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Maria Almonacid, Marie-Emilie Terret, Marie-Hélène Verlhac
To cite this version:
Maria Almonacid, Marie-Emilie Terret, Marie-Hélène Verlhac. Control of nucleus positioning in mouse oocytes. Seminars in Cell and Developmental Biology, Elsevier, 2018, 82, pp.34 - 40.
�10.1016/j.semcdb.2017.08.010�. �hal-03003488�
Control of nucleus positioning in mouse oocytes
Maria Almonacid
∗, Marie-Emilie Terret, Marie-Hélène Verlhac
CIRB,CollègedeFrance,andCNRS-UMR7241andINSERM-U1050,EquipeLabelliséeFRM,ParisF-75005,France
a r t i c l e i n f o
Articlehistory:
Received30June2017 Accepted3August2017 Availableonlinexxx
Keywords:
Mouseoocyte Nucleus Actin Advection Activediffusion
Earlyembryodevelopment
a b s t r a c t
Thepositionofthenucleusinacellcaninstructmorphogenesisinsomecases,conveyingspatialand temporalinformationandabnormalnuclearpositioningcanleadtodisease.Inoocytesfromworm,sea urchin,frogandsomefish,nucleuspositionregulatesembryodevelopment,itmarkstheanimalpoleand inDrosophilaitdefinesthefuturedorso-ventralaxisoftheembryoandoftheadultbodyplan.However, inmammals,theoocytenucleusiscentrallylocatedanddoesnotinstructanyfutureembryoaxis.Yet anoff-centernucleuscorrelateswithpooroutcomeformouseandhumanoocytedevelopment.This issurprisingsinceoocytesfurtherundergotwoextremelyasymmetricdivisionsintermsofthesizeof thedaughtercells(enablingpolarbodyextrusion),requiringanoff-centeringoftheirchromosomes.In thisreviewweaddressnotonlythebio-physicalmechanismcontrollingnucleuspositioningviaanactin- mediatedpressuregradient,butwealsospeculateonpotentialbiologicalrelevanceofnuclearpositioning inmammalianoocytesandearlyembryos.
©2017PublishedbyElsevierLtd.
Contents
1. Introduction...00
2. Anoriginalmechanismfornuclearpositioning...00
2.1. Acytoplasmicactinmeshinvolvedinnuclearcentering...00
2.2. Dynamicsofthecytoplasmicactinmeshandcreationofanactivitygradient...00
2.3. Thecytoplasmicactinmeshpropelsthecytoplasmbyadvection...00
2.4. Dynamicsofthecytoplasmicactinmeshandactivediffusion...00
2.5. Adualmodelfornuclearpositioning...00
2.6. Aconservedmechanism:pronuclearcenteringinthemousezygote ... 00
3. Biologicalsignificanceofnuclearcentering...00
4. Conclusion...00
Fundingsources ... 00
Declarationofinterestsstatement...00
Acknowledgments...00
References...00
1. Introduction
Regulation of nuclear position is essential for the achievement of a variety of cellular and developmental functions. In the female gamete of most species, nuclear positioning close to the oocyte cortex prepares for the very asymmetric divisions allowing the maintenance of maternal stores supporting embryo development.
Indeed, meiotic spindles form where the nucleus was, and thus a
∗Correspondingauthor.
E-mailaddress:[email protected](M.Almonacid).
cortically-associated spindle will promote asymmetric partitioning of the cytoplasm. In most oocyte models, nucleus positioning relies on microtubules, and, depending whether or not oocytes have kept mother centrioles, involves or not centrosomes. In echinoderms, the nucleus is maintained close to the cortex by centrosome- microtubules asters [1,2]. In Drosophila oocytes, the nucleus is pushed towards the antero-dorsal cortex by microtubules, fol- lowing two redundant pathways: a centrosomal-microtubules pathway and a pathway involving the microtubule-associated protein Mud/NuMa [3]. In C.elegans oocytes, that are devoid of cen- trosomes, nuclear positioning is Kinesin 1 dependent [4].
http://dx.doi.org/10.1016/j.semcdb.2017.08.010 1084-9521/©2017PublishedbyElsevierLtd.
