Formin homology (FH) domain proteins (e.g., FMN; 136535) play a role in cytoskeletal organization and/or establishment of cell polarity.
This is a maternal effect gene.
NCBI Summary:
This gene is a member of the formin homology protein family. The encoded protein is thought to have essential roles in organization of the actin cytoskeleton and in cell polarity. Mutations in this gene have been associated with mental retardation autosomal recessive 47 (MRT47). Alternatively spliced transcript variants have been identified. [provided by RefSeq, Mar 2015]
Role of the C-terminal extension of Formin 2 in its activation by Spire and processive assembly of actin filaments. Montaville P et al. (2015) Formin 2 (Fmn2), a member of the FMN family of formins, plays an important role in early development. This formin cooperates with profilin and Spire, a WH2 repeat protein, to stimulate assembly of a dynamic cytoplasmic actin meshwork that facilitates translocation of the meiotic spindle in asymmetric division of mouse oocytes. The KIND domain of Spire directly interacts with the C-terminal extension of the FH2 domain of Fmn2, called FSI. This direct interaction is required for the synergy between the two proteins in actin assembly. We have recently demonstrated how Spire, which caps barbed ends via its WH2 domains, activates Fmn2. Fmn2 by itself associates very poorly to filament barbed ends, but is rapidly recruited to Spire-capped barbed ends via the KIND domain, and subsequently displaces Spire from the barbed end to elicit rapid processive assembly from profilin-actin. Here we address the mechanism by which Spire and Fmn2 compete at barbed ends and the role of FSI in orchestrating this competition as well as the processivity of Fmn2. We have combined microcalorimetric, fluorescence and hydrodynamic binding assays, as well as bulk solution and single filament measurements of actin assembly, to show that removal of FSI converts Fmn2 into a capping protein. This activity is mimicked by association of KIND to Fmn2. In addition, FSI binds actin at filament barbed ends as a weak capper and plays a role in displacing the WH2 domains of Spire from actin, thus allowing the association of actin-binding regions of FH2 to the barbed end.//////////////////
Spire and formin 2 synergize and antagonize in regulating actin assembly in meiosis by a ping-pong mechanism. Montaville P 2014 et al.
In mammalian oocytes, three actin binding proteins, Formin 2 (Fmn2), Spire, and profilin, synergistically organize a dynamic cytoplasmic actin meshwork that mediates translocation of the spindle toward the cortex and is required for successful fertilization. Here we characterize Fmn2 and elucidate the molecular mechanism for this synergy, using bulk solution and individual filament kinetic measurements of actin assembly dynamics. We show that by capping filament barbed ends, Spire recruits Fmn2 and facilitates its association with barbed ends, followed by rapid processive assembly and release of Spire. In the presence of actin, profilin, Spire, and Fmn2, filaments display alternating phases of rapid processive assembly and arrested growth, driven by a 'ping-pong' mechanism, in which Spire and Fmn2 alternately kick off each other from the barbed ends. The results are validated by the effects of injection of Spire, Fmn2, and their interacting moieties in mouse oocytes. This original mechanism of regulation of a Rho-GTPase-independent formin, recruited by Spire at Rab11a-positive vesicles, supports a model for modulation of a dynamic actin-vesicle meshwork in the oocyte at the origin of asymmetric positioning of the meiotic spindle.
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Cellular localization
Cytoskeleton
Comment
Ovarian function
Oogenesis, Oocyte maturation, Early embryo development
, First polar body extrusion
Comment
Leader B, et al 2002 reported
that a
recently identified formin homology (FH) gene, formin-2 (Fmn2), is a
maternal-effect gene that is expressed in oocytes and is required for
progression through metaphase of meiosis I. Fmn2(-/-) oocytes cannot correctly
position the metaphase spindle during meiosis I and form the first polar body.
