Remodeling of cell-cell junctions in arrhythmogenic cardiomyopathy

Sex-Specific Response to A1BG Loss Results in Female Dilated Cardiomyopathy

Background Cardiac disease often manifests differently in terms of frequency and pathology between men and women. However, the mechanisms underlying these differences are not fully understood. The glycoprotein A1BG is necessary for proper cardiac function in females but not males. Despite this, the role of A1BG in the female heart remains poorly studied. Methods To determine the sex differential function of A1BG, we generated a novel conditional A1bg allele and a novel conditional A1bg Rosa26 knockin allele. Histology, electrocardiography, transcriptional profiling (RNA-seq), transmission electron microscopy, western blot analyses, mass spectrometry, and immunohistochemistry were used to assess cardiac structure and function. Results The study reveals that the absence of A1BG results in significant cardiac dysfunction in female but not male mice. Gene expression underscores that A1BG plays a critical role in metabolic processes and the integrity of intercalated discs in female cardiomyocytes. This dysfunction may be related to sex-specific A1BG cardiac interactomes and manifests as structural and functional alterations in the left ventricle indicative of dilated cardiomyopathy, thus suggesting a sex-specific requirement for A1BG in cardiac health. Conclusion The loss of A1BG in cardiomyocytes leads to dilated cardiomyopathy in females, not males.

Cheek-Pro-Heart: What Can the Buccal Mucosa Do for Arrhythmogenic Cardiomyopathy?

Arrhythmogenic cardiomyopathy (ACM) is a heart muscle disease associated with ventricular arrhythmias and a high risk of sudden cardiac death (SCD). Although the disease was described over 40 years ago, its diagnosis is still difficult. Several studies have identified a set of five proteins (plakoglobin, Cx43, Nav1.5, SAP97 and GSK3β), which are consistently re-distributed in myocardial samples from ACM patients. Not all protein shifts are specific to ACM, but their combination has provided us with a molecular signature for the disease, which has greatly aided post-mortem diagnosis of SCD victims. The use of this signature, however, was heretofore restricted in living patients, as the analysis requires a heart sample. Recent studies have shown that buccal cells behave similarly to the heart in terms of protein re-localization. Protein shifts are associated with disease onset, deterioration and favorable response to anti-arrhythmic therapy. Accordingly, buccal cells can be used as a surrogate for the myocardium to aid diagnosis, risk stratification and even monitor response to pharmaceutical interventions. Buccal cells can also be kept in culture, hence providing an ex vivo model from the patient, which can offer insights into the mechanisms of disease pathogenesis, including drug response. This review summarizes how the cheek can aid the heart in the battle against ACM.

"Orphan" Connexin43 in Plakophilin-2 Deficient Hearts Revealed by Volume Electron Microscopy

Previous studies revealed an abundance of functional Connexin43 (Cx43) hemichannels consequent to loss of plakophilin-2 (PKP2) expression in adult murine hearts. The increased Cx43-mediated membrane permeability is likely responsible for excess entry of calcium into the cells, leading to an arrhythmogenic/cardiomyopathic phenotype. The latter has translational implications to the molecular mechanisms of inheritable arrhythmogenic right ventricular cardiomyopathy (ARVC). Despite functional evidence, visualization of these "orphan" (i.e., non-paired in a gap junction configuration) Cx43 hemichannels remains lacking. Immuno-electron microscopy (IEM) remains an extremely powerful tool to localize, with nanometric resolution, a protein within its native structural landscape. Yet, challenges for IEM are to preserve the antigenicity of the molecular target and to provide access for antibodies to reach their target, while maintaining the cellular/tissue ultrastructure. Fixation is important for maintaining cell structure, but strong fixation and vigorous dehydration (as it is routine for EM) can alter protein structure, thus impairing antigen-antibody binding. Here, we implemented a method to combine pre-embedding immunolabeling (pre-embedding) with serial block-face scanning electron microscopy (SBF-SEM). We utilized a murine model of cardiomyocyte-specific, Tamoxifen (TAM) activated knockout of PKP2. Adult hearts were harvested 14 days post-TAM, at this time hearts present a phenotype of concealed ARVC (i.e., an arrhythmogenic phenotype but no overt structural disease). Thick (200 µm) vibratome slices were immunolabelled for Cx43 and treated with nanogold or FluoroNanogold, coupled with a silver enhancement. Left or right ventricular free walls were dissected and three-dimensional (3D) localization of Cx43 in cardiac muscle was performed using SBF-SEM. Reconstructed images allowed us to visualize the entire length of gap junction plaques, seen as two parallel, closely packed strings of Cx43-immunoreactive beads at the intercalated disc. In contrast, in PKP2-deficient hearts we observed bulging of the intercellular space, and entire areas where only one of the two strings could be observed, indicating the presence of orphan Cx43. We conclude that pre-embedding and SBF-SEM allowed visualization of cardiac Cx43 plaques in their native environment, providing for the first time a visual complement of functional data indicating the presence of orphan Cx43 hemichannels resulting from loss of desmosomal integrity in the heart.