2 M.Almonacidetal./SeminarsinCell&DevelopmentalBiologyxxx(2017)xxx–xxx
Fig.1. Mammalianmeiosisandearlydevelopment.Inmammals,oocytegrowthtakesplaceintheovary.Fully-grownoocytesarrestedinProphaseIresumemeiosisand undergotwosuccessiveasymmetricdivisions.Fertilizationtriggersthesecondmeioticdivision.Thezygotethenachievestheveryfirstembryonicdivision.Nuclearcentering takesplaceduringthegrowthphase.Asymmetricmeioticdivisionsoccurwithspindleandchromosomesoff-centering.Fertilizationbringsthepaternalgenomeintothe egg.Thenbothmaternalandpaternalgenomes,packedintopronuclei,mergetogethernearthecenterofthezygotetoprepareforthefirstembryonicdivision.Blue:nuclei, chromosomesandsperm.Darkblue:nucleoli.Green:spindlemicrotubules.
Fig.2. Nuclearcenteringbyacytoplasmicactinmesh.A:Infully-grownmouseoocytes,thestraightactinnucleatorFormin2nucleatesacytoplasmicactinmeshinvolved innuclearcentering.Left:Fmn2−/−oocyteslackacytoplasmicactinmeshandalsodisplayabnormallyoff-centerednuclei.Right:ReintroducingFormin2backintoFmn2
−/−oocytesrestoresthecytoplasmicactinmeshthatexertsapressuregradientonthenucleus,inducingitsrepositioningfromthecortextotheoocytecenter,asforcontrol Fmn2+/−oocytes.Singleplaneconfocalmicroscopyimages:F-actinisinwhiteandDNAisinblue.DNAformsaringaroundthenucleolusinfully-grownoocytes.ForFmn2
−/−oocytes,theonlypoolsofactinarecorticalactinandsmallseedsinthecytoplasmcorrespondingtoactin-coatedvesicles.ForFmn2+/−oocytes,twopoolsofactincoexist:
corticalactinandthecytoplasmicactinmesh.Scalebaris10m.Figurativeimages:nucleusisinblue,nucleolusindarkblueandactinfilamentsareinred.Blackarrows representthepressuregradient.B:Modelfornucleuspositioninginmouseoocytes.ThenucleusexperiencesapropulsionforceFp=−␣<v2>(␣beingconstant)resulting fromagradientofMyosinVb-dependentactivityofactinvesicles<v2>.ThispropulsionforceisbalancedbyafrictionforceFf=−V(Vnuclearvelocityandproportionalto cytoplasmicviscosity).Thus,nuclearvelocityV=−(␣/)<v2>isproportionaltotheactivitygradientofactinvesiclesandinverselyproportionaltotheviscosity.Thegradient ofactivity<v2>vanishesclosetothecenter,thuskeepingthenucleusinacentralposition.Blue:nucleus,reddots:actinvesicles,redlines:actinfilaments,purple:Myosin Vb.
In mouse oocytes, nuclear positioning occurs during oocyte growth, which takes place in the ovary. During embryonic life, a population of germ cells synchronously progress into meiosis until becoming arrested in Prophase I. Then, periodically, all along the reproductive life, some oocytes enter into a growth phase upon hormonal stimulation, increasing in size up to 80
m diameter and accumulating the reserves necessary for further embryonic devel- opment (for a review, see [5]). This growth phase prepares the female gamete for the following steps of meiotic divisions, fer- tilization and early embryonic divisions (Fig. 1). In particular, the
production of a pool of dormant mRNAs is very important since
transcription is strongly reduced at the end of oocyte growth, so
that the following steps of meiotic divisions and early embry-
onic divisions take place without transcription, until the activation
of the zygotic genome at the two-cell stage. During the growth
phase, the nucleus moves from an off-centered position to the
center of the oocyte. At the end of growth, meiosis resumes and
the oocyte undergoes two successive asymmetric divisions, with
off-centering of its genome. Thus, contrary to most oocyte mod-
els, nuclear centering in mouse and human does not position
Fig.3.Addressingactivediffusionandmechanicalpropertieswithinthemouseoocytecytoplasmusingopticaltweezers.A:Diffusive-likemotionofactinvesicles.LOG–LOG representationoftheMeanSquaredDisplacement(MSD)ofactinvesiclesfittedasafunctionofthedelayinsecondstoapower-lawmodelf(t)=atb,wheretisthetimeand b=1.03.Thepower-lawistheslopeofthelineoftheLOG–LOGplot.Aslopeof1isthesignatureofadiffusive-likemotion.B:Opticaltweezerset-uptomeasuremicro-rheology oftheoocytecytoplasm.Theopticaltrapcatchesavesicleinthecytoplasm.Themechanicalresponseismeasuredbyapplyingasinusoidalforcetothetrappedvesicleand observingthesubsequentdisplacement.
