MECHANISMS OF AGE-RELATED COGNITIVE CHANGE AND TARGETS FOR INTERVENTION: EPIGENETICS MADIHAH...

MECHANISMS OF AGE-RELATED COGNITIVE

CHANGE AND TARGETS FOR INTERVENTION: EPIGENETICS

MADIHAH MOHAMAD, EIZZATI ARIPIN, SITI ZULAIHA ABU BAKAR

AGING AND EPIGENETIC Changes- cognitive fx, brain anatomy, physiology and

neurochemistry

Rate + magnitude of changes varies across individuals, brain regions and functional domains

Epigenetic mech.- potent regulators of gene expression, unrelated to changes in DNA sequence.

“Cognitive Neuroepigenetics”- research on psychiatric illness, addiction, neurodegenerative diseases.

Research on potential epigenetic contributions to age-related cognitive change has only recently emerged

Cognitive Aging Summit II

U. of Alabama, Birmingham: survey on DNA mathylation and other epigenetic mech in learning and memory, cog epigenetic of aging

U of California, Santa Barbara: role of miRNAs affecting large scale protein networks in aging process

Broad Ins. And Massachusetts Ins. of Tech.: dev of preclinical strategies in mouse models targeting reg of histone acethylation to repress transc + translation to reduce synaptic plasticity in aged brain.

Columbia U: human cognitive aging, signif. of regional vulnerability in hippocampusin relation of age mediated effects on mediators of histone acethylation.

Concluding comments: major themes, future directions and challenges to progress.

EPIGENETIC MECHANISMS IN MEMORY FORMATION

Age-related memory decline = prominently in declarative/episodic and working memory,

memory modalities = based largely in the hippocampus and prefrontal cortex

Memory and synaptic plasticity associated with transcription of immediate-early genes (IEGs) including :

1. Arc (activity-regulated cytoskeletal gene)

2. Zi1268 (also known as nerve growth factor inducible-A, and early growth response gene)

3. BDNF (brain-derived neurotrophic factor Consolidation of memory = prevented by blocking the

expression oh these genes Normal aging = results from decreased immediate-early gene

expression (as seen in some models of memory disorders)

The relevant epigenetic mechanisms include histone posttranslational modifications and DNA methylation (recently discovered) = to control hippocampal synaptic plasticity and long-term memory formation

Involving :

1. the covalent chemical modification of histones by histone acetyltransferases and histone deacetylases (HDACs)

2. covalent modification of DNA by DNA methyltransferases Epigenetic mechanisms = powerful controllers of memory-

associated gene transcription (typically result : transcriptional silencing + loss of gene function)

aging-related cognitive dysfunction = caused by dysregulation of epigenetic control mechanisms and accumulation of aberrant epigenetic marks

transcription of key memory-promoting genes = decline during aging

An assessment of memory formation-associated DNA methylation in the aged rat hippocampus (Carol Barnes‘ research group ) = to determine if aging is associated with a disruption of epigenomic signalling

Process :

1. 1st group of animals screened using spatial version of the Morris swim task = to confirm that the aged animals exhibited impaired memory

2. Animals explored (training) a novel environment for 5 mins (a week later) = treatment that results in both new memory formation

3. Animals rested in cage for 25 mins

4. Decapitation under deep isoflurane anesthesia

5. Extraction and dissection of hippocampus into CA1 and dentate gyrus samples = to get DNA and processed for bisulfite modification, methylation state was determined for the Arc gene via sequencing of control and bisulfite-treated DNA

6. 2nd group (directly from cage) = to determine resting levels of DNA methylation of the Arc gene in hippocampus

Results :

1. Revealed a distinct pattern of methylation of the Arc gene within the aged hippocampus

2. In CA1 = young adult and aged rat showed significant and comparable demethlation of Arc DNA (in response to spatial exploration)

3. In dentate gyrus = aged rats showed less DNA methylation + significantly increased methylation of Arc gene following spatial learning

Aging = accompanied by significant alterations in epigenomic signaling + changes specifically targeting the memory-promoting gene Arc

MIRNAs Hold The Potential To Reveal A Genetic Architecture Of

Aging

Biological aging: Internal biological clock Accumulation of insults to the organism

Lifespan of a species: Biological aging

Life span of an individual: Specific environmental circumstances

(accumulated insults) Individual differences of biological clock

These 2 facets operate at every level of biological hierarchy (genes, proteins, cells, organs, systems, organisms).

The 1st of relevant genetic pathways: Discovered in 1993 Cynthia Kenyon found that a single gene mutation

in daf-2 could double the lifespan of Caenorhabditis elegans

Could be reversed by a second mutation of daf-16m.

The following relevant genetic pathway: Using the previous system Discovered by Victor Ambrose A novel class of posttranscriptional gene

regulators called miRNAs.

