Abstract Background Thoracic aortic aneurysm (TAA) is a life-threatening condition with an unpredictable lisk of rupture. Current clinical parameters have limited ability to accurately predict imminent rupture. Osteopontin (OPN) has been implicated in aortic aneurysm pathology, however, it role as a marker of imminent rupture remains. unclear. We investigated the dynamics of OPN expression dynamics in a mouse model with predictable rupture timing and validated our findings in human TAA. Methods One-month-old fibrillin-1 hypomorphic ( Fbn1 mgR/mgR ) mice were used as a TAA model; with wild-type (WT) mice served as controls. Angiotensin II (AngII) was administered to Fbn1 mgR/mgR to induce acute aortic rupture. Single-section transcriptome analysis and immunofluorescence staining were performed on ascending aortic tissue at 24 and 72 hours after AngII infusion, with pre-treatment Fbn1 mgR/mgR and WT mice serving as controls. To determine conservation in human disease, we reanalyzed publicly available single-cell RNA sequencing data from ascending thoracic aortic aneurysm (ATAA) patients. Results AngII infusion induced progressive mortality beginning at 24 hours, with approximately 60% survival at 72 hours and nearly no survival by 8 days in Fbn1 mgR/mgR mice. At this pre-rupture time point, OPN showed prominent upregulation at both mRNA and protein levels in ascending aortic tissues compared to controls. Immunofluorescence staining revealed increased OPN expression in the aortic wall, particularly in regions exhibiting structural deterioration. Reanalysis of human ATAA single-cell data showed elevated OPN expression compared to controls, with enrichment in immune cell populations, especially macrophages. Within the macrophage compartment, subcluster analysis identified a stress-responsive subpopulation (MC1) that was markedly expanded and almost exclusively composed of ATAA-derived cells, representing the primary source of OPN upregulation. Conclusions OPN upregulation represents a conserved molecular signature of the pre-rupture state in TAA across mice and humans. Our mode, which enables predictable rupture timing, allowed the capture of acute pre-rupture molecular changes, suggesting OPN as a potential biomarker for predicting imminent aortic rupture.

Mammals exhibit a circadian rhythm of bladder capacity, but whether this rhythm can be maintained through intrinsic bladder mechanisms independently of central clock-driven behavioral rhythms remains unclear. Here, we addressed this question using Bmal1 conditional knockout mice lacking central clock function in the brain. Under constant darkness, these mice lost behavioral rhythmicity but largely retained circadian variation in bladder capacity together with bladder clock gene expression. Transcriptomic analyses further identified diurnal variation in bladder extracellular matrix-related factors, suggesting a potential molecular basis for bladder capacity rhythmicity. These findings indicate that the bladder harbors a peripheral circadian mechanism that contributes to the circadian regulation of bladder capacity and provides insight into the role of peripheral clock mechanisms in bladder pathophysiology.

Abstract ECM composition and organization are greatly altered during inflammation but it is still elusive if ECM dynamics may protect tissue stem cells against aberrant inflammation. Fibulin 7 (encoded by Fbln7 ) is part of the basement membrane ECM where epidermal stem cells (EpSCs) reside. It supports the long-term potential of fast-cycling EpSCs and moderates aging-related inflammatory markers in keratinocytes. Here, we assessed fibulin 7’s role during imiquimod (IMQ)-induced inflammation in 1-year-old mouse dorsal skin. We found that loss of Fbln7 aggravates epidermal inflammation, marked by increased epidermal thickness, proliferation, and phosphorylation of JNK (c-Jun N-terminal kinase). Fast-cycling EpSCs labeled with Slc1a3 -creER-TdTomato demonstrated that IMQ-induced proliferation in Fbln7 KO mice is contributed by cell divisions in the suprabasal layers, a hallmark of inflammatory epidermal responses. EpSC transcriptomes further reveal IMQ-modulated genes that are more substantially affected in Fbln7 KO mice, including IL-17 pathway-related genes known in psoriasis pathogenesis. Mechanistically, fibulin 7 directly binds to IL-17A and decreases IL-17A-mediated p38 MAPK activation. In public human psoriasis datasets, FBLN7 is reduced in lesional skin compared with non-lesional or normal skin, and it is significantly correlated with common psoriasis-associated genes. Altogether, fibulin 7 is potentially beneficial to protect against skin inflammation.

