Fantastic muscle proteins and where to find them

Date:
June 19, 2020
Source:
Max Delbru"ck Center for Molecular Medicine in the Helmholtz
Association
Summary:
Setting out to identify all proteins that make up the sarcomere, the
basic contractile unit of muscle cells, resulted in an unexpected
revelation, providing experimental evidence that helps explain a
fundamental mystery about how muscles work.



FULL STORY ========================================================================== Researchers at the Max Delbru"ck Center for Molecular Medicine in the
Helmholtz Association (MDC) developed a mouse model that enables them to
look inside a working muscle and identify the proteins that allow the
sarcomere to contract, relax, communicate its energy needs, and adapt
to exercise. Specifically, they were able to map proteins in defined
subregions of the sarcomere, starting from the "Z-disc," the boundary
between neighboring sarcomeres. This in and of itself was a significant
step forward in the study of striated muscle.


==========================================================================
In the process, they made an unexpected discovery: myosin, one of the
three main proteins that make up striated muscle fibers, appears to
enter the Z-disc.

Models of how myosin, actin and the elastic scaffold protein titin work together have largely ignored the possibility that myosin filaments
penetrate the Z-disc structure. Only recently have German scientists
theorized that they do, but no experimental evidence has validated the
model, until now.

"This is going to be unexpected even for myosin researchers,"
says Professor Michael Gotthardt, who heads MDC's Neuromuscular and Cardiovascular Cell Biology Lab and led the research. "It gets to the very basics of how muscles generate force." Who's there? Gotthardt's team including first authors Dr. Franziska Rudolph and Dr. Claudia Fink with
the help from colleagues at the MDC and the University of Go"ttingen,
never set out to validate this theory. Their primary goal was to identify
the proteins in and near the Z-disc. To do this, they developed a mouse
model with an artificial enzyme, called BioID, inserted into the giant
protein titin. The Titin-BioID then tagged proteins close to the Z-disc.

Sarcomeres are tiny molecular machines, packed with proteins that
tightly interact. Until now it has been impossible to separate
proteins specific to the different subregions, especially in live,
functioning muscle. "Titin-BioID probes specific regions of the sarcomere structure in vivo," says Dr. Philipp Mertins, who heads MDC's Proteomics
Lab. "This has not been possible before." The team is the first to use
BioID in live animals under physiological conditions and identified 450 proteins associated with the sarcomere, of which about half were already
known. They found striking differences between heart and skeletal muscle,
and adult versus neonatal mice, which relate to sarcomere structure,
signaling and metabolism. These differences reflect the need of adult
tissue to optimize performance and energy production versus growth and remodeling in neonatal tissue.



==========================================================================
"We wanted to know who's there, know who the players are," Gotthardt says.

"Most were expected, validating our approach." The surprise The protein
that they were not expecting to see in the Z-disc was myosin, which
is integrated at the opposite site of the sarcomere. When a muscle is
triggered to move, myosin walks along actin bringing neighboring Z-discs
closer together.

This sliding of actin and myosin filaments creates the force that enables
our heart to pump blood or our skeletal muscle to maintain posture,
or lift an object.

This so-called "sliding filament model" of the sarcomere describes
force production and helps explain how force and sarcomere length
relate. However, current models have trouble predicting the behavior of
fully contracted sarcomeres. Those models have assumed myosin does not
enter the Z-disc on its walk along actin. There have been some hints
that maybe it keeps going. "But we didn't know if what we were seeing
in stained tissue samples was an artefact or real life," Gotthardt
says. "With BioID we can sit at the Z-disc and watch myosin pass by."
Gotthardt agrees with the proposed theory that myosin entering the Z-disc
can limit or dampen the contraction. This might help solve the ongoing
issue scientists have had calculating how much force a muscle fiber
can create in relation to its length and lead to a refined model of the sarcomere and possibly serve to protect muscle from excessive contraction.

Why it matters Understanding how muscle fibers extend and contract on
the molecular level under normal conditions is important so researchers
can then identify what is going wrong when muscles are damaged, diseased
or atrophy with age. Identifying which proteins are causing problems
could potentially help identify novel treatment targets for patients
with heart disease or skeletal muscle disorders.

Gotthardt and his team plan to next use BioID to study animals with
different pathologies, to see what proteins are involved in muscle
atrophy, for example.

"Maybe a protein that is not normally there goes into the sarcomere, and
it is part of the pathology," Gotthardt says. "We can find it with BioID."

========================================================================== Story Source: Materials provided by Max_Delbru"ck_Center_for_Molecular_Medicine_in_the
Helmholtz_Association. Original written by Laura Petersen. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Franziska Rudolph, Claudia Fink, Judith Hu"ttemeister, Marieluise
Kirchner, Michael H. Radke, Jacobo Lopez Carballo, Eva Wagner,
Tobias Kohl, Stephan E. Lehnart, Philipp Mertins, Michael Gotthardt.

Deconstructing sarcomeric structure-function relations in
titin-BioID knock-in mice. Nature Communications, 2020; 11 (1)
DOI: 10.1038/s41467- 020-16929-8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200619120839.htm

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