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sketch of muscle cells

Losing muscle to fat: misdirected fate of a multipotent stem cell drives LGMD2B

Fibro/adipogenic precursors (FAPs) control the onset and severity of disease in limb-girdle muscular dystrophy type 2 (LGMD2B)

Fibro/adipogenic precursors (FAPs) control the onset and severity of disease in limb-girdle muscular dystrophy type 2 (LGMD2B). a) Healthy and/or pre-symptomatic LGMD2B muscle contains resident FAPs. b) After myofiber injury, inflammatory cells invade and trigger FAP proliferation. c) In symptomatic LGMD2B muscle, there is a gradual accumulation of extracellular AnxA2, which prolongs the pro-inflammatory environment, causing excessive FAP proliferation. d) Blocking aberrant signaling due to AnxA2 buildup blocks FAP accumulation and thus preventing adipogenic loss of dysferlinopathic muscle. Credit: “Fibroadipogenic progenitors are responsible for muscle loss in limb girdle muscular dystrophy 2B.” Published online June 3, 2019, in Nature Communications. Marshall W. Hogarth, Aurelia Defour, Christopher Lazarski, Eduard Gallardo, Jordi Diaz Manera, Terence A. Partridge, Kanneboyina Nagaraju and Jyoti K. Jaiswal. https://rdcu.be/bFu9U.

Research led by faculty at Children’s National published online June 3, 2019, in Nature Communications shows that the sudden appearance of symptoms in limb-girdle muscular dystrophy type 2 (LGMD2B) is a result of impaired communication between different cell types that facilitate repair in healthy muscle. Of particular interest are the fibro/adipogenic precursors (FAPs), cells that typically play a helpful role in regenerating muscle after injury by removing debris and enhancing the fusion of muscle cells into new myofibers.

LGMD2B is caused by mutations in the DYSF gene that impair the function of dysferlin, a protein essential for repairing injured muscle fibers. Symptoms, like difficulty climbing or running, do not appear in patients until young adulthood. This late onset has long puzzled researchers, as the cellular consequences of dysferlin’s absence are present from birth and continue through development, but do not impact patients until later in life.

The study found that in the absence of dysferlin, muscle gradually increases the expression of the protein Annexin A2 which, like dysferlin, facilitates repair of injured muscle fiber. However, increasing Annexin A2 accumulates outside the muscle fiber and drives an increase in FAPs within the muscle as well as encourages these FAPs to differentiate into adipocytes, forming fatty deposits. Shutting down Annexin A2 or blocking the adipocyte fate of FAPs using an off-the-shelf medicine arrests the fatty replacement of dysferlinopathic muscle.

“We propose a feed-forward loop in which repeated myofiber injury triggers chronic inflammation which, over time, creates an environment that promotes FAPs to accumulate and differentiate into fat. This, in turn, contributes to more myofiber damage,” says Jyoti K. Jaiswal, MSc, Ph.D., a principal investigator in the Center for Genetic Medicine Research at Children’s National and the study’s senior author.

“Adipogenic accumulation becomes the nucleating event that results in an abrupt decline in muscle function in patients. This new view of LGMD2B disease opens previously unrealized avenues to intervene,” adds Marshall Hogarth, Ph.D., the study’s lead author.

Joyti Jaiswal

“We propose a feed-forward loop in which repeated myofiber injury triggers chronic inflammation which, over time, creates an environment that promotes FAPs to accumulate and differentiate into fat. This, in turn, contributes to more myofiber damage,” says Jyoti K. Jaiswal, MSc, Ph.D.

A research team led by Jaiswal collaborated with Eduard Gallardo and Jordi Diaz Manera, of Hospital de la Santa Creu in Barcelona, Spain, to examine muscle biopsies from people with LGMD2B who had mild to severe symptoms. They found that adipogenic deposits originate in the extracellular matrix space between muscle fibers, with the degree of accumulation tied to disease severity. They found a similar progressive increase in lipid accumulation between myofibers predicted disease severity in dysferlin-deficient experimental models. What’s more, this process can be accelerated by muscle injury, triggering increased adipogenic replacement in areas that otherwise would be occupied by muscle cells.

“Accumulation and adipogenic differentiation of FAPs is responsible for the decline in function for dysferlinopathic muscle. Reversing this could provide a therapy for LGMD2B, a devastating disease with no effective treatment,” predicts Jaiswal as the team continues research in this field.

Promising off-the-shelf drugs include batimastat, an anti-cancer drug that inhibits the extracellular matrix enzyme matrix metalloproteinase. This drug reduces FAP adipogenesis in vitro and lessens injury-triggered lipid formation in vivo. In experimental models, batimastat also increases muscle function.

