The Dendritic Cell Receptor DNGR-1 Promotes the Development of Atherosclerosis in Mice.

RATIONALE: Necrotic core formation during the development of atherosclerosis is associated with a chronic inflammatory response and promotes accelerated plaque development and instability. However, the molecular links between necrosis and the development of atherosclerosis are not completely understood. Clec9a (C-type lectin receptor) or DNGR-1 (dendritic cell NK lectin group receptor-1) is preferentially expressed by the CD8α+ subset of dendritic cells (CD8α+ DCs) and is involved in sensing necrotic cells. We hypothesized that sensing of necrotic cells by DNGR-1 plays a determinant role in the inflammatory response of atherosclerosis. OBJECTIVE: We sought to address the impact of total, bone marrow-restricted, or CD8α+ DC-restricted deletion of DNGR-1 on atherosclerosis development. METHODS AND RESULTS: We show that total absence of DNGR-1 in Apoe (apolipoprotein e)-deficient mice (Apoe-/-) and bone marrow-restricted deletion of DNGR-1 in Ldlr (low-density lipoprotein receptor)-deficient mice (Ldlr-/-) significantly reduce inflammatory cell content within arterial plaques and limit atherosclerosis development in a context of moderate hypercholesterolemia. This is associated with a significant increase of the expression of interleukin-10 (IL-10). The atheroprotective effect of DNGR-1 deletion is completely abrogated in the absence of bone marrow-derived IL-10. Furthermore, a specific deletion of DNGR-1 in CD8α+ DCs significantly increases IL-10 expression, reduces macrophage and T-cell contents within the lesions, and limits the development of atherosclerosis. CONCLUSIONS: Our results unravel a new role of DNGR-1 in regulating vascular inflammation and atherosclerosis and potentially identify a new target for disease modulation

SRF, un facteur de transcription crucial dans la physiologie des muscles squelettiques (Contribution au vieillissement et à l'hypertrophie)

Afin de déterminer le rôle de SRF dans la physiologie du muscle adulte, nous avons créé un modèle murin d'invalidation inductible de SRF dans les muscles squelettiques. Après induction de la perte de SRF, les muscles mutants présentent des altérations similaires à celles observées lors du vieillissement musculaire. L'expression de SRF diminuant de façon drastique avec le temps dans des muscles contrôles, cette perte de SRF avec l'âge pourrait contribuer au processus naturel du vieillissement. Afin d'évaluer le rôle de SRF dans l'hypertrophie, j'ai soumis les muscles de souris contrôles et mutantes à une surcharge et montré que seuls les muscles contrôles présentent une réponse hypertrophique. Ce défaut d'hypertrophie dans les muscles mutants est due à une altération de la prolifération et de la fusion des cellules satellites, SRF contrôlant de façon paracrine l'expression de IL6 et IL4. Nos résultats montrent que SRF est impliqué dans le maintien et l'hypertrophie musculaire.To investigate SRF function in adult skeletal muscle physiology, we developed a myofiber-specific and tamoxifen-inducibie SRF Knockout mice model. After induction of SRF loss, mutant muscles exhibits similar alterations to those observed during muscle aging. We also observed an important age-associated decrease in SRF expression in control muscles. Thus SRF loss with age could contribute to the natural muscle aging process. To assess the role of SRF during hypertrophy, I submitted muscles of mutant and control mice to an overload-induced hypertrophy and showed that only controls muscles show a hypertrophic response. The lack of hypertrophy in mutant muscles is due to an impairment of satellite cells proliferation and fusion. In fact, SRF enhance hypertrophy through the control of IL6 and IL4 expression in a paracrine fashion. Our results show that SRF is involved in skeletal muscle maintenance and hypertrophy.PARIS5-BU Méd.Cochin (751142101) / SudocSudocFranceF

New Role for Serum Response Factor in Postnatal Skeletal Muscle Growth and Regeneration via the Interleukin 4 and Insulin-Like Growth Factor 1 Pathways

Serum response factor (SRF) is a crucial transcriptional factor for muscle-specific gene expression. We investigated SRF function in adult skeletal muscles, using mice with a postmitotic myofiber-targeted disruption of the SRF gene. Mutant mice displayed severe skeletal muscle mass reductions due to a postnatal muscle growth defect resulting in highly hypotrophic adult myofibers. SRF-depleted myofibers also failed to regenerate following injury. Muscles lacking SRF had very low levels of muscle creatine kinase and skeletal alpha-actin (SKA) transcripts and displayed other alterations to the gene expression program, indicating an overall immaturity of mutant muscles. This loss of SKA expression, together with a decrease in beta-tropomyosin expression, contributed to myofiber growth defects, as suggested by the extensive sarcomere disorganization found in mutant muscles. However, we observed a downregulation of interleukin 4 (IL-4) and insulin-like growth factor 1 (IGF-1) expression in mutant myofibers which could also account for their defective growth and regeneration. Indeed, our demonstration of SRF binding to interleukin 4 and IGF-1 promoters in vivo suggests a new crucial role for SRF in pathways involved in muscle growth and regeneration

