1. Presenilin 1/2 and their mutation in Alzheimer Disease
AD WORKSHOP
Presenilin 1/2 and their mutation in Alzheimer Disease
Abinaya Chandrasekaran
September 2014

2. DEMENTIA Alzheimer’s Disease
Lewy Body Dementia
Other Dementias
Metabolic
Drugs/toxic
White matter disease
Mass effects
Depression
Infections
Parkinson’s
Vascular (Multi-infarct)
Dementia
Alzheimer’s
Disease
Early onset
Late onset
Familial AD
Fronto-
Temporal Lobe Dementias
Alzheimer Disease (AD), a progressive neurodegenerative disorder, is characterized by the presence of amyloid beta (Aβ) peptides and neurofibrillary tangles.
Affects 10% of people over the age of 65 and 50% of people over the age of 85.
09.Sep.14
AD workshop Abinaya Chandrasekaran

3. AD workshop Abinaya Chandrasekaran
Forms of Alzheimer's
Familial:
It is an inherited form of mutation.
Mutation, in one of three genes causes the disease.
PS1, PS2 and APP.
Sporadic: Is a disease, “not inherited” in any simple (or) direct pattern. A slight variation in certain genes may influence the disease. The best-studied susceptibility gene in sporadic alzheimer's disease is APOE.
APP is processed by α β γ secretases.
α- Cleaves within Aβ peptide in APP preventing Aβ peptides.
β- Cleaves N terminal generation of Aβ peptides.
γ – Cleaves C terminal of 712 residue.
[ : Short Aβ (Aβ 40) ; if 714 long Aβ(Aβ42)]
PS1- Presenilin 1
PS2- Presenilin 2
APP-Amyloid Precursor Protein
APOE-Apolipoprotein E
09.Sep.14
AD workshop Abinaya Chandrasekaran

4. AD workshop Abinaya Chandrasekaran
What are Presenilins?
Presenilin are part of ɤ-secretase complex.
Presenilin is a nine-membrane domain protein.
Cleave APP aminoterminal (~27- to 30-kDa NTF) and a carboxyterminal (~16- to 18-kDa CTF).
Mutations in presenilin cause the production of Aβ42 (Ala42) and Aβ43 peptides (insoluble forms of Aβ).
The catalytic site of presenilin consists of two critical aspartyl residues, each located on one of these noncovalently bound fragments . They are together with the surrounding amino acids (YD and GLGD), completely conserved in the presenilins and their cousins .
The presenilins in combination with other proteins generate different γ-secretases, which are involved in the regulated intramembrane proteolysis of a variety of proteins.
The two other γ-secretase components, Aph-1 and Pen-2.
 Aph-1 has likely a seven TM structure with the COOH terminus located in the cytoplasm.
Pen-2 displays a hairpin-like structure with two transmembrane domains and both termini located at the luminal side.
09.Sep.14
AD workshop Abinaya Chandrasekaran

5. AD workshop Abinaya Chandrasekaran
Mutations of PSEN1/2, disrupt the cellular protein-recycling process thereby killing nerve cells.→ Autosomal dominant forms of Familial Alzheimer’s Disease (FAD).
The molecular mechanisms by which expression of mutant PS1 causes FAD are not fully understood, but the widely held view is that mutant PS1 enhances the ratio of:
Aβ42/Aβ40 peptides
Leading to nucleation
Oligomerization
Neuropathogenicity of Aβ42 peptides
09.Sep.14
AD workshop Abinaya Chandrasekaran

6. Mutations in Alzheimers
Chromosome
Known Mutation
Gene defect
Phenotype 14
150 familial AD-causing mutations
Presenilin 1 mutation
↑Production of Aβ42 peptides 1 10
Presenilin 2 mutation 6
TREM2
↑Density of Aβ plaques 19
ApoE4 polymorphism
(ε4 allele)
↑Density of Aβ plaques & vascular deposits 21 25
β-APP mutations
09.Sep.14
AD workshop Abinaya Chandrasekaran

7. AD workshop Abinaya Chandrasekaran
Some of the mutations in the gene include: His163Arg, Ala246Glu, Leu286Val and Cys410Tyr.
Most common mutation is Glu318Gly.
Red circles indicates the mutation
09.Sep.14
AD workshop Abinaya Chandrasekaran

8. Clinical Mutation in Presenilin 1 cause AD
Mutation in PESN1 are:
Missense mutation: Single AA substitution in PESN1 .
Donor–acceptor splice mutation that causes two amino-acid substitutions and an in-frame deletion of exon 9.
Mutations in either presenilin or APP consistently increase the
relative ratio between the long (Aβ42) and short (Aβ40) amyloid
peptides (Aβ42/Aβ40).
09.Sep.14
AD workshop Abinaya Chandrasekaran

9. AD workshop Abinaya Chandrasekaran
Auguste Deter a peculiar disorder of the cerebral cortex- Century ago.
DNA extracted from a histological section of Auguste Deter’s brain.
In exon 6 they found a T→C substitution at position 526 (c.526T→C).
This change is predicted to result in a Phe176Leu aminoacid substitution in the protein.
09.Sep.14
AD workshop Abinaya Chandrasekaran

