A small proportion of Talin molecules transmit forces at developing muscle attachments in vivo

Abstract

Cells in developing organisms are subjected to particular mechanical forces that shape tissues and instruct cell fate decisions. How these forces are sensed and transmitted at the molecular level is therefore an important question, one that has mainly been investigated in cultured cells in vitro. Here, we elucidate how mechanical forces are transmitted in an intact organism. We studied Drosophila muscle attachment sites, which experience high mechanical forces during development and require integrin-mediated adhesion for stable attachment to tendons. Therefore, we quantified molecular forces across the essential integrin-binding protein Talin, which links integrin to the actin cytoskeleton. Generating flies expressing 3 Förster resonance energy transfer (FRET)-based Talin tension sensors reporting different force levels between 1 and 11 piconewton (pN) enabled us to quantify physiologically relevant molecular forces. By measuring primary Drosophila muscle cells, we demonstrate that Drosophila Talin experiences mechanical forces in cell culture that are similar to those previously reported for Talin in mammalian cell lines. However, in vivo force measurements at developing flight muscle attachment sites revealed that average forces across Talin are comparatively low and decrease even further while attachments mature and tissue-level tension remains high. Concomitantly, the Talin concentration at attachment sites increases 5-fold as quantified by fluorescence correlation spectroscopy (FCS), suggesting that only a small proportion of Talin molecules are mechanically engaged at any given time. Reducing Talin levels at late stages of muscle development results in muscle–tendon rupture in the adult fly, likely as a result of active muscle contractions. We therefore propose that a large pool of adhesion molecules is required to share high tissue forces. As a result, less than 15% of the molecules experience detectable forces at developing muscle attachment sites at the same time. Our findings define an important new concept of how cells can adapt to changes in tissue mechanics to prevent mechanical failure in vivo.

The Hippo pathway controls myofibril assembly and muscle fiber growth by regulating sarcomeric gene expression

“…Then, flight muscles build up mechanical tension, compact to about 220 µm in length, while their diameter grows to about 2000 µm 2 , and assemble the immature myofibrils at 32 h APF (Lemke et al, 2019;Weitkunat et al, 2014) (Figure 1A,C). After 32 h, flight muscles grow about 2.5 times in length while keeping the same diameter until 48 h APF.…”

Myofibril and mitochondria morphogenesis are coordinated by a mechanical feedback mechanism in muscle

“…We found that flight muscle mitochondria are in intimate contact with myofibrils and acquire an elongated shape. We have shown in the past that myofibrils are under significant mechanical tension during development and that this tension is required to build linear myofibrils 18,19 . Hence, we hypothesized that myofibril tension creates a pushing force against mitochondria that constrain them into the observed ellipsoid shape.…”

<strong>The Mechanical Basis of Memory - the MeshCODE Theory</strong>

“…Visualising these complexes is further complicated as any perturbation of the system will result in altered MeshCODE arrangements. However, the technical capabilities to observe protein states and forces acting on proteins in cells are advancing rapidly (Kumar et al, 2016;Ringer et al, 2017;Lemke et al, 2019) and used in conjunction with super-resolution microscopy techniques (Leterrier et al, 2017;Schnitzbauer et al, 2017;Jacquemet et al, 2020), optogenetic techniques (Liu et al, 2012), and the well-established strategies for studying neurotransmission (reviewed in (Kandel et al, 2014)) such conformational changes during memory formation should be detectable. Further, a number of talin-binding compounds have recently been identified (Yang et al, 2017;Bryce et al, 2019) and the effect of such compounds on learning and memory in animal systems might provide opportunities to pharmaceutically modulate these processes.…”