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Department of Cell and Molecular Biology, Northwestern University Medical School, Chicago, Illinois 60611, USA
1Correspondence: Department of Cell and Molecular Biology, Northwestern University Medical School, 303 East Chicago Avenue, Chicago, IL 60611, USA. E-mail: r-goldman{at}nwu.edu
| INTRODUCTION |
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The extent of sequence homology, the pattern of cell type specific
expression, and the similarity of intron positions of their genes are
properties that have been used to classify IF proteins into six
different types (1
2
3)
. The largest number of IF proteins
are categorized as the type I and II keratins, which are expressed
mainly in epithelial cells. The keratins are obligatory copolymers that
require both a type I and a type II protein to form IF. The expression
of specific keratin pairs is precisely regulated during development,
and this unique feature has led to their use as differentiation markers
for epithelial cells. Vimentin, desmin, glial fibrillary acidic protein
(GFAP), and peripherin are the four known type III IF proteins. Each of
these type III proteins is capable of assembling into homopolymer IF.
Among them, vimentin is unique in that it often forms a scaffold IF
network before the expression and assembly of differentiation-specific
IF proteins such as desmin, GFAP, and peripherin. Type IV IF proteins
consist of the three neurofilament subunit proteins, NF-L, NF-M, and
NF-H, as well as
-intenexin. Neurofilaments are obligatory
heteropolymers, while
-internexin is self-assembly competent
(4)
. Although the majority of IF proteins are expressed in
the cytoplasm, the type V IF proteins (the nuclear lamins) are
localized in the nucleus where they polymerize to form the nuclear
lamina. The lamina is located at the interface between chromatin and
the inner face of the nuclear envelope membrane. Recent observations
demonstrate that the nuclear lamina is not only essential for the
structural integrity of the nuclear envelope but is also involved in
the process of DNA replication during S phase of the cell cycle
(5)
. The type VI IF protein nestin has not been
characterized extensively. It was discovered as an early developmental
marker of neuroepithelial cells (3)
, which cannot form IF
on its own but is capable of coassembling with vimentin to form IF
(28)
.
The IF proteins possess both well-conserved and nonconserved
domains. The most conserved region is the central
-helical rod
domain consisting of ~310 amino acids, known to form coiled-coils in
assembled IF. This central rod is flanked at both ends by
NH2- and carboxyl-terminal non-
-helical
regions that are not as well conserved among the various types of IF
proteins. It has often been speculated that these less conserved end
domains may be involved in the cell type specific functions of IF, as
well as their higher order structure. Within the rod domain, two
stretches of sequence are highly conserved that are known to play
important roles in IF assembly (1)
. They are located at
the beginning (the IA region) and at the end (the 2B region) of the
rod. Although the details of IF structure remain to be determined, the
most widely accepted model is that IF are comprised of four
protofibrils. Individual protofibrils contain two protofilaments, each
of which is constructed from linear arrays of antiparallel and
staggered dimers. The basic building block is the dimer, consisting of
two parallel and in register protein chains (6)
.
IF are known to provide mechanical integrity to cells. In
vitro they exhibit unusual viscoelastic properties making them
much more resistant to mechanical stress when compared with
microtubules and actin-containing microfilaments (7)
.
Their mechanical role has been demonstrated at the tissue level
in vivo by the finding that numerous human blistering
diseases are caused by point mutations in epidermal keratin genes
(8)
. This is also supported by studies in which targeted
expression of dominant negative keratin mutations in mice results in
severe blistering of the skin. The latter lesions appear to be caused
by the weakening of the mechanical properties of keratin IF in
epidermal keratinocytes, with concomitant loss of cell shape and cell
lysis (8)
. In the case of type III IF, it has been shown
in mice that ablation of the desmin gene leads to the degeneration of
cardiac and skeletal muscle, suggesting that desmin plays an important
role in maintaining the mechanical integrity of muscle cells
(9)
. In a similar fashion, defective expression of
neurofilaments in animal models results in the aberrant organization of
neurofilament networks, which is sufficient to generate phenotypes
reminiscent of neurological diseases (10)
.
IF are closely associated with various types of junctional complexes at
cell surfaces, including desmosomes and hemidesmosomes in epidermal
cells (11)
and focal adhesions in fibroblasts
(12)
. These associations are mediated by a growing family
of proteins known as intermediate filament associated proteins (IFAPS;
ref 13
). These include plectin, desmoplakin, and
BPAG1n/dystonin (13)
. IFAPs have also been shown to play
an important role in the interactions among cytoskeletal systems. For
example, plectin (14) BPAG1n/dystonin (15) and fimbrin (32) possess
both IF and actin binding sites (14
, 15)
. The importance
of IFAPs in cytoskeletal integrity has been highlighted by studies of
BPAG 1-null mice and patients afflicted with muscular dystrophy
associated with epidermolysis bullosa simplex (MD-EBS). In the case of
MD-EBS, the effects are linked to the expression of truncated
plectin molecules (14
, 16)
.
Based on their biochemical properties in vitro and their
known roles in contributing to the mechanical properties of cells, many
researchers have assumed that IF are static elements of a cells
cytoskeletal repertoire. However, it has become increasingly obvious
over the past decade that IF are dynamic structures. For example,
dramatic changes in IF network organization occur in mitotic cells and
in spreading cells after cytokinesis or, in the case of cultured cells,
after trypsinization and replating (25)
. In addition, the
nuclear lamina is dismantled and dispersed throughout the cytoplasm
during mitosis and is reassembled during the reformation of daughter
cell nuclei (5)
.
