doi:10.1016/j.cell.2005.09.015


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3

able and the results will surely provide further intrigu-
ing insights.

Already, it is clear that the Dscam exon 6 array uses
a new mechanism to achieve ME splicing. Rather than
resulting from an absolute physical impediment to
splicing, ME behavior appears to arise as an intrinsic
consequence of the regulatory mechanism used to se-
lect individual exons. How the docker:selector struc-
ture might lead to derepression is one of many open
questions. The docker:selector duplex might bind to an
activator that antagonizes the repressor. Alternatively,
the single-stranded selectors might be intron-splicing
silencers to which the repressor binds. These possibili-
ties would be distinguished by the effects of selector
mutation. A particularly puzzling feature of the model is
how docker-selector pairing is regulated. The exon 6.1
selector is only 120 bases downstream of the docker,
whereas that of 6.48 is over 11 kb distant. If docker-
selector pairing were dictated on a cotranscriptional
“first-come, first-served” basis (Eperon et al., 1988),
there would be an overwhelming preference for selec-
tion of the 5# proximal exon 6 variants, but this is not
observed. Neither does the predicted thermodynamic
stability of selector:docker pairs correlate with the fre-
quency of selection of the associated exons. Both ob-
servations strongly suggest that selector:docker pair-
ing must be regulated, although the manner of such
regulation remains to be elucidated. Open questions
notwithstanding, the docker-selector model is so im-
mediately attractive that it seems surprising that it does
not obviously apply to any of the other arrays of Dro-
sophila ME exons, not even in Dscam. Perhaps the
power of persistent staring and luck (see Experimental
Procedures in Graveley, 2005) will unlock their secrets
and possibly reveal some general mechanistic prin-
ciples underlying this complex form of alternative
splicing.

Christopher W.J. Smith
Department of Biochemistry
80 Tennis Court Road
Cambridge, CB2 1GA
United Kingdom

Selected Reading

Buratti, E., and Baralle, F.E. (2004). Mol. Cell. Biol. 24, 10505–10514.

Eperon, L.P., Graham, I.R., Griffiths, A.D., and Eperon, I.C. (1988).
Cell 54, 393–401.

Graveley, B.R. (2005). Cell 123, this issue, 65–73.

Graveley, B.R., Kaur, A., Gunning, D., Zipursky, S.L., Rowen, L., and
Clemens, J.C. (2004). RNA 10, 1499–1506.

Gromak, N., Matlin, A.J., Cooper, T.A., and Smith, C.W.J. (2003).
RNA 9, 443–456.

Jin, Y., Suzuki, H., Maegawa, S., Endo, H., Sugano, S., Hashimoto,
K., Yasuda, K., and Inoue, K. (2003). EMBO J. 22, 905–912.

Jones, R.B., Wang, F., Luo, Y., Yu, C., Jin, C., Suzuki, T., Kan, M.,
and McKeehan, W.L. (2001). J. Biol. Chem. 276, 4158–4167.

Letunic, I., Copley, R.R., and Bork, P. (2002). Hum. Mol. Genet. 11,
1561–1567.

Neves, G., Zucker, J., Daly, M., and Chess, A. (2004). Nat. Genet.
36, 240–246.

Park, J.W., Parisky, K., Celotto, A.M., Reenan, R.A., and Graveley,
B.R. (2004). Proc. Natl. Acad. Sci. USA 101, 15974–15979.

Schmucker, D., Clemens, J.C., Shu, H., Worby, C.A., Xiao, J., Muda,
M., Dixon, J.E., and Zipursky, S.L. (2000). Cell 101, 671–684.

Smith, C.W.J., and Nadal-Ginard, B. (1989). Cell 56, 749–758.
DOI 10.1016/j.cell.2005.09.010

A Cellular Response to an
Internal Energy Crisis

Lack of an appropriate energy supply has been
thought to induce cell death in a nonspecific manner
by causing a decline in metabolism and a gradual
cessation of cellular function. In this issue of Cell,
Nutt et al. (2005) describe a new mechanism that di-
rectly links nutrient availability to apoptosis in Xeno-
pus oocytes and show that age-dependent changes
in the nutritional state of a cell might lead to caspase
activation and apoptotic cell death.

Under normal physiological conditions, most cell types
in our body obtain their energy from nutrients that are
present in abundance in the extracellular environment.
The availability of these nutrients to each cell is “ra-
tioned” by the limited amount of trophic factors that
control nutrient uptake. Only a few select cell types,
such as oocytes, are “self-sufficient” and rely entirely
on internal energy stores for survival. Thus, oocytes are
an interesting model in which to study the events that
occur during a cellular energy crisis. In this issue of
Cell, Nutt et al. (2005) explore the biochemical events
that take place when oocytes exhaust their internal en-
ergy stores and define a new pathway regulating cell
survival in response to nutrient depletion.

