Predictors of fracture risk in patients with systemic lupus erythematosus

Objectives Bone loss in systemic lupus erythematosus is multifactorial. Recent studies demonstrate corticosteroids, previous fractures and increasing age decrease bone mineral density. The effect of body mass index and fat mass are less well characterized. We sought to determine fracture risk factors in patients undergoing dual-energy X-ray absorptiometry scanning at a district hospital in 2004-2015. Methods Standard dual-energy X-ray absorptiometry parameters were recorded, plus rheumatoid arthritis diagnosis, smoking status, alcohol consumption, family history of fractures, history of secondary operation and corticosteroid use. Data were analyzed using Fisher's exact test for categorical data and logistic regression for continuous data. Results One hundred and fifty patients (141 women, nine men) with SLE were included; 52 (34.6%) had sustained at least one fracture. Fracture risk increased with increased age, body mass index, fat mass and average tissue thickness, and decreased lean mass (adjusted for steroid use), as well as with smoking and rheumatoid arthritis. Increased femoral and vertebral bone mineral density conversely decreased fracture risk. Conclusion Our study suggests increased age, body mass index, fat mass, smoking and/or rheumatoid arthritis increase fracture risk in SLE patients. To our knowledge, this is the first demonstration of a correlation between increased fat mass, adjusted for steroid use and fracture risk, in adults, potentially indicating a differential effect of fat on bone metabolism and lessening of lean body mass.

Structural Investigation of a Dimeric Variant of Pyruvate Kinase Muscle Isoform 2

Pyruvate kinase muscle isoform 2 (PKM2) catalyzes the terminal step in glycolysis, transferring a phosphoryl group from phosphoenolpyruvate to ADP, to produce pyruvate and ATP. PKM2 activity is allosterically regulated by fructose 1,6-bisphosphate (FBP), an upstream glycolytic intermediate. FBP stabilizes the tetrameric form of the enzyme. In its absence, the PKM2 tetramers dissociate, yielding a dimer-monomer mixture having lower enzymatic activity. The S437Y variant of PKM2 is incapable of binding FBP. Consistent with that defect, we find that S437Y exists in a monomer-dimer equilibrium in solution, with a Kd of ∼20 μM. Interestingly, however, the protein crystallizes as a tetramer, providing insight into the structural basis for impaired FBP binding of S437Y.

Bacillus anthracis Prolyl 4-Hydroxylase Interacts with and Modifies Elongation Factor Tu

Prolyl hydroxylation is a very common post-translational modification and plays many roles in eukaryotes such as collagen stabilization, hypoxia sensing, and controlling protein transcription and translation. There is a growing body of evidence that suggests that prokaryotes contain prolyl 4-hydroxylases (P4Hs) homologous to the hypoxia-inducible factor (HIF) prolyl hydroxylase domain (PHD) enzymes that act on elongation factor Tu (EFTu) and are likely involved in the regulation of bacterial translation. Recent biochemical and structural studies with a PHD from Pseudomonas putida (PPHD) determined that it forms a complex with EFTu and hydroxylates a prolyl residue of EFTu. Moreover, while animal, plant, and viral P4Hs act on peptidyl proline, most prokaryotic P4Hs have been known to target free l-proline; the exceptions include PPHD and a P4H from Bacillus anthracis (BaP4H) that modifies collagen-like proline-rich peptides. Here we use biophysical and mass spectrometric methods to demonstrate that BaP4H recognizes full-length BaEFTu and a BaEFTu 9-mer peptide for site-specific proline hydroxylation. Using size-exclusion chromatography coupled small-angle X-ray scattering (SEC-SAXS) and binding studies, we determined that BaP4H forms a 1:1 heterodimeric complex with BaEFTu. The SEC-SAXS studies reveal dissociation of BaP4H dimeric subunits upon interaction with BaEFTu. While BaP4H is unusual within bacteria in that it is structurally and functionally similar to the animal PHDs and collagen P4Hs, respectively, this work provides further evidence of its promiscuous substrate recognition. It is possible that the enzyme might have evolved to hydroxylate a universally conserved protein in prokaryotes, similar to the PHDs, and implies a functional role in B. anthracis.

Postrenal transplant metastatic colonic neoplasm: Posttransplant lymphoproliferative disorder or adenocarcinoma?

