Myosin 31DF: Biological Overview | References
Gene name - Myosin 31DF
Synonyms - myosin-IA
Cytological map position - 31F1-31F4
Function - cytoskeletal motor protein
Symbol - Myo31DF
FlyBase ID: FBgn0086347
Genetic map position - 2L:10,491,682..10,506,780 [-]
Classification - Myosin motor domain, type I myosins and Myosin tail
Cellular location - cytoplasmic
|Recent literature||Amcheslavsky, A., Wang, S., Fogarty, C. E., Lindblad, J. L., Fan, Y. and Bergmann, A. (2018). Plasma membrane localization of apoptotic caspases for non-apoptotic functions. Dev Cell 45(4): 450-464.e453. PubMed ID: 29787709
Caspases are best characterized for their function in apoptosis. However, they also have non-apoptotic functions such as apoptosis-induced proliferation (AiP), where caspases release mitogens for compensatory proliferation independently of their apoptotic role. This study reports that the unconventional myosin, Myo1D, which is known for its involvement in left/right development, is an important mediator of AiP in Drosophila. Mechanistically, Myo1D translocates the initiator caspase Dronc to the basal side of the plasma membrane of epithelial cells where Dronc promotes the activation of the NADPH-oxidase Duox for reactive oxygen species generation and AiP in a non-apoptotic manner. It is proposed that the basal side of the plasma membrane constitutes a non-apoptotic compartment for caspases. Finally, Myo1D promotes tumor growth and invasiveness of the neoplastic scrib Ras(V12) model. Together, these studies have identified a new function of Myo1D for AiP and tumorigenesis and reveal a mechanism by which cells sequester apoptotic caspases in a non-apoptotic compartment at the plasma membrane.
The internal organs of animals often have left-right asymmetry. Although the formation of the anterior-posterior and dorsal-ventral axes in Drosophila is well understood, left-right asymmetry has not been extensively studied. This study demonsrates that the handedness of the embryonic gut and the adult gut and testes is reversed (not randomized) in viable and fertile homozygous Myo31DF mutants. Myo31DF encodes an unconventional myosin, Drosophila MyoIA [also referred to as MyoID in mammals; Gillespie, 2001; Morgan, 1995), and is the first actin-based motor protein to be implicated in left-right patterning. Myo31DF is required in the hindgut epithelium for normal embryonic handedness. Disruption of actin filaments in the hindgut epithelium randomizes the handedness of the embryonic gut, suggesting that Myo31DF function requires the actin cytoskeleton. Consistent with this, it was found that Myo31DF colocalizes with the cytoskeleton. Overexpression of Myo61F (myosin-IB), another myosin I, reverses the handedness of the embryonic gut, and its knockdown also causes a left-right patterning defect. These two unconventional myosin I proteins may have antagonistic functions in left-right patterning. It is suggested that the actin cytoskeleton and myosin I proteins may be crucial for generating left-right asymmetry in invertebrates (Hozumi, 2006). An accompanying paper (Spéder, 2006, see below) demonstrates that Myo31DF regulates dextral looping of genitalia.
Mechanisms that create characteristic left-right asymmetry have been studied extensively in vertebrates. However, although the organs of many invertebrate species also have left-right asymmetry, the mechanisms by which this asymmetry arises are largely unknown. In Drosophila, several organs have left-right asymmetry, including the embryonic gut, the adult brain and the genitalia (Hozumi, 2006).
To identify genes involved in left-right asymmetry of the Drosophila embryonic gut, a genetic screen was performed using a collection of P-element lines. The embryonic gut is composed of three major parts, the foregut, midgut and hindgut, all of which have characteristic left-right asymmetry. 75.7% of homozygous Myo31DFsouther embryos show synchronous inversion of the midgut and hindgut. In these embryos, the hindgut and midgut are the mirror-image of those in wild-type embryos, rather than showing randomized patterning. In contrast, foregut handedness was normal in all cases examined, indicating that this phenotype was heterotaxic. Myo31DFsouther is a background mutation of the Gene Search Drosophila line GS14508. Deficiency mapping was used to map the cytological location of Myo31DFsouther to between 30D and 31F. Complementation tests were performed between Myo31DFsouther and lines bearing mutations that map to this region. Myo31DFsouther failed to complement Myo31DFK1 (Spéder, 2006), an allele of Myo31DF encoding Drosophila MyoID (Hozumi, 2006).
The transposable element gypsy was inserted into the 5'-untranslated region of the Myo31DF gene in Myo31DFsouther. Myo31DFL152 was one of five ethylmethanesulfonate (EMS)-induced Myo31DF alleles isolated in a large-scale EMS mutant screen. Myo31DFL152 has a base substitution that introduces a premature stop codon at amino acid 331, resulting in a putative truncated product. Myo31DF overexpression from UAS-Myo31DF driven by byn-Gal4 in the hindgut and posterior midgut and their primordial counterparts rescued the left-right defects of Myo31DFsouther embryos, indicating that Myo31DF was responsible for the heterotaxia. In Myo31DFsouther embryos, NP2432-driven expression of Myo31DF in the hindgut epithelium, but not in other parts of the embryonic gut, such as the midgut and mesoderm, was sufficient to rescue this heterotaxia, suggesting that Myo31DF is required in the hindgut epithelium. Furthermore, the frequency of the handedness defect was similar in Myo31DFL152 homozygous and Myo31DFL152/Df(2L)J2 embryos. Thus, Myo31DFL152 is probably a null mutant of Myo31DF. All homozygous Myo31DF mutants isolated in this study were viable and fertile, with normal hindgut tissue specification, suggesting that Myo31DF function is largely restricted to left-right patterning. No maternal phenotype or Myo31DF gene contribution was detected. Notably, the foregut became a mirror-image of its wild-type counterpart when Myo31DF was overexpressed in the entire embryo, but other parts of the gut were normal. Therefore, it is suggested that Myo31DF is not involved in the left-right asymmetrical development of the foregut in wild-type embryos, but can reverse foregut handedness (Hozumi, 2006).
The adult hindgut and testes, which are regenerated during metamorphosis, were examined. These also showed inversed handedness in the Myo31DF homozygote. In most Myo31DFL152 adults, the loop of the hindgut and spiral of the testes were reversed, although not always synchronously. The function of the Myo31DF gene was knocked-down using RNA interference (RNAi) in vivo. The expression of double-stranded Myo31DF RNA driven by byn-Gal4 caused inversion of the adult, but not the embryonic, hindgut. Thus, the left-right pattern involving Myo31DF is not transmitted during metamorphosis (Hozumi, 2006).
In situ hybridization revealed Myo31DF expression in the amnioproctodeal invagination at stage 6. At stages 12-14, Myo31DF messenger RNA was strongly detected in the primordial midgut and hindgut, and in the proventriculus, midgut and hindgut. A sense-strand probe of Myo31DF gave no detectable signal. Immunostaining of wild-type embryos with an anti-Myo31DF antibody (anti-Myo31DF-1P) also labelled the midgut and hindgut. These signals were absent in Myo31DFsouther and Myo31DFL152 homozygotes, indicating that the staining was specific. Myo31DF mRNA and protein were detected in a symmetrical pattern before the development of left-right asymmetry. Myo31DF protein is expressed in the adult gut (Morgan, 1995). No Myo31DF expression was detected in the foregut, which may account for the absence of any laterality defect in the foregut of Myo31DF mutants (Hozumi, 2006).
