The interphase microtubule damage checkpoint defines an S-phase commitment point and does not require p21(waf-1)

KIF7 Controls the Proliferation of Cells of the Respiratory Airway through Distinct Microtubule Dependent Mechanisms

The cell cycle must be tightly coordinated for proper control of embryonic development and for the long-term maintenance of organs such as the lung. There is emerging evidence that Kinesin family member 7 (Kif7) promotes Hedgehog (Hh) signaling during embryonic development, and its misregulation contributes to diseases such as ciliopathies and cancer. Kif7 encodes a microtubule interacting protein that controls Hh signaling through regulation of microtubule dynamics within the primary cilium. However, whether Kif7 has a function in nonciliated cells remains largely unknown. The role Kif7 plays in basic cell biological processes like cell proliferation or cell cycle progression also remains to be elucidated. Here, we show that Kif7 is required for coordination of the cell cycle, and inactivation of this gene leads to increased cell proliferation in vivo and in vitro. Immunostaining and transmission electron microscopy experiments show that Kif7^dda/dda mutant lungs are hyperproliferative and exhibit reduced alveolar epithelial cell differentiation. KIF7 depleted C3H10T1/2 fibroblasts and Kif7^dda/dda mutant mouse embryonic fibroblasts have increased growth rates at high cellular densities, suggesting that Kif7 may function as a general regulator of cellular proliferation. We ascertained that in G1, Kif7 and microtubule dynamics regulate the expression and activity of several components of the cell cycle machinery known to control entry into S phase. Our data suggest that Kif7 may function to regulate the maintenance of the respiratory airway architecture by controlling cellular density, cell proliferation, and cycle exit through its role as a microtubule associated protein.

Respiratory diseases such as lung cancer, COPD, and asthma are the second leading cause of death in the United States. These diseases are heterogeneous and arise from genetic factors, environmental hazards, or developmental abnormalities that persist throughout life. An increased understanding of the genes and cellular mechanisms regulating respiratory system homeostasis and regeneration should provide information for the development of future therapeutics. We show that the gene Kif7 regulates cell proliferation, cellular density, and intracellular signaling within the epithelial and mesenchymal cells of the respiratory airway. We expand on the known role for Kif7 in regulating microtubule architecture within ciliated cells by showing that this protein regulates cell signaling in non-ciliated secretory cells. Furthermore, we show that microtubules function to regulate the abundance and activity of several factors known to be required for proper cell cycle timing. We propose that Kif7 and microtubule dynamics hone cellular signaling necessary for control of the balance between cell proliferation and cell cycle exit, and we provide evidence that Kif7 has a critical role in the maintenance of the respiratory system in postnatal life.

Checkpoint-apoptosis uncoupling in human and mouse embryonic stem cells: a source of karyotpic instability

Karyotypic abnormalities in cultured embryonic stem cells (ESCs), especially near-diploid aneuploidy, are potential obstacles to ESC use in regenerative medicine. Events causing chromosomal abnormalities in ESCs may be related to events in tumor cells causing chromosomal instability (CIN) in human disease. However, the underlying mechanisms are unknown. Using multiparametric permeabilized-cell flow cytometric analysis, we found that the mitotic-spindle checkpoint, which helps maintain chromosomal integrity during all cell divisions, functions in human and mouse ESCs, but does not initiate apoptosis as it does in somatic cells. This allows an unusual tolerance to polyploidy resulting from failed mitosis, which is common in rapidly proliferating cell populations and which is reduced to near-diploid aneuploidy, which is also common in human neoplastic disease. Checkpoint activation in ESC-derived early-differentiated cells results in robust apoptosis without polyploidy/aneuploidy similar to that in somatic cells. Thus, the spindle checkpoint is "uncoupled" from apoptosis in ESCs and is a likely source of karyotypic abnormalities. This natural behavior of ESCs to tolerate/survive varying degrees of ploidy change could complicate genome-reprogramming studies and stem-cell plasticity studies, but could also reveal clues about the mechanisms of CIN in human tumors.

p53 deficiency and defective mitotic checkpoint in proliferating T lymphocytes increase chromosomal instability through aberrant exit from mitotic arrest

