DNA-protein cross-linking via guanine oxidation: dependence upon protein and photosensitizer

DNA-protein cross-links form when guanine undergoes a 1-electron oxidation in a flash-quench experiment, and the importance of reactive oxygen species, protein, and photosensitizer is examined here. In these experiments, a strong oxidant produced by oxidative quenching of a DNA-bound photosensitizer generates an oxidized guanine base that reacts with protein to form the covalent adduct. These cross-links are cleaved by hot piperidine and are not the result of reactive oxygen species, since neither a hydroxyl radical scavenger (mannitol) nor oxygen affects the yield of DNA-histone cross-linking, as determined via a chloroform extraction assay. The cross-linking yield depends on protein, decreasing as histone > cytochrome c > bovine serum albumin. The yield does not depend on the cytochrome oxidation state, suggesting that reduction of the guanine radical by ferrocytochrome c does not compete effectively with cross-linking. The photosensitizer strongly influences the cross-linking yield, which decreases in the order Ru(phen)(2)dppz(2+) [phen = 1,10-phenanthroline; dppz = dipyridophenazine] > Ru(bpy)(3)(2+) [bpy = 2,2'-bipyridine] > acridine orange > ethidium, in accordance with measured oxidation potentials. A long-lived transient absorption signal for ethidium dication in poly(dG-dC) confirms that guanine oxidation is inefficient for this photosensitizer. From a polyacrylamide sequencing gel of a (32)P-labeled 40-mer, all of these photosensitizers are shown to damage guanines preferentially at the 5' G of 5'-GG-3' steps, consistent with a 1-electron oxidation. Additional examination of ethidium shows that it can generate cross-links between histone and plasmid DNA (pUC19) and that the yield depends on the quencher. Altogether, these results illustrate the versatility of the flash-quench technique as a way to generate physiologically relevant DNA-protein adducts via the oxidation of guanine and expand the scope of such cross-linking reactions to include proteins that may associate only transiently with DNA.     

The chemokine receptor CXCR2 controls positioning of oligodendrocyte precursors in developing spinal cord by arresting their migration

Spinal cord oligodendrocytes originate in the ventricular zone and subsequently migrate to white matter, stop, proliferate, and differentiate. Here we demonstrate a role for the chemokine CXCL1 and its receptor CXCR2 in patterning the developing spinal cord. Signaling through CXCR2, CXCL1 inhibited oligodendrocyte precursor migration. The migrational arrest was rapid, reversible, concentration dependent, and reflected enhanced cell/substrate interactions. White matter expression of CXCL1 was temporo-spatially regulated. Developing CXCR2 null spinal cords contained reduced oligodendrocytes, abnormally concentrated at the periphery. In slice preparations, CXCL1 inhibited embryonic oligodendrocyte precursor migration, and widespread dispersal of postnatal precursors occurred in the absence of CXCR2 signaling. These data suggest that population of presumptive white matter by oligodendrocyte precursors is dependent on localized expression of CXCL1.     

Model of ionic transport for bovine ciliary epithelium: effects of acetazolamide and HCO

The possible existence of transepithelial bicarbonate transport across the isolated bovine ciliary body was investigated by employing a chamber that allows for the measurement of unidirectional, radiolabeled fluxes of CO2 + HCO. No net flux of HCO was detected. However, acetazolamide (0.1 mM) reduced the simultaneously measured short-circuit current (I(sc)). In other experiments in which (36)Cl- was used, a net Cl- flux of 1.12 microeq. h(-1). cm(-2) (30 microA/cm(2)) in the blood-to-aqueous direction was detected. Acetazolamide, as well as removal of HCO from the aqueous bathing solution, inhibited the net Cl- flux and I(sc). Because such removal should increase HCO diffusion toward the aqueous compartment and increase the I(sc), this paradoxical effect could result from cell acidification and partial closure of Cl- channels. The acetazolamide effect on Cl- fluxes can be explained by a reduction of cellular H+ and HCO (generated from metabolic CO2 production), which exchange with Na+ and Cl- via Na+/H+ and Cl-/HCO exchangers, contributing to the net Cl- transport. The fact that the net Cl- flux is about three times larger than the I(sc) is explained with a vectorial model in which there is a secretion of Na+ and K+ into the aqueous humor that partially subtracts from the net Cl- flux. These transport characteristics of the bovine ciliary epithelium suggest how acetazolamide reduces intraocular pressure in the absence of HCO transport as a driving force for fluid secretion.     