Plakophilin-2 truncating variants impair cardiac contractility by disrupting sarcomere stability and organization

Progressive loss of cardiac systolic function in arrhythmogenic cardiomyopathy (ACM) has recently gained attention as an important clinical consideration in managing the disease. However, the mechanisms leading to reduction in cardiac contractility are poorly defined. Here, we use CRISPR gene editing to generate human induced pluripotent stem cells (iPSCs) that harbor plakophilin-2 truncating variants (PKP2tv), the most prevalent ACM-linked mutations. The PKP2tv iPSC–derived cardiomyocytes are shown to have aberrant action potentials and reduced systolic function in cardiac microtissues, recapitulating both the electrical and mechanical pathologies reported in ACM. By combining cell micropatterning with traction force microscopy and live imaging, we found that PKP2tvs impair cardiac tissue contractility by destabilizing cell-cell junctions and in turn disrupting sarcomere stability and organization. These findings highlight the interplay between cell-cell adhesions and sarcomeres required for stabilizing cardiomyocyte structure and function and suggest fundamental pathogenic mechanisms that may be shared among different types of cardiomyopathies.

Genes, other than Muc5b, play a role in bleomycin-induced lung fibrosis

Idiopathic pulmonary fibrosis (IPF) is an incurable genetic disease that affects 5 million people worldwide. The gain-of-function MUC5B promoter variant rs35705950 is the dominant genetic risk factor for IPF, yet has a low penetrance. This raises the possibility that other genes and transcripts affect the penetrance of MUC5B. Previously, we have shown that the concentration of Muc5b in bronchoalveolar epithelia is directly associated with the extent and persistence of bleomycin-induced lung fibrosis in mice. In this study, we investigated whether bleomycin-induced lung injury is Muc5b dependent in genetically divergent strains of mice. Specifically, mice from the eight Diversity Outbred (DO) founders were phenotyped for Muc5b expression and lung fibrosis 3 wk after intratracheal bleomycin administration. Although we identified strains with low Muc5b expression and minimal lung fibrosis (CAST/EiJ and PWK/PhJ) and strains with high Muc5b expression and extensive lung fibrosis (NZO/H1LtJ and WSB/EiJ), there also were strains that did not demonstrate a clear relationship between Muc5b expression and lung fibrosis (129S1/SvlmJ, NOD/ShiLtJ, and C57BL/6J, A/J). Hierarchical clustering suggests that other factors may work in concert with or potentially independent of Muc5b to promote bleomycin-induced lung injury and fibrosis. This study suggests that these strains and their recombinant inbred crosses may prove helpful in identifying the genes and transcripts that interact with Muc5b and cause lung fibrosis.

Arrhythmogenic Cardiomyopathy Is a Multicellular Disease Affecting Cardiac and Bone Marrow Mesenchymal Stromal Cells

Arrhythmogenic cardiomyopathy (AC) is a familial cardiac disorder at high risk of arrhythmic sudden death in the young and athletes. AC is hallmarked by myocardial replacement with fibro-fatty tissue, favoring life-threatening cardiac arrhythmias and contractile dysfunction. The AC pathogenesis is unclear, and the disease urgently needs mechanism-driven therapies. Current AC research is mainly focused on ‘desmosome-carrying’ cardiomyocytes, but desmosomal proteins are also expressed by non-myocyte cells, which also harbor AC variants, including mesenchymal stromal cells (MSCs). Consistently, cardiac-MSCs contribute to adipose tissue in human AC hearts. We thus approached AC as a multicellular disorder, hypothesizing that it also affects extra-cardiac bone marrow (BM)-MSCs. Our results show changes in the desmosomal protein profile of both cardiac- and BM- MSCs, from desmoglein-2 (Dsg2)-mutant mice, accompanied with profound alterations in cytoskeletal organization, which are directly caused by AC-linked DSG2 downregulation. In addition, AC BM-MSCs display increased proliferation rate, both in vitro and in vivo, and, by using the principle of the competition homing assay, we demonstrated that mutant circulating BM-MSCs have increased propensity to migrate to the AC heart. Taken altogether, our results indicate that cardiac- and BM- MSCs are additional cell types affected in Dsg2-linked AC, warranting the novel classification of AC as a multicellular and multiorgan disease.

