Mitochondrial dna and aging

1. Maha Hammady
MBBCh, Demonstrator at Medical Histology & Cell
Biology Department,
Faculty of Medicine Alexandria university
Mitochondrial DNA and Aging

2. • Mitochondria are the cellular organelles
responsible for generating most of the useful
energy derived from the breakdown of lipids
and carbohydrates, which is converted to ATP
by the process of oxidative phosphorylation.

3. Ultra-structure of mitochondria:
Mitochondria are surrounded by a
double-membrane system, consisting
of inner and outer mitochondrial
membranes separated by an
intermembrane space .The inner
membrane forms numerous folds
(cristae), which extend into the
interior (or matrix) of the organelle.
Each of these components plays a
distinct functional role, with the
matrix and inner membrane
representing the major working
compartments of mitochondria.

4. Ultra-structure of mitochondria:
Mitochondria are surrounded by a
double-membrane system, consisting
of inner and outer mitochondrial
membranes separated by an
intermembrane space .The inner
membrane forms numerous folds
(cristae), which extend into the
interior (or matrix) of the organelle.
Each of these components plays a
distinct functional role, with the
matrix and inner membrane
representing the major working
compartments of mitochondria.

5. Ultra-structure of mitochondria:
1-The matrix: contains
1-the mitochondrial genetic system
2-enzymes responsible for oxidative metabolism of both fatty acids and
carbohydrates. (TriCarboxylic Acid cycle)
In the mitochondrial matrix, series of biochemical reactions, known as
TriCarboxylic Acid (TCA) cycle, convert the glycolysis-derived pyruvate into
NADH and succinate. The latter compounds are the substrates of a further
bio-chemical process called oxidative phosphorylation (OXPHOS) that takes
place at the mitochondrial Electron Transport Chain (ETC) which is located on
the inner mitochondrial membrane.
glycolysis-derived
pyruvate
NADH + succinate
Succinate being
(OXPHOS) in ETC

6. Ultra-structure of mitochondria:
2- The inner mitochondrial membrane:
• Is the principal site of ATP generation
• its surface area is substantially increased by folding into cristae , that contains ETC.
• The ETC is composed of four enzymatic Complexes (I to IV)
• The NADH is oxidized by the Complex I, and the Complex II oxidizes the succinate.

7. Ultra-structure of mitochondria:
2- The inner mitochondrial membrane:
• Electrons are transferred from Complex I and II to Complex III and from here to Complex IV
where oxygen is reduced to form H2O.
• At level of Complexes I, III and IV, electron transport is coupled to proton pumping across the
inner membrane, forming a proton gradient.
• The proton gradient is projected toward the mitochondrion matrix through ATP synthesis by
ATP synthase (Complex V) that converts the proton gradient energy into ATP.

8. Ultra-structure of mitochondria:
2- The inner mitochondrial membrane:
• Production of mitochondrial ROS occurs in ETC as a byproduct, they include superoxide (O2-)
hydrogen peroxide (H2O2) and hydroxyl radical (OH-) increases with age and senescence.
• ROS can be seen as a cause and an effect of aging and senescence. ROS are both mutagenic
molecules able to induce DNA, protein and lipid damage, and signaling molecules that
mediate important functions during normal and non-aged cell homeostasis.

9. Ultra-structure of mitochondria:
2- The inner mitochondrial membrane:
In addition, the inner mitochondrial membrane contains an unusually high percentage (greater
than 70%) of proteins, which are involved in oxidative phosphorylation as well as in the selective
transport of metabolites between the cytosol and mitochondria. Otherwise, the inner membrane
is impermeable to most ions and small molecules. The inner mitochondrial membrane is the
functional barrier to the passage of small molecules between the cytosol and the matrix, and it
maintains the proton gradient that drives oxidative phosphorylation.

