Mitochondria: Function, Genetics, Diseases Flashcards
What effect will the insufficiency of oxygen have on the mitochondria? (Or in mito disease)
The reactions of oxidative metabolism will change
i.e. Something will go wrong with:
Pyruvate conversion to acetyl coA
Running of TCA cycle to make CO2 and capture electrons from acetyl coa
NADH and FADH providing electrons to ETC to pump protons across inner membrane
What happens when the muscle is out of oxygen?
Pyruvate is converted to Lactate through lactate dehydrogenase
Glycolysis
6-C sugar is oxidized and split into 2x 3-C pyruvates
Makes 2 ATP
Fate of Pyruvate (aerobic and anerobic)
Aerobic: Pyruvate oxidized and converted to Acetyl CoA. NADH produced, CO2 produced
Anaerobic: (also in RBCs which lack mito)
Pyruvate reduced to lactate, NAD+ produced
NAD+ keeps glycolysis going
Fate of lactate
Sent via blood to liver where it’s converted to pyruvate and then glucose (Cori cycle)
Heteroplasmy of mtDNA
Causes: Fission and fusion Accumulated mutations Random segregation during mitosis Proximity to ETC (ROS) Limited protection and repair mechanisms
Threshold Effect
70-90% mutation load is associated with clinical manifestations
PTEN-induced Putative Kinase Protein 1 (PINK1)
Remove damaged mitochondria
Accumulates following mitochondrial damage
Recruits E3 ubiquitin kinase PARKIN
Parkin
E3 ubiquitin kinase
ubiquitylates mito proteins causing mito to be engulfed by isolation membrane that fuses with lysosomes
Autophagosome around mito
PGC1a (and why it’s increased)
Regulation of mito biogenesis
Increased by: endurance exercise caloric restriction energy deprivation cold exposure neuro-hormonal signaling oxidative damage
Mutations in mtDNA
Sporadic or maternal inheritance
Small mutations or large deletions
Cause defects in:
ETC components
rRNA or tRNA for synthesis in mito
Mutations in nuclear DNA
Sporadic or Mendelian inheritance
Small mutations or large deletions
Cause defects in: ETC complexes/assembly mtDNA maintenance protein import lipid dynamics biosynthesis of CoQ10
MELAS (mito encephalomyopathy, lactic acidosis, stroke-like episodes)
Point mutations in tRNA genes in mtDNA
Clinical findings:
CNS (seizures, ataxia, cortical blindess, headahces, dystonia)
Muscle weakness
Kearns-Sayre Syndrome
Large deletions in mtDNA
Clinical findings: Ataxia Muscle weakenss (ophthalmoplegia and ptosis) Pigmentary retinopathy Endocrine, blood, cardiac
PEO (progressive external ophthalmoplegia)
Autosomal dominant or recessive
Point mutations in nuclear gene encoding mtDNA polymerase (POLG)
Ptosis Myopathy Sensorineural hearing loss Ataxia Hypogonadism Cataracts Onset adolescence/adulthood
Cytochrome C Oxidase (COX)
Complex IV
Absence of COX activity reveals SDH staining
Brown: Cox working
Blue: Cox absence, shows SDH
Succinate Dehydrogenase (SDH)
Transfers electrons to complex II via FADH
Mito/mtDNA in the cell
Each mitochondrion has 2-10 copies of mtDNA
Heart
Skeletal muscle
Brain
Lung
Genetic bottleneck during oogenesis
Explains why we see difference in mutation load among children
Only a subset of heterogeneous mitochondria is passed on to a given oocyte
Mitotic segregation
Tricarboxylic Acid (TCA) Cycle or Krebs Cycle
Stepwise oxidation of acetyl coA to CO2
Electrons are harvested by reducing NAD and FAD
Products: 3NADH 1FADH 1ATP 2CO2
Where can the TCA cycle get acetyl-coa?
From pyruvate (glycolysis) Beta-oxidation of fats Breakdown of AA
ATP Production (glycolysis and oxidative phosphorylation)
2: glycolysis
24: oxidative phosphorylation
Location of ETC, TCA, b-oxidation, ATP synthase, membrane transporters
ETC: mito inner membrane on cristae
TCA: matrix
b-oxidation: matrix
membrane transporters: cristae
ATP synthase: cristae
Pyruvate dehydrogenase
Pyruvate > Acetyl CoA
Happens in matrix
TIM
Inner membrane transporters
Signal peptide determines where it goes in mito
TIM22: insert proteins into membrane; internal signal peptide
TIM23: insert proteins into matrix; pre-sequence signal peptide
TOM
Outer membrane receptor (R)
+
General import pore (GiP)
Red-ragged fibers
Seen in most patients with mtDNA defects and some with nDNA mutations
Due to accumulation of abnormal mitochondria below plasma membrane
Can lead outline of muscle fiber to become irregular “ragged”
NOT universal in mito disorders
NOT specific for primary mito diseases
mtDNA vs nDNA
mtDNA:
replicates autonomously
DNA polymerase gamma (POLG)
nDNA:
encodes >85% of mito proteins
DNA polymerase gamma (POLG)
nuclear encoded
structurally similar to bacteria DNA polymerases
Less accurate than nDNA polymerases
Diagnostic criterion for mtDNA mutation
- Disease expressed in children of both sexes or affected mother
- No evidence of paternal inheritance
NADH in metabolism
Oxidized by complex I in ETC under normal mitochondrial function
Oxidized when pyruvate is converted to lactate in defective respiratory metabolism
NAD+ needs to be regenerated to recycle back to glycolysis, where it is reduced again when electrons are harvested from C-C bonds in glucose
Serum levels
Pyruvate: unchanged in MD
Lactate: elevated in MD
Lactate:Pyruvate ratio is normal/lower (<20) in patients with pyruvate dehydrogenase deficiency (pyruvate accumulates) in comparison to those with MD, where Lactate:Pyruvate is elevated (>20) because pyruvate does not accumulate
SDH vs COX staining
SDH: shows blue
COX: shows brown
Brown covers up blue. When COX isn’t working, blue will show.
