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1594iti Page 536 Friday, November 15, 2002 12:43 PM
Published November 18, 2002
In This Issue
Suicide attempt
linked to breakdown
A COOH-terminal fragment (right) but not
full-length (left) p115 induces apoptosis.
maintaining normal Golgi apparatus
architecture, is selectively cleaved
during apoptosis. The apoptotic
proteases caspase-3 and caspase-8
cleave the protein in vitro, and a
stable cell line expressing a cleavageresistant form of p115 exhibits a
delay in Golgi fragmentation during
apoptosis. Expressing the COOHterminal cleavage fragment of p115
in cells is sufficient to induce
fragmentation, and, surprisingly,
this fragment also translocates to
the nucleus, where it induces apoptosis.
The data suggest that the p115
COOH-terminal fragment serves two
functions during apoptosis, both
disrupting the Golgi apparatus as
a dominant–negative inhibitor of p115,
and sending a signal to the nucleus to
induce other apoptotic processes. icrotubules grow persistently from
the centrosome to the cell margin,
and then enter a state of dynamic instability,
repeatedly growing and shrinking over
short distances to probe the margin of
the cell. This process is thought to be
critical for rapid sensing of cell shape
changes. On page 589, Komarova et al.
demonstrate that the cytoplasmic linker
proteins (CLIPs) are important and highly
specific regulators of dynamic instability.
CLIP-170 (red and blue) helps rescue
The work helps explain why microtubules
shrinking microtubules (green).
normally probe only the cell margin,
rather than growing and shrinking over greater distances.
The authors found that expression of a truncated form of the CLIP protein CLIP-170
acts in a dominant–negative fashion in cultured mammalian cells, knocking endogenous
CLIPs off their normal perches on the plus ends of microtubules. Though this has no
effect on the rate of microtubule growth or shortening, it significantly reduces the
frequency with which shrinking microtubules are rescued, or switched from shortening
to growth. When rescue is inhibited, the microtubules still grow persistently from
the centrosome to the cell margin; but once shortening begins, the microtubules
shrink persistently back toward the centrosome, probing the entire cytoplasm. The
results suggest that CLIPs normally bind to tubulin dimers, which then bind to
shortening microtubules to convert them to the growth phase. M
ER takes a faster track
D
uring mitosis, the movement of membrane-bound organelles within a cell is
generally believed to slow down so that organelles can partition accurately into
daughter cells. Using a new procedure for studying Xenopus oocyte extracts, Wöllert
et al. now report on page 571 that the picture is more complicated.
Previous studies have focused primarily on
microtubule-based organelle motility, whereas
the new work examined F-actin–based movement. The authors used extracts from interphase
or metaphase-arrested oocytes, and developed
an in vitro procedure to produce stable threedimensional microtubule-free F-actin networks
from the extracts. Motion analysis shows that
the movement of ER membranes and globular
vesicles on F-actin is differentially regulated
throughout the cell cycle. During mitosis, the
ER speeds up while globular vesicles slow
down. Myosin V is responsible for ER transport
in this system, and the motor appears to be
indirectly regulated by the kinase CaMKII.
An ER network is transported by myosin
Wöllert et al. propose that F-actin takes over
V during mitosis.
the job of moving the ER during mitosis—a time
when microtubules are diverted for mitotic spindle assembly and chromosome
segregation. Unlike the Golgi apparatus, the ER does not fragment into small vesicles
during mitosis, so its increase in motility and fusion may be a way of dividing the ER
equally between daughter cells. 536 The Journal of Cell Biology | Volume 159, Number 4, 2002
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he Golgi apparatus breaks down
temporarily to segregate into
daughter cells during mitosis, but
during apoptosis the organelle
fragments irreversibly. On page 637,
Chiu et al. report that cleavage of
a critical tethering protein during
apoptosis not only drives the
fragmentation of the Golgi apparatus,
but also appears to help propagate
the apoptotic signal. The work adds
to mounting evidence that the Golgi
apparatus is a perpetrator as well as a
victim of apoptosis.