Macrophages are highly dynamic cells that maintain tissue homeostasis by regulating both initiation and resolution of inflammation. During efferocytosis, macrophages recognize the 'eat me' signal, phosphatidylserine (PS), exposed at the surface of apoptotic cells, leading to the resolution of inflammation and acquisition of a pro-resolving phenotype. Inspired by this endogenous mechanism, PS-based biomaterials have demonstrated immunomodulatory potential. However, the molecular mechanisms underlying PS-mediated macrophage reprogramming remain poorly understood. Here, submicron PS-exposing polymeric particles (PSPs; ~300 nm) were developed to improve the suitability of PSP formulations for future systemic administration while preserving their immunomodulatory activity. PSPs were efficiently internalized by macrophages through both actin- and dynamin-dependent pathways. PSP treatment significantly reduced IL-6 and IL-12p70 production in LPS-stimulated macrophages, whereas induction of the classical anti-inflammatory M2 marker CD206 was limited. Transcriptomic analysis revealed coordinated attenuation of inflammatory signaling pathways, including downregulation of Myd88, Nfkb1, Rel, and Irf8, together with activation of NRF2-associated antioxidant pathways characterized by increased expression of Nfe2l2, Hmox1, Prdx1, Gclm, and Gclc. Activation of antioxidant-associated genes together with reduced Irf8 expression suggests that PSP promotes inflammatory resolution through coordinated redox adaptation and selective attenuation of inflammatory signaling. Collectively, these findings provide mechanistic insight into PS-mediated macrophage reprogramming and support the future development of systemically administered therapies for chronic inflammatory diseases, including vascular inflammatory disorders.

Aortic dissection (AD) is characterized by separation within the medial layers of the aortic wall. Pathogenic variants in the fibrillin-1 gene (FBN1), which cause Marfan syndrome, represent a major genetic cause of AD. In a recently established Fbn1G234D/G234D mouse model, intimomedial tears develop at 3 weeks of age, and 50% of mice die by 5 weeks from aortic rupture. Despite this severe phenotype, the magnitude and expansion of AD lesions, as well as the molecular alterations within the medial layers remain incompletely understood. In this study, we used three-dimensional propagation-based X-ray phase-contrast synchrotron imaging for reconstruction of the ascending aortas, together with single-cell RNA sequencing (scRNA-seq) analysis in Fbn1G234D/G234D mice. Synchrotron imaging revealed 1-2 elastic lamellar breaks evolved into widespread disruptions spanning the entire elastic lamellae, accompanied by localized adventitial thickening. scRNA-seq analysis followed by immunofluorescence staining showed upregulation of fibronectin (Fn1) in Fbn1G234D/G234D smooth muscle cells (SMCs). Consistently, increased FN1 expression was observed in human non-heritable AD samples. Furthermore, enhanced expression of fibronectin receptors and activation of focal adhesion kinase signaling suggested augmented extracellular matrix-SMC interactions during disease progression. These findings indicate that AD progression involves coordinated medial structural failure, adventitial remodeling, and fibronectin-associated SMC dysfunction.

Adenosine is essential for energy metabolism and acts as a neuromodulator in the central nervous system. The retina is a highly energy-demanding neural tissue, and dysregulation of adenosine metabolism contributes to retinal diseases. Although adenosine pathways form extensive compensatory networks, the dynamics of ecto-5 '-nucleotidase (CD73), a key enzyme for extracellular adenosine generation, remain elusive. Here, we map its spatiotemporal profile from development to adulthood using two transgenic mouse lines. We found that CD73 became expressed in the rod-photoreceptor lineage by postnatal day (P) 3 and appeared in the inner nuclear layer from P7 onward. Moreover, CD73 was transiently expressed in the astrocyte lineage between embryonic day 16.5 and P3, and the descendants accumulated in the retinal periphery. Functionally, CD73 deletion delayed rod-response recovery and shortened the implicit time in scotopic electroretinography under dim light. These findings suggest that physiological cues such as light and hypoxia may influence CD73 expression and retinal function. Our work lays the groundwork for investigating how genetic and environmental risk factors may alter adenosine metabolism in retinal diseases.