Novel pathogenic role for galectin-3 in early disease stages of arrhythmogenic cardiomyopathy

BACKGROUND

Arrhythmogenic cardiomyopathy (AC) is a myocardial disease due to desmosomal mutations whose pathogenesis is incompletely understood.

OBJECTIVE

The purpose of this study was to identify molecular pathways underlying early AC by gene expression profiling in both humans and animal models.

METHODS

RNA sequencing for differentially expressed genes (DEGs) was performed on the myocardium of transgenic mice overexpressing the Desmoglein2-N271S mutation before phenotype onset. Zebrafish signaling reporters were used for in vivo validation. Whole exome sequencing was undertaken in 10 genotype-negative AC patients and subsequent direct sequencing in 140 AC index cases.

RESULTS

Among 29 DEGs identified at early disease stages, Lgals3/GAL3 (lectin, galactoside-binding, soluble, 3) showed reduced cardiac expression in transgenic mice and in 3 AC patients who suffered sudden cardiac death without overt structural remodeling. Four rare missense variants of LGALS3 were identified in 5 human AC probands. Pharmacologic inhibition of Lgals3 in zebrafish reduced Wnt and transforming growth factor-β signaling, increased Hippo/YAP-TAZ signaling, and induced alterations in desmoplakin membrane localization, desmosome integrity and stability. Increased LGALS3 plasma expression in genotype-positive AC patients and CD98 activation supported the galectin-3 (GAL3) release by circulating macrophages pointing toward the stabilization of desmosomal assembly at the injured regions.

CONCLUSION

GAL3 plays a crucial role in early AC onset through regulation of Wnt/β-catenin signaling and intercellular adhesion.

Molecular mechanisms of arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy is a genetic disorder characterized by the risk of life-threatening arrhythmias, myocardial dysfunction and fibrofatty replacement of myocardial tissue. Mutations in genes that encode components of desmosomes, the adhesive junctions that connect cardiomyocytes, are the predominant cause of arrhythmogenic cardiomyopathy and can be identified in about half of patients with the condition. However, the molecular mechanisms leading to myocardial destruction, remodelling and arrhythmic predisposition remain poorly understood. Through the development of animal, induced pluripotent stem cell and other models of disease, advances in our understanding of the pathogenic mechanisms of arrhythmogenic cardiomyopathy over the past decade have brought several signalling pathways into focus. These pathways include canonical and non-canonical WNT signalling, the Hippo-Yes-associated protein (YAP) pathway and transforming growth factor-β signalling. These studies have begun to identify potential therapeutic targets whose modulation has shown promise in preclinical models. In this Review, we summarize and discuss the reported molecular mechanisms underlying the pathogenesis of arrhythmogenic cardiomyopathy.

Filamin C variants are associated with a distinctive clinical and immunohistochemical arrhythmogenic cardiomyopathy phenotype

BACKGROUND

Pathogenic variants in the filamin C (FLNC) gene are associated with inherited cardiomyopathies including dilated cardiomyopathy with an arrhythmogenic phenotype. We evaluated FLNC variants in arrhythmogenic cardiomyopathy (ACM) and investigated the disease mechanism at a molecular level.

METHODS

120 gene-elusive ACM patients who fulfilled diagnostic criteria for arrhythmogenic right ventricular cardiomyopathy (ARVC) were screened by whole exome sequencing. Fixed cardiac tissue from FLNC variant carriers who had died suddenly was investigated by histology and immunohistochemistry.

RESULTS

Novel or rare FLNC variants, four null and five variants of unknown significance, were identified in nine ACM probands (7.5%). In FLNC null variant carriers (including family members, n = 16) Task Force diagnostic electrocardiogram repolarization/depolarization abnormalities were uncommon (19%), echocardiography was normal in 69%, while 56% had >500 ventricular ectopics/24 h or ventricular tachycardia on Holter and 67% had late gadolinium enhancement (LGE) on cardiac magnetic resonance imaging (CMRI). Ten gene positive individuals (63%) had abnormalities on ECG or CMRI that are not included in the current diagnostic criteria for ARVC. Immunohistochemistry showed altered key protein distribution, distinctive from that observed in ARVC, predominantly in the left ventricle.

