The 47-kD lens-specific protein phakinin is a tailless intermediate filament protein and an assembly partner of filensin

Are the Head and Tail Domains of Intermediate Filaments Really Unstructured Regions?

Intermediate filaments (IFs) are integral components of the cytoskeleton which provide cells with tissue-specific mechanical properties and are involved in a plethora of cellular processes. Unfortunately, due to their intricate architecture, the 3D structure of the complete molecule of IFs has remained unresolved. Even though most of the rod domain structure has been revealed by means of crystallographic analyses, the flanked head and tail domains are still mostly unknown. Only recently have studies shed light on head or tail domains of IFs, revealing certainsecondary structures and conformational changes during IF assembly. Thus, a deeper understanding of their structure could provide insights into their function.

Effects of truncations in the N- and C-terminal domains of filensin on filament formation with phakinin in cell-free conditions and cultured cells

Filensin and phakinin are lens fiber cell-specific proteins that constitute the beaded filaments (BFs) that are critical for maintaining lens transparency. In the Shumiya cataract rat, filensin 94 kDa undergoes N- and C-terminal proteolytic processing to give a transient 50 kDa fragment and a final 38 kDa fragment, just before opacification. To characterize the effects of this processing on filensin function, recombinant proteins representing the two filensin fragments, termed Fil(30-416) and Fil(30-369), respectively, were examined. Fil(30-416) lacks the N-terminal 29 amino acids and the C-terminal 248 amino acids. Fil(30-369) lacks the N-terminal 29 residues and the C-terminal 295 residues. In cell-free assembly characterized by electron microscopy, filensin and Fil(30-416) co-polymerized with phakinin and formed rugged, entangled filaments, whereas Fil(30-369) formed only aggregates. In cultured SW-13 and MCF-7 cells expressing fluorescent fusion proteins, filensin and Fil(30-416) co-polymerized with phakinin and formed cytoplasmic sinuous filaments with different widths, while Fil(30-369) gave aggregates. Therefore, while truncation of the N-terminal 29 amino acids did not affect filament formation, truncation of the C-terminal 295 but not the 248 residues resulted in failure of filament formation. These results indicate that the tail B region (residues 370-416) of rat filensin is essential for filament formation with phakinin. Truncation of the tail B region by proteolytic processing in the cataract rat lens might interfere with BF formation and thereby contribute to opacification.

Intermediate filament proteins are reliable immunohistological biomarkers to help diagnose multiple tissue-specific diseases

Cytoskeletal networks are proteins that effectively maintain cell integrity and provide mechanical support to cells by actively transmitting mechanical signals. Intermediate filaments, which are from the cytoskeleton family and are 10 nanometres in diameter, are unlike actin and microtubules, which are highly dynamic cytoskeletal elements. Intermediate filaments are flexible at low strain, harden at high strain and resist breaking. For this reason, these filaments fulfil structural functions by providing mechanical support to the cells through their different strain-hardening properties. Intermediate filaments are suitable in that cells both cope with mechanical forces and modulate signal transmission. These filaments are composed of fibrous proteins that exhibit a central α-helical rod domain with a conserved substructure. Intermediate filament proteins are divided into six groups. Type I and type II include acidic and basic keratins, type III, vimentin, desmin, peripheralin and glial fibrillary acidic protein (GFAP), respectively. Type IV intermediate filament group includes neurofilament proteins and a fourth neurofilament subunit, α-internexin proteins. Type V consists of lamins located in the nucleus, and the type VI group consists of lens-specific intermediate filaments, CP49/phakinin and filen. Intermediate filament proteins show specific immunoreactivity in differentiating cells and mature cells of various types. Various carcinomas such as colorectal, urothelial and ovarian, diseases such as chronic pancreatitis, cirrhosis, hepatitis and cataract have been associated with intermediate filaments. Accordingly, this section reviews available immunohistochemical antibodies to intermediate filament proteins. Identification of intermediate filament proteins by methodological methods may contribute to the understanding of complex diseases.

Update of the keratin gene family: evolution, tissue-specific expression patterns, and relevance to clinical disorders

Intermediate filament (IntFil) genes arose during early metazoan evolution, to provide mechanical support for plasma membranes contacting/interacting with other cells and the extracellular matrix. Keratin genes comprise the largest subset of IntFil genes. Whereas the first keratin gene appeared in sponge, and three genes in arthropods, more rapid increases in keratin genes occurred in lungfish and amphibian genomes, concomitant with land animal-sea animal divergence (~ 440 to 410 million years ago). Human, mouse and zebrafish genomes contain 18, 17 and 24 non-keratin IntFil genes, respectively. Human has 27 of 28 type I "acidic" keratin genes clustered at chromosome (Chr) 17q21.2, and all 26 type II "basic" keratin genes clustered at Chr 12q13.13. Mouse has 27 of 28 type I keratin genes clustered on Chr 11, and all 26 type II clustered on Chr 15. Zebrafish has 18 type I keratin genes scattered on five chromosomes, and 3 type II keratin genes on two chromosomes. Types I and II keratin clusters-reflecting evolutionary blooms of keratin genes along one chromosomal segment-are found in all land animal genomes examined, but not fishes; such rapid gene expansions likely reflect sudden requirements for many novel paralogous proteins having divergent functions to enhance species survival following sea-to-land transition. Using data from the Genotype-Tissue Expression (GTEx) project, tissue-specific keratin expression throughout the human body was reconstructed. Clustering of gene expression patterns revealed similarities in tissue-specific expression patterns for previously described "keratin pairs" (i.e., KRT1/KRT10, KRT8/KRT18, KRT5/KRT14, KRT6/KRT16 and KRT6/KRT17 proteins). The ClinVar database currently lists 26 human disease-causing variants within the various domains of keratin proteins.

Quantitative Phosphoproteomic Comparison of Lens Proteins in Highly Myopic Cataract and Age-Related Cataract

Purpose

To investigate and compare the lens phosphoproteomes in patients with highly myopic cataract (HMC) or age-related cataract (ARC).

Methods

In this study, we undertook a comparative phosphoproteome analysis of the lenses from patients with HMC or ARC. Intact lenses from ARC and HMC patients were separated into the cortex and nucleus. After protein digestion, the phosphopeptides were quantitatively analyzed with TiO₂ enrichment and liquid chromatography-mass spectrometry. The potential functions of different phosphopeptides were assessed by Gene Ontology (GO) enrichment analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis.

Results

In total, 522 phosphorylation sites in 164 phosphoproteins were identified. The number of phosphorylation sites was significantly higher in the cortex than in the nucleus, in both ARC and HMC lenses. The differentially phosphorylated peptides in the lens cortex and nucleus in HMC eyes were significantly involved in the glutathione metabolism pathway. The KEGG pathway enrichment analysis indicated that the differences in phosphosignaling mediators between the ARC and HMC lenses were associated with glycolysis and the level of phosphorylated phosphoglycerate kinase 1 was lower in HMC lenses than in ARC lenses.

Conclusions

We provide an overview of the differential phosphoproteomes of HMC and ARC lenses that can be used to clarify the molecular mechanisms underlying their different phenotypes.

