Biochemical studies on degenerative neurological disorders. I. Acute experimental encephalitis

18F-FEAnGA for PET of β-glucuronidase activity in neuroinflammation

Activation of microglia is a hallmark of inflammatory, infectious, and degenerative diseases of the central nervous system. Several studies have indicated that there is an increase in release of β-glucuronidase by activated microglia into the extracellular space at the site of neuroinflammation. β-glucuronidase is involved in the hydrolysis of glycosaminoglycans on the cell surface and the degradation of the extracellular matrix. Therefore, β-glucuronidase might be a biomarker for ongoing neurodegeneration induced by neuroinflammation. In this study, we investigated whether the PET tracer (18)F-FEAnGA was able to detect β-glucuronidase release during neuroinflammation in a rat model of herpes encephalitis.

METHODS

Male Wistar rats were intranasally inoculated with herpes simplex virus 1 (HSV-1) or phosphate-buffered saline as a control. (11)C-(R)-PK11195 and (18)F-FEAnGA small-animal PET scans were acquired for 60 min. Logan graphical analysis was used to calculate (18)F-FEAnGA distribution volumes (DV(Logan)) in various brain areas.

RESULTS

After administration of (18)F-FEAnGA, the area under the activity concentration-versus-time curve of the whole brain was 2 times higher in HSV-1-infected rats than in control rats. In addition, the DV(Logan) of (18)F-FEAnGA was most increased in the frontopolar cortex, frontal cortex, bulbus olfactorius, cerebral cortex, cerebellum, and brainstem of HSV-1-infected rats, when compared with control rats. The conversion of (18)F-FEAnGA to 4-hydroxy-3-nitrobenzyl alcohol was found to be 1.6 times higher in HSV-1-infected rats than in control rats and correlated with the DV(Logan) of (18)F-FEAnGA in the same areas of the brain. Furthermore, the DV(Logan) of (18)F-FEAnGA also correlated with β-glucuronidase activity in the same brain regions. In addition, DV(Logan) of (18)F-FEAnGA showed a tendency to correlate with (11)C-(R)-PK11195 uptake (marker for activated microglia) in the same brain regions.

CONCLUSION

Despite relatively low brain uptake, (18)F-FEAnGA was able to detect an increased release of β-glucuronidase during neuroinflammation.

Lysosomal hydrolases of different classes are abnormally distributed in brains of patients with Alzheimer disease

beta-Amyloid formation requires multiple abnormal proteolytic cleavages of amyloid precursor protein (APP), including one within its intramembrane domain. Lysosomes, which contain a wide variety of proteases (cathepsins) and other acid hydrolases, are major sites for the turnover of membrane proteins and other cell constituents. Using immunocytochemistry, immunoelectron microscopy, and enzyme histochemistry, we studied the expression and cellular distributions of 10 lysosomal hydrolases, including 4 cathepsins, in neocortex from patients with Alzheimer disease and control (non-Alzheimer-disease) individuals. In control brains, acid hydrolases were localized exclusively to intracellular lysosome-related compartments, and 8 of the 10 enzymes predominated in neurons. In Alzheimer disease brains, strongly immunoreactive lysosomes and lipofuscin granules accumulated markedly in the perikarya and proximal dendrites of many cortical neurons, some of which were undergoing degeneration. More strikingly, these same hydrolases were present in equally high or higher levels in senile plaques in Alzheimer disease, but they were not found extracellularly in control brains, including those from Parkinson or Huntington disease patients. At the ultrastructural level, hydrolase immunoreactivity in senile plaques was localized to extracellular lipofuscin granules similar in morphology to those within degenerating neurons. Two cathepsins that were undetectable in neurons were absent from senile plaques. These results show that lysosome function is altered in cortical neurons in Alzheimer disease. The presence of a broad spectrum of acid hydrolases in senile plaques indicates that lysosomes and their contents may be liberated from cells, principally neurons and their processes, as they degenerate. Because cathepsins can cleave polypeptide sites on APP relevant for beta-amyloid formation, their abnormal extracellular localization and dysregulation in Alzheimer disease can account for the multiple hydrolytic events in beta-amyloid formation. The actions of membrane-degrading acid hydrolases could also explain how the intramembrane portion of APP containing the C terminus of beta-amyloid becomes accessible to proteases.

