Proliferation/Quiescence: When to start? Where to stop? What to stock?

ABSTRACT: The cell cycle is a tightly controlled series of events that ultimately lead to cell division. The literature deciphering the molecular processes involved in regulating the consecutive cell cycle steps is colossal. By contrast, much less is known about non-dividing cellular states, even if they concern the vast majority of cells, from prokaryotes to multi-cellular organisms. Indeed, cells decide to enter the division cycle only if conditions are favourable. Otherwise they may enter quiescence, a reversible non-dividing cellular state. Recent studies in yeast have shed new light on the transition between proliferation and quiescence, re-questioning the notion of cell cycle commitment. They also indicate a predominant role for cellular metabolic status as a major regulator of quiescence establishment and exit. Additionally, a growing body of evidence indicates that environmental conditions, and notably the availability of various nutrients, by impinging on specific metabolic routes, directly regulate specific cellular re-organization that occurs upon proliferation/quiescence transitions.     

How to regulate a gene: to repress or to activate?

Gene-expression responses to an input can depend on growth conditions; in this issue, Sasson et al. (2012) show that this dependence is lower when the input results in a high degree of promoter occupancy.     

Antiprion immunotherapy: to suppress or to stimulate?

Although human prion diseases are rare, they are invariably fatal, and treatments remain elusive. Hundreds of iatrogenic prion transmissions have occurred in the past two decades, and the bovine spongiform encephalopathy epidemic has raised concerns about prion transmission from cattle to humans. Research into therapeutics for prion disease is being pursued in several centres and prominently includes immunological strategies. Currently, the options that are being explored aim either to mobilize the innate and adaptive immune systems towards prion destruction or to suppress or dedifferentiate the lymphoreticular compartments that replicate prions. This article reviews the pathophysiology of prion diseases in mouse models and discusses their relevance to immunotherapeutic and immunoprophylactic antiprion strategies.     

Whatever happened to ecology?

The holistic ecology of the early days had much in common with the world-view of primal people and of the romantic poets. In the 1950s a revival of reductionist and mechanistic ideas took place. The living world, constantly changing in a total "chance direction", appeared by its very nature "totally indestructible". The view of ecological succession toward a "climax" (Odum) became unacceptable for its assumed metaphysical and teleological connotations. Ecological succession is indeed only one special instance of sequential development, that is a feature of all life processes at all levels of organisation. Even the principle of "balance" was rejected by reductionists, because it recalled divine-order. Finally, the principle that "the whole is more than the sum of its parts" was abandoned, thus denying the most basic feature of the living world: organisation. Mainstream scientific ecology no longer reflects the ecological movement of today and may serve to justify the destruction that the irresponsible activities of the multinationals are giving rise to.