T-type Ca²⁺ channels regulate the exit of cardiac myocytes from the cell cycle after birth

T-type Ca(2+) channels (TTCCs) are expressed in the fetal heart and then disappear from ventricular myocytes after birth. The hypothesis examined in this study was the α1G TTCCs' influence in myocyte maturation and their rapid withdrawal from the cell cycle after birth.

METHODS

Cardiac myocytes were isolated from neonatal and adult wild type (WT), α1G-/- and α1G over expressing (α1GDT) mice. Bromodeoxyuridine (BrdU) uptake, myocyte nucleation, cell cycle analysis, and T-type Ca(2+) currents were measured.

RESULTS

All myocytes were mono-nucleated at birth and 35% of WT myocytes expressed functional TTCCs. Very few neonatal myocytes had functional TTCCs in α1G-/- hearts. By the end of the first week after birth no WT or α1G-/- had functional TTCCs. During the first week after birth about 25% of WT myocytes were BrdU+ and became bi-nucleated. Significantly fewer α1G-/- myocytes became bi-nucleated and fewer of these myocytes were BrdU+. Neonatal α1G-/- myocytes were also smaller than WT. Adult WT and α1G-/- hearts were similar in size, but α1G-/- myocytes were smaller and a greater % were mono-nucleated. α1G over expressing hearts were smaller than WT but their myocytes were larger.

CONCLUSIONS

The studies performed show that loss of functional TTCCs is associated with bi-nucleation and myocyte withdrawal from the cell cycle. Loss of α1G TTCCs slowed the transition from mono- to bi-nucleation and resulted in an adult heart with a greater number of small cardiac myocytes. These results suggest that TTCCs are involved in the regulation of myocyte size and the exit of myocytes from the cell cycle during the first week after birth.

Volume overload induces differential spatiotemporal regulation of myocardial soluble guanylyl cyclase in eccentric hypertrophy and heart failure

Nitric oxide activation of soluble guanylyl cyclase (sGC) blunts the cardiac stress response, including cardiomyocyte hypertrophy. In the concentric hypertrophied heart, oxidation and re-localization of myocardial sGC diminish cyclase activity, thus aggravating depressed nitric oxide-cyclic guanosine monophosphate (NO-cGMP) signaling in the pressure-overloaded failing heart. Here, we hypothesized that volume-overload differentially disrupts myocardial sGC activity during early compensated and late decompensated stages of eccentric hypertrophy. To this end, we studied the expression, redox state, subcellular localization, and activity of sGC in the left ventricle of dogs subjected to chordal rupture-induced mitral regurgitation (MR). Unoperated dogs were used as Controls. Animals were studied at 4weeks and 12months post chordal rupture, corresponding with early (4wkMR) and late stages (12moMR) of eccentric hypertrophy. We found that the sGC heterodimer subunits relocalized away from caveolae-enriched lipid raft microdomains at different stages; sGCβ1 at 4wkMR, followed by sGCα1 at 12moMR. Moreover, expression of both sGC subunits fell at 12moMR. Using the heme-dependent NO donor DEA/NO and NO-/heme-independent sGC activator BAY 60-2770, we determined the redox state and inducible activity of sGC in the myocardium, within caveolae and non-lipid raft microdomains. sGC was oxidized in non-lipid raft microdomains at 4wkMR and 12moMR. While overall DEA/NO-responsiveness remained intact in MR hearts, DEA/NO responsiveness of sGC in non-lipid raft microdomains was depressed at 12moMR. Caveolae-localization protected sGC against oxidation. Further studies revealed that these modifications of sGC were also reflected in caveolae-localized cGMP-dependent protein kinase (PKG) and MAPK signaling. In MR hearts, PKG-mediated phosphorylation of vasodilator-stimulated phosphoprotein (VASP) disappeared from caveolae whereas caveolae-localization of phosphorylated ERK5 increased. These findings show that differential oxidation, re-localization, and expression of sGC subunits distinguish eccentric from concentric hypertrophy as well as compensated from decompensated heart failure.

Cardiotoxic and cardioprotective features of chronic β-adrenergic signaling

RATIONALE

In the failing heart, persistent β-adrenergic receptor activation is thought to induce myocyte death by protein kinase A (PKA)-dependent and PKA-independent activation of calcium/calmodulin-dependent kinase II. β-adrenergic signaling pathways also are capable of activating cardioprotective mechanisms.

OBJECTIVE

This study used a novel PKA inhibitor peptide to inhibit PKA activity to test the hypothesis that β-adrenergic receptor signaling causes cell death through PKA-dependent pathways and cardioprotection through PKA-independent pathways.

METHODS AND RESULTS

In PKA inhibitor peptide transgenic mice, chronic isoproterenol failed to induce cardiac hypertrophy, fibrosis, and myocyte apoptosis, and decreased cardiac function. In cultured adult feline ventricular myocytes, PKA inhibition protected myocytes from death induced by β1-adrenergic receptor agonists by preventing cytosolic and sarcoplasmic reticulum Ca(2+) overload and calcium/calmodulin-dependent kinase II activation. PKA inhibition revealed a cardioprotective role of β-adrenergic signaling via cAMP/exchange protein directly activated by cAMP/Rap1/Rac/extracellular signal-regulated kinase pathway. Selective PKA inhibition causes protection in the heart after myocardial infarction that was superior to β-blocker therapy.