Identification of a direct Aquaporin-0 binding site in the lens-specific cytoskeletal protein filensin

An interaction between the C-terminus of aquaporin-0 (AQP0) and lens beaded filament protein filensin has been reported previously; however, the region of filensin that is involved in the interaction has not been determined. This study is designed to identify the region of filensin that interacts with AQP0. Chemical crosslinking coupled with mass spectrometry was used to identify the site of interaction. The protein complex was crosslinked with zero-length crosslinker: 1-Ethyl-3-[3-dimethylaminopropyl]carbodiimide Hydrochloride (EDC). The crosslinked membrane fraction was digested by trypsin and crosslinked peptides were identified by liquid chromatography-tandem mass spectrometry. A crosslinked peptide between bovine filensin 450-465 (VKGPKEPEPPADLYTK) and bovine AQP0 239-259 (GSRPSESNGQPEVTGEPVELK) was detected. AQP0/filensin crosslinking was not detected in superficial young fiber cells, but increased with fiber cell age in the lens cortex. AQP0/filensin crosslinking and filensin truncation were observed in the same regions of the lens. This crosslinked peptide can be detected in 75 kDa gel band confirming that AQP0/filensin crosslinking can occur between AQP0 and the filensin C-terminal fragment. These results suggest that the AQP0 C-terminus directly interacts with the region of filensin that is adjacent to the major truncation site and the polybasic cluster of residues in the filensin C-terminal tail. This interaction occurs in a specific region of the lens and could only occur between AQP0 and filensin C-terminal fragment in vivo. This interaction supports the dual roles of filensin in the lens; roles that could be important during lens development.

Evolution of the vertebrate beaded filament protein, Bfsp2; comparing the in vitro assembly properties of a "tailed" zebrafish Bfsp2 to its "tailless" human orthologue

In bony fishes, Bfsp2 orthologues are predicted to possess a C-terminal tail domain, which is absent from avian, amphibian and mammalian Bfsp2 sequences. These sequences, are however, not conserved between fish species and therefore questions whether they have a functional role. For other intermediate filament proteins, the C-terminal tail domain is important for both filament assembly and regulating interactions between filaments. We confirm that zebrafish has a single Bfsp2 gene by radiation mapping. Two transcripts (bfsp2α and bfsp2β) are produced by alternative splicing of the last exon. Using a polyclonal antibody specific to a tridecameric peptide in the C-terminal tail domain common to both zebrafish Bfsp2 splice variants, we have confirmed its expression in zebrafish lens fibre cells. We have also determined the in vitro assembly properties of zebrafish Bfsp2α and conclude that the C-terminal sequences are required to regulate not only the diameter and uniformity of the in vitro assembly filaments, but also their filament-filament associations in vitro. Therefore we conclude zebrafish Bfsp2α is a functional orthologue conforming more closely to the conventional domain structure of intermediate filament proteins. Data mining of the genome databases suggest that the loss of this tail domain could occur in several stages leading eventually to completely tailless orthologues, such as human BFSP2.

Tight binding of proteins to membranes from older human cells

The lens is an ideal model system for the study of macromolecular aging and its consequences for cellular function, since there is no turnover of lens fibre cells. To examine biochemical processes that take place in the lens and that may also occur in other long-lived cells, membranes were isolated from defined regions of human lenses that are synthesised at different times during life, and assayed for the presence of tightly bound cytosolic proteins using quantitative iTRAQ proteomics technology. A majority of lens beta crystallins and all gamma crystallins became increasingly membrane bound with age, however, the chaperone proteins alpha A and alpha B crystallin, as well as the thermally-stable protein, βB2 crystallin, did not. Other proteins such as brain-associated signal protein 1 and paralemmin 1 became less tightly bound in the older regions of the lens. It is evident that protein-membrane interactions change significantly with age. Selected proteins that were formerly cytosolic become increasingly tightly bound to cell membranes with age and are not removed even by treatment with 7 M urea. It is likely that such processes reflect polypeptide denaturation over time and the untoward binding of proteins to membranes may alter membrane properties and contribute to impairment of communication between older cells.

A novel terminal web-like structure in cortical lens fibers: architecture and functional assessment

This study describes a novel cytoskeletal array in fiber cells of the ocular lens of the rat and shows its relationship to the classical terminal web of other epithelial tissues. Naive adult Sprague-Dawley rats (n = 28) were utilized. F-actin, fodrin, myosin IIA, and CP49 distribution was assessed in anterior and posterior polar sections. For functional analysis, lenses were cultured with or without cytochalasin-D for 3 hr, then processed for confocal microscopy or assessed by laser scan analysis along sutures. Phalloidin labeling demonstrated a dense mesh of F-actin adjacent to posterior sutural domains to a subcapsular depth of 400 μm. Anterior polar sections revealed a comparable actin structure adjacent to anterior suture branches however, it was not developed in superficial fibers. Fodrin and myosin were localized within the web-like actin apparatus. The data was used to construct a model showing that the cytoskeletal array is located within the blunt, variable-width fiber ends that abut at sutures such that the "terminal web" flanks the suture on either side. Treatment with cytochalasin-D resulted in partial disassembly of the "terminal web" and perturbed cellular organization. Laser scan analysis revealed that cytochalasin-D treated lenses had significantly greater focal variability than control lenses (P = 0.020). We conclude that cortical fibers of rat lenses contain a bipolar structure that is structurally and compositionally analogous to classical terminal webs. The results indicate that the lens "terminal web" functions to stabilize lens fiber ends at sutures thus minimizing structural disorder, which in turn, promotes the establishment and maintenance of lens transparency.

Seven kinds of intermediate filament networks in the cytoplasm of polarized cells: structure and function

Intermediate filaments (IFs) are involved in many important physiological functions, such as the distribution of organelles, signal transduction, cell polarity and gene regulation. However, little information exists on the structure of the IF networks performing these functions. We have clarified the existence of seven kinds of IF networks in the cytoplasm of diverse polarized cells: an apex network just under the terminal web, a peripheral network lying just beneath the cell membrane, a granule-associated network surrounding a mass of secretory granules, a Golgi-associated network surrounding the Golgi apparatus, a radial network locating from the perinuclear region to the specific area of the cell membrane, a juxtanuclear network surrounding the nucleus, and an entire cytoplasmic network. In this review, we describe these seven kinds of IF networks and discuss their biological roles.

Posttranslational modifications of the bovine lens beaded filament proteins filensin and CP49

PURPOSE

The lens beaded filament proteins filensin and CP49 are phosphorylated proteins that undergo proteolytic degradation with fiber cell age; however, the specific sites of modifications remain largely unknown. The purpose of this study was to identify posttranslational modifications (PTMs) in bovine lens beaded filament proteins.

METHODS

Filensin and CP49 were enriched by urea extraction of lens fiber cell homogenates after the water-soluble fraction was removed. The urea-soluble fraction was separated by SDS-PAGE, and the corresponding filensin and CP49 bands were digested by trypsin, Lys C, or Glu C. The enzymatic digests were analyzed by HPLC mass spectrometry.

RESULTS

The sequences of lens beaded filament proteins were systematically mapped, and putative database sequence errors of filensin were identified. The data also indicated that Met-1 of CP49 was removed and Ser2 was acetylated. Nine phosphorylation sites on filensin and seven phosphorylation sites on CP49 were identified. Filensin was found to be truncated at D431 and L39, and the resulting new N termini were N-myristoylated and N-acetylated, respectively. Truncation of CP49 occurred at D37. Aspartic acid isomerization to isoaspartic acid occurs at the major truncation sites of filensin (D431) and of CP49 (D37).

CONCLUSIONS

This study identified sites of phosphorylation and truncation in filensin and CP49 and revealed two unusual PTMs: postproteolytic N-acetylation and N-myristoylation of filensin. The detailed knowledge about these PTMs provides important information for further study of their functional consequences-for example protein redistribution during lens fiber cell differentiation and aging.

