Desmogleins are transmembrane cadherin proteins and obligate members of the desmosome, a cell-cell adhesion complex which connects adjacent cells and provides structural integrity to tissues. While Desmogleins are well-known for their importance in maintenance of cell-cell junctions, several studies have also highlighted their role in signaling crosstalk with cell-matrix adhesions and the extracellular matrix (ECM). We have recently shown that Desmoglein-2 (Dsg2) controls cell spreading on ECM substrates (fibronectin and collagen) and phosphorylation of focal adhesion proteins via Rap1 GTPase signaling. In our current study, we show that loss of Dsg2 in keratinocytes enhances the expression of ECM proteins and matrix metalloproteinases (MMPs), an effect that was not recapitulated upon loss of Desmocollin-2 (Dsc2) or loss of Dsg2 in other epithelial cell types. Signaling pathways well-known to control ECM function (TGF-β and Rho) were not involved in Dsg2-mediated changes in ECM gene expression, but an analysis of global transcriptome changes by RNA sequencing identified major changes in Nuclear Factor-kappa B (NF-κB)-mediated signaling in Dsg2-deficient cells. Interestingly, NF-κB (RelA) activation is elevated in Dsg2-deficient cells, and knockdown of RelA rescued both the enhanced expression of ECM/MMP genes and the enhanced migratory ability of Dsg2-deficient cells. Taken together, this study has identified an important link between Dsg2 and NF-κB signaling involved in controlling matrix production and remodeling, which has relevance for multiple processes in the epidermis such as wound healing and psoriasis.

Myocarditis is clinically characterized by chest pain, arrhythmias, and heart failure, and treatment is often supportive. Mutations in DSP, a gene encoding the desmosomal protein desmoplakin, have been increasingly implicated in myocarditis. To model DSP-associated myocarditis and assess the role of innate immunity, we generated engineered heart tissues (EHTs) using human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) from patients with heterozygous DSP truncating variants (DSPtvs) and a gene-edited homozygous deletion cell line (DSP-/-). At baseline, DSP-/- EHTs displayed a transcriptomic signature of innate immune activation, which was mirrored by cytokine release. Importantly, DSP-/- EHTs were hypersensitive to Toll-like receptor (TLR) stimulation, demonstrating more contractile dysfunction compared with isogenic controls. Relative to DSP-/- EHTs, heterozygous DSPtv EHTs had less functional impairment. DSPtv EHTs displayed heightened sensitivity to TLR stimulation, and when subjected to strain, DSPtv EHTs developed functional deficits, indicating reduced contractile reserve compared with healthy controls. Colchicine or NF-κB inhibitors improved strain-induced force deficits in DSPtv EHTs. Genomic correction of DSP p.R1951X using adenine base editing reduced inflammatory biomarker release from EHTs. Thus, EHTs replicate electrical and contractile phenotypes seen in human myocarditis, implicating cytokine release as a key part of the myogenic susceptibility to inflammation. The heightened innate immune activation and sensitivity are targets for clinical intervention.

