The project: a 元気 super-relaxed state in striated muscles

 Let me introduce shortly the project. The main reason why I am living in Japan now. The super short story, is that a new state has been “recently” discovered in the striated muscle. A state which is stable in the relaxed muscles, which was hiding itself because for many decades researchers were interested in the active behavior of muscles. It’s obvious isn’t it? You want to know how muscle contract, not how the can stay relaxed!!

Well, the current view is that this new state, called SUPER-RELAXED STATE, can deeply influence the active behavior of muscles, especially of cardiac muscle, causing also serious diseases when it’s equilibrium is perturbed. So, in this sense the super-relaxed state is pretty “active” or “genki” (げんき, or 元気) in Japanese.

A 元気 super-relaxed state!

So! My Global Fellowship project aims at:

(i) defining the theoretical bases of the recently discovered “mechanosensing” (MS) mechanism that acts on muscle molecular motors

(ii) applying it in a quantitative, non-phenomenological way to the cardiac muscle, via an heart simulator.

The project wants to help the understanding in myocardial mechanics thus pathologies in a patient-specific approach, and the currently developing pharmaceutical treatments based on this mechanism.

Muscle force is directly proportional to the number of myosin motors going through the so-called cross- bridge cycle, the ATP-driven interaction between myosin proteins, protruding from the thick filament, and actin proteins, forming the thin filament. Classically, the calcium concentrations ([Ca2+]) in the myofibrillar space is known to modulate the activation of the thin-filament and, through it, the number of detached but active (ON, pointing toward the thin filament) myosin motors, which generate the cross-bridges.

Historically, experimental efforts to understand striated muscle energetics focused on this ON-to-attached transition. However, in 2010 a detached state, where motors lie on the thick filament and are unable to interact with activated actin, has been identified: the super-relaxed (SRX or OFF) state.

Scheme of the multi-scale model. The OFF state is introduced at the molecular level (left-top) and the MS mechanism operate in the OFF-ON transition. This affect the sarcomere/fiber level (left top) and the whole heart contraction.

Moreover, ground-breaking data have recently proved the existence of an internal MS mechanism that relates the ratio of ON-to-OFF motors to the tension sustained by the thick myosin filament. This creates a critical cellular feedback mechanism, likely associated with the Frank-Starling law, which malfunction can be at play in hypertrophic cardiomyopathies (HCM). Accordingly, the pharmacological “stabilization” of the OFF state could prevent or reduce HCM consequences, a therapeutic option that already reached the clinical stage.

Yet the molecular bases of the MS mechanism remain mostly unknown at the molecular level (Gap1), and this limits the strategies to address cardiac pathologies related to its disfunction (Gap2). On these grounds, here we propose to shape the theoretical description of this mechanism (Objective Obj1) and to usher it into a multiscale model (Obj2) – from the molecule to the organ. Doing so will enable us to create a benchmark to drive pharmacological applications, aiming at reducing the failure rate in this drug discovery pipeline.

The concepts of MS mechanism and ON state have been introduced for the first time in 2015 (Linari et al Nature, 2015), and experimental evidence in skeletal muscle has shown that it is related to a second regulatory mechanism, in tandem with the Ca2+-mediated thin filament activation. Briefly, at higher active or passive tensions sustained by the thick filament, the MS-mechanism generates a higher probability that OFF myosin motors, not able to enter in the conventional cycle, are recruited to the ON state. As per my previous work (Marcucci Reggiani Front Physiol 2016, Marcucci et al. Sci. Reports 2017) , the first mathematical muscle model including the MS mechanism (and representing the conceptual foundation of the present proposal) separates the conventional cross-bridge cycle into two steps (Figure above): 1) a non-force generating, or “resting cycle”, including the OFF and ON states; and 2) a force-generating or “working” cycle, encompassing the ON and strongly-attached states. Force regulation in contracting muscle includes both cycles, through thick and thin filament activations.

Since its discovery, the MS-mechanism has been attracting more and more interest in the scientific community, as shown by the rapidly growing literature. This is not only because it may play a fundamental role in the physiology of basic understanding of the physiological aspects of the skeletal and cardiac muscle contraction, but also for its implications in the treatment of hypertrophic and dilated cardiomyopathies, as in the case of MyoKardia. This US-based company developed a drug for the treatment of HCM that, in essence, is a stabilizer of the OFF state. Phase III clinical trials testing the effects of this compound are about to start in EU and USA (“Explorer- HCM”). This clinical trial not only shows that this project is in line with the European strategies, but also the data from “Explorer-HCM” represents a unique opportunity for validating the pragmatic implications of the findings emanating from this project.

All these recently published experimental and theoretical evidences strongly suggest that including the MS- mechanism into the heart simulator is crucial for reliable clinical applications.
The long-term purpose of the present project is the quantitative understanding of the physiological meaning of MS mechanism in macroscopic cardiac contraction, both in physiological and pathological situations. Gathering this new knowledge will pave the way to innovative, unprecedented opportunities to treat cardiac diseases.

More specifically, the objectives of the present projects are :
Obj1) To go beyond the current phenomenological approach, modelling the OFF-ON state transition through a protein-protein interaction scheme and implement this molecular-to-fibre model into a fibre-to-heart simulator
Obj2) To characterize the influence of this molecular mechanism quantitatively and relating it to the double regulation in heart contraction, aiming at clinical outcomes in a patient-specific way
Obj3) To create a cluster of researchers with complementary expertise at the University of Padova and integrating it as a crucial node in an international network of collaborations

This blog is created as a dissemination part of the project. Here i will report my experience, both about living in Japan and the project development, mixing scientific and social information about my experience as a European researcher in this country. I will describe the daily life in Japan, inside and outside academic world.

Ok ok ok… shortly, if you enjoy science AND also traveling around the world, there is a way to be also (well) paid to do it!! The HORIZON-MSCA funding opportunities!