Robustness of differentiation cascades with symmetric stem cell division

The role of cell location and spatial gradients in the evolutionary dynamics of colon and intestinal crypts

BACKGROUND

Colon and intestinal crypts serve as an important model system for adult stem cell proliferation and differentiation. We develop a spatial stochastic model to study the rate of somatic evolution in a normal crypt, focusing on the production of two-hit mutants that inactivate a tumor suppressor gene. We investigate the effect of cell division pattern along the crypt on mutant production, assuming that the division rate of each cell depends on its location.

RESULTS

We find that higher probability of division at the bottom of the crypt, where the stem cells are located, leads to a higher rate of double-hit mutant production. The optimal case for delaying mutations occurs when most of the cell divisions happen at the top of the crypt. We further consider an optimization problem where the "evolutionary" penalty for double-hit mutant generation is complemented with a "functional" penalty that assures that fully differentiated cells at the top of the crypt cannot divide.

CONCLUSION

The trade-off between the two types of objectives leads to the selection of an intermediate division pattern, where the cells in the middle of the crypt divide with the highest rate. This matches the pattern of cell divisions obtained experimentally in murine crypts.

REVIEWERS

This article was reviewed by David Axelrod (nominated by an Editorial Board member, Marek Kimmel), Yang Kuang and Anna Marciniak-Czochra. For the full reviews, please go to the Reviewers' comments section.

Wnt Signaling: Role in Regulation of Haematopoiesis

Hematopoietic stem cells (HSCs) are a unique population of bone marrow cells which are responsible for the generation of various blood cell lineages. One of the significant characteristics of these HSCs is to self-renew, while producing differentiating cells for normal hematopoiesis. Deregulation of self-renewal and differentiation leads to the hematological malignancies. Several pathways are known to be involved in the maintenance of HSC fate among which Wnt signaling is a crucial pathway which controls development and cell fate determination. Wnt signaling also plays a major role in differentiation, self-renewal and maintenance of HSCs. Wnt ligands activate three major pathways including planar cell polarity, Wnt/β-catenin and Wnt/Ca(2+). It has been shown that Wnt/β-catenin or canonical pathway regulates cell proliferation, survival and differentiation in HSCs, deregulation of this pathway leads to hematological malignancies. Wnt non-canonical pathway regulates calcium signaling and planar cell polarity. In this review, we discuss various signaling pathways induced by Wnt ligands and their potential role in hematopoiesis.