Myo31DF protein binds to actin in an ATP-dependent manner (Morgan, 1995). The co-localization of Myo31DF was examined and the actin cytoskeleton in cultured Drosophila S2 cells. A green fluorescent protein (GFP)-tagged Myo31DF (Myo31DF-GFP) had wild-type function, given that its overexpression rescued Myo31DFsouther. Myo31DF-GFP co-localized with actin, mostly at cell protrusions. In epithelial cells of the hindgut, endogenous Myo31DF was detected as punctate staining, partly overlapping with cortical actin (Hozumi, 2006).
byn-Gal4-driven misexpression of GFP-tagged moesin, an actin-binding protein, in wild-type embryos caused a reduction in actin filaments in the apical region of the hindgut epithelium, where Myo31DF function is required. Notably, the midgut and hindgut always had the same handedness, but the handedness was random (not inversed), in wild-type and Myo31DF homozygous embryos. Embryo handedness was also affected by NP2432-driven GFP-moesin expressed in the hindgut epithelium only. However, GFP-moesin expression in the midgut only did not affect handedness (Hozumi, 2006).
To investigate further the functional link between Myo31DF and the actin cytoskeleton, the phenocritical period for inducing left-right defects was determined using Myo31DF RNAi and GFP-moesin misexpression, following the TARGET method. Both Myo31DF knockdown and GFP-moesin expression in the hindgut 0-24 h after pupation caused similar defects in adult gut handedness, suggesting that Myo31DF and the appropriate organization of actin filaments are required at the same time. GFP-moesin also affected handedness in the Myo31DF embryo, suggesting that the default handedness, which may be manifested in the Myo31DF homozygote, also depends on the actin cytoskeleton. The involvement of three Rho GTPase family proteins, Rho, Rac and Cdc42, which regulate the organization of the actin cytoskeleton, was examined. Expression of dominant-negative forms of these proteins, especially Rho, in the hindgut induced synchronous left-right defects in the embryonic midgut and hindgut. Together, these results suggest that Myo31DF depends on the actin cytoskeleton to generate left-right asymmetry (Hozumi, 2006).
Cell division and cell death do not occur during left-right asymmetric development of the hindgut. It is therefore speculated that the rearrangement of hindgut epithelial cells may be part of this process. To test this possibility, a time-lapse analysis was performed. The position of each cell was visualized by labelling the nucleus with GFP. Cell rearrangement, which coincided with the left-right bias associated with left-handed rotation of the hindgut, was suggested by the significant intercalation of some cells (Hozumi, 2006).
Another myosin I protein, Myo61F (Drosophila MyoIB, also referred to as MyoIC in mammals), has been reported in Drosophila. Myo61F protein is detected in the embryo and adult gut. To test whether Myo61F is also involved in left-right asymmetry, Myo61F was overexpressed using UAS-Myo61F or GS9889 driven by byn-Gal4. Unexpectedly, Myo61F overexpression resulted in inversion of the midgut and hindgut in both cases. In contrast, Myo31DF overexpression did not affect the handedness of these organs. These results suggest that Myo31DF and Myo61F have antagonistic functions in creating the left-right asymmetry of these organs. The involvement of Myo61F in left-right asymmetry is also supported by the finding that its knockdown by RNAi results in the left-right defect in the embryonic midgut (Hozumi, 2006).
Homozygous Myo31DF embryos show reversed handedness of embryonic and adult visceral organs, which may represent the default state of left-right asymmetry in Drosophila. This situation is similar to the function of the sinistral gene in the freshwater snail, Limnea (although the sinistral gene is required maternally) (Wood, 1997; Shibazaki, 2004). Normal handedness is still seen in 25% of Myo31DF homozygotes. It is speculate that some other myosin gene(s) has a redundant function in left-right patterning. Inversion of the anteroposterior axis does not affect laterality, suggesting that left-right pattering occurs zygotically; this is consistent with the zygotic function of Myo31DF. These results also suggest that an actin-based mechanism, which can align itself to either an anteroposterior-dorsoventral reference or the pre-existing sinistral handedness, exists to direct the rotation of the hindgut epithelium. Since myosin I proteins are involved in vesicular transport (Huber, 2000), it is proposed that Myo31DF and Myo61F, which on the basis of their structures are believed to move to the plus ends of actin filaments, carry left-right determinants with opposite activities. Thus, both left-right determinants would be concentrated in the plus ends of actin filaments that have a hypothetical planar polarity. In the Myo31DF mutant, only the opposing determinant is concentrated here, which reverses the handedness. According to this model, disruption in actin organization would result in left-right randomization, as was indeed observed experimentally (Hozumi, 2006).
Breaking left-right symmetry in Bilateria embryos is a major event in body plan organization that leads to polarized adult morphology, directional organ looping, and heart and brain function. However, the molecular nature of the determinant(s) responsible for the invariant orientation of the left-right axis (situs choice) remains largely unknown. Mutations producing a complete reversal of left-right asymmetry (situs inversus) are instrumental for identifying mechanisms controlling handedness, yet only one such mutation has been found in mice (inversin; Morgan, 1998) and snails (Ueshima, 2003; Freeman, 1982). The conserved type ID unconventional myosin 31DF gene (Myo31DF) has been identified as a unique situs inversus locus in Drosophila. Myo31DF mutations reverse the dextral looping of genitalia, a prominent left-right marker in adult flies. Genetic mosaic analysis pinpoints the A8 segment of the genital disc as a left-right organizer and reveals an anterior-posterior compartmentalization of Myo31DF function that directs dextral development and represses a sinistral default state. As expected of a determinant, Myo31DF has a trigger-like function and is expressed symmetrically in the organizer, and its symmetrical overexpression does not impair left-right asymmetry. Thus Myo31DF is a dextral gene with actin-based motor activity controlling situs choice. Like mouse inversin (Nurnberger, 2002), Myo31DF interacts and colocalizes with β-catenin, suggesting that situs inversus genes can direct left-right development through the adherens junction (Spéder, 2006).
In wild-type males, the genital plate, to which the spermiduct is attached, undergoes a 360° clockwise (dextral) rotation when viewed from the posterior pole. This directional looping is reminiscent of other coiling processes such as mammalian heart tube looping and snail spiral development. As in other species, one direction is dominant among the Drosophilidae, the dextral rotation, with no sinistral species reported to date. This study identified an insertional mutation, KG02246, which shows a striking inverted phenotype when combined with deficiencies covering the 31DF genomic region. In KG02246/Df(2L)Exel7048 or KG02246/Df(2L)J3 males, genitalia rotation is variable, with ~60% of individuals showing sinistral rotation. Imprecise excision of KG02246 generated two genomic deletions, KG022461 and KG022462, both of which presented a stronger, 100% inverted (sinistral) genitalia rotation phenotype. Thus, KG02246 alleles identify the first situs inversus mutations in Drosophila and provide genetic evidence for the existence of a left-right axis in this organism (Spéder, 2006).
Drosophila Myo31DF is a conserved myosin belonging to the Myo1D family. It is known to interact with F-actin (Morgan, 1995) and it colocalizes with actin-rich structures in different tissues. After mouse inversin, Drosophila Myo31DF represents the second situs inversus gene to be molecularly identified. The mouse inversin gene (Invs) encodes a protein with ankyrin repeats and two IQ domains (Morgan, 2002; Eley, 2004) that bind calmodulin, a Ca2+ -dependent regulatory protein (Bahler, 2002). Like inversin, Myo31DF contains two IQ domains essential for its function. Indeed, a Myo31DF form lacking IQ domains (Myo31DFDeltaIQ) is unable to rescue Myo31DF mutations (Spéder, 2006).
