Lamina-associated polypeptide (LAP) 2alpha is a chromatin-associated protein that binds A-type lamins. Mutations in both LAP2alpha and A-type lamins are linked to human diseases called laminopathies, but the molecular mechanisms are poorly understood. The A-type lamin-LAP2alpha complex interacts with and regulates retinoblastoma protein (pRb), but the significance of this interaction in vivo is unknown. Here we address the function of the A-type lamin-LAP2alpha complex with the use of LAP2alpha-deficient mice. We show that LAP2alpha loss causes relocalization of nucleoplasmic A-type lamins to the nuclear envelope and impairs pRb function. This causes inefficient cell-cycle arrest in dense fibroblast cultures and hyperproliferation of epidermal and erythroid progenitor cells in vivo, leading to tissue hyperplasia. Our results support a disease-relevant model in which LAP2alpha defines A-type lamin localization in the nucleoplasm, which in turn affects pRb-mediated regulation of progenitor cell proliferation and differentiation in highly regenerative tissues.

In mammals, including humans, nearly all physiological processes are subject to daily oscillations that are governed by a circadian timing system with a complex hierarchical structure. The central pacemaker, residing in the suprachiasmatic nucleus (SCN) of the ventral hypothalamus, is synchronized daily by photic cues transmitted from the retina to SCN neurons via the retinohypothalamic tract. In turn, the SCN must establish phase coherence between self-sustained and cell-autonomous oscillators present in most peripheral cell types. The synchronization signals (Zeitgebers) can be controlled more or less directly by the SCN. In mice and rats, feeding-fasting rhythms, which are driven by the SCN through rest-activity cycles, are the most potent Zeitgebers for the circadian oscillators of peripheral organs. Signaling through the glucocorticoid receptor and the serum response factor also participate in the phase entrainment of peripheral clocks, and these two pathways are controlled by the SCN independently of feeding-fasting rhythms. Body temperature rhythms, governed by the SCN directly and indirectly through rest -activity cycles, are perhaps the most surprising cues for peripheral oscillators. Although the molecular makeup of circadian oscillators is nearly identical in all cells, these oscillators are used for different purposes in the SCN and in peripheral organs.

Mammalian genes are often transcribed discontinuously as short bursts of RNA synthesis followed by longer silent periods. However, how these "on" and "off" transitions, together with the burst sizes, are modulated in single cells to increase gene expression upon stimulation is poorly characterized. By combining single-cell time-lapse luminescence imaging with stochastic modeling of the time traces, we quantified the transcriptional responses of the endogenous connective tissue growth factor gene to different physiological stimuli: serum and TGF-β1. Both stimuli caused a rapid and acute increase in burst sizes. Whereas TGF-β1 showed prolonged transcriptional activation mediated by an increase of transcription rate, serum stimulation resulted in a large and temporally tight first transcriptional burst, followed by a refractory period in the range of hours. Our study thus reveals how different physiological stimuli can trigger kinetically distinct transcriptional responses of the same gene.stochastic gene expression | single-cell dynamics | computational modeling

In mammals, body temperature fluctuates diurnally around a mean value of 36°C-37°C. Despite the small differences between minimal and maximal values, body temperature rhythms can drive robust cycles in gene expression in cultured cells and, likely, animals. Here we studied the mechanisms responsible for the temperaturedependent expression of cold-inducible RNA-binding protein (CIRBP). In NIH3T3 fibroblasts exposed to simulated mouse body temperature cycles, Cirbp mRNA oscillates about threefold in abundance, as it does in mouse livers. This daily mRNA accumulation cycle is directly controlled by temperature oscillations and does not depend on the cells' circadian clocks. Here we show that the temperature-dependent accumulation of Cirbp mRNA is controlled primarily by the regulation of splicing efficiency, defined as the fraction of Cirbp pre-mRNA processed into mature mRNA. As revealed by genome-wide "approach to steady-state" kinetics, this post-transcriptional mechanism is widespread in the temperature-dependent control of gene expression.[Keywords: Cirbp; splicing efficiency; temperature; circadian rhythms] Supplemental material is available for this article. Temperature is a fundamental physical parameter that influences metabolism in all organisms by modulating the rate of biochemical reactions. Endothermic organisms like mammals have acquired special thermoregulatory adaptation mechanisms in order to decrease their dependence on environmental temperature. These mechanisms maintain core body temperature (CBT) within a favorable range, allowing efficient metabolic activity throughout diurnal and seasonal cycles (for review, see Tattersall 2012).Although regulated around a species-specific set point, mammalian CBT shows diurnal oscillations of ∼0.4°C-6.0°C (for review, see Refinetti 2010). These regular oscillations are generated under the control of the circadian timekeeping system, which is composed of a master pacemaker in the brain's suprachiasmatic nucleus (SCN) and subsidiary peripheral clocks in nearly all other cells of the body (Dibner et al. 2010). Although peripheral circadian oscillators are self-sustained and cell-autonomous, they must be synchronized by the SCN and/or environmental Zeitgebers (timing cues), such as light-dark cycles, in order to maintain phase coherence and rhythmic physiology in the body. Without a functional SCN, laboratory rodents kept in constant darkness lose virtually all overt rhythms in behavior and physiology, including CBT cycles (Refinetti et al. 1994).While the period length (τ) of circadian oscillations is remarkably resilient to temperature variations, a phenomenon called temperature compensation, the phase is exquisitely sensitive to temperature perturbations, in particular in peripheral cells. In fact, in cultured fibroblasts and tissue explants, the phase of circadian clock gene expression can be perfectly synchronized by simulated CBT cycles (Brown et al. 2002;Buhr et al. 2010;Saini et al. 2012).In intact animals, the systemic synchronization cues governed by t...