on both K(m) and k(cat). Stopped-flow experiments using an alternative substrate, p-nitrobenzoylformate, lead to kinetic resolution of the fate of key thiamin diphosphate-bound intermediates. Together, the experimental results suggest the following roles for residues in the active site. The residue H70 is important for the protonation of the 2-alpha-mandelyl-ThDP intermediate, thereby assisting in decarboxylation, and for the deprotonation of the 2-alpha-hydroxybenzyl-ThDP intermediate, aiding product release. H281 is involved in protonation of the enamine. Surprisingly, S26 appears to be involved not only in substrate binding but also in other steps of the reaction.

The proteasome is a vital cellular machine that maintains protein homeostasis, which is of particular importance in multiple myeloma and possibly other cancers. Targeting proteasome 20S peptidase activity with bortezomib and carfilzomib has been widely used to treat myeloma. However, not all patients respond, and those that do eventually suffer relapse. Therefore, there is an urgent and unmet need to develop novel drugs that target proteostasis through different mechanisms. We identified quinoline-8-thiol (8TQ) as a first-in-class inhibitor of the proteasome 19S subunit Rpn11. A derivative of 8TQ, capzimin, shows >5-fold selectivity for Rpn11 over the related JAMM proteases and >2 logs less activity towards metalloenzymes. Capzimin stabilized proteasome substrates, induced an unfolded protein response, and blocked proliferation of cancer cells, including those resistant to bortezomib. Proteomic analysis revealed that capzimin stabilized a subset of polyubiquitinated substrates. Identification of capzimin offers an alternative path to develop proteasome inhibitors for cancer therapy.

Pharmacological strategies that boost intracellular NAD + are highly coveted for their therapeutic potential. One approach is activation of nicotinamide phosphoribosyltransferase (NAMPT) to increase production of nicotinamide mononucleotide (NMN), the predominant NAD + precursor in mammalian cells. A high-throughput screen for NAMPT activators and hit-to-lead campaign yielded SBI-797812, a compound that is structurally similar to active-site directed NAMPT inhibitors and blocks binding of these inhibitors to NAMPT. SBI-797812 shifts the NAMPT reaction equilibrium towards NMN formation, increases NAMPT affinity for ATP, stabilizes phosphorylated NAMPT at His247, promotes consumption of the pyrophosphate by-product, and blunts feedback inhibition by NAD + . These effects of SBI-797812 turn NAMPT into a “super catalyst” that more efficiently generates NMN. Treatment of cultured cells with SBI-797812 increases intracellular NMN and NAD + . Dosing of mice with SBI-797812 elevates liver NAD + . Small molecule NAMPT activators such as SBI-797812 are a pioneering approach to raise intracellular NAD + and realize its associated salutary effects.

Yeast pyruvate decarboxylase (YPDC), in addition to forming its metabolic product acetaldehyde, can also carry out carboligase reactions in which the central enamine intermediate reacts with acetaldehyde or pyruvate (instead of the usual proton electrophile), resulting in the formation of acetoin and acetolactate, respectively (typically, 1% of the total reaction). Due to the common mechanism shared by the acetaldehyde-forming and carboligase reactions through decarboxylation, a detailed analysis of the rates and stereochemistry of the carboligase products formed by the E477Q, D28A, and D28N active center YPDC variants was undertaken. While substitution at either position led to an approximately 2-3 orders of magnitude lower catalytic efficiency in acetaldehyde formation, the rate of acetoin formation by the E477Q and D28N variants was higher than that by wild-type enzyme. Comparison of the steady-state data for acetaldehyde and acetoin formation revealed that the rate-limiting step for acetaldehyde formation by the D28A, H114F, H115F, and E477Q variants is a step post-decarboxylation. In contrast to the wild-type YPDC and the E477Q variant, the D28A and D28N variants could synthesize acetolactate as a major product. The lower overall rate of side-product formation by the D28A variant than wild-type enzyme attests to participation of D28 in steps leading up to and including decarboxylation. The results also provide insight into the state of ionization of the side chains examined. (R)-Acetoin is produced by the variants with greater enantiomeric excess than by wild-type YPDC. (S)-Acetolactate is the predominant enantiomer produced by the D28-substituted variants, the same configuration as produced by the related plant acetolactate synthase.