Transplant recipients are vulnerable to a horde of infections and neoplastic conditions due to immunosuppression. Posttransplant lymphoproliferative disorder (PTLD) is a condition unique to the transplant recipient occurring due to monoclonal lymphocytic proliferation. It may affect any organ system with reportedly highest incidence in the gastrointestinal tract. The incidence of adenocarcinoma of the colon, however, has not been shown to be uniformly higher in transplant recipients. We report here an unusual case of adenocarcinoma of the ascending colon presenting with liver, lymph node and skin metastasis in a transplant recipient, which simulated PTLD both clinically and radiologically. For any gastrointestinal lesion in transplant recipient, the possibility of carcinoma must be considered. However, a high index of suspicion for PTLD facilitates early diagnosis since the treatment of the two conditions is starkly different.

New Mechanistic Insight from Substrate- and Product-Bound Structures of the Metal-Dependent Dimethylsulfoniopropionate Lyase DddQ

The marine microbial catabolism of dimethylsulfoniopropionate (DMSP) by the lyase pathway liberates ∼300 million tons of dimethyl sulfide (DMS) per year, which plays a major role in the biogeochemical cycling of sulfur. Recent biochemical and structural studies of some DMSP lyases, including DddQ, reveal the importance of divalent transition metal ions in assisting DMSP cleavage. While DddQ is believed to be zinc-dependent primarily on the basis of structural studies, excess zinc inhibits the enzyme. We examine the importance of iron in regulating the DMSP β-elimination reaction catalyzed by DddQ as our as-isolated purple-colored enzyme possesses ∼0.5 Fe/subunit. The UV-visible spectrum exhibited a feature at 550 nm, consistent with a tyrosinate-Fe(III) ligand-to-metal charge transfer transition. Incubation of as-isolated DddQ with added iron increases the intensity of the 550 nm peak, whereas addition of dithionite causes a bleaching as Fe(III) is reduced. Both the Fe(III) oxidized and Fe(II) reduced species are active, with similar k_cat values and 2-fold differences in their K_m values for DMSP. The slow turnover of Fe(III)-bound DddQ allowed us to capture a substrate-bound form of the enzyme. Our DMSP-Fe(III)-DddQ structure reveals conformational changes associated with substrate binding and shows that DMSP is positioned optimally to bind iron and is in the proximity of Tyr 120 that acts as a Lewis base to initiate catalysis. The structures of Tris-, DMSP-, and acrylate-bound forms of Fe(III)-DddQ reported here illustrate various states of the enzyme along the reaction pathway. These results provide new insights into DMSP lyase catalysis and have broader significance for understanding the mechanism of oceanic DMS production.

Structural analysis of cofactor binding for a prolyl 4-hydroxylase from the pathogenic bacteriumBacillus anthracis

The prolyl 4-hydroxylases (P4Hs) are mononuclear nonheme iron enzymes that catalyze the formation of 4R-hydroxyproline from many different substrates, with various biological implications. P4H is a key player in collagen accumulation, which has implications in fibrotic disorders. The stabilization of collagen triple-helical structureviaprolyl hydroxylation is the rate-limiting step in collagen biosynthesis, and therefore P4H has been extensively investigated as a potential therapeutic target of fibrotic disease. Understanding how these enzymes recognize cofactors and substrates is important and will aid in the future design of inhibitors of P4H. In this article, X-ray crystal structures of a metallocofactor- and α-ketoglutarate (αKG)-bound form of P4H fromBacillus anthracis(BaP4H) are reported. Structures of BaP4H were solved at 1.63 and 2.35 Å resolution and contained a cadmium ion and αKG bound in the active site. The αKG–Cd–BaP4H ternary complex reveals conformational changes of conserved residues upon the binding of metal ion and αKG, resulting in a closed active-site configuration required for dioxygen, substrate binding and catalysis.

Bacillus anthracis Prolyl 4-Hydroxylase Modifies Collagen-like Substrates in Asymmetric Patterns

Proline hydroxylation is the most prevalent post-translational modification in collagen. The resulting product trans-4-hydroxyproline (Hyp) is of critical importance for the stability and thus function of collagen, with defects leading to several diseases. Prolyl 4-hydroxylases (P4Hs) are mononuclear non-heme iron α-ketoglutarate (αKG)-dependent dioxygenases that catalyze Hyp formation. Although animal and plant P4Hs target peptidyl proline, prokaryotes have been known to use free l-proline as a precursor to form Hyp. The P4H from Bacillus anthracis (BaP4H) has been postulated to act on peptidyl proline in collagen peptides, making it unusual within the bacterial clade, but its true physiological substrate remains enigmatic. Here we use mass spectrometry, fluorescence binding, x-ray crystallography, and docking experiments to confirm that BaP4H recognizes and acts on peptidyl substrates but not free l-proline, using elements characteristic of an Fe(II)/αKG-dependent dioxygenases. We further show that BaP4H can hydroxylate unique peptidyl proline sites in collagen-derived peptides with asymmetric hydroxylation patterns. The cofactor-bound crystal structures of BaP4H reveal active site conformational changes that define open and closed forms and mimic "ready" and "product-released" states of the enzyme in the catalytic cycle. These results help to clarify the role of BaP4H as well as provide broader insights into human collagen P4H and proteins with poly-l-proline type II helices.

