Development and characterization of a new inbred transgenic rat strain expressing DsRed monomeric fluorescent protein.

The inbred rat is a suitable model for studying human disease and because of its larger size is more amenable to complex surgical manipulation than the mouse. While the rodent fulfills many of the criteria for transplantation research, an important requirement is the ability to mark and track donors cells and assess organ viability. However, tracking ability is limited by the availability of transgenic (Tg) rats that express suitable luminescent or fluorescent proteins. Red fluorescent protein cloned from Discosoma coral (DsRed) has several advantages over other fluorescent proteins, including in vivo detection in the whole animal and ex vivo visualization in organs as there is no interference with autofluorescence. We generated and characterized a novel inbred Tg Lewis rat strain expressing DsRed monomeric (DsRed mono) fluorescent protein under the control of a ubiquitously expressed ROSA26 promoter. DsRed mono Tg rats ubiquitously expressed the marker gene as detected by RT-PCR but the protein was expressed at varying levels in different organs. Conventional skin grafting experiments showed acceptance of DsRed monomeric Tg rat skin on wild-type rats for more than 30 days. Cardiac transplantation of DsRed monomeric Tg rat hearts into wild-type recipients further showed graft acceptance and long-term organ viability (>6 months). The DsRed monomeric Tg rat provides marked cells and/or organs that can be followed for long periods without immune rejection and therefore is a suitable model to investigate cell tracking and organ transplantation.

OCT4 expression mediates partial cardiomyocyte reprogramming of mesenchymal stromal cells.

Mesenchymal stem/stromal cells (MSCs) are in numerous cell therapy clinical trials, including for injured myocardium. Acquisition of cardiomyocyte characteristics by MSCs may improve cardiac regeneration but the mechanisms regulating this process are unclear. Here, we investigated whether the pluripotency transcription factor OCT4 is involved in the activation of cardiac lineage genetic programs in MSCs. We employed our established co-culture model of MSCs with rat embryonic cardiomyocytes showing co-expression of cardiac markers on MSCs independent of cell fusion. Bone marrow-derived MSCs were isolated from transgenic mice expressing GFP under the control of the cardiac-specific α-myosin heavy chain promoter. After 5 days of co-culture, MSCs expressed cardiac specific genes, including Nkx2.5, atrial natriuretic factor and α-cardiac actin. The frequency of GFP+ cells was 7.6±1.9%, however, these cells retained the stromal cell phenotype, indicating, as expected, only partial differentiation. Global OCT4 expression increased 2.6±0.7-fold in co-cultured MSCs and of interest, 87±5% vs 79±4% of MSCs expressed OCT4 by flow cytometry in controls and after co-culture, respectively. Consistent with the latter observation, the GFP+ cells did not express nuclear OCT4 and showed a significant increase in OCT4 promoter methylation compared with undifferentiated MSCs (92% vs 45%), inferring that OCT4 is regulated by an epigenetic mechanism. We further showed that siRNA silencing of OCT4 in MSCs resulted in a reduced frequency of GFP+ cells in co-culture to less than 1%. Our data infer that OCT4 expression may have a direct effect on partial cardiomyocyte reprogramming of MSCs and suggest a new mechanism(s) associated with MSC multipotency and a requirement for crosstalk with the cardiac microenvironment.

Mesenchymal stromal cells improve cardiac function and left ventricular remodeling in a heart transplantation model.

BACKGROUND: Ischemia/reperfusion (I/R) injury is an inevitable consequence of organ transplantation and a major determinant of patient and graft survival in heart transplantation. Bone marrow-mesenchymal stromal cell (BM-MSC) treatment is a potentially effective cell therapy for cardiac disease. We investigated the effects of intravenous delivery of BM-MSCs in the acute phase post-transplant in a heterotopic heart transplantation (HHT) model associated with I/R injury. METHODS: Hearts of wild-type Lewis (WT LEW) rats were harvested and transplanted heterotopically into the necks of recipient WT LEW rats. Forty-eight hours after HHT, BM-MSCs were injected intravenously into animals in the experimental group, whereas controls received normal saline (NS). RESULTS: Eight days after BM-MSC injection, fractional shortening of transplanted hearts was significantly higher and left ventricular systolic diameter was lower in the BM-MSC group compared with controls, whereas no differences were found 28 days after infusion. A reduction in ventricular remodeling and cardiac fibrosis was observed by histochemical analysis and confirmed by cardiac magnetic resonance imaging in the BM-MSC group. The perivascular stromal cells' density and the number of capillaries were increased whereas the number of apoptotic cells decreased significantly in transplanted hearts in the BM-MSC group compared with the NS group. CONCLUSIONS: We showed early improvement in cardiac function and subsequent enhanced ventricular remodeling, reduced cardiac fibrosis, augmented neo-vascularization and decreased cardiomyocyte apoptosis of the transplanted heart in a heterotopic transplantation model after intravenous infusion of BM-derived MSCs. Our data suggest that clinical studies with BM-MSCs are warranted to understand their effects on cardiac graft and transplant recipient survival.