In this webinar, Dr. Dennis Turner delves into dementia syndrome, the metabolic changes that occur, and the importance of proper physiological monitoring of animal models. Brain metabolism transforms with normal aging, and transient, dynamic metabolic insufficiency may underlie critical progression from aging into dementia syndrome and Alzheimer’s disease (AD). Age-related brain metabolism balances vascular-related substrate supply and transport mechanisms into extracellular space to neurons with cellular metabolic needs and utilization. Dynamic metabolic insufficiency can occur when there is intermittent supply-demand mismatch. Adequacy of neurovascular coupling to provide sufficient cerebral blood flow (CBF) to meet neuronal demand in vivo in a mouse AD model, compared to aged controls were studied. Dr. Turner’s lab analyzed the response to maximal neuronal metabolic demands, spreading depression and anoxia, using imaging, CBF measurements, and oxygen and glucose levels. These in vivo studies require human-similar anesthesia conditions, through monitoring temperature, blood pressure/pulse oximetry, and respiration, to maintain homeostasis. The lab confirmed abnormal neurovascular coupling in a mouse model of AD in response to these metabolic challenges, showing disruption much earlier in dementia than in equivalently aged individuals. Chronic metabolic treatments could influence dementia syndrome progression.

1. Copyright 2022. All Rights Reserved. Contact Presenters for Permission
Dennis Turner, MA, MD
Strategic Approaches to
Age-Related Metabolic
Insufficiency and Transition
into Dementia Syndrome
Professor
Neurosurgery
Duke University

2. Age-Related Metabolic Insufficiency and
Transition into Dementia Syndrome
 Dennis A Turner MA, MD
 Professor, Neurosurgery, Neurobiology, Biomedical
Engineering; Senior Fellow Duke Aging Center
 Duke University Medical Center, Durham, NC 27710
 dennis.turner@duke.edu
 NIH funding (PI): RO1 AG037599, R21 AG051103,
UH3 NS103468, RO1 AG074999, UH3 NS129828.
 No conflicts of interest.

3. Brain Substrate Supply and Metabolism
 Brain metabolism is based on neurovascular
coupling, which matches neuronal demand with
vascular substrate supply of oxygen/glucose.
 Age and disease can also reduce vascular supply
as well as reactivity, potentially leading to a
transient, deficient neurovascular response for
substrate supply at time of greatest demand.
 The critical distribution of supply across the blood-
brain barrier can result in neuronal damage: if
chronic this is termed metabolic insufficiency.

4. There are 3 main aspects of metabolism:
1) Neurovascular supply into the brain;
2) Utilization of substrate within cells;
3) Clearance out of the brain.
While O2 can diffuse across the blood-brain
barrier glucose requires facilitated transport using Glut-1,
ketones with MCT. Dynamic neuro-vascular coupling mediated by
neuronal demand via nitric oxide, lactate, K+, EET, PGE2, 20-HETE.

5. Neuronal Metabolic Function
 Two basic neuronal needs – energy provision
for ion pumps and ongoing cell maintenance.
 Systemic vascular supply of metabolic
substrate provides O2, glucose/ketone bodies.
 Metabolism requires specific transporters.
 Glut-1 glucose transporter across BBB and
into astrocytes; Glut-3 for entry into neurons.
 Ketones require MCT1 at BBB and astrocytes,
MCT2 for neuronal uptake (varying affinity).

6. Attwell D et al, 2010

7. Shetty et al, 2011 – normal cellular
metabolic regulation.
Mitochondria:
Foster et al, 2006

8. Theories of Aging
 Programmed involution: delayed genetic program
that is activated, like puberty, at a predetermined
time, to end cellular function (ie, via apoptosis).
 Telomeric limit on cell division number.
 Wear/tear – accumulation of somatic and
mitochondrial DNA/protein damage.
 O2 free radical –induced damage via ROS.
 Calcium theory – changes in Ca2+ buffering and
organelle function, regulation.
 Is Alzheimer’s disease accelerated aging?

9. Shetty et al, 2011 – Aging changes

10. Role of Neural Activity in Metabolism
 Neural activity requires significant energy
for pumps to maintain Na+, K+ gradients.
 Synthesis of neurotransmitters can also
generate ROS – example of dopamine.
 Neural activity critical to maintain neural
connections and brain function overall.
 Concept of reduced caloric intake, which
may lower ROS burden.