Abstract Myofibrillar myopathy 6 is a rare, autosomal-dominant neuromuscular disorder caused by an amino acid exchange Pro209Leu in the co-chaperone BAG3, which disrupts muscle protein turnover and causes severe muscle weakness and shortened lifespan. We generated transgenic mice overexpressing the human mutant BAG3P209L-GFP, which rapidly develop skeletal muscle weakness unlike controls expressing BAG3WT-GFP. Here we show that mutant mice exhibit sarcomere breakdown, inflammation, protein aggregates, centralized nuclei and mitochondrial defects in their skeletal muscles, thereby reducing contraction force by ~90%. Omics profiling uncovered impaired protein synthesis, blocked autophagy, impaired mitophagy and loss of sarcomere proteins. Pathway modulation in vitro and in vivo showed autophagy dysfunction as the primary driver for the pathology, while BAG3 knockdown gene therapy markedly restored muscle function in vivo. In summary, this model recapitulates core disease features, revealing how BAG3 aggregates and loss of BAG3 function impair autophagy to drive muscle degeneration.

Adenosine is essential to energy metabolism and neuromodulation in the central nervous system. The retina is a highly energy-demanding neural tissue, and dysregulation of adenosine signaling causes retinal diseases. However, the dynamics of ecto-5’-nucleotidase (CD73), a key enzyme for extracellular adenosine generation, remain elusive. Here, we investigate its spatiotemporal profile from development to adulthood using two transgenic mouse lines. We found that CD73 is transiently expressed in the early astrocyte lineage (Embryonic day 16.5 to Postnatal day [P] 3), becomes prominent in the rod-photoreceptor lineage by P3, and appears in the inner nuclear layer from P7 onward. CD73 deletion delays rod-response recovery and shortens the implicit time in scotopic ERG under dim light. These findings indicate that light and other physiological cues influence CD73 expression, which has significant consequences for retinal function, thereby providing a foundation for exploring disease-related alterations in CD73 and designing therapies that restore adenosine homeostasis. ### Competing Interest Statement The authors have declared no competing interest. All data generated or analysed during this study are included in the manuscript and supporting files. Japan Society for the Promotion of Science, JP22K16940, JP22K09346 Terumo (Japan) Uehara Memorial Foundation, https://ror.org/00gc20a07 Novartis Foundation, https://ror.org/04f9t1x17 Inamori Foundation, https://ror.org/00ht8je70 SENSHIN Medical Research Foundation Japan Agency for Medical Research and Development, 23bm1123032h0001

BACKGROUND:Aortic dissection (AD) is the separation of medial layers of the aorta and is a major cause of death in patients with connective tissue disorders such as Marfan syndrome. However, molecular triggers instigating AD, its temporospatial progression, and how vascular cells in each vessel layer interact and participate in the pathological process remain incompletely understood. To unravel the underlying molecular mechanisms of AD, we generated a spontaneous AD mouse model. METHODS:We incorporated a novel missense variant (p.G234D) in FBN1, the gene for fibrillin-1, identified in a patient with nonsyndromic familial AD into mice using the CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9) system. We performed molecular pathological analyses of the aortic lesions by histology, immunofluorescence staining, electron microscopy, synchrotron-based imaging, and single-cell RNA sequencing. Biochemical analysis was performed to examine the binding capacity of mutant human FBN1G234D (fibrillin 1 Gly234Asp) protein to LTBPs (latent TGFβ [transforming growth factor-beta] binding proteins), and signaling pathways in the mutant aortic wall were examined by the Western blot analysis. RESULTS:Fifty percent of the Fbn1G234D/G234D mutant mice died within 5 weeks of age from multiple intimomedial tears that expanded longitudinally and progressed to aortic rupture accompanied by massive immune cell infiltration. Fbn1G234D/G234D endothelial cells exhibited altered mechanosensing with loss of parallel alignment to blood flow and upregulation of VCAM-1 (vascular cell adhesion molecule-1) and ICAM-1 (intercellular adhesion molecule-1) as early as 1 week of age. Single-cell RNA sequencing, validated by immunostaining, revealed a cluster of monocyte/macrophage predominantly in the intima at 3 weeks of age before the dissection, and the second cluster of macrophages increased during the progression of intimomedial tears, exhibiting strong CCR2+ (C-C motif chemokine receptor 2 positive) and both M1- and M2-like features. Consistently, upregulation of MMP2/9 (matrix metalloproteinase 2 and 9) was observed. Biochemically, FBN1G234D lost the ability to bind to LTBP-1, -2, and -4, resulting in the downregulation of TGFβ signaling in the aortic wall. CONCLUSIONS:We show that interactions involving endothelial cells and macrophages/monocytes in the intima, where the extracellular matrix (ECM) microenvironment contains reduced TGFβ signaling, contribute to the initiation of AD. Our novel AD mouse model provides a unique opportunity to identify target molecules involved in the intimomedial tears that can be utilized for the development of therapeutic strategies.

