Physiological functions of

mitochondrial dynamics

David Chan

California Institute of Technology

Division of Biology and Biological Engineering

Eukaryotic cells

• Contains nucleus

• Contains membrane-bound organelles that compartmentalize

biochemical reactions

Mitochondria likely arose from an

endosymbiotic event involving a prokaryote

• Genomes of eukaryotes

show evidence of a

hybrid origin—archaeal

and bacterial.

• Genetics systems of

mitochondria (like

ribosomes) resemble

those of bacteria.

Mitochondria have many cellular functions

Molecular Biology of the Cell, Fifth Edition (© Garland Science 2008)

• “Powerhouses of the cell”

• Double-membraned

organelles

• Specialized cristae

membranes

• Cristae membranes

are enriched for OXPHOS complexes

Mitochondria have many cellular functions

• Oxidative phosphorylation• TCA/Krebs cycle

• Fatty acid oxidation• Fe-S clusters

• Apoptosis

• Calcium handling• Innate immunity

Our interests:

• Mitochondrial dynamics-Fusion

-Fission

• Mitochondrial quality control-Mitophagy

-Coordination of 2 genomes-Protein folding in matrixMolecular Biology of the Cell, Fifth Edition (© Garland Science 2008)

Efficient ATP production requires mitochondria

DiMauro and Schon (2004) N Engl J Med

Anaerobic glycolysis yields 2 ATP

molecules:

glucose + 2 NAD+ + 2 ADP +2 Pi ®

2 pyruvate + 2 NADH + 2 H+ + 2 ATP +

2 H2O

Complete oxidation of glucose to CO2

and H2O yields ~ 30 ATP molecules:

requires the TCA cycle, the electron

transport chain, and oxidative

phosphorylation, all localized to

mitochondria.

b-oxidation of fatty acids also occurs in

mitochondria.

The TCA cycle generates the activated carrier

molecules NADH and FADH2 from acetyl CoA

• Tricarboxylic acid (TCA) cycle = citrate cycle= Krebs

cycle

• acetyl CoA generated from

pyruvate or fatty acid b-oxidation

• Reduced flavin adenine dinucleotide (FADH2) and

reduced nicotinamide

adenine dincucleotide(NADH) are produced. They

are high energy electron carriers.

• NADH and FADH2 are substrates for the electron

transport chain.

Lodish et al (2016)

One turn of cycle produces 3 NADH, 1 GTP, 1 FADH2,

and releases 2 CO2.

Pyruvate

Five enzyme complexes in inner membrane

perform oxidative phosphorylation

Oxidative phosphorylation

(OXPHOS) involves 5 large enzyme

complexes of the inner membrane.

• Complexes I-IV are components of

the electron transport chain

(ETC).

• The ETC performs electron transfer

reactions coupled with proton

pumping to generate proton gradient

across the inner membrane.

• This proton gradient powers ATP

synthesis by Complex V (ATP

synthase)

Alberts et al

In the electron transport chain, electron transfer

reactions are coupled to proton pumping

Respiratory complexes in the inner

membrane generate proton gradient and

membrane potential:

• Complex I = NADH dehydrogenase

§ 2 e- transferred from NADH to CoQ

• Complex III = Cytochrome bc1 complex

§ Electrons transferred from QH2 to

cytochrome c

• Complex IV = Cytochrome c oxidase

§ Electrons transferred from

cytochrome c to O2

§ O2 is final e- acceptor

• Complex II = Succinate-CoQ reductase;

succinate dehydrogenase

§ Oxidizes fumarate to succinate, and

electrons transferred to CoQAlberts et al

In the electron transport chain, electron transfer

reactions are coupled to proton pumping

• Complexes I, III, IV pump protons into the intermembrane space, using the

energy from electron flow.

• Ubiquinone (=Coenzyme Q10) and

cytochrome c are mobile electron

carriers between the respiratory

complexes.