the genome in preparation for future asymmetric divisions. How- ever, surprisingly, nuclear centering correlates with the success of meiotic divisions in mouse and human oocytes [6,7]. Another inter- esting feature of nuclear positioning in mouse oocytes is its very original mechanism, that does not involve centrosomes and micro- tubules, as mouse oocytes lose their centrioles early during oocyte growth [8].
2. Anoriginalmechanismfornuclearpositioning
2.1. A cytoplasmic actin mesh involved in nuclear centering
The fully-grown mouse oocyte is filled with a cytoplasmic mesh of actin filaments, which is involved in spindle off-centering during asymmetric divisions [9,10]. This actin mesh is highly dynamic and is made out of actin-coated vesicles and actin filaments nucleated from the surface of these vesicles by the actin nucleator Formin 2 [11] (Fig. 2A). Formin 2 is a straight microfilament nucleator, which permits the elongation of the barbed-end via conserved FH1-FH2 domains. In mouse oocytes, Formin 2 cooperates with Spire 1/2 in a functional unit to nucleate actin filaments at the surface of actin vesicles [11,12]. Formin 2 and Spire 1/2 are known to interact with each other [13]. In Drosophila oocytes, the formin Cappuccino and Spire also work together, as suggested by their genetic and physical interaction [14,15]. Recently, it has been proposed that Formin 2 and Spire 1/2 alternately kick-off each other from the barbed end of the filament, leading to alternating phases of processive assembly and arrested growth, in a kind of “ping-pong” mechanism [16].
Oocytes invalidated for Formin 2 (Fmn2
−/
−), lacking a cyto- plasmic actin mesh, display also abnormally off-centered nuclei (Fig. 2A). Reintroducing Formin 2 back into Fmn2
−/
−oocytes allows for nucleus repositioning from the cortex to the oocyte cen- ter within 4–6 h. The mouse oocyte nucleus, about 30
m wide, moves a 20
m distance in about 5 h, with a global mean velocity of 0.07
m/min. This is slower than the movement of the meiotic spindle towards the cortex, which occurs within 2–3 h at a mean velocity of 0.12
m/min [17], maybe because spindle movement
depends on Myosin II contractility [10,18,19] and because the spin- dle moving along its long axis provides less viscous friction than a round nucleus [17].
This process is also extremely slow compared to the velocity of nucleus positioning mediated by microtubules in other model systems. For example in starfish oocytes, the sperm aster, to which the sperm nucleus is attached, reaches the oocyte center with a mean velocity of about 5
m/min [20]. The difference in efficiency of nucleus centration might reside in the 100 fold difference in the persistence length of actin filaments versus microtubules [21].
The mouse nucleus centering process is very robust, as it occurs in all Fmn2
−/
−rescued oocytes. It is also not reversible, because the nucleus remains perfectly centered after disassembly of the actin cytoplasmic mesh following Cytochalasin D treatment in wild-type oocytes.
2.2. Dynamics of the cytoplasmic actin mesh and creation of an activity gradient
High temporal resolution movies of the cytoplasmic actin mesh during the process of nuclear repositioning, using the filamentous actin probe GFP-UtrCH [22], highlight the important dynamics of actin vesicles within the meshwork.