10. Hypothesis of mutation over years
Total loss of PENS1 function in the forebrain of mice causes neurodegenerative disease in the absence of Aß.
Additional mutation in progranulin gene in patient with presenilin 1 Agr352 insertion is probably the cause of dementia.
Finally promoter polymorphism in PESN1 gene that decreases its expression, contribute to risk early onset-AD. However whether these affects amyloid generation is not yet known.
09.Sep.14
AD workshop Abinaya Chandrasekaran

11. Red circles: presenilin 1 and APP mutations associated with familial AD
Hardy and Selkoe, 2002, Science
presenilin 1 is part of
-secretase, a membrane-associated protease
09.Sep.14
AD workshop Abinaya Chandrasekaran

12. PSEN mutations are loss-of-function mutations
PSEN mutation decreases the cleavage of Notch, Syndecan and N-cadherin.
PSEN mutations result in a loss of function of the γ-secretase.
Such as: protein trafficking, apoptosis, autophagy, calcium homeostasis, β-catenin turnover, regulation of kinase pathways and tau phosphorylation.
09.Sep.14
AD workshop Abinaya Chandrasekaran

13. Loss of γ-secretase changes the Aβ42/Aβ40 ratio
The effects of PSEN clinical mutations on APP processing have been investigated by analysing the Aβ42/Aβ40 ratio.
More than 10 years ago, Jarrett and Lansbury described Aβ42 as a ‘nucleation’ factor, which notably accelerates the aggregation of Aβ into amyloid in vitro(Jarrett& Lansbury, 1993).
Aβ42, although generated by neurons at a tenfold lower rate than Aβ40, is the main component of amyloid plaques in the brains of AD patients (Iwatsubo et al, 1994).
Furthermore, expression of Aβ42—but not Aβ40—alone is sufficient to cause amyloidosis in transgenic mice (McGowan et al, 2005).
09.Sep.14
AD workshop Abinaya Chandrasekaran

14. AD workshop Abinaya Chandrasekaran
The effects of PSEN clinical mutations on APP processing were recently evaluated. An increase in Aβ42/Aβ40 was confirmed; however, this was due to a decrease in Aβ40 peptide levels in several mutants.
Importantly, all cell lines accumulated APP C-terminal fragments and showed decreased generation of the cytoplasmic AICD. →Establishing that PSEN mutations result in a loss of γ-secretase cleavage of APP. This apparently translates into an ‘incomplete digestion’ of the APP substrate, generating fewer but longer Aβs. (Qi et al, 2003; Yagishita et al, 2006).
Therefore the biochemical loss of function of presenilin can cause AD.
09.Sep.14
AD workshop Abinaya Chandrasekaran

15. AD workshop Abinaya Chandrasekaran
PS1-E280A affects both Ca2+ homeostasis and Aβ preprocessing leading to FAD and Neurodegeneration.
Characterize the phenotype of PSEN1 F177S mutation.
Excessive amyloid beta 42 production in the brain cortex corresponds with PSEN1 mutations.
09.Sep.14
AD workshop Abinaya Chandrasekaran

16. AD workshop Abinaya Chandrasekaran
Used mouse model with (Tg2576) mutation.
PS1 conformational shift is associated with amyloid pathology and precedes amyloid-β deposition in the brain.
Furthermore, also oxidative stress, a common stress characteristic of aging and AD, causes pathogenic PS1 conformational change in neurons in vitro, which is accompanied by increased Aβ42/40 ratio.
The study provides important information about the timeline of pathogenic changes in PS1 conformation during aging and suggest that structural changes in PS1/γ-secretase may represent a molecular mechanism by which oxidative stress triggers amyloid-β accumulation in aging and in sporadic AD brain.
09.Sep.14
AD workshop Abinaya Chandrasekaran

17. AD workshop Abinaya Chandrasekaran
This study indicates PS2 mutation over express APP protein in mouse model.
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AD workshop Abinaya Chandrasekaran

18. AD workshop Abinaya Chandrasekaran
Wild type PS1 prevents neurodegeneration by activating PI3K/Akt pathway.
Stimulation of PI3/Akt pathway may be used as a strategy for the treatment of Familial Alzheimer disease.
Recently, small interfering RNA (siRNA) has been found to be important in regulating protein expression.
By activating a sequence-specific RNA degradation process, siRNA post transcriptionally inhibits protein expression of the specific gene.
In this study suppression of PS1 gene in IMR-32 cells decreased the production of Aβ42.
Use of nonsteroidal anti-inflammatory drugs (NSAIDs) unexpectedly shift the cleavage products of amyloid precursor protein (APP) to shorter, less fibrillogenic form.
In this study they use fluorescence resonance energy transfer method to alter the proximity between APP and presenilin 1 conformation.
Removing the wild-type PS1 in the presence of the PS1 M146V mutation
(PS1M146V) reduce the amyloid accumulations.
This study was attributed to a reduction of γ-secretase activity rather than an increase in Aβ42.
09.Sep.14
AD workshop Abinaya Chandrasekaran

19. AD workshop Abinaya Chandrasekaran
Most of the drug development for Alzheimer's has been focused on removing amyloid from the brain. “It is better to be aimed at repairing the cellular mechanism that eliminates toxic proteins before they damage the brain."
09.Sep.14
AD workshop Abinaya Chandrasekaran