Brain Development: Normal Processes and the Effects of Alcohol ...

96 NORMA L DEVELOPMENT
where i t ha s transcriptiona l activities . p53-regulate d
genes critica l for normal brain development includ e
cell cycl e proteins p21 and murine double minut e 2 ,
the DN A repai r protei n growt h arres t an d DN A
damage-inducible gene 45 , and th e apoptosis-related
proteins insulin-lik e growth facto r binding protei n 3
and Ba x (Levine , 1997) . I n addition , p5 3 regulate s
the Fa s receptor (Somasundaram, 2000) and th e hormone
polyadenylat e cyclase-activatin g polypeptid e
receptor 1 (e.g., Cian i e t al, 1999 ; Johnson , 2001)
(see Chapters 1 5 and 16) .
p53 can play a direct role in cell death by interacting
wit h th e Be l famil y o f protein s (Chipu k e t al. ,
2003, 2004 ; Mihar a e t al , 2003) , APAF-1 , an d
apoptosis-related receptors . Thes e interaction s ar e
pro-apoptotic. p5 3 down-regulate s pro-survival Bcl-2
and Bcl-x , and up-regulate s Bax, Bid, and APAF-1 . It
also up-regulate s TRAIL-R2/DR5 , Fas , an d FasL . I n
addition, p53 represses c-Fos (Kle y et al, 1992) , thus
potentially interferin g with activato r protei n 1 and ,
hence, transcription.
Oxidative Stress
Oxygen, thoug h centra l t o life , ca n b e toxic . I n it s
ground state, it possesses two unpaired electron s with
parallel spin states. This setting makes a two-electron
reduction kineticall y unlikely ; however , sequentia l
one-electron reduction s can occur, hence generating
oxygen fre e radicals , reactive oxyge n specie s (ROS) .
In th e biologica l setting , th e initia l one-electron ex -
change generate s th e superoxid e anio n radical . Th e
protonated two-electro n reductio n produce s H 2O2
(the protonated for m o f the peroxide ion), with the final
protonate d four-electro n produc t bein g water .
The oxyge n radicals resulting from thi s process generate
a plethor a o f reactiv e specie s wit h othe r mole -
cules, suc h a s nitroge n an d iron , al l o f which ca n
produce a pro-oxidan t environment i n cells , termed
oxidative stress.
The centra l role s for ROS in apoptosis, both as initiators
an d a s signalin g event s withi n th e apoptoti c
process, ar e wel l documente d (Curti n e t al. , 2002 ;
Fleury e t al , 2002 ; Polste r an d Fisku m 2004) . Although
the specific mechanism s by which ROS elicit
and/or maintai n apoptosi s remai n undefined , thes e
compounds hav e a n effec t o n a variet y o f cellula r
components: proteins , DN A base s and sugars , polysaccharides,
an d lipids . On e ROS-relate d pathwa y
that has been connecte d t o ethanol-mediated neuro n
apoptosis i s th e productio n o f pro-apoptoti c prod -
ucts o f lipi d peroxidatio n withi n mitochondria (Ra -
machandran e t al , 2001 , 2003) . RO S reac t wit h
unsaturated fatt y acids , initiatin g a self-perpetuatin g
peroxidation o f membrane lipid s (Kappus , 1985) . In
addition t o direc t damag e t o biomembranes , thi s
ubiquitous proces s generate s highl y reactive aldehydes,
th e mos t studie d an d th e mos t toxi c bein g 4 -
hydroxynonenal (HNE ) (Esterbaue r e t al , 1990 ;
Uchida e t al, 1993) . Importantly , HN E formatio n
generates apoptotic death o f neurons, and it s production,
secondar y t o oxidativ e stress , ha s bee n com -
pellingly linke d t o neuro n deat h i n a variet y o f
neurodegenerative diseases (Zarkovic, 2003).
APOPTOTIC PATHWAY S
Caspase-Dependent: Intrinsic vs.
Extrinsic Pathways of Apoptosi s
The pathway s of apoptosis can be divided into intrinsic
an d extrinsi c on th e basi s of activation site (Budi -
hardjo e t al , 1999 ; Shiozak i an d Shi , 2004) . Th e
intrinsic, mitochondria-associate d pathwa y depend s
on Bel proteins. The extrinsi c pathway is mediated by
activation o f an apoptosis-relate d receptor. Th e path -
ways ar e no t trul y independent , a s ther e i s muc h
cross-talk, and both can activate caspas e 3 (Fig. 6-1) .
In addition, activation of the extrinsic pathway can activate
a feedback loop to the intrinsic pathway.
Intrinsic Pathways
The fundamenta l featur e o f the intrinsi c pathwa y i s
the releas e o f pro-apoptoti c compound s fro m mito -
chondria. Thi s pathwa y i s activated b y intracellula r
stress signal s includin g bu t no t limite d t o oxidativ e
stress or DNA damage. Thus, major effector s ma y be
ROSandp53.
Mitochondrial permeabilit y i s affected b y the Be l
family o f proteins . Ba x homodimer s allo w io n flu x
through th e mitochondrial membrane , an d this some -
how allow s movemen t o f cytochrom e C , possibl y
through enlargement of the voltage-dependent anio n
channels (Banerje e an d Ghosh , 2004 ) o r by opening
the mitochondria l permeabilit y transition por e (e.g. ,
Marzo et al., 1998) . Breach of the membrane cause s a
loss of membrane potentia l and release of cytochrome c,
AIF, HtrA2/Omi, and/or second mitochondria-derive d