Tracking of these actin positive vesicles revealed that they estab-
lish a velocity gradient from the cortex, where they move faster,
towards the oocyte center, where they move slower. This gradient
could be explained by an attractive effect of the cortex enriched
in actin filaments (Fig. 2A). Accordingly, the activity of actin vesi-
cles, represented by their squared velocity v
2, which is a measure
of thermal agitation of particles in a fluid, follows a gradient dis-
tribution from the cortex to the oocyte center (Fig. 2B). By using
an analogy of a perfect gas, this activity gradient of actin vesicles
< v
2> generates a propulsion force Fp =
−␣< v
2>. This propulsion
force is proportional to the gradient value and is thereby the trigger
of the nuclear movement towards the oocyte center.
4 M.Almonacidetal./SeminarsinCell&DevelopmentalBiologyxxx(2017)xxx–xxx
Fig.4. Actin-dependentpronuclearcenteringinthezygote.Inthezygote,theactivityofthecytoplasmicmeshishomogeneouswithinthecell,whereasthedensityofthe meshworkvaries.Themeshworkisdenserbetweenthepronucleiandtheclosestcortex(blackarrows).Thislocalactinenrichmentmaybeduetothecorticaldifferentiated zoneabovethesecondmeioticspindleforthefemalepronucleusandtothefertilizationconeforthemalepronucleus.Thisheterogeneitydisappearsaroundpronucleithat havemigratedtowardsthecenter.Therefore,inthezygote,theforcemovingthepronucleiisgeneratedbyadensitygradientratherthananactivitygradient.Blue:pronuclei, darkblue:nucleoli,redlines:actinfilaments.
2.3. The cytoplasmic actin mesh propels the cytoplasm by advection
The centripetal movement of the nucleus is concomitant with the appearance of a strong cytoplasmic streaming, corresponding to random movement of fluids and intracellular compartments.
Remarkably, injection into wild-type oocytes of a dominant- negative of Myosin Vb, the molecular motor driving the dynamics of the cytoplasmic actin mesh [11], fully abolishes both cytoplas- mic streaming and nuclear positioning [23]. Cytoplasmic streaming is generated by the hydrodynamic drag of fluid and cytoplasmic components around Myosin Vb-driven actin vesicles. Therefore, the whole cytoplasm is subjected to advection, a transport mode where objects are moved with a flow. Advection has been reported in many developmental contexts. For example, in the Drosophila oocyte, two successive types of streaming occur during oogenesis (for a review, see [24]): a slow streaming at stages 8 and 9, fol- lowed by the appearance of a fast rotational streaming from stage 11, which are both driven by microtubules and Kinesin 1. Dur- ing C.elegans oogenesis, an actin-dependent cytoplasmic streaming flows through the gonad [25] and in the oocyte/zygote, a circular cytoplasmic flow, driven by microtubules and Kinesin 1, reverses occasionally [4,26]. In the alga Chara corallina, an helical streaming is generated by Myosin XI sliding on actin filaments that are aligned along cylindrical cells [27]. The role of such cytoplasmic streams is not fully understood but several studies suggest they may be partic- ularly relevant during development. In Meiosis II arrested mouse oocytes, an Arp2/3-dependent fountain flow maintains the spin- dle beneath the oocyte cortex [28]. In Drosophila oocytes, the slow cytoplasmic streaming during mid-oogenesis allows the transport of the body axes determinant, mRNA oskar, to the posterior cor- tex [29], whereas the role of the later fast cytoplasmic streaming could be the mixing of cytoplasmic components provided by nurse cells [24]. In Arabidopsis, actin-Myosin XI dependent cytoplasmic streaming could be involved in plant size regulation [30].