To determine the function of Myo31DF in genitalia rotation, its expression was examined in the genital disc, the precursor of adult genitalia. The genital disc is composed of segments A8, A9 and A10, each with an anterior and a posterior. Immunostaining of wild-type flies with two polyclonal antibodies directed against overlapping regions of the Myo31DF tail domain, anti-Myo31DF-1P and anti-Myo31DF-3P, or using a Myo31DF-Gal4 enhancer-trap line (NP1548) revealed symmetrical expression of Myo31DF in a double chevron-like pattern restricted to the ventral domain of the male genital disc. This expression, starting in third instar larvae and remaining unchanged during this stage, was absent in Myo31DFK2 mutant discs. Double staining with an A8-specific marker in the genital disc (tsh-Gal4 > myr-RFP) indicated that Myo31DF is expressed in the A8 segment, with one chevron in the posterior compartment and the other in the anterior compartment. Consistently, expression of two copies of an inhibitory RNA gene (2 x Myo31DFRNAi) specifically in the A8 segment led to loss of Myo31DF expression and to inverted phenotypes that mimicked Myo31DF mutations. Together, these data identify the A8 segment as a left-right organizer that is required for situs choice in Drosophila (Spéder, 2006).
To investigate the relationship between the anterior-posterior and left-right axes, the putative domain(s) of Myo31DF function was mapped by selectively silencing the gene in different compartments, using specific Gal4 lines driving 2 x Myo31DFRNAi. The combinatorial removal of Myo31DF function in the anterior and/or posterior domains led to distinct phenotypes, indicating a dual function for the Myo31DF protein in A8. First, blocking Myo31DF function posteriorly in A8 (using hh-Gal4 or en-Gal4) resulted in a striking non-rotated genitalia phenotype. This finding was confirmed in a complementary experiment using dpp-Gal4 to rescue Myo31DFK1 in the anterior compartment. The absence of situs choice observed in these experiments indicates that posterior Myo31DF has an instructive role in dextral looping. Second, blocking Myo31DF function solely in the anterior compartment (in dpp-Gal4 > 2 x Myo31DFRNAi or in Myo31DFK1/Myo31DFK1; hh-Gal4 > Myo31DF rescued males) led to partial dextral rotation, suggesting a permissive role for Myo31DF in this compartment. Comparing this outcome to the effect of concomitant removal of both anterior and posterior functions (the only context that led to complete reversal) indicates that the function of Myo31DF in the anterior compartment is to repress sinistral looping. These experiments demonstrate that Myo31DF is essential both anteriorly and posteriorly. A model illustrating the dual function of Myo31DF in regulating dextral development within the A8 segment is presented. In this model, A8 contains information to specify both sinistral and dextral rotation, with sinistral information being anterior and dextral information posterior. Dextral information is dominant over sinistral information, and Myo31DF function is required both posteriorly, to induce dextral development, and anteriorly, to repress sinistral development. Only in the absence of Myo31DF can sinistral development occur as a default state (Spéder, 2006).
As expected of a left-right determinant with a function that precedes asymmetry, Myo31DF is expressed symmetrically in A8. In addition, as with mouse inversin (Watanabe, 2003), symmetrical overexpression does not lead to left-right defects (AbdB-Gal4, tsh-Gal4 and ptc-Gal4 lines). Another predicted feature of left-right determinants is their temporally restricted, trigger-like function, which is later relayed by mechanisms acting to maintain the initial symmetry-breaking event. To test this, temperature-shift experiments were carried out using a genetically engineered temperature-sensitive Myo31DF allele. Single temperature-shift experiments with 24-h or 6-h resolution indicated that Myo31DF function is required at day 6 of development, between 126 and 132 h. Double temperature-shift experiments allowed to determine that Myo31DF function is required for as little as 3 h within this period. These data provide high temporal resolution, allowing the temporal mapping of a left-right symmetry-breaking event and demonstrating that situs choice depends on a peak of Myo31DF function in the left-right organizer (Spéder, 2006).
Cilia have emerged as important cellular structures for generating left-right asymmetry in vertebrate embryos. To address their possible contribution to invertebrate left-right determination, genital discs were stained with GT335, an antibody that labels glutamylated tubulin in cilia across species, including Drosophila. GT335 did not detect any cilia in genital discs. Additionally, mutations in the conserved Rfx gene, which controls the formation of ciliated neurons in Drosophila, did not affect genitalia rotation. Together, these data suggest that cilia are not involved in dextral development in Drosophila. It is proposed that Drosophila uses primarily the actin cytoskeleton to determine left-right asymmetry. Consistently, the small GTPase Drac1 and the JNK pathway, known regulators of the actin cytoskeleton, showed specific genetic interactions with Myo31DF. Notably, actin is also important for situs choice in snails, suggesting that in invertebrates the actin cytoskeleton has a central role in left-right determination (Spéder, 2006).
To start investigating how Myo31DF might act to determine left-right asymmetry, two-hybrid screening was used to identify Myo31DF interactors or cargo(es). Using the Myo31DF tail domain (amino acids 737-1011) as bait, several positive clones were found encoding a carboxy-terminal fragment containing ARM repeats 6-12 of the Armadillo/β-catenin protein. This interaction was direct, as shown by glutathione S-transferase (GST)-pulldown experiments using purified proteins or S2 cell extracts. Furthermore, endogenous Myo31DF and a functional Myo31DF-GFP (green fluorescent protein) fusion protein colocalized with Armadillo at the adherens junctions in A8, suggesting that the two proteins can interact in vivo. Notably, inversin has been shown to colocalize and interact with β-catenin in vertebrate epithelial cells, indicating that an interaction with β-catenin is a common feature of both known situs inversus genes. These results are consistent with a demonstrated role of N-cadherin in left-right asymmetry (Spéder, 2006).
How could anterior-posterior organization of Myo31DF and interaction with Armadillo account for left-right asymmetry? The actin cable network might serve as a track for Myo31DF to deliver specific cargoes or vesicles at the adherens junction. Indeed, it was found that Myo31DF interacts with dynamin, consistent with the role of rat MyoID in vesicular transport (Huber, 2000). The anterior-posterior boundary itself creates an asymmetric junction that could serve as a scaffold for Myo31DF to assemble a dextral-specific complex on the anterior side of posterior Myo31DF-expressing cells. Anterior-posterior asymmetry of dextral information could later be translated into left-right asymmetry through the remodelling of cell contacts (cell intercalation or rotation), as seen in other epithelia. For example, 90° rotation of epithelial cells relative to the main body axis is observed in the Drosophila eye imaginal disc. A similar, 90° planar rotation of Myo31DF-expressing cells would result in left-right orientation of the dextral junctional complex in the tissue. In this working model, a short pulse of Myo31DF activity would be essential for spatially restricting the dextral junction, as has been observed (Spéder, 2006).
Handed asymmetry in organ shape and positioning is a common feature among bilateria (for a review see Huber, 2007), yet little is known about the morphogenetic mechanisms underlying left-right (LR) organogenesis. This study utilized the directional 360° clockwise rotation of genitalia in Drosophila to study LR-dependent organ looping. Using time-lapse imaging, it was shown that rotation of genitalia by 360° results from an additive process involving two ring-shaped domains, each undergoing 180° rotation. The results show that the direction of rotation for each ring is autonomous and strictly depends on the LR determinant myosin ID (MyoID: Myo31DF). Specific inactivation of MyoID in one domain causes rings to rotate in opposite directions and thereby cancels out the overall movement. A specific pattern of apoptosis at the ring boundaries is revealed, and this study also shows that local cell death is required for the movement of each domain, acting as a brake-releaser. These data indicate that organ looping can proceed through an incremental mechanism coupling LR determination and apoptosis. Furthermore, they suggest a model for the stepwise evolution of genitalia posture in Diptera, through the emergence and duplication of a 180° LR module (Suzanne, 2010).
Left-right (LR) asymmetric development is essential to the morphogenesis of many vital organs, such as the heart. Directional looping of LR organs is a complex morphogenetic process relying on proper coordination of early LR patterning events with late cell-tissue behaviors. In vertebrates, several developmental models have been proposed for gut coiling downstream of the Nodal-Pitx2 regulatory pathway, including intrinsic asymmetric elongation of the gut in Xenopus or extrinsic force generation by mesenchymal tissue in Zebrafish and by dorsal mesentery in the chick and mouse embryos. However, the cellular mechanisms underlying LR organ morphogenesis are mostly unknown (Suzanne, 2010).