Cerebral glycosidases in experimental Creutzfeldt-Jakob disease

In Creutzfeldt-Jakob disease (CJD), there are prominent ultrastructural alterations of the plasma membrane, which contains many glycolipids and glycoproteins. Glycosidases can degrade glycolipids and glycoproteins. Gangliosides, a subset of glycolipids, are decreased in amount at the terminal stages of CJD, and CJD infectivity is closely associated with membrane rich fractions. We therefore studied 10 glycosidases, and found a statistically significant increase in beta-xylosidase, beta-glucuronidase, N-acetyl-beta-D-glucosaminidase and N-acetyl-beta-D-galactosaminidase activities in CJD. In contrast, alpha-glucosidase, beta-glucosidase, alpha-galactosidase, alpha-mannosidase, alpha-fucosidase, and beta-galactosidase were not significantly changed. The above results are consistent with degenerative membrane changes observed morphologically, and with increased degradation of sugar residues on lipids and/or proteins. These changes may be effected by the accumulation of the CJD agent in cell membranes. We suggest that the higher activities of these enzymes in CJD may be partially responsible for some of the structural and biochemical alterations in CJD infected brains.

Subcellular pathology of human neurodegenerative disorders: Alzheimer-type dementia and Huntington's disease

Activities of enzyme markers of subcellular organelles have been measured in brain tissue from subjects with Alzheimer-type dementia (ATD) and Huntington's disease (HD). Significant increases in the activity of the lysosomal enzyme beta-glucuronidase were observed in both ATD temporal cortex and HD putamen. It is suggested that beta-glucuronidase activity may be a useful biochemical indicator of cellular damage in the CNS. A significant reduction in neutral alpha-glucosidase activity was observed in ATD temporal cortex and HD putamen. This change may reflect an alteration in glycoconjugate processing and may relate to the susceptibility of neurones to the degenerative processes of ATD and HD.

Neurotoxic effects of the combined exposure to carbon disulphide and ethanol in rats

Neurotoxic effects of the combined exposure of rats to carbon disulphide (CS2) and ethanol (EtOH) were studied. Biochemical and ultrastructural evaluation of the central nervous system (CNS) and peripheral nervous system (PNS) was performed. Male Wistar rats were exposed to CS2 vapour (0.8 mg/l air) and to 10% alcohol in the drinking water for 8 months. EtOH elevated the increase in beta-glucuronidase activity caused by CS2 in the hippocampus and in the cerebral cortex. No effect on the high-affinity synaptosomal uptake of L-glutamate and GABA was observed and no marked ultrastructural changes in the tested brain regions were found. In the peripheral nerves CS2 alone evoked axonal degeneration whereas CS2 combined with EtOH caused disturbances in myelin. Ultrastructural changes preceded biochemical alterations in the PNS and the biochemical indicators of peripheral neuropathy such as beta-glucuronidase activity and cholesterol ester content were not significantly affected. It is suggested that CS2 and EtOH combined affect both PNS and CNS to a higher extent than each of these substances alone.

The neurotoxicity of aflatoxin B1 in the rat

The effects of repeated intraperitoneal administration of aflatoxin B1 on the peripheral and central nervous systems of rats were investigated. Biochemical markers of neurotoxicity were monitored in nervous tissues following aflatoxin B1 dosage and after the cessation of aflatoxin B1 administration. Aflatoxin B1 increased the activities of beta-glucuronidase and beta-galactosidase in the central and peripheral nervous systems. Repeated exposure of rats to aflatoxin B1 also activated Na+ K+-ATPase and inhibited Mg2+-ATPase. Nervous tissue levels of DNA and total protein increased while the concentrations of RNA and phospholipid were depressed by aflatoxin B1. The alterations in these parameters were specific for each of the tissues examined during the recovery of the rats. The findings indicate that the repeated administration of aflatoxin B1 to rats results in degeneration in the central and peripheral nervous systems that may be related to the overt toxicity observed following aflatoxin administration.

A biochemical assessment of the neurotoxicity of the radiosensitizing drug misonidazole in the rat

We have obtained biochemical evidence that misonidazole when administered in large doses to rats produces a sparse dying-back peripheral neuropathy and degenerative changes in the trigeminal ganglia and cerebellum. In our experience these neurotoxic effects of misonidazole cannot be detected reliably by the use of simple behavioural and functional tests, e.g., inclined plane and narrowing bridge tests (Rose and Dewar, unpublished results). Therefore, these methods would be of limited use in the neurotoxicity screening of misonidazole analogues. On the other hand, the biochemical approach provides a convenient quantitative method which could be used as the basis for comparing the neurotoxicity of other candidate radiosensitizing drugs.

