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From the Department of Gene and Cell Medicine, Mount Sinai School of Medicine, New York, New York.
| Correspondence to: R. Jessberger, Department of Gene and Cell Medicine, Mount Sinai School of Medicine, One Gustave L. Levy Place, New York, NY 10029 (e-mail: rolf.jessberger{at}mssm.edu). |
| Received for publication June 6, 2004; accepted for publication August 30, 2004. |
Meiosis
The purpose of meiosis is to create haploid germ cells from diploid
parental cells (Figure 1).
Chromosomes are duplicated during premeiotic S-phase, which generates cells
with 4 chromatids of each type of chromosome—2 maternal and 2
paternal—similar to the mitotic S phase. In contrast to mitosis, meiotic
DNA replication is followed by 2 successive rounds of nuclear divisions called
meiosis I and II, which result in haploid gametes containing 1 copy of each
chromosome. At meiosis I, the reductional division, the homologous chromosomes
move toward opposite poles, while the sister chromatids of each homolog remain
connected, generating cells with 2 chromatids. This is followed by a second
mitosislike equational division in which sister chromatids separate from each
other and the haploid cells are generated. Errors in meiosis result in the
production of aneuploid zygotes with devastating consequences. Aneuploidy is a
key factor in 
35% of spontaneous pregnancy losses and the most commonly
recognized cause of mental retardation
(Hassold and Hunt, 2001;
Page and Hawley, 2003).
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During prophase of meiosis I, various chromatin rearrangements take place, which are, like the reductional division itself, unique to meiosis (Roeder, 1997; van Heemst and Heyting, 2000; Page and Hawley, 2003; Petronczki et al, 2003). Homologous chromosomes, each consisting of 2 sister chromatids, find each other and pair to form the "bivalent," a structure containing all 4 sister chromatids. Maternal and paternal chromosomes pair and recombine to generate new combinations of alleles. Recombination occurs between 1 sister chromatid of each homolog and also serves to keep the homologs physically connected until metaphase I. The pairing between homologous chromosomes in prophase I is stabilized by tight axial associations called synapsis and is possible within the synaptonemal complex (SC), a proteinaceous ladderlike structure that consists of 2 axial elements (AEs) connected all along their length by transverse filaments (TF). In mammals, 3 proteins were identified as components of the SC: SYCP1 (named SCP1 in the rat) is a component of the TF, and SYCP2 (SCP2) and SYCP3 (SCP3) are constituents of the AE (reviewed in Moens et al, 1998). Thus, the 4 sister chromatids are specifically structured and behave uniquely during meiosis. SMC protein complexes are central in determining these features. Unless otherwise noted, this review focuses on mammalian (mouse) cohesins and their functions.
Mitotic and Meiotic Sister Chromatid Cohesion
Generally, cohesion is essential to hold sister chromatids together until
they are separated at nuclear division. During the last few years, protein
components of the mitotic cohesin complex were identified (for recent reviews,
see Jessberger, 2002,
2003;
Hagström and Meyer,
2003). In mammalian somatic cells, the cohesin complex consists of
at least 4 different subunits, 2 of them belong to the SMC family of proteins
(Figure 2). SMC proteins are
highly dynamic proteins that modulate chromosome structures. Six members of
the family, usually called SMC1 to SMC6 fall into 2 subgroups. One member of a
subgroup heterodimerizes with 1 SMC of the other subgroup
(Figure 2B). These dimers
associate with other proteins to form large complexes and act in various
chromosome-related processes
(Table). For example, the
SMC2-SMC4 heterodimer is part of the condensin complex, which contributes to
chromosome condensation. The SMC1-SMC3 heterodimer constitutes the core of the
cohesin complex. Cohesin is an essential multiprotein complex required for
sister chromatid cohesion (ie, the ordered arrangement of the newly replicated
daughter DNA duplices in S, G2, and early M phase of the cell
cycle; Figure 2C). In the
mitotic cohesin complex, the heterodimer associates with 2 non-SMC subunits:
the Rad21 (Scc1) protein, a member of the kleisin protein family
(Schleiffer et al, 2003), and
an Scc3-type protein, of which there are several variants. In mitotic cells,
these are either the SA1 or the SA2 proteins. Homologs of all these proteins
were identified in many organisms, and they appear to be essential. If
defective, chromosomes are not segregated properly during mitosis, aneuploidy
is generated, and the cells die. There is also impaired DNA recombinational
repair, which under normal circumstances after S phase usually uses the
aligned sister chromatids. The meiotic cohesin significantly differs from its
mitotic counterparts and is specifically adapted to accommodate the unique
structural and dynamic needs of meiotic chromosomes.
