Cardiac fibroblasts convert to myofibroblasts with injury to mediate healing after acute myocardial infarction (MI) and to mediate long-standing fibrosis with chronic disease. Myofibroblasts remain a poorly defined cell type in terms of their origins and functional effects in vivo. Here we generate Postn (periostin) gene-targeted mice containing a tamoxifen-inducible Cre for cellular lineage-tracing analysis. This Postn allele identifies essentially all myofibroblasts within the heart and multiple other tissues. Lineage tracing with four additional Cre-expressing mouse lines shows that periostin-expressing myofibroblasts in the heart derive from tissue-resident fibroblasts of the Tcf21 lineage, but not endothelial, immune/myeloid or smooth muscle cells. Deletion of periostin+ myofibroblasts reduces collagen production and scar formation after MI. Periostin-traced myofibroblasts also revert back to a less-activated state upon injury resolution. Our results define the myofibroblast as a periostin-expressing cell type necessary for adaptive healing and fibrosis in the heart, which arises from Tcf21+ tissue-resident fibroblasts.

If and how the heart regenerates after an injury event is highly debated. c-kit-expressing cardiac progenitor cells have been reported as the primary source for generation of new myocardium after injury. Here we generated two genetic approaches in mice to examine if endogenous c-kit+ cells contribute differentiated cardiomyocytes to the heart during development, with aging or after injury in adulthood. A cDNA encoding either Cre recombinase or a tamoxifen inducible MerCreMer chimeric protein was targeted to the Kit locus in mice and then bred with reporter lines to permanently mark cell lineage. Endogenous c-kit+ cells did produce new cardiomyocytes within the heart, although at a percentage of ≈0.03% or less, and if a preponderance towards cellular fusion is considered, the percentage falls below ≈0.008%. In contrast, c-kit+ cells amply generated cardiac endothelial cells. Thus, endogenous c-kit+ cells can generate cardiomyocytes within the heart, although likely at a functionally insignificant level.

reverse transcribed using random oligo-dT primers and a Verso cDNA Synthesis Kit (Thermo Fisher, AB1453) according to the manufacturer's instructions. Real-time PCR was performed using SsoAdvanced SYBR Green (Bio-Rad, 6090), and Rpl7 expression was used for normalization. The following primer sets were used to identify transcripts: collagen 1a1, 5′-AATGGCACGGCTGTGTGCGA and 5′-AACGGGTCCCCTTG-GGCCTT; collagen 3a1, 5′-TCCCCTGGAATCTGTGAATC and 5′-TGAGTCGAATTGGGGAGAAT; periostin, 5′-ACGGAGCTCAGG-GCTGAAGATG and 5′-GTTTGGGCCCTGATCCCGAC.Cell death analysis. At 90% confluence, primary skin fibroblasts were treated with 200 nM staurosporine for 36 hours or vehicle (DMSO). Cell death was determined by the Muse Count & Viability Assay (Millipore, MCH100102) as previously described (62). Briefly, the medium was collected with the trypsin-liberated cells, which were centrifuged and washed twice with PBS and then incubated with the Muse Count & Viability reagent. The cells were then quantified on a Muse cell analyzer (Millipore) at 5,000 counts per sample.Statistics. One-way ANOVA with post hoc Tukey's honest significant difference (HSD) or Student's t test was used to determine statistical significance, depending on the type of data analyzed and number of comparisons. P values of less than 0.05 were considered statistically significant. Averaged data are presented with SEM to indicate variability.Study approval. Mice were observed daily and cages changed weekly by certified veterinary technicians at Cincinnati Children's Hospital Medical Center. Mice were also closely assessed for their well-being, monitored by adequate physical activity and food intake on a daily basis. Housing conditions and husbandry conformed to AAALAC standards as well as the standard guidelines from the NIH Office of Laboratory Animal Welfare (http://grants.nih.gov/grants/olaw/animal_use. htm). The institution also retains ongoing certification by AAALAC.

Cyclophilin D (which is encoded by the Ppif gene) is a mitochondrial matrix peptidyl-prolyl isomerase known to modulate opening of the mitochondrial permeability transition pore (MPTP). Apart from regulating necrotic cell death, the physiologic function of the MPTP is largely unknown. Here we have shown that Ppif -/-mice exhibit substantially greater cardiac hypertrophy, fibrosis, and reduction in myocardial function in response to pressure overload stimulation than control mice. In addition, Ppif -/-mice showed greater hypertrophy and lung edema as well as reduced survival in response to sustained exercise stimulation. Cardiomyocyte-specific transgene expression of cyclophilin D in Ppif -/-mice rescued the enhanced hypertrophy, reduction in cardiac function, and rapid onset of heart failure following pressure overload stimulation. Mechanistically, the maladaptive phenotype in the hearts of Ppif -/-mice was associated with an alteration in MPTP-mediated Ca 2+ efflux resulting in elevated levels of mitochondrial matrix Ca 2+ and enhanced activation of Ca 2+ -dependent dehydrogenases. Elevated matrix Ca 2+ led to increased glucose oxidation relative to fatty acids, thereby limiting the metabolic flexibility of the heart that is critically involved in compensation during stress. These findings suggest that the MPTP maintains homeostatic mitochondrial Ca 2+ levels to match metabolism with alterations in myocardial workload, thereby suggesting a physiologic function for the MPTP.

A critical event in ischemia-based cell death is the opening of the mitochondrial permeability transition pore (MPTP). However, the molecular identity of the components of the MPTP remains unknown. Here, we determined that the Bcl-2 family members Bax and Bak, which are central regulators of apoptotic cell death, are also required for mitochondrial pore-dependent necrotic cell death by facilitating outer membrane permeability of the MPTP. Loss of Bax/Bak reduced outer mitochondrial membrane permeability and conductance without altering inner membrane MPTP function, resulting in resistance to mitochondrial calcium overload and necrotic cell death. Reconstitution with mutants of Bax that cannot oligomerize and form apoptotic pores, but still enhance outer membrane permeability, permitted MPTP-dependent mitochondrial swelling and restored necrotic cell death. Our data predict that the MPTP is an inner membrane regulated process, although in the absence of Bax/Bak the outer membrane resists swelling and prevents organelle rupture to prevent cell death.DOI: http://dx.doi.org/10.7554/eLife.00772.001