Background Aortic dissection (AD) is the separation of medial layers of the aorta and is a major cause of death in patients with connective tissue disorders such as Marfan syndrome. However, molecular triggers instigating AD, its temporospatial progression, and how vascular cells in each vessel layer interact and participate in the pathological process remain incompletely understood. To unravel the underlying molecular mechanism of AD, we generated a spontaneous AD mouse model.Methods We incorporated a novel missense variant (p.G234D) in FBN1 , the gene for fibrillin-1, identified in a non-syndromic familial AD patient into mice using CRISPR/Cas9 system. We performed histopathological analyses of the aortic lesions by histology, immunofluorescence staining, electron microscopy, synchrotron-based imaging and single-cell (sc)RNA-sequencing. Biochemical analysis was performed to examine the binding capacity of mutant human FBN1G234D protein to latent Tgfβ binding proteins (LTBPs), and signaling pathways in the mutant aortic wall were examined by western blot analysis.Results 50% of the Fbn1 G234D/G234D mutant mice died within 5 weeks of age from multiple intimomedial tears that expanded longitudinally and progressed to aortic rupture accompanied by massive immune cell infiltration. scRNA-sequencing, validated by immunostaining, revealed a significant increase in MHC class II-positive pro-inflammatory macrophages and monocytes at the site of intima tears with upregulation of MMP2/9 and marked disruption of elastic lamina. Subendothelial matrices, such as type IV collagen and laminin, expanded into the medial layer, where fibronectin expression was highly upregulated. Fbn1 G234D/G234D endothelial cells exhibited altered mechanosensing with loss of parallel alignment to blood flow and upregulation of VCAM-1 and ICAM-1, all of which likely contributed to the infiltration of immune cells. Biochemically, FBN1G234D lost the ability to bind to latent TGFβ binding protein (LTBP)-1, -2, and -4, resulting in the downregulation of TGFβ signaling in the aortic wall.Conclusions We show that dynamic interactions involving endothelial cells (ECs) and macrophages/monocytes in the intima, where the ECM microenvironment is altered with the reduced TGFβ signaling, contributes to the initiation of AD. Our novel AD mouse model provides a unique opportunity to identify target molecules involved in the intimomedial tears that can be utilized for development of therapeutic strategies.### Competing Interest StatementThe authors have declared no competing interest.* AD : aortic dissection TAA : thoracic aortic aneurysm TAAD : thoracic aortic aneurysm and dissection AA : aortic aneurysm FBN1 : fibrillin1 TGFβ : transforming growth factor-beta LTBP : latent TGFβ binding protein ECM : extracellular matrix EC : endothelial cell SMC : smooth muscle cell

Abstract Despite widespread adoption of tissue clearing techniques in recent years, poor access to suitable light-sheet fluorescence microscopes remains a major obstacle for biomedical end-users. Here, we present descSPIM (desktop-equipped SPIM for cleared specimens), a low-cost ($20,000–50,000), low-expertise (one-day installation by a non-expert), yet practical do-it-yourself light-sheet microscope as a solution for this bottleneck. Even the most fundamental configuration of descSPIM enables multi-color imaging of whole mouse brains and a cancer cell line-derived xenograft tumor mass for the visualization of neurocircuitry, assessment of drug distribution, and pathological examination by false-colored hematoxylin and eosin staining in a three-dimensional manner. Academically open-sourced ( https://github.com/dbsb-juntendo/descSPIM ), descSPIM allows routine three-dimensional imaging of cleared samples in minutes. Thus, the dissemination of descSPIM will accelerate biomedical discoveries driven by tissue clearing technologies.