Fmn2 is required for microtubule-independent chromatin
positioning during metaphase I. Fertilization of Fmn2(-/-) oocytes results in
polyploid embryo formation, recurrent pregnancy loss and sub-fertility in
Fmn2(-/-) females. Injection of Fmn2 mRNA into Fmn2-deficient oocytes rescues
the metaphase I block. Given that errors in meiotic maturation result in
severe birth defects and are the most common cause of chromosomal aneuploidy
and pregnancy loss in humans, studies of Fmn2 may provide a better
understanding of infertility and birth defects.
Spire-Type Actin Nucleators Cooperate with Formin-2 to Drive Asymmetric Oocyte Division. Pfender S et al. Oocytes mature into eggs by extruding half of their chromosomes in a small cell termed the polar body. Asymmetric oocyte division is essential for fertility [1], but despite its importance, little is known about its mechanism. In mammals, the meiotic spindle initially forms close to the center of the oocyte. Thus, two steps are required for asymmetric meiotic division: first, asymmetric spindle positioning and second, polar body extrusion. Here, we identify Spire1 and Spire2 as new key factors in asymmetric division of mouse oocytes. Spire proteins are novel types of actin nucleators that drive nucleation of actin filaments with their four WH2 actin-binding domains [2-6]. We show that Spire1 and Spire2 first mediate asymmetric spindle positioning by assembling an actin network that serves as a substrate for spindle movement. Second, they drive polar body extrusion by promoting assembly of the cleavage furrow. Our data suggest that Spire1 and Spire2 cooperate with Formin-2 (Fmn2) to nucleate actin filaments in mouse oocytes and that both types of nucleators act as a functional unit. This study not only reveals how Spire1 and Spire2 drive two critical steps of asymmetric oocyte division, but it also uncovers the first physiological function of Spire-type actin nucleators in vertebrates.
Expression regulated by
Comment
Ovarian localization
Oocyte
Comment
Dynamic interaction of formin proteins and cytoskeleton in mouse oocytes during meiotic maturation. Kwon S et al. Formin-2 (Fmn2) nucleates actin filaments required for spindle migration during the metaphase of meiosis I in mouse oocytes. While recent studies showed that Fmn2 is involved in the formation of a dynamic actin meshwork on meiotic spindle and the migration of chromosomes, the precise location and the mechanism of action of Fmn2 in the mouse oocyte is not known. In this work, we show that Fmn2 is colocalized with spindle during metaphase I (MI) and this pattern is lost in nocodazole-treated oocytes. Fmn2 directly interacts with polymerized microtubules in vitro via a well-conserved domain called formin homology 2 (FH2). Microinjection of mRNA encoding FH1FH2 domains of Fmn2 into Fmn2-/- oocytes partially rescued the defect of polar body extrusion, while mRNAs encoding FH2 domain alone could not rescue the defect. mDia1 and mDia2, Diaphanous (Dia) subfamily of formin proteins, exhibit unique patterns of expression in mouse oocytes. While mDia1 is localized on meiotic spindle, mDia2 localization is confined in spindle poles similar to ?-tubulin. Collectively, our results suggest that the ability of Fmn2 to directly interact with microtubules and to polymerize actins via the conserved FH1FH2 domains are crucial for chromosomal migration in MI oocytes. We also show that mDia1 and mDia2 are dynamic components of meiotic spindle and pole complex during meiotic maturation of oocytes.
A new model for asymmetric spindle positioning in mouse oocytes. Schuh M et al. An oocyte matures into an egg by extruding half of the chromosomes in a small polar body. This extremely asymmetric division enables the oocyte to retain sufficient storage material for the development of the embryo after fertilization. To divide asymmetrically, mammalian oocytes relocate the spindle from their center to the cortex. In all mammalian species analyzed so far, including human, mouse, cow, pig, and hamster, spindle relocation depends on filamentous actin (F-actin). However, even though spindle relocation is essential for fertility, the involved F-actin structures and the mechanism by which they relocate the spindle are unknown. Here we show in live mouse oocytes that spindle relocation requires a continuously reorganizing cytoplasmic actin network nucleated by Formin-2 (Fmn2). We found that the spindle poles were enriched in activated myosin and pulled on this network. Inhibition of myosin activation by myosin light chain kinase (MLCK) stopped pulling and spindle relocation, indicating that myosin pulling creates the force that drives spindle movement. Based on these results, we propose the first mechanistic model for asymmetric spindle positioning in mammalian oocytes and validate five of its key predictions experimentally.