During the proliferation of T cells for successful immune responses against pathogens, the fine regulation of cell cycle is important to the maintenance of T cell homeostasis and the prevention of lymphoproliferative disorders. However, it remains to be elucidated how the cell cycle is controlled at the mitotic phase in proliferating T cells. Here, we show that during the proliferation of primary T cells, the disruption of the mitotic spindle leads to cell-cycle arrest at mitosis and that prolonged mitotic arrest results in not only apoptosis but also the form of chromosomal instability observed in human cancers. It is interesting that in response to spindle damage, the phosphorylation of BubR1, a mitotic checkpoint kinase, was significantly induced in proliferating T cells, and the expression of the dominant-negative mutant of BubR1 compromised mitotic arrest and subsequent apoptosis and thus led to the augmentation of polyploidy formation. We also show that in response to prolonged spindle damage, the expression of p53 but not of p73 was significantly induced. In addition, following sustained mitotic arrest, p53-deficient T cells were found to be more susceptible to polyploidy formation than the wild type. These results suggest that during flourishing immune response, mitotic checkpoint and p53 play important roles in the prevention of chromosomal instability and in the maintenance of the genomic integrity of proliferating T cells.

Centrosome replication, genomic instability and cancer

Karyotypic alterations, including whole chromosome loss or gain, ploidy changes, and a variety of chromosome aberrations are common in cancer cells. If proliferating cells fail to coordinate centrosome duplication with DNA replication, this will inevitably lead to a change in ploidy, and the formation of monopolar or multipolar spindles will generally provoke abnormal segregation of chromosomes. Indeed, it has long been recognized that errors in the centrosome duplication cycle may be an important cause of aneuploidy and thus contribute to cancer formation. This view has recently received fresh impetus with the description of supernumerary centrosomes in almost all solid human tumors. As the primary microtubule organizing center of most eukaryotic cells, the centrosome assures symmetry and bipolarity of the cell division process, a function that is essential for accurate chromosome segregation. In addition, a growing body of evidence indicates that centrosomes might be important for initiating S phase and completing cytokinesis. Centrosomes undergo duplication precisely once before cell division. Recent reports have revealed that this process is linked to the cell division cycle via cyclin-dependent kinase (cdk) 2 activity that couples centriole duplication to the onset of DNA replication at the G(1)/S phase transition. Alterations in G(1)/S phase regulating proteins like the retinoblastoma protein, cyclins D and E, cdk4 and 6, cdk inhibitors p16(INK4A) and p15(INK4B), and p53 are among the most frequent aberrations observed in human malignancies. These alterations might not only lead to unrestrained proliferation, but also cause karyotypic instability by uncontrolled centrosome replication. Since several excellent reports on cell cycle regulation and cancer have been published, this review will focus on the role of centrosomes in cell cycle progression, as well as causes and consequences of aberrant centrosome replication in human neoplasias.

Steel factor regulates cell cycle asymmetry

Asymmetric segregation of cell-fate determinants during mitosis (spatial asymmetry) is an essential mechanism by which stem cells are maintained while simultaneously giving rise to differentiated progenitors that ultimately produce all the specialized cells in the hematopoietic system. Temporal cell cycle asymmetry and heterogeneity are attributes of cell proliferation that are also essential for maintaining tissue organization. Hematopoietic stem cells (HSCs) are regulated by a complex network of cytokines, some of which have very specific effects, while others have very broad ranging effects on HSCs. Some cytokines, like steel factor (SLF), are known to synergize with other cytokines to produce rapid expansion of progenitor cells. Using the human growth factor-dependent MO7e cell line as a model for synergistic proliferation, we present evidence that links proliferation asymmetry to SLF synergy with GM-CSF, and suggests that temporal asymmetry and cell cycle heterogeneity can be regulated by SLF in vitro. We also show that CDK-inhibitor and cell cycle regulator, p27kip-1, may be involved in this temporal asymmetry regulation. We propose that SLF/GM-CSF synergy is, in part, due to a shift in proliferation pattern from a heterogeneous and asymmetric one to a more synchronous and symmetric pattern, thus contributing dramatically to the rapid expansion that accompanies SLF synergy observed in MO7e cells. This kinetic model of asymmetry is consistent with recent evidence showing that even though SLF synergy results in a strong proliferative signal, it does not increase primary HSC self-renewal, which is believed to be highly dependent on asymmetric divisions. The factor-dependent MO7e/SCF- synergy/asymmetry model described here may therefore be useful for studies of the effects of various cytokines on cell cycle asymmetry.