2.4. Dynamics of the cytoplasmic actin mesh and active diffusion Addressing the mechanical properties of the mouse oocyte cyto- plasm has been very informative in order to better understand the whole process of nucleus positioning. One way to gain access to the mechanical properties of a material is to track some particles embedded in it. If the particles move a lot, then the material is probably more fluid than if they move less. Calculating the Mean Squared Displacement (or MSD) of a particle, which represents the
space explored by the particle in a given amount of time, allows having a measure of the particle motion. The MSD = <r
2> = 2dDt, where d is the dimension of the system, D is the diffusion coeffi- cient and t is the time. D is inversely proportional to the viscosity, due to the Stokes-Einstein equation D = k
bT/(6
R), where k
bis the Boltzmann’s constant, T is the temperature,
is the viscosity and R is the radius of the particle. Thus, the extent of space explored by a particle is inversely proportional to the viscosity of the surround- ing material. This is the case for diffusive (or Brownian) motion, which describes the motion of a particle only driven by thermal agitation around it. In a purely viscous material, a diffusive motion is characterized by a line of slope 1 when plotting the MSD against time in a LOG–LOG representation (Fig. 3A). Brownian motion is a particular case of a more general theorem of statistical physics, the Fluctuation Dissipation Theorem (or FDT). According to the FDT, the movements of a particle provide information about the mechanical properties of the surrounding environment [31]. However, in some cases, this relationship between particle movements and environ- mental mechanical properties is not verified, what physicists call a “violation of the Fluctuation Dissipation Theorem”. One example is precisely the cell cytoplasm, where the movement of particles is the result of both a passive (or Brownian) contribution and an active one resulting from the activity of molecular motors, as in mouse oocytes. Thus, the cell cytoplasm undergoes a unique regime of “active diffusion”, with the characteristics of a diffusive motion, but which is driven by motors (for reviews on active diffusion, see [32,33]).
How to explain such an apparent paradox? In a visco-elastic material like the cytoplasm, a motion purely driven by thermal agi- tation would appear as constrained, with a slope <1 on a LOG–LOG representation of MSD (Fig. 3A), whereas an active process can result in a slope of 1, otherwise characteristic of diffusive motion.
This is exactly the case for actin vesicles in the mouse oocyte, that are characterized by an MSD plot presenting a slope of 1 (Fig. 3A). Such a discrepancy between the mechanical properties of the surrounding environment and the observed motion of a parti- cle requires to directly measuring the mechanical properties inside the cytoplasm. This is the principle of Force Spectrum Microscopy, which combines direct measurement by optical tweezers in the cytoplasm with the tracking of particles [23,34]. It allows to sepa- rate the contributions of thermal activity and motor activity in the motion of cytoplasmic particles and thus to highlight the violation of FDT as well as to detect active diffusion.
Therefore, we took the challenging approach to probe directly
the mechanical properties of the mouse oocyte cytoplasm by direct
Fig.5.Nuclearpositioninganddeterminationofthebodyaxes.DifferentlyfromtheDrosophilaoocyte,nuclearpositioninginthemouseoocytedoesnotactasamorphogen forthefuturebodyplans.Left:DeterminationofthebodyaxesinDrosophila(fromtoptobottom,eggchamberwithoocyte,cellularblastodermwithpolecellsattheposterior, adult).Intheoocyte,thenucleusmarkstheantero-dorsalcortexandestablishesthedorso-ventralpolarityoftheeggchamberandthefutureembryo.Right:Determination ofthebodyaxesinmouse(fromtoptobottom,fully-grownoocyte,blastocyst,adult).Thecentralnucleusintheoocytedoesnotpredefinethefuturedevelopmentalaxes.
Nucleiaredrawninblue,nucleoliindarkblue.
measurements using optical tweezers in wild-type oocytes, in Fmn2
−
/
−oocytes and in wild-type oocytes expressing the Myosin Vb dominant negative (Fig. 3B) [23]. The cytoplasm of oocytes express- ing Myosin Vb is 2–3 times stiffer than in other conditions. Thus, by promoting active diffusion, the dynamics of the cytoplasmic actin meshwork fluidizes the cytoplasm, thereby favoring the movement of such a big object like the mouse oocyte nucleus.
2.5. A dual model for nuclear positioning
All these data contribute to elaborate a mathematical model (Fig. 2B). Nuclear movement results from an equilibrium between the propulsion force Fp and a friction force Ff =
−V (V nuclear velocity and
proportional to cytoplasmic viscosity). Thus, nuclear velocity V =
−(
␣/
) < v
2> is proportional to the activity gradient of actin vesicles and inversely proportional to the viscosity. In this context, Myosin Vb plays a dual role:
– It generates the activity gradient ( <v
2>), which is the source of the propulsion force Fp.
– It generates advection, fluidizing the cytoplasm and thus ensur- ing a viscosity compatible with the movement of a large organelle like the nucleus.