CONCLUSIONS

These results suggest that selective block of PKA could be a novel heart failure therapy.

Limitation of individual folding resources in the ER leads to outcomes distinct from the unfolded protein response

ER stress leads to upregulation of multiple folding and quality control components, known as the unfolded protein response (UPR). Glucose Regulated Protein 78 (GRP78) (also known as binding immunoglobulin protein, BiP, and HSPA5) and GRP94 are often upregulated coordinately as part of this homeostatic response. Given that endoplasmic reticulum (ER) chaperones have distinct sets of clients, we asked how cells respond to ablation of individual chaperones. The cellular responses to silencing BiP, GRP94, HSP47, PDIA6 and OS-9, were distinct. When BiP was silenced, a widespread UPR was observed, but when GRP94 was either inhibited or depleted by RNA interference (RNAi), the expression of only some genes was induced, notably those encoding BiP and protein disulfide isomerase A6 (PDIA6). Silencing of HSP47 or OS-9 did not lead to any compensatory induction of other genes. The selective response to GRP94 depletion was distinct from a typical ER stress response, both because other UPR target genes were not affected and because the canonical UPR signaling branches were not activated. The response to silencing of GRP94 did not preclude further UPR induction when chemical stress was imposed. Importantly, re-expression of wild-type GRP94 in the silenced cells prevented the upregulation of BiP and PDIA6, whereas re-expression of an ATPase-deficient GRP94 mutant did not, indicating that cells monitor the activity state of GRP94. These findings suggest that cells are able to distinguish among folding resources and generate distinct responses.

Abstract P074: Loss of Function of TRPC4 Protects Against Cardiac Dysfunction Progression After Myocardial Infarction

The source of Ca 2+ to hypertrophic signaling after myocardial infarction (MI) is not clearly defined. Transient Receptor Potential Canonical (TRPC) channels could be an important source of hypertrophic Ca 2+ after MI. The objective of this study was to determine if TRPC 4 is a major source of Ca 2+ influx mediating cardiac dysfunction after MI.

Methods: Cardiac-specific transgenic mice that express a dominant-negative (dn) TRPC4 that reduces the activity of the TRPC1/4/5 subfamily of channels in the heart were used. MI was produced and in-vivo cardiac function was measured with ECHO. Myocytes were isolated and isoproterenol (ISO) effects on LTCC Current ( I Ca-L ), fractional shortening (FS) and Ca 2+ transients were measured 6 weeks after MI.

Results: Baseline ejection fraction (EF) and fractional shortening (FS) were greater in (dn) TRPC4 vs. WT mice. Two weeks after MI, EF and FS were significantly decreased in all animals (WT: 37.1% and 18.2%; (dn) TRPC4: 41.7% and 20.5%), but there was no significant difference between WT and (dn) TRPC4 mice. Six weeks after MI, EF and FS were significant greater in (dn) TRPC4 compared with WT mice (WT: 37.4% and 18.2%; (dn) TRPC4: 52.2% and 27.4%). Heart weight and lung weight were significantly increased after 2 weeks MI, but there were significant lower heart and lung weight in (dn) TRPC4 vs. WT mice after 6 weeks MI. I Ca-L [[Unsupported Character - Codename ­]] after 6 weeks MI was smaller than that in sham myocytes, and there was no significant difference between (dn) TRPC4 and WT myocytes. Contractions and Ca 2+ transients were significantly greater in sham and post-MI (dn) TRPC4 vs. WT myocytes. ISO increased contractions and Ca 2+ transients to a similar extent in all myocytes.

Conclusions: (dn) TRPC4 mice have greater baseline cardiac and myocyte function. While initial effects of MI were similar to control, there was improved function in these mice by 6 weeks. These results suggest that blocking TRPC4 after MI may reduce pathological cardiac remodeling.

Glucose regulated protein 94 is required for muscle differentiation through its control of the autocrine production of insulin-like growth factors

The endoplasmic reticulum chaperone GRP94 is essential for early embryonic development and in particular affects differentiation of muscle lineages. To determine why an ubiquitously expressed protein has such a specific effect, we investigated the function of GRP94 in the differentiation of established myogenic cell lines in culture. Using both genetic suppression of expression, via RNA interference, and inhibition of function, via specific chemical inhibitors, we show that GRP94 expression and activity are needed for the in vitro fusion of myoblasts precursors into myotubes and the expression of contractile proteins that mark terminal differentiation. The inhibition can be complemented by addition of insulin-like growth factors to the cultures. GRP94 is not needed for the initial steps of myogenesis, only for the steps downstream of MyoD up-regulation, coinciding with the known need for synergistic input from growth factor signaling. Indeed, GRP94 is needed for the production of insulin-like growth factors I and II (IGF-I and IGF-II) by the differentiating cells. Moreover, the depletion of the chaperone does not increase the rate of apoptosis that always accompanies myogenic differentiation. Thus, the major effect of GRP94 on muscle differentiation is mediated by its regulation of IGF production.