A structural scaffolding of intermediate filaments in health and disease

The cytoplasm of animal cells is structured by a scaffolding composed of actin microfilaments, microtubules, and intermediate filaments. Intermediate filaments, so named because their 10-nanometer diameter is intermediate between that of microfilaments (6 nanometers) and microtubules (23 nanometers), assemble into an anastomosed network within the cytoplasm. In combination with a recently identified class of cross-linking proteins that mediate interactions between intermediate filaments and the other cytoskeletal networks, evidence is reviewed here that intermediate filaments provide a flexible intracellular scaffolding whose function is to structure cytoplasm and to resist stresses externally applied to the cell. Mutations that weaken this structural framework increase the risk of cell rupture and cause a variety of human disorders.

The nuclear lamina is a meshwork of intermediate-type filaments

The nuclear lamina, a protein meshwork lining the nucleoplasmic surface of the inner nuclear membrane, is thought to provide a framework for organizing nuclear envelope structure and an anchoring site at the nuclear periphery for interphase chromatin. In several higher eukaryotic cells, the lamina appears to be a polymer comprised mainly of one to three immunologically related polypeptides of relative molecular mass (Mr) 60,000-75,000 (60-70K) termed lamins. Three lamins (A, B, and C) are typically present in mammalian somatic cells. Previous studies on nuclear envelopes of rat liver and Xenopus oocytes suggested that the lamina has a fibrillar or filamentous substructure. Interestingly, protein sequences recently deduced for human lamins A and C from complementary DNA clones indicate that both of these polypeptides contain a region of approximately 350 amino acids very similar in sequence to the coiled-coil alpha-helical rod domain that characterizes all intermediate-type filament (IF) proteins. Here we analyse the supramolecular organization of the native nuclear lamina and the structure and assembly properties of purified lamins, and show that the lamins constitute a previously unrecognized class of IF polypeptides.

Novel functions of vimentin in cell adhesion, migration, and signaling

Vimentin is the major intermediate filament (IF) protein of mesenchymal cells. It shows dynamically altered expression patterns during different developmental stages and high sequence homology throughout all vertebrates, suggesting that the protein is physiologically important. Still, until recently, the real tasks of vimentin have been elusive, primarily because the vimentin-deficient mice were originally characterized as having a very mild phenotype. Recent studies have revealed several key functions for vimentin that were not obvious at first sight. Vimentin emerges as an organizer of a number of critical proteins involved in attachment, migration, and cell signaling. The highly dynamic and complex phosphorylation of vimentin seems to be a likely regulator mechanism for these functions. The implicated novel vimentin functions have broad ramifications into many different aspects of cell physiology, cellular interactions, and organ homeostasis.

Local modulation of neurofilament phosphorylation, axonal caliber, and slow axonal transport by myelinating Schwann cells

Studies in Trembler and control mice demonstrated that myelinating Schwann cells exert a profound influence on axons. Extensive contacts between myelin and axons have been considered structural. However, demyelination decreases neurofilament phosphorylation, slow axonal transport, and axonal diameter, as well as significantly increasing neurofilament density. In control sciatic nerves with grafted Trembler nerve segments, these changes were spatially restricted: they were confined to axon segments without normal myelination. Adjacent regions of the same axons had normal diameters, neurofilament phosphorylation, cytoskeletal organization, and axonal transport rates. Close intercellular contacts between myelinating Schwann cells and axons modulate a kinase-phosphatase system acting on neurofilaments and possibly other substrates. Myelination by Schwann cells sculpts the axon-altering functional architecture, electrical properties, and neuronal morphologies.

'Hard' and 'soft' principles defining the structure, function and regulation of keratin intermediate filaments

Keratins make up the largest subgroup of intermediate filament proteins and represent the most abundant proteins in epithelial cells. They exist as highly dynamic networks of cytoplasmic 10-12 nm filaments that are obligate heteropolymers involving type I and type II keratins. The primary function of keratins is to protect epithelial cells from mechanical and nonmechanical stresses that result in cell death. Other emerging functions include roles in cell signaling, the stress response and apoptosis, as well as unique roles that are keratin specific and tissue specific. The role of keratins in a number of human skin, hair, ocular, oral and liver diseases is now established and meshes well with the evidence gathered from transgenic mouse models. The phenotypes associated with defects in keratin proteins are subject to significant modulation by functional redundancy within the family and modifier genes as well. Keratin filaments undergo complex regulation involving post-translational modifications and interactions with self and with various classes of associated proteins.

