1666iti Page 760 Wednesday, September 1, 2004 3:00 PM
In This Issue
Best stiffness for striation
The Journal of Cell Biology
too stiff (e.g., glass).
Adhesions were strongest on the stiffest substrate. Differentiation
therefore requires enough
adhesion to sense the
matrix stiffness, but not
so much that cytoskeletal
changes leading to striation are inhibited. What
translates forces felt at
adhesion sites into differentiation is unknown,
but the membrane-bound
scaffold protein N-RAP
Striation (middle) is inefficient on too soft (left) or too
is one possibility, as it
hard (right) surfaces.
both nucleates actin
filaments and regulates transcription.
differentiation of its own precursors. If
Striation was most prominent on
cardiac muscles are similarly sensitive,
substrates within just 25% of the stiffcareful application of antifibrotics may
ness of normal muscle. The authors
be needed before injections of precursor
found that mdx muscle is stiffer than
cells can regenerate tissue damaged by
this optimal range, and thus may inhibit
heart attacks. A model of cell death
O
n page 839, Bentele et al. use a mathematical model to simplify a complex biological problem—programmed cell death.
Models are mostly used to study relatively simple and well-understood biological systems. Complex systems, in
contrast, have so many unknowns that an overwhelming amount of data is needed to complete a model.
But Bentele et al. show that CD95-induced cell death can be simplified. The authors found that the activity or concentration
of many molecules involved in this death pathway (such as caspases and Bcl family proteins) are unaffected by large changes
in most parameters (including binding kinetics and reaction speeds). So they broke down their original model into
modules—groups of molecules that change in response to changes in the same set of parameters. As a result, only a subset
of molecules needs to be examined when certain parameters are changed in simulations.
Using these simulations, the group identified the pathway’s most critical molecules as those that reacted strongly to
parameter changes. The concentrations of these critical molecules were measured in lab experiments over time following
CD95 activation to estimate some of the remaining unknown parameters and thus refine the model.
Both the refined model and lab experiments predicted that a threshold concentration of CD95 ligand is required for cell
death to occur. One candidate that might
control the threshold is c-FLIP, whose binding
to the CD95-containing complex competes
with activation of caspase-8. Death simulations
run in the absence of c-FLIP abolished the
threshold. Cell death now occurred under
low concentrations of the ligand that did
not cause death in the presence of c-FLIP.
Lab experiments in which c-FLIP expression
was inhibited confirmed that c-FLIP is the
threshold switch. The authors hope that
biologists will use modeling approaches to
improve benchwork experiments for finding
A model predicts that just a little active death receptor is enough to activate
caspases for cell death if FLIP is absent (right).
the important players in complex pathways. 760 The Journal of Cell Biology | Volume 166, Number 6, 2004
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small change in substrate stiffness can
deter striated muscle differentiation,
as shown on page 877 by Engler et al.
As stiffness changes of this magnitude
are not uncommon in diseased tissues,
injections of stem cells may be useless
unless the target environment is also treated.
Muscular dystrophy patients suffer from
stiffened muscle tissue. Although muscle
precursors are abundant in mdx mice, a
muscular dystrophy model, they fail to
regenerate injured muscle. The new article
shows that this failure may be due to their
overly stiff environment, which prevents
skeletal muscle striation.
Skeletal muscle precursors spread,
assumed a spindle shape, and fused into
multinucleated cells when grown on
surfaces within a wide range of stiffness.
However, striation—the alignment of
actin and myosin into repeated units—
was blocked if the substrates were either
too soft (e.g., fibroblasts or weak gels) or
A
1666iti Page 761 Wednesday, September 1, 2004 3:00 PM
TEXT BY NICOLE LEBRASSEUR
[email protected]
Forced to bond
Hitched genes still independent
T
T
Condensation by folding
o understand how chromosomes
condense for mitosis, most researchers
pick apart DNA’s most compact form:
metaphase chromosomes. On page 775,
Kireeva et al. work from the other end
and watch condensation as it occurs.
