chickadee: Biological Overview | Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation | References

Gene name - chickadee

Synonyms -

Cytological map position - 26A1--26A1

Function - regulation of actin cytoskeleton

Keywords - cytoskeleton, oogenesis

Symbol - chic

FlyBase ID: FBgn0000308

Genetic map position - 2-[18]

Classification - profilin homolog

Cellular location - cytoplasmic



NCBI links: Precomputed BLAST | Entrez Gene | UniGene
BIOLOGICAL OVERVIEW

Recent literature
Newton, I. L., Savytskyy, O. and Sheehan, K. B. (2015). Wolbachia utilize host actin for efficient maternal transmission in Drosophila melanogaster. PLoS Pathog 11: e1004798. PubMed ID: 25906062
Summary:
Wolbachia pipientis is a ubiquitous, maternally transmitted bacterium that infects the germline of insect hosts. Estimates are that Wolbachia infect nearly 40% of insect species on the planet, making it the most prevalent infection on Earth. The bacterium, infamous for the reproductive phenotypes it induces in arthropod hosts, has risen to recent prominence due to its use in vector control. Wolbachia infection prevents the colonization of vectors by RNA viruses, including Drosophila C virus and important human pathogens such as Dengue and Chikungunya. This study presents data indicating that Wolbachia utilize the host actin cytoskeleton during oogenesis for persistence within and transmission between Drosophila melanogaster generations. Phenotypically wild type flies heterozygous for cytoskeletal mutations in Drosophila profilin chic or villin quail either clear a Wolbachia infection, or result in significantly reduced infection levels. This reduction of Wolbachia is supported by PCR evidence, Western blot results and cytological examination. This phenotype is unlikely to be the result of maternal loading defects, defects in oocyte polarization, or germline stem cell proliferation, as the flies are phenotypically wild type in egg size, shape, and number. Importantly, however, heterozygous mutant flies exhibit decreased total G-actin in the ovary, compared to control flies and chic heterozygous mutants exhibit decreased expression of profilin. Additionally, RNAi knockdown of profilin during development decreases Wolbachia titers. Evidence in support of alternative theories was analyzed to explain this Wolbachia phenotype and it was concluded that the results support the hypothesis that Wolbachia utilize the actin skeleton for efficient transmission and maintenance within Drosophila.

Chickadee, the Drosophila homolog of Profilin, a protein involved in actin polymerization, was initially characterized in a search for genes that function during cytoplasmic flow from nurse cells to the oocyte. Cytoplasmic flow along actin-based cytoskeletal microfilaments is responsible for transferring cytoplasmic components from nurse cells to oocytes. This process is critical: it ensures that the oocyte is supplied with sufficient stores for initial zygotic development. In chickadee mutants, females are sterile and there is a disruption of the nurse cell cytoplasmic flow and lack of nurse cell cytoplasmic actin networks. Thus chickadee is a perfect candidate to study as a gene involved in actin based dynamics in oocytes (Cooley, 1992)

Chickadee is implicated in several stages of bulk transport (known as 'dumping') of nurse cell contents into the oocyte. In chickadee mutants some nurse cell cytoplasm flows into the oocyte, although the transfer is incomplete. As the microtubule based cytoskeleton is implicated in nurse cell dumping (See beta1 tubulin), it is presumed that both actin based microfilaments and tubulin based microtubules (Theurkauf, 1994) are involved in dumping. What aspect of the actin based cytoskeleton is involved in cytoplasmic dumping? It is likely that sub-cortical actin-based nurse-cell microfilaments play a role in cytoplasmic flow into the oocyte. (There are there are more than three types of cytoskeleton: for example, actin based microfilaments, microtubules, and subcortical microfilaments. See cytoskeleton for a more detailed description of these three). A nonmuscle myosin is found to associate with subcortical actin but not with cytoplasmic networks. These subcortical actin filaments are very sensitive to cytochalasin treatment. Thus, contraction of the subcortical actin could play a role in the bulk movement of nurse cells into the oocyte (Cooley, 1992 and references).