Point mutations in human keratin 14 genes of epidermolysis bullosa simplex patients: genetic and functional analyses

Previously we demonstrated that transgenic mice expressing mutant basal epidermal keratin genes exhibited a phenotype resembling a group of autosomal dominant human skin disorders known as epidermolysis bullosa simplex (EBS). EBS diseases affect approximately 1: 50,000 and are of unknown etiology, although all subtypes exhibit blistering arising from basal cell cytolysis. We now demonstrate that two patients with spontaneous cases of Dowling-Meara EBS have point mutations in a critical region in one (K14) of two basal keratin genes. To demonstrate function, we engineered one of these point mutations in a cloned human K14 cDNA, and showed that a K14 with an Arg-125----Cys mutation disrupted keratin network formation in transfected keratinocytes and perturbed filament assembly in vitro. Since we had previously shown that keratin network perturbation is an essential component of EBS diseases, these data suggest that the basis for the phenotype in this patient resides in this point mutation.

Microtubules and the endoplasmic reticulum are highly interdependent structures

The interrelationships of the endoplasmic reticulum (ER), microtubules, and intermediate filaments were studied in the peripheral regions of thin, spread fibroblasts, epithelial, and vascular endothelial cells in culture. We combined a fluorescent dye staining technique to localize the ER with immunofluorescence to localize microtubules or intermediate filaments in the same cell. Microtubules and the ER are sparse in the lamellipodia, but intermediate filaments are usually completely absent. These relationships indicate that microtubules and the ER advance into the lamellipodia before intermediate filaments. We observed that microtubules and tubules of the ER have nearly identical distributions in lamellipodia, where new extensions of both are taking place. We perturbed microtubules by nocodazole, cold temperature, or hypotonic shock, and observed the effects on the ER distribution. On the basis of our observations in untreated cells and our experiments with microtubule perturbation, we conclude that microtubules and the ER are highly interdependent in two ways: (a) polymerization of individual microtubules and extension of individual ER tubules occur together at the level of resolution of the fluorescence microscope, and (b) depolymerization of microtubules does not disrupt the ER network in the short term (15 min), but prolonged absence of microtubules (2 h) leads to a slow retraction of the ER network towards the cell center, indicating that over longer periods of time, the extended state of the entire ER network requires the microtubule system.

Mice lacking vimentin develop and reproduce without an obvious phenotype

To address the biological role of vimentin in the context of the living organism, we have introduced a null mutation of the vimentin gene into the germ line of mice. Surprisingly, animals homozygous for this mutation developed and reproduced without an obvious phenotype. Immunoblotting, immunofluorescence, and immunogold labeling analysis confirmed the absence of vimentin and of the corresponding filament network. Furthermore, no compensatory expression of another intermediate filament could be demonstrated. While these results leave open the question of the possible role of vimentin in unusual situations or pathological conditions, they show that a conspicuous developmental and cell-specific structure that is an integral part of the cytoskeleton can be eliminated without apparent effect on mouse reproduction and development.

Intermediate filaments: from cell architecture to nanomechanics

Intermediate filaments (IFs) constitute a major structural element of animal cells. They build two distinct systems, one in the nucleus and one in the cytoplasm. In both cases, their major function is assumed to be that of a mechanical stress absorber and an integrating device for the entire cytoskeleton. In line with this, recent disease mutations in human IF proteins indicate that the nanomechanical properties of cell-type-specific IFs are central to the pathogenesis of diseases as diverse as muscular dystrophy and premature ageing. However, the analysis of these various diseases suggests that IFs also have an important role in cell-type-specific physiological functions.

X-ray diffraction from intraneuronal paired helical filaments and extraneuronal amyloid fibers in Alzheimer disease indicates cross-beta conformation

