KChIP2, a gene encoding three auxiliary subunits of Kv4.2 and Kv4.3, is preferentially expressed in the adult heart, and its expression is downregulated in cardiac hypertrophy. Mice deficient for KChIP2 exhibit normal cardiac structure and function but display a prolonged elevation in the ST segment on the electrocardiogram. The KChIP2(-/-) mice are highly susceptible to the induction of cardiac arrhythmias. Single-cell analysis revealed a substrate for arrhythmogenesis, including a complete absence of transient outward potassium current, I(to), and a marked increase in action potential duration. These studies demonstrate that a defect in KChIP2 is sufficient to confer a marked genetic susceptibility to arrhythmias, establishing a novel genetic pathway for ventricular tachycardia via a loss of the transmural gradient of I(to).

LIM domain containing proteins play critical roles in animal development and cellular differentiation. Here, we describe the cloning and expression patterns of three members of the four and a half LIM domain-only protein family, FHL1, 2, and 3, from mouse. A comparison of embryonic expression patterns of these three highly-related genes indicates that they are expressed in an overlapping pattern in the developing cardiovascular system, and skeletal muscle. In adult tissues, the three genes are expressed in a predominant and overlapping manner in cardiac and skeletal muscle. Of the three genes, FHL2 appears to have the most restricted expression pattern during development, in heart, blood vessels, and skeletal muscle. Expression in heart is highest in cardiac septa and in the region adjacent to the atrio-ventricular ring, suggesting a potential role in septation or conduction system development. In the heart, FHL1expression was observed strongly in developing outflow tract, and to a lesser extent in myocardium. FHL3 displays low and ubiquitous expression during mouse development. Cardiac ventricular expression of FHL1, but not FHL2 or FHL3, was upregulated in two mouse models of cardiac hypertrophic and dilated cardiomyopathy. Taken together, these data indicate the potential importance of this FHL family in the development and maintenance of the cardiovascular system and striated muscle, and suggest that FHL1 may play a role in the development of heart disease.

The requirement for atrial function in developing heart is unknown. To address this question, we have generated mice deficient in atrial myosin light chain 2 (MLC2a), a major structural component of the atrial myofibrillar apparatus. Inactivation of the Mlc2a gene resulted in severely diminished atrial contraction and consequent embryonic lethality at ED10.5-11.5, demonstrating that atrial function is essential for embryogenesis. Our data also address two longstanding questions in cardiovascular development: the connection between function and form during cardiac morphogenesis, and the requirement for cardiac function during vascular development. Diminished atrial function in MLC2a-null embryos resulted in a number of consistent secondary abnormalities in both cardiac morphogenesis and angiogenesis. Our results unequivocally demonstrate that normal cardiac function is directly linked to normal morphogenic development of heart and vasculature. These data have important implications for the etiology of congenital heart disease.

LIM domain-containing proteins play critical roles in vertebrate development and cellular differentiation. Recently, four members of the four and one-half LIM protein (FHL) family have been identified and cloned. One of these, FHL2, is expressed in a restricted manner in the cardiovascular system throughout development into adulthood, suggesting that FHL2 may play an important role in cardiovascular development and function. Here we describe the generation and analysis of mice carrying a null mutation of the FHL2 gene. FHL2-deficient mice are viable and maintain normal cardiac function both before and after acute mechanical stress induced by aortic constriction. These data suggest that FHL2 is not essential for cardiac development and function.The LIM domain has been found in variety of proteins (8,9,13,22) and mediates protein-protein interaction (1). Some LIM proteins are transcription factors involved in cell lineage determination and pattern formation. Others are associated with the cytoskeleton and play a role in adhesion plaque and actin microfilament organization (4,8,16,23).Functional roles for LIM domain proteins have been demonstrated by genetic studies. Disruption of several homeodomain-containing LIM genes has demonstrated a critical role in development of neuronal lineages (16). Mice homozygous for deficiency of the LIM-only protein LMO2 die at embryonic day 10.5 due to lack of erythropoiesis (21). Muscle LIM protein, which is expressed abundantly in heart and skeletal muscle, consists of two LIM domains only (2). Recently it has been shown that mice lacking muscle LIM protein develop dilated cardiomyopathy with hypertrophy and heart failure after birth (3).We hypothesized that other LIM domain-containing proteins may also play important roles in cardiac function. Characterization of these proteins will improve our understanding of the function of LIM domains and may identify candidate genes for cardiomyopathy. A combined GenBank and expressed sequence tag database search revealed a newly identified group of LIM-only proteins with four and one-half LIM domains (the FHL family) (5, 14), which are enriched in striated muscle. This group consists of four family members. Recently, we reported the expression patterns of murine FHL family members, FHL1, -2, and -3, which suggest important functions of this family in skeletal muscle and the cardiovascular system (7).To address the in vivo functions of FHL2, we generated an FHL2-deficient mouse through homologous recombination in embryonic stem (ES) cells. In this study, we demonstrate that FHL2-deficient mice exhibit no obvious phenotype before or at 15 months of age compared to their wild-type littermates. The hearts and blood vessels of homozygous null mice appear normal by histological analysis; hearts of homozygous null mice exhibit normal function on echocardiographic and electrocardiographic analyses, with normal heart weight/body weight ratios, compared to hearts from wild-type littermates. Moreover, homozygous null mice respond to acute pressure ov...