Li-Jan Chang

Brain Cytochrome Oxidase in Alzheimer's Disease

Abstract: A recent demonstration of markedly reduced (‐50%) activity of cytochrome oxidase (CO; complex 4), the terminal enzyme of the mitochondrial enzyme transport chain, in platelets of patients with Alzheimers disease (AD) suggested the possibility of a systemic and etiologically fundamental CO defect in AD. To determine whether a CO deficiency occurs in AD brain, we measured the activity of CO in homogenates of autopsied brain regions of 19 patients with AD and 30 controls matched with respect to age, postmortem time, sex, and, as indices of agonal status, brain pH and lactic acid concentration. Mean CO activity in AD brain was reduced in frontal (‐26%; p < 0.01), temporal (‐17%; p < 0.05), and parietal (‐16%; not significant, p= 0.055) cortices. In occipital cortex and putamen, mean CO levels were normal, whereas in hippocampus, CO activity, on average, was nonsignificantly elevated (20%). The reduction of CO activity, which is tightly coupled to neuronal metabolic activity, could be explained by hypofunction of neurons, neuronal or mitochondrial loss, or possibly by a more primary, but region‐specific, defect in the enzyme itself. The absence of a CO activity reduction in all of the examined brain areas does not support the notion of a generalized brain CO abnormality. Although the functional significance of a 16‐26% cerebral cortical CO deficit in human brain is not known, a deficiency of this key energy‐metabolizing enzyme could reduce energy stores and thereby contribute to the brain dysfunction and neurodegenerative processes in AD.

Decreased brain protein levels of cytochrome oxidase subunits in Alzheimer's disease and in hereditary spinocerebellar ataxia disorders: A nonspecific change?

Abstract : Controversy exists as to the clinical importance, cause, and disease specificity of the cytochrome oxidase (CO) activity reduction observed in some patients with Alzheimers disease (AD). Although it is assumed that the enzyme is present in normal amount in AD, no direct measurements of specific CO protein subunits have been conducted. We measured protein levels of CO subunits encoded by mitochondrial (COX I, COX II) and nuclear (COX IV, COX VIc) DNA in autopsied brain of patients with AD whom we previously reported had decreased cerebral cortical CO activity. To assess disease specificity, groups of patients with spinocerebellar ataxia type I and Friedreichs ataxia were also included. As compared with the controls, mean protein concentrations of all four CO subunits were significantly decreased (‐19 to ‐47%) in temporal and parietal cortices in the AD group but were not significantly reduced (‐12 to ‐17%) in occipital cortex. The magnitude of the reduction in protein levels of the CO subunits encoded by mitochondrial DNA (‐42 to ‐47%) generally exceeded that encoded by nuclear DNA (‐19 to ‐43%). In the spinocerebellar ataxia disorders, COX I and COX II levels were significantly decreased in cerebellar cortex (‐22 to ‐32%) but were normal or close to normal in cerebral cortex, an area relatively unaffected by neurodegeneration. We conclude that protein levels of mitochondrial‐ and nuclear‐encoded CO subunits are moderately reduced in degenerating but not in relatively spared brain areas in AD and that the decrease is not specific to this disorder. The simplest explanation for our findings is that CO is decreased in human brain disorders as a secondary event in brain areas having reduced neuronal activity or neuronal/synaptic elements consequent to the primary neurodegenerative process.

Brain serotonin transporter in human methamphetamine users

RationaleResearch on methamphetamine (MA) toxicity primarily focuses on the possibility that some of the behavioural problems in human MA users might be caused by damage to brain dopamine neurones. However, animal data also indicate that MA can damage brain serotonin neurones, and it has been suggested that cognitive problems and aggression in MA users might be explained by serotonergic damage. As information on the brain serotonin system in human MA users is fragmentary, our objective was to determine whether protein levels of serotonin transporter (SERT), a key marker of serotonin neurones, are decreased in brain of chronic MA users.MethodsSERT immunoreactivity was measured using an immunoblotting procedure in autopsied brain of 16 chronic MA users testing positive for the drug in blood and brain and matched controls.ResultsSERT levels were non-significantly decreased (−14% to −33%) in caudate, putamen and thalamus (normal in hippocampus), and, unlike the robust striatal dopamine reduction, there was marked overlap between control and MA user ranges. Concentrations of SERT were significantly decreased (−23% to −39%) in orbitofrontal and occipital cortices (normal in frontopolar and temporal cortices).ConclusionsOur data suggest that MA might modestly damage brain serotonin neurones and/or inhibit SERT protein expression, with cerebral cortex being more affected than sub-cortical regions. The SERT reduction in orbitofrontal cortex complements other data suggesting involvement of this area in MA-related behaviour. Decreased brain SERT could also be related to the clinical finding that treatment with a selective serotonin re-uptake inhibitor might increase relapse to MA.

Distribution of Monoamine Oxidase Proteins in Human Brain: Implications for Brain Imaging Studies

Positron emission tomography (PET) imaging of monoamine oxidases (MAO-A: [11C]harmine, [11C]clorgyline, and [11C]befloxatone; MAO-B: [11C]deprenyl-D2) has been actively pursued given clinical importance of MAOs in human neuropsychiatric disorders. However, it is unknown how well PET outcome measures for the different radiotracers are quantitatively related to actual MAO protein levels. We measured regional distribution (n = 38) and developmental/aging changes (21 hours to 99 years) of both MAOs by quantitative immunoblotting in autopsied normal human brain. MAO-A was more abundant than MAO-B in infants, which was reversed as MAO-B levels increased faster before 1 year and, unlike MAO-A, kept increasing steadily to senescence. In adults, regional protein levels of both MAOs were positively and proportionally correlated with literature postmortem data of MAO activities and binding densities. With the exception of [11C]befloxatone (binding potential (BP), r = 0.61, P = 0.15), correlations between regional PET outcome measures of binding in the literature and MAO protein levels were good (P < 0.01) for [11C]harmine (distribution volume, r = 0.86), [11C]clorgyline (λk3, r = 0.82), and [11C]deprenyl-D2 (λk3 or modified Patlak slope, r = 0.78 to 0.87), supporting validity of the latter imaging measures. However, compared with in vitro data, the latter PET measures underestimated regional contrast by ~2-fold. Further studies are needed to address cause of the in vivo vs. in vitro nonproportionality.