Previous studies have demonstrated that oocytes
can be induced to undergo apoptosis regulated by cas-
pases, a family of aspartic proteases necessary for the
execution of apoptotic cell death, as well as by both
the pro- and antiapoptotic members of the Bcl-2 family
of apoptotic regulators. To address how the availability
of nutrients may regulate oocyte apoptosis, the authors
used oocyte extracts from the frog Xenopus laevis.
They demonstrated that depletion of stores of glucose-
6-phosphate (G6P), an intermediate of glucose metab-
olism, caused the loss of an inhibitory phosphorylation
of caspase-2 by the Ca2+/Calmodulin dependent kinase
II (CaMKII), thereby activating caspase-2 and resulting
in apoptosis. They also showed that caspase-2 activa-
tion occurs upstream of apoptotic events in the mito-
chondria. This suggests that activation of this protease
is an initiating event linking glucose depletion to the
induction of apoptosis in oocytes. Caspase-2 is an up-
stream caspase involved in the initiation of apoptosis
induced by cellular stress caused by factors such as
DNA damage (Tinel and Tschopp, 2004). However, the
phenotype of mice deficient in caspase-2 revealed sur-
prisingly little other than the resistance of their oocytes
to DNA damage and naturally occurring cell death
(Bergeron et al., 1998). The data in the Nutt et al. paper



Cell
4

may finally explain the phenotype of these mice. If oo-
cytes undergo apoptosis in response to insufficient nu-
trients, then the loss of caspase-2, a proposed media-
tor of cell death induced by nutrient depletion, would
result in resistance to apoptosis.

Although it is apparent from this study that, during
oocyte apoptosis, caspase-2 operates upstream of mito-
chondrial events and may exert its effects through reg-
ulation of the Bcl-2 family, it is not clear which members
might be involved. Although the Bcl-2 family member
Bad may mediate the apoptotic response to glucose
deprivation, Bad is regulated through phosphorylation
rather than by cleavage by caspases (Danial et al.,
2003). Other proapoptotic Bcl-2 family proteins known
to be regulated by caspase-dependent cleavage, such
as Bid, are possible candidates. The substrate specific-
ity of caspase-2 is unusual, and very few substrates of
caspase-2 have been identified. In the presence of
active caspase-2, there is still a lag time until induction
of apoptosis in oocyte extracts, indicating that addi-
tional signaling steps might also be involved in the sup-
pression of apoptosis by glucose in this system.

Nutt et al. (2005) suggest that a sensor of intracellular
energy levels may be an important direct regulator of
the apoptotic cascade in oocytes. It is not clear what
might serve as the upstream molecular sensor of glu-
cose levels signaling to caspase-2. Although the ratio
of ATP to ADP and cAMP levels have been previously
reported to mediate cellular responses to energy and
nutrients, they are unlikely to play a direct role in this
case. The authors demonstrate that suppression of cas-
pase-2-mediated apoptosis by G6P depends on the con-
tinued operation of the pentose cycle and production of
NADPH. In fact, NADPH can suppress caspase-2 activa-
tion in oocyte extracts in the absence of G6P. There-
fore, the energy sensor operating in this system may
be measuring NADPH or the ratio of NADPH to NADP+.

The findings described by Nutt et al. (2005) may be
applicable to other cellular systems. Caspase-2 is in-
volved in the programmed cell death of mouse sympa-
thetic neurons deprived of nerve growth factor (NGF).
Given that NGF also regulates glucose uptake in neu-
rons, it is conceivable that caspase-2 could be acti-
vated in sympathetic neurons deprived of NGF due to
the reduction in intracellular glucose levels. Other cas-
pases may also be involved in mediating apoptosis in-
duced by energy crisis. For example, caspase-8 has
been implicated in the apoptosis of human hepatoma
cells induced by glucose starvation (Suzuki et al.,
2003). In this system, caspase-8 is activated upstream
of apoptotic events in mitochondria. Caspase-8 activa-
tion is suppressed by a new member of the AMP-
dependent protein kinase family, ARK5. Although the
mechanism of inhibition of caspase-8 by ARK5 in this
system is not clear, this kinase is known to block the
activity of caspase-6 through phosphorylation (Suzuki
et al., 2004). Therefore, altering the phosphorylation
state of caspases may represent a common mecha-
nism to control the induction or repression of apoptosis
in response to cellular energy levels.