Studies designed to determine the properties of IF during interphase
have been carried out using microinjection techniques. Experiments
involving the microinjection of soluble biotinylated vimentin and type
I keratin subunits have revealed that endogenous polymerized IF can
incorporate exogenous subunits (17
, 18)
. However, if the
amount of microinjected type I keratin is above a certain
concentration, there is a rapid induction of the disassembly of
endogenous keratin tonofibrils. Presumably, this is because of the
formation of types I and II heterodimers, which are required for
keratin IF polymerization. Based on these findings, we have speculated
that an excess of type I protein could effectively remove type II from
an exchangeable pool of subunits, thereby displacing additional subunit
proteins from keratin IF, ultimately resulting in their disassembly
(17)
. These microinjection results suggest that there is
an equilibrium state between exchangeable subunits and polymerized IF.
By far the most convincing evidence supporting the dynamic properties
of IF in vivo has been derived from fluorescence recovery
after photobleaching (FRAP) experiments. These experiments on IF in
living cells were initially carried out following the microinjection of
rhodamine-conjugated vimentin (19)
. The results show that
fluorescent vimentin fibrils are capable of recovering their
fluorescence after photobleaching. This fluorescence recovery appears
to be uniform all along the bleach zones, indicating that in
vivo IF are apolar filaments. Unfortunately, in these studies only
the initial phases of recovery could be monitored because of the low
level of fluorescence emission of the rhodamine-tagged vimentin
fibrils. This made it impossible to measure the t1/2
for full recovery (19)
. More recently, we have been able
to track total recovery of fluorescence in bleach zones across vimentin
fibrils tagged with green fluorescent protein (GFP)-vimentin (see
below; refs 20
, 21
).
| RECENT INSIGHTS INTO THE MOTILE PROPERTIES OF IF IN LIVING CELLS |
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In addition to the above, rapid movements of short vimentin IF,
termed squiggles, are observed. These are seen primarily in the
peripheral regions of the cytoplasm where they translocate at an
average rate of 3.3 ± 1.9 µm/min with speeds ranging from
1.311.4 µm/min (Fig. 1j-l
). These latter
movements are almost completely inhibited when microtubules are
disassembled with nocodazole (21)
, indicating the
involvement of microtubule-based motors (see below).
The opportunity to study individual vimentin fibrils in cells
transfected with GFP-vimentin for longer time intervals in
vivo has also permitted us to calculate the t1/2
for full recovery of fluorescence after photobleaching. FRAP analyses
of GFP-vimentin fibrils yield a t1/2 = 5 ±
3 min. This rate is similar to those obtained for microtubules and
microfilaments in vivo (21)
, indicating that
the speeds of subunit/polymer exchange for all three cytoskeletal
elements are comparable.
The assembly of IF in vitro has been studied extensively,
and the accumulated data suggest that the process requires no energy
and no accessory proteins. However, details of the in vivo
assembly process remain largely unknown. To begin to investigate this
process, we have monitored the behavior of GFP-vimentin by time-lapse
confocal microscopy in spreading BHK-21 cells following trypsinization
and replating. During the first 3045 min of spreading, the majority
of the GFP-vimentin is localized as a filamentous aggregate in the
perinuclear area (25)
. However, in the peripheral regions
of the cell, most of the GFP-vimentin is present as nonfilamentous
vimentin particles (also called dots (20)
; Fig. 2a, b
). Within 11.5 h after replating, the vimentin
particles in the periphery appear to be transformed into the short
fibrous squiggles (see above; ref 21
). After 3 h, as
cells spread even further, the number of particles and squiggles
decreases, apparently replaced by the long vimentin fibrils that typify
the IF networks of spread cells. From these time-lapse observations, it
appears that the assembly of the vimentin IF network involves three
morphologically distinct steps: nonfilamentous particles, short fibrous
squiggles, and long fibrils (20)
.
|
Time-lapse observations have also provided us with remarkable new
insights into the behavior of the vimentin particles. At any given time
during the early stages of cell spreading, these particles can be seen
to undergo either vibrational movements with no net translocation or
rapid and unidirectional movements. The same particle can switch back
and forth between these two types of movements. Time-lapse measurements
of these rapidly moving particles give an average of 0.55 ± 0.24
µm/s with peak velocities of 1 µm/s. The movement of the particles
is along relatively straight paths primarily oriented toward the
peripheral regions of the cell. In a few cases, vimentin particles move
away from the cell surface toward the nucleus (Fig. 2c-e
).
Furthermore, the rapid translocation is abolished after the treatment
of cells with nocodazole, suggesting that particle motility depends on
microtubules. Taken together, these observations suggest the
involvement of a plus-end directed microtubule-based motor in the
transport of nonfilamentous vimentin from the perinuclear to the
peripheral region of the cell. Indeed, immunofluorescence observations
using antibodies specific for conventional kinesin indicate that the
majority of vimentin particles in the peripheral regions of the cell
colocalize with conventional kinesin (20)
. Overall, our
observations of GFP-vimentin in live spreading cells suggest that IF
assembly in vivo is a dynamic process involving
kinesin, microtubules, and an energy source such as ATP (20
, 21)
.
| THE USE OF MIMETIC PEPTIDES TO DISASSEMBLE AND EXPLORE THE FUNCTIONS OF IN VIVO |
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| SUMMARY AND CONCLUSIONS |
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| ACKNOWLEDGMENTS |
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| REFERENCES |
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-actinin along stress fibers. Exp. Cell Res. 167,95-105[Medline]
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