Cell cycle regulation (cyclins, checkpoints) Knowledge-Based Study

Study notes

1. Molecular Control of the Cell Cycle

   The eukaryotic cell cycle is a tightly regulated sequence of events comprising interphase ($G_1$, $S$, and $G_2$ phases) and the mitotic phase (M phase). Progression through these phases is not automatic; rather, it is directed by an internal molecular control system that functions much like an automated timing device.

   The core molecular engine consists of two key classes of regulatory proteins:

   • Cyclins: Regulatory proteins whose concentrations fluctuate cyclically throughout the cell cycle. They are synthesized and degraded at specific times to regulate phase transitions.
   • Cyclin-Dependent Kinases (CDKs): Enzymes that remain at relatively constant concentrations throughout the cell cycle. CDKs are inactive on their own and require binding with a specific cyclin partner to become enzymatically active.

   When a cyclin binds to its matching CDK, the active complex phosphorylates specific target proteins, altering their activity to trigger downstream events necessary for the next cell cycle stage. For example, MPF (Maturation-Promoting Factor or M-phase-promoting factor) is a cyclin-CDK complex formed by Cyclin B and CDK1 that triggers the cell's passage past the $G_2$ checkpoint into mitosis by phosphorylating proteins such as nuclear lamins, facilitating nuclear envelope breakdown.

   Major Cell Cycle Checkpoints

   To ensure that essential processes occur accurately and in the correct order, the cell cycle possesses built-in surveillance mechanisms known as checkpoints. At these control points, molecular stop and go signals regulate progression based on cell size, nutritional status, DNA integrity, and chromosome alignment.

   1. The $G_1$ Checkpoint (Restriction Point): Regarded as the master checkpoint in animal cells. It assesses whether cell size is adequate, nutrients are sufficient, growth factors are present, and DNA is undamaged. If a cell receives a go-ahead signal, it usually completes $S$, $G_2$, and M phases. In the absence of a go-ahead signal, the cell exits the active cycle and enters a non-dividing, quiescent state called the $G_0$ phase.
   2. The $G_2$ Checkpoint: Operates at the threshold between $G_2$ and mitosis. This checkpoint checks for the completeness and fidelity of DNA replication during $S$ phase, as well as cell size. DNA damage or unreplicated segments activate inhibitory proteins that halt MPF activation, delaying entry into mitosis until repairs are complete.
   3. The M Checkpoint (Spindle Assembly Checkpoint): Occurs during metaphase of mitosis. It monitors the physical attachment of kinetochores from all sister chromatids to spindle microtubules. Anaphase is initiated only after every kinetochore is under tension from both spindle poles, preventing non-disjunction and subsequent aneuploidy.

   Checkpoint Signaling and Clinical Relevance

   When DNA damage is detected during interphase, sensor proteins activate a signal transduction cascade that stabilizes tumor suppressor proteins, such as p53. Known as the 'guardian of the genome,' p53 functions as a transcription factor that upregulates genes encoding CDK inhibitors, such as p21.

   • CDK Inhibitors (CKIs): Proteins like p21 bind directly to cyclin-CDK complexes, inhibiting their kinase activity and arresting the cell cycle to allow DNA repair.
   • Apoptosis: If the DNA damage is severe or irreparable, p53 activates pro-apoptotic genes (such as Bax), triggering programmed cell death to eliminate the defective cell and prevent the propagation of mutations.

   A breakdown in cell cycle regulation is a hallmark of tumorigenesis. Loss-of-function mutations in tumor suppressor genes (e.g., TP53 or RB1) or gain-of-function mutations in proto-oncogenes (which produce hyperactive positive regulators) lead to uncontrolled cell division, failure of checkpoint arrest, genomic instability, and ultimately cancer development.