The desmosome is a cell-cell adhesion complex which facilitates the mechanical stability of tissues and cell-cell communication. Desmosome function depends upon a tripartite organizational structure wherein transmembrane cadherins (Desmoglein and Desmocollin) link adjacent cells in the extracellular space, armadillo proteins (Plakophilin and Plakoglobin) stabilize the intracellular plaque, and the cytolinker Desmoplakin (DP) connects the plaque to the intermediate filament network. In addition to their central role in maintaining cell-cell junction integrity, desmosomal cadherins also coordinate biological processes such as proliferation, apoptosis, differentiation and cell migration. In our study, we sought to investigate the signaling mechanisms involved in control of actin architecture via desmosomal cadherins, using A431 cells lacking Desmoglein-2 (Dsg2 KO), generated via CRISPR-mediated knock-out. Compared to control cells, Dsg2 KO cells displayed a significant increase in spreading area on both fibronectin and collagen. As these experiments were performed in singly spreading cells, these experiments have identified a novel cell-autonomous, cell-cell adhesion-independent role for Dsg2 in regulation of cell spreading. Spreading changes in Dsg2 KO cells were dependent on Rap1 GTPase, as siRNA-mediated knockdown of Rap1 or pharmacological inhibition of Rap1 in Dsg2 KO cells rescued the increase in spreading area. Rap1 activity was significantly increased in Dsg2KO cells, but no change was observed in the activity of other GTPases tested (Rho, Rac and Cdc42). Changes in Dsg2-mediated cell spreading was also shown to require the Rap GEF PDZ-GEF2 (but not PDZ-GEF1 or other Rap GEFs), and PDZ-GEF2 localization at the cell periphery markedly increases upon loss of Dsg2. Further investigation into signaling pathways known to affect cell spreading identified a role for TGF-beta signaling, but neither Erk or p38 MAPK were involved. In singly-spreading cells, the expression of other desmosomal proteins is unchanged in Dsg2 KO cells (mRNA and protein levels), and the localization of Plakophilin-2, Plakoglobin and Desmoplakin is also not significantly perturbed (specifically in singly-spreading cells). Nevertheless, siRNA-mediated knockdown of DP was able to rescue Dg2KO-dependent increases in cell spreading (while siRNA for Plakophilin-2, Plakoglobin and Plakophilin-3 did not). In addition, DP siRNA was also able to rescue changes in localization of PDZ-GEF2 seen upon loss of Dsg2. Taken together, these data have identified novel cell-cell adhesion independent role for Dsg2 and DP in mediating cell spreading via Rap1 signaling, which provides significant insight into the signaling mechanisms via which desmosomal cadherins control cell-matrix attachment and cytoskeletal architecture.

Desmosomes have a central role in mediating extracellular adhesion between cells, but they also coordinate other biological processes such as proliferation, differentiation, apoptosis and migration. In particular, several lines of evidence have implicated desmosomal proteins in regulating the actin cytoskeleton and attachment to the extracellular matrix, indicating signaling crosstalk between cell–cell junctions and cell–matrix adhesions. In our study, we found that cells lacking the desmosomal cadherin Desmoglein-2 (Dsg2) displayed a significant increase in spreading area on both fibronectin and collagen, compared to control A431 cells. Intriguingly, this effect was observed in single spreading cells, indicating that Dsg2 can exert its effects on cell spreading independent of cell–cell adhesion. We hypothesized that Dsg2 may mediate cell–matrix adhesion via control of Rap1 GTPase, which is well known as a central regulator of cell spreading dynamics. We show that Rap1 activity is elevated in Dsg2 knockout cells, and that Dsg2 harnesses Rap1 and downstream TGFβ signaling to influence both cell spreading and focal adhesion protein phosphorylation. Further analysis implicated the Rap GEF PDZ-GEF2 in mediating Dsg2-dependent cell spreading. These data have identified a novel role for Dsg2 in controlling cell spreading, providing insight into the mechanisms via which cadherins exert non-canonical junction-independent effects.