Premature aging in skeletal muscle lacking serum response factor

International audienceAging is associated with a progressive loss of muscle mass, increased adiposity and fibrosis that leads to sarcopenia. At the molecular level, muscle aging is known to alter the expression of a variety of genes but very little is known about the molecular effectors involved. SRF (Serum Response Factor) is a crucial transcription factor for muscle-specific gene expression and for post-natal skeletal muscle growth. To assess its role in adult skeletal muscle physiology, we developed a post-mitotic myofiber-specific and tamoxifen-inducible SRF knockout model. Five months after SRF loss, no obvious muscle phenotype was observed suggesting that SRF is not crucial for myofiber maintenance. However, mutant mice progressively developed IIB myofiber-specific atrophy accompanied by a metabolic switch towards a more oxidative phenotype, muscular lipid accumulation, sarcomere disorganization and fibrosis. After injury, mutant muscles exhibited an altered regeneration process, showing smaller regenerated fibers and persistent fibrosis. All of these features are strongly reminiscent of abnormalities encountered in aging skeletal muscle. Interestingly, we also observed an important age associated decrease in SRF expression in mice and human muscles. Altogether, these results suggest that a naturally occurring SRF down-regulation precedes and contributes to the muscle aging process. Indeed, triggering SRF loss in the muscles of mutant mice results in an accelerated aging process

Srf-Dependent Paracrine Signals Produced by Myofibers Control Satellite Cell-Mediated Skeletal Muscle Hypertrophy

SummaryAdult skeletal muscles adapt their fiber size to workload. We show that serum response factor (Srf) is required for satellite cell-mediated hypertrophic muscle growth. Deletion of Srf from myofibers and not satellite cells blunts overload-induced hypertrophy, and impairs satellite cell proliferation and recruitment to pre-existing fibers. We reveal a gene network in which Srf within myofibers modulates interleukin-6 and cyclooxygenase-2/interleukin-4 expressions and therefore exerts a paracrine control of satellite cell functions. In Srf-deleted muscles, in vivo overexpression of interleukin-6 is sufficient to restore satellite cell proliferation but not satellite cell fusion and overall growth. In contrast cyclooxygenase-2/interleukin-4 overexpression rescue satellite cell recruitment and muscle growth without affecting satellite cell proliferation, identifying altered fusion as the limiting cellular event. These findings unravel a role for Srf in the translation of mechanical cues applied to myofibers into paracrine signals, which in turn will modulate satellite cell functions and support muscle growth

Impaired sarcomere ultrastructure in SRF deficient muscles.

<p>(A) Representative electron micrographs of longitudinal sections of control and mutant Plantaris muscles. Well-structured sarcomere units are observed in control myofibers 13 months after TAM injection. Major changes to the ultrastructure in the male mutant Plantaris muscle are illustrated by sarcomere unit disorganisation (left panel) with the presence of tubular aggregates (right panel). Bars: 0.2 µm. (B) Modified Gomori trichrome staining of transversal sections of control and mutant Tibialis muscles 13 months after TAM injection. Arrowhead indicates the presence of tubular aggregates in the mutant muscle. Bars: 50 µm. (C) Quantification of the frequency of tubular aggregate formation in control (white) and mutant (black) TA muscles 13 months after TAM injection (n = 3 for control and mutant; 700 fibers were analysed for each muscle). The frequency was determined by plotting the number of tubular aggregates per individual fiber versus the percentage of fibers with tubular aggregates. *P<0.05 over control.</p

Progressive atrophy of MyHC-IIB fibers and metabolic switch in SRF deficient muscles.

<p>(A) Quantitative real-time PCR was performed on RNAs prepared from gastrocnemius muscles of control (white) and mutant (black) mice 5 months (5 m.) and 13 months (13 m.) after TAM injection (n = 4). Mean expression levels for <i>SRF</i> mRNAs was normalized using <i>Cyclophilin</i> transcripts as a reference. (B, C) Frequency histogram showing distributions of muscle fiber CSA in control (white) and mutant (black) plantaris muscles 5 (B) and 13 (C) months after TAM injection (n = 4 for control and mutant at each time; 600 fibers were analysed for each muscle) (D) Immunostaining analysis using MyHC-IIA,-IIB antibody (left and middle panels) and SDH staining (right panel) of transversal sections of control and mutant plantaris muscles 13 months after TAM injection. Bars: 50 µm. (E) Mean myofiber CSA of MyHC-IIA or -IIB expressing myofibers from control (white) and mutant (black) 5 and 13 months after TAM injection (n = 4, 600 myofibers were measured for each muscle). *P<0.05 over controls 13 months after TAM injection.</p

Fibrosis accumulation and alteration of lipid metabolism in SRF deficient muscles.

<p>(A) Sirius Red staining of transversal sections of control and mutant TA muscles 13 months after TAM injection. Bars: 50 µm. (B) Oil Red O staining of transversal sections of control and mutant TA muscles 13 months after TAM injection. Nuclei are counter-stained with DAPI. Bars: 50 µm. (C) Immunoblot analysis of protein extracts from control (Cnt) and mutant (Mut) muscles 8 months after TAM injection using anti-SRF, anti-SREBP1c and anti-FAS antibodies. Anti-GAPDH is used as a loading control.</p