Abstract 3775: Cardiomyocyte-Specific Deletion of Inhibitor of Kappa-B Kinase-β (IKKβ) In Vivo Reveals Essential Role in Calcium Handling

Abstract

The serine-threonine kinase, IKKβ, is required in most settings for activation of NF-κB transcription factors, which regulate a wide range of essential processes from cell survival to inflammation. In the heart, while IKKβ and NF-κB are activated in many diseases conditions, whether this activation is adaptive or deleterious remains unclear. To determine the role of IKKβ in the heart, we generated cardiac-specific knockout mice utilizing cardiomyocyte (CM)-specific expression of either Cre or an activatable Cre-fusion protein, producing constitutive (c) or tamoxifen-inducible (i) cardiac-specific IKKβ knockouts (CSIKO), respectively. cCSIKO were born at the expected Mendelian frequency and developed normally. IKKβ expression was dramatically reduced in both cCSIKO and (after tamoxifen) iCSIKO hearts. As expected, CM NF-κB activation after in vivo LPS treatment was markedly impaired in both cCSIKO and iCSIKO mice. Interestingly, both lines had cardiac dysfunction. No evidence of increased apoptosis was observed by TUNEL staining in cCSIKO. Studies in isolated CM from cCSIKO demonstrated reduced decreased peak shortening (2.3% vs. 3.7%) and calcium transient amplitude (0.30 vs 0.46, p<0.03 for both) without a change in normalized kinetics. Protein expression of the calcium handling proteins (L-type calcium channel [LTCC], calsequestrin, sodium-calcium exchanger, and phospho- to dephosphorylated ryanodine receptors) was unchanged in whole heart lysates. However confocal immunohistochemistry demonstrated a markedly abnormal distribution of the LTCC α2 subunit in CSIKO hearts despite intact T-tubules and dyads by electron microscopy. In conclusion, conditional deletion of IKKβ reveals an essential and unanticipated role in regulating CM calcium handling that appears mediated by alterations in LTCC subunit trafficking. Understanding the role of IKKβ and NF-κB in regulating cardiac function has important practical implications for therapeutic strategies targeting this pathway.