BPAG1a and b associate with EB1 and EB3 and modulate vesicular transport, Golgi apparatus structure, and cell migration in C2.7 myoblasts.

BPAG1a and BPAG1b (BPAG1a/b) constitute two major isoforms encoded by the dystonin (Dst) gene and show homology with MACF1a and MACF1b. These proteins are members of the plakin family, giant multi-modular proteins able to connect the intermediate filament, microtubule and microfilament cytoskeletal networks with each other and to distinct cell membrane sites. They also serve as scaffolds for signaling proteins that modulate cytoskeletal dynamics. To gain better insights into the functions of BPAG1a/b, we further characterized their C-terminal region important for their interaction with microtubules and assessed the role of these isoforms in the cytoskeletal organization of C2.7 myoblast cells. Our results show that alternative splicing does not only occur at the 5' end of Dst and Macf1 pre-mRNAs, as previously reported, but also at their 3' end, resulting in expression of additional four mRNA variants of BPAG1 and MACF1. These isoform-specific C-tails were able to bundle microtubules and bound to both EB1 and EB3, two microtubule plus end proteins. In the C2.7 cell line, knockdown of BPAG1a/b had no major effect on the organization of the microtubule and microfilament networks, but negatively affected endocytosis and maintenance of the Golgi apparatus structure, which became dispersed. Finally, knockdown of BPAG1a/b caused a specific decrease in the directness of cell migration, but did not impair initial cell adhesion. These data provide novel insights into the complexity of alternative splicing of Dst pre-mRNAs and into the role of BPAG1a/b in vesicular transport, Golgi apparatus structure as well as in migration in C2.7 myoblasts.

Nervous-tissue-specific elimination of microtubule-actin crosslinking factor 1a results in multiple developmental defects in the mouse brain.

The microtubule-actin crosslinking factor 1 (MACF1) is a ubiquitous cytoskeletal linker protein with multiple spliced isoforms expressed in different tissues. The MACF1a isoform contains microtubule and actin-binding regions and is expressed at high levels in the nervous system. Macf1-/- mice are early embryonic lethal and hence the role of MACF1 in the nervous system could not be determined. We have specifically knocked out MACF1a in the developing mouse nervous system using Cre/loxP technology. Mutant mice died within 24-36h after birth of apparent respiratory distress. Their brains displayed a disorganized cerebral cortex with a mixed layer structure, heterotopia in the pyramidal layer of the hippocampus, disorganized thalamocortical and corticofugal fibers, and aplastic anterior and hippocampal commissures. Embryonic neurons showed a defect in traversing the cortical plate. Our data suggest a critical role for MACF1 in neuronal migration that is dependent on its ability to interact with both microfilaments and microtubules.

Alterations in lipid metabolism gene expression and abnormal lipid accumulation in fibroblast explants from giant axonal neuropathy patients.

BACKGROUND: Giant axonal neuropathy (GAN) is a hereditary neurological disorder that affects both central and peripheral nerves. The main pathological hallmark of the disease is abnormal accumulations of intermediate filaments (IFs) in giant axons and other cell types. Mutations in the GAN gene, encoding gigaxonin, cause the disease. Gigaxonin is important in controlling protein degradation via the ubiquitin-proteasome system. The goal of this study was to examine global alterations in gene expression in fibroblasts derived from newly identified GAN families compared with normal cells. RESULTS: We report the characterization of fibroblast explants obtained from two unrelated GAN patients. We identify three novel putative mutant GAN alleles and show aggregation of vimentin IFs in these fibroblasts. By microarray analysis, we also demonstrate that the expression of lipid metabolism genes of the GAN fibroblasts is disrupted, which may account for the abnormal accumulations of lipid droplets in these cells. CONCLUSION: Our findings suggest that aberrant lipid metabolism in GAN patients may contribute to the progression of the disease.

Bim is elevated in Alzheimer's disease neurons and is required for beta-amyloid-induced neuronal apoptosis.

