Plectin-Mediated Intermediate Filament Functions: Why Isoforms Matter

Identification of AGO2 and PLEC genes polymorphisms in Hu sheep and their relationship with body size traits

The body size traits are major traits in livestock, which intuitively displays the development of the animal's bones and muscles. This study used PCR amplification, Sanger sequencing, KASPar genotyping, and quantitative real-time reverse transcription PCR (qRT-PCR) to analyze the Single-nucleotide polymorphism and expression characteristics of Argonaute RISC catalytic component 2 (AGO2) and Plectin (PLEC) genes in Hu sheep. Two intron mutations were found in Hu sheep, which were AGO2 g.51700 A > C and PLEC g.23157 C > T, respectively. Through association analysis of two mutation sites and body size traits, it was found that AGO2 g.51700 A > C mainly affects the chest and cannon circumference of Hu sheep of while PLEC g.23157 C mainly affects body height and body length. The combined genotypes of AGO2 and PLEC genes with body size traits showed SNPs at the AGO2 g.51700 A > C and PLEC g.23157 C > T loci significantly improved the body size traits of Hu sheep. In addition, the AGO2 gene has the highest expression levels in the heart, rumen, and tail fat, and the PLEC gene is highly expressed in the heart. These two loci can provide new research ideas for improving the body size traits of Hu sheep.

Cancer cell extravasation requires iplectin-mediated delivery of MT1-MMP at invadopodia

BACKGROUND

Invadopodia facilitate cancer cell extravasation, but the molecular mechanism whereby invadopodia-specific proteases such as MT1-MMP are called to invadopodia is unclear.

METHODS

Mass spectrometry and immunoprecipitation were used to identify interactors of MT1-MMP in metastatic breast cancer cells. After identification, siRNA and small molecule inhibitors were used to assess the effect these interactors had on cellular invasiveness. The chicken embryo chorioallantoic membrane (CAM) model was used to assess extravasation and invadopodia formation in vivo.

RESULTS

In metastatic breast cancer cells, MT1-MMP was found to associate with plectin, a cytolinker and scaffolding protein. Complex formation between plectin and MT1-MMP launches invadopodia formation, a subtype we termed iplectin (i = invadopodial). iPlectin delivers MT1-MMP to invadopodia and is indispensable for regulating cell surface levels of the enzyme. Genetic depletion of plectin with siRNA reduced invadopodia formation and cell invasion in vitro. In vivo extravasation efficiency assays and intravital imaging revealed iplectin to be a key contributor to invadopodia ultrastructure and essential for extravasation. Pharmacologic inhibition of plectin using the small molecule Plecstatin-1 (PST-1) abrogated MT1-MMP delivery to invadopodia and extravasation efficiency.

CONCLUSIONS

Anti-metastasis therapeutic approaches that target invadopodia are possible by disrupting interactions between MT1-MMP and iplectin.

CLINICAL TRIAL REGISTRATION NUMBER

NCT04608357.

Plectin: Dual Participation in Tumor Progression

The plectin gene can encode a cytoskeletal linking protein, plectin, known for its interaction with three critical components of the cellular cytoskeleton: intermediate filaments, microtubules, and actin filaments. In recent years, more and more studies have reported that plectin is closely related to tumorigenesis and development, exhibiting both tumor-suppressive and tumor-promoting functions. Here, we first introduce the molecular structure and function of plectin, and then we summarize the current understanding of the crucial role of plectin in cancer progression. Finally, we also discuss the possible reasons for the different roles of plectin expression in various types of cancer and highlight the double-edged sword role of plectin in tumor progression. The review aims to deepen the comprehensive understanding of plectin's role in cancer and further help to develop novel therapeutic strategies and drug targets.

Intermediate filaments at a glance

Intermediate filaments (IFs) comprise a large family of versatile cytoskeletal proteins, divided into six subtypes with tissue-specific expression patterns. IFs have a wide repertoire of cellular functions, including providing structural support to cells, as well as active roles in mechanical support and signaling pathways. Consequently, defects in IFs are associated with more than 100 diseases. In this Cell Science at a Glance article, we discuss the established classes of IFs and their general features, their functions beyond structural support, and recent advances in the field. We also highlight their involvement in disease and potential use as clinical markers of pathological conditions. Finally, we provide our view on current knowledge gaps and the future directions of the IF field.

