Cell Cycle And Its Regulators
One of the fundamental functions of cells is to multiply by cell division and maintain their by proliferation. Cell cycle is the study of coordinated series of events that lead to cell replication (mitosis), and the time period between successive divisions of a cell to accurately segregate into daughter cells.
Many of the events in mitosis are ancient in origin and highly conserved; however, there have been advances in our understanding of molecular events and their regulators in cell cycle in health and in disease, particularly in cancer, cell injury and repair.
Read And Learn More: General Pathology Notes
With this perspective, the focus in learning here is directed at the following key aspects:
- Overview of cell cycle
- Checkpoints in the cell cycle
- Intrinsic controls: Checkpoint regulators
- Extrinsic controls: Growth factors and receptors
1. Overview Of Cell Cycle:
Cells begin their life in a cyclical pattern, during which there are checkpoints to ensure that it progresses through the stages of division appropriately. There are two broad phases in cell cycle
seen as interphase and mitosis.
Interphase:
This is the phase in which the cell grows and DNA replication occurs. The cell spends more time in interphase than in mitosis phase.
Interphase includes 3 stages:
1. Gap 1 (G1 phase)
2. Synthesis (S phase), and
3. Gap 2 (G2 phase).
- G1 phase:
- This is the stage in which the cell prepares for DNA replication. The cell grows by building up proteins and organelles. The period in G1 is more variable and is generally the longest.
- G1 phase contains checkpoint to assess its readiness for DNA replication i.e. before entry into S phase (G1-S checkpoint).
- S phase:
- The cell growth that started in G1 phase is continued further.
- More importantly, replication of chromosomal DNA within the cell occurs i.e. in S phase the cell contains double the number of chromosome.
- G2 phase:
- In this phase, the cell continues to grow and prepare for mitosis.
- There is checkpoint at G2 phase to assess the cell for accurate replication of chromosomes i.e. before the cell is ready to enter into mitosis (G2-M checkpoint.
Mitosis:
Following interphase, mitosis occurs in sequential stages.
The salient features of these stages are as under:
- Prophase:
- During this phase, chromatin condensation starts. The chromosomes appear discrete and become visible on light microscopy as long thread-like.
- At this stage, each chromosome has two identical chromatids joined together at the centromere.
- Nucleolus breaks down and disappears.
- Near the nucleus lies a single centrosome which is composed of a pair of centrioles having cylindrical proteins; this forms the coordination centre for development of microtubules that create mitotic spindle for separation of chromosomes later.
- In late prophase, kinetochores are formed; these are proteinaceous microtubule-binding structures formed on the chromosomal centromere.
- Prometaphase:
- In this stage, the nuclear envelop disintegrates into small membrane vesicles.
- Kinetochore microtubules connect to the chromosomal kinetochores and form mitotic spindle, in such a way that each polar microtubule finds.
- And interacts with the corresponding polar microtubules from the opposite side.
- Metaphase:
- In this stage, the paired chromosomes split up at the centromeres and start moving to the opposite sides of the cell due to pulling action by the mitotic spindle.
- The movement of chromosomes to the opposite side aligns them along the centre of the cell called the metaphase plate or equatorial plane.
- At this stage, equitable distribution of chromosomes at the mitotic spindle is assessed by the metaphase checkpoint.
- Anaphase:
- The newly formed identical daughter chromosomes move to the opposite ends of the cell.
- Polar microtubules push the cell to elongate.
- By the end of anaphase, the chromosomes have reached the opposite ends of the cell, segregated and condensed, ready to form the new nucleus on both ends.
- Telophase:
- The cell is elongated further by polar microtubules.
- Using the membrane vesicles of the parent cell nucleus, the new nuclear membrane starts to form on both ends of the cell around the segregated daughter chromosomes.
- The nucleolus also reappears; the plasma membrane too separates.
- Thus, with formation of two daughter cells having their own identical nuclei, the process of mitosis is complete.
The cell may now remain in resting stage (Gap 0 or G0) or may re-enter mitosis.
2. Checkpoints In Cell Cycle:
Checkpoints are major internal surveillance mechanisms which condition of the cell before the next major event of the cell cycle.
At each checkpoint, specialised monitor and evaluate the proteins determine whether the conditions exist for the cell to proceed to the next stage, or further progression in the cell cycle is to be halted.
Checkpoint in cell cycle is significant for the following reasons:
- To detect any error in major events in cell cycle, and delay progression till internal remedial mechanism fixes the error.
- To halt the cell cycle if the problem cannot be corrected.
- To maintain the genomic composition accurately.
- Cell cycle checkpoints are valid for one round of DNA replication per cycle only.
