- Email: [email protected]
Nuclear Division: Giving Daughters Their Fair Share
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decades. Only five years ago, a concise study [19] describing all medulla neurons connected to R7 and R8 could claim to list the entire colour vision pathway of Drosophila. The new study by Schnaitmann and colleagues [8] now shows convincingly that, contrary to these expectations, photoreceptors R1–6 do indeed contribute to colour vision in Drosophila. Using a blind mutant and GAL4-drivers they generated flies with restricted sets of functional photoreceptors and tested their colour discrimination. Flies with functional ‘yellow’ ommatidia, but not those with ‘pale’ ommatidia, discriminated green and blue as well as normal flies, even with inversed intensities. As expected, flies which had no functional receptors except R7 and R8 in ‘yellow’ ommatidia also did well. However, even flies which only had functional receptors R8 and R1–6 in ‘yellow’ ommatidia could do the job. This came as a surprise. It implies that the broadly tuned receptors R1–6 contribute to both the achromatic pathway and the colour vision pathway in flies. Schnaitmann et al. [8] went one step further and generated flies lacking neurons in the lamina. They showed that the colour vision pathway depends on neurons known as ‘lamina monopolar cells’ to convey the signals from R1–6 to the medulla, where they can be compared neurally with signals from R7 and R8. Further studies can now unravel the full colour vision pathway of Drosophila. The results by Schnaitmann and colleagues [8] strongly suggest that flies may have a rather conserved insect colour vision system. Thus, anything we learn from Drosophila will help us to understand
colour vision not only in this tiny fly that did not seem to care much about colour, but even in bees and other insects. More generally, we learn that flies use information more efficiently than previously thought. The analogy that fly receptors R1–6 serve a similar function as human rods, while fly receptors R7 and R8 are comparable to our cones, no longer holds. More adequately, flies use their receptors in a similar way as we use our cones: all receptors are involved in colour vision, and most — in flies six out of eight receptors in each ommatidium, in humans the red and green cones (93% of all cones) — are additionally used for achromatic vision, in a parallel pathway. Birds remain the challenge: why do the animals that have the sharpest vision of all use only half of their cones — the double cones — for high acuity achromatic vision? Or did we, just as in fruit flies, miss something? The new results on Drosophila [8] have challenged a paradigm: parallel visual pathways may share the same input more often than we thought. 1. Frisch, K.v. (1914). Der Farbensinn und Formensinn der Biene. Zool. J. Physiol. 37, 1–238. 2. Vorobyev, M., and Osorio, D. (1998). Receptor noise as a determinant of colour thresholds. Proc. R. Soc. B 265, 351–358. 3. Kelber, A., Vorobyev, M., and Osorio, D. (2003). Colour vision in animals – behavioural tests and physiological concepts. Biol. Rev. 78, 81–118. 4. Borst, A. (2009). Drosophila’s view on insect vision. Curr. Biol. 19, R36–R47. 5. Schu¨mperli, R.A. (1973). Evidence for colour vision in Drosophila melanogaster through spontaneous phototactic choice behaviour. J. Comp. Physiol. 86, 77–94. 6. Menne, D., and Spatz, H.-C. (1977). Colour vision in Drosophila melanogaster. J. Comp. Physiol. A 114, 301–312. 7. Schnaitmann, C., Vogt, K., Triphan, T., and Tanimoto, H. (2010). Appetitive and aversive
How do nuclear components, apart from chromosomes, partition equally to daughter nuclei during mitosis? In Schizosaccharomyces japonicus, the conserved LEM-domain nuclear envelope protein Man1 ensures the formation of identical daughter nuclei by coupling nuclear pore complexes to the segregating chromosomes.
When we consider what constitutes a successful mitosis, we immediately
9.
10.
11.
12.
13.
14.
15.
16.
References
Nuclear Division: Giving Daughters Their Fair Share
Alison D. Walters and Orna Cohen-Fix
8.
think of the correct segregation of chromosomes into two daughter nuclei. However, it takes more than chromosomes to make a nucleus. The
17.
18.