Methionine oxidation within the cerebroside-sulfate activator protein (CSAct or Saposin B)

The cerebroside-sulfate activator protein (CSAct or Saposin B) is a small water-soluble glycoprotein that plays an essential role in the metabolism of certain glycosphingolipids, especially sulfatide. Deficiency of CSAct in humans leads to sulfatide accumulation and neurodegenerative disease. CSAct activity can be measured in vitro by assay of its ability to activate sulfatide-sulfate hydrolysis by arylsulfatase A. CSAct has seven methionine residues and a mass of 8,845 Da when deglycosylated. Mildly oxidized, deglycosylated CSAct (+16 Da), separated from nonoxidized CSAct by reversed-phase high-performance liquid chromatography (RP-HPLC), showed significant modulation of the in vitro activity. Because oxidation partially protected against CNBr cleavage and could largely be reversed by treatment with dithiothreitol, it was concluded that the major modification was conversion of a single methionine to its sulfoxide. High-resolution RP-HPLC separated mildly oxidized CSAct into seven or more different components with shorter retention times than nonoxidized CSAct. Mass spectrometry showed these components to have identical mass (+16 Da). The shorter retention times are consistent with increased polarity accompanying oxidation of surface-exposed methionyl side chains, in general accordance with the existing molecular model. A mass-spectrometric CNBr mapping protocol allowed identification of five of the seven possible methionine-sulfoxide CSAct oxoforms. The most dramatic suppression of activity occurred upon oxidation of Met61 (26% of control) with other residues in the Q60MMMHMQ66 motif falling in the 30-50% activity range. Under conditions of oxidative stress, accumulation of minimally oxidized CSAct protein in vivo could perturb metabolism of sulfatide and other glycosphingolipids. This, in turn, could contribute to the onset and progression of neurodegenerative disease, especially in situations where the catabolism of these materials is marginal.     

Antibody-Secreting Cell Responses and Protective Immunity Assessed in Gnotobiotic Pigs Inoculated Orally or Intramuscularly with Inactivated Human Rotavirus

Newborn gnotobiotic pigs were inoculated twice perorally (p.o.) (group 1) or intramuscularly (i.m.) (group 2) or three times i.m. (group 3) with inactivated Wa strain human rotavirus and challenged with virulent Wa human rotavirus 20 to 24 days later. To assess correlates of protection, antibody-secreting cells (ASC) were enumerated in intestinal and systemic lymphoid tissues from pigs in each group at selected postinoculation days (PID) or postchallenge days. Few virus-specific ASC were detected in any tissues of group 1 pigs prior to challenge. By comparison, groups 2 and 3 had significantly greater numbers of virus-specific immunoglobulin M (IgM) ASC in intestinal and splenic tissues at PID 8 and significantly greater numbers of virus-specific IgG ASC and IgG memory B cells in spleen and blood at challenge. However, as for group 1, few virus-specific IgA ASC or IgA memory B cells were detected in any tissues of group 2 and 3 pigs. Neither p.o. nor i.m. inoculation conferred significant protection against virulent Wa rotavirus challenge (0 to 6% protection rate), and all groups showed significant anamnestic virus-specific IgG and IgA ASC responses. Hence, high numbers of IgG ASC or memory IgG ASC in the systemic lymphoid tissues at the time of challenge did not correlate with protection. Further, our findings suggest that inactivated Wa human rotavirus administered either p.o. or parenterally is significantly less effective in inducing intestinal IgA ASC responses and conferring protective immunity than live Wa human rotavirus inoculated orally, as reported earlier (L. Yuan, L. A. Ward, B. I. Rosen, T. L. To, and L. J. Saif, J. Virol. 70:3075–3083, 1996). Thus, more efficient mucosal delivery systems and rotavirus vaccination strategies are needed to induce intestinal IgA ASC responses, identified previously as a correlate of protective immunity to rotavirus.     

HtrA1 is upregulated during RANKL-induced osteoclastogenesis,and negatively regulates osteoblast differentiation and BMP2-induced Smad1/5/8, ERK and p38 phosphorylation

Bone remodeling is regulated by secreted factors in the bone microenvironment. However, data regarding osteoclast-derived factors that influence osteoblast differentiation are lacking. Here, we show that HtrA1 is produced as a secreted protein during osteoclastogenesis, and negatively regulates osteoblast differentiation. Exogenous addition of recombinant HtrA1 attenuates osteoblast differentiation and BMP2-induced Smad1/5/8, ERK1/2 and p38 phosphorylation in pre-osteoblasts. Our studies imply a unique mode of crosstalk in which HtrA1 is produced by both osteoclasts and osteoblasts and negatively regulates osteoblast differentiation, suggesting that HtrA1 may mediate the fine tuning of paracrine and autocrine regulations during bone remodeling processes.