The actin based microfilament cytoskeleton plays an additional role in cytoplasmic dumping. At stage 11, the nurse cells dump their contents into the oocyte through cytoplasmic bridges termed ring canals. Microfilament bundles form in the nurse cells during this process and are apparently required to hold the nurse cell nuclei in place so that they do not obstruct the ring canals and allow rapid flow of nurse cell cytoplasm into the oocyte. It is thought that these cytoplasmic microfilament bundles are non-contractile and serve a structural function (Mahajan-Miklos, 1994). Mutants in chickadee, quail and singed affect actin bundle formation. Profilin, encoded by chickadee, is presumably required for the polymerization of the actin filaments that compose the bundles (Cooley, 1992), while a villin-related protein encoded by quail and a fascin-related protein encoded by singed are thought to be required to cross-link the actin filaments to form the actin bundles. Two components of the actin-lined ring canals have also been identified - an adducin-like protein encoded by hu-li tai shao and a protein containing scruin repeats encoded by kelch (Mahajan-Milos, 1994 and Manseau, 1996 and references).

The actin and tubulin based microfilament components of the cytoskeleton are intimately associated in oocytes; any discussion of one without the other is clearly incomplete. The rapid cytoplamic streaming that occurs during the microfilament-dependent rapid transfer of cytoplasm from nurse cells into the oocytes is dependent on microtubules. This is known since streaming is inhibitable by colcemid, which functions to disrupt microtubules. Mutations in cappuccino and spire repress this microtubule-based ooplasmic streaming. In capu and spir mutants, the bundling of the microtubules at the cortex of the oocyte and streaming of the oocyte cytoplasm occurs prematurely. The effects on capu and spir mutations suggest that these genes are involved in microtubule processes. However, chickadee mutants share the premature streaming phenotype with capu and spir. The mutant phenotype of these three genes is due to a premature bundling of microtubules. Normally microtubules are found at the cortex of the oocyte from stages 8 through 10. In chic and capu mutants, long tubulin-staining fibers are found throughout the oocyte. It is concluded that a protein that interacts with the actin based cytoskeleton, Chickadee, is also involved in maintainence of the tubulin based cytoskeleton. In fact, mutations in chic result in the mislocalization of Staufen, which normally localizes to the posterior pole. Although the phenotype is quite variable, there is a close relationship between the effects of chic on the distribution of microtubules and on the distribution of Staufen (Manseau, 1996).

Therefore, Chickadee, a protein involved in actin cytoskeletal dynamics, is involved in maintainence of the tubulin based cytoskeleton. How can this been? It has been found that Cappuccino interacts directly with profilin. The fact that Capu and Chickadee interact directly, suggests that Capu and Chic may affect the same processes through this interaction. If this is true, then these two proteins may serve as the interface between the actin and tubulin based cytoskeletons (Manseau, 1996).

Another candidate for an protein that interacts with Chicadee may be Diaphanous, which functions in Drosophila to promote cytokinesis, the final separation process that occurs between daughter cells in mitosis. A mammalian protein, p140mDia, has been identified as a downstream effector of Rho, a small GTPase that regulates cell morphology, adhesion and cytokinesis through the actin cytoskeleton. (For more information see Drosophila Rac1). p140mDia is a mammalian homolog of Diaphanous. p140mDia binds selectively to the GTP-bound form of Rho and also binds to profilin, the homolog of Chickadee in mammals. The interactions among Rho, Diaphanous and profilin suggest that Rho regulates actin polymerization by targeting profilin via p140mDia beneath the specific plasma membranes, and that Diaphanous and profilin play a joint role in cytokinesis (Watanabe, 1997).

In Drosophila, Chickadee is involved in proliferation of germ-line cells in both males and females. In the adult female, the germarium is devoid of germline material, resulting in empty follicle cells stacks. Testes of mutant flies contain a few spermatid bundles; testes from older males are markedly smaller than wild type and appear agametic (Verheyen, 1994).

