Poly-ADP-ribosylation-mediated degradation of ARTD1 by the NLRP3 inflammasome is a prerequisite for osteoclast maturation

Evidence implicates ARTD1 in cell differentiation, but its role in skeletal metabolism remains unknown. Osteoclasts (OC), the bone-resorbing cells, differentiate from macrophages under the influence of macrophage colony-stimulating factor (M-CSF) and receptor-activator of NF-κB ligand (RANKL). We found that M-CSF induced ADP-ribosyltransferase diphtheria toxin-like 1 (ARTD1) auto-ADP-ribosylation in macrophages, a modification that marked ARTD1 for cleavage, and subsequently, for degradation upon RANKL exposure. We established that ARTD1 proteolysis was NLRP3 inflammasome-dependent, and occurred via the proteasome pathway. Since ARTD1 is cleaved at aspartate²¹⁴, we studied the impact of ARTD1 rendered uncleavable by D214N substitution (ARTD1^D214N) on skeletal homeostasis. ARTD1^D214N, unlike wild-type ARTD1, was resistant to cleavage and degradation during osteoclastogenesis. As a result, ARTD1^D214N altered histone modification and promoted the abundance of the repressors of osteoclastogenesis by interfering with the expression of B lymphocyte-induced maturation protein 1 (Blimp1), the master regulator of anti-osteoclastogenic transcription factors. Importantly, ARTD1^D214N-expressing mice exhibited higher bone mass compared with controls, owing to decreased osteoclastogenesis while bone formation was unaffected. Thus, unless it is degraded, ARTD1 represses OC development through transcriptional regulation.

Virulence of a phosphoribosylaminoimidazole carboxylase-deficient Candida albicans strain in an immunosuppressed murine model of systemic candidiasis

The relative pathogenicities of three Candida albicans strains differing in the function of ADE2 (the gene encoding phosphoribosylaminoimidazole carboxylase) were evaluated in a murine candidiasis model. C. albicans strain CAI7 (ade2/ade2), previously constructed by site-specific recombination, was avirulent in immunosuppressed mice compared to the parent strain, CAF2-1, and a heterozygous ADE2/ade2 strain obtained by transforming CAI7 with a wild-type allele. The reduced virulence of CAI7 was correlated with the inability to proliferate in either synthetic medium or serum without the exogenous addition of >10 microg of adenine/ml. The loss of virulence upon site-specific disruption of the ade2 locus, and the restoration of wild-type virulence with the repair of just one ade2 allele, confirmed that the ADE2 gene and de novo purine biosynthesis were required for Candida pathogenicity. The potential of the phosphoribosylaminoimidazole carboxylase enzyme as a novel target for antifungal drug discovery is discussed.

Sequence analysis of the Candida albicans ADE2 gene and physical separation of the two functionally distinct domains of the phosphoribosylaminoimidazole carboxylase

An ADE2 genomic clone from the pathogenic fungus, Candida albicans, was isolated by complementation of an Escherichia coli purK mutant and the gene was analysed by DNA sequencing. A 1707 bp open reading frame was identified encoding a polypeptide of 569 amino acids with significant homology to all the known yeast ADE2 genes. Sequence homology to both the E. coli purE and purK genes suggests that the C. albicans ADE2 gene is the result of an evolutionary fusion. The amino-acid sequence comparison showed that the N-terminal domain of the Ade2 protein has a 52.5% identity to purK, whereas the C-terminal domain has a distinct 64.3% identity to purE. In order to establish the functional relationship of these two regions, deletion mutants of the Ade2 protein were prepared by recombinant expression of the functional domains, which were tested by complementation of their respective E. coli auxotrophs.

Epidermal growth factor precursor is present in a variety of human renal cyst fluids

Progressive human renal cystic diseases are characterized by proliferation of the epithelial cells lining the cyst. The kidney synthesizes epidermal growth factor and its presence in renal cyst fluid might contribute to renal epithelial cell proliferation. We screened autosomal dominant polycystic kidney disease, acquired renal cystic disease, the von Hippel-Lindau syndrome, multilocular cystic nephroma, multicystic dysplastic kidney disease and simple cyst fluids for the presence of epidermal growth factor by radioreceptor assay, specific radioimmunoassay, immunoprecipitation and immunoblotting. Multiple epidermal growth factor immunoreactive species of approximately 180 kD. and lower molecular weights were present in almost all cyst fluids examined, suggesting endogenous synthesis and limited proteolysis of epidermal growth factor precursor protein in cyst fluid. Tamm-Horsfall protein was detected by enzyme-linked immunosorbent assay in most cysts (for example 55 of 59 autosomal dominant polycystic kidney disease samples). The majority of simple and autosomal dominant polycystic kidney disease renal cysts contained high sodium ion concentration, epidermal growth factor precursor protein and Tamm-Horsfall protein, characteristic of the early thick ascending limb. Rather than the mere presence of epidermal growth factor in renal cyst fluids, increased sensitivity to epidermal growth factor or other mitogens present in renal cyst fluid may be pathogenic in progressive renal cystic disease.