Strategic Positioning and Biased Activity of the Mitochondrial Calcium Uniporter in Cardiac Muscle

Control of myocardial energetics by Ca2+ signal propagation to the mitochondrial matrix includes local Ca2+ delivery from sarcoplasmic reticulum (SR) ryanodine receptors (RyR2) to the inner mitochondrial membrane (IMM) Ca2+ uniporter (mtCU). mtCU activity in cardiac mitochondria is relatively low, whereas the IMM surface is large, due to extensive cristae folding. Hence, stochastically distributed mtCU may not suffice to support local Ca2+ transfer. We hypothesized that mtCU concentrated at mitochondria-SR associations would promote the effective Ca2+ transfer. mtCU distribution was determined by tracking MCU and EMRE, the proteins essential for channel formation. Both proteins were enriched in the IMM-outer mitochondrial membrane (OMM) contact point submitochondrial fraction and, as super-resolution microscopy revealed, located more to the mitochondrial periphery (inner boundary membrane) than inside the cristae, indicating high accessibility to cytosol-derived Ca2+ inputs. Furthermore, MCU immunofluorescence distribution was biased toward the mitochondria-SR interface (RyR2), and this bias was promoted by Ca2+ signaling activity in intact cardiomyocytes. The SR fraction of heart homogenate contains mitochondria with extensive SR associations, and these mitochondria are highly enriched in EMRE. Size exclusion chromatography suggested for EMRE- and MCU-containing complexes a wide size range and also revealed MCU-containing complexes devoid of EMRE (thus disabled) in the mitochondrial but not the SR fraction. Functional measurements suggested more effective mtCU-mediated Ca2+ uptake activity by the mitochondria of the SR than of the mitochondrial fraction. Thus, mtCU "hot spots" can be formed at the cardiac muscle mitochondria-SR associations via localization and assembly bias, serving local Ca2+ signaling and the excitation-energetics coupling.

Unique Features of Cortical Bone Stem Cells Associated With Repair of the Injured Heart

RATIONALE

Adoptive transfer of multiple stem cell types has only had modest effects on the structure and function of failing human hearts. Despite increasing the use of stem cell therapies, consensus on the optimal stem cell type is not adequately defined. The modest cardiac repair and functional improvement in patients with cardiac disease warrants identification of a novel stem cell population that possesses properties that induce a more substantial improvement in patients with heart failure.

OBJECTIVE

To characterize and compare surface marker expression, proliferation, survival, migration, and differentiation capacity of cortical bone stem cells (CBSCs) relative to mesenchymal stem cells (MSCs) and cardiac-derived stem cells (CDCs), which have already been tested in early stage clinical trials.

METHODS AND RESULTS

CBSCs, MSCs, and CDCs were isolated from Gottingen miniswine or transgenic C57/BL6 mice expressing enhanced green fluorescent protein and were expanded in vitro. CBSCs possess a unique surface marker profile, including high expression of CD61 and integrin β4 versus CDCs and MSCs. In addition, CBSCs were morphologically distinct and showed enhanced proliferation capacity versus CDCs and MSCs. CBSCs had significantly better survival after exposure to an apoptotic stimuli when compared with MSCs. ATP and histamine induced a transient increase of intracellular Ca(2+) concentration in CBSCs versus CDCs and MSCs, which either respond to ATP or histamine only further documenting the differences between the 3 cell types.

CONCLUSIONS

CBSCs are unique from CDCs and MSCs and possess enhanced proliferative, survival, and lineage commitment capacity that could account for the enhanced protective effects after cardiac injury.

Abstract 240: Strategic Distribution of the Mitochondrial Ca2+ Uniporter in Cardiac Mitochondria

Propagation of SR-derived Ca2+ signals to the mitochondrial matrix is an important tuning tool of cardiac energy metabolism (excitation-bioenergetics coupling). Effective activation of the low-affinity mitochondrial Ca2+ uniporter (mtCU) is supported by high [Ca2+] nanodomains where SR cisternae are tethered to mitochondria. Cardiac mitochondria have large inner membrane (IMM) surface with extensive cristae folding. However, recent comparative patch clamp studies found cardiac mtCU current density extremely low.