Formin-2 is required for spindle migration and for the late steps of cytokinesis in mouse oocytes. Dumont J et al. Female meiotic divisions in higher organisms are asymmetric and lead to the formation of a large oocyte and small polar bodies. These asymmetric divisions are due to eccentric spindle positioning which, in the mouse, requires actin filaments. Recently Formin-2, a straight actin filaments nucleator, has been proposed to control spindle positioning, chromosome segregation as well as first polar body extrusion in mouse oocytes. We reexamine here the possible role of Formin-2 during mouse meiotic maturation by live videomicroscopy. We show that Formin-2 controls first meiotic spindle migration to the cortex but not chromosome congression or segregation. We also show that the lack of first polar body extrusion in fmn2(-/-) oocytes is not due to a lack of cortical differentiation or central spindle formation but to a defect in the late steps of cytokinesis. Indeed, Survivin, a component of the passenger protein complex, is correctly localized on the central spindle at anaphase in fmn2(-/-) oocytes. We show here that attempts of cytokinesis in these oocytes abort due to phospho-myosin II mislocalization.
Follicle stages
Antral, Preovulatory
Comment
PtdIns(3,4,5)P3 is constitutively synthesized and required for spindle translocation during meiosis in mouse oocytes. Zheng P et al. Prior to ovulation, mammalian oocytes complete their first meiotic division and arrest at metaphase II. During this marked asymmetric cell division, the meiotic spindle moves dramatically from the center of the oocyte to the cortex to facilitate segregation of half of its chromosomal content into the diminutive first polar body. Recent investigations have documented critical roles for filamentous actin (F-actin) in meiotic spindle translocation. However, the identity of the upstream regulators responsible for these carefully orchestrated movements has remained elusive. Utilizing fluorescence-tagged probes and time-lapse confocal microscopy, we document that phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P3) is constitutively synthesized with spatial and temporal dynamics similar to that of F-actin and Formin 2 (Fmn2). Blockage of PtdIns(3,4,5)P3 synthesis by LY294002, a specific inhibitor of phosphoinositide 3-kinases (PI3K), disrupts cytoplasmic F-actin organization and meiotic spindle migration to the cortex. F-actin nucleator Fmn2 and Rho GTPase Cdc42 play roles in mediating the effect of PtdIns(3,4,5)P3 on F-actin assembly. Moreover, the spatial and temporal dynamics of PtdIns(3,4,5)P3 is impaired by depletion of MATER or Filia, two oocyte proteins encoded by maternal effect genes. Thus, PtdIns(3,4,5)P3 is synthesized during meiotic maturation and acts upstream of Cdc42 and Fmn2, but downstream of MATER/Filia proteins to regulate the F-actin organization and spindle translocation to the cortex during mouse oocyte meiosis.
Phenotypes
Mutations
1 mutations
Species: mouse
Mutation name: None
type: null mutation fertility: infertile - ovarian defect Comment:Leader B, et al 2002 reported
that a
recently identified formin homology (FH) gene, formin-2 (Fmn2), is a
maternal-effect gene that is expressed in oocytes and is required for
progression through metaphase of meiosis I. Fmn2(-/-) oocytes cannot correctly
position the metaphase spindle during meiosis I and form the first polar body.
Fmn2 is required for microtubule-independent chromatin
positioning during metaphase I. Fertilization of Fmn2(-/-) oocytes results in
polyploid embryo formation, recurrent pregnancy loss and sub-fertility in
Fmn2(-/-) females. Injection of Fmn2 mRNA into Fmn2-deficient oocytes rescues
the metaphase I block. Given that errors in meiotic maturation result in
severe birth defects and are the most common cause of chromosomal aneuploidy
and pregnancy loss in humans, studies of Fmn2 may provide a better
understanding of infertility and birth defects.