11. Enhanced Lifespan
 Caloric restriction enhances longevity in
many species – not clear in humans.
 Mechanisms may include inducing ketone
use rather than glucose in mitochondria.
 Species with shorter lifespans show more
effect, particularly if started young.
 May be related to decreased neural activity,
decreased ROS production, changes in
mitochondrial function.

12. Mattson et al, 2019: Energy
restriction improves lifespan
through fasting, ketones,
Increased energy efficiency.

13. Successful Aging
 Neurons slowly die with aging, but remaining
cells show dendritic hypertrophy (Pyapali &
Turner, 1996) and axon collateral sprouting.
 This resilient compensation requires
maintained neuronal activity to occur.
 Balance between sufficient neural activity to
initiate compensation for ongoing cell loss vs
too much activity that can damage cells.
 If resilient, compensation may maintain overall
brain function.

14. Types of Metabolism
 Glycolysis provides fast ATP but with
inefficient glucose utilization – localized near
membrane for Na+-K+ ATPases.
 Aerobic glycolysis provides critical
mitochondrial precursor pyruvate.
 Oxidative phosphorylation of pyruvate (in
mitochondria) most efficient.
 Muscle has glycolytic (lactate releasing) and
oxidative types – similar concept of lactate
shuttle in brain from astrocytes to neurons.

15. In Vivo, In Vitro Respirometry
 Calculation of O2 utilization and CO2 production
for energy and respiratory quotient.
 In vivo values relate to diet, exercise, with
muscle metabolism as main contribution.
 Generally decreased oxidative capacity in
aging – mitochondrial regulation, turnover.
 PET studies with O2
15 show declining cerebral
metabolic rate of O2 in aging.
 Seahorse analysis for in vitro cells/slices for
balance of glycolysis/oxidative metabolism.

16. Alternatives to In Vivo Physiology
 Innate, detailed cellular metabolism can be
evaluated in vitro (ie, tissue slices) with fixed
substrate levels (but artificial supply).
 Gradients present within tissue slices due to
diffusion, neuronal activity.
 This approach can separate substrate supply
and vascular pathology from innate cell
metabolism with aging – example of
genetically-limited vascular collaterals but
similar intrinsic metabolic function.

17. CNS Metabolism in Aging
 In brain slices can limit either O2 or glucose –
if limit glucose then young slices rapidly show
enhanced oxidative metabolism.
 In rat aged brain slices baseline glycolysis
rate is high and tissue cannot fully switch to
oxidative metabolism – energy inefficiency
due to increased reliance on glycolysis.
 Aged brain also develops NADH
hyperoxidation after even short periods of
hypoxia due to enhanced ROS generation.

18. Foster et al, 2005: O2 gradients in slices; Foster et al, 2008:
Enhanced NADH hyperoxidation in aged hippocampal slices.

19. Galeffi et al: 2015, low glucose enhances O2 utilization in
young but not old slices – reduced ability to convert to
oxidative metabolism from baseline glycolysis.

20. Transition from Aging to AD
 Is aging alone a model for AD?
 Need for comparison of age-matched controls
in metabolic studies to either human or animal
model studies as pathology progresses.
 Metabolic demands on brain can be graded:
sensory inputs mild, direct electrical
stimulation moderate, spreading depression a
strong demand due to cell depolarization.
 Analysis of whether supply can match demand
may require a strong metabolic need.

21. Metabolism Research Approaches
 Invasive experiments require humane, reliable and
reproducible anesthesia, with constant vital sign
recording and maintenance (as in humans).
 Cerebral blood flow [CBF] estimated by laser
doppler or laser speckle, similar to CT perfusion.
 Neuronal activity induces CBF demand through
electrical or optical stimulation, sensory inputs, K+-
induced spreading depolarization, anoxia.
 Dynamic neurovascular coupling measured in
response to neuronal demand to estimate metabolic
responses, as well as at baseline or resting levels.

22. Preclinical In Vivo Physiology
 Detailed in vivo analysis of neuronal demand,
neurovascular coupling, and substrates
typically requires general anesthesia.
 Anesthetic drugs can interfere with vascular
responses and also be neuroprotective–
isofluorane vs propofol and chloralose.
 Vital sign monitoring critical under anesthesia:
temperature, pulse, respirations, blood
pressure, oxygenation - pulse oximetry.