Both macrochaete and microchaete bristles on the head, thorax, legs and wings are affected by chickadee mutation. The bristle shaft is formed as a cytoplasmic extension of the trichogen cell. Its structure is provided by a core of microtubules surrounded by fiber bundles that are in fact actin filament bundles. The ridges seen on the bristle cuticle are formed by cytoplasm of the trichogen cell protruding between the actin filament bundles: therefore these ridges are indicative of the number of bundles formed. chic mutant bristles are thicker and shorter than normal with sharp bends, kinks and forked ends. The ridges on mutant bristles are often thinner, more numerous and disorganized. Mutant bristles contain abundant actin filament bundles, but they are more numerous and somewhat thinner than wild type. Thus the aberrant external ridge morphology correlates with the condition of the underlying actin filament bundles (Verheyen, 1994).

How does profilin/Chickadee function to regulate the actin cytoskeleton? To approach this question one must know something about actin, the substrate of profilin in mammals. Actin is an ATPase that goes through a cycle of nucleotide binding and hydrolysis. ATP hydrolysis is associated with actin polymerization, and ATP-actin polymerizes faster and at a lower critical concentration than ADP-actin. Profilin binding to an actin monomer decreases 1000-fold the affinity of actin for its bound nucleotide. Since ATP is generally present in large excess over ADP, this will have the effect inside cells of replacing bound ADP with ATP. In the absence of profilin, nucleotide exchange on an actin monomer is relative slow. These observations led to a model in which profilin can locally promote actin filament growth (Theriot, 1993).

Profilin functions in another pathway to promote filament growth. It has been shown that the profilin-actin complex adds directly onto the barbed end of growing actin filaments. Since profilin has a relatively low affinity for the barbed end of filaments, the profilin dissociates, leaving the actin filament one subunit longer. It is though that the pathway for barbed-end elongation involving profilin is more thermodynamically favored than the pathway without it; consequently, the final amount of free unpolymerized actin monomer is lower in the presence of profilin than in its absence (Theriot, 1993).

Profilin was originally identified as a component of cell extracts that inhibit actin filament growth in vitro. It was initially assumed that profilin was a major sequestering factor in most cells, and sequestering was considered to be profilin's primary function. However, there is not nearly enough profilin in a typical cell for this, and another protein that occurs in higher abundance (thymosin beta4) may be responsible for monomer sequestration (Theriot, 1993).

The actin-binding protein profilin is required for germline stem cell maintenance and germ cell enclosure by somatic cyst cells

Specialized microenvironments, or niches, provide signaling cues that regulate stem cell behavior. In the Drosophila testis, the JAK-STAT signaling pathway regulates germline stem cell (GSC) attachment to the apical hub and somatic cyst stem cell (CySC) identity. This study demonstrates that chickadee, the Drosophila gene that encodes profilin, is required cell autonomously to maintain GSCs, possibly facilitating localization or maintenance of E-cadherin to the GSC-hub cell interface. Germline specific overexpression of Adenomatous Polyposis Coli 2 (APC2) rescued GSC loss in chic hypomorphs, suggesting an additive role of APC2 and F-actin in maintaining the adherens junctions that anchor GSCs to the niche. In addition, loss of chic function in the soma resulted in failure of somatic cyst cells to maintain germ cell enclosure and overproliferation of transit-amplifying spermatogonia (Shields, 2014).

Chickadee, the only Drosophila profilin homolog, is required cell intrinsically for GSC maintenance in the testis. As profilin is a regulator of actin filament polymerization and filamentous actin (F-actin) plays a crucial role in the development and stabilization of cadherin-catenin-mediated cell-cell adhesion, profilin likely maintains attachment of Drosophila male GSCs to the hub through its effect on F-actin, which concentrates at the hub-GSC interface where localized adherens junctions anchor GSCs to hub cells. It is proposed that profilin-dependent stabilization of F-actin at the GSC cortex next to the hub may help localize E-cadherin and APC2 to the junctional region. E-cadherin and APC2 in turn may recruit β-catenin/Armadillo, stabilizing the adherens junctions that attach GSCs to the hub. Chickadee may thus facilitate maintenance of GSCs through a cascade of interactions leading to localization and/or retention of both E-cadherin and β-catenin at the hub-GSC interface (Shields, 2014).

