MSE-2020 L10

Mitochondria Part 1

Andrea Schimper

One of the earliest to make the connection between cyanobacteria (photosynthesis-capable bacteria) and energy-producing organelles (organelles) in 1888

Konstantin Merechkowski

Developed the Schrimp’s ideas into the theory of symbiogenesis in 1905

Symbiogenesis

Evolutionary process where organisms, initially living symbiotically, become intergrated and lead to the formation of new species, structures or functions

Iron-Sulphur Cluster proteins

• Versatile class of bio-inorganic cofactors

• Form relay for long-rage electron transfer in enzymes

  • NADH dehydrogenase (Complex I)

  • Ribonucleotide reductase (dNDP synthesis)

• Mitochondria involved in their maturation

What enzymes do iron-sulphur cluster proteins form long-range electron transfer for?

• NADH dehydrogenase (aka Complex I)

• Ribonucleotide reductase (dNDP synthesis)

Mitochondria Structure

• Double membrane

• Smooth outer memberane

• Inner membrane folded into cristae

  • Where mitochondrial matrix is, has enzymes for ATP synthesis

• Contain own DNA and ribosomes so they can create their own proteins and repelicate independently of the cell

Outer Mitochondrial Membrane (OMM)

• Semi-permeable membrane, separates mitochondrial contents from cell cytosol

• Regulates exchange of metabolites, ions and proteins

• Helps mitochondrial fission and fusion

• Has various receptors e.g. VDAC and TOM, for importing proteins and metabolites in mitochondria

• Initiates apoptosis by releasing pro-apoptopic proteins from intermembrane space into cytoplasm e.g. Cytochrome C

Voltage Dependant Anion Channel (VDAC)

• Helps import proteins and metabolites into mitochondria

• Receptor found in the OMM

Translocase of the OMM Complex (TOM)

• Helps import proteins and metabolites into mitochondria

• Receptor found in the OMM

Other mitochondrial functions

• Proteins containing heme and porphyrin synthesis

• Act as initiators and transducers of cell signalling (Ca2+, ROS, NADH)

• Synthesis of MAVS in immune system

Mitochondrial Antiviral Signalling Protein (MAVS)

• Protein that helps innate response to viral infections

• On OMM, scaffold to recruit other proteins involved in antiviral response e.g. IRF1, IRF3, NF-kB

• When activated by viral RNA, triggers signalling cascade > Type I interferons and cytokines are produced to protect cells and alert immune system

Jak/STAT

1. Interferon signalling triggers formation of ISGF3, a transcriptor complex

2. ISGF3 binds to ISRE

3. Protein kinase R induced by interferon, phosphorylates eIF2α, downregulating protein synthesis

4. 2’-5’-oligoadenylate synthetase oligomerases activate RNases to degrade RNA

ISGF3

• Heterotrimeric transcription factor

• Involved in Jak/STAT pathway

• Consists of phosphorylated STAT1, STAT2 and IRF9

ISRE

Interferon-stimulated response element

Two Membrane Complexes Involved in CA2+ Movement in and out of Mitochondria

• Mitochondrial Calcium Uniporter (MCU)

• Sodium-Calcium Exchanger (NCX)

What happens when cytosolic Ca2+ levels are low?

• MICU1 and MICU2 bind to MCU, blocking its function

• MICU1 & 2 = Mitochondrial calcium uptake 1&2 protein

What happens when cytosolic CA2+ levels increase?

Ca-bound MICU1 and MICU2 open MCU channel

What does the Sodium-Calcium Exchanger do?

NCX removes Ca2+ from matric at a slower rate than how it enters through the MCU

Apoptosis Histology

1. Cell shrinks, chromatin condenses

2. Membrane starts blebbing, organelles disintegrate

3. Nucleus and organelles collapse, membrane still blebbing

4. Apoptopic bodies form

5. Macrophages phagocytose apoptopic bodies

6. No inflammation yay 🎉

Is apoptosis constant?

• Yes, balance between mitosis and apoptosis

• Dysregulation in balance between cell cycle arrest and apoptosis plays important role in cancer

What activates apoptosis?

• Extrinsic stimuli e.g. Death receptors

• Intrinsic pathway e.g. intracellular stress

What proteins do mitochondria contain in their intermembrane space that initiate apoptosis?

• Cytochrome C

• Apoptosis-inducing factor (AIF)

• Second mitochondria derived activator of caspases (SMAC)

How does Cytochrome C work?

• Initiates apoptosis

• Inner membrane depolarisation and free radical generation releases Cytochrome C (mitochondrial damage)

• Initiates caspase cascade (protein cleavage) by forming a apoptosome

How does SMAC work?

• Protein for apoptosis initiation

• Inhibits IAPs (Inhibitor of Apoptosis Proteins)

How does AIF work?

• Protein for apoptosis initiation

• Migrates to the nucleus and signals for chromosomes to condense and fragment

Bcl-2 Family

• Family of proteins that control apoptosis

• Includes both pro-apoptopic and anti-apoptopic members, which regulate apoptosis intrinsic pathways

• In normal conditions, anti-apoptopic members dominate and promote cell survival

Anti-apoptopic members of Bcl-2 family

• Bcl-2, Bcl-XL

• Promote cell survival by inhibiting activity of pro-apoptopic members

• Bind to sequestering pro-apoptopic proteins to prevent them from forming pores in OMM

Pro-apoptopic members of Bcl-2 family

• Bax, Bak

• Promote apoptosis by forming pores in OMM

• Leads to release of pro-apoptopic proteins e.g. Cytochrome C into cytoplasm