- Any error in major checkpoints has catastrophic consequences for the cell, for example, Directing cell to programmed cell death (apoptosis) or its transformation to cancerous cell.
Following are major checkpoints in the cell cycle which are of critical significance in assessment of the appropriateness of the cell for continuing the next step of cell cycle:
- Cell size control: This checkpoint is observed at G1-S for ensuring that each daughter cell gets the appropriate amount of genetic and biosynthetic material, and that there is coordinated cell size with cell cycle progression.
- DNA damage response: During interphase, any DNA damage elicits a cell cycle arrest response that allows the intrinsic DNA repair pathways to operate. This checkpoint is at G1-S when DNA damage is repaired. Restriction point at G1 phase in the cell cycle is the stage at which the cell is committed irreversibly to the cell cycle. If the conditions are not appropriate, the cell is not allowed to proceed to S phase.
- Appropriateness of DNA replication: This is a G2-M checkpoint that cells with unrepaired DNA are not allowed to proceed for replicative polymerases and associated proteins. Checkpoint also ensures that the cell does not enter mitosis if replication is not completed.
- Mitotic spindle checkpoint: Also called M checkpoint located in the metaphase of mitosis, it determines whether all chromatids are correctly attached to the corresponding spindle microtubule before the cell enters the anaphase.
Thus, in summary, there are three major checkpoints:
- G1-S checkpoint (for assessing cell size and integrity of DNA),
- G2-M checkpoint (for proper chromosomal duplication), and
- Mitotic spindle checkpoint (appropriate attachment of kinetochores to the mitotic spindle).
3. Intrinsic Controls: Checkpoint Regulators
Cell cycle is regulated internally at major checkpoints by activator and inhibitor protein molecules: cyclins and cyclin-dependent kinases (CDKs) are together called activators or positive regulators, and inhibitors of CDK (CDKIs) and certain protein molecules are called negative regulators.
- Cyclins are a family of proteins having no enzymatic activity of their own but activate CDKs binding to them. They are so named due to their cyclic production and degradation.
- CDKs are a family of multifunctional kinases i.e. they are phosphorylating enzymes that transfer phosphate groups from ATP to specific amino acids in the substrates involved in the
cell cycle. - As their name indicates, the kinase activity of CDKs is dependent upon cyclins. Over 20 cyclins have been identified (named alphabetically as cyclin A, B, C, etc) and over a dozen CDKs
(named numerically as CDK1, 2, 3 etc). - Each cyclin is paired with specific one or more CDKs, forming a complex with relevant cyclin. However, CDK1 and CDK2 bind to multiple cyclins (A, B, D, E), while CDK4 and CDK6 pair with cyclin-D only.
- On completion of the phosphorylating activity of CDK, the associated cyclin is degraded. Accordingly, there is fall in level of cyclin at the end of each checkpoint.
- CDKIs are proteins which regulate the cell cycle by modulating cyclin-CDK complex activity. Like CDKs, there are several CDKIs. Best known examples are two families of CDKI:
- CDKN1 family having 3 members which inhibit multiple CDKs: p21 (CDKN1A), p27 (CDKN1B), and p57 (CDKN1C). This family of CDKI can inhibit all CDKs.
- CDKN2 family (INK4 inhibitor family) has 3 members: p16 (CDKN2A), p15 (CDKN2B),
and p18 (CDKN2C). This family of inhibitors acts on cyclin D-CDK4 and cyclin D-CDK6.
Specific Regulatory Molecules:
Major cyclins, CDKs and CDKIs act as regulators in different phases of the cell cycle and at
specific checkpoints as under
- At G1 phase: Activation by pairing complex of cyclin E-CDK2 , Inhibition by CDKIs: INK4 family (p16, p15, p18)
- At G1-S checkpoint: Activation by pairing complex of cyclin D-CDK4, cyclinD-CDK6, and cyclin E-CDK2
- CDKIs: CDKN1 family (p21, p27, p57)
- At S phase: Activation by pairing complex of cyclin A-CDK1, cyclin , A-CDK2 Inhibition by CDKIs: CDKN1 family (p21, p27, p57)
- At G2-M and mitotic spindle checkpoint: Activation by pairing complex of cyclin B-CDK1, Inhibition by CDKIs: CDKN1 family (p21, p27, p57) .
- Negative regulatory protein molecules: Two of the important negative regulatory molecules of the cell cycle are retinoblastoma protein (pRb) and p53 (called tumour-suppressor proteins) common in humans.
Regular of cell cycle phase and checkpoint:
However, in brief their function in the cell cycle is as under:
- pRb exerts its negative effect on other positive regulatory proteins.
