One of the fundamental principles in developmental biology is that cell cycle exit must precede differentiation, because the regulatory networks that drive cell proliferation are incompatible with those that direct differentiation. When differentiated cells are forced to re-enter the cell cycle, they typically undergo programmed cell death. For example, ectopic expression of cyclin D1 in differentiating retinal photoreceptors results in apoptosis (Skapek et al., 2001
). In addition, programmed cell death associated with inappropriate proliferation of differentiated neurons in other regions of the central nervous system (CNS) contributes to human neurodegenerative disorders such as Alzheimer's disease (Yang et al., 2001
Studies on progenitor cells in developing embryonic tissues have provided some key insight into the mechanism underlying the coordination of proliferation and differentiation. When differentiation is induced prematurely in proliferating progenitor cells, it is accompanied by concomitant cell cycle withdrawal. For example, in developing muscle cells, MyoD induces myocyte differentiation and binds to Rb to prevent further rounds of cell division (Gu et al., 1993
). This tightly coordinated regulation of proliferation and differentiation is not limited to the developing embryo. In self-renewing adult tissues such as the hematopoietic system, cell cycle exit precedes differentiation. To retain its long-term self-renewing capability while generating terminally differentiated cell types, the hematopoietic system relies on a small number of stem cells. Hematopoietic stem cells are thought to divide asymmetrically to generate another stem cell and a daughter cell that produces differentiated hematopoietic cells. A similar mechanism is used by neural stem cells in the adult CNS in regions where neurogenesis occurs (Reynolds and Weiss, 1992
). For example, neural stem cells lining the ventricles of the brain give rise to additional stem cells and committed neural precursors that migrate along the rostral migratory stream to differentiate and integrate into the olfactory bulb (Lois and Alvarez-Buylla, 1993
; Luskin, 1993
). Therefore, in both embryonic progenitor cells and adult stem cells, cell cycle exit precedes differentiation.
Examples of dedifferentiation and transdifferentiation have provided important insight into the reverse process. When differentiated cells re-enter the cell cycle, this event is preceded by cellular, molecular, and morphologic dedifferentiation. For example, radial glial cells in the retina (Müller glia) are capable of generating neurons in response to injury or exogenous growth factors (Fausett and Goldman, 2006
; Fischer et al., 2002
; Ooto et al., 2004
). Neurogenesis from retinal radial glia is accompanied by dedifferentiation, proliferation, and subsequent neuronal cell fate specification and maturation. Similar dedifferentiation processes have been reported in other systems such as muscle (Odelberg et al., 2000
) providing a strong foundation for the widely held belief in developmental biology that a cell must exit the cell cycle before it can differentiate and that a differentiated cell cannot proliferate while maintaining its molecular, cellular, and morphologic features.
Cancer represents one of the best examples of deregulated proliferation during development and in adults. As mentioned above, differentiated cells usually undergo programmed cell death when they re-enter the cell cycle inappropriately. Therefore, it is not surprising that genetic perturbations in genes that control cell death occur during tumorigenesis. Yet, even in cancer cells, differentiation and proliferation are, for the most part, considered incompatible. Specifically, tumor cells with molecular or histologic features of differentiated cells are generally less aggressive and less invasive than those resembling progenitor or stem cells. One of the best examples of this is chronic myeloid leukemia (CML) (Faderl et al., 1999
). Early-stage CML is associated with few signs or symptoms, because the cancer cells differentiate and remain relatively benign. However, in the later stage of CML, which is called blast crisis, the cancer cells fail to differentiate and are much more aggressive and invasive. Chemotherapy that induces tumor cell differentiation has proven to be an effective treatment for CML establishing the clinical significance of the inverse correlation between tumor cell differentiation and long-term survival of patients.
Rb and its related family members (p107 and p130) lie at the heart of the cell cycle machinery that executes cell cycle exit in coordination with terminal differentiation during development. Interestingly, studies in the developing CNS and other tissues have suggested that the Rb family also regulates differentiation in some tissues. In the developing retina, the individual roles of the Rb family members and their complex genetic compensation and redundancy have been extensively studied (Chen et al., 2004
; Donovan et al., 2006
; MacPherson et al., 2004
; Zhang et al., 2004a
). In postmitotic differentiated neurons of the inner nuclear layer (INL), p130 is expressed redundantly with Rb (Donovan et al., 2006
). Although proliferation is mildly deregulated in Rb;p130
-deficient retinae during development (MacPherson et al., 2004
), compensation by p107 facilitates relatively normal neurogenesis in the INL (Donovan et al., 2006
). Additional studies have shown that p107
is haploinsufficient for proliferative control in Rb
-deficient retina (Donovan et al., 2006
In this study, we tested the effect of reduced Rb family function in the developing retina by characterizing retinal development in p107-single (Chx10-Cre;RbLox/Lox;p107+/−;p130−/−) mice. We find that horizontal interneurons in the INL of the p107-single retina differentiate normally during development and form appropriate synaptic connections, but several weeks later, they re-enter the cell cycle and clonally expand. Dividing cells maintain all of the features of differentiated neurons including neurites and synapses. This is the first example of a differentiated neuron undergoing continued cell division while maintaining the hallmarks of mature neurons. The proliferating horizontal cells form highly aggressive retinoblastomas with many of the features of differentiated horizontal cells.