Differential effects of ischemia and reperfusion on c-Jun N-terminal kinase isoform protein and activity.

Activation of the c-Jun N-terminal (JNK) or stress-activated protein kinases (SAPK) is associated with a wide range of disparate cellular responses to extracellular stimuli, including either induction of or protection from apoptosis. This study investigates the effect of ischemia and reperfusion on JNK isoform activities using a reversible rabbit spinal cord ischemia model. High basal JNK activity, attributed to the p46 JNK1 isoform, was expressed in the CNS of untreated rabbits. JNK activity decreased in the lumbar spinal cord of rabbits occluded for 15-60 min. During reperfusion animals occluded for 15 min recovered neurological function and JNK activity returned to normal levels. In contrast animals occluded for 60 min remained permanently paraplegic and JNK activity was half the control activity after 18 h of reperfusion. In these animals proteolytic fragments of JNK1 and JNK3 were observed and protein levels, but not activity, of JNK isoforms increased in a detergent-insoluble fraction. Two novel c-Jun (and ATF-2) kinase activities increased during reperfusion of animals occluded for 60 min. An activity designated p46(slow) was similar in M(r) to a JNK2 isoform induced in these animals. A second 30-kDa activity associated with the detergent-insoluble fraction co-migrated with a JNK3 N-terminal fragment. The results show that JNK1 is active in the normal CNS and increased activity is not associated with durations of ischemia and reperfusion that induce cell death. However, specific JNK isoform activation may participate in the cell death pathways as increased activity of novel c-Jun (ATF-2) kinase activities was observed in paraplegic animals.

Dephosphorylation of tau during transient forebrain ischemia in the rat.

The effect of transient cerebral ischemia on phosphorylation of the microtubule-associated protein (MAP) tau was investigated using the rat four-vessel occlusion model. Phosphorylation of tau is proposed to regulate its binding to microtubules, influencing the dynamics of microtubule assembly necessary for axonal growth and neurite plasticity. In this study, tau was rapidly dephosphorylated during ischemia in the hippocampus, neocortex, and striatum. Dephosphorylation of tau was observed within 5 min of occlusion and increased after 15 min in all three brain regions, regardless of their relative vulnerability to the insult. Thus, dephosphorylation of tau is an early marker of ischemia and precedes the occlusion time required to cause extensive neuronal cell death in this model. On restoration of blood flow for a little as 15 min, tau was phosphorylated at a site(s) that causes a reduction in its electrophoretic mobility. The dephosphorylation/phosphorylation of tau may alter its distribution between axon and cell body, and affect its susceptibility to proteolysis. These changes would be expected to influence microtubule stability, possibly contributing to disruption of axonal transport, but also allowing neurite remodeling in a regenerative response.

Effect of cerebral ischemia on calcium/calmodulin-dependent protein kinase II activity and phosphorylation.

The effects of cerebral ischemia on calcium/calmodulin-dependent kinase II (CaM kinase II) were investigated using the rat four-vessel occlusion model. In agreement with previous results using rat or gerbil models of cerebral ischemia or a rabbit model of spinal cord ischemia, this report demonstrates that transient forebrain ischemia leads to a reduction in CaM kinase II activity within 5 min of occlusion onset. Loss of activity from the cytosol fractions of homogenates from the neocortex, striatum, and hippocampus correlated with a decrease in the amount of CaM kinase alpha and beta isoforms detected by immunoblotting. In contrast, there was an apparent increase in the amount of CaM kinase alpha and beta in the particulate fractions. The decrease in the amount of CaM kinase isoforms from the cytosol but not the particulate fractions was confirmed by autophosphorylation of CaM kinase II after denaturation and renaturation in situ of the blotted proteins. These results indicate that ischemia causes a rapid inhibition of CaM kinase II activity and a change in the partitioning of the enzyme between the cytosol and particulate fractions. CaM kinase II is a multifunctional protein kinase, and the loss of activity may play a critical role in initiating the changes leading to ischemia-induced cell death. To identify a structural basis for the decrease in enzyme activity, tryptic peptide maps of CaM kinase II phosphorylated in vitro were compared. Phosphopeptide maps of CaM kinase alpha from particulate fractions of control and ischemic samples revealed not only reduced incorporation of phosphate into the protein but also the absence of a limited number of peptides in the ischemic samples. This suggested that certain sites are inaccessible, possibly due to a conformational change, a covalent modification of CaM kinase II, or steric hindrance by an associated molecule. Verifying one of these possibilities should help to elucidate the mechanism of ischemia-induced modulation of CaM kinase II.

Light and heavy lysosomes: characterization of N-acetyl-beta-D-hexosaminidase isolated from normal and I-cell disease lymphoblasts.

We previously reported that I-cell disease lymphoblasts maintain normal or near-normal intracellular levels of lysosomal enzymes, even though N-acetylglucosamine-1-phosphotransferase activity is severely depressed or absent (Little et al., Biochem. J., 248, 151-159, 1987). The present study, employing subcellular fractionation on colloidal silica gradients, indicates that both light and heavy lysosomes isolated from I-cell disease and pseudo-Hurler polydystrophy lymphoblasts possess normal specific activity levels of N-acetyl-beta-D-hexosaminidase, alpha-D-mannosidase and beta-D-glucuronidase. These current findings are in contrast to those of cultured fibroblasts from the same patients, where decreased intralysosomal enzyme activities are found. Column chromatography on Ricinus communis revealed that N-acetyl-beta-D-hexosaminidase in both heavy and light I-cell disease lysosomal fractions from lymphoblasts possesses an increased number of accessible galactose residues (30-50%) as compared to the enzyme from the corresponding normal controls. Endo-beta-N-acetylglucosaminidase H treatment of N-acetyl-beta-D-hexosaminidase from the I-cell lysosomal fractions suggests that the majority of newly synthesized high-mannose-type oligosaccharide chains are modified to complex-type carbohydrates prior to being transported to lysosomes. This result from lymphoblasts differs from previous findings with fibroblasts, where N-acetyl-beta-D-hexosaminidase from I-cell disease and pseudo-Hurler polydystrophy lysosomes exhibited properties associated with predominantly high-mannose-type oligosaccharide chains.(ABSTRACT TRUNCATED AT 250 WORDS)

Inactivation and subcellular redistribution of Ca2+/calmodulin-dependent protein kinase II following spinal cord ischemia.

