G. Allen et al., Oscillating on borrowed time: Diffusible signals from immortalized suprachiasmatic nucleus cells regulate circadian rhythmicity in cultured fibroblasts, J NEUROSC, 21(20), 2001, pp. 7937-7943
The capacity to generate circadian rhythms endogenously and to confer this
rhythmicity to other cells was compared in immortalized cells derived from
the suprachiasmatic nucleus (SCN) and a fibroblast line to differentiate SC
N pacemaker properties from the oscillatory behavior of non-clock tissues.
Only SCN2.2 cells were capable of endogenously generating circadian rhythms
in 2-deoxyglucose uptake and Per gene expression. Similar to SCN function
in vivo, SCN2.2 cells imposed rhythms of metabolic activity and Per gene ex
pression on cocultured NIH/3T3 fibroblasts via a diffusible signal. The con
ferred rhythms in NIH/3T3 cells were phase delayed by 4-12 hr relative to S
CN2.2 circadian patterns, thus resembling the phase relationship between SC
N and peripheral tissue rhythms in vivo. Sustained metabolic rhythmicity in
NIH/3T3 cells was dependent on continued exposure to SCN2.2-specific outpu
ts. In response to a serum shock the NIH/3T3 fibroblasts exhibited recurren
t oscillations in clock gene expression, but not in metabolic activity. The
se molecular rhythms in serum-shocked fibroblasts cycled in a phase relatio
nship similar to that observed in the SCN in vivo; peak Per1 and Per2 mRNA
expression preceded the rhythmic maxima in Cry1 and Cry2 mRNA levels by 4 h
r. Despite these clock gene oscillations the serum-shocked NIH/3T3 cells fa
iled to drive circadian rhythms of Per1 and Per2 expression in cocultures o
f untreated fibroblasts, suggesting that expression and circadian regulatio
n of the Per and Cry genes are not sufficient to confer pacemaker function.
Therefore, SCN-specific outputs are necessary to drive circadian rhythms o
f metabolic activity, and these output signals are not a direct product of
clock gene oscillations.