Blocking Rac1 function in precursors of the indirect flight muscle of Drosophila severely disrupts muscle formation. The DLM fibers that develop using larval scaffolds are reduced in number and fiber size, while the DVMs, which develop using founder cells, are mostly absent. These adult muscle phenotypes are in part due to a reduced myoblast pool present at the third larval instar. BrDU labeling studies indicated that this is primarily due to a reduction in proliferation. In addition, DVM myoblasts display altered morphology and are unable to segregate into primordia. This defect precedes the evident block in fusion. We also show that the recently described DVM founder cells can be labeled with 22C10 and beta-3 tubulin, and that they are present under conditions of dominant negative Rac1(N17) expression. Despite the presence of founder cells, DVM fiber formation is rarely observed. Although DLM myoblasts are able to segregate around their larval scaffolds, the pace of fusion is reduced and consequently there is a delay in DLM fiber formation. Thus, in addition to its well-established role in fusion, Rac1 is also involved in the regulation of myoblast proliferation and segregation during adult myogenesis. These are two new roles for Rac1 in Drosophila.

During insect myogenesis, myoblasts are organized into a pre-pattern by specialized organizer cells. In the Drosophila embryo, these cells have been termed founder cells and play important roles in specifying muscle identity and in serving as targets for myoblast fusion. A group of adult muscles, the dorsal longitudinal (flight) muscles, DLMs, is patterned by persistent larval scaffolds; the second set, the dorso-ventral muscles, DVMs is patterned by mono-nucleate founder cells (FCs) that are much larger than the surrounding myoblasts. Both types of organizer cells express Dumbfounded, which is known to regulate fusion during embryonic myogenesis. The role of DVM founder cells as well as the DLM scaffolds was tested in genetic ablation studies using the UAS/Gal4 system of targeted transgene expression. In both cases, removal of organizer cells prior to fusion, causes formation of supernumerary fibers, suggesting that cells in the myoblast pool have the capacity to initiate fiber formation, which is normally inhibited by the organizers. In addition to the large DVM FCs, some (smaller) cells in the myoblast pool also express Dumbfounded. We propose that these cells are responsible for seeding supernumerary fibers, when DVM FCs are eliminated prior to fusion. When these cells are also eliminated, myogenesis fails to occur. In the second set of studies, targeted expression of constitutively active Ras(V12) also resulted in the appearance of supernumerary fibers. In this case, the original DVM FCs are present, suggesting alterations in cell fate. Taken together, these data suggest that DVM myoblasts are able to respond to cues other than the original founder cell, to initiate fusion and fiber formation. Thus, the role of the large DVM founder cells is to generate the correct number of fibers, but they are not required for fiber formation itself. We also present evidence that the DVM FCs may arise from the leg imaginal disc.

Indirect flight muscles (IFMs) in adult Drosophila provide the key power stroke for wing beating. They also serve as a valuable model for studying muscle development. An age-dependent decline in Drosophila free flight has been documented, but its relation to gross muscle structure has not yet been explored satisfactorily. Such analyses are impeded by conventional histological preparations and imaging techniques that limit exact morphometry of flight muscles. In this study, we employ microCT scanning on a tissue preparation that retains muscle morphology under homeostatic conditions. Focusing on a subset of IFMs called the dorsal longitudinal muscles (DLMs), we find that DLM volumes increase with age, partially due to the increased separation between myofibrillar fascicles, in a sex-dependent manner. We have uncovered and quantified asymmetry in the size of these muscles on either side of the longitudinal midline. Measurements of this resolution and scale make substantive studies that test the connection between form and function possible. We also demonstrate the application of this method to other insect species making it a valuable tool for histological analysis of insect biodiversity.

In 2017, an inborn error of metabolism caused by recessive mutations in SGPL1 was discovered. The disease features steroid-resistant nephrotic syndrome, adrenal insufficiency, and neurological defects. The latter can include sensorineural hearing loss, cranial nerve defects, peripheral neuropathy, abnormal brain development, seizures and/or neurodegeneration. SGPL1 encodes the pyridoxal-5’-phosphate (PLP) dependent enzyme sphingosine phosphate lyase (SPL), and the condition is now referred to as SPL insufficiency syndrome (SPLIS). SPL catalyzes the final step in the degradative pathway of sphingolipids in which the bioactive sphingolipid sphingosine-1-phosphate (S1P) is irreversibly degraded to a long chain aldehyde and phosphoethanolamine (PE). SPL guards the only exit point for sphingolipid metabolism, and its inactivation leads to accumulation of various types of sphingolipids which have biophysical roles in plasma membrane rafts and myelin, and signaling roles in cell cycle progression, vesicular trafficking, cell migration, and programmed cell death. In addition, the products of the SPL reaction have biological functions including regulation of autophagic flux, which is important in axonal and neuronal integrity. In this review, the neurological manifestations of SPLIS will be described, and insights regarding the neurological consequences of SPL insufficiency from the study of brain-specific SPL knockout mice and Drosophila SPL mutants will be summarized.