In addition to Jaiswal, Hogarth, Gallardo and Diaz Manera, other study co-authors include Aurelia Defour, Christopher Lazarski, Terence A. Partridge and Kanneboyina Nagaraju, all of Children’s National.

Financial support for research described in this post was provided by the Muscular Dystrophy Association under awards MDA477331 and MDA277389, the National Institute of Arthritis and Musculoskeletal and Skin Diseases under award R01AR055686 and the National Institutes of Health under awards K26OD011171, R24HD050846 and P50AR060836.

Stanley Thomas Fricke

Using IR imaging to improve lead apron inspection

Stanley Thomas Fricke

“When I researched how lead aprons are inspected, I learned that a combination of tactile and visual inspection is considered the gold standard. But many of the smallest holes can be missed this way,” says Stanley Thomas Fricke, Nucl. Eng., Ph.D., radiation safety officer at Children’s National Health System and study senior author.

Workers inspecting the lead aprons that shield patients from radiation during imaging tend to use tactile and visual inspections to find defects, running their fingers over the aprons since fingertips can detect even subtle changes to a surface. Yet findings from a new study could influence changes in this approach to improve inspection performance and better protect patients and inspectors.

Infrared (IR) thermal imaging is a much better detective, with 50 percent of study participants picking out all holes intentionally drilled into a test apron compared with just 6 percent of participants who detected the same defects using the tactile method, according to research published online Nov. 8, 2017 in Journal of the American College of Radiology. In addition to being a more accurate way to detect subtle defects, the IR imaging technology also reduces ionizing radiation exposure for inspectors checking the protective power of lead aprons.

“When I researched how lead aprons are inspected, I learned that a combination of tactile and visual inspection is considered the gold standard. But many of the smallest holes can be missed this way,” says Stanley Thomas Fricke, Nucl. Eng., Ph.D., radiation safety officer at Children’s National Health System and study senior author. “Unlike the fingertips, infrared light can penetrate the lead apron’s protective outer fabric and illuminate defects that are smaller than the defect size now used to reject a protective apron. This work challenges conventional wisdom and offers an inexpensive, readily available alternative.”

According to the study team, a growing number of health care settings use radiation-emitting imaging, from the operating room to the dentist’s office. Lead aprons and gonadal shields lower radiation doses experienced by health care staff and patients. In compliance with regulators, these protective devices are inspected regularly. A layer of lead inside keeps patients’ exposure to ionizing radiation at the lowest detectable level. The aprons are covered with nylon or polyester fabric for the patients’ comfort and for ease of cleaning.

“It is standard for health care institutions to use a tactile-visual approach to inspect radiation protective apparel,” Fricke says. “While increasingly common, that inspection method can allow aprons with holes and tears to slip by undetected due to the large surface area that needs to be inspected, the outer fabric that encloses the protective apron and other factors.”

Fricke recalled a news clip from years ago about an IR camera used to film swimmers at the pool that, like Superman’s powerful vision, could see through pool-goers’ clothing. The manufacturer quickly recalled the camera. But the IR technology is a perfect fit for inspectors looking for defects hidden under a lead apron’s fabric cover.

To validate this inspection alternative, the team drilled a series of nine holes ranging from 2 mm to 35 mm in diameter into a “phantom” lead apron and enclosed it within fabric that typically covers the protective shielding. The research team stapled the phantom apron to a wooden frame and placed dry wall under the frame.

Two of 31 radiation workers picked out all nine holes by touch and recorded the holes and their locations on written questionnaires.

For the IR method, the team used an infrared light to illuminate the lead apron from behind and relied on an infrared imaging camera to record 10 seconds of video from which still images were exported. Ten of 20 radiation technologists, radiology nurses and medical doctors identified all nine holes using those color photographs and recorded their entries on a questionnaire. An additional 20 percent identified eight of nine intentional defects to the phantom apron.

In both the tactile and IR groups, all participants found the largest hole and correctly recorded its location.

“Using the tactile method for inspection, most staff who work regularly with radiation-emitting devices were able to identify defects that would cause a lead apron to be rejected, which is 11 mm holes for thyroid shields and 15 mm holes for aprons,” Fricke says. “However, it is standard for these well-used aprons to develop smaller holes—which, over time, become bigger holes. Here at Children’s National, we care about every photon that touches a child.”

In the next phase of the research, the team will explore infrared flash photography, cooling the apron material and the impact of high-resolution cameras with greater depth of field.