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Although the protein composition of the mitotic cohesin complex is well understood, it is not yet entirely clear how cohesin interacts with the 2 sister chromatids. Recent data derived from the crystal structure of bacterial SMC hinge domains (Haering et al, 2002) and supported by biochemical protein interaction studies on yeast cohesin subunits (Gruber et al, 2003) suggest that cohesin forms a large proteinaceous ring within which sister chromatids could be entrapped after DNA replication (Figure 2C). Embracing of DNA strands by the cohesin ring creates a link between sister chromatids, which might be more topological than chemical (ie, through protein-DNA binding). In vivo evidence for this elegant model still needs to be substantiated.
The meiotic cohesin, however, differs significantly from its mitotic
counterparts. In fact, not just 1 cohesin but several cohesin complexes are
found in meiotic cells (Table).
All are built around the SMC1-SMC3 heterodimer. In mammalian meiotic cells,
the SMC1 protein exists in 2 variants: the canonical SMC1, called SMC1
,
and a meiosis-specific variant, SMC1ß
(Revenkova et al, 2001).
SMC1ß has only been found in vertebrate cells. There is no published
evidence for SMC3 variants, but the RAD21 protein coexists with a
meiosis-specific paralog named REC8. REC8 is highly conserved through
evolution, and orthologs from yeast, plants, and vertebrates are described
(DeVeaux et al, 1992;
Klein et al, 1999;
Watanabe and Nurse, 1999; Cai et al, 2003;
Lee et al, 2003). The SA1 and
SA2 proteins coexist with their meiosis-specific variant STAG3
(Pezzi et al, 2000; Prieto et al, 2001).
Establishing Meiotic Sister Chromatid Cohesion
In somatic cells, cohesion between sister chromatids is established during
S phase, and only the fully assembled cohesin complex can provide cohesion.
Data obtained by studying the yeast Schizosaccharomyces pombe
suggested that at least Rec8 associates with chromosomes during the last
premeiotic round of replication, just before entry into meiosis
(Watanabe and Nurse, 1999). In
mice, the equivalent might be spermatogonial cells that are about to enter
preleptotene. Although REC8 was not observed in spermatogonia,
immunofluorescence studies showed its presence in preleptotene cells
(Eijpe et al, 2003).
Interestingly, SMC1ß was not found at this early stage of meiosis but
appeared later in leptotene cells (Figure
3). Therefore, REC8, possibly within a complex with SMC1,
might provide an initial basis for sites of meiotic cohesion, onto which the
complex assembles. In preleptotene REC8 forms AE-like filamentous chromatin
structures, which are, however, still quite diffuse. SMC1 exists in
preleptotene cells; thus, other cohesin components presumably are present in
those cells as well, including SMC3 (Figure
3). In leptotene, zygotene, and pachytene, all known cohesin
components localize to the AEs, although with different characteristics.
Unlike SMC1ß or SMC3, the SMC1 protein, which is detectable only
until the end of prophase I, does not localize as uniformly throughout the
chromosomal axes; rather, it appears concentrated in certain areas, displaying
a dotlike pattern (Eijpe et al,
2000). This, together with its absence in later meiosis, led to
the suggestion that SMC1ß rather than SMC1 is primarily
responsible for sister chromatid cohesion during meiosis
(Revenkova et al, 2001).