In Drosophila, directional clockwise (or dextral) rotation of the genital plate and gut has been shown only recently to be controlled by the LR determinant myosin ID (MyoID). In myoID mutant flies, LR morphological markers are inverted, leading to counterclockwise (or sinistral) looping of the genital plate, spermiduct, gut, and testis. This indicates that myoID is a unique situs inversus gene in Drosophila. Intriguingly, the expression of MyoID is restricted to two rows of cells within the A8 segment of the genital disc (the analia and genitalia precursor), with one row of expression in the anterior compartment (A8a) and the other in the posterior compartment (A8p) (Suzanne, 2010).
Removal of myoID function specifically in the A8 segment is sufficient to provoke the complete inversion of rotation (360° counterclockwise) of the genitalia and sinistral looping of the spermiduct to which it is attached. The A8 segment therefore represents a LR organizer controlling the directional rotation of the whole genitalia in Drosophila (Suzanne, 2010).
Because circumrotation (the process of 360° rotation) may result from a number of different morphogenetic processes, not deducible from the simple observation of the final adult phenotype, a new and innocuous imaging method was developed to follow the rotation in living pupae (Suzanne, 2010).
To be able to analyze the movement of distinct domains in live developing genitalia, time-lapse imaging was coupled with genital disc 'painting' by expressing different fluorophores in various regions of the genitalia precursor. Analysis of wildtype live genitalia through this method revealed their spatial and temporal organization during rotation. It was first determined that rotation begins at around 25 hr after puparium formation (APF) and lasts 12-15 hr. At 25 hr APF, the genital disc is organized into concentric rings, which, from anterior to posterior, include an A8a ring, an A8p ring, and a large central disc composed of A9-A10 tissues. The analysis of rotation in live pupae coupled to manual tracking allowed the identification of two distinct moving domains: a large posterior domain comprising A8p-A9-A10 (hereafter referred to as A8p) and a smaller anterior domain made of A8a. The A8p domain moves first and is followed by A8a, which starts moving later on. During the entire process, cells from the abdomen, to which the genital disc is connected, remain immobile. The finding of two rotating domains, A8a and A8p, was unexpected. It reveals a complex rotational activity of the genitalia and rules out a simple model in which the genital plate would rotate by 360° as a whole. To further understand how rotation occurs, timelapse imaging of the full, 15-hr-long rotation was performed. This analysis revealed that each ring had a different rotational activity. When viewed from the posterior pole, the A8p ring undergoes 360° clockwise rotation, while the A8a ring makes a 180° clockwise rotation. Whereas the rotation of the central part (A8p-A10) of the disc was inferred from the looping of the spermiduct around the gut, the 180° rotation of A8a was not predicted and could only be revealed by time-lapse analysis because this compartment solely gives rise to a tiny and colorless part of the cuticle. Altogether, these in vivo analyses show that rotation of genitalia in Drosophila is a composite process involving two compartments of the A8 segment, A8a and A8p, each expressing a row of MyoID at its anterior boundary and having its own rotational behavior (Suzanne, 2010).
These findings raise the questions of the contribution of each of the two rings to the entire rotation and of how they interact during rotation. In order to address this question, the intrinsic or real rotational activity of A8a and A8p was determined. So far, each ring movement was analyzed relative to the same immobile referential: the abdomen. Although this referential allows the real movement of A8a to be determined, it cannot be used to determine that of A8p, because A8p moves relatively to a mobile referential, i.e., A8a, to which it is attached. To determine the real movement of A8p, it is thus essential to analyze its angular movement relative to A8a, in other words A8a contribution to motion must be subtracted from the apparent A8p movement. To do so, movies were analyzed by setting A8a as a referential and by determining the angular movement of A8p. Reassessing A8p movement through this approach revealed that A8p rotates clockwise only by 180° relative to A8a. The new angular velocity curve of A8p fits almost perfectly with that of A8a, indicating that both movements have similar features. Importantly, these data also indicate that the observed 360° clockwise rotation is the result of a composite process involving two additive 180° clockwise components: a 180° rotation of the A8a relative to the abdomen and an 180° rotation of A8p relative to A8a (Suzanne, 2010).
To further determine the autonomy of each ring relatively to the other, the role of the LR determinant MyoID in this process was dissected by specifically inactivating myoID in either A8a or A8p or in both. By convention, the presence or absence of myoID is represented by a + or - sign, respectively. Accordingly, the wild-type context is noted 'A8a+A8p+' and the myoID mutant 'A8a-A8p-.' Upon specific inactivation of myoID in the A8a domain (A8a-A8p+ context), the adults showed an apparent 'nonrotation phenotype' (0°, no spermiduct looping and genitalia correctly oriented). However, time-lapse imaging revealed that both rings were spinning, although in opposite directions: the A8a domain rotated counterclockwise by 180° (-180°), whereas the A8p domain rotated clockwise by 180° (+180°, real movement). Reciprocally, the inactivation of myoID in the A8p domain (A8a+A8p- context) also led to an apparent nonrotation phenotype. In this context, the behavior of each domain was inverted compared to the previous condition, with the A8a domain rotating clockwise by 180° (+180°) whereas the A8p domain rotated anticlockwise by 180° (-180°, relative or real movement). In both cases, the movement of each ring is consistent with its myoID genotype and the 'dextralizing' activity of this gene. The strict dependence on MyoID for the direction of the rotation is further confirmed in flies where both A8a and A8p were mutants for myoID (myoIDk1). The rotation is often incomplete in this genotype because of the hypomorphic nature of the myoIDk1 allele analyzed; however, both domains show an anticlockwise movement. Therefore, in all genetic contexts analyzed, all parameters of the rotation remain unaffected except the direction of rotation, as illustrated by the perfect mirror image of the angular velocity curves (Suzanne, 2010).
These experiments reveal that each ring adopts an independent 180° movement relative to more anterior structures (A8a relative to the abdomen and A8p relative to A8a): clockwise in the presence of MyoID, anticlockwise in its absence. When both movements are unidirectional, the net rotation is circumrotation (± 360°), whereas upon opposite movements of A8a and A8p, the net rotation is zero (0°), leading to an apparent nonrotation phenotype. Therefore, the net rotation (or apparent rotation = R) can be modeled through a simple equation in which R equals the addition of A8a and A8p movements, with MyoID acting as a sign function (Suzanne, 2010).
It was next of interest to characterize potential cellular mechanisms acting downstream of LR determination during genitalia rotation. In particular, the cellular events responsible for uncoupling rings at the onset of their rotation was determined. Initial insights came from blocking apoptosis, which leads to genitalia rotation defects, but the role of apoptosis in the process is not completely understood. To determine the morphogenetic function of the apoptotic pathway during genitalia rotation, the spatial and temporal requirements for apoptosis were first characterized by analyzing the expression pattern of hid and reaper (rpr) in the genital disc, using two reporter lines. Both reporters were strongly expressed in the A9 and A10 segments. However, in the A8 segment, only hid expression is observed. This coincides with the phenotype of misrotated genitalia observed specifically when hid function is altered but not in rpr mutants. Then the pattern and timing of cell death was determined in the genital disc. To do so, nuclear fragmentation was followed, and an in vivo reporter of caspase activation (the apoliner construct) was used. At the onset of rotation, a large number of apoptotic cells was detected on the most ventral part of the genital disc, first within the A8p ring bordering A8a, coinciding with the beginning of A8p movement. These data indicate an overlap between the apoptotic field and the domain of MyoID expression. These results have been further confirmed by the detection of apoptotic cells by TUNEL staining of fixed pupal genital discs. Later on, a new wave of apoptosis was detected in the most anterior part of the A8a ring, at the junction between A8a and the abdomen. In contrast, only marginal if any apoptosis was detected before and at the end of rotation. Therefore, two waves of cell death are taking place in the A8 segment, coinciding spatially and temporally with the rotation of A8a and A8p rings (Suzanne, 2010).