Down's Syndrome Individuals Begin Life with Normal Levels of Brain Cholinergic Markers

Abstract We measured the activities of the cholinergic marker enzymes choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) in autopsied brains of seven infants (age range 3 months to 1 year) with Downs syndrome (DS), a disorder in which virtually all individuals will develop by middle age the neuropathological changes of Alzheimers disease accompanied by a marked brain cholinergic reduction. When compared with age‐matched controls cholinergic enzyme activity was normal in all brain regions of the individuals with infant DS with the exception of above‐normal activity in the putamen (ChAT) and the occipital cortex (AChE). Our neurochemical observations suggest that DS individuals begin life with a normal complement of brain cholinergic neurons. This opens the possibility of early therapeutic intervention to prevent the development of brain cholinergic changes in patients with DS.

Low striatal serotonin transporter protein in a human polydrug MDMA (ecstasy) user: a case study

Evidence that the widely used methamphetamine analog MDMA (3,4-methylenedioxymethamphetamine, ecstasy) might damage brain serotonin neurones in humans is derived from imaging investigations showing variably decreased binding of radioligands to the serotonin transporter (SERT), a marker of serotonin neurones. However, in humans, it is not known whether low SERT binding reflects actual loss of SERT protein itself. As this question can only be answered in post-mortem brain, we measured protein levels of SERT and that of the rate-limiting serotonin-synthesizing enzyme tryptophan hydroxylase (TPH) in autopsied brain of a high-dose MDMA user. As compared with control values, SERT protein levels were markedly (−48% to −58%) reduced in striatum (caudate, putamen) and occipital cortex and less affected (−25%) in frontal and temporal cortices, whereas TPH protein was severely decreased in caudate and putamen (−68% and −95%, respectively). The magnitude of the striatal SERT protein reduction was greater than the SERT binding decrease typically reported in imaging studies. Although acknowledging limitations of a case study, these findings extend imaging data based on SERT binding and suggest that high-dose MDMA exposure could cause loss of two key protein markers of brain serotonin neurones, a finding compatible with either physical damage to serotonin neurones or downregulation of components therein.

Distribution of Vesicular Monoamine Transporter 2 Protein in Human Brain: Implications for Brain Imaging Studies:

The choice of reference region in positron emission tomography (PET) human brain imaging of the vesicular monoamine transporter 2 (VMAT2), a marker of striatal dopamine innervation, has been arbitrary, with cerebellar, whole cerebral, frontal, or occipital cortices used. To establish whether levels of VMAT2 are in fact low in these cortical areas, we measured VMAT2 protein distribution by quantitative immunoblotting in autopsied normal human brain (n = 6). Four or five species of VMAT2 immunoreactivity (75, 55, 52, 45, 35 kDa) were detected, which were all markedly reduced in intensity in nigrostriatal regions of patients with parkinsonian conditions versus matched controls (n = 9 to 10 each). Using the intact VMAT2 immunoreactivity, cerebellar and cerebral neocortices had levels of the transporter > 100-fold lower than the VMAT2-rich striatum and with no significant differences among the cortical regions. We conclude that human cerebellar and cerebral cortices contain negligible VMAT2 protein versus the striatum and, in this respect, all satisfy a criterion for a useful reference region for VMAT2 imaging. The slightly lower PET signal for VMAT2 binding in occipital (the currently preferred reference region) versus cerebellar cortex might not therefore be explained by differences in VMAT2 protein itself but possibly by other imaging variables, for example, partial volume effects.

Glycerophosphoethanolamine concentration is elevated in brain of patients with dominantly inherited olivopontocerebellar atrophy

We measured the concentration of glycerophosphoethanolamine (GPEA), a membrane breakdown product, in autopsied brain of 10 patients with dominantly inherited olivopontocerebellar atrophy (OPCA), a cerebellar ataxia disorder. As compared with the controls, mean GPEA levels were significantly elevated by 37-69% in 11 of the 15 brain areas examined, including extracerebellar brain regions in which no neuronal cell loss could be detected by semiquantitative estimation. Our data suggest the possibility of altered membrane phospholipid metabolism in OPCA which could be a contributing factor in the neuronal cell death.

Cerebellar glutamate metabolizing enzymes in spinocerebellar ataxia type I

We measured the levels of three glutamate metabolizing enzymes, namely, glutamate dehydrogenase (GDH), aspartate aminotransferase (AAT), and glutamine synthetase (GS) in cerebellar and occipital cortices of nine patients with dominantly-inherited olivopontocerebellar atrophy (OPCA; spinocerebellar ataxia type I). As compared with the controls, mean GDH activities in cerebellar cortex of the OPCA patients were normal whereas levels of AAT (−17%) and the glial enzyme GS (−27%) were significantly reduced. No statistically significant changes were observed in occipital cortex, a morphologically unaffected brain area. We suggest that the decreased GS levels could reflect impaired capacity of astrocytes to metabolize glutamate which might contribute to the degenerative processes in OPCA cerebellum.