Shape transitions in developing organisms can be driven by active stresses, notably, active contractility generated by myosin motors. The mechanisms generating tissue folding are typically studied in epithelia. There, the interaction between cells is also coupled to an elastic substrate, presenting a major difficulty for studying contraction induced folding. Here we study the contraction and buckling of active, initially homogeneous, thin elastic actomyosin networks isolated from bounding surfaces. The network behaves as a poroelastic material, where a flow of fluid is generated during contraction. Contraction starts at the system boundaries, proceeds into the bulk, and eventually leads to spontaneous buckling of the sheet at the periphery. The buckling instability resulted from system self-organization and from the spontaneous emergence of density gradients driven by the active contractility. The buckling wavelength increases linearly with sheet thickness. Our system offers a well-controlled way to study mechanically induced, spontaneous shape transitions in active matter.

In the developing retina, the production of ganglion cells is dependent on the proneural proteins NGN2 and ATH5, whose activities define stages along the pathway converting progenitors into newborn neurons. Crossregulatory interactions between NGN2, ATH5 and HES1 maintain the uncommitted status of ATH5-expressing cells during progenitor patterning, and later on regulate the transition from competence to cell fate commitment. Prior to exiting the cell cycle, a subset of progenitors is selected from the pool of ATH5-expressing cells to go through a crucial step in the acquisition of a definitive retinal ganglion cell fate. The selected cells are those in which the upregulation of NGN2, the downregulation of HES1 and the autostimulation of ATH5 are coordinated with the progression of progenitors through the last cell cycle. This coordinated pattern initiates the transcription of ganglion cell-specific traits and determines the size of the ganglion cell population.

The atonal homolog 5 (ATH5) protein is central to the transcriptional network regulating the specification of retinal ganglion cells, and its expression comes under the spatiotemporal control of several basic helix-loop-helix (bHLH) proteins in the course of retina development. Monitoring the in vivo occupancy of the ATH5 promoter by the ATH5, Ngn2, and NeuroM proteins and analyzing the DNA motifs they bind, we show that three evolutionarily conserved E-boxes are required for the bHLH proteins to control the different phases of ATH5 expression. E-box 4 mediates the activity of Ngn2, ATH5, and NeuroM along the pathway leading to the conversion of progenitors into newborn neurons. E-box 1, by mediating the antagonistic effects of Ngn2 and HES1 in proliferating progenitors, controls the expansion of the ATH5 expression domain in early retina. E-box 2 is required for the positive feedback by ATH5 that underlies the up-regulation of ATH5 expression when progenitors are going through their last cell cycle. The combinatorial nature of the regulation of the ATH5 promoter suggests that the bHLH proteins involved have no assigned E-boxes but use a common set at which they either cooperate or compete to finely tune ATH5 expression as development proceeds.Retina development in vertebrates relies on regulatory proteins, most of which are widely expressed in the developing nervous system. However, the expression of the basic helixloop-helix protein atonal homolog 5 (ATH5) 3 appears to be restricted to retina ontogenesis. ATH5 activates neurogenesis and is required for the production of retinal ganglion cells (RGCs) (1-4). It is transiently expressed during the period of development when RGCs are produced and underlies the pathway leading to the conversion of proliferating progenitors into newborn RGCs (5, 6). Retinotopic differences in the timing of RGC production may reflect the wave-like expression of ATH5 (5, 7). In zebrafish, a signal from the optic stalk appears to induce the first patch of ATH5-expressing cells, and the continued spread of ATH5 expression beyond that patch may require a cascade of cell-to-cell interactions (7,8). It has been suggested that Sonic hedgehog derived from newborn RGCs drives a self-propagating wave of ATH5 expression and RGC production across the zebrafish retina (9, 10). In contrast, related experiments in zebrafish and chick highlight the importance of intrinsic factors for triggering ATH5 expression and neurogenesis (11-13).The presence of consensus E-box binding sites in the highly conserved upstream sequence of the ATH5 gene suggests that bHLH transcription factors are directly involved in the regulation of ATH5. This idea is supported by experiments showing the selective binding of neuronal bHLH proteins to the upstream sequence of ATH5 and the simultaneous changes in the expression level of ATH5 in response to several of these proteins (5, 6, 14 -16). These findings indicate a requirement for different combinations of bHLH proteins in regulating the different phases of ATH5 expression...

Genetic studies in Drosophila and in vertebrates have implicated basic helix-loop-helix (bHLH) transcription factors in neural determination and differentiation. In this report, we analyze the role that several bHLH proteins play in the transcriptional control of differentiation in chick retina. Our experimental system exploits the properties of the promoter for the beta 3 subunit of the neuronal acetylcholine receptors, important components of various phenotypes in the CNS of vertebrates. The beta 3 subunit contributes to define ganglion cell identity in retina and its promoter, whose activation is an early marker of ganglion cell differentiation, is under the specific control of the chick atonal homolog ATH5. Functional analysis of the ATH5 promoter indicates that interactions between ATH5 and several other bHLH transcription factors underlie the patterning of the early retinal neuroepithelium and form a regulatory cascade leading to transcription of the gene for beta 3. ATH5 appears to coordinate the transcriptional pathways that control pan-neuronal properties with those that regulate the subtype-specific features of retinal neurons.