Page 230 - Hematology_ Basic Principles and Practice ( PDFDrive )

182 Part II Cellular Basis of Hematology

dwarfism, is linked to mutation or depletion of ORC1, which impairs cyclin B, leading to nuclear localization of cyclin B and CDC25C.
cell cycle progression. Cyclin B–CDK1 can phosphorylate serine/threonine residues in
During the transition from G 1 to S phase, the MCM complex is many cellular proteins that are essential for mitosis. Phosphorylation
phosphorylated by CDC7 and cyclin E–CDK2, leading to the of lamins by cyclin B–CDK1 promotes nuclear lamina disassembly
recruitment of CDC45 and the four-protein DNA replication and envelope breakdown, which occurs during prophase. Phosphory-
complex GINS. Together, CDC45, MCM, and GINS represent the lation of some kinesin motor proteins that function in spindle
CMG complex, and dissociation of the two MCM hexamers enables assembly, such as KIF11 and CENPE, by cyclin B–CDK1 increases
the formation of two preinitiation complexes at a replication origin. their efficiency of microtubule binding and increases processive
Cyclin E–CDK2 phosphorylates Treslin (TICRR) and RECQL4 motility along microtubules. Other kinesins that function in midzone
that, with TOPBP1 and MCM10, facilitate the formation of the formation, such as KIF23, are blocked in their activity by cyclin
preinitiation complex. After replication origins are licensed, ORC1 B–CDK1–mediated phosphorylation.
phosphorylation by cyclin A–CDK1 inhibits rebinding of ORC1 to After chromosome condensation, centrosome separation, and
replication origins, and CDT1 is blocked by Geminin (GMNN) and nuclear envelope breakdown during prophase, chromosomes become
sent to proteasomal degradation by SCF SKP2 to prevent relicensing attached to microtubules of the mitotic spindle apparatus in prometa-
during S phase. phase. Kinetochores were originally called centromeres because they
In S phase, initiation of DNA replication by CDK2 promotes are located at the center of chromosomes. They are formed by cen-
helicase activation, which leads to unwinding of DNA and recruit- tromere proteins during G 2 phase and prophase and link chromosomes
ment of DNA polymerases alpha (POLA), delta (POLD), and epsilon to the mitotic spindle apparatus, so that sister chromatids become
(POLE), together with PCNA (proliferating cell nuclear antigen) and attached to opposite poles. In metaphase, the chromosomes align at
RFC (replication factor C). The MCM complex translocates from the equatorial plate, and cyclin B–CDK1 levels start to decline by
replication origins to unwind double-stranded DNA, whereas POLA APC/C CDC20 -mediated degradation of cyclin B. The APC/C CDC20
binds to single-stranded DNA and initiates DNA synthesis on both further targets securin (PTTG1) and thereby activates separase
the leading and lagging strands by providing an RNA primer and (ESPL1), which cleaves the cohesin complexes linking sister chroma-
synthesizing the first bases of DNA. POLE and POLD elongate these tids. Once CDK1 activity is low, APC/C CDH1 becomes active and
−5
primers with a base substitution error rate of approximately 10 . removes CDC20, PLK1, and Aurora kinases, as well as the mitotic
When they occasionally incorporate a false nucleotide, it is usually cyclins and CDKs.