Fibrillin-1, an extracellular matrix (ECM) protein encoded by the FBN1 gene, serves as a microfibril scaffold crucial for elastic fiber formation and homeostasis in pliable tissue such as the skin. Aside from causing Marfan syndrome, some mutations in FBN1 result in scleroderma, marked by hardened and thicker skin which limits joint mobility. Here, we describe a tight skin phenotype in the Fbn1G234D/G234D mice carrying a corresponding variant of FBN1 in the hybrid1 domain that was identified in a patient with familial aortic dissection. Unlike scleroderma, skin thickness and collagen fiber abundance do not change in the Fbn1G234D/G234D mutant skin. Instead, increased collagen cross-links were observed. In addition, short elastic fibers were sparsely located underneath the panniculus muscle layer, and an abundance of thin, aberrant elastic fibers was increased within the subcutaneous fascia, which may have tightened skin attachment to the underlying skeletal muscle. Structurally, Fbn1G234D/G234D microfibrils have a disrupted shoulder region that shares similarities with hybrid1 deletion mutant microfibrils. We then demonstrate the consequence of fibrillin-1 G234D mutation on dermal fibroblast functions. Mutant primary fibroblasts produce fewer elastic fibers, exhibit slower migration and increased cell stiffness. Moreover, secretome from mutant fibroblasts are marked by enhanced secretion of ECM, ECM-modifying enzymes, proteoglycans and cytokines, which are pro-tissue repair/fibrogenic. The transcriptome of mutant fibroblasts displays an increased expression of myogenic developmental and immune-related genes. Our study proposes that imbalanced ECM homeostasis due to a fibrillin-1 G234Dmutation impacts fibroblast properties with potential ramifications on skin function.

Background: Marfan syndrome (MFS) is a genetic disorder caused by mutations in the gene encoding theextracellular matrix Fibrillin-1 (FBN1), resulting in systemic connective tissue abnormalities. The mostcommon life-threatening conditions are aortic rupture and/or dissection. Aortic dissections show a highmortality rate, but the details of molecular triggers are unknown, particularly in acute aortic dissection(AAD). Raman spectroscopy is a technique for analyzing molecular structures from Raman spectra. It hasrecently attracted attention in the biology field because it can non-invasively distinguish between diseaseand non-disease areas. Objectives: This study aimed to identify the signals that induce changes in the aortic wall prior to acuteaortic dissection using genetic analysis and Raman spectroscopy with preservation of positional information. Methods: An acute aortic dissection model was created by using MFS model mice (Fbn1 mgR/mgR ) byadministration of angiotensin II (AngII) via osmotic pump (Fbn1 mgR/mgR -AAD). Aortas from wild-type(WT), Fbn1 mgR/mgR and Fbn1 mgR/mgR -AAD mice were harvested, and frozen sections were prepared. Thevascular wall of each mouse was assessed histologically, followed by spatial transcriptomic analysis andRaman microscopy measurements. Multivariate data analysis was performed on the obtained Ramanspectra. Mass spectrometry (MS) imaging was utilized for lipidome analysis. Results: Label-free imaging of extracellular substrates (collagen fibres, elastic fibres, versican, andaggrecan), lipids, and cells was produced in WT, Fbn1 mgR/mgR and Fbn1 mgR/mgR - AAD aortic tissue. Spatialtranscriptomic analysis identified differentially expressed genes among these aortas. We identified markedlipid accumulations in the acute aortic dissection models with Raman and MS imaging.

Mesenchymal stem cells (MSCs) are multipotent and considered to be of great potential for regenerative medicine. We could show recently (Breitbach, Kimura et al. 2018) that a subpopulation of MSCs as well as sinusoidal endothelial cells (sECs) in the bone marrow (BM) of CD73-EGFP reporter mice could be labeled in vivo. We took advantage of this model to explore the plasticity and osteogenic potential of CD73-EGFP(+) MSCs in vitro and their role in the regenerative response upon bone lesion in vivo. Herein we show that isolated CD73EGFP(+) MSCs displayed more pronounced stemness and stronger in vitro differentiation capacity into the osteogenic lineage compared to CD73-EGFP MSCs. In a bone fracture model, endogenous BM-resident CD73-EGFP(+) MSCs were found to migrate to the fracture site and differentiate into cartilage and bone cells. Our analysis also showed that CD73-EGFP(+) sECs contributed to the neovascularization of the fracture site. In addition, grafting of CD73-EGFP(+) MSCs into acute bone lesions revealed their capacity to differentiate into chondrocytes and osteocytes in vivo and their contribution to callus formation in the regeneration process of fracture healing. Thus, CD73(+) MSCs display enhanced stemness and osteogenic differentiation potential in vitro and in vivo illustrating a prominent role of the CD73(+) MSC subpopulation to promote fracture repair.