We hypothesized that efficient excitation-bioenergetics coupling would require strategic positioning of the mtCU. We thus analyzed the distribution of mtCU’s pore protein, MCU in rat and mouse cardiomyocytes and cardiac membrane fractions. Immunofluorescence (IF) using the commercial Sigma antibody showed significant non-mitochondrial (nuclear, cyto/sarcoplasmic) crossreactions as verified in MCU-KO MEFs and cardiomyocytes. To isolate mitochondrial labeling, we performed IF of isolated mitochondria (mtIF) and immunogold electron microscopy of freshly isolated cardiomyocytes. mtIF showed ~50% of MCU-positive structures colocalized with RyR2, in line with a location preference to SR-mitochondria associations. This distribution bias was also reflected in the immunogold TEM analysis. Moreover, WB showed both MCU and calsequestrin being enriched in the same sub-mitochondrial membrane fraction separated on sucrose gradient. The prevalence of MCU and even more the essential MCU regulator (EMRE) was higher in the mitochondria “contaminating” the heavy SR (SR-associated mito membranes) than in the crude mitochondrial fraction. [Ca2+] activation of mitochondrial Ca2+ uptake at similar IMM potentials was as effective in suspensions of SR-associated mito membranes as in the crude mitochondrial fraction.

Collectively, our data suggest a location bias of the functionally mandatory mtCU channel components MCU and EMRE toward SR-mitochondrial associations. As the bias is much stronger for EMRE, the component that is required for MCU to function as a channel, it raises the possibility that EMRE availability may be a physiological limiting factor in regional mtCU activity in the cardiac muscle mitochondria.

Abstract 157: Unique Features of Cortical Bone Stem Cells Associated With Enhanced Cardiac Repair

Rationale: Adoptive transfer of bone marrow and cardiac derived stem cells (CDCs) into failing human hearts has been shown to be safe, yet these cells have only induced modest improvements after myocardial infarction (MI). Recently we have shown in a mouse model that cortical bone derived stem cells (CBSCs) induced a greater enhancement of cardiac function after MI through enhanced paracrine signaling and transdifferentiation of CBSCs into new cardiac tissue. However, the reparative potential of CBSCs relative to other stem cell types including bone marrow derived mesenchymal stem cells (MSCs) and CDCs is not known.

Objective: To characterize surface marker expression, proliferation, survival, migration and differentiation capacity of swine CBSCs relative to MSCs and CDCs.

Methods and Results: CBSCs, MSCs and CDCs were isolated from Gottingen miniswine. CBSCs were morphologically distinct from MSCs and CDCs, with differences in length to width ratio and overall cell surface area. Cell surface marker profiling using RT-PCR analysis revealed that CBSCs express some of the classical MSC markers such as CD106, CD271, CD105, CD90 and CD29 and are negative for CD45 and CD11-b. CBSCs had an enhanced proliferation capacity versus CDCs and MSCs, measured by CyQuant assay. Concurrently CBSCs had significantly decreased population-doubling time (3.57 and 1.26 fold decrease) as compared to MSC and CDCs. CBSCs exhibit enhanced survival after exposure to apoptotic stimuli as compared to MSCs measured by Annexin-V staining. A significantly greater % of CBSCs expressed markers of cardiac lineage commitment when exposed to dexamethasone than did CDCs or MSCs. Markers of cardiac lineages including GATA-4, α SMA, Troponin T, sm22 were measured with RT-PCR and immunocytochemistry.

Conclusion: CBSCs have enhanced proliferative, survival capacity and cardiac lineage commitment versus CDCs and MSCs that could account for their enhanced effects on cardiac regeneration after myocardial infarction.