• Proton-motive force (-220 mV) across

IM= electric potential (-160 mV) + the

proton gradient (DpH, -60 mV)

Alberts et al

The proton motive force is used to drive ATP synthesis by Complex V

Complex V is the ATP synthase:

ATP synthesis by ATP synthase is coupled to the

movement of protons down their concentration

gradient.

"Respiratory control":

If ATP synthesis is stopped, proton pumping by the

ETC also stops. The proton motive force builds up and

prevents further functioning of the ETC.

This regulatory phenomenon can be demonstrated by

chemical inhibitors; e.g., oligomycin.

ATP export is driven by the proton-motive force:

• ATP export via the ATP/ADP antiporter [ATP (-4) has

one more negative charge than ADP (-3)]

• Keeps high ATP/ADP ratio in the cytosolAlberts et al

Mitochondria have their own genome

Mammalian mtDNA is a circular 16.6 kb

genome.

Encodes 37 genes:

13 polypeptides, 2 rRNA, 22 tRNA

Each gene essential for respiratory

function.

Mishra & Chan (2014) Nat Rev Mol Cell Biol

• The 5 respiratory chain complexes

are enriched in the

cristae membranes.

• Only complex II is

entirely nuclearly

encoded.

Complexes I, III, IV, and V are partially encoded by

mtDNA

Mishra & Chan (2014) Nat Rev Mol Cell Biol

mtDNA mutations cause maternally

inherited mitochondrial diseases

16.6 kb genome encodes 37

genes:

13 polypeptides, 2 rRNAs, 22

tRNAs

Each essential for respiratory

function.

mtDNA diseases are diverse, but

have common features:

• maternal inheritance

• respiratory chain defects

• neuro-muscular symptoms

DiMauro and Schon (2004) N Engl J Med

Maternally inherited mtDNA disease is

one type of mitochondrial disease

Maternal inheritance of MERFF (myoclonic epilepsy with ragged red fibers):

Caused by mtDNA point mutation

Wallace et al. (1988) Cell

Dominant optic atrophy: Caused by nuclear mutation in

OPA1

Hudson et al. (2007) Brain

DiMauro and Schon (2004) N Engl J Med

Clinical features of mitochondrial diseases

KSS denotes the Kearns–Sayre

syndrome; PEO progressive

external ophthalmoplegia; PS Pear-son s syndrome; MERRF myoclonic

epilepsy with ragged-red fibers; MELAS mitochondrial

encephalomyopathy, lactic acidosis,

and strokelike episodes; AID aminoglycoside-induced deafness;

NARP neuropathy, ataxia, and retinitis pigmentosa; MILS

maternally inherited Leigh s syndrome; and LHON Leber s hereditary optic neuropathy.

Weakness/paralysis of

muscles controlling eye

movement

Drooping of eyelid

Chan & Schon (2012) Dev Cell

Multiple mechanism to ensure

uniparental inheritance of mtDNA

Biogenesis of mtDNA gene products

To make 13 polypeptides, mtDNA

encodes 2 rRNAs and 22 tRNAs.

The mitochondrial ribosomes are

formed by rRNAs encoded by mtDNA and proteins subunits

encoded by nuclear DNA.

The tRNAs are charged by

aminoacyl tRNA synthetases encoded by nuclear DNA*.

Humans have complete set of mito-tRNAs. But many organism

do not, and then some tRNAsare imported from cytosol.

*Therefore, there must be compatibility between

mitochondrial and nuclear genomes.

