The AE3 Cl − /HCO 3 − exchanger is abundantly expressed in the sarcolemma of cardiomyocytes, where it mediates Cl − -uptake and HCO 3 − -extrusion. Inhibition of AE3-mediated Cl − /HCO 3 − exchange has been suggested to protect against cardiac hypertrophy; however, other studies indicate that AE3 might be necessary for optimal cardiac function. To test these hypotheses we crossed AE3-null mice, which appear phenotypically normal, with a hypertrophic cardiomyopathy mouse model carrying a Glu180Gly mutation in α-tropomyosin (TM180). Loss of AE3 had no effect on hypertrophy; however, survival of TM180/AE3 double mutants was sharply reduced compared with TM180 single mutants. Analysis of cardiac performance revealed impaired cardiac function in TM180 and TM180/AE3 mutants. TM180/AE3 double mutants were more severely affected and exhibited little response to β-adrenergic stimulation, a likely consequence of their more rapid progression to heart failure. Increased expression of calmodulin-dependent kinase II and protein phosphatase 1 and differences in methylation and localization of protein phosphatase 2A were observed, but were similar in single and double mutants. Phosphorylation of phospholamban on Ser16 was sharply increased in both single and double mutants relative to wild-type hearts under basal conditions, leading to reduced reserve capacity for β-adrenergic stimulation of phospholamban phosphorylation. Imaging analysis of isolated myocytes revealed reductions in amplitude and decay of Ca 2+ transients in both mutants, with greater reductions in Address correspondence to Gary E. Shull; Department of Molecular Genetics, Biochemistry and Microbiology, University of Cincinnati, College of Medicine, 231 Albert Sabin Way, Cincinnati, Ohio 45267-0524 Telephone: 513 558-0056, Fax: 513 558-1885, shullge@ucmail.uc.edu. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. TM180/AE3 mutants, consistent with the greater severity of their heart failure phenotype. Thus, in the TM180 cardiomyopathy model, loss of AE3 had no apparent anti-hypertrophic effect and led to more rapid decompensation and heart failure. NIH Public Access

Cl−/HCO−3 exchangers are expressed abundantly in cardiac muscle, suggesting that HCO−3 extrusion serves an important function in heart. Mice lacking Anion Exchanger Isoform 3 (AE3), a major cardiac Cl−/HCO−3 exchanger, appear healthy, but loss of AE3 causes decompensation in a hypertrophic cardiomyopathy (HCM) model. Using intra-ventricular pressure analysis, in vivo pacing, and molecular studies we identified physiological and biochemical changes caused by loss of AE3 that may contribute to decompensation in HCM. AE3-null mice had normal cardiac contractility under basal conditions and after β-adrenergic stimulation, but pacing of hearts revealed that frequency-dependent inotropy was blunted, suggesting that AE3-mediated HCO−3 extrusion is required for a robust force-frequency response (FFR) during acute biomechanical stress in vivo. Modest changes in expression of proteins that affect Ca2+-handling were observed, but Ca2+-transient analysis of AE3-null myocytes showed normal twitch-amplitude and Ca2+-clearance. Phosphorylation and expression of several proteins implicated in HCM and FFR, including phospholamban (PLN), myosin binding protein C, and troponin I were not altered in hearts of paced AE3-null mice; however, phosphorylation of Akt, which plays a central role in mechanosensory signaling, was significantly higher in paced AE3-null hearts than in wild-type controls and phosphorylation of AMPK, which is affected by Akt and is involved in energy metabolism and some cases of HCM, was reduced. These data show loss of AE3 leads to impaired rate-dependent inotropy, appears to affect mechanical stress-responsive signaling, and reduces activation of AMPK, which may contribute to decompensation in heart failure.

Silent β-thalassemia (β-thal) is a group of mutations affecting the β-globin gene that cannot be differentiated in heterozygote states from normal conditions by using conventional criteria for the diagnosis of β-thal trait. Here we report the existence of two silent β-thal mutations in the population of Bahrain, one at -101 (C>T) and the other at -71 (C>T). We screened 126 healthy individuals with high-normal Hb A2 levels and found a frequency of 23.0% for both of these mutations (8.0% for -71 and 15.0% for -101). Heterozygotes for either of these two mutations have an overlapping level of Hb A2 ranging from 3.1-3.9% (3.7 ± 0.3) with some cases displaying microcytic and hypochromic anemia, mainly attributed to coinherited defective α-thal genes. Interestingly, each of these mutations were discovered in combination with the sickle cell allele displaying distinct high performance liquid chromatography (HPLC) profiles, different from those observed in the typical sickle cell disease or the sickle cell trait conditions. These investigations are invaluable to provide appropriate counseling for partners undergoing premarital counseling and to understand the molecular basis of mild and atypical forms of sickle cell disease.

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