Objective ·To investigate the cardiac cellular composition and molecular alterations in a dilated cardiomyopathy (DCM) mouse model carrying the Lamin A/C p.Asn195Lys (Lmna N195K) mutation, single-nucleus RNA sequencing (snRNA-seq) was employed to elucidate the mutation′s impact on the cardiac multicellular ecosystem. The pathological phenotypes were systematically characterized in terms of cell type proportions, gene expression profiles, and intercellular communication networks. Methods ·A homozygous Lmna N195K mutant mouse model was generated using clustered regularly interspaced short palindromic repeats-associated protein 9 (CRISPR-Cas9) technology, with wild-type littermates serving as controls. Cardiac function was evaluated by echocardiography and electrocardiography, while histological analyses, including hematoxylin and eosin (HE) staining, Sirius Red staining, wheat germ agglutinin (WGA) staining, and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay, were performed to assess tissue pathology. Nuclei were isolated from cardiac tissue for snRNA-seq, and Seurat (v4.4.0) was applied for cell clustering, differential expression analysis, and subpopulation identification. Pseudotime analysis was used to trace cell state transitions, and CellChat (v1.6.1) was employed to infer intercellular communication networks. Quantitative reverse transcription polymerase chain reaction (RT-qPCR) and Western blotting (WB) were conducted to assess the expression of fibrosis markers and LMNA proteins, respectively. Results ·Homozygous Lmna N195K mutant mice exhibited chamber dilation, impaired systolic function, severe cardiac fibrosis, and high mortality. Histological analysis revealed disorganized cardiomyocyte alignment, significantly increased fibrotic area, elevated apoptosis, and reduced LMNA protein expression in the mutant group. SnRNA-seq identified nine major cardiac cell types, including cardiomyocytes, endothelial cells, and fibroblasts. In the mutant group, the proportion of metabolically active cardiomyocytes (CM2) was markedly decreased (P=0.005), while contractile cardiomyocytes (CM3) were increased (P=0.004). Nearly all fibroblasts were converted into a profibrotic subpopulation FB4 (P=0.001). Among endothelial cells, the extracellular matrix (ECM)-secreting subpopulation EC3 was significantly expanded (P=0.007), suggesting the occurrence of endothelial-to-mesenchymal transition (EndoMT). Differential expression analysis revealed dysregulation of genes related to fibrosis, energy metabolism, and cardiac injury. Intercellular communication analysis indicated significant activation of receptor-ligand signaling pathways, including periostin(POSTN)-integrin, vascular endothelial growth factor A(VEGFA)-vascular endothelial growth factor receptor 1(VEGFR1), and Laminin Subunit Alpha 2(LAMA2)-Integrin signaling pathways. Conclusion ·The Lmna N195K mutation disrupts the cardiac multicellular ecosystem, characterized by cardiomyocyte subpopulation imbalance (decreased metabolic cells and increased contractile cells), fibroblast conversion to profibrotic phenotypes, and endothelial EndoMT with enhanced ECM secretion. These alterations, reinforced by aberrant intercellular communication networks, collectively drive fibrosis and pathological remodeling, ultimately leading to dilated cardiomyopathy. This study reveals the impact of Lmna N195K mutation on cardiac cell composition and function at the single cell level, offering critical insights into the cellular basis of LMNA related DCM.