Desmosomes are protein complexes crucial for maintaining cell-cell adhesion and integrity of tissues. These complexes are made up of proteins from three families: transmembrane cadherins (Desmoglein and Desmocollin) link adjacent cells in the extracellular space, armadillo proteins (Plakophilin and Plakoglobin) stabilize the intracellular plaque, and the cytolinker Desmoplakin (DP) connects the plaque to the intermediate filament network. Desmosomal proteins have also been shown to coordinate gene expression pathways required for processes such as proliferation, differentiation and cell migration. In particular, several lines of evidence have linked desmosomal proteins to gene expression of extracellular matrix (ECM) proteins. Loss of Plakophilin-2 or Desmoplakin causes increases in expression of fibronectin and collagen, while in contrast, loss of plakoglobin results in a significant decrease in expression of fibronectin. These data indicate that individual components of the desmosome control ECM gene expression via distinct cellular signaling networks. In our study, we sought to investigate the role of desmosomal cadherins in ECM gene expression, via use of A431 cells lacking Desmoglein-2, generated via CRISPR-mediated knock-out. Compared to control cells, Dsg2 KO cells demonstrated a dramatic increase in expression of Fibronectin (Fn1) mRNA, and relatively minor changes in expression of Collagen 1 (Col1a1) and Collagen 2 (Col2a1) mRNA. An increase in expression of Fibronectin protein levels in Dsg2-deficient cells was also observed via western blot. We show that loss of Dsg2 KO also caused a significant increase in mRNA expression of the pro-fibrotic signaling molecule transforming growth factor beta 2 (Tgfb2), but not Tgfb1 or Tgfb3. Increased expression of Fn1, Col1a1, Col2a1 and Tgfb2 was also observed upon siRNA-mediated knockdown of Dsg2 in both A431 cells and HaCaT keratinocytes, verifying that these changes are not clone-specific or due to off-target CRISPR effects. siRNA-mediated knockdown of other desmosomal proteins could not rescue Fn1 or Tgfb2 increases in Dsg2 KO cells, indicating that changes are not due to mislocalization of other desmosomal components. An extensive analysis of signaling pathways known to be involved in regulation of ECM gene expression uncovered an important role for PI3-Kinase and Akt signaling, as inhibition of either signaling protein resulted in a rescue of Dg2KO-mediated increases in Fn1 gene expression. Moreover, we have documented a significant increase in phosphorylation of Akt in Dsg2-deficient cells. Taken together, our study highlights a novel role for Dsg2 in mediating ECM gene expression via PI3-K/Akt signaling, adding significant insight into the mechanisms by which desmosomal cadherins control the adhesive behavior of cancer cells.

Desmoplakin (DP) is an obligate component of desmosomal cell-cell junctions that links the adhesion plaque to the cytoskeletal intermediate filament network. While a central role for DPin maintaining the structure and stability of the desmosome is well established, recent work has indicated that DP's functions may extend beyond cell-cell adhesion. In our study, we show that loss of DP results in a significant increase in cellular migration, as measured by scratch wound assays, Transwell migration assays, and invasion assays. Loss of DP causes dramatic changes in actin cytoskeleton morphology, including enhanced protrusiveness, and an increase in filopodia length and number. Interestingly, these changes are also observed in single cells, indicating that control of actin morphology is a cell-cell adhesion-independent function of DP. An investigation of cellular signaling pathways uncovered aberrant Rac and p38 mitogen-activated protein kinase (MAPK) activity in DP knockdown cells, restoration of which is sufficient to rescue DP-dependent changes in both cell migration and actin cytoskeleton morphology. Taken together, these data highlight a previously uncharacterized role for the desmosomal cytolinker DP in coordinating cellular migration via p38 MAPK and Rac signaling.

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.

Members of the desmosome protein family are integral components of the cardiac area composita, a mixed junctional complex responsible for electromechanical coupling between cardiomyocytes. In this study, we provide evidence that loss of the desmosomal armadillo protein Plakophilin-2 (PKP2) in cardiomyocytes elevates transforming growth factor β1 (TGF-β1) and p38 mitogen-activated protein kinase (MAPK) signaling, which together coordinate a transcriptional program that results in increased expression of profibrotic genes. Importantly, we demonstrate that expression of Desmoplakin (DP) is lost upon PKP2 knockdown and that restoration of DP expression rescues the activation of this TGF-β1/p38 MAPK transcriptional cascade. Tissues from PKP2 heterozygous and DP conditional knockout mouse models also exhibit elevated TGF-β1/p38 MAPK signaling and induction of fibrotic gene expression in vivo. These data therefore identify PKP2 and DP as central players in coordination of desmosome-dependent TGF-β1/p38 MAPK signaling in cardiomyocytes, pathways known to play a role in different types of cardiac disease, such as arrhythmogenic or hypertrophic cardiomyopathy.

