Brain Energy Metabolism, Cognitive Function and Down-regulated Oxidative Phosphorylation in Alzheimer Disease

NEURODEGENERATION, Vol. 5, pp 473–476 (1996)

Brain Energy Metabolism, Cognitive Function and Down-regulated Oxidative Phosphorylation in Alzheimer Disease Stanley I. Rapoport, Kimmo Hatanpää, Daniel R. Brady and Krish Chandrasekaran Laboratory of Neurosciences, National Institute on Aging, National Institutes of Health, Bethesda, MD 20892 USA Reduced brain glucose utilization in early stages of Alzheimer disease, as measured with in vivo positron emission tomography, reflects potentially reversible down-regulation of gene expression for oxidative phosphorylation within neuronal mitochondria. Such down-regulation may occur when neuronal energy demand is first reduced by synaptic dysfunction or loss. © 1996 Academic Press Limited

Key words: Alzheimer disease, positron emission tomography, brain, energy metabolism, oxidative phosphorylation, mitochondria

THE MAMMALIAN BRAIN is distinguished by its high rate of glucose consumption and high enzyme activities for mitochondrial oxidative phosphorylation (OXPHOS) to produce ATP. ATP is the energy source for active ion pumping to maintain resting membrane potential and is consumed mainly at synapses and dendrites (Sokoloff, 1991). Because glucose is the major substrate for brain metabolism, in vivo brain imaging using positron emission tomography (PET) to measure the cerebral metabolic rate for glucose (rCMRglc) or regional cerebral blood flow (rCBF), which is coupled to this rate, makes it possible to examine brain functional activity in life. Furthermore, in diseases like Alzheimer disease (AD), metabolic or flow deficits can be localized and quantified in relation to dementia severity, can be related to cognitive and behavioral changes, and can be used to understand mechanisms and potential reversibility of functional failure. Comparison of PET decrements in life to changes in post-mortem brain involving OXPHOS and glucose transport suggests that, at least early in AD, the decrements are potentially reversible. We discuss this hypothesis in this paper. Correspondence to: Stanley e-mail. [email protected] © 1996 Academic Press Limited 1055-8330/96/040473 1 4 $25.00/0

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Resting metabolism and cognitive correlations Cross-sectional PET studies demonstrate reduced resting-state rCMRglc throughout the neocortex of AD patients; the reductions are related to dementia severity (Rapoport, 1991). Metabolic vulnerability is selective, as neocortical association areas are affected earlier and more severely than are primary visual, auditory or somatosensory areas, whereas thalamic and basal ganglia nuclei are relatively spared. Comparison with post-mortem studies shows that metabolically affected areas including entorhinal cortex, posterior hippocampus and amygdaloid complex demonstrate large numbers of neurofibrillary tangles (NFTs) and neuronal loss (Lewis et al., 1987; DeCarli et al., 1992). The affected regions constitute a telencephalic system that preferentially expanded during human evolution, suggesting that AD is a phylogenic human disease (Rapoport, 1990). Metabolic reductions in individual AD patients can be quite heterogeneous, without regard to age of onset or sporadic or familial genesis (Grady et al., 1988). Four independent rCMRglc patterns have been identified by a principal components analysis, and each has been shown to correspond to a specific cognitive and behaviour deficit profile (Grady et al., 1990). In the most common group, rCMRglc is reduced in superior and inferior parietal lobules and posterior medial tem-

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poral lobe, and patients demonstrate depression and poor visuospatial and memory performance. An abnormal left-right metabolic asymmetry is common in AD, and can be related to specific patterns of cognitive deficits (Haxby et al., 1990). As predicted from lesion and stimulation studies, moderately demented AD patients with lower right than left-sided rCMRglc have worse scores on visuospatial tests (e.g. Range Drawing) than on language tests (e.g. Syntax Comprehension), whereas the reverse is true in patients with lower left-sided rCMRglc. Once a cortical asymmetry is established in a patient, even a patient with only a memory deficit, its direction remains constant for many years, suggesting that the AD process in each hemisphere progresses at a common rate in a given patient. In patients with only a memory deficit, the metabolic asymmetry is not correlated with differences between visuospatial and language deficits, because the deficits don’t exist, but the metabolic asymmetry does predict which deficits will appear some 1–3 years later (Grady et al., 1988; Haxby et al., 1990). Thus, a metabolic asymmetry in a patient with a diagnosis of ‘possible’ AD can convert the diagnosis to ‘probable’ AD. A ‘probable’ diagnosis now requires a cognitive deficit in addition to memory failure (McKhann et al., 1984).