19.
visual training in freely moving Drosophila. Front. Behav. Neurosci. 4, 10. Schnaitmann, C., Gerbers, C., Wachtler, T., and Tanimoto, H. (2013). Color discrimination with broadband photoreceptors. Curr. Biol. 23, 2375–2382. Osorio, D., and Vorobyev, M. (2005). Photoreceptor spectral sensitivities in terrestrial animals: adaptations for luminance and colour vision. Proc. R. Soc. B 272, 1745–1752. Nilsson, D.-E., and Osorio, D. (1997). Homology and parallelism in arthropod sensory processing. In Arthropod Relationships, R.A. Fortey and R.H. Thomas, eds. (London: Chapman Hall), pp. 333–347. Kelber, A. (2006). Invertebrate colour vision. In Invertebrate Vision, E.J. Warrant and D.-E. Nilsson, eds. (Cambridge: Cambridge University Press), pp. 250–290. Marshall, N.J., Kent, J. and Cronin, T. (1999). Visual adaptations in crustaceans: spectral sensitivity in diverse habitats. In Adaptive Mechanisms in the Ecology of Vision (eds. S. N. Archer et al.), pp. 285–27. Kluwer, Dordrecht. Gribakin, F.G. (1972). The distribution of the long wave photoreceptors in the compund eye of the honey bee as revealed by selective osmic staining. Vision Res. 12, 1225–1230. Arikawa, K. (2003). Spectral organisation of the eye of a butterfly, Papilio. J. Comp. Physiol. A 189, 791–800. White, R.H., Xu, H., Munch, T.A., Bennett, R.R., and Grable, E.A. (2003). The retina of Manduca sexta: rhodopsin expression, the mosaic of green-, blue- and UV-sensitive photoreceptors, and regional specialization. J. Exp. Biol. 206, 3337–3348. Wakakuwa, M., Kurasawa, M., Giurfa, M., and Arikawa, K. (2005). The compound eye of the honeybee Apis mellifera is composed of three spectrally distinct types of ommatidia. Naturwissenschaften 92, 464–467. Hardie, R.C. (1986). The photoreceptor array of the dipteran retina. Trends Neurosc. 9, 419–423. Strausfeld, N.J., and Lee, J.-K. (1991). Neuronal basis for parallel visual processing in the fly. Visual Neurosci. 7, 13–33. Morante, J., and Desplan, C. (2008). The color-vision circuit in the medulla of Drosophila. Curr. Biol. 18, 553–565.
Lund Vision Group, Department of Biology, Lund University So¨lvegatan 35, 22362 Lund, Sweden. E-mail: [email protected], [email protected] http://dx.doi.org/10.1016/j.cub.2013.10.025
integrity of the daughter nuclei and the organization of the chromatin within them rely on the presence of an intact nuclear envelope (NE). The NE is a double lipid bilayer, with an outer membrane that is continuous with the ER, and an inner nuclear membrane (INM) that contains proteins that interact with chromatin and other nuclear components. The NE is perforated by nuclear pore complexes (NPCs) that allow selective passage of proteins between the nucleoplasm and cytoplasm. In metazoans, a filamentous network,
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Figure 1. Anaphase in wild-type and man1D S. japonicus. (A) In wild-type cells, the expansion of the NE (in green) is limited. Consequently, in early anaphase (top panel) the nucleus is shaped as a diamond (or prolate spheroid) and cannot elongate any further. At this point the sister chromatids (in blue) are segregated to the two poles and the NPCs (in red) are absent from the nucleus mid-section, which is occupied by the nucleolus (in yellow). The chromosome segments that extend into the nucleolus represent the DNA region coding for the ribosomal RNA. The nucleus can fully elongate in late anaphase thanks to the rupturing of the NE, which typically happens around the middle of the nucleus (bottom panel). The parental nucleolus is left in the nucleus mid-section and begins to disassemble. New nucleoli form in the daughter nuclei. (B) In a man1D cell, sister chromatids separate normally in early anaphase but the NPCs remain in the nucleus mid-section. In late anaphase the nucleus ruptures, as in wild-type cells, but the rupture site is randomly positioned, often away from the nuclear mid-section. The parental nucleolus fails to disassemble and segregates to one of the daughter nuclei.