The model also predicts that the propulsion force Fp vanishes at the oocyte center, as does the gradient (Fig. 2B), thus keeping the nucleus in a central position. Interestingly, this mechanism is necessary to move the nucleus to the center, but not for maintain-
ing it. Indeed, removing both the actin mesh and the microtubules has no effect on nuclear positioning: in wild-type oocytes treated with Cytochalasin D and Nocodazole, the nucleus stays at the cen- ter [23]. This suggests a high intrinsic viscosity of the cytoplasm, independent on actin and microtubules. Fully-grown oocytes have accumulated a high quantity of dormant RNAs engaged into RNPs (RiboNucleoProteins) that are necessary for the further steps of meiotic divisions and the first steps of embryonic development.
These dormant RNAs/RNPs could provide the viscosity of this matrix, as it has been observed for C. elegans oocytes [35].
Another interesting feature of this mechanism is its non- specificity. Due to the physical nature of the gradient, we suspect that any object placed within the gradient would find its way to the center. In agreement with this, injected fluorescent latex beads were found at the center 18 h after injection instead of 5 h for the nucleus [23]. The size of the object may be a critical parameter and further work will be required to fully characterize this gradient and its properties.
2.6. A conserved mechanism: pronuclear centering in the mouse zygote
Later during development, in the mouse zygote, the migration of the male and female pronuclei shares striking similarities with nuclear positioning in fully-grown oocytes [36]. Following fertiliza- tion, the two pronuclei assemble at the periphery of the embryo.
Then, they migrate towards the embryo center in 12–15 h where
they remain roughly positioned until the merging of the two sets
6 M.Almonacidetal./SeminarsinCell&DevelopmentalBiologyxxx(2017)xxx–xxx
of chromosomes into the metaphase plate [36]. Pronuclear migra-
tion also occurs in the absence of true centrosomes and astral microtubules since the centrioles brought by the sperm are rapidly degraded [37] and rebuilt de novo at the 64-cell stage [38].
Interestingly, as for oocyte nuclear positioning, this coarse pronuclear centering in the mouse zygote depends on F-actin- and Myosin Vb driven cytoplasmic flows [36]. However, some differ- ences can be pinpointed. In the case of oocyte nuclear centering, the trigger of nuclear movement is the anisotropy in actin vesi- cles activity, whereas the density of actin vesicles remains constant across the cytoplasm [23]. Conversely, in the zygote, the activ- ity of the cytoplasmic mesh is homogeneous within the cell; the anisotropy triggering nuclear movement comes from the heteroge- neous density of the actin mesh. Indeed, the meshwork between the pronuclei and the closest cortex is denser than the meshwork on the other side of the pronuclei (Fig. 4). This probably corresponds to a local actin enrichment due to the presence of maternal and paternal chromosomes underneath the cortex (for the female pronucleus, the cortical differentiated zone above the second meiotic spindle and for the male pronucleus, the fertilization cone formed upon sperm entry [39]). This heterogeneity is lost around pronuclei that have migrated towards the center (Fig. 4).
Accordingly, the model describing pronuclear movement is dif- ferent from the one describing oocyte nuclear movement. Here again, by keeping the analogy of the cytoplasm as a perfect gas, the pressure resulting from the agitation of n particles in the gas is P = nk
bT
e, where k
bis the Boltzmann’s constant and T
eis the effective temperature of the particle. k
bT
erepresents the activity of particles, like < v
2> in the previous oocyte nuclear migration model. But here, < v
2> and so k
bT
eremain constant throughout the cytoplasm, so that the gradient in the number n of actin- positive foci directly results in a pressure gradient. In other words, in the zygote, the force moving the pronuclei stems from a density gradient whereas in the Prophase I arrested oocyte, the force mov- ing the nucleus comes from a velocity gradient. Interestingly, the model predicts that the force moving the pronuclei depends both on the pressure gradient P and on the volume (and thus the size) of the object. This could explain why the pronuclei move slower towards the center than the bigger Prophase I nucleus, and raises the question of object size dependence also for the nuclear center- ing mechanism in fully-grown oocytes. An alternative explanation could be that an activity gradient, as in the case of Prophase I nuclear centering, is more efficient than a density gradient.
