Keratocan-deficient mice display alterations in corneal structure

Keratocan (Kera) is a cornea-specific keratan sulfate proteoglycan (KSPG) in the adult vertebrate eye. It belongs to the small leucine-rich proteoglycan (SLRP) gene family and is one of the major components of extracellular KSPG in the vertebrate corneal stroma. The Kera gene is expressed in ocular surface tissues including cornea and eyelids during morphogenesis. Corneal KSPGs play a pivotal role in matrix assembly, which is accountable for corneal transparency. In humans, mutations of the KERA gene are associated with cornea plana (CNA2) that manifests decreases in vision acuity due to the flattened forward convex curvature of cornea. To investigate the biological role of the Kera gene and to establish an animal model for corneal plana, we generated Kera knockout mice via gene targeting. Northern and Western blotting and immunohistochemical analysis showed that no Kera mRNA or keratocan protein was detected in the Kera-/- cornea. The expression levels of other SLRP members including lumican, decorin, and fibromodulin were not altered in the Kera-/- cornea as compared with that of the wild-type littermates. Mice lacking keratocan have normal corneal transparency at the age of 12 months. However, they have a thinner corneal stroma and a narrower cornea-iris angle of the anterior segment in comparison to the wild-type littermates. As demonstrated by transmission electron microscopy, Kera-/- mice have larger stromal fibril diameters and less organized packing of collagen fibrils in stroma than those of wild type. Taken together, our results showed that ablation of the Kera gene resulted in subtle structural alterations of collagenous matrix and did not perturb the expression of other SLRPs in cornea. Keratocan thus plays a unique role in maintaining the appropriate corneal shape to ensure normal vision.     

Histone deacetylase 4 interacts with 53BP1 to mediate the DNA damage response

Anumber of proteins are recruited to nuclear foci upon exposure to double-strand DNA damage, including 53BP1 and Rad51, but the precise role of these DNA damage-induced foci remain unclear. Here we show in a variety of human cell lines that histone deacetylase (HDAC) 4 is recruited to foci with kinetics similar to, and colocalizes with, 53BP1 after exposure to agents causing double-stranded DNA breaks. HDAC4 foci gradually disappeared in repair-proficient cells but persisted in repair-deficient cell lines or cells irradiated with a lethal dose, suggesting that resolution of HDAC4 foci is linked to repair. Silencing of HDAC4 via RNA interference surprisingly also decreased levels of 53BP1 protein, abrogated the DNA damage-induced G2 delay, and radiosensitized HeLa cells. Our combined results suggest that HDAC4 is a critical component of the DNA damage response pathway that acts through 53BP1 and perhaps contributes in maintaining the G2 cell cycle checkpoint.     

APP processing is regulated by cytoplasmic phosphorylation

Amyloid-beta peptide (Abeta) aggregate in senile plaque is a key characteristic of Alzheimer's disease (AD). Here, we show that phosphorylation of amyloid precursor protein (APP) on threonine 668 (P-APP) may play a role in APP metabolism. In AD brains, P-APP accumulates in large vesicular structures in afflicted hippocampal pyramidal neurons that costain with antibodies against endosome markers and the beta-secretase, BACE1. Western blot analysis reveals increased levels of T668-phosphorylated APP COOH-terminal fragments in hippocampal lysates from many AD but not control subjects. Importantly, P-APP cofractionates with endosome markers and BACE1 in an iodixanol gradient and displays extensive colocalization with BACE1 in rat primary cortical neurons. Furthermore, APP COOH-terminal fragments generated by BACE1 are preferentially phosphorylated on T668 verses those produced by alpha-secretase. The production of Abeta is significantly reduced when phosphorylation of T668 is either abolished by mutation or inhibited by T668 kinase inhibitors. Together, these results suggest that T668 phosphorylation may facilitate the BACE1 cleavage of APP to increase Abeta generation.     