Biochemical, Kinetic, and Spectroscopic Characterization of Ruegeria pomeroyi DddW--A Mononuclear Iron-Dependent DMSP Lyase

The osmolyte dimethylsulfoniopropionate (DMSP) is a key nutrient in marine environments and its catabolism by bacteria through enzymes known as DMSP lyases generates dimethylsulfide (DMS), a gas of importance in climate regulation, the sulfur cycle, and signaling to higher organisms. Despite the environmental significance of DMSP lyases, little is known about how they function at the mechanistic level. In this study we biochemically characterize DddW, a DMSP lyase from the model roseobacter Ruegeria pomeroyi DSS-3. DddW is a 16.9 kDa enzyme that contains a C-terminal cupin domain and liberates acrylate, a proton, and DMS from the DMSP substrate. Our studies show that as-purified DddW is a metalloenzyme, like the DddQ and DddP DMSP lyases, but contains an iron cofactor. The metal cofactor is essential for DddW DMSP lyase activity since addition of the metal chelator EDTA abolishes its enzymatic activity, as do substitution mutations of key metal-binding residues in the cupin motif (His81, His83, Glu87, and His121). Measurements of metal binding affinity and catalytic activity indicate that Fe(II) is most likely the preferred catalytic metal ion with a nanomolar binding affinity. Stoichiometry studies suggest DddW requires one Fe(II) per monomer. Electronic absorption and electron paramagnetic resonance (EPR) studies show an interaction between NO and Fe(II)-DddW, with NO binding to the EPR silent Fe(II) site giving rise to an EPR active species (g = 4.29, 3.95, 2.00). The change in the rhombicity of the EPR signal is observed in the presence of DMSP, indicating that substrate binds to the iron site without displacing bound NO. This work provides insight into the mechanism of DMSP cleavage catalyzed by DddW.

Rheostat re-wired: alternative hypotheses for the control of thioredoxin reduction potentials

Thioredoxins are small soluble proteins that contain a redox-active disulfide (CXXC). These disulfides are tuned to oxidizing or reducing potentials depending on the function of the thioredoxin within the cell. The mechanism by which the potential is tuned has been controversial, with two main hypotheses: first, that redox potential (Em) is specifically governed by a molecular 'rheostat'-the XX amino acids, which influence the Cys pKa values, and thereby, Em; and second, the overall thermodynamics of protein folding stability regulates the potential. Here, we use protein film voltammetry (PFV) to measure the pH dependence of the redox potentials of a series of wild-type and mutant archaeal Trxs, PFV and glutathionine-equilibrium to corroborate the measured potentials, the fluorescence probe BADAN to measure pKa values, guanidinium-based denaturation to measure protein unfolding, and X-ray crystallography to provide a structural basis for our functional analyses. We find that when these archaeal thioredoxins are probed directly using PFV, both the high and low potential thioredoxins display consistent 2H+:2e- coupling over a physiological pH range, in conflict with the conventional 'rheostat' model. Instead, folding measurements reveals an excellent correlation to reduction potentials, supporting the second hypothesis and revealing the molecular mechanism of reduction potential control in the ubiquitous Trx family.

Conversion of differentiated cancer cells into cancer stem-like cells in a glioblastoma model after primary chemotherapy

Glioblastoma multiforme patients have a poor prognosis due to therapeutic resistance and tumor relapse. It has been suggested that gliomas are driven by a rare subset of tumor cells known as glioma stem cells (GSCs). This hypothesis states that only a few GSCs are able to divide, differentiate, and initiate a new tumor. It has also been shown that this subpopulation is more resistant to conventional therapies than its differentiated counterpart. In order to understand glioma recurrence post therapy, we investigated the behavior of GSCs after primary chemotherapy. We first show that exposure of patient-derived as well as established glioma cell lines to therapeutic doses of temozolomide (TMZ), the most commonly used antiglioma chemotherapy, consistently increases the GSC pool over time both in vitro and in vivo. Secondly, lineage-tracing analysis of the expanded GSC pool suggests that such amplification is a result of a phenotypic shift in the non-GSC population to a GSC-like state in the presence of TMZ. The newly converted GSC population expresses markers associated with pluripotency and stemness, such as CD133, SOX2, Oct4, and Nestin. Furthermore, we show that intracranial implantation of the newly converted GSCs in nude mice results in a more efficient grafting and invasive phenotype. Taken together, these findings provide the first evidence that glioma cells exposed to chemotherapeutic agents are able to interconvert between non-GSCs and GSCs, thereby replenishing the original tumor population, leading to a more infiltrative phenotype and enhanced chemoresistance. This may represent a potential mechanism for therapeutic relapse.