Information about the structure of the paired helical filaments (PHF) that accumulate within human neurons and the amyloid fibers that accumulate in the extracellular spaces between neurons in Alzheimer disease has so far depended on electron microscopy of thin-sectioned or negatively stained material. To determine the protein conformation of these abnormal fibers, we have obtained x-ray diffraction patterns from unfixed human brain fractions highly enriched in PHF and from purified amyloid cores isolated from senile plaques. The predominant x-ray scatter evident from both types of samples, either wet or dry, is a sharp reflection at 4.76-A spacing and a diffuse one at about 10.6-A spacing. These features are characteristic of a beta-pleated sheet type of protein conformation. In doubly oriented dried pellets of PHF fractions, the two reflections are accentuated at right angles to each other and the arc at 4.76-A spacing is in the fiber direction indicating a cross-beta conformation. From the integral widths of the reflections we estimate the cross-beta crystallite to be about 80 A long in the fiber direction and about 40 A thick. These dimensions correspond to approximately four pleated sheets, each of which consists of approximately 16 hydrogen-bonded polypeptide chains running normal to the fiber direction. The cross-beta conformation of PHF and amyloid fibers that we have found from x-ray diffraction is in contrast to the predominant alpha-helical coiled-coil conformation of the neurofilaments with which they share epitopes and from which they have been postulated to derive.

Ubiquitin is a common factor in intermediate filament inclusion bodies of diverse type in man, including those of Parkinson's disease, Pick's disease, and Alzheimer's disease, as well as Rosenthal fibres in cerebellar astrocytomas, cytoplasmic bodies in muscle, and mallory bodies in alcoholic liver disease

Polyclonal antibodies were raised which have a high affinity for conjugated ubiquitin. Immunocytochemistry was performed on paraffin sections of tissues showing well-characterized inclusion bodies. Ubiquitin was found as a component of the intermediate filament inclusion bodies characteristic of several major diseases including Lewy bodies of Parkinson's disease, Pick bodies of Pick's disease, Mallory bodies of alcoholic liver disease, cytoplasmic bodies of a specific myopathy, and Rosenthal fibres within astrocytes. Ubiquitin was also present in the three histological lesions characteristic of Alzheimer's disease. These observations suggest a fundamental role for ubiquitin in the formation of intermediate filament inclusion bodies in man, and have implications regarding the pathogenesis of these important diseases.

Soma neurofilament immunoreactivity is related to cell size and fibre conduction velocity in rat primary sensory neurons

1. Intracellular recordings were made in dorsal root ganglia in vitro at 37 degrees C. The L4, L5 and L6 ganglia from 46- to 51-day-old female Wistar rats were used. In each neuron conduction velocity (CV) was measured and fluorescent dye was injected. Later the intensity of the immunoreactivity to RT97 (a monoclonal antibody to the phosphorylated 200 kDa neurofilament subunit) as well as the cell size (cross-sectional area at the nuclear level) were measured in the dye-injected neurons. RT97 was used to distinguish between the L (light, neurofilament-rich) and the SD (small dark, neurofilament-poor) neuronal somata. 2. Neurons were classified as C neurons (CV less than 1.3 m/s), C/A delta neurons (1.3-2 m/s), A delta neurons (2-12 m/s) or A alpha/beta neurons (greater than 12 m/s). 3. All A-fibre somata were RT97 positive (L) and all C-fibre somata were RT97 negative (SD), although in the C/A delta group both positive and negative neurons were seen. Thus, RT97-negative somata had C (unmyelinated) or C/A delta fibres, while RT97-positive somata had A (myelinated) or C/A delta fibres. 4. The size distributions of A neurons and C neurons were consistent with their classification as L- and SD-cell neurons respectively. The size distribution of A delta cells was skewed with a peak of small cells and a tail of medium-sized cells. 5. There was a loose positive correlation between cell size and fibre CV. 6. RT97 intensity was positively correlated with CV if all neurons were considered together, but no correlation was seen within the C, A delta or A alpha/beta CV groups. 7. RT97 intensity was positively correlated with cell size when all neurons were considered together. Although no correlation was seen within the C or the A delta CV groups, a clear positive correlation was seen for A alpha/beta neurons. 8. The relationship of RT97 intensity to cell size was not demonstrably altered by axotomy, time in vitro or the presence of intracellular dye in control experiments. 9. RT97-negative and -positive neurons could be seen in neonatal rat ganglia. Their size distributions resembled, respectively, the SD- and L-neuron populations at this age. RT97 immunoreactivity may therefore be a useful predictor of the cell type and myelinated state which a sensory cell is destined to reach in the adult rat.

Neuronal intermediate filaments

Neurofilaments (NFs) are the most abundant structural components in large-diameter myelinated axons. Assembled as obligate heteropolymers requiring NF-L and substoichiometric amounts of NF-M and/or NF-H, NF investment into axons is essential for establishment of axonal caliber, itself a key determinant of conduction velocity. Use of transgenic mice to increase axonal accumulation of NFs or to express mutant NFs subunits has proven that aberrant organization or assembly of NFs is sufficient to cause disease arising from selective dysfunction and degeneration of motor neurons. Because aberrant accumulation of NFs is a common pathology in a series of motor neuron diseases-including amyotrophic lateral sclerosis-NF misaccumulation, and the resultant disruption in axonal transport, is probably a key intermediate in the pathogenesis of these diseases.