known as the nuclear lamina, underlies the INM. The NE breaks down at the onset of mitosis; membrane and membrane-associated proteins are absorbed into the ER while soluble proteins diffuse throughout the cytoplasm [1,2]. At the end of mitosis these NE components are retrieved in a poorly understood process to assemble the NEs of the two new daughter nuclei. Yeast undergo mitosis in a somewhat different fashion. In most of the commonly studied yeast, the nuclear lamina is absent, the NE remains intact throughout the entire cell cycle, and the nucleus divides by a process of expansion followed by fission (called ‘closed mitosis’, in contrast to ‘open mitosis’ where the NE breaks down). The mechanisms ensuring that each daughter nucleus receives half of the nuclear membrane and its protein components (e.g., NPCs) are largely unknown. There are also examples of organisms where mitosis is neither fully open (as in metazoa), nor fully closed (as in budding or fission yeast). One such
example is Schizosaccharomyces japonicus. In this particular yeast, the NE does not expand during mitosis, and instead it ruptures near the nucleus mid-section as the anaphase spindle elongates [3,4] (Figure 1A). The nucleolus does not divide but remains in the nucleus mid-section and then disassembles. Interestingly, the NPCs, which are uniformly distributed around the interphase nucleus, are absent from the mid-section of the anaphase nucleus (Figure 1A). Once anaphase is complete, the NE is resealed to form two daughter nuclei of equal size. The mechanism of this semi-open mitosis raises several interesting questions: how are NPCs cleared from the mid-section? Do they move through the NE or are they disassembled and new ones reassemble elsewhere? And how do cells ensure an even split of the nuclear membrane and other NE components? A study by Yam et al. [5] published in a recent issue of Current Biology reveals that the key to equal division of the nucleus in
S. japonicus lies with the highly conserved LEM-domain protein Man1. NPCs of higher eukaryotes are largely immobile, likely due to their association with the nuclear lamina. The little movement that these NPCs do exhibit happens in conjunction with the underlying lamin network [6]. In budding yeast, which lack lamins, NPCs are much more mobile [7,8]. How, then, are NPCs cleared from the nuclear mid-section in S. japonicus? To distinguish between NPC movement and NPC disassembly followed by reassembly, Yam et al. [5] fused a photo-convertible fluorescent protein to an NPC subunit and examined the fate of NPCs at the center of the mitotic nucleus after they had been photo-converted. Not only did these NPCs move away from the mid-section and towards the nuclear poles, they did so in a manner that was coincident with the poleward movement of the chromosomes. Motor-dependent NPC movement in yeast has been reported previously [9], but these movements were on a much smaller scale than mitotic NPC movement in S. japonicus in terms of the number of NPCs that moved coordinately and the distance that they travelled. Thus, the NPC movement observed in S. japonicus likely involved a different mechanism. Since NPCs moved along with chromosomes, the authors hypothesized that this movement may be mediated by one or more proteins that link chromosomes to NPCs. LEM-domain proteins appear to fit the bill: members of this protein family (named after its founding members LAP2, Emerin and Man1) localize to the INM and help to organize and regulate chromatin at the NE, thereby playing roles in transcriptional regulation, recombination and DNA replication ([10] and references therein). They perform these functions through a transmembrane domain that anchors them in the NE and a w40 amino-acid LEM domain that either binds DNA directly [11] or, in metazoans, binds to chromatin through the small DNA-binding protein BAF ([10] and references therein). Yam et al. found that one of the S. japonicus LEM-domain proteins, Man1, was required for several aspects of nuclear division: man1D cells failed to disassemble their nucleolus at mitosis, produced
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unequal sized daughter nuclei, formed an NE break in random locations and did not clear NPCs from the nucleus mid-section [5] (Figure 1B). Chromosome segregation, on the other hand, was largely unaffected. These observations suggested that Man1 might mediate NPC movement by linking NPCs to chromosomes. But is the effect of Man1 on NPC movement direct, or is the NPC movement defect in the man1D mutant an indirect consequence of its many other failures in mitotic processes? To address this, Yam et al. [5] designed an artificial tether that linked chromosomes with NPCs independently of Man1. This tether was able to rescue the NPC movement defect of man1D cells, suggesting that Man1 physically couples chromosome segregation to the movement of NPCs away from the nuclear mid-section. The observation that LEM-domain proteins can perform ‘lamina-like’ functions in yeast is not a new one: the LEM domain proteins Heh1 and Heh2 have previously been shown to affect NPC distribution in S. cerevisiae [12], and LEM domain proteins are known to tether telomeres