From this perspective, condensation
looks like a folding continuum with
intermediates that do not fit the favored
radial loop model.
The authors used serial section microscopy to examine chromosomes at stages
of prophase, when most condensation
occurs. At even the earliest stages, 10- and
30-nm chromatin fibers are folded into
larger 100-nm fibers. In middle prophase,
chromatids of 200–250 nm are present
that appear to form from the folding of
the 100-nm fibers. A further doubling in
T
diameter occurs by late prophase.
Radial loop models propose that
chromatin loops of fixed size are the
repeating subunit of condensed chromosomes. Loops were imagined to be pulled
together by a protein scaffold (including
topoisomerase II and condensin), to
which the loops were attached. But
Kireeva et al. see that topoisomerase II
and condensin are dispersed unevenly
in foci on the chromosomes until condensation is nearly complete, at late prophase.
The authors do not contest that
metaphase chromosomes decondensed
in vitro show chromatin loops that likely
result from the cross-linking of fibers by
scaffold proteins. But they stress that
formation of the scaffold axis and its
cross-linking to chromatin occur after
100-nm chromatin fibers (arrows) in middle
prophase will later fold into larger fibers.
chromatid axis formation and most condensation, which they propose is driven
by levels of folding. Topoisomerase II
and condensin may lock these folds into
a stable structure. In This Issue 761
Downloaded from jcb.rupress.org on July 31, 2017
ranscribed genes move away from heterochromatin
even if their silent neighbors do not, as shown by Zink
et al. (page 815).
Transcriptional status is closely related to nuclear positioning.
Silenced genes, for example, are often associated with heterochromatin at the nuclear periphery, whereas active genes
occupy different nuclear domains. The new results show that
even close linkage to genes that are not transcribed does not
prevent an activated gene from leaving heterochromatin.
The authors imaged
three adjacent genes,
CFTR (mutations in
which cause cystic
fibrosis), and its closest
neighbors, GASZ and
CORTBP2, in various
cell types. When none
A gene’s association with perinuclear
of the genes were
heterochromatin (bars) is not restricted
expressed, all three
by its neighbor’s location.
were closely associated with the nuclear envelope and peripheral heterochromatin. In cells that transcribed only one or two of the
genes, only the active ones were found in the nuclear interior,
separated from heterochromatin.
Repositioning might be controlled by histone modifications,
which can be stably inherited through mitosis. Chemically
induced histone acetylation pushed CFTR from the periphery
into the interior. CFTR transcription was not activated,
at least in the short term, but positioning may be important
for maintaining transcriptional status. If so, gene therapy
strategies for cystic fibrosis may need to overcome this
additional layer of complexity. he bonds between leukocytes and endothelial cells last longer
when under some strain, as shown by Yago et al. (page 913).
The results explain why these white blood cells attach to and roll
along the vasculature only when blood flow is strong enough.
Most explanations of
this flow-enhanced adhesion suggest that flow
increases the number of
bonds that form between
L-selectin on leukocytes
and PSGL-1 or other
ligands on vascular cells,
possibly by rotating or
deforming the blood cell.
But some scientists believe L-selectin bonds hold longer as force
increases up to an optimum shear.
that force generated from
flow might also increase the lifetime of existing bonds.
The new results show that catch bonds—those whose lifetimes
are lengthened by force—between L-selectin and PSGL-1 control
leukocyte rolling. The authors correlated the lasting power of
individual bonds with the rolling stability of the cells. As the force
imposed on bonds increased, their lifetimes increased. The blood
cells thus rolled more slowly on PSGL-1 substrates. Slow rolling
allows leukocytes to respond to chemokines and traverse the endothelium. The force requirement probably prevents inflammation
and leukocyte clumping at vascular blockages.
Above optimum shear, when blood cells roll most slowly, catch
bonds became slip bonds, whose lifetimes are shortened by force.
Rolling velocities thus increased, and the cells detached from the
substrate. The transition to slip bonds may explain why leukocytes
usually do not adhere in arteries, where blood flow is very strong.