Insights into the Regulatory Properties of Human Adenylyl Cyclase Type 9

Abstract

Membrane-bound adenylyl cyclase (AC) isoforms have distinct regulatory mechanisms that contribute to their signaling specificity and physiologic roles. Although insight into the physiologic relevance of AC9 has progressed, the understanding of AC9 regulation is muddled with conflicting studies. Currently, modes of AC9 regulation include stimulation by Gαs, protein kinase C (PKC) βII, or calcium-calmodulin kinase II (CaMKII) and inhibition by Gαi/o, novel PKC isoforms, or calcium-calcineurin. Conversely, the original cloning of human AC9 reported that AC9 is insensitive to Gαi inhibition. The purpose of our study was to clarify which proposed regulators of AC9 act directly or indirectly, particularly with respect to Gαi/o. The proposed regulators, including G proteins (Gαs, Gαi, Gαo, Gβγ), protein kinases (PKCβII, CaMKII), and forskolin, were systematically evaluated using classic in vitro AC assays and cell-based cAMP accumulation assays in COS-7 cells. Our studies show that AC9 is directly regulated by Gαs with weak conditional activation by forskolin; other modes of proposed regulation either occur indirectly or possibly require additional scaffolding proteins to facilitate regulation. We also show that AC9 contributes to basal cAMP production; knockdown or knockout of endogenous AC9 reduces basal AC activity in COS-7 cells and splenocytes. Importantly, although AC9 is not directly inhibited by Gαi/o, it can heterodimerize with Gαi/o-regulated isoforms, AC5 and AC6.

Fig. 1.

AC9 is conditionally activated by forskolin. (A) Western blot using α-FLAG (top) and goat α-AC9 (bottom) for membranes from Sf9 cells expressing vector control (β-gal) or human AC9. (B and C) Dose-response curves of Gαs stimulation of AC9 and β-gal Sf9 membranes in the presence or absence of 50 µM forskolin (Fsk) (B) and membranes from HEK293 cells expressing AC9 in the presence or absence of 100 µM forskolin (C) [activity from control membranes (D) was subtracted]. The inset in (B) shows individual replicates for 3 µM Gαs with or without forskolin. (D) AC activity of membranes isolated from HEK293 cells expressing pcDNA3 (background AC activity). Data are shown as means ± S.D. Statistical analyses in (B) through (D) were performed with two-way ANOVA followed by the Sidak multiple-comparisons test of experimental means comparing vehicle and forskolin-stimulated groups at each concentration indicated. n = 3 to 4 with experiments performed in duplicate. *P < 0.05; **P < 0.01; ***P < 0.001. ns, not significant.

Fig. 2.

AC9 is less sensitive to Gαs compared with AC6 and is insensitive to Gβγ. (A) Dose-response curves of Gαs stimulation of membranes from Sf9 cells expressing AC9, AC6, or β-gal. (B) Dose-response curves of Gαs stimulation of Sf9 membranes expressing AC9 or β-gal in the presence and absence of 100 nM Gβ₁γ₂. Data are shown as means ± S.D. Statistical analyses in (B) were performed with two-way ANOVA followed by a Sidak multiple-comparisons test comparing experimental means of vehicle- and Gαs- or Gβ₁γ₂-stimulated groups at each concentration indicated. n = 3. No statistical difference was found in the presence of Gβγ.

Fig. 3.

AC9 is not directly regulated by Gαi/o in vitro. (A) Structure-based alignment of amino acids that correspond to the Gαi binding site in AC5. (B) Gαs dose-response curves for AC9 and β-gal Sf9 membranes in the presence and absence of 300 nM Gαi. (C) Gαi dose-response curves for AC9 and β-gal Sf9 membranes in the presence of 300 nM Gαs. (D) E. coli vs. Sf9 purified Gαi isoforms. AC9 Sf9 membranes were stimulated with 300 nM Gαs in the presence or absence of 300 nM Gαi₁ (E. coli or Sf9) or Gαi₃. (E) AC6 membranes with 50 nM Gαs ± 300 nM Gαi proteins served as a positive control. (F) Gαo regulation of AC9 Sf9 membranes stimulated with 300 nM Gαs. (G) AC1 membranes stimulated with 100μM Ca²⁺/300 nM CaM with 1 μM Gαo served as a positive control. Data are shown as means ± S.D. Statistical analyses were performed as follows: (B) and (D), two-way ANOVA followed by a Sidak multiple-comparisons test comparing the vehicle and Gαi/o groups at each concentration indicated; (C), (E), and (F), one-way ANOVA followed by a Tukey multiple-comparisons test comparing experimental means; and (G), t test comparing Ca²⁺/CaM with or without Gαo. For all experiments, n = 3 to 4, performed in duplicate or triplicate. *P < 0.05; **P < 0.01; ***P < 0.001. No statistical difference was found for Gαi/o under any condition with AC9 (B–D and F). ns, not significant.

Fig. 4.

