Mitophagy and Mitoptosis in Disease Processes - Protein Misfolding and Cellular Stress in Disease and Aging - page 95

for other uses. Mitochondria could become targets of autophagic
degradation during basal, constitutive turnover, during starva-
tion-induced turnover, and turnover of dysfunctional mitochon-
dria ( 30 ). How mitochondria are selected and/or recognised by
the autophagic machinery is currently under active investigation.
The presence of mitochondria within autophagosomes
arising from macroautophagic activity in cells was first observed
by electron microscopy (EM) in mammalian cells ( 31 ) and later
in yeast ( 32 ). EM remains a primary technique for detection of
mitophagy since it offers the potential to yield information on
membrane interactions contributing to mitophagy (see, for
example, Chap. 19). Other techniques that can be used to
monitor mitophagy in yeast and mammalian cells include:
(1) fluorescence microscopy, (2) immunofluorescence, and
(3) following protein degradation by western immunoblotting
( 33 ); however, they reveal less information concerning the mechanism
of the process.

4.1. Factors Involved
in Mitophagy

Mitochondrial damage or dysfunction could lead to one or more
changes in mitochondrial function, morphology, membrane
potential, ATP or ROS production, and Ca 2+ homeostasis.
Presumably such changes are recognised and cells activate
autophagy in response. Mitophagy in this context acts as a mech-
anism for mitochondrial quality control (i.e. maintaining cellular
homeostasis) and therefore is an important cytoprotective
response ( 29 ) (Fig. 1 ).

A recent study tracked individual photolabelled mitochondria
through fusion and fission events. It was demonstrated that fis-
sion yields some daughter mitochondria having decreased mem-
brane potential and which are less likely to become involved in a
subsequent fusion event ( 34 ). These observations have led to the
proposal of a mechanism by which fusion and fission allow for
sequestration of damaged mitochondrial components into daugh-
ter mitochondria that are then eliminated by autophagy ( 35 ).

4.1.1. The Importance
of Fission and Fusion

As indicated above, mitochondrial respiration produces ROS,
including H 2 O 2 and the superoxide anion, especially if respiration
is inhibited or otherwise disordered ( 36 ). ROS causes oxidative
damage to mitochondrial proteins or mtDNA resulting in abnor-
mal mitochondrial protein synthesis, which may then lead to
defects in protein folding, aggregate formation. Under condi-
tions of oxidative stress, the opening of non-specific aqueous
pores, including mitochondrial permeability transition (MPT)
can occur which can provide a signal leading to induction of
mitophagy. Pharmacological inhibitors of MPT, such as cyclosporin
A (CsA) can block mitophagy ( 36, 37 ).

4.1.2. ROS and
Mitochondrial Permeability
Transition