Astrocytes block axonal regeneration in mammals by activating the physiological stop pathway

Regenerating sensory axons in the dorsal roots of adult mammals are stopped at the junction between the root and spinal cord by reactive astrocytes. Do these cells stop axonal elongation by activating the physiological mechanisms that normally operate to stop axons during development, or do they physically obstruct the elongating axons? In order to distinguish these possibilities, the cytology of the axon tips of regenerating axons that were stopped by astrocytes was compared with the axon tips that were physically obstructed at a cul-de-sac produced by ligating a peripheral nerve. The terminals of the physically obstructed axon tips were distended with neurofilaments and other axonally transported structures that had accumulated when the axons stopped elongating. By contrast, neurofilaments did not accumulate in the tips of regenerating axons that were stopped by spinal cord astrocytes at the dorsal root transitional zone. These axo-glial terminals resembled the terminals that axons make on target neurons during normal development. On the basis of these observations, astrocytes appear to stop axons from regenerating in the mammalian spinal cord by activating the physiological stop pathway that is built into the axon and that normally operates when axons form stable terminals on target cells.

Neurofilaments are obligate heteropolymers in vivo

Neurofilaments (NFs), composed of three distinct subunits NF-L, NF-M, and NF-H, are neuron-specific intermediate filaments present in most mature neurons. Using DNA transfection and mice expressing NF transgenes, we find that despite the ability of NF-L alone to assemble into short filaments in vitro NF-L cannot form filament arrays in vivo after expression either in cultured cells or in transgenic oligodendrocytes that otherwise do not contain a cytoplasmic intermediate filament (IF) array. Instead, NF-L aggregates into punctate or sheet like structures. Similar nonfilamentous structures are also formed when NF-M or NF-H is expressed alone. The competence of NF-L to assemble into filaments is fully restored by coexpression of NF-M or NF-H to a level approximately 10% of that of NF-L. Deletion of the head or tail domain of NF-M or substitution of the NF-H tail onto an NF-L subunit reveals that restoration of in vivo NF-L assembly competence requires an interaction provided by the NF-M or NF-H head domains. We conclude that, contrary to the expectation drawn from earlier in vitro assembly studies, NF-L is not sufficient to assemble an extended filament network in an in vivo context and that neurofilaments are obligate heteropolymers requiring NF-L and NF-M or NF-H.

Desmin myopathy, a skeletal myopathy with cardiomyopathy caused by mutations in the desmin gene

BACKGROUND

Myofibrillar myopathies, often referred to as desmin-related myopathies, are a heterogeneous group of inherited or sporadic distal-onset skeletal myopathies associated with cardiomyopathy. Among the myofibrillar proteins that characteristically accumulate within the muscle fibers of affected patients, the one found most consistently is desmin, a muscle-specific intermediate-filament protein responsible for the structural integrity of the myofibrils. Skeletal and cardiac myopathy develops in mice that lack desmin, suggesting that mutations in the desmin gene may be pathogenic.

METHODS

We examined 22 patients from 8 families with dominantly inherited myofibrillar or desmin-related myopathy and 2 patients with sporadic disease and analyzed the desmin gene for mutations, using complementary DNA (cDNA) amplified from muscle-biopsy specimens and genomic DNA extracted from blood lymphocytes. Restriction-enzyme analysis was used to confirm the mutations. Expression vectors containing normal or mutant desmin cDNA were introduced into cultured cells to determine whether the mutant desmin formed intermediate filaments.

RESULTS

Six missense mutations in the coding region of the desmin gene that cause the substitution of an amino acid were identified in 11 patients (10 members of 4 families and 1 patient with sporadic disease); a splicing defect that resulted in the deletion of exon 3 was identified in the other patient with sporadic disease. Mutations were clustered in the carboxy-terminal part of the rod domain, which is critical for filament assembly. In transfected cells, the mutant desmin was unable to form a filamentous network. Seven of the 12 patients with mutations in the desmin gene had cardiomyopathy.

