Mechanistic Insights of the LEMD2 p.L13R Mutation and Its Role in Cardiomyopathy

Illuminating Cardiac Remodeling: Insights From [18F]-Fluorodeoxyglucose Positron Emission Tomography Imaging in Plakoglobin-Associated Arrhythmogenic Cardiomyopathy

BACKGROUND

Arrhythmogenic cardiomyopathy (ACM) is a genetic heart muscle disease, which presents with arrhythmias and sudden cardiac death, along with progressive cardiac remodeling and myocardial inflammation. This study aims to elucidate the patterns of [18F]-fluorodeoxyglucose ([18F]-FDG) uptake in a mouse model of plakoglobin-associated cardiac disease to better understand its diagnostic potential.

METHODS AND RESULTS

Plakoglobin (Jup) knockout mice developed a cardiomyopathy that presented an ACM-like phenotype at 6 weeks of age. Flow cytometry experiments showed a significant increase of immune cells, for example, an expansion of proinflammatory and tissue-injury macrophages. In vivo positron emission tomography and ex vivo autoradiography showed increased [18F]-FDG uptake in genotype positive hearts. A correlative analysis between [18F]-FDG positivity and macrophage infiltration using CD68 and CD206 staining did not show colocalization. CD68 and CD206 positivity was primarily observed within the fibrotic scar, whereas [18F]-FDG uptake was predominantly identified in CD68 and CD206-negative tissue areas. Instead, [18F]-FDG signal seemed to originate from cardiomyocytes adjacent to areas of fibrotic remodeling. Morphometric analysis revealed hypertrophy of these cardiomyocytes, which may reflect metabolic remodeling as a compensatory response.

CONCLUSIONS

In our murine model of Jup-related ACM, strong cardiac [18F]-FDG uptake was detected, which colocalized with regional hypertrophic cardiomyocytes rather than inflammatory cells. These findings indicate that [18F]-FDG positron emission tomography is a valuable tool for identifying and localizing hypermetabolic areas associated with cardiac remodeling in ACM, providing insights into disease mechanisms and potential diagnostic strategies.

Gpr55 deficiency crucially alters cardiomyocyte homeostasis and counteracts angiotensin II induced maladaption in female mice

BACKGROUND AND PURPOSE

Cannabis stimulates several G-protein-coupled-receptors and causes bradycardia and hypotension upon sustained consumption. Moreover, in vitro studies suggest an interference of cannabinoid-signalling with cardiomyocyte contractility and hypertrophy. We aimed at revealing a functional contribution of the cannabinoid-sensitive receptor GPR55 to cardiomyocyte homeostasis and neurohumorally induced hypertrophy in vivo.

EXPERIMENTAL APPROACH

Gpr55-/- and wild-type (WT) mice were characterized after 28-day angiotensin II (AngII; 1·μg·kg-1 min-1) or vehicle infusion. In isolated adult Gpr55-/- and WT cardiomyocytes, mitochondrial function was assessed under naïve conditions, while cytosolic Ca2+ handling was additionally determined following application of the selective GPR55 antagonist CID16020046.

KEY RESULTS

Gpr55 deficiency did not affect angiotensin II (AngII) mediated hypertrophic growth, yet, especially in females, it alleviated maladaptive pro-hypertrophic and -inflammatory gene expression and improved inotropy and adrenergic responsiveness compared to WT. In-depth analyses implied increased cytosolic Ca2+ concentrations and transient amplitudes, and accelerated sarcomere contraction kinetics in Gpr55-/- myocytes, which could be mimicked by GPR55 blockade with CID16020046 in female WT cells. Moreover, Gpr55 deficiency up-regulated factors involved in glucose and fatty acid transport independent of the AngII challenge, accelerated basal mitochondrial respiration and reduced basal protein kinase (PK) A, G and C activity and phospholemman (PLM) phosphorylation.

CONCLUSIONS AND IMPLICATIONS

Our study suggests GPR55 as crucial regulator of cardiomyocyte hypertrophy and homeostasis presumably by regulating PKC/PKA-PLM and PKG signalling, and identifies the receptor as potential target to counteract maladaptation, adrenergic desensitization and metabolic shifts as unfavourable features of the hypertrophied heart in females.

Unveiling the roles of LEMD proteins in cellular processes

Proteins localized in the inner nuclear membrane (INM) engage in various fundamental cellular processes via their interactions with outer nuclear membrane (ONM) proteins and nuclear lamina. LAP2-emerin-MAN1 domain (LEMD) family proteins, predominantly positioned in the INM, participate in the maintenance of INM functions, including the reconstruction of the nuclear envelope during mitosis, mechanotransduction, and gene transcriptional modulation. Malfunction of LEMD proteins leads to severe tissue-restricted diseases, which may manifest as fatal deformities and defects. In this review, we summarize the significant roles of LEMD proteins in cellular processes, explains the mechanisms of LEMD protein-related diseases, and puts forward questions in less-explored areas like details in tissue-restricted phenotypes. It intends to sort out previous works about LEMD proteins and pave way for future researchers who might discover deeper mechanisms of and better treatment strategies for LEMD protein-related diseases.

A novel role for CSA in the regulation of nuclear envelope integrity: uncovering a non-canonical function

Cockayne syndrome (CS) is a premature ageing condition characterized by microcephaly, growth failure, and neurodegeneration. It is caused by mutations in ERCC6 or ERCC8 encoding for Cockayne syndrome B (CSB) and A (CSA) proteins, respectively. CSA and CSB have well-characterized roles in transcription-coupled nucleotide excision repair, responsible for removing bulky DNA lesions, including those caused by UV irradiation. Here, we report that CSA dysfunction causes defects in the nuclear envelope (NE) integrity. NE dysfunction is characteristic of progeroid disorders caused by a mutation in NE proteins, such as Hutchinson-Gilford progeria syndrome. However, it has never been reported in Cockayne syndrome. We observed CSA dysfunction affected LEMD2 incorporation at the NE and increased actin stress fibers that contributed to enhanced mechanical stress to the NE. Altogether, these led to NE abnormalities associated with the activation of the cGAS/STING pathway. Targeting the linker of the nucleoskeleton and cytoskeleton complex was sufficient to rescue these phenotypes. This work reveals NE dysfunction in a progeroid syndrome caused by mutations in a DNA damage repair protein, reinforcing the connection between NE deregulation and ageing.