24. Alzheimer’s Disease (AD)
 Multiple theories to explain cell damage and
cognitive loss – herpes encephalitis, low
metabolism, inflammation, plaques/tangles.
 Risk factors include education, diet, age,
vascular factors, diabetes, physical activity,
APOE ε4 and genetic factors, small number of
familial Alzheimer’s (PS1, APP).
 Metabolism and blood supply critical since low
glucose utilization – is there metabolic
insufficiency during high demand periods?.

25. AD Animal Models
 Dr. C Colton has developed a combined
familial APP mutation with human levels of
nitric oxide via NOS2 (inflammatory NOS).
 This CVN-AD model shows age-related,
plaques, phosphorylated tau, behavioral
abnormalities, all at predictable ages.
 Moderate to severe AD changes by 36-52
weeks, so neurovascular supply analyzed at 3
main ages (young, middle, aged > 40 weeks),
in response to strong metabolic demand.

26. Left: plaques at various ages in
CVN-AD model and (below)
AT8 insoluble tau (Colton et al,
2014).

27. Turner et al (Alz & Dementia 2021):
total cerebral blood flow reduced after
KCl spreading depression in both aged
and in middle-aged CVN Alzheimer’s
mouse model.

28. Alzheimer’s Disease
 CVN Alzheimer’s model shows reduced,
premature aging changes in CBF response
to KCl spreading depression.
 Stroke, hemorrhage also major risk factors
for worsening metabolism, dementia.
 Unlike successful aging, reduced ability to
compensate to maintain neurons.
 Degeneration of neurons, atrophy, reduced
glucose metabolism early hallmarks.

29. Components of Metabolic Insufficiency
 Abnormal blood vessels and their decreased
reactivity to neuronal demand may limit supply.
 Changes in Glut-1 at BBB may limit glucose
uptake into the extracellular space.
 Decreased glucose utilization (ie, clinical FDG-
PET data) may result from both factors.
 Further, age-related changes to less efficient
glucose metabolism may increase neuronal
need for glucose concurrent with limited supply
– transient hypoglycemia/damage.

30. Limits of In Vivo Physiology
 Anesthesia alters physiological responses but
is required for humane, invasive electrode
placement, monitoring – awake state requires
non-invasive imaging/monitoring.
 Cerebral blood vessel diameter can be
estimated by imaging with skull fixed
microscopy in awake, behaving animals.
 Laser doppler can be measured by flexible,
fiber optic cables to allow movement.
 Acute vs chronic research analysis.

31. Multiple, small substrate electrodes can be placed to detect deficient
metabolism: glass recording, O2 glass, glutamate, glucose, and
cerebral blood flow. Spreading depolarization (SD) and
peri-infarct depolarization (PID) show opposite blood flow.

32. Treatment of Metabolic Insufficiency
 Prevention of vascular pathology and stroke –
hypertension, diabetes control.
 Enhancing blood flow directly with extracranial
brain stimulation approaches.
 Enhancing pericyte, blood vessel response to
neuronal demand – restoring K+ channels.
 Improving glucose uptake at BBB (GLP-1).
 Enhancing metabolism at mitochondrial level.

34. Turner et al (2020): Extracranial brain
stimulation (10 Hz tACS) enhances cerebral
blood flow in a dose - dependent manner
(left). Above shows relationship between the
Intracranial electrical field and CBF.

35. 1) Understanding dynamic brain metabolism requires physiological,
real-time analysis of supply, demand, and clearance.
2) Balance between dynamic brain metabolic supply and demand
critical to explore transient energy failure and metabolic insufficiency.
3) Important to integrate intrinsic cellular utilization within neurons
and astrocytes with neurovascular supply limitations, as these are
synergistic factors in separating aging from Alzheimer’s disease.
Summary

36. Research Collaborators
 Aging studies: Kelley Foster, PhD; Francesca
Galeffi MD, PhD; Gowri Pyapali, PhD; Matthew
Sadgrove, PhD; Pavan Shetty, PhD.
 CVN Alzheimer’s model studies: Carol Colton,
PhD; Simone Degan, PhD; Yu Feng, MD;
Ulrike Hoffmann MD, PhD.
 Extracranial brain stimulation studies: Steve
Schmidt PhD; Angel Peterchev PhD.

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