CONCLUSIONS

Mutations in the desmin gene affecting intermediate filaments cause a distinct myopathy that is often associated with cardiomyopathy and is termed "desmin myopathy." The mutant desmin interferes with the normal assembly of intermediate filaments, resulting in fragility of the myofibrils and severe dysfunction of skeletal and cardiac muscles.

Desmin cytoskeleton linked to muscle mitochondrial distribution and respiratory function

Ultrastructural studies have previously suggested potential association of intermediate filaments (IFs) with mitochondria. Thus, we have investigated mitochondrial distribution and function in muscle lacking the IF protein desmin. Immunostaining of skeletal muscle tissue sections, as well as histochemical staining for the mitochondrial marker enzymes cytochrome C oxidase and succinate dehydrogenase, demonstrate abnormal accumulation of subsarcolemmal clumps of mitochondria in predominantly slow twitch skeletal muscle of desmin-null mice. Ultrastructural observation of desmin-null cardiac muscle demonstrates in addition to clumping, extensive mitochondrial proliferation in a significant fraction of the myocytes, particularly after work overload. These alterations are frequently associated with swelling and degeneration of the mitochondrial matrix. Mitochondrial abnormalities can be detected very early, before other structural defects become obvious. To investigate related changes in mitochondrial function, we have analyzed ADP-stimulated respiration of isolated muscle mitochondria, and ADP-stimulated mitochondrial respiration in situ using saponin skinned muscle fibers. The in vitro maximal rates of respiration in isolated cardiac mitochondria from desmin-null and wild-type mice were similar. However, mitochondrial respiration in situ is significantly altered in desmin-null muscle. Both the maximal rate of ADP-stimulated oxygen consumption and the dissociation constant (K(m)) for ADP are significantly reduced in desmin-null cardiac and soleus muscle compared with controls. Respiratory parameters for desmin-null fast twitch gastrocnemius muscle were unaffected. Additionally, respiratory measurements in the presence of creatine indicate that coupling of creatine kinase and the adenine translocator is lost in desmin-null soleus muscle. This coupling is unaffected in cardiac muscle from desmin-null animals. All of these studies indicate that desmin IFs play a significant role in mitochondrial positioning and respiratory function in cardiac and skeletal muscle.

Rapid movements of vimentin on microtubule tracks: kinesin-dependent assembly of intermediate filament networks

The assembly and maintenance of an extended intermediate filament (IF) network in fibroblasts requires microtubule (MT) integrity. Using a green fluorescent protein-vimentin construct, and spreading BHK-21 cells as a model system to study IF-MT interactions, we have discovered a novel mechanism involved in the assembly of the vimentin IF cytoskeleton. This entails the rapid, discontinuous, and MT-dependent movement of IF precursors towards the peripheral regions of the cytoplasm where they appear to assemble into short fibrils. These precursors, or vimentin dots, move at speeds averaging 0.55 +/- 0.24 micrometer/s. The vimentin dots colocalize with MT and their motility is inhibited after treatment with nocodazole. Our studies further implicate a conventional kinesin in the movement of the vimentin dots. The dots colocalize with conventional kinesin as shown by indirect immunofluorescence, and IF preparations from spreading cells are enriched in kinesin. Furthermore, microinjection of kinesin antibodies into spreading cells prevents the assembly of an extended IF network. These studies provide insights into the interactions between the IF and MT systems. They also suggest a role for conventional kinesin in the distribution of non-membranous protein cargo, and the local regulation of IF assembly.

The bacterial cytoskeleton: an intermediate filament-like function in cell shape

Various cell shapes are encountered in the prokaryotic world, but how they are achieved is poorly understood. Intermediate filaments (IFs) of the eukaryotic cytoskeleton play an important role in cell shape in higher organisms. No such filaments have been found in prokaryotes. Here, we describe a bacterial equivalent to IF proteins, named crescentin, whose cytoskeletal function is required for the vibrioid and helical shapes of Caulobacter crescentus. Without crescentin, the cells adopt a straight-rod morphology. Crescentin has characteristic features of IF proteins including the ability to assemble into filaments in vitro without energy or cofactor requirements. In vivo, crescentin forms a helical structure that colocalizes with the inner cell curvatures beneath the cytoplasmic membrane. We propose that IF-like filaments of crescentin assemble into a helical structure, which by applying its geometry to the cell, generates a vibrioid or helical cell shape depending on the length of the cell.