Victor Ambrose’s discovery: miRNAs form RNA-RNA duplex housed in RNA-induced

silencing complex partially silencing the translation of target mRNAs

A single miRNA targets multiple mRNAs Recent findings suggest that their exquisite tissue

specificity may open a door to the congnitive aging problem

Although several studies indicate that miRNA profiles change with age, the precise association of specific aging models is unclear

A coherent set of pathways related to aging will emerge miRNA levels can be exogenously manipulated;

Upregulated – delivery of precursor miRNAs Downregulated – delivery of locked nucleic acid antisense

sequences

Sets of miRNA targets are often functionally related

Demonstrated property in cancer: Sets of mRNA targets that are all related to p53

pathway in onco-miR, miR-21 (16) miR-128 = tumor suppressor miRNA that targets

the functionally related genes within tyrosine kinase receptor pathways

Shows the role of miRNAs in modulating entire networks in a distributed and robust manner by main small changes in protein levels among many components of network

miRNA approach to modulating function differs radically from a classic pharmacological approach

The flaws in pharmaceutical approach: Pathways are highly redundant Inhibiting any single component compensatory

response Identify & manipulate miRNAs that target

multiple mRNAs (related to aging) modulate some facets of aging

Genes related to aging at a cellular level are tumor suppressor genes: Frequent miRNA target Their depression reduced level of specific

miRNAs accelerate an aging phenotype e.g. genes at the Ink4/Arf locus activated

proliferation reduced anticancer mechanism may contribute to the attrition of stem cells with aging

miRNAs have important roles in stem cells as pluripotent cells pass through stages of increasingly restricted potential until they reach terminal differentiation

Biological aging begins the moment cells exit pluripotency

Pluripotency terminal differentiation can be tracked by a set of miRNA changes

Each discrete stage in a cell’s lineage is marked by a defining miRNA profile

Unraveling the complex target networks of miRNAs could offer important new insight into the aging process

Histone Acetylation and Histone Deacetylases in Mouse Models

of Neurodegenration

Alzheimer’s disease (AD): Age related neurodegenrative disorder associated

with severe memory impairment Prominent feature – the progressive loss of

forebrain neurons and deterioration of learning and memory

No significantly effective treatment yet The development of alternative therapeutic

approaches is an absolute necessity

Epigenetic: Study of changes in gene expression that are

mediated by mechanisms other than changes in DNA sequence

e.g. chromatin remodeling – patterns of gene expression are modulated via the alteration of chromatin structure

Increased histone acetylation more relaxed chromatin structure increased gene expression

Histone acetylation – regulated by the opposing activities of 2 groups of enzymes (the histone acetyltransferase + HDACs)

Class I HDACs (HDAC 1, 2, 3): Primarily found within nucleus Regulate histone acetylation + suppress gene

expression Recruited to the promoter regions of genes via

transcriptional repressor and corepressor proteins Recent studies – histone acetylation – learning and

memory Increased histone acetylation after various

learning paradigms After HDACi treatment, facilitation of synaptic

plasticity and memory formation Thus, increased histone acetylation facilitates

cognitive function

CK-p25 mouse model experiment: Nonselective HDACi sodium butyrate improves

cognitive performance (even after severe neurodegeneration)

HDACis suberoylanilide hydroxamic acid + phenylbutyrate reinstate learning behaviour in AD mouse

They showed elevated H4 acetylation + increased production of proteins implicated in synaptic function

Treatment with HDACis has emerged as a promising new strategy for therapeutic intervention in neurodegeneration

Overexpression of HADC2 in mouse neurons striking impairment of memory formation + synaptic plasticity (not observed in overexpression of HDAC1) + reduced hippocampal H4K12 and H4K5 acetylation

*other marks not affected HDAC2 knockout mice (not HDAC1 knockout

mice) increased H4K12 and H4K5 acetylation + enhanced learning, memory, synaptic plasticity + rare model of cognitive enhancement

HDAC2: Learning and memory Synaptic plasticity Regulation of H4K12 (dysregulation is implicated

in age-associated memory impairment) Enriched on the promoters of genes that are

implicated in synaptic remodeling and plasticity or that are regulated by neuronal activity (based on immunoprecipitation)

Administration of suberoylanilide hydroxamic acid fails to further increase synaptic plasticity in HDAC2 knockout mice HDAC2 appears to be the major target of HDACi in eliciting memory enhancement

Conclusion from the observations: Dysregulation of chromatin remodeling cognitive

impairment Chronic abnormalities in histone acetylation

(dysfunction of HDAC or histone acetyltransferase enzymes) aberrant expression of genes for learning and memory + synaptic plasticity + synaptogenesis brain in a “locked” state (i.e. probability of the activity-dependent expression of plasticity is reduced)

The Dentate Gyrus in Cognitive Aging: Is Histone Acetylation The Molecular Link??

The Dentate Gyrus and Cognitive Aging Frontal cortex and the hippocampal formation:

strongly implicated in age-related memory decline

Hippocampal formation made up of: Entorhinal cortex Dentate gyrus CA1 and CA3 pyramidal cell fields subiculum

The Dentate Gyrus and Cognitive Aging

The Dentate Gyrus and Cognitive Aging

The Dentate Gyrus and Cognitive Aging Each hippocampal subregion expresses a

unique malecular profile this is why individual subregions are differentially vulnerable to disease

Age-related hippocampal dysf(x) due to: Absence of neuron loss Pathognomonic histological features

The Aging Dentate Gyrus and Histone Acetylation Histone acetylation epigenetically regulates

transcription Dentate gyrus differentially engages this

pathway Unique feature of dentate gyrus is, it supports

neurogenesis late into development, even into postnatal period

Histone acetylation is a critical pathway for neuronal differentioation

Age- related defects in histone acetylation play an important role in age-related dentate gyrus dysf(x)

The Aging Dentate Gyrus and Histone Acetylation Aging dentate gyrus age-related changes in

molecules that regulate histone acetylation Therapeutic intervention:

Any interventions that ameliorate age-related hippocampal dysf(x), will improve the f(x) of dentate gyrus (via histone acetylation pathway)

Physical excersice Improving glucose control in DM

Concluding Comments Cognitive aging is multifarious phenomenon Key challenges:

To identify the precise epigenetic changes To determine the time scale of their influence To define how epigenetic mechanisms achieve

specificity in the coordination of experience-dependent gene expression profile