Abnormal bone architecture in mice expressing MyD88 in cells of the osteoclast lineage.

The adapter protein myeloid differentiation primary response gene 88 (MyD88) links the intracellular domains of interleukin receptors 1 and 18, and most Toll-like receptors (TLRs) to interleukin 1 receptor associated kinase (IRAK) signaling and subsequent NF-κB-mediated transcription. Previous work showed that mice with global deficiency of MyD88 (MyD88-/-) have osteopenic cancellous bone along with a reduction in osteoblastic but also osteoclastic surfaces. To further elucidate the role of MyD88 in bone, we utilized mice with osteoclast-restricted MyD88 expression in bone (MyD88OC). Bones of MyD88OC and wild type (wt) mice were examined by microCT analysis. Mechanical properties of bones were tested by three-point bending, and gene expression measured using quantitative real-time polymerase chain reaction. In MyD88OC mice, no osteopenic traits were observed, however, a drastic reduction in geometric parameters was detected. In trabecular bone a loss of connectivity density (-44%, p less than 0.0001) was measured and in cortical bone Imax (-31%, p less than 0.0001), Imin (-20%, p less than 0.001), J (-26%, p less than 0.0001) were reduced. Mechanical testing showed increased load to failure (77%, p less than 0.01) and decreased deflection at failure (-68%, p less than 0.01) of the femur. On the molecular level, relative gene expression analysis showed a (-29%, p less than 0.01) reduction in receptor activator of nuclear factor κ B ligand (RANKL) and no difference in osteoprotegerin (OPG) or RANK. Further, the bone resorption markers cathepsin K (CTSK) and tartrate-resistant acid phosphatase 5 (TRAP) were unchanged. In contrast, the bone formation markers collagen type 1 (COL1A1) and osteocalcin (OC) were decreased by -72% (p less than 0.0001) and -82% (p less than 0.0001), respectively. Together, our data suggests that the function of MyD88 in osteoclasts is sufficient to maintain bone mass, while it fails to preserve bone geometry, likely through dysfunctions in osteoblasts.

MicroCT morphometry analysis of mouse cancellous bone: intra- and inter-system reproducibility.

The agreement between measurements and the relative performance reproducibility among different microcomputed tomography (microCT) systems, especially at voxel sizes close to the limit of the instruments, is not known. To compare this reproducibility 3D morphometric analyses of mouse cancellous bone from distal femoral epiphyses were performed using three different ex vivo microCT systems: GE eXplore Locus SP, Scanco µCT35 and Skyscan 1172. Scans were completed in triplicate at 12 µm and 8 µm voxel sizes and morphometry measurements, from which relative values and dependence on voxel size were examined. Global and individual visually assessed thresholds were compared. Variability from repeated scans at 12 µm voxel size was also examined. Bone volume fraction and trabecular separation values were similar, while values for relative bone surface, trabecular thickness and number varied significantly across the three systems. The greatest differences were measured in trabecular thickness (up to 236%) and number (up to 218%). The relative dependence of measurements on voxel size was highly variable for the trabecular number (from 0% to 20% relative difference between measurements from 12 µm and 8 µm voxel size scans, depending on the system). The intra-system reproducibility of all trabecular measurements was also highly variable across the systems and improved for BV/TV in all the systems when a smaller voxel size was used. It improved using a smaller voxel size in all the other parameters examined for the Scanco system, but not consistently so for the GE or the Skyscan system. Our results indicate trabecular morphometry measurements should not be directly compared across microCT systems. In addition, the conditions, including voxel size, for trabecular morphometry studies in mouse bone should be chosen based on the specific microCT system and the measurements of main interest.

Functional interaction of poly(ADP-ribose) with the 20S proteasome in vitro.

The nuclear enzyme poly(ADP-ribosyl) transferase (pADPRT) catalyzes the formation of poly(ADP-ribose) from NAD+. Several nuclear proteins and pADPRT itself are targets for the modification by poly(ADP-ribosyl)ation. It is demonstrated here that poly(ADP-ribose) or pADPRT automodified with poly(ADP-ribose) interacts noncovalently with the 20S proteasome in vitro. The interaction of pADPRT with the 20S proteasome requires the long ADP-ribose polymers formed by automodification of the pADPRT with poly(ADP-ribose). As a result pADPRT automodified with short ADP-ribose oligomers is unable to associate with the 20S proteasome. The interaction with poly(ADP-ribose) causes a specific stimulation of the peptidase activity of the 20S proteasome. Modified pADPRT does not serve as a substrate for the degradation by the 20S proteasome. No covalent modification of the 20S proteasome by ADP-ribosylation was observed. The results may point to a functional relationship between pADPRT and the 20S proteasome in a pathway protecting the cell from oxidative damage.