Given that rings are initially part of the same epithelium and move independently later, it was reasoned that local cell death may be a mechanism to provide the degree of liberty necessary for proper movement. To test this hypothesis, cell death was inhibited in each compartment separately by expressing the caspase inhibitor p35. Interestingly, inactivation of apoptosis in either A8p or A8a leads to a similar phenotype, with flies showing a high proportion of half-rotated genitalia (180° rotation), suggesting that rotation was blocked in the ring deficient for apoptosis. This has been further demonstrated by following the rotation process in vivo, when apoptosis is specifically blocked in the A8a. In this genetic context, the A8a ring stayed mostly still during the whole process, whereas A8p rotated normally. The resulting 180° rotation is thus exclusively due to the movement of one ring, i.e., A8p, in which apoptosis is unaffected. Inhibiting apoptosis in both domains strongly aggravates the phenotype, with 40% of the flies showing nonrotated genitalia (0°), suggestive of an additive phenotype. The rest of the population had 90° rotated genitalia, which may be due to incomplete inhibition of apoptosis. Alternatively, it is possible that some rotation occurs without apoptosis thanks to tissue elasticity. In any case, the results indicate that cell death is required in each ring for separating them from the neighboring tissues and allowing their free rotation. Consistently, nuclei fragmentation and cell death occur normally in a myoID mutant background. Because local cell death is not likely to provide a direct force for rotation, it is proposed that it contributes to the release of rings from neighboring tissues (Suzanne, 2010).
This study has revealed that organ looping can proceed through discrete steps, breaking down circumrotation into the simple building blocks of 180° each. The incremental nature of genitalia rotation is indeed based upon two 180° LR modules, sharing identical angular velocity and range as well as requirement for MyoID and apoptosis. Modularity in morphogenesis provides interesting control mechanisms (through addition or substraction) and therefore plasticity to the process, both at the organism level and during evolution. Entomologists have described different patterns of genitalia rotation in Diptera, ranging from 0° to 360°, that evolved together with changes in mating position. Interestingly, in the Brachycera suborder, to which Drosophilidae belong, we notice that most ancestral species have a nonrotated genitalia (Stratiomyomorpha and Tabanomorpha), whereas 180° and 360° rotation have appeared progressively later in evolution (in Muscomorpha and Cyclorrhapha, respectively). Together with this sequential organization of rotation amplitude in the phylogenetic tree, these data strongly support a model by which the 360° rotation observed in Brachycera ('modern Diptera') would result from the emergence (transition from 0 to 180°) and duplication (transition from 180° to 360°) of a 180° L/R module (Figure S3), thus providing a simple additive model for both the origin of circumrotation and the evolution of genitalia rotation and mating position. However, it should be noted that alternative mechanisms maylead to a similar pattern of genitalia rotation among Diptera (Suzanne, 2010).
The incremental model presented here also offers a solution to the apparent paradox of circumrotation and the question of its elusive utility, illustrated by the fact that both 360° rotation and the absence of rotation lead to the same final posture of genitalia. A facultative role of 360° rotation is further supported by the finding that D. melanogaster males with nonrotating genitalia (A8a-A8p+ or A8a+A8p-) are normally fertile (data not shown). An incremental origin of 360° rotation in which a second half-turn would be added to the existing 180° rotation would well explain this paradox. Thus, circumrotation can be viewed as recapitulating the evolutionary history of genitalia rotation in Brachycera, and its logic would reveal a case of 'retrograde evolution,' in which duplication of a functional module is used to revoke a previous evolutionary step (Suzanne, 2010).
Finally, this analysis of genitalia rotation highlights a new mechanism of morphogenesis relying on a combination of LR patterning and apoptosis. In this process, a new role for apoptosis is revealed as a releasing mechanism allowing the sliding of two parts of an organ. It will be interesting to test in the future whether this releasing role of apoptosis is used more generally, in other morphogenetic movements requiring important cellular rearrangement (Suzanne, 2010).
Some organs in animals display left-right (LR) asymmetry. To better understand LR asymmetric morphogenesis in Drosophila, LR directional rotation of the hindgut epithelial tube was studied. Hindgut epithelial cells adopt a LR asymmetric (chiral) cell shape within their plane, referred to as planar cell-shape chirality (PCC). Drosophila E-cadherin (DE-Cad) is distributed to cell boundaries with LR asymmetry, which is responsible for the PCC formation. Myosin ID (Myosin 31DF) switches the LR polarity found in PCC and in DE-Cad distribution, which coincides with the direction of rotation. An in silico simulation showed that PCC is sufficient to induce the directional rotation of this tissue. Thus, the intrinsic chirality of epithelial cells in vivo is an underlying mechanism for LR asymmetric tissue morphogenesis (Taniguchi, 2011).
Directional left-right (LR) asymmetry is widely found in animals, such as in the position and structure of the heart, spleen, gut, and lung in vertebrates. The mechanisms of LR axis formation are well understood in some vertebrates, and the cellular basis for LR symmetry breaking, including cell polarities, is beginning to be elucidated. Drosophila shows a directional LR asymmetry of certain organs, including the embryonic hindgut. Although some unique features of Drosophila laterality development have been revealed, such as the involvement of myosin ID (MyoID), the detailed mechanisms of its LR asymmetric development remain largely unknown (Taniguchi, 2011).
The Drosophila embryonic hindgut begins as a symmetric midline structure that curves ventrally at stage 12. It subsequently makes a 90° left-handed rotation, forming a rightward curving structure by stage 13. The hindgut epithelium, but not the overlying visceral muscles, is responsible for this rotation, which is not accompanied by cell proliferation or cell death. Therefore, it was speculated that the hindgut epithelial cells themselves might have LR polarity, which could contribute to the rotation (Taniguchi, 2011).
To analyze LR polarity in the hindgut epithelial cells, the locations of the centrosomes, which reflect cell polarity in other systems, was examined. Each cell's centroid was calculated with respect to its boundaries and the relative position of the centrosome was plotted, labeled with green fluorescent protein (GFP)-centrosomin. In wild-type animals, the relative position of the centrosome was significantly biased to the right-posterior region. These results suggest that hindgut epithelial cells adopt a LR polarity within their plane before the hindgut rotates (Taniguchi, 2011).
It was speculated that this LR polarity would be reflected in the cell shape and participate directly in the left-handed rotation. To address this, the angle was measured between apical cell boundaries and the antero-posterior (AP) axis of the hindgut epithelial tube before rotation (late-stage 12). These apical cell boundaries corresponds to the zonula adherens (ZA). Cell-boundary angles of -90° to 0° to the AP axis were more frequent than those of 0° to 90°, indicating that hindgut epithelial cells have a LR-biased planar cell shape. This LR bias was designated as planar cell-shape chirality (PCC), because the mirror image of the cell's planar shape does not overlap with its original cell shape (Taniguchi, 2011).
Previous studies demonstrated that the hindgut rotates right-handedly in embryos homozygous for Myo31DF, which encodes MyoID. In Myo31DFL152 homozygotes, the distribution of angle x° was reversed from that of wild type, although the LR bias became less prominent. The reversed PCC in Myo31DFL152 was rescued by the overexpression of Myo31DFGFP. Rho family guanosine triphosphatases, including Rho1 and Rac1, regulate the organization of the actin cytoskeleton. It was previously shown that overexpression of a dominant-negative Rho1 (Rho1.N19) or Rac1 (Rac1.N17) in the hindgut epithelium disrupts the hindgut's LR asymmetry. No PCC was observed in these epithelial cells, suggesting that PCC formation depends on the actin cytoskeleton. These results support the suggestion that PCC could determine the subsequent laterality of the hindgut (Taniguchi, 2011).