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Some data suggest physical interactions between components of the SC and cohesin proteins (Eijpe et al, 2000; Lee et al 2003), but the assembly of filamentous cohesin structures on meiotic chromatin does not depend on SC formation. Mice lacking the SYCP3 protein, a component of the AE, still assemble filamentous cohesin structures, although the SC is absent (Pelttari et al, 2001). Thus a "cohesin core" exists that is believed to layer itself on top of and parallel to the AE. This cohesin core might, together with SC proteins, regulate the structure of meiotic chromosomes (eg, their compaction).
In early meiosis, there are at least 2 types of cohesin complexes, probably
more, but the situation is not yet clear. There is certainly an SMC1-
and at least 1 SMC1ß-based complex. Both SMC1 variants dimerize with SMC3
but might associate with different non-SMC proteins. Recent
coimmunoprecipitation experiments indicate the existence of 2
SMC1-based complexes in testis, bringing the total number of
cohesinlike complexes to at least 3
(Revenkova et al, 2004). One
SMC1 complex seems to reflect the mitotic RAD21-type complex
(Prieto et al, 2002;
Eijpe et al, 2003; Revenkova et al, 2004;
Xu et al, 2004), whereas the
other contains the meiotic STAG3 and REC8 proteins
(Revenkova et al, 2004). However, in another report no coimmunoprecipitation of SMC1 with REC8
was observed (Lee et al,
2003). Reports also conflict on the chromosomal localization and
stage-specific existence of RAD21. One paper suggests that a mammalian complex
made of RAD21, SMC3, SMC1ß, and an undetermined fourth subunit persists
at the centromeres until the metaphase to anaphase II transition
(Xu et al, 2004). Another
publication claims that RAD21 is displaced from the centromeres in telophase I
and is not present at meiosis II centromeres
(Parra et al, 2004). In
fission yeast, Rec8 was found to associate either with a STAG3 ortholog
(Rec11) or with an ortholog of the mitotic SA1 or SA2 cohesin subunits (Psc3;
Kitajima et al, 2003). Thus it
appears as if many of the potential combinations of the various cohesin
subunits—meiosis-specific or ubiquitous—are realized in germ
cells. The exact composition of each complex and their chromosomal
localization; stage-specific occurrence; and relative contribution to sister
chromatid cohesion, AE formation, and meiotic recombination are a matter of
current debate and will be further discussed below.
It is important to distinguish between sister chromatid cohesion in chromosome arms and centromeric cohesion. Although arm cohesion along with chiasmata is dissolved in metaphase I, centromeric cohesion is maintained until metaphase II to keep the sister chromatids connected for their shared journey toward a pole in anaphase I. Only in anaphase II are sister chromatids separated to allow their migration to opposite poles and formation of the haploid cells.
REC8 might not only provide an early basis for cohesin assembly but seems
also to be involved in ensuring cohesion throughout meiosis. In mammalian
cells, SMC1, SMC1ß, and SMC3 are initially found all along the
prophase chromosomes, possibly contributing to ubiquitous cohesion along the
chromosomal axis. Evidence gathered from analysis of a mouse deficient in
SMC1ß points to an essential role of SMC1ß in sister chromatid
cohesion starting in pachytene (Revenkova
et al, 2004). Without SMC1ß, the mutant pachytene
spermatocytes enter into apoptosis but can be driven prematurely into
metaphase by okadaic acid treatment. Here they visibly lack all sister
chromatid cohesion, whereas mutant zygotene cells treated with okadaic acid
maintained cohesion. SMC1ß and SMC3 dissociate from the chromosome arms
in late prophase I, whereas REC8 persists along the chromosome arms until
metaphase I (Eijpe et al, 2003;
Lee et al, 2003). At least in
S pombe, the 2 different Scc3-like subunits with which Rec8
associates mark differently located Rec8 complexes: the Rec11-Rec8 complex
localizes to chromosome arms, and the Psc3-Rec8 complex to the centromeres
(Kitajima et al, 2003).
