Volume regulation of the heart: Molecular mechanisms and strain sensing by the giant protein titin.
CIC nanoGUNE Seminars
- Speaker
-
Pieter de Tombe, Loyola University, Chicago, USA
- When
-
2014/04/28
13:00 - Place
- nanoGUNE seminar room, Tolosa Hiribidea 76, Donostia - San Sebastian
- Add to calendar
- iCal
**Host** : R. Perez-Jimenez
The cellular basis of the Frank-Starling mechanism is sarcomere length (SL)
modulation of myofilament Ca2+ sensitivity (LDA). The molecular mechanism(s)
that underlie LDA are unknown, but recent evidence has implicated the giant
protein titin as possible sarcomeric strain sensor responsible for LDA by an,
as of yet unidentified, signal transduction pathway. Accordingly, the aim of
the present study was to elucidate the impact of SL (from slack=2.0 µm to
stretch=2.4 µm) on LDA and sarcomere structure in isolated rat myocardium
from either wild-type (WT) or mutant (HM) rats expressing a giant splice
isoform of titin. At stretch, WT muscles showed reduced increase in passive
tension (-20%) and twitch force (-58%); also, LDA was significantly blunted at
the myofilament level. Time-resolved small angle x-ray diffraction of intact
twitching muscles during diastole revealed at stretch a significant increase
in intensity and spacing of: myosin M2 (+121% & +0.4%) and troponin T3 (+174%
& +1.0%); Myosin binding protein C (MyoBPC) also trended to increase (C1%C2).
These SL dependent changes in sarcomere structure were absent in HM muscles.
Cross-bridge radial spacing (layer line analysis) was significantly reduced at
stretch in WT (-8.0%); in HM muscles radial spacing was further, but similarly
reduced at both slack and stretch (~-20%). Equatorial spacings and intensity
ratios were similar between WT and HM at both slack and stretch. Electron
density reconstruction revealed, only in WT, increased mass in both thick and
thin filament, and the appearance of an as of yet unidentified moiety spanning
the space between the thick and thin filaments at stretch. These results were
independently confirmed in skinned myocyte fragments in which endogenous TnC
was replaced with fluorescently labeled TnC employing confocal microscopy.
Conclusion: Stretch induces structural changes in both thick and thin
filaments mediated by titin strain. Moreover, MyoBPC may interact with actin
to mediate LDA.