The alternative cell cycle-independent functions that core cell cycle regulators play in neurons are best appreciated by first considering the context in which they carry out their well-established roles – the cell cycle (). A fundamental requirement for any proliferating cell, whether a fibroblast or a neural progenitor, is to replicate its DNA and divide. Comprised of 4 distinct stages, the unidirectional progression of the eukaryotic cell cycle is ensured by checkpoints and the oscillating expression of cell cycle proteins.
During the first gap phase of the cell cycle, or G1, cells assimilate environmental signals that allow them to progress through the “restriction point,” a point after which a cell is committed to divide. G1 progression is promoted by cyclin-dependent kinase (CDK) 4/cyclin D and CDK6/cyclin D, but also kept in check by CDK inhibitors (CKIs) of the In
hibitor of k
rame (INK4a) and Cip/Kip families, which inhibit CDK-cyclin complexes. A major obstacle for progressing into S-phase involves the derepression of E2F transcription factors. Kept inactive by the Retinoblastoma (Rb) tumor suppressor protein, E2F proteins are activated as Rb becomes hyper-phosphorylated by CDK4, CDK6, and CDK2/cyclin E over the course of G1 progression (Nevins, 2001
). Derepressed E2F proteins can then proceed to induce downstream target genes required for subsequent cell cycle progression, including cyclins (D, E, and A), DNA polymerase, CDC6, mini-chromosome maintenance (MCM) proteins, and origin recognition complex (ORC) proteins. Another important feature of G1 is the preparation of DNA replication origins, or DNA licensing, through the recruitment of pre-replication complexes ().
Once cells have passed the restriction point, they commit to DNA replication and cell division. DNA replication and centrosome duplication occur in S-phase, which is driven by CDK2/cyclin E and CDK2/cyclin A. DNA replication initiates on multiple origins located throughout the genome bound by pre-replication complexes formed in G1. DNA polymerase, the enzyme responsible for DNA replication, is recruited to origins by the concerted actions of protein kinases, including Cdc7, CDK2/cyclin E, and CDK2/cyclin A (Woo and Poon, 2003
). Once replication origins fire, re-replication of DNA is prevented via phosphorylation of replication complex components by S-phase CDKs. Given the importance of faithful genome replication, cells have evolved quality control mechanisms, or checkpoints, to ensure sufficient time to repair any damage DNA accrued during or following replication (i.e., intra-S-phase and G2/M checkpoints, respectively). The importance of these quality control mechanisms are underscored by the various diseases, including cancer, that result from the absence of key checkpoint proteins.
Once the entire genome is duplicated, cells enter a second gap phase, or G2, during which cells verify the fidelity of DNA replication prior to mitosis. If DNA is somehow damaged during replication, cells arrest at the G2/M checkpoint and repair the damage. Once DNA replication fidelity in confirmed in G2, cells undergo mitosis and equally partition genomic material into daughter cells. Mitosis is comprised of 4 distinct phases: prophase, metaphase, anaphase, and telophase, followed by cytokinesis, or cell division. Cells achieve many feats within the span of about an hour during mitosis, including nuclear envelope breakdown, chromosome condensation, chromosome alignment at the metaphase plate, sister chromatid separation, reformation of the nuclear envelope, and cell division. Proper execution of mitotic events are monitored and controlled by the mitotic spindle checkpoint, a mechanism to ensure that kinetochores, chromosomal structures to which spindle fibers attach, are properly attached to the mitotic spindle.
These basic cell cycle concepts and mechanisms, most of which derive from studies in transformed cells, hold true in neural progenitors of the developing brain. However, the context of the developing brain provides an extra layer of spatiotemporal control on the cell cycle not observed in a culture dish. For instance, the G1 phase of the cell cycle plays a crucial role in determining when a neural progenitor will undergo cell cycle exit and neuronal differentiation, or neurogenesis. During the period of neurogenesis, which peaks at around E14 in mice, G1 length in progenitors increases, and this correlates with increased cell cycle exit (Takahashi et al., 1995
). Supporting this, artificially lengthening the G1 phase of the cell cycle can induce neurogenesis (Calegari and Huttner, 2003
). Spatially, distinct cell cycle phases in neural progenitors are carried out with positional discrimination in the proliferative ventricular zone in a process called interkinetic nuclear migration (). This spatiotemporal coordination of neural progenitor cell cycle dynamics in the developing brain ensures that a precise number of neurons and specific neuronal subtypes are generated.