Perhaps the most prominent role for the SMC1ß complex is in centromeric cohesion because the protein, along with REC8 and SMC3, remains associated with the centromeres until anaphase II (Revenkova et al, 2001). In SMC1ß-deficient mice, spermatocytes die in pachytene of apoptosis. Oocytes develop further but terminate in metaphase II, in which the total absence of sister chromatid cohesion leads to complete separation of individual chromatids. Expectedly, neither male nor female mutant mice are fertile (Revenkova et al, 2004).
If the SMC1ß complex becomes critical for cohesion in spermatocytes
around pachytene, then other cohesin complexes should support cohesion early
in meiosis. The SMC1-based complexes likely provide this function.
SMC1 is detected early, and along with STAG3, disappears gradually in
late prophase to metaphase I (Prieto et
al, 2001) when arm cohesion is dissolved. RAD21 also dissociates
from chromosome arms but, according to one report, stays at the centromeres
until the metaphase II to anaphase II transition, similar to REC8, SMC3, and
SMC1ß (Xu et al, 2004).
This poses the question of whether 2 SMC1ß-SMC3 complexes are at the
centromeres—1 with REC8 and 1 with RAD21 as a kleisin subunit—or
whether some other cohesinlike entity is still present. Because removal of
Rec8 causes premature sister chromatid dissociation in yeast
(Klein et al, 1999), Rad21p
alone seems not to be sufficient to maintain centromeric cohesion. However,
recent data on different localization of REC8 and RAD21 in the centromeric
region suggest that these proteins perform different functions
(Xu et al, 2004). Because REC8
localizes to sites that flank the kinetochores, it could allow monopolar
attachment of kinetochores to the spindle microtubules for segregation of
homologs in meiosis I, similar to the proposal for fission yeast Rec8
(Yokobayashi et al, 2003).
According to Xu et al (2004)
RAD21 localizes to the centromeres in meiosis II and thus could be involved in
bipolar attachment of the kinetochores to the tubules for segregation of
sister chromatids. In contrast, Parra et al
(2004) suggest the presence of
RAD21 only at meiosis I centromeres and its contribution to monopolar rather
than bipolar spindle attachment. In an earlier paper, Prieto et al
(2002) proposed that RAD21,
within an SMC1-SMC3-SA2 complex only localizes to chromosomes late in
prophase I and disappears in metaphase I when chromosomes condense.
Accordingly, in early prophase, RAD21 and SA2 were seen only loosely attached
to chromosomes or dispersed in the nuclei
(Eijpe et al, 2000). This
contrasts with the paper by Xu et al
(2004), in which the presence
of RAD21 was reported along the chromosome arms in parallel with SCP3 from
leptotene.
Another dissenting view on the involvement of cohesins in centromeric cohesion until meiosis II is presented by Parra et al (2004). Because these authors do not observe SMC1, SMC3, REC8, STAG3, and RAD21 at the centromeres of meiosis II, they suggest that cohesins might not be required at all for centromeric cohesion in meiosis II. This is in contrast to studies in Saccharomyces cerevisiae and S pombe (Klein et al, 1999; Watanabe and Nurse, 1999) and to reports on the presence of SMC1ß, SMC3, REC8, and RAD21 at meiosis II centromeres in mammalian meiocytes (Eijpe et al, 2000, 2003; Revenkova et al, 2001; Lee et al, 2003; Xu et al, 2004). It is also difficult to reconcile with the loss of all cohesion, including centromeric cohesion in SMC1ß-deficient meiocytes (Revenkova et al, 2004). These differences likely are based on different techniques used to analyze chromosomes: squashing (Parra et al, 2004) vs spreading (the other groups).
As confusing as these different reports might be, they agree on a meiotic role for RAD21 and REC8 and on the formation of a number of different cohesinlike complexes in mammalian meiocytes. The need to clarify this rather complex and dynamic, and still quite diffuse, picture of the function of individual cohesins is obvious.