CONCLUSIONS

ACM associated with FLNC variants presents with a distinctive phenotype characterized by Holter arrhythmia and LGE on CMRI with unremarkable ECG and echocardiographic findings. Clinical presentation in asymptomatic mutation carriers at risk of sudden death may include abnormalities which are currently non-diagnostic for ARVC. At the molecular level, the pathogenic mechanism related to FLNC appears different to classic forms of ARVC caused by desmosomal mutations.

Disruption of Ca2+i Homeostasis and Connexin 43 Hemichannel Function in the Right Ventricle Precedes Overt Arrhythmogenic Cardiomyopathy in Plakophilin-2-Deficient Mice

BACKGROUND

Plakophilin-2 (PKP2) is classically defined as a desmosomal protein. Mutations in PKP2 associate with most cases of gene-positive arrhythmogenic right ventricular cardiomyopathy. A better understanding of PKP2 cardiac biology can help elucidate the mechanisms underlying arrhythmic and cardiomyopathic events consequent to PKP2 deficiency. Here, we sought to capture early molecular/cellular events that can act as nascent arrhythmic/cardiomyopathic substrates.

METHODS

We used multiple imaging, biochemical and high-resolution mass spectrometry methods to study functional/structural properties of cells/tissues derived from cardiomyocyte-specific, tamoxifen-activated, PKP2 knockout mice (PKP2cKO) 14 days post-tamoxifen injection, a time point preceding overt electrical or structural phenotypes. Myocytes from right or left ventricular free wall were studied separately.

RESULTS

Most properties of PKP2cKO left ventricular myocytes were not different from control; in contrast, PKP2cKO right ventricular (RV) myocytes showed increased amplitude and duration of Ca²⁺ transients, increased Ca²⁺ in the cytoplasm and sarcoplasmic reticulum, increased frequency of spontaneous Ca²⁺ release events (sparks) even at comparable sarcoplasmic reticulum load, and dynamic Ca²⁺ accumulation in mitochondria. We also observed early- and delayed-after transients in RV myocytes and heightened susceptibility to arrhythmias in Langendorff-perfused hearts. In addition, ryanodine receptor 2 in PKP2cKO-RV cells presented enhanced Ca²⁺ sensitivity and preferential phosphorylation in a domain known to modulate Ca²⁺ gating. RNAseq at 14 days post-tamoxifen showed no relevant difference in transcript abundance between RV and left ventricle, neither in control nor in PKP2cKO cells. Instead, we found an RV-predominant increase in membrane permeability that can permit Ca²⁺ entry into the cell. Connexin 43 ablation mitigated the membrane permeability increase, accumulation of cytoplasmic Ca²⁺, increased frequency of sparks and early stages of RV dysfunction. Connexin 43 hemichannel block with GAP19 normalized [Ca²⁺]_i homeostasis. Similarly, protein kinase C inhibition normalized spark frequency at comparable sarcoplasmic reticulum load levels.

CONCLUSIONS

Loss of PKP2 creates an RV-predominant arrhythmogenic substrate (Ca²⁺ dysregulation) that precedes the cardiomyopathy; this is, at least in part, mediated by a Connexin 43-dependent membrane conduit and repressed by protein kinase C inhibitors. Given that asymmetric Ca²⁺ dysregulation precedes the cardiomyopathic stage, we speculate that abnormal Ca²⁺ handling in RV myocytes can be a trigger for gross structural changes observed at a later stage.

Reversing Mechanoinductive DSP Expression by CRISPR/dCas9-mediated Epigenome Editing

RATIONALE

DSP (desmoplakin), the most abundant component of desmosomes, which maintain the mechanical integrity of epithelium, is a genome-wide association study-identified genetic risk locus in human idiopathic pulmonary fibrosis (IPF). Subjects with IPF express a significantly higher level of DSP than control subjects.

OBJECTIVES

Determine potential mechanisms by which DSP is regulated in lung fibrosis.

METHODS

Matrigel-coated soft and stiff polyacrylamide gels were made to simulate the stiffness of normal and fibrotic lungs. Quantitative chromatin immunoprecipitation and electrophoretic mobility shift assay were used to evaluate transcription factor binding to the DSP promoter. Targeted DNA methylation was achieved by CRISPR (clustered regularly interspaced short palindromic repeats)/dCas9 (deactivated CRISPR-associated protein-9 nuclease)-mediated Dnmt3A (DNA methyltransferase 3A) expression under the guidance of sequence-specific single guide RNAs.