Brain activation in Alzheimer disease Brain activation using PET can be used to examine the degree to which reduced resting metabolism or flow can be reversed in AD and to understand mechanisms of functional failure. Occipitotemporal cortical regions that subserve object recognition are activated in mildly–moderately demented AD patients as much as in controls, when both groups perform a face recognition task with equal accuracy (Table 1) (Grady et al., 1993; Rapoport & Grady, 1993). In support of a retained capacity for activation, visual cortical regions having reduced resting rCMRglc in AD patients have been shown to be activated as much as in controls during passive auditory-visual stimulation (Pietrini et al., 1996). Using a variable intensity (parametric) stimulus, rCBF was measured in mildly–moderately demented patients and controls subjected to patterned flashes at different frequencies (Mentis et al., 1996). In the controls, striate cortex rCBF increased linearly from 0 to 7 Hz, then fell at higher frequencies. In the AD patients, the decline began after 4 Hz and a middle temporal region (V5/MT) that reflected activation of the mag-

Table 1. rCBF in occipitotemporal visual association cortex (Brodmann areas 19 and 37) in Alzheimer patients and normal volunteers performing a control or face-matching task Task

Normal (n 5 13) Alzheimer (n 5 11) subjects patients rCBF (ml/100 g/min)

Baseline control task 48.4 6 0.9 Face matching task 53.1 6 0.9 Difference ∆ rCBF, face matching-control task 4.7 6 0.2

40.1 6 1.1* 44.1 6 1.2* 4.0 6 0.4

*Mean 6 SE differs significantly from control mean, P , 0.001. From Rapoport (unpublished) and (Rapoport & Grady, 1993).

nocellular visual system in the controls at the 1 Hz flash frequency (in which light movement was evident) was not activated. These results suggest a selective failure of the magnocellular visual system in AD, which unlike the parvocellular system can respond to frequencies above 8 Hz (Livingstone & Hubel, 1988), and more generally a failure of high frequency synapses.

Down-regulation of oxidative phosphorylation (OXPHOS) underlies PET decrements Post-mortem studies of the AD brain have led us to hypothesize that changes in rCMRglc or rCBF measured with PET during life are accompanied by downregulation of expression of mitochondrial and nuclear genes coding for glucose transporters and subunits of enzymes of OXPHOS. As down-regulation is a normal physiological response to reduced neuronal energy demand (see below), the in vivo brain changes at least early in AD are likely secondary to reduced demand and are potentially reversible. Such changes have been ascribed to primary deficits in glucose transport (Simpson et al., 1994) or in OXPHOS, the latter deficits resulting from accumulation of mutations due to free radical damage (Mecocci et al., 1994) or mitochondrial deletions (Corral-Debrinski et al., 1994), but we believe these interpretations to be unlikely (Chandrasekaran et al., in press). The activity of cytochrome oxidase (COX), the ratelimiting enzyme complex of OXPHOS within mitochondria, and levels of mRNAs for Complex I–V subunits of the electron transport chain, coded by mitochondrial DNA (mtDNA) or nuclear DNA (nDNA), are reduced in the AD brain (Table 2). Thus, levels of mRNA for the mtDNA-encoded ND1 and

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Functional failure in Alzheimer disease brain Table 2. Mitochondrial and nuclear DNA markers in midtemporal cortex of Alzheimer disease brain, and in lateral geniculate nucleus of monkey brain seven days after injecting tetrodotoxin into vitreous humor of one eye