A biochemical method for assessing the neurotoxic effects of misonidazole in the rat

A proven biochemical method for assessing chemically induced neurotoxicity has been applied to the study of the toxic effects of misonidazole (MISO) in the rat. This involves the fluorimetric measurement of beta-glucuronidase and beta-galactosidase activities in homogenates of rat nervous tissue. The tissues analysed were sciatic/posterior tibial nerve (SPTN) cut into 4 sections, trigeminal ganglia and cerebellum. MISO administered i.p. to Wistar rats in doses greater than 300 mg/kg/day for 7 consecutive days produced maximal increases in both beta-glucuronidase and beta-galactosidase activities in th SPTN at 4 weeks (140-180% of control values). The highest increases were associated with the most distal secretion of the nerve. Significant enzyme-activity changes were also found in the trigeminal ganglia and cerebellum of MISO-dosed rats. The greatest activity occurred 4-5 weeks after dosing, and was dose-related. It is concluded that, in the rat, MISO can produce biochemical changes consistent with a dying-back peripheral neuropathy, and biochemical changes suggestive of cerebellar damage. This biochemical approach would appear to offer a convenient quantitative method for the detection of neurotoxic effects of other potential radio-sensitizing drugs.

Brain lysosomal glycosidase activity in immunosuppressed mice infected with avirulent Semliki forest virus

Mice infected with an avirulent strain of Semliki forest virus show an increase in the activity of some of the brain lysosomal glycosidases. The increase in activity of these enzymes has been correlated with the histological, virological, and serological changes that result from the infection in the presence and absence of immunosuppression. Semliki forest virus alone caused the development of a mild encephalitis with perivascular infiltration, microgliosis, astrocyte hypertrophy, and a focal spongiform encephalopathy, together with an increased activity of brain N-acetyl-beta-D-glucosaminidase and beta-glucuronidase. Antilymphocyte serum given after infection marginally affected the course of the disease. Cyclophosphamide markedly delayed the development of the spongy changes in the increase in enzyme activities, but not the perivascular infiltration. It is suggested that the increased activity of the lysosomal glycosidases studied may be linked both to the development of a successful immune response and to the focal spongiform changes produced by the infection.

Myelin composition in acute and chronic multiple sclerosis in relation to cerebral lysosomal activity

The neuropathology of 3 cases of acute multiple sclerosis was correlated with biochemical analyses. Astrocytosis was a characteristic feature of the diffuse demyelinating lesions in one case and lymphocytic cuffing characterized the well-defined plaques present in the white matter of the other two cases. No abnormalities were found in the protein or lipid composition of isolated myelin, despite a wide range of recovery. Nevertheless, the gel electrophoretic protein pattern of white matter adjacent to plaque areas showed selective loss of myelin basic protein. Lysosomal acid proteinase and beta-glucuronidase levels were very significantly increased in all white matter samples in which astrocytosis was a major neuropathological feature. Levels were less markedly raised in samples containing discrete active plaques. Enzyme changes were also found in the apparently normal white matter of 2 of the cases. Acid proteinase activity was in the normal range but the activities of beta-glucuronidase and acetylcholine esterase were elevated. The significance of these results in relation to glial cell activity in the early stages of demyelination is discussed.

Effect of an inapparent viral encephalitis on the levels of lysosomal glycosidases in mouse brain

The changes in the activity of several lysosomal glycosidases of mouse brain which occured during an inapparent infection with the A774 strain (avirulent) of Semliki Forest Virus (SFV) have been related to the histopathological and viral changes caused by the disease. N-acetyl-beta-D-glucosaminidase, N-acetyl-beta-D-galactosaminidase and beta-glucuronidase were significantly elevated between post-inoculation day 7 and 28. Lesions characteristic of encephalitis were also observed between these times. Histochemical and biochemical of encephalitis were also observed between these times. Histochemical and biochemical observations showed that not all areas of brain were affected equally; the cerebellum, parts of the mid-brain and the spinal cord showed the most sevre biochemical and histochemical changes, whilst histopathological lesions were more evenly distributed. The biochemical results have been related to the histological, histochemical and virological findings and the production of glycosidases from 2 or more cellcular types has been postulated.