LEMD2-associated progeroid syndrome: Expanding the phenotype of the nuclear envelopathy caused by a defect in LEMD2 gene

Nuclear envelopathies are rare genetic diseases that compromise the integrity of the nuclear envelope. Patients with a defect in LEM domain nuclear envelope protein 2 (LEMD2) leading to LEMD2-associated progeroid syndrome are exceedingly scarce in number, yet they exhibit shared clinical features including skeletal abnormalities and a prematurely-aged appearance. Our study broadens the understanding of LEMD2-associated progeroid syndrome by detailing its phenotypic and molecular characteristics in the first female and fourth reported case, highlighting a distinct impact on metabolic functions. The patient's history revealed growth delay, facial and skeletal abnormalities, and recurrent abdominal pain crises caused by hepatomegaly. Comparisons with the previously documented cases emphasized similarities in skeletal and facial features while showcasing unique variations, notably in cardiac and hepatic manifestations. In vitro experiments conducted on patient-derived peripheral blood and urinary epithelial cells and LEMD2-downregulated HepG2 cells confirmed abnormalities in the structure of the nuclear envelope in all three tissue-types. Overall, our work offers a comprehensive profile of a patient with LEMD2-related syndrome, emphasizing the hepatic involvement in the disease and broadening our understanding of clinical and molecular implications. This study not only contributes specific insights into LEMD2-related conditions but also underscores potential therapeutic paths for disorders affecting nuclear envelope dynamics.

Pervasive nuclear envelope ruptures precede ECM signaling and disease onset without activating cGAS-STING in Lamin-cardiomyopathy mice

Nuclear envelope (NE) ruptures are emerging observations in Lamin-related dilated cardiomyopathy, an adult-onset disease caused by loss-of-function mutations in Lamin A/C, a nuclear lamina component. Here, we test a prevailing hypothesis that NE ruptures trigger the pathological cGAS-STING cytosolic DNA-sensing pathway using a mouse model of Lamin cardiomyopathy. The reduction of Lamin A/C in cardio-myocyte of adult mice causes pervasive NE ruptures in cardiomyocytes, preceding inflammatory transcription, fibrosis, and fatal dilated cardiomyopathy. NE ruptures are followed by DNA damage accumulation without causing immediate cardiomyocyte death. However, cGAS-STING-dependent inflammatory signaling remains inactive. Deleting cGas or Sting does not rescue cardiomyopathy in the mouse model. The lack of cGAS-STING activation is likely due to the near absence of cGAS expression in adult cardiomyocytes at baseline. Instead, extracellular matrix (ECM) signaling is activated and predicted to initiate pro-inflammatory communication from Lamin-reduced cardiomyocytes to fibroblasts. Our work nominates ECM signaling, not cGAS-STING, as a potential inflammatory contributor in Lamin cardiomyopathy.

How nuclear envelope dynamics can direct laminopathy phenotypes

The nuclear envelope separates the genome from the cytoplasmic environment. However, the nuclear envelope is also physically associated with the genome and exerts influence on gene expression and genome modification. The nucleus is dynamic, changing shape and responding to cell movement, disassembling and assembling during cell division, and undergoing rupture and repair. These dynamics can be impacted by genetic disease, leading to a family of diseases called laminopathies. Their disparate phenotypes suggest that multiple processes are affected. We highlight three such processes here, which we believe can be used to classify most of the laminopathies. While much still needs to be learned, some commonalities between these processes, such as proteins involved in nuclear envelope formation and rupture repair, may drive a variety of laminopathies. Here we review the latest information regarding nuclear dynamics and its role in laminopathies related to mutations in the nuclear lamina and linker of nucleoskeleton and cytoskeleton complex (LINC) proteins.

Natural History of Dilated Cardiomyopathy Due to c.77T>C (p.Val26Ala) in Emerin Protein

(1) Introduction: Dilated cardiomyopathy (DCM) mainly affects young individuals and is the main indication of heart transplantation. The variant c.77T>C (p.Val26Ala) of the gene coding for emerin (EMD) in chromosome Xq28 has been catalogued as a pathogenic variant for the development of DCM, exhibiting an X-linked inheritance pattern. (2) Methods: A retrospective study was conducted covering the period 2015-2023 in patients with DCM of genetic origin. The primary endpoint was patient age at onset of the first composite major cardiac event, in the form of a first episode of heart failure, malignant ventricular arrhythmia, or end-stage heart failure, according to the presence of truncating variant in titin gene (TTNtv) versus the p.Val26Ala mutation in the EMD protein. (3) Results: A total of 31 and 22 patients were included in the EMD group and TTNtv group, respectively. The primary endpoint was significantly higher in the EMD group, with a hazard ratio of 4.16 (95% confidence interval: 1.83-9.46; p = 0.001). At 55 years of age, all the patients in the EMD group had already presented heart failure, nine presented malignant ventricular arrhythmia (29%), and 13 required heart transplantation (42%). (4) Conclusions: DCM secondary to the c.77T>C (p.Val26Ala) mutation in the EMD gene is associated to an increased risk of major cardiac events compared to patients with DCM due to TTNtv, with a large proportion of transplanted patients in the fifth decade of life.