Date Published: February 21, 2018
Publisher: Public Library of Science
Author(s): Daniel W. Fisher, Phillip Luu, Neha Agarwal, Jonathan E. Kurz, Dane M. Chetkovich, Xuan-Zheng Shi.
Hyperpolarization-activated Cyclic Nucleotide-gated (HCN) channels are important regulators of excitability in neural, cardiac, and other pacemaking cells, which are often altered in disease. In mice, loss of HCN2 leads to cardiac dysrhythmias, persistent spike-wave discharges similar to those seen in absence epilepsy, ataxia, tremor, reduced neuropathic and inflammatory pain, antidepressant-like behavior, infertility, and severely restricted growth. While many of these phenotypes have tissue-specific mechanisms, the cause of restricted growth in HCN2 knockout animals remains unknown. Here, we characterize a novel, 3kb insertion mutation of Hcn2 in the Tremor and Reduced Lifespan 2 (TRLS/2J) mouse that leads to complete loss of HCN2 protein, and we show that this mutation causes many phenotypes similar to other mice lacking HCN2 expression. We then demonstrate that while TRLS/2J mice have low blood glucose levels and impaired growth, dysfunction in hormonal secretion from the pancreas, pituitary, and thyroid are unlikely to lead to this phenotype. Instead, we find that homozygous TRLS/2J mice have abnormal gastrointestinal function that is characterized by less food consumption and delayed gastrointestinal transit as compared to wildtype mice. In summary, a novel mutation in HCN2 likely leads to impaired GI motility, causing the severe growth restriction seen in mice with mutations that eliminate HCN2 expression.
Hyperpolarization-activated Cyclic Nucleotide-gated (HCN) channels are primarily expressed in neural and cardiac tissues where they influence the excitability of both pacemaking and non-pacemaking cells by conducting a depolarizing, hyperpolarization-activated inward current (Ih)[1, 2]. HCN channels form both hetero- and homo-tetramers in a “dimer-of-dimers” configuration from four pore-forming subunits, HCN1-4[1, 2]. Differing expression patterns, channel gating kinetics, interaction with accessory subunits[2–4], and responsivity to modulators such as cyclic nucleotides distinguish the effects of each subunit on cellular and somatic processes[1, 2].
In this report, we have identified a pathologic mutation in Hcn2 in the TRLS/2J mouse line and demonstrated that the phenotype of this mouse is similar to other mice with mutations leading to loss of HCN2 expression. Through investigation of insulin, pituitary hormones, and thyroid hormones, we present evidence that nutritional deficiency due to reduced energy intake rather than primary pancreatic, pituitary, or thyroid dysfunction causes the small size in Trls2-/- mice. Finally, we show that altered GI motility is a plausible explanation for the reduced feeding and growth deficits in these mice.