Carelli & Chan (2014) Neuron

Taylor & Turnbull (2005) Nat Rev Genetics

Nuclear versus mitochondrial genomes

Organization of mtDNA genomes into nucleoids

Chen et al. (2007) Cell

• mtDNA nucleoids are protein/mtDNA complexes

• each contains one to several mtDNA molecules

• mtDNA compacted by TFAM; no histones

Lewis MR, Lewis WH (1914) Science; Am J Anatomy (1915)

Live chick embryonic cells, mitochondria stained with Janus Green:

“granules can be seen to fuse together into rods or chains,

and these elongate into threads, which in turn anastomose

with each other and may unite into a complicated network, which in turn may again break down into threads, rods, loops

and rings”

Early observations of mitochondrial dynamics

Two mouse lines to track mitochondrial

dynamics

• PhAM: Photo-Activatible Mitochondria• mito-Dendra2: photoconvertible fluorophore

localized to mitochondrial matrixPham et al. (2012) Genesis

PhAMexcised: constitutive labeling

PhAMfloxed: Cre-dependent labeling

Mitochondria are dynamic organelles:fusion and fission

matrix

Molecular identification of fusion machinery

Hales & Fuller (1997) Cell

Male fzo mutants are sterile and have mitochondrial fusion

defect in post-meiotic spermatocytes.

WT fzo

Molecules necessary for mitochondrial fusion

Chen et al (2003) J. Cell BiolFusion and fission control mitochondrial size, shape, and number.

WT Mfn1 Mfn2

Visualization of OM fusion in OPA1-null cells

Song et al., MBoC (2009)

An in vitro assay to distinguish outer

membrane versus inner membrane

fusion

• OM fusion requires mitofusins

• IM fusion requires OPA1

• Respiratory substrates stimulate IM fusion.

Mishra et al., Cell Metabolism (2014)

Mitofusins and OPA1 act at distinct steps in mitochondrial fusion

• Mitofusins are involved in outer membrane

fusion, consistent with a role in mitochondrial tethering.

• OPA1 is only required for inner membrane fusion.

Song et al (2009) MBC

Chan (2019) Ann Rev Pathology

Similar observations in yeast:

Jodi Nunnari (UC Davis)

Mammalian mitochondrial fission requires

recruitment of Drp1

Mff: van der Bliek; MiDs: Ryan and Nister

Otera et al (2010)

Chan (2012) Ann Rev Genetics

Perturbations in mitochondrial dynamics cause neurodegenerative disease

• Mutations in OPA1 cause dominant optic atrophy

§ most common inherited optic neuropathy§ degeneration of retinal ganglion cells

• Mutations in Mfn2 cause Charcot-Marie-Tooth disease 2A

§ neuropathy of long peripheral nerves§ degeneration of axons (versus

demyelination)

• Mutations in fission genes

§ Homozygous Mff causes neuromuscular disease

§ Dominant negative mutation in Drp1 causes

neonatal lethality with microcephaly and multi-organ dysfunction; also intractable

epilepsy

Benefits of dynamics for mitochondria

Chan (2019) Ann Rev Pathology

Manipulation of mitochondrial morphology

by control of fusion/fission ratios

Chen and Chan (2004) Curr. Topics Dev. Biol.

Wild-type fibroblast

Normal fusion

Normal fissionNormal fusion

Low fission

dnDrp1DM

fn2

DM

fn1

Low fusion

Normal fission

dnDrp1

dnDrp1

Low fusion

Low fission

Cerebellar defect in Mfn2 mutant mice

Hsiuchen Chen

Cerebellar histology

Internal granular layer

Purkinje cell bodies

Molecular layer

White matter

Purkinje cell degeneration in cerebella lacking Mfn2

• reduced dendritic arbor

• reduced dendritic spines

• Purkinje cell loss

Chen, McCaffery, Chan (2007) Cell

wt

calbindin

L7 mutant

calbindin

L7 mutant

DAPI

Degeneration of established Purkinje

cells due to loss of Mfn21 month 3 months 6 months

Mitochondrial defects in Purkinje cells

Loss of fusion leads to heterogeneity in

membrane potential and reduced respiration

• mitochondria

• membrane potential sensitive dye

Chen et al. (2005) J. Biol. Chem.

mtDNA nucleoid-deficient mitochondria

accumulate in mitofusin-null cells

mtDNA nucleoid-deficient mitochondria

accumulate in OPA1-null cells