Changes in Traction Force Mediated Through the Actin-Myosin Interface Controls Inside-Out Integrin Signaling Leading to Cardiac Hypertrophy

Abstract

Thrombospondin-4 (TSP4) is a matricellular protein found in the heart and upregulated in heart failure; however, its role in cardiac regulation is unknown. Our previous work showed that mice lacking the TSP4 gene (tsp4-/-) respond normally to acute (seconds) pressure overload, but rapidly deteriorate minutes after. Thus, we hypothesize that TSP4 is involved in the slow force response (SFR), the second phase of stretch-mediated adaptation to loading, discovered a century ago, yet not fully understood. One proposed mechanism for the SFR is an increase in ERK phosphorylation that leads to increased Na þ /H þ exchanger-1 activity. The latter increases intracellular Na þ , causing the Na þ /Ca 2þ exchanger (reverse mode) to rise intracellular Ca 2þ , therefore increasing force. To investigate this, cardiac papillary muscles were isolated from tsp4-/and wild-type (WT) littermates. Muscles were stretched from 92% of the length that generated maximum force (L max ) to 98% L max . This length was maintained for 15 min while force and Ca 2þ transients (fura-2AM) were simultaneously recorded. The immediate rise in force without change in Ca 2þ (Frank-Starling) was similar in both groups. However, whereas a positive SFR occurred in WT (33 5 7%, n=5), tsp4-/muscle displayed a negative SFR (À14 5 2%, n=5). The difference in force was accompanied by a rise in Ca 2þ in WT but not in tsp4-/-(7 5 2% vs. 15 2%, P<0.05). Next, hearts were excised from mice 15 minutes following transaortic constriction (pressure overload) and phosphorylation of ERK1/2 and Akt was found to be decreased in the tsp4-/mice (P<0.05). We conclude that TSP4 is necessary for cardiac adaptation to stretch. Its absence blunts the SFR (force and Ca 2þ rise). Moreover, TSP4 seems to be involved in this mechanism upstream of ERK1/2 and Akt.