To identify genes involved in PCC formation, a screen was carried out for mutations affecting LR asymmetry of the hindgut. It was found that shotgun (shg) mutations (shgR758, shgR1232, and shgR69, a null allele) disrupted the laterality of the hindgut. shg encodes DE-Cad, a conserved transmembrane protein that mediates cell-cell adhesion in the epithelium. Genetic analyses suggested that DE-Cad functions downstream of MyoID, and both are required in the hindgut epithelium just before its rotation for normal LR asymmetric development. In shgR69 homozygotes, the angle x° did not demonstrate LR asymmetry, indicating that PCC was not formed in this mutant. This PCC defect in shgR69 homozygotes was rescued by the overexpression of shgDECH (Taniguchi, 2011).
To understand how DE-Cad contributes to PCC formation, whether the distribution of DE-Cad showed LR polarity in hindgut epithelial cells was examined. For this, the mean of DE-Cad's relative intensity at the ZA of each cell boundary in the hindgut epithelium was at late-stage 12 was calculated. In wild type, the mean intensity was significantly greater at the cell boundaries with an angle x° of -90° to 0° than in those with 0 to 90° angles, whereas this situation was reversed in Myo31DFL152 homozygotes. Rose diagrams depicting the intensity of DE-Cad in the cell boundaries bundled for 30° intervals showed that DE-Cad was enriched in cell boundaries with an angle x° of -90° to -30°. Conversely, in Myo31DFL152 homozygotes, this situation was reversed. This reversed bias was restored to the wild-type situation by overexpressing Myo31DFGFP in the hindgut epithelium (Taniguchi, 2011).
It was then asked whether the LR bias of DE-Cad distribution was attributable to a cell-autonomous function of MyoID. To address this, a new system was developed for generating a mosaic hindgut epithelium composed of Myo31DFL152 homozygous cells with (+) or without (-) Myo31DFmEGFP overexpression. In the hindgut epithelium, the cell boundaries were classified into three types according to the cell type on either side: +/+, green; +/-, yellow; -/-, magenta. Cell boundaries of +/+ showed the wild-type LR bias of DE-Cad localization, which was reversed in the -/- boundaries. The +/- cell boundaries did not show a statistically significant LR bias. Thus, the LR asymmetry of DE-Cad distribution at each cell boundary is attributable to the concordance of LR polarity in two adjacent cells (Taniguchi, 2011).
To gain insight into how MyoID reverses the LR asymmetric distribution of DE-Cad, defects were sought in endocytic trafficking, because DE-Cad's localization to the ZA is controlled by its recycling. Rab11-positive recycling endosomes became fewer in the apical-middle part of cells in Myo31DFL152 homozygotes compared with wild type, and this defect was restored by Myo31DFGFP expression. These results may suggest that MyoID is involved in the recycling of DE-Cad (Taniguchi, 2011).
Besides the angle x°, the length was also measured of cell boundaries at the ZA of the hindgut epithelial cells. In wild-type animals, the cell boundaries gradually expanded from late-stage 12 to late-stage 13. In homozygous Myo31DFL152 or shgR69 embryos, the length was greater than in wild type at all stages examined. This increase was rescued by the overexpression of Myo31DFGFP or shgDECH in the respective mutant background. Thus, DE-Cad and MyoID appear to restrict the expansion of these cell boundaries, suggesting that these proteins introduce cortical tension, possibly with LR asymmetry (Taniguchi, 2011).
To evaluate the idea that PCC is involved in the hindgut LR asymmetric development, an in silico simulation model was built of the PCC of the hindgut epithelial cells and the directional rotation of the tube composed of these cells. This model consisted of two epithelial sheets composed of model cells, forming the dorsal and ventral arcs of a tube with boundary cells separating the sheets, as found in vivo. In this simulation, the number of cells along the AP and LR sides was set to mimic the in vivo situation, and a statistical LR shape bias was not introduced initially. In vivo, DE-Cad was enriched at cell boundaries with an angle x of -90° to 0° and might restrict the cell-boundary expansion. Therefore, in this simulation, the constriction of cell boundaries was maximized at -45° to the AP axis of the hindgut epithelial tube, and the maximized value was twofold greater than at -135° or +45°. This parameter introduced PCC in the modeled epithelial cells (corresponding to late-stage 12) (Taniguchi, 2011).
Because DE-Cad and MyoID were required before but not during the left-handed rotation of the hindgut epithelium, no LR bias was added to the cell-boundary constriction during the epithelial remodeling. The removal of LR bias subsequently led the modeled epithelial cells to assume stable cell shapes that were mostly regular hexagons (corresponding to late-stage 13). This progressive transition in cell shape was also observed in vivo. This simulation reproduced the 90° left-handed rotation of the epithelial tube in silico, suggesting that PCC is sufficient to explain this rotation in vivo. In addition, LR asymmetric changes in the cell-boundary length observed in the hindgut epithelium in vivo were recapitulated in the simulation, supporting the validity of the model (Taniguchi, 2011).
This study has reported PCC as a previously unknown mechanism of LR asymmetric morphogenesis. Various mutants of genes encoding the core components of planar cell polarity (PCP), a well-understood type of epithelial planar polarity, did not affect the laterality of the Drosophila embryonic gut, suggesting that PCC is not simply a variant of PCP. Although the importance of single-cell chirality has not been studied in multicellular organisms in vivo, intrinsic cell chirality has been found in the LR-polarized protrusion of neutrophil-like cells in vitro. Therefore, cell chirality may be a general property of animal cells. These findings demonstrate a contribution of such chirality to LR asymmetric morphogenesis (Taniguchi, 2011).
In bilateria, positioning and looping of visceral organs requires proper left-right (L/R) asymmetry establishment. Recent work in Drosophila has identified a novel situs inversus gene encoding the unconventional type ID myosin (MyoID). In myoID mutant flies, the L/R axis is inverted, causing reversed looping of organs, such as the gut, spermiduct and genitalia. MyoID has been shown to interact physically with β-Catenin, suggesting a role of the adherens junction in Drosophila L/R asymmetry. This study shows that DE-Cadherin co-immunoprecipitates with MyoID and is required for MyoID L/R activity. It was further demonstrated that MyoIC (Myo61F), a closely related unconventional type I myosin, can antagonize MyoID L/R activity by preventing its binding to adherens junction components, both in vitro and in vivo. Interestingly, DE-Cadherin inhibits MyoIC, providing a protective mechanism to MyoID function. Conditional genetic experiments indicate that DE-Cadherin, MyoIC and MyoID show temporal synchronicity for their function in L/R asymmetry. These data suggest that following MyoID recruitment by β-Catenin at the adherens junction, DE-Cadherin has a twofold effect on Drosophila L/R asymmetry by promoting MyoID activity and repressing that of MyoIC. Interestingly, the product of the vertebrate situs inversus gene inversin also physically interacts with β-Catenin, suggesting that the adherens junction might serve as a conserved platform for determinants to establish L/R asymmetry both in vertebrates and invertebrates (Petzoldt, 2012).