Functions of the intermediate filament cytoskeleton in the eye lens

Intermediate filaments (IFs) are a key component of the cytoskeleton in virtually all vertebrate cells, including those of the lens of the eye. IFs help integrate individual cells into their respective tissues. This Review focuses on the lens-specific IF proteins beaded filament structural proteins 1 and 2 (BFSP1 and BFSP2) and their role in lens physiology and disease. Evidence generated in studies in both mice and humans suggests a critical role for these proteins and their filamentous polymers in establishing the optical properties of the eye lens and in maintaining its transparency. For instance, mutations in both BFSP1 and BFSP2 cause cataract in humans. We also explore the potential role of BFSP1 and BFSP2 in aging processes in the lens.

The structure of the cytoplasm of lens fibers as determined by conical tomography

Studies using conventional electron microscopy describe the cytoplasm of lens fiber cells as having essentially an amorphous structure. We hypothesized that significant structural detail might have been lost as a result of projecting the entire thickness of the section (50-100 nm) onto a single plane (the "projection artifact"). To test this hypothesis, we studied the 3D-structure of rat lens cortical fibers before and after extracting the "soluble" crystallins with low ionic strength buffers to make "ghosts." Tomographic series in conical geometry were collected at 55 degrees tilts and by 5 degrees rotations until completing a 360 degrees turn by low dose methods. They were aligned using fiduciary points, reconstructed with the weighted back projection algorithm and refined by projection matching. Analysis of the 3D-maps included semiautomatic density segmentation using a computer program based on the watershed algorithm. We found that the cytoplasm of cortical fibers, though appearing amorphous in regions of the highest density, was in fact comprised of an ordered structure resembling a "clustered matrix." The matrix was comprised of thin ( approximately 6 nm diameter) filaments bent sharply at 110-120 degrees angles and studded with cube-shaped particles (the "beaded" filaments). In cortical fibers, the particles measured a=14+/-2, b=13+/-2 and c=10+/-2.4 nm (n=30, mean+/-SD) and were spaced at distances measuring 27.5+/-2.4 nm apart (n=8, mean+/-SD), center-to-center. The matrix was formed as "beaded" filaments, bound to clusters of "soluble" proteins, crossed each other at nearly perpendicular angles. The matrix also made contact with the plasma membrane at a large number of distinct regions. We thus concluded that the cytoplasm of cortical lens fibers is comprised of a cytoskeletal matrix of "beaded" filaments that organize the "soluble" crystallins in separate regions. The association of this matrix with the plasma membrane allows the lens to maintain its structural integrity, while its association with crystallins yields its long-term transparency. Loss of either function likely would play a significant role in cataract formation.

Lens intermediate filaments

The ocular lens assembles two separate intermediate filament systems sequentially with differentiation. Canonical 8-11 nm IFs composed of Vimentin are assembled in lens epithelial cells and younger fiber cells, while the fiber cell-specific beaded filaments are switched on as fiber cell elongation initiates. Some of the key features of both filament systems are reviewed.

The function of filensin and phakinin in lens transparency

PURPOSE

Beaded filaments are lens cell-specific intermediate filaments composed of two proteins: filensin and phakinin (CP49). Filensin and phakinin are believed to function in the maintenance of lens transparency. To elucidate the function of filensin and phakinin at the molecular level, we examined the degradation of these two proteins in normal and cataractous rat lenses.

METHODS

A hereditary cataract model, the Shumiya cataract rat (SCR), was used for these studies. Anti-filensin antibodies were raised against three different regions of the protein, the rod domain, the inner region of the tail domain, and the outer region of the tail domain. Anti-filensin and anti-phakinin antibodies were used to examine the conformation of degradation of filensin and phakinin by western blot analysis and fluorescent immunocytochemistry of cryosectioned lenses.

RESULTS

In the normal lens, filensin was processed from a 94 kDa protein to proteins of 50 kDa and 38 kDa. Similarly, phakinin was processed from a 49 kDa protein to one of 40 kDa. The concentrations of filensin and phakinin in the rat lens cortex fluctuated with age and decreased during cataractogenesis. The 50 kDa form of filensin decreased significantly before opacification. In the normal lens, phakinin, the filensin rod domain, and the filensin inner tail domain localized to membrane lining regions in the shallow cortex and to the central region of the cytoplasm in the deep cortex. The COOH-terminal domain of filensin localized to the membrane lining region in the deep cortex. In pre-cataractous lenses, phakinin and the filensin rod domain localized primarily to the membranes lining the shallow cortex region and were distributed throughout the cytoplasm of lens fiber cells in the deep cortex.

CONCLUSIONS

The 50 kDa form of filensin is important for the localization of beaded filaments in lens fiber cells and for lens transparency.

Identifying the role of specific motifs in the lens fiber cell specific intermediate filament phakosin

PURPOSE

Phakosin and filensin are lens fiber cell-specific intermediate filament (IF) proteins. Unlike every other cytoplasmic IF protein, they assemble into a beaded filament (BF) rather than an IF. Why the lens fiber cell requires two unique IF proteins and why and how they assemble into a structure other than an IF are unknown. In this report we test specific motifs/domains in phakosin to identify changes that that have adapted phakosin to lens-specific structure and function.

METHODS

Phakosin shows the highest level of sequence identity to K18, whose natural assembly partner is K8. We therefore exchanged conserved keratin motifs between phakosin and K18 to determine whether phakosin's divergent motifs could redirect the assembly of chimeric K18 and K8. Modified proteins were bacterially expressed and purified. Assembly competence was assessed by electron microscopy.

RESULTS

Substitution of the phakosin helix initiation motif (HIM) into K18 does not alter assembly with K8, establishing that the radical divergence in phakosin HIM is not by itself the mechanism by which IF assembly is redirected to BF assembly. Unexpectedly, K18 bearing phakosin HIM resulted in normal IF assembly, despite the presence of an otherwise disease-causing R-C substitution, and two helix-disrupting glycines. This disproves the widely held belief that mutation of the R is catastrophic to IF assembly. Additional data are presented that suggest normal IF assembly is dependent on sequence-specific interactions between the IF head domain and the HIM.

CONCLUSIONS

In the lens fiber cell, two members of the IF family have evolved to produce BFs instead of IFs, a change that presumably adapts the IF to a fiber cell-specific function. The authors establish here that the most striking divergence seen in phakosin is not, as hypothesized, the cause of this altered assembly outcome. The authors further establish that the HIM of IFs is far more tolerant of mutations, such as those that cause some corneal dystrophies and Alexander disease, than previously hypothesized and that normal assembly involves sequence-specific interactions between the head domain and the HIM.

Intermediate filaments: a historical perspective

Intracellular protein filaments intermediate in size between actin microfilaments and microtubules are composed of a surprising variety of tissue specific proteins commonly interconnected with other filamentous systems for mechanical stability and decorated by a variety of proteins that provide specialized functions. The sequence conservation of the coiled-coil, alpha-helical structure responsible for polymerization into individual 10 nm filaments defines the classification of intermediate filament proteins into a large gene family. Individual filaments further assemble into bundles and branched cytoskeletons visible in the light microscope. However, it is the diversity of the variable terminal domains that likely contributes most to different functions. The search for the functions of intermediate filament proteins has led to discoveries of roles in diseases of the skin, heart, muscle, liver, brain, adipose tissues and even premature aging. The diversity of uses of intermediate filaments as structural elements and scaffolds for organizing the distribution of decorating molecules contrasts with other cytoskeletal elements. This review is an attempt to provide some recollection of how such a diverse field emerged and changed over about 30 years.