The molecules that mediate neuron death in Alzheimer's disease (AD) are largely unknown. We report that beta-amyloid (Abeta), a death-promoting peptide implicated in the pathophysiology of AD, induces the proapoptotic protein Bcl-2 interacting mediator of cell death (Bim) in cultured hippocampal and cortical neurons. We further find that Bim is an essential mediator of Abeta-induced neurotoxicity. Our examination of postmortem AD human brains additionally reveals upregulation of Bim in vulnerable entorhinal cortical neurons, but not in cerebellum, a region usually unaffected by AD. Accumulating evidence links inappropriate induction/activation of cell cycle-related proteins to AD, but their roles in the disease have been unclear. We find that the cell cycle molecule cyclin-dependent kinase 4 (cdk4) and its downstream effector B-myb, are required for Abeta-dependent Bim induction and death in cultured neurons. Moreover, neurons that overexpress Bim in AD brains also show elevated levels of the cell cycle-related proteins cdk4 and phospho-Rb. Our observations indicate that Bim is a proapoptotic effector of Abeta and of dysregulated cell cycle proteins in AD and identify both Bim and cell cycle elements as potential therapeutic targets.

Molecular characterization of the genetic lesion in Dystonia musculorum (dt-Alb) mice.

Dystonia musculorum (dt) is an inherited autosomal recessive neuropathy in mice. Homozygous animals display primarily sensory neurodegeneration resulting in a severe loss of coordination. Several dt strains exist, including spontaneous mutants dt-Alb (Albany), dt-J (Jackson Labs), and dt-Frk (Frankel), and a transgene insertion mutant, Tg4. They contain mutations in the gene encoding Bullous Pemphigoid Antigen 1 (BPAG1), or dystonin. BPAG1 is a member of the plakin family of cytolinker proteins. BPAG1 is alternatively spliced to produce several isoforms, including the major brain-specific isoform, BPAG1a. The neurological phenotype observed in dt-Alb mice is thought to result from the absence of BPAG1a protein in the developing nervous system. The goal of this study was to determine the precise molecular nature of the dt-Alb mutation and examine residual BPAG1 expression in homozygous dt-Alb mice. A combination of molecular biological strategies revealed that the dt-Alb lesion is a deletion-insertion eliminating a large part of the coding region of BPAG1a. The molecular lesion in the dt-Alb BPAG1 allele is expected to render it completely non-functional. Although transcripts corresponding to BPAG1 segments still remaining in homozygous dt-Alb mice could be detected by RT-PCR, there was no positive signal for BPAG1 in the brain of dt-Alb mice by Northern blotting. Western blotting with polyclonal anti-BPAG1 antibodies confirmed the absence of functional BPAG1 protein (full-length or truncated) in the dt-Alb brain. Our identification of the 5' junction of the dt-Alb insertion makes it possible to genotype dt-Alb animals by standard PCR.

The role of microtubule actin cross-linking factor 1 (MACF1) in the Wnt signaling pathway.

MACF1 (microtubule actin cross-linking factor 1) is a multidomain protein that can associate with microfilaments and microtubules. We found that MACF1 was highly expressed in neuronal tissues and the foregut of embryonic day 8.5 (E8.5) embryos and the head fold and primitive streak of E7.5 embryos. MACF1(-/-) mice died at the gastrulation stage and displayed developmental retardation at E7.5 with defects in the formation of the primitive streak, node, and mesoderm. This phenotype was similar to Wnt-3(-/-) and LRP5/6 double-knockout embryos. In the absence of Wnt, MACF1 associated with a complex that contained Axin, beta-catenin, GSK3beta, and APC. Upon Wnt stimulation, MACF1 appeared to be involved in the translocation and subsequent binding of the Axin complex to LRP6 at the cell membrane. Reduction of MACF1 with small interfering RNA decreased the amount of beta-catenin in the nucleus, and led to an inhibition of Wnt-induced TCF/beta-catenin-dependent transcriptional activation. Similar results were obtained with a dominant-negative MACF1 construct that contained the Axin-binding region. Reduction of MACF1 in Wnt-1-expressing P19 cells resulted in decreased T (Brachyury) gene expression, a DNA-binding transcription factor that is a direct target of Wnt/beta-catenin signaling and required for mesoderm formation. These results suggest a new role of MACF1 in the Wnt signaling pathway.

Dissecting the sequence specific functions of alternative N-terminal isoforms of mouse bullous pemphigoid antigen 1.