The umbrella cell keratin network: organization as a tile-like mesh, formation of a girded layer in response to bladder filling, and dependence on the plectin cytolinker

The keratin cytoskeleton and associated desmosomes contribute to the mechanical stability of epithelial tissues, but their organization in bladder umbrella cells and their responses to bladder filling are poorly understood. Using super-resolution confocal microscopy, along with 3D image reconstruction and platinum replica electron microscopy, we observed that the apical keratin network of umbrella cells was organized as a dense tile-like mesh comprised of tesserae bordered on their edges by cortical actin filaments, filled with woven keratin filaments, and crosslinked by plectin. A band of keratin was also observed at the cell periphery that was linked to the junction-associated actin ring by plectin. During bladder filling, the junction-localized desmosomal necklace expanded, and a subjacent girded layer was formed that linked the keratin network to desmosomes, including those at the umbrella cell-intermediate cell interface. Disruption of plectin led to focal keratin network dissolution, loss of the junction-associated band of keratin, perturbation of tight junction continuity, and loss of cell-cell cohesion. Our studies reveal a novel tile-like organization of the umbrella cell keratin cytoskeleton that is dependent on plectin, that reorganizes in response to bladder filling, and that likely serves to maintain umbrella cell continuity in the face of mechanical distension.

Vimentin is a key regulator of cell mechanosensing through opposite actions on actomyosin and microtubule networks

The cytoskeleton is a complex network of interconnected biopolymers consisting of actin filaments, microtubules, and intermediate filaments. These biopolymers work in concert to transmit cell-generated forces to the extracellular matrix required for cell motility, wound healing, and tissue maintenance. While we know cell-generated forces are driven by actomyosin contractility and balanced by microtubule network resistance, the effect of intermediate filaments on cellular forces is unclear. Using a combination of theoretical modeling and experiments, we show that vimentin intermediate filaments tune cell stress by assisting in both actomyosin-based force transmission and reinforcement of microtubule networks under compression. We show that the competition between these two opposing effects of vimentin is regulated by the microenvironment stiffness. These results reconcile seemingly contradictory results in the literature and provide a unified description of vimentin's effects on the transmission of cell contractile forces to the extracellular matrix.

Neurofilament Biophysics: From Structure to Biomechanics

Neurofilaments (NFs) are multisubunit, neuron-specific intermediate filaments consisting of a 10-nm diameter filament "core" surrounded by a layer of long intrinsically disordered protein (IDP) "tails." NFs are thought to regulate axonal caliber during development and then stabilize the mature axon, with NF subunit misregulation, mutation, and aggregation featuring prominently in multiple neurological diseases. The field's understanding of NF structure, mechanics, and function has been deeply informed by a rich variety of biochemical, cell biological, and mouse genetic studies spanning more than four decades. These studies have contributed much to our collective understanding of NF function in axonal physiology and disease. In recent years, however, there has been a resurgence of interest in NF subunit proteins in two new contexts: as potential blood- and cerebrospinal fluid-based biomarkers of neuronal damage, and as model IDPs with intriguing properties. Here, we review established principles and more recent discoveries in NF structure and function. Where possible, we place these findings in the context of biophysics of NF assembly, interaction, and contributions to axonal mechanics.

Plectin Deficiency in Fibroblasts Deranges Intermediate Filament and Organelle Morphology, Migration, and Adhesion

Plectin, a highly versatile and multifunctional cytolinker, has been implicated in several multisystemic disorders. Most sequence variations in the human plectin gene (PLEC) cause epidermolysis bullosa simplex with muscular dystrophy (EBS-MD), an autosomal recessive skin-blistering disorder associated with progressive muscle weakness. In this study, we performed a comprehensive cell biological analysis of dermal fibroblasts from three different patients with EBS-MD, where PLEC expression analyses revealed preserved mRNA levels in all cases, whereas full-length plectin protein content was significantly reduced or completely absent. Downstream effects of pathogenic PLEC sequence alterations included massive bundling of vimentin intermediate filament networks, including the occurrence of ring-like nuclei-encasing filament bundles, elongated mitochondrial networks, and abnormal nuclear morphologies. We found that essential fibroblast functions such as wound healing, migration, or orientation upon cyclic stretch were significantly impaired in the cells of patients with EBS-MD. Finally, EBS-MD fibroblasts displayed reduced adhesion capacities, which could be attributed to smaller focal adhesion contacts. Our study not only emphasizes plectin's functional role in human skin fibroblasts, it also provides further insights into the understanding of EBS-MD-associated disease mechanisms.