Insights into the beaded filament of the eye lens

Filensin (BFSP1) and CP49 (BFSP2) represent two members of the IF protein superfamily that are thus far exclusively expressed in the eye lens. Mutations in both proteins cause lens cataract and careful consideration of the detail of these cataract phenotypes alerts us to several interesting features concerning the function of filensin (BFSP1) and CP49 (BFSP2) in the lens. With the first filensin (BFSP1) mutation now having been reported to cause a recessive cataract phenotype, there is the suggestion that the mutation could predispose heterozygote carriers to the early onset of age-related nuclear cataract. In the case of CP49 (BFSP2), there are now three unrelated families who have been identified with a common E233 Delta mutation. Very interestingly this is linked to myopia in one family. Despite the apparent phenotypic differences of the filensin (BFSP1) and CP49 (BFSP2) mutations, the data are still consistent with the beaded filament proteins being essential for lens function and specifically contributing to the optical properties of the lens. The fact that none of the mutations thus far reported affect either the conserved LNDR or TYRKLLEGE motifs that flank the central rod domain supports the view that this pair of IF proteins have unusual structural features and a distinctive assembly mechanism. The multiple sequence divergences suggest these proteins have been adapted to the specific functional requirements of lens fibre cells, a function that can be traced from squid to man.

Transfer of lens-specific transcripts to retinal RNA samples may underlie observed changes in crystallin-gene transcript levels after ischemia

PURPOSE

Retinal ischemia appears to lead to alterations in retinal transcript levels of a group of genes known to be abundantly expressed in the lens. Our purpose is to study whether these alterations are truly the result of retinal ischemia or whether they could be caused by contamination of the retinal tissue with trace amounts of lens tissue.

METHODS

Changes occurring in the retinal gene expression profile after induction of retinal ischemia were assessed by oligonucleotide microarrays and by real-time quantitative PCR.

RESULTS

Microarray analysis of the retinal gene expression profile after 5 or 60 min ischemia showed altered transcript levels for a group of genes with functions related to "structural constituent of eye lens" (23 genes, predominantly crystallins). Subsequent qPCR assays for this set of genes showed extremely high variations in transcript levels between individual animals of both control and ischemia-treated groups. However, the relative transcript levels, or expression profile, of these genes was constant in all samples. The transcript levels of these genes were on average 2624-times higher in tissue samples isolated from the superficial layers of the total lens. Moreover, all 23 genes had high expression levels in lens compared to retina as was shown by microarray.

CONCLUSIONS

From these data, it appears plausible that during isolation of the retina, trace amounts of lens tissue may end up in the studied retinal samples. This would explain the high level of variability in transcript levels of genes, the strong correlation of relative levels between samples, and the link with lens-specific function of the "altered" genes. Changes in crystallin gene expression in other models of retinal degeneration have been reported and a careful examination of the transcript level of other lens-specific genes is essential to rule out a possible confounding effect of lens-material transfer.

A novel mutation and large size polymorphism affecting the V2 domain of keratin 1 in an African-American family with severe, diffuse palmoplantar keratoderma of the ichthyosis hystrix Curth-Macklin type

Keratin gene mutations affecting nonhelical head and tail domains are not usually associated with prominent skin blistering and keratin filament clumping. Instead, they have been associated with several distinct clinical phenotypes, such as epidermolysis bullosa simplex with mottled pigmentation (mutation P25L in the V1 domain of keratin 5), epidermolysis bullosa simplex with migratory circinate erythema (frameshift mutation c1649delG in the V2 domain of keratin 5), striate palmoplantar keratoderma (PPK), and ichthyosis hystrix Curth-Macklin (different frameshift mutations in the V2 domain of keratin 1 (K1)). We have studied a family with severe, diffuse, nonepidermolytic PPK and verrucous hyperkeratotic plaques over the joints and in flexures and identified a new KRT1 gene mutation that is predicted to completely alter the K1 tail domain. In addition, a new K1 size polymorphism has been detected, which is especially prevalent among the African-American population. These results further emphasize the functional importance of the nonhelical tail domain in keratin molecules despite the obvious variability in the number of glycine loop motifs and underscore the broad phenotypic spectrum of disorders due to dominant keratin tail mutations.

Detection of gender differences in rat lens proteins using 2-D-DIGE

The glass-like transparency of the human eye lens is achieved by the tight packing of abundant crystallin proteins. However, the precise role of the accessory non-crystallin proteins is not well understood. We have carried out 2-DE mapping of these proteins in rat lens. This showed the presence of the high molecular weight filamentous structural proteins spectrin, filensin, tubulin, vimentin, actin and phakinin as well as several forms of potential crystallin oligomers comprised of alphaA, betaB1, betaA1 and betaA4 chains. Other proteins that were present include, heat shock protein 71, WD repeat protein 1, and several enzymes including alpha-enolase, pyruvate kinase, transketolase and aldose reductase. 2-D-DIGE analysis revealed several expression differences between the lens proteomes of male and female rats. Female rat lenses contained lower levels of aldose reductase, increased proteolyic fragments of the structural proteins filensin, vimentin and phakinin and higher levels of potential alphaA, betaB1 and betaA1 crystallin oligomers. Taken together these findings suggest that there are potential differences in oxidative stress regulation between male and female rat lenses, which may have implications on susceptibility to cataract formation. Future studies aimed at elucidating pre-cataractic changes in the non-crystallin proteins described here may facilitate identification of novel markers involved in cataractogenesis.

Structural stability of wild type and mutated alpha-keratin fragments: molecular dynamics and free energy calculations

The objective of this study is to investigate the influence of point mutations on the structural stability of coiled coil fragments of the human hair intermediate filament by molecular dynamics simulations and free energy calculations. Mutations in the helix termination motif of human hair keratin gene hHb6 seem to be connected to the hereditary hair dystrophy Monilethrix. The most common mutations reported are Glu413Lys and Glu413Asp, located at the C-terminal end of the coiled coil 2B rod domain of the IF. According to our simulations, significant conformational changes of the side chains at the mutation and neighboring sites occur due to the Glu413Lys mutation. Furthermore, the differences in electrostatic interactions cause a large change in free energy during transformation of Glu413 to Lys calculated by the thermodynamic integration approach. It is speculated that the structural rearrangement necessary to adapt the interactions in the mutated coiled coil leads to changes in the IF assembly or its stability. The second mutation, Glu413Asp, only leads to a small value of the calculated free energy difference that is within the error limits of the simulations. Thus, it has to be concluded that this mutation does not affect the coiled coil stability.

Bfsp2 mutation found in mouse 129 strains causes the loss of CP49' and induces vimentin-dependent changes in the lens fibre cell cytoskeleton

Here we report the first natural mutation in the mouse Bfsp2 gene. Characterisation of mouse Bfsp2 in the 129X1/SvJ revealed a mutation that deleted the acceptor site of exon 2. This results in exon 1 being erroneously spliced to exon 3 causing a frameshift in the reading frame and the introduction of a stop codon at position 2 of exon 3 in the Bfsp2 transcript. RT-PCR studies of lens RNA isolated from 129S1/SvImJ, 129S2/SvPas and 129S4/SvJae strains confirmed the presence of this mutation in these diverse 129 strains and similar mutations were found in both CBA and 101 strains, but not in C3H or C57BL/6J mouse strains. This mutation is predicted to result in a severely truncated protein product called CP49, comprising essentially only exon 1, but polyclonal antibodies to CP49 failed to detect either full length or fragments of CP49 in extracts made from either 129S1/SvImJ or 129S4/SvJae suggesting that these 129 strains lack CP49 protein. Like the knockout of Bfsp2 reported recently, filensin protein levels and its proteolytic processing were altered also in the 129S1/SvImJ and 129S4/SvJae strains compared to C57BL/6J. Electron microscopy of the lens cytoskeleton from 129S2/SvPas revealed similar morphological changes in the cytoskeleton as compared to the CP49 knockout, with beaded and intermediate filaments being apparently replaced by poorly defined filament-like material. Vimentin was a key component of this residual material as shown by immunoelectron microscopy and by the generation of a CP49/vimentin double knockout mouse. This report of a natural mutation in Bfsp2 in the 129 and other mouse strains also has important implications for lens studies that have used the 129X1/SvJ strain in knockout strategies.

