Fundamental Mechanisms Governing Muscle Formation and Maintenance

Our lab investigates genes and networks regulating muscle identity, muscle size, and muscle organelle positioning, particularly myonuclei movement and placement.

In the Drosophila embryo, a repeated pattern of 30 distinct muscle fibers is present in each abdominal hemisegment. Despite similarities, such as shared expression of contractile proteins, each muscle fiber can be distinguished by properties like its size, shape, orientation, number of nuclei, innervation, and tendon attachment site (see Figure 4).

Muscle identity (transcriptional and chromatin regulation)

Muscle fibers arise by the iterative fusion of two types of myoblasts, called founder cells (FCs) and fusion competent myoblasts (FCMs), to form a syncytium. FCs are thought to contain all the information needed to make a muscle of a particular size and shape. This information is encoded by DNA-binding transcription factors, such as Krüppel, Slouch, and Apterous, expressed in incompletely overlapping subsets of FCs, known as the FC identity genes.

Using a novel molecular screen, we have identified new transcription factors, such as Alhambra and Charlatan, and, for the first time, chromatin regulators such as Sin3a and Kdm2 that are enriched in different populations of FCs (see Figure 5). We have found that the chromatin regulators buffer the function of the transcriptional regulators, assuring specific muscle identities. Current investigation is directed at finding how morphological information is translated from the identity regulators to the cellular processes that control muscle size and shape.

Muscle cell size (membrane dynamics, actin regulation, and cellular trafficking along MTs)

As in humans, muscle cells in Drosophila form by the fusion of myoblasts. Thirty FCs seed the abdominal muscle pattern and recruit neighboring FCMs to form multinucleate muscle fibers. Certain muscles stereotypically contain as few as three nuclei (indicative of two fusion events), whereas others consistently incorporate up to 25 nuclei (24 fusion events).

Using genetic screens and time-lapse videography, we have identified an actin structure — the F-actin focus — that marks the fusion site. The F-actin focus forms the backbone of an invasive podosome-like structure that is required for fusion to progress. Using different genetic screens, we have shown that this structure can be modulated by several additional actin regulators as well as by PI(4,5)P2 signaling.

Current experiments are aimed at identifying additional components of the fusion process as well as the mechanism by which muscle cells “count” and determine the characteristic number of fusion events for a particular muscle.

Muscle subcellular organization (nuclear positioning, cytoskeletal organization, other organelle placement)

Highlighting the importance of proper intracellular organization, many muscle diseases are characterized by mispositioned myonuclei. We have found that proper positioning of myonuclei is dependent upon microtubule organization, microtubule-associated proteins such as Ensconsin, and the microtubule motor proteins Kinesin-1 and cytoplasmic Dynein. Mutations in these proteins affect myonuclear position and muscle function (see Figure 7, 8).

We have identified at least two distinct mechanisms by which these proteins move and position myonuclei. These motors exert forces both directly on the nuclear surface and from the cell cortex via microtubules. Current projects investigate additional components required for myonuclear movement, the contributions of other tissues to nuclear placement, and the physiological changes that occur in muscle with displaced nuclei.

Dobi, KC, Halfon MS, Baylies MK. 2014 Whole-genome analysis of muscle founder cells implicates the chromatin regulator sin3a in muscle identity. Cell Reports. 2014 Jul 30. pii: S2211-1247(14)00572-5. PMID:25088419. PMCID: PMC – in process.

Bothe, I., Deng, S., Baylies MK. 2014 PI(4,5)P2 regulates myoblast fusion through Arp2/3 regulator localization at the fusion site. Development. 141(11):2289-301. PMID: 24821989. PMCID: PMC4034421

Folker, E., Schulman, V., Baylies MK. 2014 Translocating myonuclei have distinct leading and lagging edges which require Kinesin and Dynein. Development. 141(2):355-66. doi: 10.1242/dev.095612. Epub 2013 Dec 11. PMID: 24335254. PMCID: PMC3879816

Folker, E. and Baylies, MK. 2013 Nuclear positioning in muscle development and disease. Frontiers in Physiology. 2013 Dec 12;4:363. eCollection 2013. PMID: 24376424. PMCID: PMC3859928. Editors’ feature with over 800 hits thus far, making it one of the most visited reviews.

Folker, E., Schulman, V., Baylies MK. 2012 Muscle length and myonuclear position are independently regulated by distinct Dynein pathways. Development. 139(20):3827-37. PMID: 22951643. PMCID: PMC3445310

Metzger T, Gache V, Xu M, Cadot B, Folker ES, Richardson BE, Gomes ER, Baylies MK. 2012 MAP and Kinesin-dependent nuclear positioning is required for skeletal muscle function. Nature. 484:120-124. PMID: 22425998. PMCID: PMC3321085

Nowak SJ, Aihara H, Gonzalez K, Nibu Y, Baylies MK. 2012 Akirin links Twist-regulated transcription with Brahman chromatin remodeling complex during embryogenesis. PloS Genetics: 8, e1002547, 2012. PMID: 22396663. PMCID: PMC3291577

Dobi, KC, Metzger, T, and Baylies MK. 2011 Characterization of the early steps of muscle morphogenesis in a Drosophila primary culture system. Fly (Austin) 5, 68-75. PMID: 21339707. PMCID: PMC3127058

Nowak, S., Nahirney, P, Hadjantonakis, A.K. and Baylies, MK. 2009. Nap1-mediated actin remodeling is essential for mammalian myoblast fusion. Journal of Cell Science. 122: 3282-3293. PMID: 19706686. PMCID: PMC2736864

Wong, MC, Castanon, I. and Baylies, MK. 2008 Daughterless dictates Twist activity in a context dependent manner during somatic myogenesis. Developmental Biology, 317:417-429. PMID: 18407256. PMCID:PMC2435306

Richardson, B, Beckett, K, Nowak, S. and Baylies, MK. 2007 SCAR/WAVE and Arp2/3 are critical for cytoskeletal remodeling at the site of myoblast fusion. Development. 134:4357-4367. PMID: 18003739. PMCID: PMC2880884

Beckett, K. and Baylies, MK 2007 3D analysis of founder cell and fusion competent myoblasts arrangements outlines a new model of myoblast fusion. Developmental Biology. 309:113-125. PMID:17662708. PMCID: PMC2709992

Carmena, A. and Baylies, M.K. 2006 The PDZ protein Canoe/AF-6 links RAS-MAPK, Notch and Wingless/Wnt signaling pathways by directly interacting RAS, Notch and Dishevelled. PLoS ONE: 1, e66. PMID: 17183697. PMCID: PMC1762375