Marker

% decrease in % decrease in LGN Alzheimer TTX treated temporal vs cortex vs control control monkeya

COX enzyme activity COX protein mtDNA COX I mRNA (mtDNA)c COX III mRNA (mtDNA) ND1 mRNA (mtDNA) ND4 mRNA (mtDNA) 12S rRNA (mtDNA) 28S rRNA (nDNA) COX IV mRNA (nDNA) COX VIII mRNA (nDNA) ATPsyn.β mRNA (nDNA) β-actin mRNA (nDNA) LDH-B mRNA (nDNA)

20–25 ND n.s. 58 6 3 54 6 5 50–60 60 6 8 n.s. n.s. 40 6 8 ND 50–60 n.s. n.s.

23 6 1b 23 6 2 26 6 4 49 6 3 ND ND ND ND ND 18 6 3 29 6 3 ND ND ND

Mean 6 SEM; n.s., not significant; ND, not determined. Parentheses identify whether mRNA is encoded by mitochondrial DNA (mtDNA) or nuclear DNA (nDNA). TTX, tetrodotoxin; LGN, lateral geniculate nucleus. Table from Chandrasekaran et al. (in press) and aHevner & WongRiley (1993). See text for additional references. b c

ND4 subunits of Complex I, and for COX I and III subunits of Complex IV are all significantly reduced. Remarkably, the nDNA-encoded mRNAs for COX IV and for the beta subunit of ATP synthase (ATPsynβ) of Complex V are reduced as well. The molecular changes are specific and unrelated to mitochondrial drop-out, as mtDNA-encoded 12S rRNA and total mtDNA are normal. Nor are they related to a general reduction in nuclear transcription, as nDNA-encoded mRNA for β-actin, lactic acid dehydrogenase-B, and 28S rRNA are unchanged (Fukuyama et al., 1996; Hatanpää et al., 1996; Chandrasekaran et al., in press). Also shown by Table 2, a comparable pattern of loss of COX enzyme activity and mRNAs coded by nDNA or mtDNA for COX subunits is found in the monkey lateral geniculate nucleus after 3–7 days of reduced retinal electrical activity due to chronic retinal exposure to tetrodotoxin (Hevner & Wong-Riley, 1993). These reductions are reversed after the tetrodotoxin has been removed (Wong-Riley, 1989), suggesting that they reflect physiological down-regulation of COX subunit gene expression in response to reduced neuronal energy demand. Mechanisms involved may include changes in the precursor pool for mitochon-

drial peptides or proteins (Liu & Wong-Riley, 1994) or regulation by transcriptional or post-transcriptional factors produced by the nuclear genome (Scarpulla, 1996). Physiological down-regulation of expression of genes coding for GLUT1 and GLUT3, glucose transporters at the blood-brain barrier and at neurons, respectively, also likely occurs in AD (Kalaria & Harik, 1989; Simpson et al., 1994; Chandrasekaran et al., in press). Staging of neuronal functional failure in AD was estimated by using in situ hybridization to measure levels of mtDNA-encoded COX III mRNA and 12S rRNA and of poly(A)1 mRNA coded by mtDNA plus nDNA in pyramidal neurons of midtemporal AD cortex (Hatanpää et al., 1996). Intracellular NFTs were quantified using an appropriate antibody. Pyramidal neurons without NFTs showed reduced COX III mRNA compared with NFT-free neurons in control brain, whereas mRNA for mtDNA-encoded 12S rRNA was normal. When NFTs filled less than 50% of the neuronal cytoplasm, COX III mRNA was further reduced while poly(A)1 mRNA and 12S rRNA remained unchanged. With more than 50% of neuronal cytoplasm filled with NFTs, poly(A)1 mRNA and 12S rRNA were reduced as well. Thus, mitochondrial transcriptional capacity, evidenced by normal levels of 12S rRNA, is maintained in the initial stages of AD (prior to NFTs filling more than 50% of neuronal cytoplasm), when decreased COX III mRNA likely reflects physiological down-regulation of COX expression due to reduced neuronal demand.

