| Cell cycle exit ("birth") of retinal progenitor cells (RPCs) is considered a watershed that is preceded by changing levels of cell cycle regulators, and followed rapidly by induction of a post M-phase differentiation cascade. Yet the actual dynamics of these events are largely unclear, thus whether mitosis separates pre- and post- birth differentiation cascades is unproven. We characterized the regulation of many division and differentiation markers relative to each other and final mitosis. Unexpectedly, classic "cell cycle" markers were present well beyond exit (e.g. Ki67, Pcna), early embryonic RPCs expressed "differentiation" markers that later labeled post-mitotic neurons exclusively (e.g. Brn3b, Tubb3, Ptf1a), and factors detected just after cell birth in the embryo were induced well beyond M-phase post-natally (e.g. Nrl, Crx). Thus, the dynamics of birth-associated events shift dramatically during development, even to either side of mitosis. Instead of mitosis behaving as a cog that activates post-exit differentation events we suggest that a common trigger induces both the exit and differentiation programs in RPCs, precisely coordinating their startpoints, but that each subsequent cascade unfolds independently. This model explains the convergence of birth and differentiation but also their temporal maliability. This view fits with our observation that in the absence of the Rb tumor suppressor, differentiation still initiates even without cell cycle exit. Finally, neoplastic transformation in the mouse retina requires loss of Rb and its relative p107, and emerging tumor features suggest an amacrine cell-of-origin. We studied Rb/p107 null clones, and noted two striking features. First, despite initial expansion of aberrantly dividing differentiating cells, apoptosis pruned clones precisely to wild type sizes. "Cell competition" maintains tissue size by selecting fitter over weaker progenitors; our data provide a unique example of competition among differentiating cells. Second, despite normal numbers of amacrine cells per Rb/p107 null clone, more clones contained amacrine cells and fewer had bipolar cells. Both this effect and ectopic division were E2f1-dependent. Thus, the oncogenic initiation event in mouse retinoblastoma triggers a very early fate switch, even before neoplastic transformation, broadening the possibilities for the cell-of-origin of retinoblastoma, and arguing that even very early stage tumors cannot be used to define cancer origin. |