Neither CaMKII nor PKCβII directly regulates AC9. (A) AC9 membranes were stimulated with 100 nM Gαs in the presence or absence of 200 µM Ca²⁺/1 µM calmodulin (Ca²⁺/CaM), 100 ng CaMKII, or Ca²⁺/CaM plus CaMKII. The inset shows ³²P labeling of MBP by CaMKII, performed in duplicate. As a positive control, AC3 was stimulated with 100 nM Gαs in the presence of 200 µM Ca²⁺/1 µM CaM in the presence or absence of 100 ng CaMKII. (B) AC2, AC9, and β-gal membranes were stimulated with 100 nM Gαs in the presence or absence of 1 µM PMA or PMA plus purified PKCβII. The inset shows ³²P labeling of MBP by PKCβII, performed in duplicate. Data are shown as means ± S.D. Statistical analyses were performed as follows: (A), two-way ANOVA followed by a Sidak multiple-comparisons test; (B), a paired t test comparing the control and the kinase group; and (C), one-way ANOVA followed by a Tukey multiple-comparisons test. For all experiments, n = 3, performed in duplicate. *P < 0.05; **P < 0.01. ns, not significant.

Fig. 5.

Regulation of endogenous and overexpressed AC9 in COS-7 cells. (A) Representative traces of cells expressing μOR with or without AC6 or AC9 treated with 1 µM isoproterenol (ISO; black square) ± 10 μM DAMGO. Arrows indicate the addition of drugs (ISO/DAMGO). (B) Quantitation of individual cell traces. Data points were averaged (avg) from the 12- to 14-minute timepoints of ISO (1 µM) and ISO plus 10 μM DAMGO. (C) Western blot of COS-7 whole-cell lysates, confirming expression of endogenous or exogenous AC9 (top: rabbit anti-AC9) and AC6 (bottom). (D) Confirmation of endogenous AC9 protein by Western blot of membranes isolated from COS-7 cells, or cells expressing an AC9 siRNA or control siRNA. The Na-K ATPase served as a loading control for COS-7 membranes. Data are shown as means ± S.D. Statistical analyses in (B) were performed with two-way ANOVA followed by a Sidak multiple-comparisons test comparing ISO and ISO plus DAMGO. n = 3–5 with experiments performed in duplicate. **P < 0.01. au, arbitrary unit; KD, knockdown; ns, not significant.

Fig. 6.

Basal AC activity is dependent on AC9 in COS-7 cells and mouse splenocytes. (A) AC assay of membranes from COS-7 cells stably expressing control or AC9 shRNA/siRNA stimulated with 400 nM Gαs or 50 µM forskolin. (B) Whole-cell basal cAMP accumulation was measured in COS-7 cells expressing control shRNA, AC9 shRNA, or overexpressing (OVE) AC9 using a GloSensor bioluminescent cAMP reporter. Cells were pretreated with 1 mM IBMX for 10 minutes. (C) Quantitative real-time PCR of AC isoforms present in spleen from AC9^−/− mice, normalized to wild-type (WT) expression levels (n = 3). (D) AC assay of WT and AC9 knockout (KO) spleen membranes stimulated with 400 nM Gαs or 50 µM forskolin in the presence or absence of the P-site inhibitor, Ara-A (300 nM). (E and F) Whole-cell cAMP accumulation of mouse splenocytes isolated from WT and AC9KO mice treated with 1 mM IBMX ± 1 μM ISO. Data are shown as means ± S.D. Statistical analysis were performed as follows: (A) and (D), two-way ANOVA followed by a Sidak multiple-comparison test [comparisons within Ara-A treatments in (D) were made separately]; (B), (E), and (F), an unpaired t test of means comparing the indicated groups; and (C), one-way ANOVA followed by a Tukey multiple-comparisons test. For all, experiments were performed in duplicate. n = 3 to 4. *P < 0.05; **P < 0.01; ***P < 0.001. KO, knockout; ns, not significant.

Fig. 7.

AC9 homo- and heterodimers. Quantification of COS-7 cells expressing BiFC constructs for AC9, AC5, and AC6. One of two halves of a fluorescent protein: the N-terminal portion (VN) or the C-terminal portion (VC) is fused in frame to the C terminus of each AC isoform, except where noted. (A) The homodimers are expressing both AC-VN and AC-VC for the given isoform. AC9-VN alone is a negative control. (B) Heterodimer formation used AC-VN (top one listed) from one isoform with AC-VC from a second isoform. For example, AC5-AC9 corresponds to cotransfection of AC5-VN and AC9-VC. AC5* is N-terminally tagged with VC. (C and D) Cell extracts from cells transiently transfected with Flag-AC9 (fAC9) with or without AC5 (C) or Flag-AC9 with or without AC6 (D) were subjected to immunoprecipitation with FLAG-agarose and Western blotted for FLAG, AC5, or AC6. As positive controls, Flag-AC5 and YFP-AC6 were immunoprecipitated with FLAG-agarose or anti-GFP and protein G. The delta symbol indicates a nonspecific band; no endogenous AC6 is ever observed in pulldowns of Flag-AC9. Note, although the FLAG tag on the control Flag-AC5 can be used for immunoprecipitation, it is not readily detected by Western blotting. Data are shown as means ± S.D. Statistical analyses in (A) and (B) were performed with one-way ANOVA followed by the Dunnet multiple-comparisons test comparing AC9 VN to each condition indicated. n = 3–5 with experiments performed in duplicate. *P < 0.05; **P < 0.01; ***P < 0.001. GFP, green fluorescent protein; ns, not significant.