The roles of four of the active center groups with potential acid-base properties in the region of pH optimum of pyruvate decarboxylase from Saccharomyces cerevisiae have been studied with the substitutions Asp28Ala, His114Phe, His115Phe, and Glu477Gln, introduced by site-directed mutagenesis methods. The steady-state kinetic constants were determined in the pH range of activity for the enzyme. The substitutions result in large changes in k(cat) and k(cat)/S(0.5) (and related terms), indicating that all four groups have a role in transition state stabilization. Furthermore, these results also imply that all four are involved in some manner in stabilizing the rate-limiting transition state(s) both at low substrate (steps starting with substrate binding and culminating in decarboxylation) and at high substrate concentration (steps beginning with decarboxylation and culminating in product release). With the exception of some modest effects, the shapes of neither the bell-shaped k(cat)/S(0.5)-pH (and related functions) plots nor the k(cat)-pH plots are changed by the substitutions. Yet, the fractional activity still remaining after substitutions virtually rules out any of the four residues as being directly responsible for initiating the catalytic process by ionizing the C2H. There is no effect on the C2 H/D exchange rate exhibited by the D28A and E477Q substitutions. These results strongly imply that the base-induced deprotonation at C2 is carried out by the only remaining base, the iminopyrimidine tautomer of the coenzyme, via intramolecular proton abstraction. The first product is released as CO(2) rather than HCO(3)(-) by both wild-type and E477Q and D28A variants, ruling out several mechanistic alternatives.

The ABCs of PXE Pseudoxanthoma elasticum (PXE) is a genetic disorder caused by mutations in ABCC6 that is characterized by calcium deposition outside of the skeletal system, specifically in the blood vessels, skin, and eyes. Using patient-derived fibroblasts and genetic knockout mouse models, Ziegler et al. demonstrate that ABCC6 mutant cells are osteogenic and that loss of ABCC6 reduces pyrophosphate, an inhibitor of calcification. In mice, ectopic calcification was seen only when ABCC6 was deleted jointly from the liver and from Wnt1+ cells, suggesting systemic and local contributions to the phenotype. Treating mice and cells with a tissue-nonspecific alkaline phosphatase (TNAP) inhibitor prevented pyrophosphate degradation and ectopic calcification progression. Biallelic mutations in ABCC6 cause pseudoxanthoma elasticum (PXE), a disease characterized by calcification in the skin, eyes, and blood vessels. The function of ATP-binding cassette C6 (ABCC6) and the pathogenesis of PXE remain unclear. We used mouse models and patient fibroblasts to demonstrate genetic interaction and shared biochemical and cellular mechanisms underlying ectopic calcification in PXE and related disorders caused by defined perturbations in extracellular adenosine 5′-triphosphate catabolism. Under osteogenic culture conditions, ABCC6 mutant cells calcified, suggesting a provoked cell-autonomous defect. Using a conditional Abcc6 knockout mouse model, we excluded the prevailing pathogenic hypothesis that singularly invokes failure of hepatic secretion of an endocrine inhibitor of calcification. Instead, deficiency of Abcc6 in both local and distant cells was necessary to achieve the early onset and penetrant ectopic calcification observed upon constitutive gene targeting. ABCC6 mutant cells additionally had increased expression and activity of tissue-nonspecific alkaline phosphatase (TNAP), an enzyme that degrades pyrophosphate, a major inhibitor of calcification. A selective and orally bioavailable TNAP inhibitor prevented calcification in ABCC6 mutant cells in vitro and attenuated both the development and progression of calcification in Abcc6−/− mice in vivo, without the deleterious effects on bone associated with other proposed treatment strategies.