Conclusions The AD pattern of reduced expression of nuclear and mitochondrial genes involving OXPHOS and of nuclear genes coding for glucose transporters is consistent with physiological down-regulation of brain glucose delivery and energy production in response to reduced neuronal demand. Reduced demand most likely follows synaptic dysfunction and loss, as ATP is consumed mainly at synapses and there is evidence of synaptic loss early and throughout the disease course (DeKosky & Scheff, 1990; Scheff et al., 1990). A comparable pattern of reduced expression of OXPHOS markers appears in the primate lateral geniculate nucleus following inhibition of retinal activity. Thus, further examination of transcriptional and post-transcriptional regulation in the AD brain might identify changes comparable to those following visual deprivation. Because reduced OXPHOS is likely

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not the primary cause of AD, pharmacological modulation of OXPHOS may not provide a cure. However, this approach may slow down disease progression by providing protection from sudden energy metabolic insults such as ischaemia and excitotoxicity. Physiological down-regulation of mechanisms regulating rCMRglc and OXPHOS in AD, with potential for reversibility, is consistent with evidence that AD patients with mild–moderate dementia, in response to cognitive or sensory stimulation, are capable of increasing rCMRglc and rCBF in brain areas that show reduced metabolism or flow at rest. The increments are quantitatively equivalent to increments in healthy control subjects during comparable stimulation paradigms.

References Chandrasekaran K, Hatanpää K, Brady DR, Rapoport SI (in press) Evidence for physiological down-regulation of brain oxidative phosphorylation in Alzheimer disease. Exp Neurol Chandrasekaran K, Hatanpää K, Rapoport SI, Brady DR (in press) Decreased expression of nuclear and mitochondrial DNAencoded genes of oxidative phosphorylation in association neocortex of Alzheimer disease. Mol Brain Res Corral-Debrinski M, Horton T, Lott MT, Shoffner JM, McKee AC, Beal MF, Garham BH, Wallace DC (1994) Marked changes in mitochondrial DNA deletion levels in Alzheimer brains. Genomics 23:471–476 DeCarli CS, Atack JR, Ball MJ, Kaye JA, Grady CL, Fewster P, Pettigrew KD, Rapoport SI, Schapiro MB (1992) Post-mortem regional neurofibrillary tangle densities but not senile plaque densities are related to regional cerebral metabolic rates for glucose during life in Alzheimer’s disease patients. Neurodegeneration 1:113–121 DeKosky ST, Scheff SW (1990) Synapse loss in frontal cortex biopsies in Alzheimer’s disease: correlation with cognitive severity. Ann Neurol 27:457–464 Fukuyama R, Hatanpää K, Rapoport SI, Chandrasekaran K (1996) Gene expression of ND4, a subunit of complex I of oxidative phosphorylation in mitochondria, is decreased in temporal cortex of brains of Alzheimer’s disease patients. Brain Res 713:290–293 Grady CL, Haxby JV, Horwitz B, Gillette J, Salerno JA, GonzalezAviles A, Carson RE, Herscovitch P, Schapiro MB, Rapoport SI (1993) Activation of cerebral blood flow during a visuoperceptual task in patients with Alzheimer-type dementia. Neurobiol Aging 14:35–44 Grady CL, Haxby JV, Horwitz B, Sundaram M, Berg G, Schapiro M, Friedland RP, Rapoport SI (1988) Longitudinal study of the early neuropsychological and cerebral metabolic changes in dementia of the Alzheimer type. J Clin Exp Neuropsychol 10:576–596 Grady CL, Haxby JV, Schapiro MB, Gonzalez-Aviles A, Kumar A, Ball MJ, Heston L, Rapoport SI (1990) Subgroups in dementia of the Alzheimer type identified using positron emission tomography. J Neuropsychiatry Clin Neurosci 2:373–384 Hatanpää K, Brady DR, Stoll J, Rapoport SI, Chandrasekaran K (1996) Neuronal activity and early neurofibrillary tangles in Alzheimer’s disease. Ann Neurol 40:411–420