removed by an exonuclease associated with these DNA polymerases. In anaphase, the spindle midzone forms between the separating
This so-called proofreading, together with DNA mismatch repair chromosomes by coordinated actions of the microtubule cross-linker
−9
mechanisms, leads to mutation rates of as low as 10 per base and PRC1, the kinesin KIF4, and the multiprotein complexes central-
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per cell division cycle. spindlin and CPC (chromosomal passenger complex). Centralspin-
An immediate consequence of DNA replication is the disruption dlin consists of the kinesin KIF23 and the Rho GTPase RACGAP1,
of chromatin in front of the replication fork, which results in release and the CPC consists of Aurora kinase B, the scaffold INCENP,
of parental histones. When the replication fork has passed, chromatin survivin (BIRC5), and CDCA8. Phosphorylation by CPC promotes
is rapidly reassembled onto old as well as newly replicated DNA, and recruitment of centralspindlin to the spindle midzone through KIF23
in the reassembled chromatin, half of the histones are recycled from and allows for multimerization and accumulation of centralspindlin,
the parental chromatin, whereas the other half are newly synthesized. which in turn promotes localization of the RhoGEF ECT2 (epithelial
Then, the multiprotein complex cohesin mediates cohesion between cell transforming 2) to microtubules. ECT2 then fuels RhoA, which
replicated sister chromatids in S phase, which is essential for chromo- promotes the recruitment of effector contractile ring proteins. As the
some segregation in M phase. ring closes, the spindle midzone is remodeled to form the densely
In addition to DNA replication, the centrosome also needs to be packed telophase midbody, which organizes the intracellular bridge.
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replicated to allow the formation of two daughter cells. The centro- At this time in telophase, nuclear membranes form to envelop each
some is the major microtubule-organizing center and consists of two of the two separated sets of chromosomes, which also begin to
centrioles. Phosphorylation by cyclin E–CDK2 promotes the forma- decondense. This is soon followed by the abscission event near the
tion of one procentriole at each centriole. During S and G 2 phases, midbody, which completes mitosis.
procentrioles elongate until they reach the length of the older centri-
oles. In early M phase, the two centrosomes separate to promote
cytokinesis. CELL CYCLE CHECKPOINTS
Competence
MITOSIS
Cells require the presence of nutrients and growth factors to switch
The decision to enter mitosis is mediated by a network of proteins from quiescence to a state of proliferation. When cells sense that
that regulate activation of the cyclin B–CDK1 complex. In principle, conditions are suitable for proliferation, they leave quiescence into
a feedback system between CDK1, WEE1/CDC25C, and the G 1 phase and become competent to enter the cell cycle. G 1 has been
CDK1-counteracting phosphatases generates a bistable switch, subdivided into segments and regulatory points based largely on the
leading to robust directionality that prevents cells from going back study of the proliferative response of cells to sequential application
and forth between G 2 and M phases. Cyclin B and CDK1 levels of different growth factors, nutrients, and metabolic inhibitors. From
increase in late S phase and G 2 phase, and the activity of newly made the standpoint of cell cycle regulation, a particularly important point
cyclin B–CDK1 complexes is blocked through phosphorylation by in G 1 is the restriction point, or R, which occurs near the G 1 –S
WEE1. Activation of the cyclin B–CDK1 complex is the key to cell boundary. The period after mitosis, when cells can enter quiescence,
entry into mitosis and occurs just before mitosis through the action is termed G 1 pm (postmitosis), and the period between quiescence
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of CDC25C, causing dephosphorylation of CDK1. The activities of and S phase is termed G 1ps (pre-DNA synthesis) (Fig. 17.5).
WEE1 and CDC25C are themselves regulated with phosphorylation, Notably, nearly all of the variability in the length of G 1 can be
inhibiting WEE1 function and enhancing CDC25C function. Once accounted for by the G 1ps interval. Experiments have shown that, to
a small amount of cyclin B–CDK1 is activated, it can phosphorylate leave quiescence and to enter the cell cycle, cells require growth
CDC25C and create a self-amplifying feedback loop that generates signals either continuous for several hours during G 1 or, alternatively,
more active cyclin B–CDK1 from the large preexisting stock of as two discrete pulses of approximately 1 hour in duration and with
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inactive complex. What starts this sequence of events by initially a pause of several hours in between. To become competent for cell
phosphorylating and activating CDC25C is unclear, but PLK1 is a cycle entry, initial activation of the MAPK pathway by mitogen
candidate kinase that can phosphorylate WEE1, CDC25C, and signals is required that in turn activates essential metabolic programs.