Recently, the SMC1ß cohesin complex was also shown to be essential for a number of other events in meiosis (Revenkova et al, 2004). Spermatocytes or oocytes deficient in this protein fail to form AEs and SCs of proper length: in a unique phenotype, their chromosomes are about 50% shorter than those of wild-type meiocytes (Figure 4). Thus, it appears as if this cohesin codetermines the length compaction of meiotic chromosomes. It might do so by supporting the packaging of chromatin into the AEs (eg, at the base of chromatin loops that emanate from the AEs). If so, chromatin loops in SMC1ß-deficient mice should be enlarged. Measurements by fluorescent in situ hybridization (FISH) of chromatin surrounding the AEs support this hypothesis: the chromatin clouds in the mutant are about 2-fold more extended from the AEs than those of wild-type spermatocytes. Assuming that the intrinsic compaction of DNA in the loops is the same in both wild-type and mutant spermatocytes, a role for SMC1ß in determining the ratio of chromatin in loops vs chromatin in the filamentous core in early meiosis is indicated. The shortened chromosomes fail in several other respects. Their telomeres do not properly attach to the nuclear periphery as they do in wild-type spermatocytes. Bouquet formation (ie, bundling of the telomeres at one spot on the nuclear periphery characteristic for early prophase I) is impaired. Furthermore, synapsis of homologs is incomplete in the mutant. Both phenotypes could be a consequence of aberrant chromosome structure. For example, the shortened chromosomes might fail to reach the nuclear periphery because of steric problems, and those who do succeed might have similar steric problems in finding the homolog partner for synapsis. Finally, sister chromatid cohesion gets lost in late prophase at both the chromosome arms and the centromeres. Together, it became clear that the meiosis-specific SMC1ß protein plays essential roles in meiotic chromosome dynamics and structure (Revenkova et al, 2004).
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Why has the vertebrate meiocyte acquired an additional SMC1 protein when
other organisms perform meiosis fine with just SMC1? In human, mouse,
rat, and presumably other vertebrates, the SMC1 gene is located on the
X chromosome and might thus be silenced during early meiosis. SMC1ß,
whose gene resides on chromosome 15 in mouse (on chromosome 22 in human),
could represent a transposed variant generated during evolution to ensure
availability of an SMC1 protein all throughout meiosis. SMC1ß might later
then have acquired additional functions even early in meiosis, as illustrated
by the roles described here.
Maintenance and Dissolution of Meiotic Sister Chromatid Cohesion
Maintenance of sister chromatid cohesion at the centromeres and destruction
of cohesion along the arms is achieved through differential removal of
cohesins from the arms and centromeres. The question arises as to the
mechanism that triggers this stepwise dissolution of cohesion. During mitosis
in yeast, sister chromatids are separated after cleavage of Rad21p by
separase, a cysteine protease that belongs to the same protease family as
caspases (reviewed in Uhlmann,
2001; Haering and Nasmyth,
2003). Separase becomes active after dissociation from its
inhibitor called securin, which is degraded at the metaphase to anaphase
transition. In vertebrates, however, the majority of cohesin is removed by a
separase-independent mechanism as chromosome condensation starts in prophase.
Only a small fraction remains associated with the centromeric region and
dissociates at the metaphase to anaphase transition. Dissociation of cohesin
during the first stage in prophase requires a Polo-like/Cd5 kinase, which
phosphorylates RAD21 (Waizenegger et al,
2000; Sumara et al,
2002).