Haxby JV, Grady CL, Koss E, Horwitz B, Heston L, Schapiro M, Friedland RP, Rapoport SI (1990) Longitudinal study of cerebral metabolic asymmetries and associated neuropsychological patterns in early dementia of the Alzheimer type. Arch Neurol 47:753–760 Hevner RF, Wong-Riley MTT (1993) Mitochondrial and nuclear gene expression for cytochrome oxidase subunits are disproportionately regulated by functional activity in neurons. J Neurosci 13:1805–1819 Kalaria RN, Harik SI (1989) Reduced glucose transporter at the blood-brain barrier and in cerebral cortex in Alzheimer disease. J Neurochem 53:1083–1088 Lewis DA, Campbell MJ, Terry RD, Morrison JH (1987) Laminar and regional distributions of neurofibrillary tangles and neuritic plaques in Alzheimer’s disease: a quantitative study of visual and auditory cortices. J Neurosci 7:1799–1808 Liu S, Wong-Riley M (1994) Nuclear-encoded mitochondrial precursor protein: intramitochondrial delivery to dendrites and axon terminals of neurons and regulation of neuronal activity. J Neurosci 14:5338–5351 Livingstone M, Hubel D (1988) Segregation of form, color, movement, and depth: anatomy, physiology, and perception. Science 240:740–749 McKhann G, Drachman D, Folstein M, Katzman R, Price D, Stadlan EM (1984). Clinical diagnosis of Alzheimer’s disease: Report of the NINCDS-ADRDA work group under the auspices of Department of Health and Human Services task force on Alzheimer’s disease. Neurology 34:939–944 Mecocci P, MacGarvey U, Beal MF (1994) Oxidative damage to mitochondria DNA is increased in Alzheimer disease. Ann Neurol 36:747–751 Mentis MJ, Horwitz B, Grady CL, Alexander GE, VanMeter JW, Maisog JM, Pietrini P, Schapiro MB, Rapoport SI (1996) Visual cortical dysfunction in Alzheimer’s disease evaluated with a temporally graded stress test during PET. Am J Psychiatry 153:32–40 Pietrini P, Dani A, Furey M, Freo U, Horwitz B, Mentis M, Alexander GE, Grady CL, Schapiro MB, Rapoport SI (1996) Brain stimulation reverses metabolic reductions in Alzheimer disease. Biol Psychiatry 39:660 Rapoport SI (1990) Integrated phylogeny of the primate brain, with special reference to humans and their diseases. Brain Res Revs 15:267–294 Rapoport SI (1991) Positron emission tomography in Alzheimer’s disease in relation to disease pathogenesis: a critical review. Cerebrovasc Brain Metab Rev 3:297–335 Rapoport SI, Grady CL (1993) Parametric in vivo brain imaging during activation to examine pathological mechanisms of functional failure in Alzheimer disease. Int J Neurosci 70:39–56 Scarpulla RC (1996). Nuclear respiratory factors and the pathways of nuclear-mitochondrial interaction. Trends Cardiovasc Med 6:39–45 Scheff SW, DeKosky ST, Price DA (1990) Quantitative assessment of cortical synaptic density in Alzheimer’s disease. Neurobiol Aging 11:29–37 Simpson IA, Chundu KR, Davies-Hill T, Honer WG, Davies P (1994) Decreased concentrations of GLUT1 and GLUT3 glucose transporters in the brains of patients with Alzheimer’s disease. Ann Neurol 35:546–551 Sokoloff L (1991) Relationship between functional activity and energy metabolism in the nervous system: whether, where and why. In Lassen NA, Ingvar DH, Raichle ME, Friberg L (eds) Brain Work and Mental Activity. Quantitative Studies with Radioactive Tracers. Alfred Benzon Symposium VIII. Munksgaard, Copenhagen, pp 52–67 Wong-Riley MTT (1989) Cytochrome oxidase: an endogenous metabolic marker for neuronal activity. Trends Neuroscience 12:94–101