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

188 ETHANOL-AFFECTE D DEVELOPMENT
study show s tha t ethano l increase s th e numbe r o f
cells that exit the cell cycl e (Siegenthaler an d Miller,
2005a).
A third featur e contributin g to neurona l produc -
tion i s the LF . Thi s differ s fro m th e exi t fraction i n
that i t encompasses cel l cycl e exi t and th e proces s of
moving fro m th e proliferativ e zones . Th e L F rise s
steadily over the period of neuronal generation (Miller,
1993, 1999b ; Takahashi et al, 1996) . Ethanol does not
affect th e L F during this period (Miller , 1993) . Inasmuch
a s ethano l doe s affec t th e numbe r o f exitin g
cells, thi s lac k o f increas e i n th e L F implie s tha t
ethanol induce s postmitoti c cell s t o remai n i n th e
proliferative zones for a longer time before commenc -
ing their migration.
Various cell cycle-relate d proteins ar e affecte d b y
ethanol: th e expressio n o f cyclin A (a regulator o f S
and G2), cycli n Dl, and cdk2 (proteins that regulate
the progressio n of cells from G l t o S ) i s reduced b y
ethanol exposur e (Li et al, 2001, 2002; Siegenthale r
and Miller , 2005a) . Cerebellar expressio n o f cycli n
D2, cdk4, and cdk6 (regulators of Gl) ar e unaffected
by ethanol (L i et al, 2002). By contrast, p27 ( a protein
involve d in movin g cells from G l ou t o f the cycle)
i s depresse d b y ethano l i n th e V Z o f cortica l
expiants (Siegenthale r an d Miller , 2005a ) and i n developing
rat cerebellum (L i et al., 2002). Thus, the effects
of ethanol on cell cycle-related protein s not only
corroborate, bu t provid e a mechanis m fo r ethanol -
induced increases in the Tc and cell cycle exit and a
decrease i n the GF.
Cell Culture Studies
Studies i n cel l cultur e model s replicat e th e i n viv o
data and provide systems in which to explore cellular
and molecula r mechanism s underlyin g th e ethanol -
induced changes . Variou s cell s hav e bee n use d a s
models o f proliferatin g neura l precursors—primar y
cultures of neurons (Jacobs and Miller , 2001; Miller,
2003a) an d astrocyte s (Kenned y and Mukerji , 1985 ;
Guerri e t al. , 1990 ; Snyde r e t al, 1992 ; Kòtte r an d
Klein, 1999 ; Lu o an d Miller , 1999a ; Cost a an d
Guizzetti, 2002 ; Mille r an d Luo , 2002a ) an d neu -
ronal (Lu o an d Miller , 1997a , 1999b ; Mille r an d
Luo, 2002b ) o r glia l (Wazir i e t al , 1981; Isenber g
et al. , 1992 ; Resnicof f et al, 1994 , 1996b ; Lu o an d
Miller, 1996 ) cance r cel l lines . There i s consensus
that ethanol increase s the time these neural cells take
to double the size of their populations.
Doubling tim e i s a comple x yardstic k reflectin g
the su m o f simpler parameters. On e o f these parame -
ters is an increase in the Tc, and most of this increase
is from a lengthened Gl (Guerr i et al., 1990 ; Luo an d
Miller, 1997a ; Jacobs and Miller , 2001). Remarkably,
the effect s o f ethanol o n th e proliferatio n o f dissoci -
ated neuron s an d B10 4 neuroblastom a cell s strongl y
parallel thos e o f V Z cell s i n viv o (cf . Mille r an d
Nowakowski, 1991 ; Lu o an d Miller , 1997a ; Jacob s
and Miller, 2001). Another factor is the GF. Alteration
in the GF has an invers e effect on doubling time. For
example, a s i n th e cas e o f B10 4 cells , a n ethanol -
induced decrease in the numbe r of cycling cells con -
tributes t o a n increas e i n doublin g tim e (Lu o an d
Miller, 1997a) . A third facto r definin g th e doublin g
time i s the incidenc e o f cell death . Deat h normall y
occurs amon g proliferatin g population s (Blaschk e
et al., 1996; Thomaidou et al, 1997; Jacobs and Miller,
2000) (see Chapter 5) . Ethanol affects th e balance between
cell proliferation and death (Jacobs and Miller,
2001; Miller , 2003a) . For example , ethano l cause s a
steady decreas e i n th e numbe r o f primar y culture d
cortical neurons . Mos t (two-thirds ) of the decreas e is
due to compromised cell proliferation, whereas the remainder
i s from ethanol-induce d cel l death. Ethano l
affects neurona l generatio n an d deat h i n th e imma -
ture PSN in a similar manner (Miller , 1999a).
LIGAND-MEDIATED SYSTEM S
Many factors may contribute t o the differentia l effec t
of ethano l o n specifi c population s o f proliferating
cells. A major playe r is the myria d of growth factor s
that tightly regulate cell proliferation. This section focuses
o n a smal l numbe r o f growth factor s tha t ha s
been studie d i n relatio n t o th e effect s o f ethanol .
These factor s includ e platelet-derive d growt h facto r
(PDGF), basi c fibroblas t growt h facto r (bFG F o r
FGF-2), insulin-lik e growth factor (IGF)- l an d IGF -
2, an d transformin g growth facto r (TGF ) pi . The y
can b e classifie d as mitogeni c (FGF-2 , IGF-1 , an d
PDGF) or anti-mitogenic (TGFp).
Mitogenic Factors
Platelet-Derived Growth Factor
The cyclin g activit y o f neura l cell s i s potentl y pro -
moted by PDGF. It is noteworthy that PDGF receptors