10. Ultra-structure of mitochondria:
• 3- the outer mitochondrial membrane
• In contrast to the inner membrane, the outer mitochondrial
membrane is highly permeable to small molecules. This is
because it contains proteins called porins, which form
channels that allow the free diffusion of small molecules.

11. The genetic system of mitochondria:
• Mitochondria are thought to have evolved from bacteria that developed a symbiotic
relationship in which they lived within larger cells (endosymbiosis).
• mitochondria are unique among the cytoplasmic organelles that they contain their
own DNA

12. The genetic system of mitochondria:
Mitochondrial DNA Nuclear DNA
circular chain of DNA linear chromosomes
multiple copies per organelle (from 1000 to 10,000) in
each cell.
Two copies of each chromosome present
about 16 kb 3 billion base pairs
Contains 37 genes Contains approximately 20,000 - 30,000 genes
1-only a small number of proteins
(only encodes 13 protein that form subunits of the
ETC Complexes I , III , IV and V
2-all of the mitochondrial ribosomal RNAs and most of
the transfer RNAs
In contrast to the rRNAs and tRNAs encoded by the
mitochondrial genome, nuclear genes encode all of
the proteins needed for transcription and translation,
including mitochondrial RNA polymerase, ribosomal
proteins, and translation factors which are around
1500 different proteins(~99% of mitochondrial
proteins
The mitochondrial genes encoding these proteins are
transcribed and translated within mitochondria
they are synthesized on free cytosolic ribosomes and
imported into mitochondria
Maternally inherited Maternally and paternally inherited

13. The genetic system of mitochondria:
Mitochondrial DNA Nuclear DNA

14. The Mitochondrial Genome in Aging
• Aging: is generalized and
progressive impairment of
bodily functions and is
frequently accompanied by
the high incidence of
several diseases affecting all
organs and tissues.
• Inspite of intense research,
the mechanisms that
underlay these age-
dependent changes are still
largely unknown.

15. The Mitochondrial Genome in Aging
• Primary cell population life span is determined by a limited number of cell
duplications, called Hayflick limit (Hayflick and Moorhead, 1961). After
the Hayflick limit, cells enter an irreversible cell cycle arrest in the G1
phase and no longer respond to growth factors, a state called replicative
senescence. Replicative senescence is believed to be triggered by the
shortening of chromosome ends (telomeres).

16. The Mitochondrial Genome in Aging
• Cellular senescence in general is
irreversible cell cycle arrest in the G1
phase, accompanied by over-production
of ROS and low ATP synthesis.
• Cellular senescence onset occurs earlier
than the Hayflick limit and is not induced
by telomere erosion. Cellular senescence
is induced by a number of endogenous
and exogenous factors, such as ROS, UV
light, ionizing radiations and DNA
damage.

17. The Mitochondrial Genome in Aging
• The onset of replicative and cellular senescence has been widely studied
and documented to involve multiple pathways which partially overlap and
converge on mitochondrial genome (mtDNA) which shows that aging is a
complex and multifaceted phenomenon.

18. The Mitochondrial Genome in Aging
1-The reactive oxygen species:
• Mitochondrial Free Radicals Theory of Aging
(MFRTA) indicates mitochondrial ROS as the
main responsible factor of aging. (aging
tissues harbor dysfunctional mitochondria
producing ROS at high rates)
• A tight inverse correlation was observed
between the animal maximum life span and
ROS production at Complex I. (Complex I is in
close proximity or even in contact with the
mtDNA, as the latter is localized on the
internal surface of the inner membrane)

19. The Mitochondrial Genome in Aging
2-Mutations of Mitochondrial Genome:
• 8-oxo-7,8-dihydro-2 deoxyguanosine (8-oxodG) is one of the
most abundant mutations caused by oxidative conversion of
guanosine.
• mtDNA 8-oxodG accumulation in the heart and brain showed
an inverse correlation with maximum life span in mammals of
various sizes. (The curve that best fits life span with mtDNA
damage is not linear, but rather exponential, describing life
expectancy as rapidly decreasing at high levels of mtDNA
mutations.)