In meiotic yeast cells, Rec8p disappears from chromosome arms at the onset of anaphase I, but centromeric Rec8p persists until the metaphase II to anaphase II transition (Watanabe and Nurse, 1999), as observed in mouse spermatocytes (Eijpe et al, 2003). Dissociation of yeast Rec8p from chromosome arms is a result of cleavage by separase (Buonomo et al, 2000). Cleavage of Rec8 along chromosome arms and at the centromeres by separase is also required in fission yeast for segregation of homologs and of sister chromatids in meiosis I and II, respectively (Kitajima et al, 2003). Similarly, in Caenorhabditis elegans mutants, the chromosomes fail to separate during meiosis I because of inactivation of separase (Siomos et al, 2001) or of APC/C needed for separase activation (Golden et al, 2000). In contrast, it seems that segregation of homologs and of sister chromatids are differentially regulated in higher eukaryotes. Both the separase and the APC/C are dispensable for progression from meiosis I to II in Xenopus laevis oocytes (Peter et al, 2001; Taieb et al, 2001). However, in yeast and nematodes, APC/C activity is required for the metaphase I to anaphase I transition. The authors suggest that either differences based on the enormous size of the oocyte or divergent evolution might have caused the different experimental outcomes. Mechanisms that rule cohesion dissociation in mammalian meiosis resemble those shown for yeast and C elegans. In mouse oocytes, the meiosis I to meiosis II transition also depends on APC/C activity. Inactivation of separase results in perturbed chromosome segregation in meiosis I (Terret et al, 2003).
If separase activity removes REC8 from chromosomal arms at the metaphase I to anaphase I transition, how are REC8 and cohesin protected at the centromere, where it must stay until anaphase II? On the basis of studies in C elegans, it has been proposed that aurora-B kinase AIR-2 regulates the selective release of chromosome cohesion during both meiotic divisions, probably by phosphorylation of REC-8 (Rogers et al, 2002). In mouse spermatocytes, aurora-B relocalizes along the chromosome during meiosis, being present at the right place and time for regulating sister chromatid cohesion during both meiotic divisions (Parra et al, 2003). Therefore, it is possible that differential phosphorylation of cohesin complexes on arms and centromeres could result in 2-step cohesion dissolution in the first and second meiotic divisions. Proteins localizing to centromeres could also act as factors protecting centromeric cohesion. The meiosis-specific Spo13p in budding yeast (Shonn et al, 2002) and the Mei-S322 protein of Drosophila melanogaster (Moore et al, 1998) have been suggested as potential candidates. The recent identification of the meiosis-specific protein Sgo1 and its characterization as a protector of the centromeric Rec8 cohesin in fission and budding yeasts brought more insight into the molecular mechanism responsible for protecting centromeric cohesion during meiosis I (Kitajima et al, 2004; Katis et al, 2004). The amount of centromeric Rec8 is reduced and meiotic cohesion fails to persist at centromeres after meiosis I in mutant Sgo1 cells. It was suggested that Sgo1 acts as a shield for Rec8, protecting it physically from separase action.
Cohesins in Meiotic Recombination and Segregation of Homologs
Reciprocal exchange of DNA between the maternal and paternal chromosomes is
essential for creating diversity and is a major driving force of evolution.
The formation of crossovers gives rise to a protein-DNA structure called
chiasmata, which also functions to physically connect the homologs in prophase
I so that they can be properly aligned in metaphase for subsequent
segregation. Without chiasmata, no orderly chromosome segregation is possible.
Evidence is accumulating for an important role of cohesins in meiotic DNA
recombination. An early indication for such a role originated from studies in
S cerevisiae (Klein et al,
1999) since Smc3 or Rec8 mutants are
recombination-deficient. In C elegans, silencing of REC-8 expression
by RNA interference (RNAi) not only caused defects in chromosome synapsis at
pachytene, but also accumulation of unrepaired DNA double-strand breaks
(Pasierbeck et al, 2001), pointing to a DNA repair function of REC-8. In
vertebrates, the localization of SMC1ß and SMC3 to bridges between
chromosomes, which are visible in pachytene and diplotene and are thought to
represent sites of chiasmata, suggested that these cohesins have some role in
meiotic recombination in higher organisms as well
(Revenkova et al, 2001; Eijpe et al, 2003). Notably,
the cohesin REC8 appears to be absent from these chromosomal bridges, posing
the question of whether there exists a specialized SMC complex that localizes
to and possibly acts on chiasmata. Very recently, data obtained through
analysis of the SMC1ß-deficient mouse support the idea of a role of
cohesins in meiotic recombination
(Revenkova et al, 2004). Sites
of chiasmata can be visualized by labeling the mismatch repair proteins MLH1
and MLH3 in pachytene meiocytes. The absence of SMC1ß eliminates MLH1 or
MLH3 signals in spermatocytes and significantly reduces their number in
oocytes. Interestingly, proteins that appear at early steps during meiotic
recombination (ie, at initiation when double-strand breaks are formed) appear
unaffected by the SMC1ß deficiency. Reduced numbers of chiasmata can be
observed also in metaphase I oocytes, in which some homologs fall apart
instead of being kept together via their chiasmata. Thus, the SMC1ß
complex functions in DNA recombination after its initiation to allow stable
formation of chiasmata. How could this cohesin complex possibly support
chiasmata? It might, for example, be directly involved in the assembly of
proteins required for processing of the early intermediates in the DNA
recombination process, such as holiday junction processing enzymes. Or cohesin
might compose (part of) the "road-blocks" that prevent chiasmata
from migrating and falling off the ends of chromosomes.