Bullous pemphigoid antigen 1 (BPAG1) is a member of the plakin family of proteins that is involved in cross-linking the cytoskeletal elements and attaching them to cell junctions. BPAG1 null mice develop severe degeneration of sensory neurons that was attributed in part due to the absence of a splice variant called BPAG1a that harbors an actin-binding domain at the N-terminus. Additional alternative splicing also results in BPAG1a isoforms with different first exons, leading to three additional types of BPAG1a called isoforms 1, 2 and 3 (or BPAG1a1, BPAG1a2, and BPAG1a3). These unique N-terminal extensions of the BPAG1a isoforms are of variable length. In this study, we characterized these N-terminal isoforms and evaluated the influence of these unique N-terminal sequences to the actin-binding properties. The unique N-terminal region of isoform 1 is very short and was not expected to affect the property of the ABD that followed it. In contrast, transfection studies and mutagenesis analyses signified that the N-terminal sequences of isoform 2 had the ability to bundle actin filaments and the N-terminal region that contained isoform 3 showed cortical localization. Isoforms 1, 2 and 3 also displayed differential tissue expression profiles. Taken together, these data suggested that the unique N-terminal regions of these isoforms have different roles that may be tailored to meet tissue specific functions.

Isolation of intermediate filaments.

Intermediate filaments (IFs) are found in most eukaryotic cells and are made up of various IF proteins. IFs are highly insoluble in conventional extraction buffers and are therefore commonly purified under denaturing condition. Purified IF proteins can be reassembled into filaments by dialysis. At least 65 IF proteins are found in humans, and the procedures for the purification of each subunit vary somewhat, although many basic steps are similar. To illustrate the isolation of IFs, a detailed protocol is described for purifying neurofilament proteins (NFL, NFM, and NFH subunits) from bovine spinal cord. These three proteins form the predominant IF network in mature neurons. An alternative method for the purification of NFL from a prokaryotic expression system is also included. The isolation of recombinant proteins from bacteria is quite straightforward and may therefore be the method of choice for producing and purifying IFs. Finally, there is a discussion of the purification methods of other IF proteins.

Microtubule actin crosslinking factor 1b: a novel plakin that localizes to the Golgi complex.

MACF1 (microtubule actin crosslinking factor), also called ACF7 (actin crosslinking family 7) is a cytoskeletal linker protein that can associate with both actin filaments and microtubules. We have identified a novel alternatively spliced isoform of MACF1. We named this isoform MACF1b and renamed the original isoform MACF1a. MACF1b is identical to MACF1a, except that it has a region containing plakin (or plectin) repeats in the middle of the molecule. MACF1b is ubiquitously expressed in adult tissues with especially high levels in the lung. We studied the subcellular localization of MACF1b proteins in mammalian cell lines. In two lung cell lines, MACF1b was chiefly localized to the Golgi complex. Upon treatments that disrupt the Golgi complex, MACF1b redistributed into the cytosol, but remained co-localized with the dispersed Golgi ministacks. MACF1b proteins can be detected in the enriched Golgi fraction by western blotting. The domain of MACF1b that targets it to the Golgi was found at the N-terminal part of the region that contains the plakin repeats. Reducing the level of MACF1 proteins by small-interfering RNA resulted in the dispersal of the Golgi complex.

A pathogenic peripherin gene mutation in a patient with amyotrophic lateral sclerosis.

Peripherin is a neuronal intermediate filament protein that is expressed chiefly in motor neurons and other nerve cells that project into the peripheral nervous system. Transgenic mice that over-express peripherin develop motor neuron degeneration, suggesting that mutations in peripherin could contribute to the development of motor neuron disease. In this paper, we report the identification of a homozygous mutation in the peripherin gene (PRPH) in a patient with amyotrophic lateral sclerosis (ALS). The mutation resulted in a substitution of aspartate with tyrosine at amino acid position 141, which is located within the first linker region of the rod domain. Immunocytochemical analysis of the spinal cord of the patient upon autopsy revealed distinctive large aggregates within the cell bodies of residual spinal motor neurons that contained peripherin and was also immunoreactive with antibodies to the neurofilament proteins. In order to study the effect of the mutation on peripherin assembly, we performed transient transfections. Unlike wild-type peripherin, which self-assembles to form a filamentous network, the mutant peripherin was prone to form aggregates in transfected cells, indicating that the mutation adversely affects peripherin assembly. Moreover, the neurofilament light (NF-L) protein was not able to rescue the mutant protein from forming aggregates. These data imply that mutation of PRPH is a contributing factor for ALS.

Mutant small heat-shock protein 27 causes axonal Charcot-Marie-Tooth disease and distal hereditary motor neuropathy.