MEASUREMENTS AND MAIN RESULTS

Stiff matrix promotes DSP gene expression in both human and rodent lung epithelial cells as compared with soft matrix. A conserved region in the proximal DSP promoter is hypermethylated under soft matrix conditions and becomes hypomethylated/demethylated under stiff matrix conditions. Demethylation of this conserved DSP promoter region is associated with transactivation of transcription factor EGR1 (early growth response protein 1), resulting in EGR1-dependent DSP overexpression. Targeted DNA methylation by CRISPR/dCas9/Dnmt3A-mediated epigenome editing blocks EGR1 binding to the DSP promoter and inhibits stiff matrix-induced DSP overexpression.

CONCLUSIONS

DSP is a matrix stiffness-regulated mechanosensitive gene. CRISPR/dCas9-Dnmt3A-mediated epigenome editing reverses DSP overexpression by reestablishment of the epigenetic control of DSP under the mechanically homeostatic environment. It provides a useful tool for investigations of the functional role of DSP in the pathogenesis of lung fibrosis.

Desmoplakin maintains gap junctions by inhibiting Ras/MAPK and lysosomal degradation of connexin-43

Desmosomal mutations result in potentially deadly cardiocutaneous disease caused by electrical conduction defects and disruption of gap junctions. Kam et al. demonstrate a mechanism whereby loss of the intermediate filament anchoring protein desmoplakin stimulates Cx43 turnover by increasing K-Ras expression, marking Cx43 for lysosomal degradation through ERK1/2 phosphorylation.

Desmoplakin (DP) is an obligate component of desmosomes, intercellular adhesive junctions that maintain the integrity of the epidermis and myocardium. Mutations in DP can cause cardiac and cutaneous disease, including arrhythmogenic cardiomyopathy (ACM), an inherited disorder that frequently results in deadly arrhythmias. Conduction defects in ACM are linked to the remodeling and functional interference with Cx43-based gap junctions that electrically and chemically couple cells. How DP loss impairs gap junctions is poorly understood. We show that DP prevents lysosomal-mediated degradation of Cx43. DP loss triggered robust activation of ERK1/2–MAPK and increased phosphorylation of S279/282 of Cx43, which signals clathrin-mediated internalization and subsequent lysosomal degradation of Cx43. RNA sequencing revealed Ras-GTPases as candidates for the aberrant activation of ERK1/2 upon loss of DP. Using a novel Ras inhibitor, Ras/Rap1-specific peptidase (RRSP), or K-Ras knockdown, we demonstrate restoration of Cx43 in DP-deficient cardiomyocytes. Collectively, our results reveal a novel mechanism for the regulation of the Cx43 life cycle by DP in cardiocutaneous models.

Desmoplakin Variants Are Associated with Idiopathic Pulmonary Fibrosis

RATIONALE

Sequence variation, methylation differences, and transcriptional changes in desmoplakin (DSP) have been observed in patients with idiopathic pulmonary fibrosis (IPF).

OBJECTIVES

To identify novel variants in DSP associated with IPF and to characterize the relationship of these IPF sequence variants with DSP gene expression in human lung.

METHODS

A chromosome 6 locus (7,370,061-7,606,946) was sequenced in 230 subjects with IPF and 228 control subjects. Validation genotyping of disease-associated variants was conducted in 936 subjects with IPF and 936 control subjects. DSP gene expression was measured in lung tissue from 334 subjects with IPF and 201 control subjects.

MEASUREMENTS AND MAIN RESULTS

We identified 23 sequence variants in the chromosome 6 locus associated with IPF. Genotyping of selected variants in our validation cohort revealed that noncoding intron 1 variant rs2744371 (odds ratio = 0.77, 95% confidence interval [CI] = 0.66-0.91, P = 0.002) is protective for IPF, and a previously described IPF-associated intron 5 variant (rs2076295) is associated with increased risk of IPF (odds ratio = 1.36, 95% CI = 1.19-1.56, P < 0.001) after controlling for sex and age. DSP expression is 2.3-fold increased (95% CI = 1.91-2.71) in IPF lung tissue (P < 0.0001). Only the minor allele at rs2076295 is associated with decreased DSP expression (P = 0.001). Staining of fibrotic and normal human lung tissue localized DSP to airway epithelia.