Because there is increasing evidence for function(s) of cohesins in DNA repair, in particular the repair of double-strand breaks through homologous recombination between sister chromatids (Jessberger, 2002), meiotic SMC proteins could also be involved in DNA repair in meiocytes.
This may not only be true for the SMC1-SMC3 heterodimer in the context of cohesin or a related complex, but also for the third SMC heterodimer, the SMC5-SMC6. That dimer, which is known to act in DNA repair in mitotic cells and associates with other repair proteins, probably has additional chromosomal housekeeping functions and is highly expressed in the testis (Taylor et al, 2001; McDonald et al, 2003). However, a recent paper (Pebernard et al, 2004) suggests that in fission yeast an SMC5-SMC6 complex with associated proteins Nse1, Nse2, and Nse3 is important for meiotic chromosome segregation and recombination.
Is Chromosome Condensation in Meiosis Mediated by SMC Proteins?
Chromosome condensation during cell division leads from diffuse interphase
chromatin to a rod-shaped metaphase chromosome (for review, see
Swedlow and Hirano, 2003) and
is required for separation of sister chromatids in mitosis, or of homologs and
later of sister chromatids in meiotic anaphase I and II, respectively. The
axial and lateral compaction of chromosomes is coordinated with decatenation
of chromosomes and sister chromatids. Not much is known about proteins and
processes that contribute specifically to meiotic chromosome condensation, but
it is instructive to very briefly summarize the most important features of
mitotic chromosome condensation.
In 1997, a 5-subunit protein complex called condensin was isolated and subsequently characterized as a major protein component of mitotic chromosomes (Hirano et al, 1997). Besides 3 non-SMC proteins, the evolutionarily highly conserved condensin harbors 2 SMC proteins—that is, a heterodimer of SMC2 (CAP-C) and SMC4 (CAP-E; Table).
Deficiencies in individual condensin subunits, all of which are essential in S cerevisiae, lead to severe defects in chromosome segregation. Examination of yeast or D melanogaster mutants revealed failures of chromosome condensation, including a more diffuse appearance from prophase to anaphase, and the persistence of chromosome bridges in anaphase and telophase, resulting from impaired sister chromatid resolution (Strunnikov et al, 1995; Freeman et al, 2000; Steffensen et al, 2001). Interestingly, chromosome compaction is delayed but still maintained in the fly mutant. Similarly, RNAi depletion of SMC4 or SMC2 (called MIX-1 in C elegans) suggests that condensin is not crucial for chromosome compaction but rather indispensable for separation of the sister chromatids (Hagström et al, 2002). Recently, a conditional "knockout" of the SMC2 (ScII) subunit of condensin was created in a chicken preB cell line (Hudson et al, 2003). Again, chromosome condensation was delayed but not eliminated, sister chromatids failed to segregate, and the distribution of topoisomerase II was altered. Similar observations were reported also for D melanogaster cells deficient in SMC4 (Coelho et al, 2003). Because topoisomerase II is important for resolving entanglements between sister chromatids, its mislocalization or failure to be recruited could be a prime reason for the defect in chromosome separation. Vertebrates contain a second condensin complex, built on the same SMC2 and SMC4 proteins, that associates however with an entirely different set of other proteins (Ono et al, 2003). Both condensin complexes contribute to the formation of the proper shape of chromosomes (Ono et al, 2003). However, because they share the SMC2 subunit, both complexes are affected by the SMC2 deletion in the chicken cell line. It emerges from these recent studies that condensin, although important for promoting a chromosome structure supportive of chromosome segregation, is not essential for chromosome condensation in vertebrate cells.