20. The Mitochondrial Genome in Aging
2-Mutations of Mitochondrial Genome:
• The guanosine oxidative damage accumulation rate in mtDNAwas 3 to 9
times higher than in nuclear DNA, suggesting that the mitochondrial
genome is more exposed to the ROS toxic effect than the nuclear genome.
• Complex I has the highest efficiency reduction in aged tissues, endorsing a
direct effect of mtDNA mutations on ETC functionality
• The ROS-induced DNA mutations, if not promptly repaired, are propagated
and fixed at each mtDNA replication cycle ( 8-oxodG is paired with an
adenine instead of cytosine, which results in a G to T point mutation)

21. The Mitochondrial Genome in Aging
• The “ROS vicious cycle” theory suggests that mtDNA mutations induced by
ROS bring about mitochondria dysfunction and ETC impairment that in
turn increment ROS production and result in further mtDNA mutations
and damage mitochondrial components. This vicious cycle would be
ineffectively contrasted by the ROS-scavenging enzymes, whose function
declines with age

22. The Mitochondrial Genome in Aging
3- Tissue and cell-type specific mtDNA mutations
and somaticmosaicism
• In humans, mutations have also been shown to
occur at hotspots located mostly in the mtDNA
regulatory regions. Strikingly, each tissue seems
to hostspecific mutation hotspots. (the tissue
specific environment might apply a selective
pressure to favour the establishment and
propagation of pointmutations at specific
positions.)

23. The Mitochondrial Genome in Aging
3- Tissue and cell-type specific mtDNA
mutations and somaticmosaicism
• The total amount of mtDNA mutation also
seems to vary among different tissues, with
skeletal and cardiac muscles, liver and kidney
that are mostly affected by somatic mtDNA
mutations compared to other organs, such as
the skin and the lungs (somatic mosaicism)

24. The Mitochondrial Genome in Aging
4- mtDNA repair:
The mtDNA lesions caused by oxidative stress are repaired in the
mitochondria by the Base Excision Repair (BER) mechanism. The
importance of mitochondrial BER in the aging phenomenon is
high-lighted by the loss of function of BER associated with
certain diseases(progeroid syndromes):
-Cockayne Syndrome

25. The Mitochondrial Genome in Aging
4- mtDNA repair:
The mtDNA lesions caused by oxidative stress are repaired in the
mitochondria by the Base Excision Repair (BER) mechanism. The
importance of mitochondrial BER in the aging phenomenon is
high-lighted by the loss of function of BER associated with
certain diseases(progeroid syndromes):
-Xeroderma Pigmentosum

26. The Mitochondrial Genome in Aging
4- mtDNA repair:
The mtDNA lesions caused by oxidative stress are repaired in the
mitochondria by the Base Excision Repair (BER) mechanism. The
importance of mitochondrial BER in the aging phenomenon is
highlighted by the loss of function of BER associated with certain
diseases(progeroid syndromes):
-Trichothyiodystrophy

27. The Mitochondrial Genome in Aging
4- mtDNA repair:
• At least four glycosylase enzymes are involved in BER-
mediated mtDNA repair and their activity varies amoung
different tissues.
• The diverse impact of mtDNA mutation accumulation trend in
different organs and cell types might underlie and complicate
the understanding of the aging process.
• In fact, the high mtDNA mutation rate and the relatively low
DNA repair capacity might underlie the important age-related
dysfunction of vital organs such as brain and heart.

28. The Mitochondrial Genome in Aging
5- mtDNA content :
• The mtDNA content was also found
to decline with age in humans, as
younger individuals harbor a higher
number of mtDNA molecules per
cell
• the striking exception for
centenarians, that show no
reduction in mtDNA content. These
findings suggest the beneficial
effect of high mtDNA content to
exceptionally long living people.