Mechanisms by which microtubule plus ends interact with regions of cell-cell contact during tissue development and morphogenesis are not fully understood. We characterize a previously unreported interaction between the microtubule binding protein end-binding 1 (EB1) and the desmosomal protein desmoplakin (DP), and demonstrate that DP-EB1 interactions enable DP to modify microtubule organization and dynamics near sites of cell-cell contact. EB1 interacts with a region of the DP N terminus containing a hotspot for pathogenic mutations associated with arrhythmogenic cardiomyopathy (AC). We show that a subset of AC mutations, in addition to a mutation associated with skin fragility/woolly hair syndrome, impair gap junction localization and function by misregulating DP-EB1 interactions and altering microtubule dynamics. This work identifies a novel function for a desmosomal protein in regulating microtubules that affect membrane targeting of gap junction components, and elucidates a mechanism by which DP mutations may contribute to the development of cardiac and cutaneous diseases.

The process of fibrosis, described as accumulation of myofibroblasts and excessive deposition of extracellular matrix components, is a key development in the progression of multiple different types of cardiac disease. Nevertheless, little is known about the molecular mechanisms which cause the onset of fibrosis in cardiac disease. Fibrosis is a significant component of arrhythmogenic cardiomyopathy (AC), a genetic disorder characterized by replacement of healthy cardiomyocytes (CMs) with fibrous tissue, leading to arrhythmia and in certain cases, sudden death. AC is often characterized as a “disease of the desmosome”, as mutations for all obligate desmosome proteins have been found in cases of AC, including the desmosome armadillo proteins Plakophilin-2 (PKP2) and Plakoglobin (PG). PKP2 and PG are multi-functional proteins involved in both mechanical stabilization of the cardiac area composita, as well as mediation of desmosome-related signaling pathways. We have determined that loss of PKP2 or PG in neonatal CMs causes an aberrant increase in gene expression of pro-fibrotic stimuli such as transforming growth factor beta 1 (TGF-beta1) and Interleukin-6 (IL-6). In addition, p38 MAPK, a known mediator of inflammatory fibrosis, is activated upon loss of PKP2/PG. We hypothesize that mutation or loss of PKP2 or PG cause the recruitment and activation of cardiac fibroblasts via pro-fibrotic TGF-beta and p38MAPK signaling, resulting in pathological fibrosis characteristic of AC. Indeed, conditioned media from PKP2-silenced CMs causes an increase in fibronectin gene expression by freshly isolated cardiac fibroblasts. Our future experiments will investigate whether inhibition of TGF-beta or p38MAPK signaling can alleviate fibrotic gene production. By highlighting a novel link between desmosome armadillo proteins and pro-fibrotic signaling in cardiac tissue, this study provides mechanistic insights into the pathogenesis of AC, as well as advances our knowledge of potential therapeutic targets for combating fibrosis in multiple different types of heart disease or injury.

Genetic disorders of theRas/MAPK pathway, termed RASopathies, produce numerous abnormalities, including cutaneous keratodermas.The desmosomal cadherin, desmoglein-1 (DSG1), promotes keratinocyte differentiation by attenuating MAPK/ERK signaling and is linked to striate palmoplantar keratoderma (SPPK).This raises the possibility that cutaneous defects associated with SPPK and RASopathies share certain molecular faults.To identify intermediates responsible for executing the inhibition of ERK by DSG1, we conducted a yeast 2-hybrid screen.The screen revealed that Erbin (also known as ERBB2IP), a known ERK regulator, binds DSG1.Erbin silencing disrupted keratinocyte differentiation in culture, mimicking aspects of DSG1 deficiency.Furthermore, ERK inhibition and the induction of differentiation markers by DSG1 required both Erbin and DSG1 domains that participate in binding Erbin.Erbin blocks ERK signaling by interacting with and disrupting Ras-Raf scaffolds mediated by SHOC2, a protein genetically linked to the RASopathy, Noonan-like syndrome with loose anagen hair (NS/LAH).DSG1 overexpression enhanced this inhibitory function, increasing Erbin-SHOC2 interactions and decreasing Ras-SHOC2 interactions.Conversely, analysis of epidermis from DSG1-deficient patients with SPPK demonstrated increased Ras-SHOC2 colocalization and decreased Erbin-SHOC2 colocalization, offering a possible explanation for the observed epidermal defects.These findings suggest a mechanism by which DSG1 and Erbin cooperate to repress MAPK signaling and promote keratinocyte differentiation.

