A combination of notch signaling, preferential adhesion and endocytosis induces a slow mode of cell intercalation in the Drosophila retina

Movement of epithelial cells in a tissue occurs through neighbor exchange and drives tissue shape changes. It requires intercellular junction remodeling, a process typically powered by the contractile actomyosin cytoskeleton. This has mostly been investigated in homogeneous epithelia where intercalation takes minutes. However, in some tissues, intercalation involves different cell types and can take hours. Whether slow and fast intercalation share the same mechanisms remains to be examined. To address this issue, we use the fly eye, where the cone cells exchange neighbors over approximately 10 hours to shape the lens. We uncover three pathways regulating this slow mode of cell intercalation. Firstly, we find a limited requirement for MyosinII. In this case, mathematical modeling predicts an adhesion dominant intercalation mechanism. Genetic experiments support this prediction and reveal a role for adhesion through the Nephrin proteins Roughest and Hibris. Secondly, we find cone cell intercalation is regulated by the Notch-signaling pathway. Thirdly, we show endocytosis is required for membrane removal and Notch activation. Altogether, our work indicates that adhesion, endocytosis and Notch can induce junction remodeling over long-time scales.

Light-mediated planar polarization of cone photoreceptor cilia contributes to visual acuity in mammals

Planar cell polarity (PCP) is essential to optimize information processing and functional response in many tissues. While the fly eye is a classic example of PCP, it remains unknown whether PCP exists in the mammalian retina and whether it plays a part in vision. Here we used 3D reconstructions of the mouse retina to show that the basal body of cone photoreceptor cilia, but not rods, is systematically located on the side of the cell facing the centre of the retina. We further show that light is required during a critical window of development to establish cone PCP, and that both cone transducin and the G-protein signaling modulator protein 2 are required to mediate this effect. Importantly, we report that disruption of cone PCP impairs visual acuity. This work uncovers a non-canonical PCP pathway, mediated by light, and identifies cone PCP as a feature supporting mammalian vision.

New Tools for the Optimized Follow-Up of Imminent Impactors

The solar system is populated with, other than planets, a wide variety of minor bodies, the majority of which are represented by asteroids. Most of their orbits are comprised of those between Mars and Jupiter, thus forming a population named Main Belt. However, some asteroids can run on trajectories that come close to, or even intersect, the orbit of the Earth. These objects are known as Near Earth Asteroids (NEAs) or Near Earth Objects (NEOs) and may entail a risk of collision with our planet. Predicting the occurrence of such collisions as early as possible is the task of Impact Monitoring (IM). Dedicated algorithms are in charge of orbit determination and risk assessment for any detected NEO, but their efficiency is limited in cases in which the object has been observed for a short period of time, as is the case with newly discovered asteroids and, more worryingly, imminent impactors: objects due to hit the Earth, detected only a few days or hours in advance of impacts. This timespan might be too short to take any effective safety countermeasure. For this reason, a necessary improvement of current observation capabilities is underway through the construction of dedicated telescopes, e.g., the NEO Survey Telescope (NEOSTEL), also known as “Fly-Eye”. Thanks to these developments, the number of discovered NEOs and, consequently, imminent impactors detected per year, is expected to increase, thus requiring an improvement of the methods and algorithms used to handle such cases. In this paper we present two new tools, based on the Admissible Region (AR) concept, dedicated to the observers, aiming to facilitate the planning of follow-up observations of NEOs by rapidly assessing the possibility of them being imminent impactors and the remaining visibility time from any given station.

Some assembly required: building the fly eye for motion detection and colour discrimination

Among the many eyes that have evolved on Earth, the insect compound eye is the most abundant. Its crystal-like lattice structure is a feat of engineering that has evolved over millions of years, and is exquisitely adapted to detect moving objects and discriminate colours. This enables many behaviours, including foraging for food, finding a mate and avoiding predators. Our understanding of how the compound eye is built and works has been greatly expanded by studying the humble fruit fly, Drosophila melanogaster. The simple outward appearance of the fly eye belies a host of sophisticated features. Through the precise arrangement of photosensitive cells in the retina and their connections to the brain, the fly eye packs an astonishing amount of hardware into a very tiny volume. In this primer, we introduce the molecular pathways that underpin the building and inner workings of the fly eye.

Interdependent regulation of stereotyped and stochastic photoreceptor fates in the fly eye

Diversification of neuronal subtypes often requires stochastic gene regulatory mechanisms. How stochastically expressed transcription factors interact with other regulators in gene networks to specify cell fates is poorly understood. The random mosaic of color-detecting R7 photoreceptor subtypes in Drosophila is controlled by the stochastic on/off expression of the transcription factor Spineless (Ss). In SsON R7s, Ss induces expression of Rhodopsin 4 (Rh4), whereas in SsOFF R7s, the absence of Ss allows expression of Rhodopsin 3 (Rh3). Here, we find that the transcription factor Runt, which is initially expressed in all R7s, activates expression of Spineless in a random subset of R7s. Later, as R7s develop, Ss negatively feeds back onto Runt to prevent repression of Rh4 and ensure proper fate specification. Together, stereotyped and stochastic regulatory inputs are integrated into feedforward and feedback mechanisms to control cell fate.

Genetic interactions between Protein Kinase D and Lobe mutants during eye development of Drosophila melanogaster

Background In Drosophila, the development of the fly eye involves the activity of several, interconnected pathways that first define the presumptive eye field within the eye anlagen, followed by establishment of the dorso-ventral boundary, and the regulation of growth and apoptosis. In Lobe (L) mutant flies, parts of the eye or even the complete eye are absent because the eye field has not been properly defined. Manifold genetic interactions indicate that L influences the activity of several signalling pathways, resulting in a conversion of eye tissue into epidermis, and in the induction of apoptosis. As information on the molecular nature of the L mutation is lacking, the underlying molecular mechanisms are still an enigma. Results We have identified Protein Kinase D (PKD) as a strong modifier of the L mutant phenotype. PKD belongs to the PKC/CAMK class of Ser/Thr kinases that have been involved in diverse cellular processes including stress resistance and growth. Despite the many roles of PKD, Drosophila PKD null mutants are without apparent phenotype apart from sensitivity to oxidative stress. Here we report an involvement of PKD in eye development in the sensitized genetic background of Lobe. Absence of PKD strongly enhanced the dominant eye defects of heterozygous L2 flies, and decreased their viability. Moreover, eye-specific overexpression of an activated isoform of PKD considerably ameliorated the dominant L2 phenotype. This genetic interaction was not allele specific but similarly seen with three additional, weaker L alleles (L1, L5, LG), demonstrating its specificity. Conclusions We propose that PKD-mediated phosphorylation is involved in underlying processes causing the L phenotype, i.e. in the regulation of growth, the epidermal transformation of eye tissue and apoptosis, respectively.