Reducing the Excess Activin Signaling Rescues Muscle Degeneration in Myotonic Dystrophy Type 2 Drosophila Model

Abstract

Expanded non-coding RNA repeats of CCUG are the underlying genetic causes for myotonic dystrophy type 2 (DM2). There is an urgent need for effective medications and potential drug targets that may alleviate the progression of the disease. In this study, 3140 small-molecule drugs from FDA-approved libraries were screened through lethality and locomotion phenotypes using a DM2 Drosophila model expressing 720 CCTG repeats in the muscle. We identified ten effective drugs that improved survival and locomotor activity of DM2 flies, including four that share the same predicted targets in the TGF-β pathway. The pathway comprises two major branches, the Activin and BMP pathways, which play critical and complex roles in skeletal development, maintenance of homeostasis, and regeneration. The Drosophila model recapitulates pathological features of muscle degeneration in DM2, displaying shortened lifespan, a decline in climbing ability, and progressive muscle degeneration. Increased levels of p-smad3 in response to activin signaling were observed in DM2 flies. Decreased levels of activin signaling using additional specific inhibitors or genetic method ameliorated climbing defects, crushed thoraxes, structure, and organization of muscle fibers. Our results demonstrate that a decrease in activin signaling is sufficient to rescue muscle degeneration and is, therefore, a potential therapeutic target for DM2.

Development of Therapeutic Approaches for Myotonic Dystrophies Type 1 and Type 2

“…A search for the drugs that reduce MBNL1 binding to CCUG repeats by the screening of the compound libraries identified various small molecules that could reduce the number of CCUG foci and correct mis-splicing associated with MBNL1 sequestration in DM2 cells [ 59 , 90 , 91 , 92 , 93 , 94 , 95 ]. These compounds were tested in pre-clinical studies mainly in DM2 cell lines and in the mutant CCUG-expressing flies addressing their effects on the splicing of the MBNL1 targets and reduction in CCUG foci.…”

“…It would be important to examine the effect of these molecules in vivo using DM2 mouse models. Importantly, some identified compounds, which inhibit TGFβ-activin signaling, rescued muscle degeneration in DM2 flies [ 91 ]. Although MBNL1 is sequestered by the mutant CCUG repeats, the role of this event in DM2 pathogenesis is not fully understood.…”

Drosophila Models Reveal Properties of Mutant Lamins That Give Rise to Distinct Diseases

“…Drosophila is a powerful organism for modeling human disease, including neuromuscular disorders [ 107 , 108 , 109 , 110 , 111 , 112 , 113 ]. Such models have provided novel insights into disease mechanisms and identified potential drug targets and treatments [ 64 , 113 , 114 , 115 , 116 , 117 , 118 ]. In addition, Drosophila is one of a few selected model organisms used by the Undiagnosed Disease Network (UDN) to functionally test DNA sequence variants of uncertain significance in candidate disease genes ( ).…”

Modeling Myotonic Dystrophy Type 2 Using Drosophila melanogaster

“…The larval muscle fillet can be stained using a variety of techniques to visualize muscle structure and specific markers. For example, fluorescent dyes, such as rhodamine phalloidin, can be used to label muscular actin filaments, while antibodies against specific muscle proteins can be used to identify cell types or structural components [ 36 , 56 ]. Confocal microscopy is then used to capture high-resolution images of the muscle fillet, allowing the analysis of muscle structure and function at the cellular and subcellular levels ( Figure 2 E) [ 36 ].…”

“…For example, fluorescent dyes, such as rhodamine phalloidin, can be used to label muscular actin filaments, while antibodies against specific muscle proteins can be used to identify cell types or structural components [ 36 , 56 ]. Confocal microscopy is then used to capture high-resolution images of the muscle fillet, allowing the analysis of muscle structure and function at the cellular and subcellular levels ( Figure 2 E) [ 36 ]. At the functional level, neurophysiological techniques involving the neuromuscular junction (NMJ) can provide insights into the communication between motor neurons and muscles.…”