- Rb is active when in dephosphorylated state and it binds to transcription factors, most often E2F.
- While transcription factor E2F in itself activates genes specific for protein production in G1-S transition stage, binding of Rb to E2F blocks protein production.
- As the cell grows in size, Rb is slowly phsophorylated, inactivated, and E2F released, and thus protein production by the cell resumes.
- p53 is a multi-functional protein. It acts in G1 when the cell is preparing to enter mitosis.
- If DNA damage is detected, p53 halts the cell cycle and synthesises enzymes for DNA repair.
- If DNA damage cannot be repaired, p53 can trigger cell death by apoptosis so that damaged DNA is not passed to the next progeny of cells.
- As p53 levels rise, it stimulates production of CDK inhibitor molecule p21, that enforces the halt in the cell cycle by inhibiting the activity of the cyclin CDK complex.
- Higher levels of p53 and p21 do not let the cell cycle proceed to S phase. This is the reason why p53 is called ‘the protector of the genome’.
4. Extrinsic Controls: Growth Factors And Receptors:
While above discussion on regulation of the cell cycle is focussed on internal regulatory mechanisms in the cell cycle (by cyclins, CDKs, CDKIs, protein molecules), the activity of cell growth and cell division is also regulated by external factors such as growth factors and their receptors.
A growth factor (GF) is a substance, either a protein molecule or a hormone, capable of regulating cell proliferation and differentiation i.e. GFs act as cell signalling molecule.
Each GF has a specific cell surface receptor (GF-R) where GF binds, and that initiates or blocks cell proliferation as per the nature and mode of action of GF.
On binding of GF to its corresponding receptor on the cell’s surface, GF-R is activated. The activated GF-R then activates one or more intracellular proteins or the substrate proteins
For Example,platelet-derived growth factor (PDGF) binds to its receptor (PDGF-R), and activates intracellular protein substrates such as Ras protein and Src protein initiating cell division, followed by its inactivation by GTPase activation protein (GAP).
Features and significance of major growth factors and their receptors:
Actions:
The major actions of GFs are as follows:
- Recruiting cells from resting phase to cell cycle (i.e. G0 to G1).
- Actions in growth of cellular components for cells preparing for mitosis such as proteins, and organelles (i.e. G1 to S phase).
- Release the blocks in cell cycle.
- Prevent cell death by apoptosis.
- Role in cellular differentiation and movement.
- Major role in tissue repair and in development of tumours.
Types And Examples:
- Growth Factors: This includes GFs interacting with specific receptors on the cell surface for example,Epidermal growth factor (EGF), transforming growth factor-α and β (TGF-α and β), plateletderived growth factor (PDGF), hepatocyte growth factor (HGF), fibroblast growth factor (FGF), vascular endothelial growth factor (VEGF).
- Growth Factor Receptors: These are cell surface receptors for specific GFs having intrinsic tyrosine kinase activity for phosphorylation; these are trans-membranous receptors having extracellular and cytoplasmic domain for example, EGF-R, PDGF-R. Salient features and significance of major examples of GFs and their receptors are summed up in above table.
Cell Cycle and its Regulators:
- Cell cycle is coordinated series of events that lead to cell replication, and the time period between successive divisions of a cell to accurately segregate into daughter cells.
- The cell cycle has two main stages, each with its phases: interphase (G1, S and G2 phases) and mitosis (prophase, prometaphase, metaphase, anaphase and telophase).
- Checkpoints are major internal surveillance mechanisms which monitor and evaluate the condition of the cell before the next major event of the cell cycle.
There are 3 main checkpoints:
- G1-S (for assessing cell size and integrity of DNA), G2-M (for proper chromosomal duplication), and mitotic spindle checkpoint (appropriate attachment of
kinetochores to the mitotic spindle). - Internal regulators of the cell cycle are various proteinmolecules: activators or positive regulators are cyclins and cyclin-dependent kinases, while negative regulators are CDK inhibitors and certain protein molecules (pRb and p53).
- Cell cycle is also controlled by external regulators which are growth factors (GFs) and their receptors (GF-Rs).
- GFs are protein molecules or hormones, capable of regulating cell proliferation and differentiation; they have corresponding receptors.
- Most of the GFRs have intrinsic tyrosine kinase activity of phosphorylation.
- Major examples of GFs are: EGF and TGF-α (corresponding shared receptor EGFR) active in tissue repair, TGF-β (for ECM synthesis), PDGF and VEGF (for angiogenesis), HGF (for epithelial and endothelial cell proliferation), and FGF (in granulation tissue)
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