Plectin plays a role in the migration and volume regulation of astrocytes: a potential biomarker of glioblastoma

BACKGROUND

The expression of aquaporin 4 (AQP4) and intermediate filament (IF) proteins is altered in malignant glioblastoma (GBM), yet the expression of the major IF-based cytolinker, plectin (PLEC), and its contribution to GBM migration and invasiveness, are unknown. Here, we assessed the contribution of plectin in affecting the distribution of plasmalemmal AQP4 aggregates, migratory properties, and regulation of cell volume in astrocytes.

METHODS

In human GBM, the expression of glial fibrillary acidic protein (GFAP), AQP4 and PLEC transcripts was analyzed using publicly available datasets, and the colocalization of PLEC with AQP4 and with GFAP was determined by immunohistochemistry. We performed experiments on wild-type and plectin-deficient primary and immortalized mouse astrocytes, human astrocytes and permanent cell lines (U-251 MG and T98G) derived from a human malignant GBM. The expression of plectin isoforms in mouse astrocytes was assessed by quantitative real-time PCR. Transfection, immunolabeling and confocal microscopy were used to assess plectin-induced alterations in the distribution of the cytoskeleton, the influence of plectin and its isoforms on the abundance and size of plasmalemmal AQP4 aggregates, and the presence of plectin at the plasma membrane. The release of plectin from cells was measured by ELISA. The migration and dynamics of cell volume regulation of immortalized astrocytes were assessed by the wound-healing assay and calcein labeling, respectively.

RESULTS

A positive correlation was found between plectin and AQP4 at the level of gene expression and protein localization in tumorous brain samples. Deficiency of plectin led to a decrease in the abundance and size of plasmalemmal AQP4 aggregates and altered distribution and bundling of the cytoskeleton. Astrocytes predominantly expressed P1c, P1e, and P1g plectin isoforms. The predominant plectin isoform associated with plasmalemmal AQP4 aggregates was P1c, which also affected the mobility of astrocytes most prominently. In the absence of plectin, the collective migration of astrocytes was impaired and the dynamics of cytoplasmic volume changes in peripheral cell regions decreased. Plectin's abundance on the plasma membrane surface and its release from cells were increased in the GBM cell lines.

CONCLUSIONS

Plectin affects cellular properties that contribute to the pathology of GBM. The observed increase in both cell surface and released plectin levels represents a potential biomarker and therapeutic target in the diagnostics and treatment of GBMs.

Desmin and Plectin Recruitment to the Nucleus and Nuclei Orientation Are Lost in Emery-Dreifuss Muscular Dystrophy Myoblasts Subjected to Mechanical Stimulation

In muscle cells subjected to mechanical stimulation, LINC complex and cytoskeletal proteins are basic to preserve cellular architecture and maintain nuclei orientation and positioning. In this context, the role of lamin A/C remains mostly elusive. This study demonstrates that in human myoblasts subjected to mechanical stretching, lamin A/C recruits desmin and plectin to the nuclear periphery, allowing a proper spatial orientation of the nuclei. Interestingly, in Emery-Dreifuss Muscular Dystrophy (EDMD2) myoblasts exposed to mechanical stretching, the recruitment of desmin and plectin to the nucleus and nuclear orientation were impaired, suggesting that a functional lamin A/C is crucial for the response to mechanical strain. While describing a new mechanism of action headed by lamin A/C, these findings show a structural alteration that could be involved in the onset of the muscle defects observed in muscular laminopathies.