3. Biologicalsignificanceofnuclearcentering
The robustness of the nuclear centering machinery in the mouse oocyte is in apparent contradiction with the following steps of meiotic divisions, which are highly asymmetric, with chromosome off-centering. This could explain why the cytoplasmic mesh is dis- mantled prior meiosis resumption, to favor symmetry breaking preceding spindle migration and asymmetric division [40]. One can wonder about the biological significance of nuclear centering in this context.
In most types of oocytes, nuclear positioning acts as a develop- mental cue that predefines the future developmental axes of the embryo (Fig. 5). For example, in the Drosophila oocyte, the nucleus moves from the posterior to the antero-dorsal cortex. This local- ization initiates the establishment of the dorso-ventral polarity of the egg chamber and the future embryo. In echinoderm oocytes, the position of the nucleus marks the animal pole [1,2]. In the C.elegans oocyte, the nucleus undergoes migration to the future site of spindle attachment at the anterior cortex and thus defines the antero-posterior axis of the future embryo [4].
However, in mammals, the position of the nucleus does not prefigure the adult body plan. Indeed, developmental axes are established later during development, after embryo implantation [41]. Yet the correlation between centering and the success of meiotic divisions [6,7] suggests a possible developmental role for nuclear centering.
In this context, an attractive hypothesis is that nuclear position- ing in the mouse oocyte, and more precisely its force generated mechanism, could regulate the “competence” of the oocyte, i.e its ability to be fit for fertilization and for later embryonic devel- opment. Random cytoplasmic mixing concomitant to nuclear centering could contribute to the homeostasis of the oocyte by avoiding unphysiological compartmentalization of the cytoplasm [42,43]. Indeed, it has been shown that the transport of small molecules or nutrients is enhanced by active diffusion within the cell [34]. Altogether, this could maintain the oocyte in a non- polarized state, not only at the cortical level [44], but also in the cytoplasm, where all subcellular compartments are micronized [42,43]. As a consequence, all the cytoplasmic components would be homogeneously distributed, and nuclear central position would contribute to the creation of such an isotropic landscape. Homoge- nization of the cytoplasm could be vital for mammalian eggs, which compared to Xenopus oocytes do not possess a huge amount of vitel- lus, a major source of energy required to sustain the long Metaphase II arrest as well as early stages of embryo development. For exam- ple, homogenization of mitochondria in Prophase I, coupled to their specific retention in the oocyte cytoplasm during the two asymmet- ric divisions [42] might be essential elements to ensure sufficient energy supplies for mouse oocytes. This is totally different from other model systems, like the Drosophila oocyte. These oocytes also possess a cytoplasmic actin mesh nucleated by the formin Cappuc- cino, but differently from the mouse oocyte, this mesh prevents the appearance of premature fast microtubule-dependent cytoplasmic streaming that would lead to the dispersion of important develop- mental determinants like the bicoid and oskar mRNAs that define the anterior-posterior axis of the embryo [24,45]. It is intriguing that apparently similar cytoskeletal structures, but in two different organisms, serve such opposite developmental purposes.
4. Conclusion
Nuclear positioning in the mouse oocyte involves a novel mech- anism that relies on actin vesicles dynamics and is independent from centrosomes and associated microtubules.
It is the very first description of an activity gradient of actin vesicles producing a force triggering the movement of the oocyte nucleus, a very large organelle, the size of a somatic cell. Differently from directional transport by molecular motors along microtubules or microfilaments, the source of motion here is the activity of actin vesicles induced by Myosin Vb, that generates advection by drag- ging the fluid and other cytoplasmic components around moving vesicles, a hallmark of active diffusion.
Such a mechanism is particularly important in large cells devoid of centrosomes, where dynamic actin meshes generate forces and motion across long distances, as an alternative to microtubule- based transport.
Beyond these implications in cell biology, nuclear positioning in the mouse oocyte could have potential developmental roles, by keeping the oocyte in a non-polarized state, which, unlike in other organisms, may favor early embryonic development.
Fundingsources
This work was supported by grants from the Fondation
pour la Recherche Médicale (FRM Label to M-H Verlhac-
Acknowledgments
We thank Nature Cell Biology for the permission to reproduce parts of figures from [23] in Fig. 3.
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