Protein-protein interaction of the human poly(ADP-ribosyl)transferase depends on the functional state of the enzyme.

Poly(ADP-ribosyl)transferase (pADPRT) is a nuclear protein which catalyzes the polymerization of ADP-ribose using NAD+ as substrate, as well as the transfer of ADP-ribose polymers to itself and other protein acceptors. The catalytic activity of pADPRT strictly depends on the presence of DNA single-strand breaks. In this report, protein-protein interaction of pADPRT was found to depend on both the extent of automodification with poly(ADP-ribose) and the presence of DNA. Specific binding of radiolabeled pADPRT to transblotted proteins was first tested in blot overlay experiments. For radiolabeling, use was made of the ability of the enzyme to incorporate [32P]ADP-ribose from [32P]NAD+. Varying the concentration of NAD+, two different forms of automodified pADPRT were obtained: oligo(ADP-ribosyl)ated pADPRT with less than 20 ADP-ribose units per chain, and poly(ADP-ribosyl)ated pADPRT with polymer lengths of up to 200 ADP-ribose residues. Interaction of these probes with transblotted HeLa nuclear extracts, purified histones, and distinct regions of recombinant pADPRT was investigated. While the oligo(ADP-ribosyl)ated enzyme associated preferentially with transblotted purified histones, or pADPRT present in HeLa nuclear extracts, poly(ADP-ribosyl)ated pADPRT bound to a variety of transblotted proteins in the nuclear extracts. In the presence of DNA, both the oligo- and the poly(ADP-ribosyl)ated enzymes bound to the transblotted recombinant zinc finger domain of pADPRT even at high salt concentrations. In the absence of DNA, the transblotted automodification domain of pADPRT appeared to be the region involved in self-association. In another set of experiments, unmodified or poly(ADP-ribosyl)ated pADPRT was immobilized on Sepharose. Affinity precipitation of recombinant pADPRT domains confirmed the specific interaction of pADPRT with its zinc finger region and the automodification domain, whereas no interaction was observed with the NAD+ binding domain. Affinity precipitation of HeLa nuclear extracts with poly(ADP-ribosyl)ated pADPRT-Sepharose led to the enrichment of a number of proteins, whereas nuclear proteins bound to the unmodified pADPRT-Sepharose in a smaller extent. The results suggest that protein-protein interaction of the human pADPRT is governed by its functional state.

NAD+ analogs substituted in the purine base as substrates for poly(ADP-ribosyl) transferase.

Poly(ADP-ribosyl) transferase (pADPRT) catalyzes the transfer of the ADP-ribose moiety from NAD+ onto proteins as well as onto protein-bound ADP-ribose. As a result, protein-bound polymers of ADP-ribose are formed. pADPRT itself contains several acceptor sites for ADP-ribose polymers and may attach polymers to itself (automodification). In this study the influence of substitutions in the purine base of NAD+ on the polymerization reaction was investigated. The adenine moiety of NAD+ was replaced by either guanine, hypoxanthine or 1,N6-ethenoadenine. These analogs served as substrates for polymer synthesis as judged from the extent of automodification of the enzyme and the sizes of the polymers formed. Time course experiments revealed that 1,N6-etheno NAD+ (epsilon-NAD+) and nicotinamide hypoxanthine dinucleotide (NHD+) were rather poor substrates as compared to NAD+. Synthesis of GDP-ribose polymers from nicotinamide guanine dinucleotide (NGD+) was more efficient, but still significantly slower than poly(ADP-ribosyl)ation of the enzyme using NAD+. The size of the different polymers appeared to correlate with these observations. After 30 min of incubation in the presence of 1 mM substrate, polymers formed from epsilon-NAD+ or NHD+ contained up to 30 epsilon-ADP-ribose or IDP-ribose units, respectively. Using NGD+ as substrate polymers consisted of more than 60 GDP-ribose units, an amount similar to that achieved by poly(ADP-ribosyl)ation in the presence of only 0.1 mM NAD+ as substrate. These results suggest that the presence of an amino group in the purine base of NAD+ may facilitate catalysis. Substitution of the nicotinamide moiety of NAD+ with 3-acetylpyridine had no detectable effect on polymer formation. Oligomers of GDP-ribose and epsilon-ADP-ribose exhibited a slower mobility in polyacrylamide gels as compared to ADP-ribose or IDP-ribose oligomers. This feature of the two former analogs as well as their markedly attenuated polymerization by pADPRT provide valuable tools for the investigation of the enzymatic mechanism of this protein. Moreover, polymers of epsilon-ADP-ribose may be useful for studying enzymes degrading poly(ADP-ribose) owing to the fluorescence of this analog. Digestion of epsilon-ADPR polymers with snake venom phosphodiesterase was accompanied by a significant fluorescence enhancement.