How exactly condensin establishes such structures, how it promotes formation of mitotic chromosomes is not yet understood. In vitro, condensin changes the topology of certain DNA substrates (Kimura and Hirano, 1997; Kimura et al, 1999). This activity might contribute to the function of condensin in shaping the mitotic—and likely meiotic—chromosome structure.
The involvement of condensin in meiosis is not well understood. Meiotic functions of condensin in S cerevisiae were recently elucidated by the work of Yu and Koshland (2003), who characterized meiotic aberrations in conditional condensin mutants. In the absence of functional condensin subunits in meiosis I, the homologs paired and chromosomes condensed, but both pairing and condensation were significantly less efficient than in the wild type. Assembly of the SC was highly inefficient, demonstrating a role of condensin in recruitment of meiosis-specific chromosome proteins. In anaphase I and II, the mutants exhibited a high incidence of bridges between chromosomes. Importantly, this phenotype was dependent on recombination because in double mutants, defective also in initiation of recombination, this chromosome bridging was reduced to approximately the wild-type level. Thus the physical linkages between chromosomes, which stay unresolved in condensin mutants, appear to be generated by incomplete or erroneous processing of recombination intermediates.
The above-mentioned RNAi depletion of SMC2 (MIX-1) and SMC4 in the germline of C elegans embryos (Hagström et al, 2002) allowed observation of the consequences of condensin deficiency in meiotic cells. Meiosis I seems to proceed normally, but at anaphase of meiosis II, the chromosomes failed to segregate, leaving chromatin bridges between the second polar body and the maternal pronucleus. Probably, as in the other systems already mentioned, condensin is required for sister chromatid resolution, but it remains unclear why condensin deficiency has no striking effect on the first meiotic division. One explanation invokes the existence of a second condensinlike complex in meiosis I.
Little more is known about SMC2 or SMC4 in meiosis or related processes. In the plant A thaliana, 2 highly homologous SMC2 genes exist. Mutations in both genes cause impaired male and female gametogenesis and embryonic lethality (Siddiqui et al, 2003). Expression of the non-SMC condensin subunit XCAP-D2 in X laevis oocytes was analyzed, and the protein was shown to massively accumulate during oocyte maturation (Watrin et al, 2003). Inhibition of expression by injection of antisense oligonucleotides did not affect overall compactness of the chromosomes in the subsequent metaphase, but caused aberrant, ovoid-shaped chromosomes and prevented spontaneous separation of sister chromatid arms, which occurs in untreated oocytes.
Conclusions
Over the last few years, evidence has been generated that clearly
demonstrates important functions of cohesin and related SMC protein complexes
in meiosis. Specific SMC complexes are essential for 1) chromosome segregation
in both meiotic divisions, 2) establishing the proper chromosome structure
with full-length AEs and SCs in prophase I, 3) sister chromatid cohesion in
meiosis I and II, 4) chromosome movements such as the leptotene/zygotene
telomere attachment to the nuclear periphery and bouquet formation, and 5)
progression of DNA recombination (ie, reciprocal exchange).
Most of these processes are essential for proper chromosome segregation. If defective, nondisjunction of chromosomes could result, and it is thus not far-fetched to propose that malfunctioning of an SMC complex in meiotic cells constitutes one of the causes for the frequent human meiotic missegregation syndromes, such as the well-known trisomies (Hassold and Hunt, 2001).
Footnotes
Work in the authors' laboratory was supported by a grant from the National Institutes of Health to R.J. (GM62517).
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