E-cadherin plays a crucial role in maintaining hub-GSC attachment. GSC clones mutant for E-cadherin are not maintained. In addition, germline overexpression of E-cadherin delayed GSC loss in stat-depleted GSCs. The results indicate that profilin function is required in GSCs for proper localization of E-cadherin to the hub-GSC interface. Several studies have shown that the actin cytoskeleton plays a crucial role in assembly and stability of adherens junctions. A favored model in the field is that actin filaments indirectly anchor and reinforce E-cadherin-mediated cell junctions by forming an intracellular scaffold for E-cadherin molecules. Indeed, binding to F-actin stabilized E-cadherin and promoted its clustering. Furthermore, the actin cytoskeleton participates in proper localization of E-cadherin molecules to cell-cell contacts. In chic/profilin mutant GSCs, disruption of actin polymerization at the cell cortex leading to local F-actin disorganization may destabilize E-cadherin and reduce its ability to localize to the GSC-hub junction, form clusters and build adequate adherens junctions (Shields, 2014).

Destabilization of E-cadherin may contribute to the mislocalization of APC2 seen in chic mutant GSCs, as E-cadherin recruits APC2 to cortical sites in GSCs. Raising possibilities of a more direct link, actin filaments have been shown to be required for association of APC2 with adherens junctions in the Drosophila embryo and ovary. Treatment of embryos with actin-depolymerizing drugs resulted in complete delocalization of APC2 from adhesive zones and diffuse APC2 staining throughout the cell. Moreover, in ovaries of chic1320/chic221 females, APC2 was substantially delocalized from the plasma membranes of nurse cells and their ring canals, and increased levels of cytoplasmic APC2 staining were observed. Similarly, this study found that APC2 was delocalized from the hub-GSC interface in larval testes of chic11/chic1320 hypomorphs (Shields, 2014).

In several studies, delocalization of APC2 from junctional membranes correlated with detachment of β-catenin/Armadillo from adherens junctions. APC2 co-localizes with Armadillo and E-cadherin at adherens junctions of Drosophila epithelial cells, nurse cells in Drosophila ovaries and at the hub-GSC interface in Drosophila testes. Disruption of APC2 function resulting in significant reduction of junctional APC2 was accompanied by delocalization of junctional Armadillo and increased levels of free cytoplasmic Armadillo in embryonic epithelial cells and ovaries. In a previous study, which used chic1320/chic221 strong loss-of-function mutants, the delocalizing effect on junctional Armadillo was variable, presumably due to incomplete penetrance of chic mutant effects. Although this study did not observe significant disruption in Armadillo staining along the hub-GSC interface of testes from chic hypomorphs, this may be due to incomplete penetrance. In addition, the Armadillo protein detected could be localized to the cortex of hub cells rather than GSCs (Shields, 2014).

The finding that germline specific overexpression of APC2 in chic11/chic1320 hypomorphs partially rescued GSC loss is consistent with a previously proposed model that actin filaments shuttle APC2 to adherens junctions and APC2 in turn recruits cytoplasmic Armadillo to junctional membranes, reinforcing the adherens junctions. It is possible that in chic11/chic1320 hypomorphs, residual actin filaments associated with adherens junctions between the hub and GSCs are sufficient to shuttle the increased amounts of cytoplasmic APC2 to adherens junctions. This APC2 may in turn recruit free cytoplasmic Armadillo to the hub-GSC interface, locally stabilizing the adherens junctions and anchoring GSCs to their niche. Notably, however, germline specific overexpression of APC2 in testes of strong loss-of-function chic1320/chic221 mutants failed to rescue GSC loss. Thus either, adequate levels of actin filament polymerization may be required for the proposed translocation of junctional proteins to the plasma membrane, or APC2 function/localization may not be the only or even the major cell-autonomous target of profilin function important for maintaining GSCs. Indeed, loss of APC2 function did not lead to GSC loss. It is suggested that the localized cortical F-actin underlying adherens junctions at the GSC-hub interface, best candidate for the most direct target of chic function, strongly stabilizes adherens junctions between GSCs and the hub, with high levels of cortical APC2 able to in part make up for weak chic function by also stabilizing adherens junctions (Shields, 2014).