Bioinformatic analysis of smoothelin family members supports tissue-specific functions of unique C-terminal calponin homology domains

Smoothelins are cytoskeletal proteins with a single C-terminal calponin homology domain type 2 (CHD2). Little is known about the significance of variation in SMTN CHD2 domains, addressed here through analysis of public databases. A conserved 152 nt penultimate constitutive exon present in all SMTNs encodes helices II-IV of CHD2 with high identity (nt/aa 63/65%). Variable CHD2s of SMTN (helices IV-VI) are generated by alternative splicing of 165 nt exon E20. E20 and the CHD2 it encodes have high homology with the terminal constitutive exon of SMTNL1 (E8; nt/aa 72/75% identity). Unique to these CHD2 variants are a conserved extended nine amino acid C-terminal tail containing KTKK ubiquitination motifs. When E20 of SMTN is skipped (SMTN E20-), constitutive terminal E21 codes for helices IV-VI of CHD2. SMTN E21 has high identity with the terminal exon of SMTNL2 (E8; nt/aa 75/81% identity of aligned sequences) except for coding for a unique extended C-terminus (24 nt; 8aa) conserved only in mammals. SMTN isoform expression is tissue-specific: SMTNE20- and SMTNE20+ are highly expressed in SMC and non-muscle cells, respectively, while SMTNL1 + 2 are highly expressed in skeletal muscle cells. Tissue-specific expression of SMTN CHD2s with unique helices IV-VI suggest tissue-specific functions that require further study.

The cyto-linker and scaffolding protein "plectin" mis-localization leads to softening of cancer cells

Discovering tools to prevent cancer progression requires understanding the fundamental differences between normal and cancer cells. More than a decade ago, atomic force microscopy (AFM) revealed cancer cells' softer body compared to their healthy counterparts. Here, we investigated the mechanism underlying the softening of cancerous cells in comparison with their healthy counterparts based on AFM high resolution stiffness tomography and 3D confocal microscopy. We showed microtubules (MTs) network in invasive ductal carcinoma cell cytoskeleton is basally located and segmented for around 400 nm from the cell periphery. Additionally, the cytoskeleton scaffolding protein plectin exhibits a mis-localization from the cytoplasm to the surface of cells in the carcinoma which justifies the dissociation of the MT network from the cell's cortex. Furthermore, the assessment of MTs' persistence length using a worm-like-chain (WLC) model in high resolution AFM images showed lower persistence length of the single MTs in ductal carcinoma compared to that in the normal state. Overall, these tuned mechanics support the invasive cells to ascertain more flexibility under compressive forces in small deformations. These data provide new insights into the structural origins of cancer aids in progression.

Vimentin single cysteine residue acts as a tunable sensor for network organization and as a key for actin remodeling in response to oxidants and electrophiles

Cysteine residues can undergo multiple posttranslational modifications with diverse functional consequences, potentially behaving as tunable sensors. The intermediate filament protein vimentin has important implications in pathophysiology, including cancer progression, infection, and fibrosis, and maintains a close interplay with other cytoskeletal structures, such as actin filaments and microtubules. We previously showed that the single vimentin cysteine, C328, is a key target for oxidants and electrophiles. Here, we demonstrate that structurally diverse cysteine-reactive agents, including electrophilic mediators, oxidants and drug-related compounds, disrupt the vimentin network eliciting morphologically distinct reorganizations. As most of these agents display broad reactivity, we pinpointed the importance of C328 by confirming that local perturbations introduced through mutagenesis provoke structure-dependent vimentin rearrangements. Thus, GFP-vimentin wild type (wt) forms squiggles and short filaments in vimentin-deficient cells, the C328F, C328W, and C328H mutants generate diverse filamentous assemblies, and the C328A and C328D constructs fail to elongate yielding dots. Remarkably, vimentin C328H structures resemble the wt, but are strongly resistant to electrophile-elicited disruption. Therefore, the C328H mutant allows elucidating whether cysteine-dependent vimentin reorganization influences other cellular responses to reactive agents. Electrophiles such as 1,4-dinitro-1H-imidazole and 4-hydroxynonenal induce robust actin stress fibers in cells expressing vimentin wt. Strikingly, under these conditions, vimentin C328H expression blunts electrophile-elicited stress fiber formation, apparently acting upstream of RhoA. Analysis of additional vimentin C328 mutants shows that electrophile-sensitive and assembly-defective vimentin variants permit induction of stress fibers by reactive species, whereas electrophile-resistant filamentous vimentin structures prevent it. Together, our results suggest that vimentin acts as a break for actin stress fibers formation, which would be released by C328-aided disruption, thus allowing full actin remodeling in response to oxidants and electrophiles. These observations postulate C328 as a "sensor" transducing structurally diverse modifications into fine-tuned vimentin network rearrangements, and a gatekeeper for certain electrophiles in the interplay with actin.