Charcot-Marie-Tooth disease (CMT) is the most common inherited neuromuscular disease and is characterized by considerable clinical and genetic heterogeneity. We previously reported a Russian family with autosomal dominant axonal CMT and assigned the locus underlying the disease (CMT2F; OMIM 606595) to chromosome 7q11-q21 (ref. 2). Here we report a missense mutation in the gene encoding 27-kDa small heat-shock protein B1 (HSPB1, also called HSP27) that segregates in the family with CMT2F. Screening for mutations in HSPB1 in 301 individuals with CMT and 115 individuals with distal hereditary motor neuropathies (distal HMNs) confirmed the previously observed mutation and identified four additional missense mutations. We observed the additional HSPB1 mutations in four families with distal HMN and in one individual with CMT neuropathy. Four mutations are located in the Hsp20-alpha-crystallin domain, and one mutation is in the C-terminal part of the HSP27 protein. Neuronal cells transfected with mutated HSPB1 were less viable than cells expressing the wild-type protein. Cotransfection of neurofilament light chain (NEFL) and mutant HSPB1 resulted in altered neurofilament assembly in cells devoid of cytoplasmic intermediate filaments.

Protein products of human Gas2-related genes on chromosomes 17 and 22 (hGAR17 and hGAR22) associate with both microfilaments and microtubules.

The human Gas2-related gene on chromosome 22 (hGAR22) encodes two alternatively spliced mRNA species. The longer mRNA encodes a protein with a deduced molecular mass of 36.3 kDa (GAR22alpha), whereas the shorter mRNA encodes a larger protein with a deduced molecular mass of 72.6 kDa (GAR22beta). We show that both hGAR22 proteins contain a calponin homology actin-binding domain and a Gas2-related microtubule-binding domain. Using rapid amplification of cDNA ends, we have cloned the mouse orthologue of hGAR22, mGAR22, and found its protein products to be extremely well conserved. We also report the cDNA cloning of a human Gas2-related gene on chromosome 17 (hGAR17). hGAR17 also encodes two protein isoforms. The overall cytoskeletal binding properties of the hGAR17 and hGAR22 proteins are remarkably similar. hGAR17 mRNA expression is limited to skeletal muscle. Although hGAR22 and mGAR22 mRNAs are expressed nearly ubiquitously, mGAR22 protein can only be detected in testis and brain. Furthermore, only the beta isoform is present in these tissues. GAR22beta expression is induced in a variety of cultured cells by growth arrest. The absolute amounts of GAR22beta protein expressed are low. The beta isoforms of hGAR17 and hGAR22 appear to be able to crosslink microtubules and microfilaments in transfected cells. This finding suggests that the physiological functions of these proteins may involve integration of these two components of the cytoskeleton.

Effects of Charcot-Marie-Tooth-linked mutations of the neurofilament light subunit on intermediate filament formation.

Neurofilaments (NFs) are the major intermediate filaments (IFs) of mature neurons. They play important roles in the structure and function of axons. Recently, two mutations in the neurofilament light (NFL) subunit have been identified in families affected by Charcot-Marie-Tooth (CMT) neuropathy type 2. We have characterized the effects of these NFL mutations on the formation of IF networks using a transient transfection system. Both mutations disrupted the self-assembly of human NFL. The Q333P mutant in the rod domain of NFL also disrupted the formation of rat and human NFL/NFM heteropolymers. The phenotypes produced by the P8R mutation in the head domain of NFL were less severe. The P8R mutant NFL co-polymerized with NFM to form bundled filaments and, less often, aggregates. Our results suggest that alterations in the formation of a normal IF network in neurons elicited by these NFL mutations may contribute to the development of Charcot-Marie-Tooth neuropathy.

Plakins: a family of versatile cytolinker proteins.

By connecting cytoskeletal elements to each other and to junctional complexes, the plakin family of cytolinkers plays a crucial role in orchestrating cellular development and maintaining tissue integrity. Plakins are built from combinations of interacting domains that bind to microfilaments, microtubules, intermediate filaments, cell-adhesion molecules and members of the armadillo family. Plakins are involved in both inherited and autoimmune diseases that affect the skin, neuronal tissue, and cardiac and skeletal muscle. Here, we describe the members of the plakin family and their interaction partners, and give examples of the cellular defects that result from their dysfunction.