The genetic basis of epidermolytic hyperkeratosis: a disorder of differentiation-specific epidermal keratin genes

Epidermolytic hyperkeratosis (EH) is a skin disease characterized by keratin filament clumping and degeneration in terminally differentiating epidermal cells. We have discovered that the genetic basis for EH resides in mutations in differentiation-specific keratins. Two of six distinct incidences of EH had a keratin 10 (K10) point mutation in a highly conserved arginine. Remarkably, this same residue is mutated in the basal epidermal K14 in three incidences of another skin disease, epidermolysis bullosa simplex (EBS). By genetic engineering, gene transfection, and 10 nm filament assembly, we show that this mutation is functionally responsible for the keratin filament clumping that occurs in basal (EBS) or suprabasal (EH) cells. These studies strengthen the link between filament perturbations, cell fragility, and degeneration first established with EBS. They also suggest a correlation between filament disorganization and either cytokinesis or nuclear shape, giving rise to the seemingly binucleate cells typical of EH.

Intermediate filament reorganization during mitosis is mediated by p34cdc2 phosphorylation of vimentin

As cells enter mitosis, the intermediate filament (IF) networks of interphase BHK-21 cells are depolymerized to form cytoplasmic aggregates of disassembled IFs, and the constituent IF proteins, vimentin and desmin are hyperphosphorylated at several specific sites. We have characterized one of two endogenous vimentin kinases from a particulate fraction of mitotic cell lysates. Through several purification steps, vimentin kinase activity copurifies with histone H1 kinase and both activities bind to p13suc1-Sepharose. The final enriched kinase preparation consists primarily of p34cdc2 and polypeptides of 65 and 110 kd. The purified kinase complex phosphorylates vimentin in vitro at a subset of sites phosphorylated in vivo during mitosis. Furthermore, phosphorylation of in vitro polymerized vimentin IFs by the purified kinase causes their disassembly. Therefore, vimentin is a substrate of p34cdc2 and phosphorylation of vimentin contributes to M phase reorganization of the IF network.

Specific in vivo phosphorylation sites determine the assembly dynamics of vimentin intermediate filaments

Intermediate filaments (IFs) continuously exchange between a small, depolymerized fraction of IF protein and fully polymerized IFs. To elucidate the possible role of phosphorylation in regulating this equilibrium, we disrupted the exchange of phosphate groups by specific inhibition of dephosphorylation and by specific phosphorylation and site-directed mutagenesis of two of the major in vivo phosphorylation sites determined in this study. Inhibition of type-1 (PP1) and type-2A (PP2A) protein phosphatases in BHK-21 fibroblasts with calyculin-A, induced rapid vimentin phosphorylation in concert with disassembly of the IF polymers into soluble tetrameric vimentin oligomers. This oligomeric composition corresponded to the oligopeptides released by cAMP-dependent kinase (PKA) following in vitro phosphorylation. Characterization of the (32)P-labeled vimentin phosphopeptides, demonstrated Ser-4, Ser-6, Ser-7, Ser-8, Ser-9, Ser-38, Ser-41, Ser-71, Ser-72, Ser-418, Ser-429, Thr-456, and Ser-457 as significant in vivo phosphorylation sites. A number of the interphase-specific high turnover sites were shown to be in vitro phosphorylation sites for PKA and protein kinase C (PKC). The effect of presence or absence of phosphate groups on individual subunits was followed in vivo by microinjecting PKA-phosphorylated (primarily S38 and S72) and mutant vimentin (S38:A, S72:A), respectively. The PKA-phosphorylated vimentin showed a clearly decelerated filament formation in vivo, whereas obstruction of phosphorylation at these sites by site-directed mutagenesis had no significant effect on the incorporation rates of subunits into assembled polymers. Taken together, our results suggest that elevated phosphorylation regulates IF assembly in vivo by changing the equilibrium constant of subunit exchange towards a higher off-rate.

Neurofilament phosphorylation: a new look at regulation and function

Dynamic remodeling of cytoskeleton architecture is necessary for axonal growth and guidance, signal transduction and other fundamental aspects of neuron function. Protein phosphorylation plays a key part in these remodeling processes. Since neurofilaments are major cytoskeletal constituents and are among the most highly phosphorylated neuronal proteins, the control of their behavior serves as a possible model for understanding how phosphorylation regulates the many other phosphoproteins in the cytoskeleton. Recent studies show that neurofilament protein subunits are phosphorylated on both their amino-terminal head domains and carboxy-terminal tails by different protein kinases. This review considers the implications of this complex regulation for neurofilament function in normal neurons and in disease states characterized by neurofibrillary pathology.