Z-Disk-Associated Plectin (Isoform 1d): Spatial Arrangement, Interaction Partners, and Role in Filamin C Homeostasis

Plectin, a highly versatile cytolinker protein, is crucial for myofiber integrity and function. Accordingly, mutations in the human gene (PLEC) cause several rare diseases, denoted as plectinopathies, with most of them associated with progressive muscle weakness. Of several plectin isoforms expressed in skeletal muscle and the heart, P1d is the only isoform expressed exclusively in these tissues. Using high-resolution stimulated emission depletion (STED) microscopy, here we show that plectin is located within the gaps between individual α-actinin-positive Z-disks, recruiting and bridging them to desmin intermediate filaments (IFs). Loss of plectin in myofibril bundles led to a complete loss of desmin IFs. Loss of Z-disk-associated plectin isoform P1d led to disorganization of muscle fibers and slower relaxation of myofibrils upon mechanical strain, in line with an observed inhomogeneity of muscle ultrastructure. In addition to binding to α-actinin and thereby providing structural support, P1d forms a scaffolding platform for the chaperone-assisted selective autophagy machinery (CASA) by directly interacting with HSC70 and synpo2. In isoform-specific knockout (P1d-KO) mouse muscle and mechanically stretched plectin-deficient myoblasts, we found high levels of undigested filamin C, a bona fide substrate of CASA. Similarly, subjecting P1d-KO mice to forced swim tests led to accumulation of filamin C aggregates in myofibers, highlighting a specific role of P1d in tension-induced proteolysis activated upon high loads of physical exercise and muscle contraction.

Plectin

Prechova et al. introduce the giant cytoskeletal crosslinker protein plectin.

Protein arginine deiminase 2 (PAD2) modulates the polarization of THP-1 macrophages to the anti-inflammatory M2 phenotype

Background

Macrophages are effector cells of the innate immune system that undergo phenotypical changes in response to organ injury and repair. These cells are most often classified as proinflammatory M1 and anti-inflammatory M2 macrophages. Protein arginine deiminase (PAD), which catalyses the irreversible conversion of protein-bound arginine into citrulline, is expressed in macrophages. However, the substrates of PAD and its role in immune cells remain unclear. This study aimed to investigate the role of PAD in THP-1 macrophage polarization to the M1 and M2 phenotypes and identify the citrullinated proteins and modified arginines that are associated with this biological switch using mass spectrometry.

Results

Our study showed that PAD2 and, to a lesser extent, PAD1 and PAD4 were predominantly expressed in M1 macrophages. We showed that inhibiting PAD expression with BB-Cl-amidine decreased macrophage polarization to the M1 phenotype (TNF-α, IL-6) and increased macrophage polarization to the M2 phenotype (MRC1, ALOX15). This process was mediated by the downregulation of proteins involved in the NF-κβ pathway. Silencing PAD2 confirmed the activation of M2 macrophages by increasing the antiviral innate immune response and interferon signalling. A total of 192 novel citrullination sites associated with inflammation, cell death and DNA/RNA processing pathways were identified in M1 and M2 macrophages.

Conclusions

We showed that inhibiting PAD activity using a pharmacological inhibitor or silencing PAD2 with PAD2 siRNA shifted the activation of macrophages towards the M2 phenotype, which can be crucial for designing novel macrophage-mediated therapeutic strategies. We revealed a major citrullinated proteome and its rearrangement following macrophage polarization, which after further validation could lead to significant clinical benefits for the treatment of inflammation and autoimmune diseases.

Collagen IV and Podocyte-Related Gene Variants in Patients with Concurrent IgA Nephropathy and Thin Basement Membrane Nephropathy

INTRODUCTION

IgA nephropathy is the most common primary glomerulonephritis among adults in clinic. Thin basement membrane nephropathy is often underestimated or even omitted if it coincides with IgA nephropathy. Therefore, it is necessary to study the epidemiological, clinical, and molecular characteristics of the concurrence of this entity.

METHODS

Eight patients with concurrent IgA nephropathy and thin basement membrane nephropathy (IgA-T) were retrospectively analyzed based on their clinicopathological characteristics. Genetic analysis was performed using whole-exome sequencing and Sanger's sequencing. Data of the patients with IgA nephropathy and normal basement membrane (IgA-N) and variants in the local in-house database were used as controls. All candidate variants were assessed in silico.