Damage formation and repair efficiency in the p53 gene of cell lines and blood lymphocytes assayed by multiplex long quantitative polymerase chain reaction

We examined ultraviolet (UV) irradiation and cisplatin treatment damage formation and repair efficiency in the p53 tumor suppressor gene of various cultured cell lines and lymphocytes using a nonradioactive multiplex long quantitative polymerase chain reaction (QPCR) assay, which amplified a 7-kb fragment of the target gene and a 500-bp fragment of the template control to successfully increase the sensitivity and reliability of the assay. The multiplex long QPCR detected a lesion frequency of 0.63 lesions/10kb/10J/m(2) in the p53 gene of fibroblast cells. In addition, the multiplex long QPCR assay detected pronounced differences in the repair of UV damage in the p53 gene among repair-proficient CRL-1475 cells and repair-deficient XP-A and XP-C cells. The multiplex long QPCR assay was also evaluated as a sensitive assay for the detection of DNA damage induced by cisplatin. The data indicated that the lesion frequency in the p53 gene was 1.27-1.75 times higher in the H23 cisplatin-sensitive cell than in the H1435 cisplatin-resistant cell at the IC(70) dose. After 8-h and 24-h repair periods, only 13 and 75% of cisplatin-induced damage had been removed in the H23 cells, whereas these values were 92 and 100% in the H1435 cells. In addition, our data indicate that multiplex long QPCR is a sensitive method for validly estimating repair in freshly isolated lymphocytes. The results suggest that the current protocol of the multiplex long QPCR method can be used to assess the damage formation and repair efficiency of various agents at biologically relevant doses and to allow a more precise determination of gene-specific repair in disease susceptibility and drug resistance in epidemiological studies.     

Changes in root cap pH are required for the gravity response of the Arabidopsis root

Although the columella cells of the root cap have been identified as the site of gravity perception, the cellular events that mediate gravity signaling remain poorly understood. To determine if cytoplasmic and/or wall pH mediates the initial stages of root gravitropism, we combined a novel cell wall pH sensor (a cellulose binding domain peptide-Oregon green conjugate) and a cytoplasmic pH sensor (plants expressing pH-sensitive green fluorescent protein) to monitor pH dynamics throughout the graviresponding Arabidopsis root. The root cap apoplast acidified from pH 5.5 to 4.5 within 2 min of gravistimulation. Concomitantly, cytoplasmic pH increased in columella cells from 7.2 to 7.6 but was unchanged elsewhere in the root. These changes in cap pH preceded detectable tropic growth or growth-related pH changes in the elongation zone cell wall by 10 min. Altering the gravity-related columella cytoplasmic pH shift with caged protons delayed the gravitropic response. Together, these results suggest that alterations in root cap pH likely are involved in the initial events that mediate root gravity perception or signal transduction.     

Nigericin inhibits insulin-stimulated glucose transport in 3T3-L1 adipocytes

We used nigericin, a K+/H+ exchanger, to test whether glucose transport in 3T3-L1 adipocytes was modulated by changes in intracellular pH. Our results showed that nigericin increased basal but decreased insulin-stimulated glucose uptake in a time- and dose-dependent manner. Whereas the basal translocation of GLUT1 was enhanced, insulin-stimulated GLUT4 translocation was inhibited by nigericin. On the other hand, the total amount of neither transporter protein was altered. The finding that insulin-stimulated phosphoinositide 3-kinase (PI 3-kinase) activity was not affected by nigericin implies that nigericin exerted its inhibition at a step downstream of PI 3-kinase activation. At maximal dose, nigericin rapidly lowered cytosolic pH to 6.7; however, this effect was transient and cytosolic pH was back to normal in 20 min. Removal of nigericin from the incubation medium after 20 min abolished its enhancing effect on basal but had little influence on its inhibition of insulin-stimulated glucose transport. Moreover, lowering cytosolic pH to 6.7 with an exogenously added HCl solution had no effect on glucose transport. Taken together, it appears that nigericin may inhibit insulin-stimulated glucose transport mainly by interfering with GLUT4 translocation, probably by a mechanism not related to changes in cytosolic pH.