Cardiomyocyte Membrane Structure and cAMP Compartmentation Produce Anatomical Variation in β2AR-cAMP Responsiveness in Murine Hearts

Figures

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  Figure 1. Contraction and cAMP Production in Response to β2AR Stimulation (Rat Apical and Basal Cells) (A) Cell shortening increases after β2AR stimulation (1 μM ISO + 300 nM CGP). N, n = (6) 11/9, p < 0.001, apex versus base, unpaired t test. (B) Relative change in speed of relaxation. N, n = (6) 11/9, p < 0.001, apex versus base, unpaired t test. (C) Representative traces showing the β2AR FRET response measured by the cytosolic cAMP sensor cEPAC2-camps in apical and basal cardiomyocytes. (D) Calculation of the amount of cAMP production after β2AR stimulation relative to absolute maximum of production after stimulation with NKH477. N, n = (10) 36/34, not statistically significant (NS), apex versus base, unpaired t test. (E and F) Example traces of FRET responses after local stimulation of T-tubules or crest regions in apical (E) or basal (F) cells. (G) Comparison of apical (tubule/crest, N, n = (9) 15/13, NS) and basal (tubule/crest, N, n = (10) 21/20, p < 0.05, unpaired t test) responses. Error bars indicate SEM. See also Figure S2.

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  Figure 2. Intracellular Organization of Apical and Basal Myocytes from Rats (A) Representative SICM images of the surface topography of apical and basal cardiomyocytes. (B) Quantification of the degree of myocyte surface structural organization by Z-groove index (apical/basal, N, n = (5) 45/44, NS, unpaired t test). (C) Tubular membrane organization of di-8ANNEPPS-stained apical and basal cardiomyocytes. Insets show the areas chosen to make binary images for analysis. (D and E) Quantification of the relative regularity (D) and density (E) of T-tubules (N, n = (3) 15, p < 0.001). (F) Examples of electron micrographs showing plasma membrane cross-sections of apical and basal myocytes; N = 10. (G) Quantification of the density of caveolae in apical and basal cardiomyocytes from EM images. p < 0.001, unpaired t test. (H) Examples of immunostaining of Cav3 in apical and basal myocytes. (I) Morphometric quantification of the regularity of Cav3 staining (from immunostaining micrographs). (J) Quantification of the density of Cav3 staining. N, n = (3) 12/13, NS, unpaired t test. Error bars indicate SEM.

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  Figure 3. Role of Caveolae in Restraining Contraction of Basal Myocytes following β2AR Stimulation (A) Effect of MCD (1–2 mM) pre-treatment on the shortening responses of apical and basal cardiomyocytes from rats following β2AR stimulation with 1 μM ISO + 300 nM CGP. N, n = (3) 6/6, p < 0.001, apex versus base, unpaired t test. (B) Effect of MCD pre-treatment on the relative change in relaxation speed of rat apical and basal cardiomyocytes following β2AR stimulation. N, n = (3) 6/6, p < 0.05, p < 0.001, unpaired t test. (C) The influence of genetic KO of Cav3 on the shortening responses of apical and basal cardiomyocytes from mice following β2AR stimulation. N, n = (3) 6/6, p < 0.01, unpaired t test. (D) Effects of conditional genetic KO of Cav3 on the relative change in relaxation speed of apical and basal cardiomyocytes following β2AR stimulation. N, n = (4) 9/10, NS, unpaired t test. (E) Western blot analysis of Cav3 levels in cardiac myocytes isolated following tamoxifen treatment of cav3flox/flox/Mer-αMHC−MerCre+/− mice. (F) Quantification of the amount of Cav3 on the blots in (E) by densitometry. Error bars indicate SEM. See also Figures S2–S4.

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  Figure 4. Role of Caveolae/Cholesterol in Compartmentation of β2AR Signaling in the Apex and Base (A–C) Representative traces showing rat apical and basal cell cAMP responses with or without MCD (1–2 mM) pre-treatment following β2AR stimulation with 100 nM ISO + 100 nM CGP, as measured with a cytosolic (cEPAC2) FRET biosensor (A), in the PKA_RII compartment (RII_EPAC) (B), and in the plasma membrane compartment (pmEPAC2) (C). (D) Quantification of cAMP responses corrected to total cAMP output. Apex/base cEPAC2, N, n = (5) 22/14, (5) 10/7; RII_EPAC, N, n = (5) 20/20, 11/15; and pmEPAC2, N, n = (5) 16/18, 15/15; unpaired t test; ∗p < 0.05. Error bars indicate SEM. See also Figures S4 and S5.

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  Figure 5. Investigating the Role of PDE4 in Compartmentation of β2AR Signaling in the Apex and Base by Inhibition with ROLI (A) Representative trace showing the FRET responses of rat apical and basal cells following the stimulation of β2AR (100 nM ISO + 100 nM CGP) and subsequent inhibition of PDE4 by ROLI (5 μM) infusion, measured in cytosolic and RII_PKA compartments. (B) Quantification of the relative increase in FRET response following infusion of ROLI. N, n = (6)21/22, n = 13/13, unpaired t test, p < 0.05. (C) Effect of ROLI pre-treatment on the shortening responses of apical and basal cardiomyocytes following β2AR stimulation. N, n = (3)5, unpaired t test, ∗∗∗p < 0.001. (D) Effect of ROLI pre-treatment on the relative change in relaxation speed of apical and basal cardiomyocytes following β2AR stimulation. Unpaired t test, ∗p < 0.05, ∗∗∗p < 0.001. (E) Specific PDE4 and total PDE activity in apical and basal cell caveolar membrane preparations. N, n = (6)6, unpaired t test, ∗p < 0.05. Error bars indicate SEM. See also Figures S2 and S6.

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  Figure 6. Imaging Regional Differences in β2AR-cAMP Responses in a Langendorff-Perfused Mouse Heart (A) Images demonstrating the selection of regions for FRET analysis (these are broadly representative of the regions dissected for cells in other studies) to allow assessment of FRET responses. Images show the yellow fluorescence channel (FRET acceptor). (B) Representative traces presenting the responses of the apical and basal portions of the myocardium under different pharmacologic stimuli. (C) Quantification of FRET responses to β2AR stimulation (100 nM ISO + 100 nM CGP) and ROLI (5 μM) from the apical and basal sectors. N, n = (8) 8, p < 0.05, unpaired t test; #p < 0.05, paired t test, CGP+ISO versus CGP+ISO+ROLI steps. Error bars indicate SEM. See also Figure S7.

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  Figure 7. Simplified Depiction of the Differences in β2AR Nanodomains in Apical and Basal Cardiac Myocytes Top: the scenario within relatively well organized basal cardiomyocytes. A high degree of membrane organization in basal cardiomyocytes allows tight control of the cAMP level because of enhanced efficacy of PDE4-mediated cAMP hydrolysis. As a result, the cAMP produced by β2AR stimulation cannot modulate cell contractility to a great extent. Bottom: the scenario of less organized apical cardiomyocytes. Apical cardiomyocyte membranes lack extensive nanodomain compartmentation, and, as a result, β2AR-stimulated cAMP is able to reach deeper from the membrane and increases cellular contractility via PKA-mediated control of the excitation contraction (EC) coupling machinery.