to the NE during interphase [13]. However, the Yam et al. study [5] reveals a novel function for a LEM-domain protein in coupling NPC inheritance to chromosome segregation. Along with NPCs, nuclear membrane must also be evenly distributed among the daughters. In most organisms mitosis normally results in two equally sized daughter nuclei, regardless of whether they undergo open, semi-open or closed mitosis. Little is known about how the ‘right’ amount of membrane is used to reassemble the NE around the two sets of daughter chromosomes after open mitosis, although it is likely that components of the INM and nuclear lamina play an important role [14,15]. In the case of closed mitosis, the mechanism by which the nuclear fission point is determined remains unclear. In their recent study, Yam et al. [5] show that man1D cells form nuclei that are smaller and of unequal size. While the cause of this uneven NE distribution remains unknown, it is interesting to note that in wild-type cells the NE rupture site is equidistant between the two extreme ends of the nucleus (Figure 1), coinciding with
the telomere of the last segregating chromosome. In man1D mutants, by contrast, the site of rupture is asymmetrically localized, suggesting that the Man1-dependent positioning of the rupture site dictates the amount of NE inherited by the two daughter nuclei. Interestingly, the artificial tether described above that rescued the NPC movement in man1D cells also corrected the NE rupture site. Thus, Man1 may affect the amount of NE inherited by the daughter nuclei by properly demarcating the rupture site through chromosome–NE attachments, although at this point other mechanisms cannot be excluded. To date, many examples of how the NE can organize and regulate chromatin function have been identified. This recent study of the mitotic nucleus in S. japonicus provides an interesting example of how chromosome movement, via Man1, is being used to organize the NE in order to correctly partition NE components. A number of interesting questions arise from studies in S. japonicus. For example, why develop a mechanism to ensure the inheritance of NPCs from mother to daughters, when the cell can reassemble NPCs de novo? Perhaps inserting NPCs de novo is an energetically expensive process, or even an impossible feat if there aren’t enough NPCs, making the import of NPC components that must be inserted into the nucleoplasmic side of the NE too inefficient. Along these lines, a possible explanation for the unequal and reduced sizes of daughter nuclei in man1D cells is that a reduction in the number of NPCs present in the NE causes decreased import of a factor that determines nuclear size, as lamin B does in metazoa [16]. How Man1 affects nucleolar disassembly and the mechanism by which Man1 determines the site of nuclear rupture also remain unclear. Yam et al. suggest that the attachment of the last segregating chromosomes to the NE generates a force that aids rupture. However, it is also possible that the Man1-dependent clearance of NPCs from the centre of the nucleus weakens the NE, favoring rupture at this location. Although many questions remain to be answered, it is clear that LEM-domain proteins play a conserved role in regulating NE dynamics in variant forms of mitosis.
References 1. Mattaj, I.W. (2004). Sorting out the nuclear envelope from the endoplasmic reticulum. Nat. Rev. Mol. Cell Biol. 5, 65–69. 2. Ellenberg, J., Siggia, E.D., Moreira, J.E., Smith, C.L., Presley, J.F., Worman, H.J., and Lippincott-Schwartz, J. (1997). Nuclear membrane dynamics and reassembly in living cells: targeting of an inner nuclear membrane protein in interphase and mitosis. J. Cell Biol. 138, 1193–1206. 3. Aoki, K., Hayashi, H., Furuya, K., Sato, M., Takagi, T., Osumi, M., Kimura, A., and Niki, H. (2011). Breakage of the nuclear envelope by an extending mitotic nucleus occurs during anaphase in Schizosaccharomyces japonicus. Genes Cells 16, 911–926. 4. Yam, C., He, Y., Zhang, D., Chiam, K.H., and Oliferenko, S. (2011). Divergent strategies for controlling the nuclear membrane satisfy geometric constraints during nuclear division. Curr. Biol. 21, 1314–1319. 5. Yam, C., Gu, Y., and Oliferenko, S. (2013). Partitioning and remodeling of the Schizosaccharomyces japonicus mitotic nucleus require chromosome tethers. Curr. Biol. 23, 2303–2310. 6. Daigle, N., Beaudouin, J., Hartnell, L., Imreh, G., Hallberg, E., Lippincott-Schwartz, J., and Ellenberg, J. (2001). Nuclear pore complexes form immobile networks and have a very low turnover in live mammalian cells. J. Cell Biol. 154, 71–84. 7. Belgareh, N., and Doye, V. (1997). Dynamics of nuclear pore distribution in nucleoporin mutant yeast cells. J. Cell Biol. 136, 747–759. 8. Bucci, M., and Wente, S.R. (1997). In vivo dynamics of nuclear pore complexes in yeast. J. Cell Biol. 136, 1185–1199. 9. Steinberg, G., Schuster, M., Theisen, U., Kilaru, S., Forge, A., and Martin-Urdiroz, M. (2012). Motor-driven motility of fungal nuclear pores organizes chromosomes and fosters nucleocytoplasmic transport. J. Cell Biol. 198, 343–355. 10. Brachner, A., and Foisner, R. (2011). Evolvement of LEM proteins as chromatin tethers at the nuclear periphery. Biochem. Soc. Trans. 