A novel cell-cell junction system: the cortex adhaerens mosaic of lens fiber cells

The anucleate prismoid fiber cells of the eye lens are densely packed to form a tissue in which the plasma membranes and their associated cytoplasmic coat form a single giant cell-cell adhesive complex, the cortex adhaerens. Using biochemical and immunoprecipitation methods in various species (cow, pig, rat), in combination with immunolocalization microscopy, we have identified two different major kinds of cortical complex. In one, the transmembrane glycoproteins N-cadherin and cadherin-11 [which also occur in heterotypic ('mixed') complexes] are associated with alpha- and beta-catenin, plakoglobin (proportions variable among species), p120ctn and vinculin. The other complex contains ezrin, periplakin, periaxin and desmoyokin (and so is called the EPPD complex), usually together with moesin, spectrin(s) and plectin. In sections through lens fiber tissue, the short sides of the lens fiber hexagons appear to be enriched in the cadherin-based complexes, whereas the EPPD complexes also occur on the long sides. Moreover, high resolution double-label fluorescence microscopy has revealed, on the short sides, a finer, almost regular mosaicism of blocks comprising the cadherin-based, catenin-containing complexes, alternating with patches formed by the EPPD complexes. The latter, a new type of junctional plaque ensemble of proteins hitherto known only from certain other cell types, must be added to the list of major lens cortex proteins. We here discuss its possible functional importance for the maintenance of lens structure and functions, notably clear and sharp vision.

Functional analysis of keratin components in the mouse hair follicle inner root sheath

BACKGROUND

Recently, a family of novel type I keratins of the inner root sheath of the hair follicle were discovered, increasing the number of keratins known to be expressed in the hair follicle. The mouse database shows three keratins that are possible orthologues of these inner root sheath keratins. The sequences of these keratins include rather unusual changes to a highly conserved motif at the end of the alpha-helical rod domain of the proteins, thought to be important in filament assembly.

OBJECTIVES

To investigate whether these keratins are expressed in the inner root sheath and to determine whether they assemble normally.

METHODS

To investigate this, polyclonal antibodies were raised for immunolocalization of the keratins and their cDNAs were cloned for transfection into cultured cells.

RESULTS

At least two of these keratins were expressed in the inner root sheath but the timing of expression of the different keratins was variable. Transfection of the relevant cDNAs into cells in culture indicated that these keratins were capable of integrating into existing keratin networks without disruption, but that de novo filament assembly with the type II inner root sheath keratin, mK6irs, was poor.

CONCLUSIONS

These results provide further evidence of the complexity of keratin expression in the three concentric layers of the inner root sheath.

Bfsp2 mutation found in mouse 129 strains causes the loss of CP49 and induces vimentin-dependent changes in the lens fibre cell cytoskeleton

Here we report the first natural mutation in the mouse Bfsp2 gene. Characterisation of mouse Bfsp2 in the 129X1/SvJ revealed a mutation that deleted the acceptor site of exon 2. This results in exon 1 being erroneously spliced to exon 3 causing a frameshift in the reading frame and the introduction of a stop codon at position 2 of exon 3 in the Bfsp2 transcript. RT-PCR studies of lens RNA isolated from 129S1/SvImJ, 129S2/SvPas and 129S4/SvJae strains confirmed the presence of this mutation in these diverse 129 strains and similar mutations were found in both CBA and 101 strains, but not in C3H or C57BL/6J mouse strains. This mutation is predicted to result in a severely truncated protein product called CP49, comprising essentially only exon 1, but polyclonal antibodies to CP49 failed to detect either full length or fragments of CP49 in extracts made from either 129S1/SvImJ or 129S4/SvJae suggesting that these 129 strains lack CP49 protein. Like the knockout of Bfsp2 reported recently, filensin protein levels and its proteolytic processing were altered also in the 129S1/SvImJ and 129S4/SvJae strains compared to C57BL/6J. Electron microscopy of the lens cytoskeleton from 129S2/SvPas revealed similar morphological changes in the cytoskeleton as compared to the CP49 knockout, with beaded and intermediate filaments being apparently replaced by poorly defined filament-like material. Vimentin was a key component of this residual material as shown by immunoelectron microscopy and by the generation of a CP49/vimentin double knockout mouse. This report of a natural mutation in Bfsp2 in the 129 and other mouse strains also has important implications for lens studies that have used the 129X1/SvJ strain in knockout strategies.

Knockout of the intermediate filament protein CP49 destabilises the lens fibre cell cytoskeleton and decreases lens optical quality, but does not induce cataract

In this report, the phenotype associated with the first targeted knockout of the lens specific intermediate filament gene CP49 is described. Several surprising observations have been made. The first was that no cataract was observed despite the fact that the beaded filaments of the lens fibre cells had been disrupted. Light scatter and the lens optical properties had, however, deteriorated in the CP49 knockout lenses compared to litter mate controls. These changes were accompanied by dramatic changes in plasma membrane organisation of the fibre cells as revealed by detailed morphological examinations and providing the second surprising result. The CP49 knockout mouse is therefore an important model to study the functional link between lens transparency, the cytoskeleton and plasma membrane organisation.

Intermediate-filament expression in ocular tissue

Intermediate-filament proteins (IFPs) occur in the intracellular cytoskeleton of eukaryotic cells, and their expression in diverse tissues is related both to embryology as well as to differentiation. Although the available information concerning their functional properties in vivo is still incomplete, antibodies against individual IFPs are commonly used in immunohistochemical procedures as markers for differentiation, and these antibodies are of outstanding value in the routine histopathological evaluation of tumor specimens. This review presents a compilation of the currently available data concerning IFP expression in normal and diseased ocular tissues. Representatives of every known class of IFP have been detected in normal ocular tissues. The external epithelia exhibit complex expression patterns of cytokeratin (CK) polypeptides, with CK3 and CK12 being specific markers of the corneal epithelium. Recent research has revealed that single mutant CK polypeptides may play a role in the pathogenesis of corneal dystrophies. The internal ocular epithelia reveal simple but specific patterns of IFP expression, these comprising simple-epithelial CKs and/or the mesenchymal IFP, vimentin. The IFP complement of the neuronal structures of the eye embraces several distinct IFP classes and reflects the diversity of the cell types present at these sites. With respect to ocular tumors, the IFP profile of melanomas might be correlated with metastatic potential. In conclusion, IFP analysis may be able to cast light on the pathogenesis of ocular diseases, as well as being a valuable adjunct in ophthalmopathological diagnosis.