CONCLUSIONS

Sequence variants in DSP are associated with IPF, and rs2076295 genotype is associated with differential expression of DSP in the lung. DSP expression is increased in IPF lung and concentrated in the airway epithelia, suggesting a potential role for DSP in the pathogenesis of IPF.

Loss of plakoglobin immunoreactivity in intercalated discs in arrhythmogenic right ventricular cardiomyopathy: protein mislocalization versus epitope masking

AIMS

To examine the relevance and cause of reduced plakoglobin IF in intercalated discs for arrhythmogenic right ventricular cardiomyopathy (ARVC) and ARVC-like disease in mouse and human.

METHODS AND RESULTS

Normalized semi-quantitative IF measurements were performed in a standardized format in desmoglein 2-mutant mice with an ARVC-like phenotype (n = 6) and in cardiac biopsies from humans with ARVC and non-ARVC heart disease (n = 10). Reduced plakoglobin staining was detectable in ARVC only with one antibody directed against a defined epitope but not with three other antibodies reacting with different epitopes of plakoglobin.

CONCLUSIONS

Reduced plakoglobin staining in intercalated discs of heart tissue from human ARVC patients and in a murine ARVC model is caused by alterations in epitope accessibility and not by protein relocalization.

GSK3- and PRMT-1-dependent modifications of desmoplakin control desmoplakin-cytoskeleton dynamics

Phosphorylation and methylation of desmoplakin are required for proper junction assembly and adhesion strengthening, and inhibition of these modifications might contribute to skin and heart diseases.

Intermediate filament (IF) attachment to intercellular junctions is required for skin and heart integrity, but how the strength and dynamics of this attachment are modulated during normal and pathological remodeling is poorly understood. We show that glycogen synthase kinase 3 (GSK3) and protein arginine methyltransferase 1 (PRMT-1) cooperate to orchestrate a series of posttranslational modifications on the IF-anchoring protein desmoplakin (DP) that play an essential role in coordinating cytoskeletal dynamics and cellular adhesion. Front-end electron transfer dissociation mass spectrometry analyses of DP revealed six novel serine phosphorylation sites dependent on GSK3 signaling and four novel arginine methylation sites including R2834, the mutation of which has been associated with arrhythmogenic cardiomyopathy (AC). Inhibition of GSK3 or PRMT-1 or overexpression of the AC-associated mutant R2834H enhanced DP–IF associations and delayed junction assembly. R2834H blocked the GSK3 phosphorylation cascade and reduced DP–GSK3 interactions in cultured keratinocytes and in the hearts of transgenic R2834H DP mice. Interference with this regulatory machinery may contribute to skin and heart diseases.

Comparative proteomic analysis of human lung telocytes with fibroblasts

Telocytes (TCs) were recently described as interstitial cells with very long prolongations named telopodes (Tps; http://www.telocytes.com). Establishing the TC proteome is a priority to show that TCs are a distinct type of cells. Therefore, we examined the molecular aspects of lung TCs by comparison with fibroblasts (FBs). Proteins extracted from primary cultures of these cells were analysed by automated 2-dimensional nano-electrospray ionization liquid chromatography tandem mass spectrometry (2D Nano-ESI LC-MS/MS). Differentially expressed proteins were screened by two-sample t-test (P < 0.05) and fold change (>2), based on the bioinformatics analysis. We identified hundreds of proteins up- or down-regulated, respectively, in TCs as compared with FBs. TC proteins with known identities are localized in the cytoskeleton (87%) and plasma membrane (13%), while FB up-regulated proteins are in the cytoskeleton (75%) and destined to extracellular matrix (25%). These identified proteins were classified into different categories based on their molecular functions and biological processes. While the proteins identified in TCs are mainly involved in catalytic activity (43%) and as structural molecular activity (25%), the proteins in FBs are involved in catalytic activity (24%) and in structural molecular activity, particularly synthesis of collagen and other extracellular matrix components (25%). Anyway, our data show that TCs are completely different from FBs. In conclusion, we report here the first extensive identification of proteins from TCs using a quantitative proteomics approach. Protein expression profile shows many up-regulated proteins e.g. myosin-14, periplakin, suggesting that TCs might play specific roles in mechanical sensing and mechanochemical conversion task, tissue homoeostasis and remodelling/renewal. Furthermore, up-regulated proteins matching those found in extracellular vesicles emphasize TCs roles in intercellular signalling and stem cell niche modulation. The novel proteins identified in TCs will be an important resource for further proteomic research and it will possibly allow biomarker identification for TCs. It also creates the premises for understanding the pathogenesis of some lung diseases involving TCs.