This study showns DE-Cadherin has a dual role in promoting MyoID activity. DE-Cadherin directly stabilizes MyoID at the adherens junctions and inhibits the antagonistic activity of MyoIC, thus eliciting MyoID function. MyoID activity and organization are dependent on the level of MyoIC protein. In MyoIC gain of function, MyoID intracellular pattern is modified with a reduced overall signal and the protein level remains unchanged. In MyoIC loss of function, MyoID is not detected by immunohistochemistry and the protein level measured by western blot is reduced. Nevertheless, in MyoIC loss of function, sufficient MyoID activity remains as confirmed by the wild-type rotation phenotype. Thus, MyoID activity is only impaired when MyoIC is in large excess (Petzoldt, 2012).
It is important to note that these results were obtained through two independent approaches, either by direct modulation of MyoIC expression levels or indirectly by affecting DE-cadherin, silencing of which in the A8 segment leads to an increase of MyoIC (and of MyoID) levels. These results indicate that DE-Cadherin negatively regulates both MyoID and MyoIC levels. Taken together, these data indicate that DE-Cadherin controls both the L/R determinant MyoID and its repressor MyoIC activity and protein levels (Petzoldt, 2012).
Specific depletion of DE-Cadherin in the A8 segment, the L/R organizer, leads to a no-rotation phenotype. Could stronger depletion of the adherens junction components lead to a sinistral phenotype? In other words, does DE-Cadherin depletion also affect sinistral development or pathway, which is taking over in absence of MyoID? To tackle this question, both DE-Cadherin and MyoID were depleted. It was found that, unlike the sole depletion of MyoID that leads to a majority of sinistral flies, the double depletion leads to a majority of non-rotated flies. These data therefore indicate that DE-Cadherin acts both on the dextral (MyoID-dependent) and on the sinistral pathways, making it a general regulator of L/R asymmetry in flies (Petzoldt, 2012).
Adherens junctions have previously been connected to MyoID-mediated L/R asymmetry establishment through biochemical analysis, which identified a physical interaction between the MyoID-tail domain and β-Catenin in vitro. This study demonstrates a role of the adherens junction in the Drosophila L/R pathway and places the adherens junction component DE-Cadherin as a regulator of both unconventional myosins MyoIC and MyoID. DE-Cadherin dually promotes MyoID and β-Catenin association at the adherens junctions and inhibits the antagonistic function of MyoIC. Indeed, a partial colocalization of MyoID and DE-Cadherin was observed and the concomitant exclusion of MyoIC from the adherens junction. Further supporting this model is the additional biochemical evidence that MyoID, but not MyoIC, can directly interact with DE-Cadherin and β-Catenin in vitro. Excess of MyoIC is able to disrupt this interaction both in vivo and in vitro, leading to MyoID loss-of-function phenotypes. This disruption can be rescued by MyoID overexpression. It is proposed that MyoID L/R function depends on its physical interaction with adherens junction through both β-Catenin and DE-Cadherin. Upon adherens junction reduction, MyoID does not associate with β-Catenin, MyoIC is no longer repressed and in turn inhibits MyoID activity, leading to L/R defects. Finally, the temporal synchrony of MyoID, MyoIC and DE-Cadherin requirement in genitalia rotation implies a concomitant activity of these proteins in L/R establishment (Petzoldt, 2012).
Interestingly, MyoIC has been reported to affect L/R looping of the embryonic gut, another marker of L/R asymmetry in Drosophila. It was shown that MyoIC overexpression also causes fully penetrant embryonic gut inversion. Thus, the antagonistic function of MyoIC appears to be conserved in Drosophila L/R tissues. Furthermore, it was recently shown that DE-Cadherin is required for L/R asymmetry establishment of the Drosophila hindgut (Taniguchi, 2011). The dextral curving of the hindgut was lost in a myoID and DE-cadherin mutant background accompanied by a loss of L/R biased asymmetric cell shape and a differential DE-Cadherin localization. It was suggested that both factors are implicated in the creation of asymmetric cortical tension prior to asymmetric curving of the hindgut. Taken together, these data indicate that the Drosophila L/R tissues, hindgut and genitalia, show a similar dependency on adherens junction and type I unconventional myosins for L/R determination (Petzoldt, 2012).
Is the role of adherens junctions in L/R asymmetry establishment conserved among species? Interestingly, despite the apparent differences between vertebrates and invertebrates mechanisms of L/R asymmetry establishment, a common theme can be found in the only two molecularly described situs inversus genes to date. In addition to myoID, the mouse inversin gene also causes constant L/R axis inversion in homozygous mutants. Interestingly, the inversin protein was shown to co-precipitate with β-catenin and N-cadherin and to localize to the adherens junction of polarized epithelial cells (Nurnberger, 2002). Therefore, both situs inversus proteins interact with β-catenin and associate with adherens junction molecules, such as N- or E-cadherin. Additionally, N-cadherin was reported to play a role in L/R asymmetry establishment in chicken, as N-cadherin absence at the Hensen's node leads the randomization of L/R asymmetry, similar to the current results showing a role of DE-Cadherin in L/R determination in Drosophila. The Drosophila inversin homologue diego belongs to the planar cell polarity gene family. Depletion of diego, or of any other of PCP gene, in the L/R organizer does not affect the directionality of genitalia rotation, suggesting that Diego does not play a critical role in Drosophila L/R asymmetry. In conclusion, the situs inversus proteins MyoID and Inversin, which play a central and upstream role in the establishment of L/R asymmetry, both require an interaction with the adherens junctions for their function (Petzoldt, 2012).
It is important to note that the L/R axis is established after and, most importantly, relatively to the other two main axes, the anterior-posterior and dorsal-ventral axes. Indeed, L/R determinants have to be oriented along existing spatial coordinates so that the L/R axis is positioned perpendicular to existing axes. It is proposed that the association of MyoID (and more generally of situs inversus proteins) with the adherens junctions is an essential mechanism to orient MyoID activity along the apical-basal axis, which in epithelia is perpendicular to the anterior-posterior axis and is thus equivalent to a dorsal-ventral axis. Therefore, association of MyoID with the adherens junctions would represent a way to orient and polarize its activity. Following polarization of MyoID in epithelia, the intrinsic chirality of myosins, through their directional activity towards one end of the actin filaments, could create de novo an asymmetric axis similar to the F-molecule model proposed by Brown and Wolpert (Brown, 1990; Petzoldt, 2012 and references therein).
In conclusion, the current findings reveal an important link between the adherens junction and L/R asymmetry determinant in Drosophila and suggest an evolutionary conservation in vertebrates and invertebrates of the interactions between the adherens junction and the situs inversus proteins linking cell architecture and polarity to the patterning of the L/R axis. Future work on the molecular targets of MyoID and Inversin will help understand how the L/R axis becomes oriented to lead to stereotyped asymmetric organogenesis (Petzoldt, 2012).
Left-right (LR) asymmetry is essential for organ development and function in metazoans, but how initial LR cue is relayed to tissues still remains unclear. This study proposes a mechanism by which the Drosophila LR determinant Myosin ID (MyoID) transfers LR information to neighboring cells through the planar cell polarity (PCP) atypical cadherin Dachsous (Ds). Molecular interaction between MyoID and Ds in a specific LR organizer controls dextral cell polarity of adjoining hindgut progenitors and is required for organ looping in adults. Loss of Ds blocks hindgut tissue polarization and looping, indicating that Ds is a crucial factor for both LR cue transmission and asymmetric morphogenesis. It was further shown that the Ds/Fat and Frizzled PCP pathways are required for the spreading of LR asymmetry throughout the hindgut progenitor tissue. These results identify a direct functional coupling between the LR determinant MyoID and PCP, essential for non-autonomous propagation of early LR asymmetry (Gonzalez-Morales, 2015).
This work has revealed the existence of an hindgut-specific LR organizer having transient activity. LR information is transferred non-autonomously from this organizing center to the target tissue, involving a unique MyoID-Ds interaction taking place at a PCP signaling boundary (the H1/H2 boundary). Propagation of this initial LR information to the developing hindgut requires both Ds/Ft global and core Fz PCP signaling. Notably, these results suggest that MyoID can act as a directional cue to bias planar cell polarity (Gonzalez-Morales, 2015).