Direct coupling of cellular glucose metabolism to
caspase activation has a number of important implica-
tions. First, it suggests that glucose metabolism may
play a larger part in tissue homeostasis than previously

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ppreciated. Although glucose is generally in abundant
upply in the extracellular environment of a multicellular
rganism, the uptake of glucose is regulated by extra-
ellular trophic factors and their intracellular signaling
ediators (recently reviewed in Hammerman et al.,

004). The best characterized of these mediators is the
roto-oncogene Akt. Akt can directly regulate cellular
lucose uptake by inducing expression of the glucose
ransporter, Glut1, at the plasma membrane and tar-
eting hexokinase activity to the mitochondria, possi-
ly in part through its ability to influence the inhibitory
hosphorylation of Bad. The ability of activated Akt to
romote cell survival has been recently demonstrated
o depend on glucose availability because, in glucose-
ree media, constitutively active Akt is unable to sup-
ort sustained cell viability despite the availability of an
lternative energy source. The importance of sustained
lucose metabolism for the effects of Akt is further il-

ustrated by the fact that enhancement of glucose up-
ake and utilization can partially restore cell viability in
he absence of Akt function (Rathmell et al., 2003).
hus, we can consider glucose as an essential cofactor
f Akt signaling. It is interesting to speculate that per-
aps the reason that the prosurvival effects of Akt de-
end on the presence of glucose is because glucose is

equired to suppress the caspase-2-mediated apo-
totic pathway. This hypothesis would further predict
hat, perhaps in caspase-2-deficient cells, Akt activity
ould no longer require the presence of glucose.
Furthermore, coupling metabolic state and cell sur-

ival may provide a mechanism for regulating cell num-
ers during aging at both the cellular and organismal

evel. Nutt et al. (2005) noted an age-related decrease
n the activity of G6P dehydrogenase in murine oocytes,
hich could contribute to their decreased viability. It is
ttractive to speculate that a similar mechanism may
perate in other tissue types such as neurons or hepa-
ocytes, which are particularly sensitive to glucose
evels and are affected by deleterious age-related
hanges. Proteins involved in Ca2+ signaling (including
n isoform of CaMKII), cAMP signaling, MAP kinase
ignaling, cell survival, and mitochondrial metabolism
re among those downregulated in response to DNA
amage during aging of the human brain (Lu et al.,
004). These changes could directly contribute to the
lowing of cellular metabolism observed during normal
ging, leading to lower internal levels of glucose and,

n sensitive tissues, contributing to the decline in cell
esistance to proapoptotic stimuli and eventual cell
oss. Additionally, age-related changes have been ob-
erved in hormone and growth-factor expression,
hich could indirectly affect both survival and cellular
etabolism through their effects on Akt and other sur-

ival pathways. In either case, the gradual decline in
etabolism and intracellular glucose could directly lead

o cell death through either activation of caspase-2 or an
lternative proapoptotic pathway.
It has long been appreciated that nutrient depriva-

ion due to either growth-factor withdrawal, embolism-
nduced loss of blood supply, or age- and disease-
elated changes in metabolism can lead to cell death.
owever, it has been assumed that lack of nutrients is

ikely to induce cell death due to the gradual shutdown
f cellular metabolism and consequent cessation of all



Previews
5

cellular functions. The data presented by Nutt et al.
(2005) demonstrate that nutrient deprivation is, instead,
directly linked through caspase-2 to the apoptotic ma-
chinery and that active suppression of this pathway by
continuous glucose metabolism is required for survival.
The fact that this pathway can be activated due to age-
related changes also suggests that metabolic decline
may contribute directly to cellular and organismal aging
by inducing caspase activation.

Marta M. Lipinski and Junying Yuan
Department of Cell Biology
Harvard Medical School
240 Longwood Avenue
Boston, Massachusetts 02115

Selected Reading

Bergeron, L., Perez, G.I., Macdonald, G., Shi, L., Sun, Y., Jurisicova,
A., Varmuza, S., Latham, K.E., Flaws, J.A., Salter, J.C., et al. (1998).
Genes Dev. 12, 1304–1314.

Danial, N.N., Gramm, C.F., Scorrano, L., Zhang, C.Y., Krauss, S.,
Ranger, A.M., Datta, S.R., Greenberg, M.E., Licklider, L.J., Lowell,
B.B., et al. (2003). Nature 424, 952–956.

Hammerman, P.S., Fox, C.J., and Thompson, C.B. (2004). Trends
Biochem. Sci. 29, 586–592.

Lu, T., Pan, Y., Kao, S.Y., Li, C., Kohane, I., Chan, J., and Yankner,
B.A. (2004). Nature 429, 883–891.

Nutt, L.K., Margolis, S.S., Jensen, M., Herman, C.E., Dunphy, W.G.,
Rathmell, J.C., and Kornbluth, S. (2005). Cell 123, this issue, 89–103.

Rathmell, J.C., Fox, C.J., Plas, D.R., Hammerman, P.S., Cinalli,
R.M., and Thompson, C.B. (2003). Mol. Cell. Biol. 23, 7315–7328.

Suzuki, A., Kusakai, G., Kishimoto, A., Lu, J., Ogura, T., and Esumi,
H. (2003). Oncogene 22, 6177–6182.

Suzuki, A., Kusakai, G., Kishimoto, A., Shimojo, Y., Miyamoto, S.,
Ogura, T., Ochiai, A., and Esumi, H. (2004). Oncogene 23, 7067–7075.

Tinel, A., and Tschopp, J. (2004). Science 304, 843–846.

DOI 10.1016/j.cell.2005.09.015


	A Cellular Response to an Internal Energy Crisis
	Selected Reading