Reversible spinal cord ischemia in rabbits induced a rapid loss of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) activity measured as incorporation of phosphate into exogenous substrates. About 70% of the activity was lost from the cytosolic fraction of spinal cord homogenates after 15 min of ischemia preceding irreversible paraplegia, which takes 25 min in this model. The loss of enzyme activity correlated with a loss of in situ renaturable autophosphorylation activity and a loss of CaM kinase II alpha and beta subunits in the cytosol detected by immunoblotting. CaM kinase II activity in the particulate fraction also decreased but the protein levels of the alpha and beta subunits increased. Thus ischemia resulted in an inactivation of CaM kinase II and a sequential or concurrent subcellular redistribution of the enzyme. However, denaturation and renaturation in situ of the CaM kinase subunits immobilized on membranes partly reversed the apparent inactivation of the enzyme in the particulate fraction. CaM kinase II activity was restored after reperfusion following short (< or = 25 min) durations of ischemia but not after longer durations (60 min) that result in irreversible paraplegia. The ischemia-induced inactivation of CaM kinase II, which phosphorylates proteins regulating many cellular processes, may be important in the cascade of events leading to delayed neuronal cell death.

Properties of N-acetylglucosamine 1-phosphotransferase from human lymphoblasts.

Human lymphoblast and fibroblast cell lines from a patient with I-cell disease and normal individuals were characterized with respect to certain properties of UDP-N-acetylglucosamine:lysosomal enzyme precursor N-acetylglucosamine phosphotransferase. The enzyme isolated from normal lymphoblast and fibroblast cell lines expressed similar kinetic properties, substrate specificities and subcellular localizations. Coincident with the severe reduction of N-acetylglucosamine phosphotransferase activity in both I-cell fibroblast and lymphoblast cell lines, there was an increased secretion of several lysosomal enzymes compared to normal controls. Subsequent examination of N-acetyl-beta-D-hexosaminidase secreted by the I-cell lymphoblasts demonstrated a significant increase in adsorption of the I-cell enzyme to Ricinus communis agglutinin, a galactose-specific lectin. However, the I-cell lymphoblasts did not exhibit the significant decrease in intracellular lysosomal activities seen in I-cell fibroblasts. Our results suggest that lymphoblasts not only represent an excellent source for the purification of N-acetylglucosamine phosphotransferase, but in addition, represent a unique system for studying alternate mechanisms involved in the targeting of lysosomal enzymes.

Characterization of lysosomes and lysosomal enzymes from Chediak-Higashi-syndrome cultured fibroblasts.

Chediak-Higashi-syndrome cultured skin fibroblasts were used to study the possible involvement of lysosomal enzymes and lysosomal dysfunction in this disorder. Our evidence indicated that Chediak-Higashi fibroblasts displayed a significant decrease in the specific activity of the acidic alpha-D-mannosidase (pH 4.2) compared with normal controls. Additional studies revealed a small, but significant, decrease in the rate of degradation of 125I-labelled beta-D-glucosidase that had been endocytosed into Chediak-Higashi cells.

Uptake of alpha-D-mannosidase and beta-D-glucosidase from Dictyostelium discoideum via the phosphohexosyl receptor on normal human fibroblasts.

alpha-D-Mannosidase and beta-D-glucosidase from Dictyostelium discoideum are efficiently endocytosed into mutant human fibroblasts through a saturable, mannose 6-phosphate (Man-6-P)-inhibitable uptake system (Freeze, H. H., Kaplan, A., and Miller, A. L. (1980) J. Biol. Chem. 255, 11081-11084). We have extended this study using both of these active, purified enzymes and 125I-labeled beta-glucosidase for uptake into normal human fibroblasts. The pH optimum of uptake is 6.0 for both enzymes and greater than 95% is inhibited by 2 mM Man-6-P (Ki = 5 X 10(-5) M). A variety of mono-and diesterified mannans or mannan derivatives also inhibited uptake of the enzymes. Both enzymes compete with each other for uptake (Ki, 2.0 X 10(-9) M) and have Kuptake of 1.0-2.2 X 10(-9) M and a Vmax of 0.35-0.48 pmol/mg of cell protein/h. The specific binding of 125I-beta-glucosidase to fibroblasts was measured at 0-4 degrees C and found to have a Kd of 1.0 X 10(-9) M with approximately 15,900 +/- 900 receptors/cell. The receptors could be internalized every 5-7 min at saturating concentrations of enzyme at 37 degrees C. The 125I-beta-glucosidase previously bound to the cells at 4 degrees C could be released by continued incubation at 4 degrees C in the presence of Man-6-P, however, after brief warming to 37 degrees C followed by reincubation at 4 degrees C, Man-6-P could no longer release the ligand. Chloroquine inhibited 95% of the uptake of 125I-beta-glucosidase at 50 microM. Following internalization of the enzyme, it is degraded to trichloroacetic acid-soluble fragments with a half-life of approximately 6.5 h. These data suggest that the slime mold enzymes are bound to the same receptors which function in the uptake of mammalian lysosomal enzymes and make the slime mold lysosomal enzymes useful models to study uptake involving this receptor in normal human fibroblasts.