39, 1735–1741. 11. Steglich, B., Filion, G.J., van Steensel, B., and Ekwall, K. (2012). The inner nuclear membrane proteins Man1 and Ima1 link to two different types of chromatin at the nuclear periphery in S. pombe. Nucleus 3, 77–87. 12. Yewdell, W.T., Colombi, P., Makhnevych, T., and Lusk, C.P. (2011). Lumenal interactions in nuclear pore complex assembly and stability. Mol. Biol. Cell 22, 1375–1388. 13. Grund, S.E., Fischer, T., Cabal, G.G., Antunez, O., Perez-Ortin, J.E., and Hurt, E. (2008). The inner nuclear membrane protein Src1 associates with subtelomeric genes and alters their regulated gene expression. J. Cell Biol. 182, 897–910. 14. Anderson, D.J., Vargas, J.D., Hsiao, J.P., and Hetzer, M.W. (2009). Recruitment of functionally distinct membrane proteins to chromatin mediates nuclear envelope formation in vivo. J. Cell Biol. 186, 183–191. 15. Asencio, C., Davidson, I.F., SantarellaMellwig, R., Ly-Hartig, T.B., Mall, M., Wallenfang, M.R., Mattaj, I.W., and Gorjanacz, M. (2012). Coordination of kinase and phosphatase activities by Lem4 enables nuclear envelope reassembly during mitosis. Cell 150, 122–135. 16. Levy, D.L., and Heald, R. (2010). Nuclear size is regulated by importin alpha and Ntf2 in Xenopus. Cell 143, 288–298.
The Laboratory of Cell and Molecular Biology, NIDDK, NIH, Bethesda, MD 20892, USA. E-mail: [email protected], ornac@helix. nih.gov http://dx.doi.org/10.1016/j.cub.2013.10.041
decades. Only five years ago, a concise study [19] describing all medulla neurons connected to R7 and R8 could claim to list the entire colour vision pathway of Drosophila. The new study by Schnaitmann and colleagues [8] now shows convincingly that, contrary to these expectations, photoreceptors R1–6 do indeed contribute to colour vision in Drosophila. Using a blind mutant and GAL4-drivers they generated flies with restricted sets of functional photoreceptors and tested their colour discrimination. Flies with functional ‘yellow’ ommatidia, but not those with ‘pale’ ommatidia, discriminated green and blue as well as normal flies, even with inversed intensities. As expected, flies which had no functional receptors except R7 and R8 in ‘yellow’ ommatidia also did well. However, even flies which only had functional receptors R8 and R1–6 in ‘yellow’ ommatidia could do the job. This came as a surprise. It implies that the broadly tuned receptors R1–6 contribute to both the achromatic pathway and the colour vision pathway in flies. Schnaitmann et al. [8] went one step further and generated flies lacking neurons in the lamina. They showed that the colour vision pathway depends on neurons known as ‘lamina monopolar cells’ to convey the signals from R1–6 to the medulla, where they can be compared neurally with signals from R7 and R8. Further studies can now unravel the full colour vision pathway of Drosophila. The results by Schnaitmann and colleagues [8] strongly suggest that flies may have a rather conserved insect colour vision system. Thus, anything we learn from Drosophila will help us to understand
colour vision not only in this tiny fly that did not seem to care much about colour, but even in bees and other insects. More generally, we learn that flies use information more efficiently than previously thought. The analogy that fly receptors R1–6 serve a similar function as human rods, while fly receptors R7 and R8 are comparable to our cones, no longer holds. More adequately, flies use their receptors in a similar way as we use our cones: all receptors are involved in colour vision, and most — in flies six out of eight receptors in each ommatidium, in humans the red and green cones (93% of all cones) — are additionally used for achromatic vision, in a parallel pathway. Birds remain the challenge: why do the animals that have the sharpest vision of all use only half of their cones — the double cones — for high acuity achromatic vision? Or did we, just as in fruit flies, miss something? The new results on Drosophila [8] have challenged a paradigm: parallel visual pathways may share the same input more often than we thought. 1. Frisch, K.v. (1914). Der Farbensinn und Formensinn der Biene. Zool. J. Physiol. 37, 1–238. 2. Vorobyev, M., and Osorio, D. (1998). Receptor noise as a determinant of colour thresholds. Proc. R. Soc. B 265, 351–358. 3. Kelber, A., Vorobyev, M., and Osorio, D. (2003). Colour vision in animals – behavioural tests and physiological concepts. Biol. Rev. 78, 81–118. 4. Borst, A. (2009). Drosophila’s view on insect vision. Curr. Biol. 19, R36–R47. 5. Schu¨mperli, R.A. (1973). Evidence for colour vision in Drosophila melanogaster through spontaneous phototactic choice behaviour. J. Comp. Physiol. 86, 77–94. 6. Menne, D., and Spatz, H.-C. (1977). Colour vision in Drosophila melanogaster. J. Comp. Physiol. A 114, 301–312. 7. Schnaitmann, C., Vogt, K., Triphan, T., and Tanimoto, H. (2010). Appetitive and aversive
How do nuclear components, apart from chromosomes, partition equally to daughter nuclei during mitosis? In Schizosaccharomyces japonicus, the conserved LEM-domain nuclear envelope protein Man1 ensures the formation of identical daughter nuclei by coupling nuclear pore complexes to the segregating chromosomes.