Although much is known about signaling factors downstream of Rho GTPases that contribute to epidermal differentiation, little is known about which upstream regulatory proteins (guanine nucleotide exchange factors [GEFs] or GTPase-activating proteins [GAPs]) are involved in coordinating Rho signaling in keratinocytes. Here we identify the GEF breakpoint cluster region (Bcr) as a major upstream regulator of RhoA activity, stress fibers, and focal adhesion formation in keratinocytes. Loss of Bcr reduced expression of multiple markers of differentiation (such as desmoglein-1 [Dsg1], keratin-1, and loricrin) and abrogated MAL/SRF signaling in differentiating keratinocytes. We further demonstrated that loss of Bcr or MAL reduced levels of Dsg1 mRNA in keratinocytes, and ectopic expression of Dsg1 rescued defects in differentiation seen upon loss of Bcr or MAL signaling. Taken together, these data identify the GEF Bcr as a regulator of RhoA/MAL signaling in keratinocytes, which in turn promotes differentiation through the desmosomal cadherin Dsg1.

BACKGROUND:Rho GTPases control many cellular processes, including cell survival, gene expression and migration. Rho proteins reside mainly in the cytosol and are targeted to the plasma membrane (PM) upon specific activation by guanine nucleotide exchange factors (GEFs). Accordingly, most GEFs are also cytosolic or associated with the PM. However, Net1, a RhoA-specific GEF predominantly localizes to the cell nucleus at steady-state. Nuclear localization for Net1 has been seen as a mechanism for sequestering the GEF away from RhoA, effectively rendering the protein inactive. However, considering the prominence of nuclear Net1 and the fact that a biological stimulus that promotes Net1 translocation out the nucleus to the cytosol has yet to be discovered, we hypothesized that Net1 might have a previously unidentified function in the nucleus of cells.PRINCIPAL FINDINGS:Using an affinity precipitation method to pulldown the active form of Rho GEFs from different cellular fractions, we show here that nuclear Net1 does in fact exist in an active form, contrary to previous expectations. We further demonstrate that a fraction of RhoA resides in the nucleus, and can also be found in a GTP-bound active form and that Net1 plays a role in the activation of nuclear RhoA. In addition, we show that ionizing radiation (IR) specifically promotes the activation of the nuclear pool of RhoA in a Net1-dependent manner, while the cytoplasmic activity remains unchanged. Surprisingly, irradiating isolated nuclei alone also increases nuclear RhoA activity via Net1, suggesting that all the signals required for IR-induced nuclear RhoA signaling are contained within the nucleus.CONCLUSIONS/SIGNIFICANCE:These results demonstrate the existence of a functional Net1/RhoA signaling pathway within the nucleus of the cell and implicate them in the DNA damage response.

What are desmosomes? Desmosomes are specialized adhesive protein complexes that localize to intercellular junctions and are responsible for maintaining the mechanical integrity of tissues. The term 'desmosome' was coined by Josef Schaffer in 1920 and has its origins in the Greek words for bond (desmo) and body (soma). Desmosomes are also known as maculae adherentes, which is Latin for 'adhering spot'. Unlike adherens junctions (AJs), which connect to the actin cytoskeleton network, desmosomal junctions are tethered to the intermediate filament network.

We have recently shown that a fraction of the total cellular pool of the small GTPase RhoA resides in the nucleus, and that the nuclear guanine nucleotide exchange factor (GEF) Net1 has a role in the regulation of its activity. In this protocol, we describe a method to measure both the activities of the nuclear pools of RhoA and Rho GEFs. This process required the development of a nuclear isolation protocol that is both fast and virtually free of cytosolic and membrane contaminants, as well as a redesign of existing RhoA and Rho GEF activity assays so that they work in nuclear samples. This protocol can be also used for other Rho GTPases and Rho GEFs, which have also been found in the nucleus. Completion of the procedure, including nuclear isolation and RhoA or Rho GEF activity assay, takes 1 h 40 min. We also include details of how to perform a basic assay of whole-cell extracts.