Accommodation: its relation to refractive errors, amblyopia and biometric parameters

AIM

To study accommodation in relation to different refractive errors, amblyopia and to measure the anatomical changes in the accommodating eye

MATERIALS AND METHODS

We studied the amplitude of accommodation (AA) in 150 patients in the age group 11 ± 30 years which included emmetropes, myopes, hypermetropes and hypermetropic amblyopes using the Royal Air Force (RAF) rule. The anterior chamber depth (ACD), axial length (AxL) and lens thickness (LT) changes during accommodation were measured using an A-scan. Myopes and hypermetropes were further divided based on the amount of refractive error : less than 2D, 2 -4D and greater than 4D.

RESULTS

Corrected low myopes had the highest accommodation amplitude (p less than 0.05) followed by emmetropes. Corrected hypermetropes were found to have the lowest amplitude of accommodation (p less than 0.05). The amblyopic eye had a significantly low AA compared to the non-amblyopic eye (p less than 0.05). ACD decreased (p less than 0.05) and LT increased (p less than 0.05) during accommodation. The AxL increase was maximum in myopes (p less than 0.05) followed by hypermetropes but the change was not significant in hypermetropes (p greater than 0.05).

CONCLUSION

The amblyopic eye has low amplitudes of accommodation proving the benefit of near adds in amblyopic patients. Prolonged near work might induce myopia in susceptible eyes by increasing the axial length.

Presbyopia and its anatomical and physiological variants

AIM

To study the various ocular anatomical and physiological parameters in presbyopia.

MATERIALS AND METHODS

We studied the various ocular anatomical and physiological parameters like corneal curvature (keratometry readings: K1 and K2), central corneal thickness (CCT), anterior chamber depth (ACD), lens thickness (LT) and axial length (AL) in 100 presbyopic patients between 35 - 55 years of age. The patients were divided into two age groups: I (35 ± 44 years) and II (45-55 yrs). ACD, AL and LT were measured using an Ascan. CCT was measured with ultrasonic pachymetry.

RESULTS

The CCT decreased (BE), LT increased and ACD decreased (RE) significantly with increasing age (p less than 0.05). There was no significant difference in males and females. Nearly 3/4th of the total increase in lens thickness was responsible for the decrease in the anterior chamber depth and the rest, 1/4th , goes posteriorly. Corneal curvature and AL showed no significant change with age.

CONCLUSIONS

The mean of CCT decreased significantly with advancing age. As age increased, the mean value of lens thickness increased and anterior chamber depth decreased. Nearly 3/4th of total increase in LT was anteriorly, decreasing the ACD. Corneal curvature and AL has no relation with age.

Structural basis of regiospecificity of a mononuclear iron enzyme in antibiotic fosfomycin biosynthesis

Hydroxypropylphosphonic acid epoxidase (HppE) is an unusual mononuclear iron enzyme that uses dioxygen to catalyze the oxidative epoxidation of (S)-2-hydroxypropylphosphonic acid (S-HPP) in the biosynthesis of the antibiotic fosfomycin. Additionally, the enzyme converts the R-enantiomer of the substrate (R-HPP) to 2-oxo-propylphosphonic acid. To probe the mechanism of HppE regiospecificity, we determined three X-ray structures: R-HPP with inert cobalt-containing enzyme (Co(II)-HppE) at 2.1 Å resolution; R-HPP with active iron-containing enzyme (Fe(II)-HppE) at 3.0 Å resolution; and S-HPP-Fe(II)-HppE in complex with dioxygen mimic NO at 2.9 Å resolution. These structures, along with previously determined structures of S-HPP-HppE, identify the dioxygen binding site on iron and elegantly illustrate how HppE is able to recognize both substrate enantiomers to catalyze two completely distinct reactions.

Structural analysis of a Ni-methyl species in methyl-coenzyme M reductase from Methanothermobacter marburgensis

We present the 1.2 Å resolution X-ray crystal structure of a Ni-methyl species that is a proposed catalytic intermediate in methyl-coenzyme M reductase (MCR), the enzyme that catalyzes the biological formation of methane. The methyl group is situated 2.1 Å proximal of the Ni atom of the MCR coenzyme F(430). A rearrangement of the substrate channel has been posited to bring together substrate species, but Ni(III)-methyl formation alone does not lead to any observable structural changes in the channel.