The maladaptive remodeling of vessel walls with neointima formation is a common feature of proliferative vascular diseases. It has been proposed that neointima formation is caused by the dedifferentiation of mature smooth muscle cells (SMCs). Recent evidence suggests that adventitial cells also participate in neointima formation; however, their cellular dynamics are not fully understood. In this study, we utilized a lineage tracing model of platelet-derived growth factor receptor alpha (PDGFRa) cells and examined cellular behavior during homeostasis and injury response. PDGFRa marked adventitial cells that were largely positive for Sca1 and a portion of medial SMCs, and both cell types were maintained for 2 years. Upon carotid artery ligation, PDGFRa-positive (+) cells were slowly recruited to the neointima and exhibited an immature SMC phenotype. In contrast, in a more severe wire denudation injury, PDGFRa+ cells were recruited to the neointima within 14 days and fully differentiated into SMCs. Under pressure overload induced by transverse aortic constriction, PDGFRa+ cells developed marked adventitial fibrosis. Taken together, our observations suggest that PDGFRa+ cells serve as a reservoir of adventitial cells and a subset of medial SMCs and underscore their context-dependent response to vascular injuries.

An amino acid exchange (P209L) in the HSPB8 binding site of the human co-chaperone BAG3 gives rise to severe childhood cardiomyopathy. To phenocopy the disease in mice and gain insight into its mechanisms, we generated humanized transgenic mouse models. Expression of human BAG3P209L-eGFP in mice caused Z-disc disintegration and formation of protein aggregates. This was accompanied by massive fibrosis resulting in early-onset restrictive cardiomyopathy with increased mortality as observed in patients. RNA-Seq and proteomics revealed changes in the protein quality control system and increased autophagy in hearts from hBAG3P209L-eGFP mice. The mutation renders hBAG3P209L less soluble in vivo and induces protein aggregation, but does not abrogate hBAG3 binding properties. In conclusion, we report a mouse model mimicking the human disease. Our data suggest that the disease mechanism is due to accumulation of hBAG3P209L and mouse Bag3, causing sequestering of components of the protein quality control system and autophagy machinery leading to sarcomere disruption.

The 5,6,7,8-tetrahydroimidazo[1,2-a]pyrazine derivative BIM-46174 and its dimeric form BIM-46187 (1) are heterocyclized dipeptides that belong to the very few cell-permeable compounds known to preferentially silence G alpha(q)proteins. To explore the chemical space of G alpha(q)inhibitors of the BIM chemotype, a combinatorial approach was conducted towards a library of BIM molecules. This library was evaluated in a second messenger-based fluorescence assay to analyze the activity of G alpha(q)proteins through the determination of intracellularmyo-inositol 1-phosphate. Structure-activity relationships were deduced and structural requirements for biological activity obtained, which were (i) a redox reactive thiol/disulfane substructure, (ii) anN-terminal basic amino group, (iii) a cyclohexylalanine moiety, and (iv) a bicyclic skeleton. Active compounds exhibited cellular toxicity, which was investigated in detail for the prototypical inhibitor1. This compound affects the structural cytoskeletal dynamics in a G alpha(q/11)-independent manner.

The co-chaperone BAG3 (Bcl-2 associated athanogene 3) is strongly expressed in cross-striated muscles and plays a key role in the turnover of muscle-proteins as a member of the CASA (chaperone-assisted selected autophagy) complex. An amino acid exchange (P209L) in the human BAG3 gene, caused by a single base mutation, gives rise to a severe dominant childhood muscular dystrophy, restrictive cardiomyopathy, and respiratory insufficiency. To get deeper insights into the pathophysiological mechanisms of the disease, we generated a transgenic mouse model of the human mutation BAG3 P209L , in which a fusion protein consisting of the human BAG3 P209L and the green fluorescent protein eGFP can be conditionally overexpressed. Ubiquitous overexpression of BAG3 P209L -eGFP leads to a severe phenotype between the second and fourth week of life, including decreased body weight, skeletal muscle weakness, and heart failure. Echocardiography revealed that the BAG3 P209L -mice suffer from restrictive cardiomyopathy and Sirius-red-staining of heart tissue showed extensive fibrosis. In cardiomyocytes, isolated from hearts of transgenic mice overexpressing BAG3 wt -eGFP or BAG3 P209L -eGFP, BAG3 wt -eGFP stringently localizes to sarcomeres and intercalated discs, whereas cardiomyocytes from BAG3 P209L -eGFP mice displayed formation of BAG3 containing aggregates and disruption of sarcomeres in vivo . While BAG3 P209L -eGFP binding to á-Hsp70, Filamin C and á-HspB8 was unchanged it was less soluble than BAG3 and had a tendency to aggregate, thereby sequestering BAG3 and its clients. Depletion of the BAG3 pool leads to an impairment of CASA and accumulation of damaged proteins, causing sarcomere disintegration leading to restrictive cardiomyopathy.