Human synemin gene generates splice variants encoding two distinct intermediate filament proteins

Intermediate filament (IF) proteins are constituents of the cytoskeleton, conferring resistance to mechanical stress, and are encoded by a dispersed multigene family. In man we have identified two isoforms (180 and 150 kDa) of the IF protein synemin. Synemin alpha and beta have a very short N-terminal domain of 10 amino acids and a long C-terminal domain consisting of 1243 amino acids for the alpha isoform and 931 amino acids for the beta isoform. An intronic sequence of the synemin beta isoform is used as a coding sequence for synemin alpha. Both mRNA isoforms (6.5 and 7.5 kb) result from alternative splicing of the same gene, which has been assigned to human chromosome 15q26.3. Analyses by Northern and Western blot revealed that isoform beta is the predominant isoform in striated muscles, whereas both isoforms (alpha and beta) are present in almost equal quantities in smooth muscles. Co-transfection and immunolabeling experiments indicate that both synemin isoforms are incorporated with desmin to form heteropolymeric IFs. Furthermore synemin and desmin are found aggregated together in certain pathological situations.

The intermediate filament protein consensus motif of helix 2B: its atomic structure and contribution to assembly

Nearly all intermediate filament proteins exhibit a highly conserved amino acid motif (YRKLLEGEE) at the C-terminal end of their central alpha-helical rod domain. We have analyzed its contribution to the various stages of assembly by using truncated forms of Xenopus vimentin and mouse desmin, VimIAT and DesIAT, which terminate exactly before this motif, by comparing them with the wild-type and tailless proteins. It is surprising that in buffers of low ionic strength and high pH where the full-length proteins form tetramers, both VimIAT and DesIAT associated into various high molecular weight complexes. After initiation of assembly, both VimIAT and DesIAT aggregated into unit-length-type filaments, which rapidly longitudinally annealed to yield filaments of around 20 nm in diameter. Mass measurements by scanning transmission electron microscopy revealed that both VimIAT and DesIAT filaments contained considerably more subunits per cross-section than standard intermediate filaments. This indicated that the YRKLLEGEE-motif is crucial for the formation of authentic tetrameric complexes and also for the control of filament width, rather than elongation, during assembly. To determine the structure of the YRKLLEGEE domain, we grew crystals of peptides containing the last 28 amino acid residues of coil 2B, chimerically fused at its amino-terminal end to the 31 amino acid-long leucine zipper domain of the yeast transcription factor GCN4 to facilitate appropriate coiled-coil formation. The atomic structure shows that starting from Tyr400 the two helices gradually separate and that the coiled coil terminates with residue Glu405 while the downstream residues fold away from the coiled-coil axis.

Mapping of the human CP49 gene and identification of an intragenic polymorphic marker to allow genetic linkage analysis in autosomal dominant congenital cataract

The CP49 protein is an intermediate filament protein expressed specifically in the lens fibre cells of the lens, where it is an important cytoplasmic structural component. Dominant-negative mutations in other intermediate filament proteins, such as keratins, cause disorders characterised by dense cytoplasmic aggregates in specific cell types. The CP49 gene is therefore a good candidate for dominantly inherited forms of cataract. To allow genetic linkage analysis of families with autosomal dominant cataract with respect to CP49, a highly polymorphic intragenic microsatellite marker for this gene has been developed. In addition, both low and high resolution radiation hybrid mapping of the CP49 gene has been completed, placing it very close to microsatellite marker D3S1290 on human chromosome 3q. Furthermore, using the intragenic CP49 microsatellite, linkage was excluded in four families with genetically uncharacterized forms of autosomal dominant congenital cataract.

Up-regulation of novel intermediate filament proteins in primary fiber cells: an indicator of all vertebrate lens fiber differentiation?

The early embryonic development and expression patterns of the eye lens specific cytoskeletal proteins, CP49 and CP95, were determined for the chick and were found to be similar in both human and mouse. These proteins, as well as their homologs in other species, are obligate polymerization partners which form unique filamentous structures termed "beaded filaments." CP49 and CP95 appeared as protein products after 3 days of embryonic development in the chick during the elongation of primary fiber cells. Although limited data were obtained for human embryos at these early developmental timepoints, they were consistent with the interpretation that the up-regulation of these lens specific proteins began only after the initiation of lens vesicle closure. In situ hybridization with the mouse lens confirmed that message levels for beaded filament proteins were greatly elevated in differentiating primary fiber cells. Nuclease protection assays established that mRNA levels for CP49 remained relatively constant while CP95 mRNA levels increased once the process of secondary fiber formation was under way. Although present in relatively low abundance, the mRNA for a unique splice variant of CP49, CP49(INS), was also detected early in embryonic development and into adulthood. Peptide-specific antibodies directed against unique predicted sequences were able to confirm the protein expression of CP49(INS) in both embryonic and adult chick lens cells. These data present the first detailed study of the expression of CP49 and CP95 during early lens development. They suggest that the up-regulated expression of CP49 and CP95 could serve as pan-specific markers for all vertebrate lens fiber development.

The eye lens cytoskeleton

During lens cell differentiation there are a number of very characteristic morphological changes that occur. These include a 50- to 100-fold increase in cell length as the equatorial lens epithelial cells differentiate into fibre cells and the loss of the cellular organelles such as mitochondria, nuclei, Golgi apparatus and endoplasmic reticulum. Coincident with these changes are dramatic alterations in the organisation of the lens fibre cell cytoskeleton and in particular the lens-specific intermediate filament network comprising CP49 and filensin. Cell shape and cell polarisation as well as tissue integrity are all processes that depend upon the cytoskeleton and are therefore important to the lens. The unique aspects of the lenticular cytoskeleton are the subject of this review.

Kinetics of cyclic AMP-dependent accumulation of novel intermediate filament proteins in cultured chick lens cells

PURPOSE

To refine the parameters affecting the accumulation of cytoskeletal markers of lens fiber terminal differentiation.

METHODS

Primary cultures of chick lens annular pad cells were treated with a lipid soluble cyclic AMP analog under various culture conditions. The accumulation of beaded filament proteins, unique markers of lens fiber terminal differentiation, was quantified with an ELISA assay. The incorporation of beaded filament proteins into macromolecular structures was followed with immunofluorescence microscopy.

RESULTS

In a time- and dose-dependent manner, beaded filament protein levels were increased in cyclic nucleotide treated cells. The addition of serum to treated cells caused a further dose-dependent increase in beaded filament protein levels. The continuous presence of cyclic nucleotides for maximal beaded filament protein accumulation was also established. At the light microscopic level, cyclic nucleotide treatment produced much more extensive multilayering of cells and lentoid formation. Macromolecular structures containing beaded filament proteins also increased in both abundance and complexity after cyclic nucleotide treatment and were restricted to the multilayers/lentoids.

CONCLUSIONS

These results indicate that multiple mechanisms (including cyclic AMP, serum factors, and the degree of cell-cell interactions) affect the accumulation of beaded filament proteins during the normal differentiation of lens fibers.

Characterization of the chicken transitin gene reveals a strong relationship to the nestin intermediate filament class

Our laboratory previously reported that transitin is a radial glial intermediate filament protein sharing the basic structural features common to all intermediate filament (IF) proteins. It contains an alpha-helical core domain flanked by a short nonhelical head and a long COOH-terminal tail. The core sequence of transitin shows the greatest similarity to Xenopus tanabin and to rat and human nestin. We also reported that transitin has multiple splice variants derived from the deletion or inclusion of a leucine-zipper heptad repeat domain in the COOH-terminal tail. In the present study, we provide new evidence to support the classification of nestin and transitin in the same group of IF proteins based on the number and position of its introns. In addition, we suggest that the different isoforms of transitin are produced by a splicing mechanism that recognizes consensus 5' and 3' splice sites contained within the coding sequence of the leucine-zipper heptad repeat domain.