So far, only a role for the core PCP pathway in cilia positioning and LR asymmetry had been reported in mouse, chick, and Xenopus. This study revealed a role of the Fat/Ds PCP pathway in LR asymmetry. The atypical cadherin Ds is essential for early LR planar polarization of hindgut precursors and later on for looping morphogenesis. Ds has a cell-non-autonomous function, allowing transfer of LR information from the H1 domain to H2 hindgut precursor cells. Ds, therefore, represents a critical relay factor acting at the boundary between, and linking, a LR organizer and its target tissue (Gonzalez-Morales, 2015).
In addition to a MyoID-dependent function in H1, the mislooped phenotype induced upon Ds silencing in the H2 domain suggests that Ds also has a MyoID-independent activity in H2 cells, likely through interaction with other PCP genes. Indeed, reducing the activity of PCP global or core gene functions reveals that the two pathways are important in the H2 region for adult hindgut looping. However, the results reveal important differences in the way these pathways control hindgut asymmetry. First, although the adult phenotype is similar upon silencing of one or the other pathway, the early polarization of H2 cells in pupae (10 hr APF) is only affected when knocking down the activity of Ds, Ft, and Fj. These results show that the Ds/Ft pathway, but not the core pathway, is required for establishing early LR polarity. Second, the phenotype is quantitatively different, since silencing of the Ds, Ft, or Fj PCP gene led to a consistent and very strong phenotype, while reducing Fz PCP signaling had a significantly less penetrant one. These data suggest a partly overlapping function of both PCP signaling pathways for late hindgut morphogenesis. Therefore, the following sequential model is proposed: in H1 cells, MyoID interacts with the Ds intracellular domain, which becomes 'biased' toward dextral through a currently unknown mechanism. This initial LR bias is then transmitted across the H1/H2 boundary through Ds/Ft heterophilic interaction. Then, boundary H2 cells relay the initial bias and spread it to the remaining H2 cells through classical Ds/Ft PCP. It is interesting that the local signaling boundary suggested by this model is consistent with recent studies showing that Ds can propagate polarity information in a range of up to eight cells, a distance that is consistent with the size of the H2 domain at 10 hr APF. Once initial polarity has been set up through the Ds/Ft pathway, this is further relayed to and/or amplified by the core pathway. Notably, a similar two-step mechanism has also been proposed for the wing and could apply to other tissues (Gonzalez-Morales, 2015).
The discovery of a coupling between the MyoID dextral factor and Ds is a nice example of crosstalk between existing signaling modules. In the simplest crosstalk model, the role of MyoID would just be to bias or tilt Ds function toward one side, possibly through Ds localization and/or activity polarization along the LR axis. Using both in vitro and in vivo assays, this study has shown that interaction between Ds and MyoID requires Ds intracellular domain, supporting a cytoplasmic interaction between the two proteins. These results, along with recent findings, suggest that Ds may represent a general platform for myosin function in different tissues. In particular, the intracellular domain of Ds was found to bind to the unconventional myosin Dachs, controlling Dachs polarized localization, which is important for subsequent cell rearrangements underlying thorax morphogenesis. However, in contrast to thoracic Dachs, MyoID is not obviously polarized in H1 cells, suggesting that the interaction between myosins and Ds may involve different mechanisms. Additionally, no LR polarized localization of MyoID or Ds was observed in H1 cells, although the existence of subtle asymmetries undetectable by available tools cannot be excluded. Nevertheless, alternative means to generate the LR bias in H1 include: (1) LR polarized expression of an unknown asymmetric factor or (2) LR asymmetric activity of Ds. These interesting possibilities are consistent with recent work showing that some type I myosins can generate directed spiral movement of actin filaments in vitro. It is tempting to speculate that, similarly, MyoID putative chiral activity could be translated into Ds asymmetrical function along the LR axis. Future work will explore this possibility as well as others to unravel the molecular basis of MyoID LR biasing activity in the H1 organizer (Gonzalez-Morales, 2015).
The identification of the H1 domain as a specific adult tissue LR organizer demonstrates the existence of multiple independent tissue and stage-specific LR organizers in flies. This situation echoes what is known in other models, including vertebrates, in which at least two phases of asymmetry establishment can be distinguished. A first pre-gastrula phase, as early as the four-cell stage in Xenopus, involves the generation of asymmetric gradients of ions. Then, a second phase takes place at gastrulation and involves Nodal flow and asymmetric cell migration, eventually leading to asymmetric expression of the nodal gene in the left lateral plate mesoderm. In Drosophila, some interesting common and specific features can be drawn out by comparing the hindgut and terminalia organizers. The first major common feature is the fact that both organizers rely on MyoID function, showing the conserved role of this factor in Drosophila LR asymmetry. Second, the two organizers show temporal disconnection, acting much earlier than LR morphogenesis, which is expected of a structure providing directionality to tissues per se (24 hr for terminalia and ~72 hr for hindgut looping). Such temporal disconnection of MyoID function with late morphogenesis is also observed in the terminalia where a peak of MyoID activity precedes terminalia rotation by 24 hr. Time lag in MyoID function requires LR cue transmission and maintenance in developing tissues until directional morphogenesis. The finding of a role of Ds and PCP in hindgut LR asymmetry provides a simple mechanism by which initial LR information is maintained and transmitted across tissue through long-range PCP self-propagation (Gonzalez-Morales, 2015).
Notably, the two organizers also show distinct features. In terminalia, MyoID has a cell-autonomous function in two adjacent domains. In addition, the terminalia organizer is permanent, developing as an integral component of the adult tissue. In contrast, MyoID in the imaginal ring has a cell-non-autonomous function. Indeed, a striking feature of the hindgut organizer is its transience as it detaches from the hindgut precursors 50 r before full looping morphogenesis prior to its degradation and elimination; hence, the need to transfer LR information to the H2 hindgut primordium. An interesting question then is whether the MyoID-Ds/PCP interaction is conserved in terminalia. This study has shown that the terminali rotation requires the activity of DE-cadherin; however, invalidation of the atypical cadherins Ds or Ft or core PCP signaling in the terminalia organizer did not affect asymmetry. The fact that PCP does not have a general role in Drosophila LR asymmetry is not altogether surprising, as MyoID cell-autonomous function in terminalia and organizer persistence does not require that LR information be transferred to and stored in other parts of the tissue, as is the case in the hindgut. Therefore, despite conservation of the MyoID-dependent upstream dextral cue, significant differences in downstream morphogenetic pathways imply alternative cellular mechanisms controlling cue transmission and maintenance (Gonzalez-Morales, 2015).
The LR signaling module, comprising the dextral determinant MyoID and the still-unknown sinistral determinant, can therefore be coupled to distinct morphogenetic modules, including PCP, as shown in this study. It is suggested that coupling between LR asymmetry and PCP might be observed in processes requiring long-distance patterning of tissues and organ precursors, both in invertebrate and vertebrate models. Understanding organ LR morphogenesis clearly requires studying diverse and complementary models. In this context, the multiplicity of LR organizers discovered in Drosophila represents a powerful model to study the diversity in the coupling of LR organizers with downstream programs responsible for late tissue morphogenesis. In particular, the Drosophila hindgut represents an invaluable model for studying the genetic basis and molecular mechanisms coupling LR asymmetry with PCP patterning (Gonzalez-Morales, 2015).
Search PubMed for articles about Drosophila Myosin 31DF
Bahler, M. and Rhoads, A. (2002) Calmodulin signaling via the IQ motif. FEBS Lett. 513: 107-113. PubMed ID: 11911888
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date revised: 15 June 2018
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