Assessment of Cell Death: Apoptosis, Necrosis, or In Between - Animal Models of Acute Neurological Injuries: Injury and Mechanistic Assessments - page 17

a knowledge base will provide the foundation for developing more
effi cacious therapeutics. In addition to the issues, potential pitfalls,
and caveats discussed here in the context of ischemia in the CNS,
the reader would be wise to familiarize him- or herself with a
recently published review on the use and interpretation of various
assays for cell death ( 26 ).

Apoptosis (Greek: apo—from, ptosis—falling) is the most well-
characterized form of PCD, and this is refl ected in the large (and
continually growing) number of tools and reagents available to
study it. Cells progressing through apoptosis can, in general, be
divided into early, middle, and late stages, each of which has (gen-
erally, but not exclusively) characteristic biochemical and/or mor-
phological hallmarks. These include translocation and exposure of
phosphatidylserine (PS) from the inner to the outer leaf of the
plasma membrane, cellular shrinkage, cleavage of chromatin, nuclear
condensation, and chromatin condensation (pyknotic bodies).
In virtually all ischemic insults, both intrinsic and extrinsic apoptotic
pathways are triggered ( 27, 28 ). The intrinsic pathway is mediated
by the release of cytochrome C from the inner mitochondrial
membrane into the cytosol where, as a component of the apoptosome,
it triggers the activation caspase 3, a key effector of death-inducing
proteolytic activity. This pathway comprises a major fraction of
regulated apoptotic death after most types of experimental and
clinical ischemic insults.
Activation of the extrinsic or “death receptor” apoptotic pathway
usually temporarily lags intrinsic pathways, following in a matter of
hours as the effects of the insult radiate outward, and dead/necrotic
tissue accumulates at the core of the infarct. Activation of resident
microglia, edemia-mediated infi ltration of leukocytes, and release
of infl ammatory cytokines and chemokines, particularly TNF-

4.2. Apoptosis

α

and IL-1 , can trigger the extrinsic pathway through binding to

“death receptors” of the tumor necrosis receptor superfamily
(TNFR). The ultimate fate of such cells depends on the integra-
tion of various cellular conditions and events, including parallel pro-
survival signaling (i.e., NF-

β

B) and overall energy levels, but hinges
on the extent to which the formation of a caspase-8-containing
death-inducing signaling complex (DISC), proceeds beyond cer-
tain threshold of survival. Cells in which this threshold of DISC
accumulation is exceeded will undergo apoptosis. Those in which
NF

κ

B and other pro-survival signaling prevent the attainment of
this threshold can either survive or may still undergo regulated cell
death via necroptosis.

κ

4.3. Autophagy

Autophagy (Greek: self digestion; also known as Type II cell death),
an evolutionarily conserved adaptive response to both intra- and
extracellular stress (such as starvation, accumulation of mis-folded
proteins, or other macromolecular lesions resulting from oxidative