Dynamic Filopodia Transmit Intermittent Delta-Notch Signaling to Drive Pattern Refinement during Lateral Inhibition

Abstract

The organization of bristles on the Drosophila notum has long served as a popular model of robust tissue patterning. During this process, membrane-tethered Delta activates intracellular Notch signaling in neighboring epithelial cells, which inhibits Delta expression. This induces lateral inhibition, yielding a pattern in which each Delta-expressing mechanosensory organ precursor cell in the epithelium is surrounded on all sides by cells with active Notch signaling. Here, we show that conventional models of Delta-Notch signaling cannot account for bristle spacing or the gradual refinement of this pattern. Instead, the pattern refinement we observe using live imaging is dependent upon dynamic, basal actin-based filopodia and can be quantitatively reproduced by simulations of lateral inhibition incorporating Delta-Notch signaling by transient filopodial contacts between nonneighboring cells. Significantly, the intermittent signaling induced by these filopodial dynamics generates a type of structured noise that is uniquely suited to the generation of well-ordered, tissue-wide epithelial patterns.

A dynamical model of ommatidial crystal formation

“…The majority of the parameter sets that gave the correct pattern were biologically plausible, with decay lengths (i.e., the typical distance over which morphogen concentration drops by 1/e) and time scales for concentration changes consistent with the known properties of the proteins (3,4,(27)(28)(29). Working parameter sets shared several characteristics.…”

“…Similarly, a picture of planar cells communicating only with their nearest neighbors likely underestimates the range of Notch signaling. For example, more cells might be in contact across the entire depth of the epithelium than can be represented in a planar model, whether through filipodia (25,27) or through other complex packing geometries. Indeed, genetic mosaic studies suggest that Dl's nonautonomy extends beyond obviously adjacent cells (19,45).…”

“…We condense the known players into several lumped variables, each of which can be thought of as representing a given type of regulatory interaction rather than a specific gene: The variable u takes the place of all nonautonomous inhibition and h of the several nonautonomous activators that drive the MF; s implements delayed, cell-autonomous positive feedback; and a self-activates and is a marker of the R8 fate. Both h and u are allowed to move from cell to cell; our mathematical formulation is sufficiently broad to encompass both simple molecular diffusion and more general forms of nondirectional transport (25)(26)(27). Because the R8 pattern is ultimately defined on a single-cell level, we use a disordered array of discrete cells rather than a continuum model.…”

Dispatched mediates Hedgehog basolateral release to form the long-range morphogenetic gradient in the Drosophila wing disk epithelium

“…In contrast to the previously described apical cytonemes (52), the extensions we visualize are mainly found at the basal part of the disk epithelium. Interestingly, in the context of Notch signaling, basal actin-based filopodia are important for lateral inhibition between nonneighboring cells (53,54). However, further investigation will be necessary to demonstrate the implication of cytonemes in Hh gradient formation.…”

New Model for Understanding Mechanisms of Biological Signaling: Direct Transport via Cytonemes

“…22 They have been observed in multiple biological systems but in many cases their functions are not clear. 22,23,25,26,28,30,33 These tubular objects are supported by actin networks, they can extend and retract quite fast, and their tips can attach to other cells. 22 The idea behind the direct delivery mechanism is that the morphogens are moving along the cytonemes, probably along the actin filaments with the help from motor proteins, starting from the source cell directly to the specific cell.…”