Maintenance of hub-GSC attachment appears to be a key role of STAT in GSCs. The finding that STAT binds to a site near the upstream promoter of the chic gene raises the possibility that STAT might foster GSC attachment to the hub in part by ensuring high levels of transcription of profilin in GSCs. However, activation of STAT is clearly not the only regulatory influence on profilin expression as profilin is an essential gene expressed in many cell types, including those in which STAT is not active or detected. It is likely that transcription factors other than STAT turn on profilin expression in many cell types and that STAT acts along with other regulators to reinforce profilin expression in GSCs and CySCs. Conversely, overexpression of profilin was not sufficient to re-establish attachment of stat-depleted GSCs, suggesting that STAT probably regulates a number of genes to ensure that GSCs remain within the stem cell niche (Shields, 2014).

Loss of chic function in somatic cyst cells impaired the ability of cyst cells to build and/or maintain the cytoplasmic extensions through which they embrace and enclose spermatogonial cysts. Two somatic cyst cells normally surround each gonialblast and enclose its mitotic and meiotic progeny throughout Drosophila spermatogenesis. The cyst cells co-differentiate with the germ cells they enclose. Several lines of evidence support the model that either the ability of somatic cyst cells to enclose germ cells or their ability to send signals to adjacent germ cells is important to restrict proliferation and promote differentiation of germ cells. In either case, activation of EGFR in cyst cells is required for cyst cells to enclose germ cells and/or send the signals for germ cells to differentiate. The similarities in phenotype between loss of chic function and loss of EGFR activation in somatic cyst cells raise the possibility that chic/profilin may act downstream of activated EGFR to modulate the actin cytoskeleton for the remodeling of cyst cells to form or maintain the cytoplasmic extensions that enclose germ cells. Indeed, activated EGFR is known in other systems to tyrosine phosphorylate phospholipase C-γ1 (PLC-γ1), a soluble enzyme in quiescent cells like daughter cyst cells, activating it to catalyze hydrolysis of the membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP2), which binds profilin protein with high affinity, which inhibits the interaction between profilin and actin. The hydrolysis of PIP2 by activated PLC-γ1 results in localized release of profilin and other actin-binding proteins, enabling them to interact with actin and participate in cytoskeletal rearrangement and membrane protrusion. Thus, based on biochemical analysis in other systems, a link between EGFR activation and profilin leading to local remodeling of the actin cytoskeleton is plausible in somatic cyst cells, although it remains to be directly tested (Shields, 2014).


GENE STRUCTURE

Two transcripts, of 1.0 and 1.2 kb are present. The complementary DNAs reveal two different 5' exons, suggesting that the two transcripts are products of transcription from alternative promoters. The longer transcript is present in wild-type males and females and also present in RNA isolated from chickadee mutant flies. It is abundant in ovaries but also present in ovarectomized females. The shorter 1.0 kb transcript, however, is virtually ovary specific. It is concluded that the phenotype displayed by female-sterile alleles of chickadee is caused by the absence of the ovary-specific transcript of a gene that is also transcribed from a different promoter (Cooley, 1992). chickadee is immediately adjacent to the eukaryotic initiation factor, eIF4A (Verheyen, 1994).

Bases in 5' UTR - 215 (ovarian transcript) and 341 (zygotic transcript)

Exons - Four, with alternative first exons

Bases in 3' UTR - 347


PROTEIN STRUCTURE

Amino Acids - 126

Structural Domains

Drosophila Chickadee is 40% identical to profilins from Saccharomyces cerevisiae, Physarum and Acanthamoeba. The homology increases to greater tha 60% similarity when conserved amino acid substitutions are considered. Profilin from mouse and human is 15 amino acids longer than the protein from lower eukaryotes. Alignment allowing gaps in the proteins shows 25% identity and 50% similarity of the fly protein to profilin of mouse and humans. The homology extends throughout the protein including very high conservation at the carboxy terminus. The region of Acanthamoeba profilin (from amino acids 95-125) has been shown to contain an actin binding site (Cooley, 1992 and references).


chickadee: Evolutionary Homologs | Regulation | Developmental Biology | Effects of Mutation | References

date revised: 17 July 97  

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