When we consider what constitutes a successful mitosis, we immediately
9.
10.
11.
12.
13.
14.
15.
16.
References
Nuclear Division: Giving Daughters Their Fair Share
Alison D. Walters and Orna Cohen-Fix
8.
think of the correct segregation of chromosomes into two daughter nuclei. However, it takes more than chromosomes to make a nucleus. The
17.
18.
19.
visual training in freely moving Drosophila. Front. Behav. Neurosci. 4, 10. Schnaitmann, C., Gerbers, C., Wachtler, T., and Tanimoto, H. (2013). Color discrimination with broadband photoreceptors. Curr. Biol. 23, 2375–2382. Osorio, D., and Vorobyev, M. (2005). Photoreceptor spectral sensitivities in terrestrial animals: adaptations for luminance and colour vision. Proc. R. Soc. B 272, 1745–1752. Nilsson, D.-E., and Osorio, D. (1997). Homology and parallelism in arthropod sensory processing. In Arthropod Relationships, R.A. Fortey and R.H. Thomas, eds. (London: Chapman Hall), pp. 333–347. Kelber, A. (2006). Invertebrate colour vision. In Invertebrate Vision, E.J. Warrant and D.-E. Nilsson, eds. (Cambridge: Cambridge University Press), pp. 250–290. Marshall, N.J., Kent, J. and Cronin, T. (1999). Visual adaptations in crustaceans: spectral sensitivity in diverse habitats. In Adaptive Mechanisms in the Ecology of Vision (eds. S. N. Archer et al.), pp. 285–27. Kluwer, Dordrecht. Gribakin, F.G. (1972). The distribution of the long wave photoreceptors in the compund eye of the honey bee as revealed by selective osmic staining. Vision Res. 12, 1225–1230. Arikawa, K. (2003). Spectral organisation of the eye of a butterfly, Papilio. J. Comp. Physiol. A 189, 791–800. White, R.H., Xu, H., Munch, T.A., Bennett, R.R., and Grable, E.A. (2003). The retina of Manduca sexta: rhodopsin expression, the mosaic of green-, blue- and UV-sensitive photoreceptors, and regional specialization. J. Exp. Biol. 206, 3337–3348. Wakakuwa, M., Kurasawa, M., Giurfa, M., and Arikawa, K. (2005). The compound eye of the honeybee Apis mellifera is composed of three spectrally distinct types of ommatidia. Naturwissenschaften 92, 464–467. Hardie, R.C. (1986). The photoreceptor array of the dipteran retina. Trends Neurosc. 9, 419–423. Strausfeld, N.J., and Lee, J.-K. (1991). Neuronal basis for parallel visual processing in the fly. Visual Neurosci. 7, 13–33. Morante, J., and Desplan, C. (2008). The color-vision circuit in the medulla of Drosophila. Curr. Biol. 18, 553–565.