The authors found that the tethering
protein p115, which is essential for
T
CLIPs to the rescue
1594iti Page 537 Friday, November 15, 2002 12:43 PM
Published November 18, 2002
TEXT BY ALAN W. DOVE
[email protected]
Selecting synapse start sites
I
n the central nervous system, synapses
form when specific organelles and
proteins are recruited to a site where
the axon of one cell contacts a dendrite
of another. But how do the cells coordinate the extracellular formation of a
contact with the intracellular movement
of organelles? On page 649, Sytnyk
functional synapses.
NCAM appears to be attached to
TGN organelles before entering the
contact site. This suggests a model in
which intracellular motor proteins
move the TGN organelles along the
intracellular sides of the neurites, pulling
NCAM across the extracellular surface
of the membrane at the same time. At
a developing contact site, the NCAM
molecules of the two cells cluster,
trapping the TGN organelles and
stabilizing the nascent synapse.
At later developmental stages in
hippocampal cultures, NCAM is
responsible for stabilization only on
the postsynaptic sides of synapses,
suggesting that it may serve additional
functions once the synapse has formed.
Previous work has already shown that
NCAM is required for the proper mobilization and cycling of synaptic vesicles
at neuromuscular junctions. A chaperone in motoneuron disease
ice with the progressive motor neuronopathy (pmn) mutation
develop a degenerative and fatal motoneuron disease
shortly after birth, making them a popular animal model for
studying human neurodegenerative diseases such as spinal
muscular atrophy. Two independent reports (Bömmel et al.,
page 563; Martin, N., et al. 2002. Nat. Genet. 32:443–447)
now identify the genetic defect in pmn mice as a point mutation
in a gene for a tubulin-specific chaperone. The result suggests
that motoneuron axons, which can reach lengths of greater than
one meter in adult humans, may put unusual demands on the
microtubule cytoskeleton.
Fine-scale mapping and sequencing revealed a single missense
mutation in the Tbce gene in pmn mice, resulting in the substitution
of tryptophan for glycine in the tubulin-specific chaperone
protein CofE. The wild-type glycine residue is strictly conserved
among vertebrates, raising the possibility that a similar defect
may be responsible for some of the unexplained sporadic cases
of human motoneuron diseases.
After identifying the mutation, Bömmel et al. isolated and
cultured motoneurons from embryonic pmn and wild-type
mice, and found that the mutant neurons grow shorter axons
and exhibit axonal swellings. The pmn motoneurons appear to
survive as well as wild-type motoneurons, suggesting that the
condition seen in the mutant mice is a consequence of axonal
defects rather than neuronal death.
The findings suggest a relatively straightforward explanation
M
Mutant spinal neurons (right) have shorter, swollen axons.
for the pmn phenotype: a defective microtubule chaperone
protein could interfere with the proper assembly of the
microtubule heterodimers required for normal axonal transport,
leading to shorter axons. Microtubules are also essential for
fundamental cellular processes such as mitotic spindle assembly,
so it is unclear how pmn mutant mice manage to develop
normally. One possibility is that neuron-specific isoforms of
tubulin may have a more stringent requirement for CofE
during assembly. In This Issue 537
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NCAM anchors TGN organelles (red) at
contact sites.
et al. describe the interaction of neural
cell adhesion molecule (NCAM)
clusters with intracellular organelle
aggregates derived from the transGolgi network (TGN). The work is
the first demonstration that recognition
molecules such as NCAM directly link
extracellular signals to intracellular
organelle movement at developing
synapses.
As NCAM was believed to stabilize
the contact structure in synaptogenesis,
the authors examined the movements
and localization of NCAM and the TGN
organelles in cultured hippocampal
neurons. The results show that spectrin
links the TGN organelles directly to
clusters of NCAM, and that NCAM is
one of the first proteins to accumulate at
sites where neurites contact each other.
The trapping of NCAM and its associated
organelles at contact sites is followed by
the development of the contacts into