Transcription of messenger RNAs (mRNAs) for Notch signaling molecules oscillates with 2-hour cycles, and this oscillation is important for coordinated somite segmentation. However, the molecular mechanism of such oscillation remains to be determined. Here, we show that serum treatment of cultured cells induces cyclic expression of both mRNA and protein of the Notch effector Hes1, a basic helix-loop-helix (bHLH) factor, with 2-hour periodicity. Cycling is cell-autonomous and depends on negative autoregulation of hes1 transcription and ubiquitin-proteasome-mediated degradation of Hes1 protein. Because Hes1 oscillation can be seen in many cell types, this clock may regulate timing in many biological systems.

Spontaneous collective motion, as in some flocks of bird and schools of fish, is an example of an emergent phenomenon. Such phenomena are at present of great interest and physicists have put forward a number of theoretical results that so far lack experimental verification. In animal behaviour studies, large-scale data collection is now technologically possible, but data are still scarce and arise from observations rather than controlled experiments. Multicellular biological systems, such as bacterial colonies or tissues, allow more control, but may have many hidden variables and interactions, hindering proper tests of theoretical ideas. However, in systems on the subcellular scale such tests may be possible, particularly in in vitro experiments with only few purified components. Motility assays, in which protein filaments are driven by molecular motors grafted to a substrate in the presence of ATP, can show collective motion for high densities of motors and attached filaments. This was demonstrated recently for the actomyosin system, but a complete understanding of the mechanisms at work is still lacking. Here we report experiments in which microtubules are propelled by surface-bound dyneins. In this system it is possible to study the local interaction: we find that colliding microtubules align with each other with high probability. At high densities, this alignment results in self-organization of the microtubules, which are on average 15 µm long, into vortices with diameters of around 400 µm. Inside the vortices, the microtubules circulate both clockwise and anticlockwise. On longer timescales, the vortices form a lattice structure. The emergence of these structures, as verified by a mathematical model, is the result of the smooth, reptation-like motion of single microtubules in combination with local interactions (the nematic alignment due to collisions)--there is no need for long-range interactions. Apart from its potential relevance to cortical arrays in plant cells and other biological situations, our study provides evidence for the existence of previously unsuspected universality classes of collective motion phenomena.

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Notch signaling components such as the basic helix-loop-helix gene Hes1 are cyclically expressed by negative feedback in the presomitic mesoderm (PSM) and constitute the somite segmentation clock. Because Hes1 oscillation occurs in many cell types, this clock may regulate the timing in many biological systems. Although the Hes1 oscillator is stable in the PSM, it damps rapidly in other cells, suggesting that the oscillators in the former and the latter could be intrinsically different. Here, we have established the real-time bioluminescence imaging system of Hes1 expression and found that, although Hes1 oscillation is robust and stable in the PSM, it is unstable in the individual dissociated PSM cells, as in fibroblasts. Thus, the Hes1 oscillators in the individual PSM cells and fibroblasts are intrinsically similar, and these results, together with mathematical simulation, suggest that cell-cell communication is essential not only for synchronization but also for stabilization of cellular oscillators.Hes1 ͉ oscillation ͉ luciferase ͉ basic helix-loop-helix gene S omites, precursors for the segmental structures such as the vertebral column, ribs, and skeletal muscles, are generated in a head-to-tail order by periodic segmentation of the anterior end of the presomitic mesoderm (PSM). This periodic event is regulated by the somite segmentation clock, which is composed of Notch and Wnt signaling molecules (1-6). In the PSM, Notch components such as the basic helix-loop-helix genes Hes1 and Hes7 are cyclically expressed, and each cycle leads to segmentation of a bilateral pair of somites (7-11). This oscillatory expression occurs in a synchronous manner but with the caudalto-rostral phase delay, resulting in wave-like propagation of the expression domains from the caudal to the rostral direction. It has been shown that this oscillatory expression depends upon a negative feedback loop (12-18).Interestingly, Hes1 oscillation occurs in many cell types in addition to the PSM after serum treatment or activation of Notch signaling, suggesting that this clock may regulate the timing in many biological systems (12). Although Hes1 oscillation is stable in the PSM, it is damped after three to six cycles in other cells, raising the possibility that the Hes1 oscillator of the PSM cells is intrinsically different from that of other cell types (8, 12). However, the damping could result not only from damped oscillation in each cell but also from desynchronization between cells, and it is not clear which is the case. It was shown that PSM cells could become desynchronized when they are dissociated (19), but the nature of the segmentation clock in individual PSM cells remains to be determined.To understand the dynamics of the somite segmentation clock, we attempted real-time imaging of Hes1 expression in the PSM and the dissociated individual PSM cells. Here, we found that Hes1 oscillation is stable (both the period and amplitude are relatively constant) in the PSM but unstable (the period and amplitude are variable) in the individu...