Lund Vision Group, Department of Biology, Lund University So¨lvegatan 35, 22362 Lund, Sweden. E-mail: [email protected], [email protected] http://dx.doi.org/10.1016/j.cub.2013.10.025
integrity of the daughter nuclei and the organization of the chromatin within them rely on the presence of an intact nuclear envelope (NE). The NE is a double lipid bilayer, with an outer membrane that is continuous with the ER, and an inner nuclear membrane (INM) that contains proteins that interact with chromatin and other nuclear components. The NE is perforated by nuclear pore complexes (NPCs) that allow selective passage of proteins between the nucleoplasm and cytoplasm. In metazoans, a filamentous network,
Current Biology Vol 23 No 23 R1046
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Figure 1. Anaphase in wild-type and man1D S. japonicus. (A) In wild-type cells, the expansion of the NE (in green) is limited. Consequently, in early anaphase (top panel) the nucleus is shaped as a diamond (or prolate spheroid) and cannot elongate any further. At this point the sister chromatids (in blue) are segregated to the two poles and the NPCs (in red) are absent from the nucleus mid-section, which is occupied by the nucleolus (in yellow). The chromosome segments that extend into the nucleolus represent the DNA region coding for the ribosomal RNA. The nucleus can fully elongate in late anaphase thanks to the rupturing of the NE, which typically happens around the middle of the nucleus (bottom panel). The parental nucleolus is left in the nucleus mid-section and begins to disassemble. New nucleoli form in the daughter nuclei. (B) In a man1D cell, sister chromatids separate normally in early anaphase but the NPCs remain in the nucleus mid-section. In late anaphase the nucleus ruptures, as in wild-type cells, but the rupture site is randomly positioned, often away from the nuclear mid-section. The parental nucleolus fails to disassemble and segregates to one of the daughter nuclei.
known as the nuclear lamina, underlies the INM. The NE breaks down at the onset of mitosis; membrane and membrane-associated proteins are absorbed into the ER while soluble proteins diffuse throughout the cytoplasm [1,2]. At the end of mitosis these NE components are retrieved in a poorly understood process to assemble the NEs of the two new daughter nuclei. Yeast undergo mitosis in a somewhat different fashion. In most of the commonly studied yeast, the nuclear lamina is absent, the NE remains intact throughout the entire cell cycle, and the nucleus divides by a process of expansion followed by fission (called ‘closed mitosis’, in contrast to ‘open mitosis’ where the NE breaks down). The mechanisms ensuring that each daughter nucleus receives half of the nuclear membrane and its protein components (e.g., NPCs) are largely unknown. There are also examples of organisms where mitosis is neither fully open (as in metazoa), nor fully closed (as in budding or fission yeast). One such
example is Schizosaccharomyces japonicus. In this particular yeast, the NE does not expand during mitosis, and instead it ruptures near the nucleus mid-section as the anaphase spindle elongates [3,4] (Figure 1A). The nucleolus does not divide but remains in the nucleus mid-section and then disassembles. Interestingly, the NPCs, which are uniformly distributed around the interphase nucleus, are absent from the mid-section of the anaphase nucleus (Figure 1A). Once anaphase is complete, the NE is resealed to form two daughter nuclei of equal size. The mechanism of this semi-open mitosis raises several interesting questions: how are NPCs cleared from the mid-section? Do they move through the NE or are they disassembled and new ones reassemble elsewhere? And how do cells ensure an even split of the nuclear membrane and other NE components? A study by Yam et al. [5] published in a recent issue of Current Biology reveals that the key to equal division of the nucleus in
S. japonicus lies with the highly conserved LEM-domain protein Man1. NPCs of higher eukaryotes are largely immobile, likely due to their association with the nuclear lamina. The little movement that these NPCs do exhibit happens in conjunction with the underlying lamin network [6]. In budding yeast, which lack lamins, NPCs are much more mobile [7,8]. How, then, are NPCs cleared from the nuclear mid-section in S. japonicus? To distinguish between NPC movement and NPC disassembly followed by reassembly, Yam et al. [5] fused a photo-convertible fluorescent protein to an NPC subunit and examined the fate of NPCs at the center of the mitotic nucleus after they had been photo-converted. Not only did these NPCs move away from the mid-section and towards the nuclear poles, they did so in a manner that was coincident with the poleward movement of the chromosomes. Motor-dependent NPC movement in yeast has been reported previously [9], but these movements were on a much smaller scale than mitotic NPC movement in S. japonicus in terms of the number of NPCs that moved coordinately and the distance that they travelled. Thus, the NPC movement observed in S. japonicus likely involved a different mechanism. Since NPCs moved along with chromosomes, the authors hypothesized that this movement may be mediated by one or more proteins that link chromosomes to NPCs. LEM-domain proteins appear to fit the bill: members of this protein family (named after its founding members LAP2, Emerin and Man1) localize to the INM and help to organize and regulate chromatin at the NE, thereby playing roles in transcriptional regulation, recombination and DNA replication ([10] and references therein). They perform these functions through a transmembrane domain that anchors them in the NE and a w40 amino-acid LEM domain that either binds DNA directly [11] or, in metazoans, binds to chromatin through the small DNA-binding protein BAF ([10] and references therein). Yam et al. found that one of the S. japonicus LEM-domain proteins, Man1, was required for several aspects of nuclear division: man1D cells failed to disassemble their nucleolus at mitosis, produced
