Abstract-Depressed contractility is a central feature of the failing human heart and has been attributed to altered [Ca 2ϩ

Hyperpolarization-activated, cyclic-nucleotide-gated (HCN) channels mediate the depolarizing cation current (termed I(h) or I(f)) that initiates spontaneous rhythmic activity in heart and brain. This function critically depends on the reliable opening of HCN channels in the subthreshold voltage-range. Here we show that activation of HCN channels at physiologically relevant voltages requires interaction with phosphoinositides such as phosphatidylinositol-4,5-bisphosphate (PIP(2)). PIP(2) acts as a ligand that allosterically opens HCN channels by shifting voltage-dependent channel activation approximately 20 mV toward depolarized potentials. Allosteric gating by PIP(2) occurs in all HCN subtypes and is independent of the action of cyclic nucleotides. In CNS neurons and cardiomyocytes, enzymatic degradation of phospholipids results in reduced channel activation and slowing of the spontaneous firing rate. These results demonstrate that gating by phospholipids is essential for the pacemaking activity of HCN channels in cardiac and neuronal rhythmogenesis.

Key Words: CaMKII Ⅲ calcineurin Ⅲ NFAT Ⅲ myocytes Ⅲ heart disease I ntermittent changes in the amplitude and duration of the systolic Ca 2ϩ transient are the principle mechanism for regulating the strength of contraction (contractility) of the heart in health. Cardiovascular diseases that cause persistent increases in systolic wall stress require sustained increases in Ca 2ϩ influx and sarcoplasmic reticulum uptake, storage and release to produce the necessary increases in [Ca 2ϩ ] required to maintain the pump function of the heart under these conditions. 1 The persistent increases in [Ca 2ϩ ] that are required to maintain cardiac pump function in pathological cardiovascular stress also activate complex signaling pathways that lead to cardiac hypertrophy, structural and functional remodeling, and cell death. 2 The signaling cascades that link changes in myocyte Ca 2ϩ to activation of hypertrophic and survival signaling are the topic of this study.Increases in myocyte [Ca 2ϩ ] activate both the type 2B Ca 2ϩ /calmodulin (CaM)-dependent phosphatase, calcineurin (Cn), and the Ca 2ϩ /CaM-dependent protein kinase II (CaMKII). Activation of these signaling pathways is linked to electric and contractile disturbances in pathological cardiac hypertrophy. 3,4 Activated Cn induces pathological hypertrophy by dephosphorylation and subsequent nuclear translocation of transcription factors associated with the NFAT family (nuclear factor of activated T cells) that activates specific hypertrophic target genes. 3,[5][6][7][8] In mice, transgenic cardiac overexpression of a constitutively active form of Cn has been shown to cause hypertrophy, mechanical dysfunction, arrhythmias, and premature death. 3,9 In larger animal models 10 and in patients, 11 increased Cn activity has been linked to structural heart disease and the development of heart failure.