RESULTS

The clinical manifestations of patients with IgA-T were hematuria, proteinuria, and renal insufficiency. Histopathological analysis showed mild mesangial hyperplasia, focal segmental glomerulosclerosis, podocyte activation, and foot process fusion. Crescent was rarely seen. COL4A and/or podocyte cytoskeleton and mitochondria-related gene variants were detected in seven IgA-T patients. Three patients exhibited pathogenic variants of COL4A, including a new variant. All IgA-T and one IgA-N patient possessed ITGB4 and/or PLEC variants, but there was no corresponding genotype-phenotype relationship. Six patients possessed other podocyte cytoskeleton and mitochondria-related gene variants such as NPHS2, SRGAP1, MYO1E, MYO1C, WT1, and COQ9, which were first reported in patients with IgA-T and were not in controls. Altogether, there were no significant differences in the degrees of proteinuria, serum creatinine, and eGFR during the follow-up period of 5-10 years, but there was a significant difference in the degree of proteinuria between IgA-T patients with podocyte-related gene variants and IgA-N patients. In the IgA-T group, patients with podocyte-related gene variants seemed predisposed to progress than patients without those variants, with higher proteinuria and serum creatinine and reduced eGFR.

CONCLUSION

Concurrent thin basement membrane nephropathy and/or heterozygous COL4A gene pathogenic variants do not necessarily predict the short-term progress of sporadic IgA nephropathy in adults. Predisposition factors for this disease progression should be considered for detecting the variants of COL4A and podocyte cytoskeleton and mitochondria-related genes simultaneously, which also manifests the complexity and heterogeneity of IgA nephropathy with concurrent thin basement membrane nephropathy.

Acute Hypoxia Alters Extracellular Vesicle Signatures and the Brain Citrullinome of Naked Mole-Rats (Heterocephalus glaber)

Peptidylarginine deiminases (PADs) and extracellular vesicles (EVs) may be indicative biomarkers of physiological and pathological status and adaptive responses, including to diseases and disorders of the central nervous system (CNS) and related to hypoxia. While these markers have been studied in hypoxia-intolerant mammals, in vivo investigations in hypoxia-tolerant species are lacking. Naked mole-rats (NMR) are among the most hypoxia-tolerant mammals and are thus a good model organism for understanding natural and beneficial adaptations to hypoxia. Thus, we aimed to reveal CNS related roles for PADs in hypoxia tolerance and identify whether circulating EV signatures may reveal a fingerprint for adaptive whole-body hypoxia responses in this species. We found that following in vivo acute hypoxia, NMR: (1) plasma-EVs were remodelled, (2) whole proteome EV cargo contained more protein hits (including citrullinated proteins) and a higher number of associated KEGG pathways relating to the total proteome of plasma-EVs Also, (3) brains had a trend for elevation in PAD1, PAD3 and PAD6 protein expression, while PAD2 and PAD4 were reduced, while (4) the brain citrullinome had a considerable increase in deiminated protein hits with hypoxia (1222 vs. 852 hits in normoxia). Our findings indicate that circulating EV signatures are modified and proteomic content is reduced in hypoxic conditions in naked mole-rats, including the circulating EV citrullinome, while the brain citrullinome is elevated and modulated in response to hypoxia. This was further reflected in elevation of some PADs in the brain tissue following acute hypoxia treatment. These findings indicate a possible selective role for PAD-isozymes in hypoxia response and tolerance.

Genetic Insights into Primary Restrictive Cardiomyopathy

Restrictive cardiomyopathy is a rare cardiac disease causing severe diastolic dysfunction, ventricular stiffness and dilated atria. In consequence, it induces heart failure often with preserved ejection fraction and is associated with a high mortality. Since it is a poor clinical prognosis, patients with restrictive cardiomyopathy frequently require heart transplantation. Genetic as well as non-genetic factors contribute to restrictive cardiomyopathy and a significant portion of cases are of unknown etiology. However, the genetic forms of restrictive cardiomyopathy and the involved molecular pathomechanisms are only partially understood. In this review, we summarize the current knowledge about primary genetic restrictive cardiomyopathy and describe its genetic landscape, which might be of interest for geneticists as well as for cardiologists.