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unequal sized daughter nuclei, formed an NE break in random locations and did not clear NPCs from the nucleus mid-section [5] (Figure 1B). Chromosome segregation, on the other hand, was largely unaffected. These observations suggested that Man1 might mediate NPC movement by linking NPCs to chromosomes. But is the effect of Man1 on NPC movement direct, or is the NPC movement defect in the man1D mutant an indirect consequence of its many other failures in mitotic processes? To address this, Yam et al. [5] designed an artificial tether that linked chromosomes with NPCs independently of Man1. This tether was able to rescue the NPC movement defect of man1D cells, suggesting that Man1 physically couples chromosome segregation to the movement of NPCs away from the nuclear mid-section. The observation that LEM-domain proteins can perform ‘lamina-like’ functions in yeast is not a new one: the LEM domain proteins Heh1 and Heh2 have previously been shown to affect NPC distribution in S. cerevisiae [12], and LEM domain proteins are known to tether telomeres to the NE during interphase [13]. However, the Yam et al. study [5] reveals a novel function for a LEM-domain protein in coupling NPC inheritance to chromosome segregation. Along with NPCs, nuclear membrane must also be evenly distributed among the daughters. In most organisms mitosis normally results in two equally sized daughter nuclei, regardless of whether they undergo open, semi-open or closed mitosis. Little is known about how the ‘right’ amount of membrane is used to reassemble the NE around the two sets of daughter chromosomes after open mitosis, although it is likely that components of the INM and nuclear lamina play an important role [14,15]. In the case of closed mitosis, the mechanism by which the nuclear fission point is determined remains unclear. In their recent study, Yam et al. [5] show that man1D cells form nuclei that are smaller and of unequal size. While the cause of this uneven NE distribution remains unknown, it is interesting to note that in wild-type cells the NE rupture site is equidistant between the two extreme ends of the nucleus (Figure 1), coinciding with
the telomere of the last segregating chromosome. In man1D mutants, by contrast, the site of rupture is asymmetrically localized, suggesting that the Man1-dependent positioning of the rupture site dictates the amount of NE inherited by the two daughter nuclei. Interestingly, the artificial tether described above that rescued the NPC movement in man1D cells also corrected the NE rupture site. Thus, Man1 may affect the amount of NE inherited by the daughter nuclei by properly demarcating the rupture site through chromosome–NE attachments, although at this point other mechanisms cannot be excluded. To date, many examples of how the NE can organize and regulate chromatin function have been identified. This recent study of the mitotic nucleus in S. japonicus provides an interesting example of how chromosome movement, via Man1, is being used to organize the NE in order to correctly partition NE components. A number of interesting questions arise from studies in S. japonicus. For example, why develop a mechanism to ensure the inheritance of NPCs from mother to daughters, when the cell can reassemble NPCs de novo? Perhaps inserting NPCs de novo is an energetically expensive process, or even an impossible feat if there aren’t enough NPCs, making the import of NPC components that must be inserted into the nucleoplasmic side of the NE too inefficient. Along these lines, a possible explanation for the unequal and reduced sizes of daughter nuclei in man1D cells is that a reduction in the number of NPCs present in the NE causes decreased import of a factor that determines nuclear size, as lamin B does in metazoa [16]. How Man1 affects nucleolar disassembly and the mechanism by which Man1 determines the site of nuclear rupture also remain unclear. Yam et al. suggest that the attachment of the last segregating chromosomes to the NE generates a force that aids rupture. However, it is also possible that the Man1-dependent clearance of NPCs from the centre of the nucleus weakens the NE, favoring rupture at this location. Although many questions remain to be answered, it is clear that LEM-domain proteins play a conserved role in regulating NE dynamics in variant forms of mitosis.
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The Laboratory of Cell and Molecular Biology, NIDDK, NIH, Bethesda, MD 20892, USA. E-mail: [email protected], ornac@helix. nih.gov http://dx.doi.org/10.1016/j.cub.2013.10.041