The pathway of assembly of intermediate filaments from recombinant alpha-internexin

The pathway of filament assembly from the neuronal intermediate filament alpha-intermexin was investigated. Optimal assembly occurred in solutions of pH 6.5 to 7 and moderate ionic strength at 37 degrees C. Short filaments formed upon dialysis at 24 degrees C, which elongated further when incubated at 37 degrees C. Soluble forms of alpha-internexin were characterized by analytical ultracentrifugation and electron microscopy. In 10 mM Tris, pH 8, conditions that favor formation of tetramers and other small oligomers for other intermediate filament proteins, alpha-internexin formed 10.5 S particles, apparently unit-length half-filaments in the form of rods 10.6 nm in diameter and 68 nm long. Dialysis vs the same buffer with added 10 mM NaCl yielded 16 S rods, probably unit-length filaments, of the same length but 13.0 nm in diameter. At 50 mM NaCl, rods about 13 nm in diameter and heterogeneous in length were observed in electron micrographs, apparently formed from longitudinal annealing of unit-length rods. The results favor a model of assembly in which coiled coil dimers aggregate laterally to form first "unit-length half-filaments" (Herrmann, H., and Aebi, U. (1998) Curr. Opin. Struct. Biol. 8, 177-185) and then "unit-length filaments," which subsequently elongate by annealing.

Common and variant properties of intermediate filament proteins from lower chordates and vertebrates; two proteins from the tunicate Styela and the identification of a type III homologue

The chordates combine the vertebrates and the invertebrate phyla of the cephalo- and urochordates (tunicates). Two cytoplasmic intermediate filament (IF) proteins of the urochordate Styela plicata are characterized by cDNA cloning, gene organization, tissue specific expression patterns in the adult animal and the self assembly properties of the recombinant proteins. In line with metazoan phylogeny St-A and St-B have the short length version of the coil 1b domain found in all vertebrate and cephalochordate IF proteins while protostomic IF proteins have the longer length version with an extra 42 residues. St-A is the first IF protein from a lower chordate which can be unambiguously related to a particular vertebrate IF subfamily. St-A shares 46% sequence identity with desmin, displays the N-terminal motif necessary for filament assembly of type III proteins and forms normal homopolymeric 10 nm filaments in vitro. St-A but not St-B is present in smooth muscle cells of the body wall musculature. St-A and St-B are found as separate networks in some interior epithelia. St-B shares 30 to 35% identity with keratin 8, St-A and desmin and does not form IF under in vitro assembly conditions. Its relation to a particular vertebrate IF type or to the eight currently known IF proteins from the cephalochordate Branchiostoma remains unresolved. The striking relation between St-A and desmin predicts that the common progenitor of the urochordate (tunicate) and the cephalochordate/vertebrate lineages already possessed a type III homologue. Unlike in vertebrates intron patterns cannot be used to classify the tunicate IF genes. Although St-A is a type III homologue its gene shows an intron position which in vertebrates is restricted to keratin type II genes.

Analysis of eight cDNAs and six genes for intermediate filament (IF) proteins in the cephalochordate Branchiostoma reveals differences in the IF multigene families of lower chordates and the vertebrates

We report the sequences of seven new cytoplasmic intermediate filament (IF) proteins of the cephalochordate Branchiostoma. The eight sequences currently known describe four subfamilies (A, B, C and D). All eight IF proteins show the short-length version of the coil 1b subdomain found in vertebrates and lack the additional 42 residues present in all nuclear lamins and the protostomic IF proteins. Although the lancelet is considered to be the closest relative of the vertebrates, it is difficult to relate its IF subfamilies unambiguously to a particular type I-IV subfamily of vertebrates. C1 and C2 have tail domains with two 64 residue repeats of coiled coil-forming ability, a structural feature unknown for IF proteins from vertebrates or protostomia. The epidermal protein D1 shows only a slightly better identity score with vertebrate type II keratins than with type III proteins, but the D1 gene organization is that of type III proteins. The same holds for A1, A2, B1, B2 and C2 genes, although the latter has an additional and uniquely positioned intron. Antibodies (Ab) raised against recombinant C2 and D1 proteins reveal these proteins in epidermis, some internal epithelia and parts of the spinal cord. The results on exonic sequences, gene organization and expression suggest that Branchiostoma IF proteins may retain a largely archetypal condition, whereas the vertebrates have established the well-known type I-IV IF system.

The chicken CP49 gene contains an extra exon compared to the human CP49 gene which identifies an important step in the evolution of the eye lens intermediate filament proteins

The gene structure for chicken CP49 gene is presented. It differs from the human CP49 gene with the presence of an extra exon in helix IB and the apparent loss of an intron, intron H. The CP49 gene localises to chromosome 2 in the chicken genome where it is flanked by homologues that map to human chromosome 10p13 (VIM) 6p24-p23 (BMP6). Two transcripts, CP49 and CP49ins, are produced from the single chicken CP49 gene. The difference is a 49-amino-acid insertion in helix IB of CP49 that is encoded by a novel exon found in the chicken CP49 gene. An extended helix IB is believed to be a characteristic of the ancestral intermediate filament protein as it is found in many invertebrate intermediate filament proteins but has been lost from all vertebrate intermediate filament proteins except the nuclear lamins. Although the intron position and length of the helix IB insert sequences in CP49ins differ to those found both in the invertebrate intermediate filament proteins and the vertebrate lamins, the CP49 gene is the first vertebrate cytoplasmic intermediate filament protein to be described with an extended helix IB. The chicken CP49 gene is also the first where differential splicing can remove such a feature. Human and bovine CP49 appear to have lost the helix IB insert sequences, and so the avian CP49 gene provides an interesting evolutionary link between the eye lens proteins and the ancestral intermediate filament protein.

Gene structure and sequence comparisons of the eye lens specific protein, filensin, from rat and mouse: implications for protein classification and assembly

The full length cDNA sequences of rat and mouse filensin are presented, as well as the structure of the rat filensin gene. This gene spanned 31 kb and included seven introns. The first six introns were conserved in position and phase with those found in the intermediate filament (IF) protein genes of the type II (type II keratin), type III (vimentin) and type V (lamin). The last intron of the filensin was unique. As none of the filensin intron positions coincided with those unique to type I, II or IV genes, it appears that filensin is most similar to type III genes. Comparison of the deduced amino acid sequences for rat and mouse filensin with those of cow and chick, and with other species of IF proteins, indicated the C-terminal non-alpha-helical tail domain of filensin to be one of the most divergent yet found in the vertebrate IF family. The tail domain had three conserved regions which are interrupted with two regions with lower identity. Two motifs, (1) PGDVPDGxxISKAF; and (2) KVEVVESIEKxxxxxIQTYEETxxIVET, were identified as sequences which were particularly highly conserved across species. Coassembly studies using CP49 and a physiologically derived 53 kDa-fragment of filensin showed the motif (2) was not required for filament assembly in vitro. These data strengthen the view that the C-terminal non-alpha-helical domain of filensin contributes in more than one way to filensin function in the lens.

Actin filament bundles in cortical fiber cells of the rat lens

The distribution and organization of actin filament bundles were studied in cortical fiber cells of rat lenses at various ages (3 days to 2.5 months old), using thin-section electron microscopy, immunocytochemistry and immunoblotting. Electron microscopy showed that actin bundles were regularly found along cortical fiber cell membranes of the lens at all ages studied. The actin bundles were commonly arranged in three distinct units, one bundle in each fiber cell, located at the intersections where three hexagonal fiber cells meet as seen in cross sections. These actin bundles were approximately 150 nm in diameter and were composed of 7-nm small filaments. They were aligned parallel to the long axis of fiber cells as judged by both cross and longitudinal sections. The outside border of each bundle was always surrounded by a zone of 10-nm intermediate filaments which have the same orientation as that of the actin bundles. In longitudinal sections, elongated actin bundles were always parallel to the cell membranes. A number of individual actin bundles sometimes were found to form a chain with periodic short intervals. In addition, actin bundles were frequently associated with adherens junctions near the intersections and other regions of fiber cell membranes. By immunoelectron microscopy, we demonstrated that these filament bundles indeed contained actins. By rhodamine-phalloidin labelling, we found that labeled actin bundles appeared as large, distinct dots at the corners of hexagonal fiber cells in all ages studied. In addition, non-bundle F-actins were labeled preferentially along the cell membranes of the short sides of hexagonal fiber cells. This resulted in a unique zigzag pattern of actin labeling commonly seen in the cortical fiber cells of a mature rat lens. Finally, we showed that alpha-actinin was associated with the actin bundles in the fiber cells by immunofluorescent double labeling and immunoblotting. It is suggested that this unique arrangement of actin bundles in fiber cells may provide a stabilizing structure for forming a sharp angle at each corner of fiber cells, thereby the hexagonal shape of the cells can be maintained.

The mouse filensin gene: structure and evolutionary relation to other intermediate filament genes

Filensin and phakinin are two lens-specific members of the intermediate filament (IF) superfamily of proteins. They coassemble to form a beaded submembraneous filamentous network, the beaded filaments (BFs). The low sequence homology and differences in assembly compared to other IF proteins do not allow their classification in any of the five IF subgroups. The organization of the phakinin gene exon/intron boundaries provides evidence that this partner may be sharing a common origin with type I cytokeratin genes. Here we report the molecular cloning, sequence and characterization of the mouse filensin gene. The filensin gene consists of 8 exons and 7 introns, with 6 introns interrupting its rod domain in a highly conserved manner characteristic of type III IF genes, like vimentin, desmin, or peripherin. Of the two tail domain exons the one adjacent to the rod domain, compares to exon 7 of the non-neuronal cytoplasmic IF gene of helix aspersa and to the lamin region bridging the end of the rod domain to the nuclear localization signal. Altogether, these observations indicate that the lens beaded filaments form an independent class of IF.

Quantification and regulation of mRNAs encoding beaded filament proteins in the chick lens

PURPOSE

To quantify the expression of beaded filament protein mRNA levels in regions of the chick lens and to examine the in vitro regulation of message and protein levels using cell culture techniques.

METHODS

RNase protection assays and Northern blotting were used to quantify beaded filament protein mRNA levels in dissected lenses. Cultured cells were assayed for mRNA with RNase protection and for protein with Western blotting and ELISA techniques after treatment with cAMP analogs.

RESULTS

Beaded filament protein message levels were greatly up-regulated in cortical fiber cells compared to annular pad cells. Full length messages were also detected in nuclear fiber cells. The presence of an unusual form of the CP49 message with a lamin-like insert, CP49INS, was also established. Both message and protein levels were subject to regulation in response to elevated intracellular cAMP levels.

CONCLUSIONS

The accumulation of beaded filament protein levels during fiber cell development may be due to the increased cAMP-mediated transcription of message. The presence of CP49INS may lend new insight into mechanisms of intermediate filament assembly.

Implications of intermediate filament protein phosphorylation

Intermediate filament (IF) proteins, a large family of tissue specific proteins, undergo several posttranslational modifications, with phosphorylation being the most studied modification. IF protein phosphorylation is highly dynamic and involves the head and/or tail domains of these proteins, which are the domains that impart most of the structural heterogeneity and hence presumed tissue specific functions. Although the function of IF proteins remains poorly understood, several regulatory roles for IF protein phosphorylation have been identified or are emerging. Those roles include filament disassembly and reorganization, solubility, localization within specific cellular domains, association with other cytoplasmic or membrane associated proteins, protection against physiologic stress and mediation of tissue-specific functions. Understanding the mechanistic and functional aspects of IF protein phosphorylation is providing insights not only regarding the function of this modification, but also regarding the function of IF proteins.

The cytoskeleton and disease: genetic disorders of intermediate filaments

Specialized cytoskeletons play many fascinating roles, including mechanical integrity and wound-healing in epidermal cells, cell polarity in simple epithelia, contraction in muscle cells, hearing and balance in the inner ear cells, axonal transport in neurons, and neuromuscular junction formation between muscle cells and motor neurons. These varied functions are dependent upon cytoplasmic networks of actin microfilaments (6 nm), intermediate filaments (10 nm) and microtubules (23 nm), and their many associated proteins. In this chapter, I review what is known about the cytoskeletons of intermediate filaments and their associated proteins. I focus largely on epidermal cells, which devote most of their protein-synthesizing machinery to producing an extensive intermediate filament network composed of keratin. Recent studies have shown that many of the devastating human disorders that arise from degeneration of this cell type have as their underlying basis either defects in the genes encoding keratins or abnormalities in keratin IF networks. I discuss what we know about the functions of IFs, and how the link to genetic disease has enhanced this understanding.

Intermediate filaments in the nervous system: implications in cancer

In this review, we describe the different intermediate filament (IF) proteins, their assembly into IFs, the functions of IFs and their relation to disease with a particular emphasis on the intermediate filaments expressed in the nervous system. In the mammalian nervous system, seven intermediate filament proteins are known to be expressed in neurons or neuroblasts. These include the three neurofilament triplet proteins, which are present in both central and peripheral neurons; alpha-internexin, which is the first neuronal intermediate filament protein expressed in the developing mammalian nervous system and present primarily in CNS neurons in the adult nervous system; peripherin, which is most abundant in the PNS; vimentin, which is expressed in neuronal progenitor cells along with nestin, as well as in a few adult neurons. In contrast to these neuron-specific IF proteins, the glial fibrillary acidic protein (GFAP) is glial specific and expressed in mature astrocytes. Vimentin and nestin are also expressed in glial progenitor cells and vimentin is expressed along with GFAP in some mature astrocytes. As a whole, the expression of IF proteins is tissue specific and developmentally regulated. As a result, IF proteins are good markers for determining the cell origin and differentiation status of tumor cells. For example, peripherin is expressed in neuroblastomas, GFAP in astrocytomas and neurofilaments in tumors of neuronal origin. However, tumor cells may express IF patterns which are irrelevant to their cell origin. Therefore, one has to be very careful in using IF patterns as sole indicators of cell origin and differentiation status of tumors.

Gene structure and cDNA sequence identify the beaded filament protein CP49 as a highly divergent type I intermediate filament protein

The fiber cell of the vertebrate ocular lens assembles a cytoskeletal structure, the beaded filament, which contains two proteins unique to the fiber cell: CP49 (phakinin) and CP115/CP95 (filensin). We report here the complete primary sequence and gene structure for human CP49. These data show that CP49 is a member of the intermediate filament family, but highly unusual in several regards. 1) CP49 primary sequence does not permit unambiguous assignment to any existing class of intermediate filament protein, but exhibits a gene structure that is identical to the Type I cytokeratins. 2) CP49 essentially lacks one of the three major domains that characterize all intermediate filament proteins, the carboxyl-terminal tail domain. 3) CP49 shows substitutions at 3 of 4 residues in the otherwise highly conserved intermediate filament protein motif LNDR. Notably, this divergence includes an Arg to Cys substitution that has only been observed in the mutant human cytokeratin K14, a mutation shown to cause the skin blistering seen in the genetic disorder Dowling-Meara epidermolysis bullosa simplex.