Delilah, prospero, and D-Pax2 constitute a gene regulatory network essential for the development of functional proprioceptors.

Coordinated animal locomotion depends on the development of functional proprioceptors. While early cell-fate determination processes are well characterized, little is known about the terminal differentiation of cells within the proprioceptive lineage and the genetic networks that control them. In this work we describe a gene regulatory network consisting of three transcription factors-Prospero (Pros), D-Pax2, and Delilah (Dei)-that dictates two alternative differentiation programs within the proprioceptive lineage in Drosophila. We show that D-Pax2 and Pros control the differentiation of cap versus scolopale cells in the chordotonal organ lineage by, respectively, activating and repressing the transcription of dei. Normally, D-Pax2 activates the expression of dei in the cap cell but is unable to do so in the scolopale cell where Pros is co-expressed. We further show that D-Pax2 and Pros exert their effects on dei transcription via a 262 bp chordotonal-specific enhancer in which two D-Pax2- and three Pros-binding sites were identified experimentally. When this enhancer was removed from the fly genome, the cap- and ligament-specific expression of dei was lost, resulting in loss of chordotonal organ functionality and defective larval locomotion. Thus, coordinated larval locomotion depends on the activity of a dei enhancer that integrates both activating and repressive inputs for the generation of a functional proprioceptive organ.

A newly identified type of attachment cell is critical for normal patterning of chordotonal neurons.

This work describes unknown aspects of chordotonal organ (ChO) morphogenesis revealed in post-embryonic stages through the use of new fluorescently labeled markers. We show that towards the end of embryogenesis a hitherto unnoticed phase of cell migration commences in which the cap cells of the ventral ChOs elongate and migrate towards their prospective attachment sites. This migration and consequent cell attachment generates a continuous zigzag line of proprioceptors, stretching from the ventral midline to a dorsolateral position in each abdominal segment. Our observation that the cap cell of the ventral-most ChO attaches to a large tendon cell near the midline provides the first evidence for a direct physical connection between the contractile and proprioceptive systems in Drosophila. Our analysis has also provided an answer to a longstanding enigma that is what anchors the neurons of the ligamentless ventral ChOs on their axonal side. We identified a new type of ChO attachment cell, which binds to the scolopale cells of these organs, thus behaving like a ligament cell, but on the other hand exhibits all the typical features of a ChO attachment cell and is critical for the correct anchoring of these organs.

Deconstructing the complexity of regulating common properties in different cell types: lessons from the delilah gene.

To understand development we need to understand how transcriptional regulatory mechanisms are employed to confer different cell types with their unique properties. Nonetheless it is also critical to understand how such mechanisms are used to confer different cell types with common cellular properties, such as the ability to adhere to the extracellular matrix. To decode how adhesion is regulated in cells stemming from different pedigrees we analyzed the regulatory region that drives the expression of Dei, which is a transcription factor that serves as a central determinant of cell adhesion, particularly by inducing expression of ßPS-integrin. We show that activation of dei transcription is mediated through multiple cis regulatory modules, each driving transcription in a spatially and temporally restricted fashion. Thus the dei gene provides a molecular platform through which cell adhesion can be regulated at the transcriptional level in different cellular milieus. Moreover, we show that these regulatory modules respond, often directly, to central regulators of cell identity in each of the dei-expressing cell types, such as D-Mef2 in muscle cells, Stripe in tendon cells and Blistered in wing intervein cells. These findings suggest that the acquirement of common cellular properties shared by different cell types is embedded within the unique differentiation program dictated to each of these cells by the major determinants of its identity.

Visualization of proprioceptors in Drosophila larvae and pupae.

Proprioception is the ability to sense the motion, or position, of body parts by responding to stimuli arising within the body. In fruitflies and other insects proprioception is provided by specialized sensory organs termed chordotonal organs (ChOs). Like many other organs in Drosophila, ChOs develop twice during the life cycle of the fly. First, the larval ChOs develop during embryogenesis. Then, the adult ChOs start to develop in the larval imaginal discs and continue to differentiate during metamorphosis. The development of larval ChOs during embryogenesis has been studied extensively. The centerpiece of each ChO is a sensory unit composed of a neuron and a scolopale cell. The sensory unit is stretched between two types of accessory cells that attach to the cuticle via specialized epidermal attachment cells. When a fly larva moves, the relative displacement of the epidermal attachment cells leads to stretching of the sensory unit and consequent opening of specific transient receptor potential vanilloid (TRPV) channels at the outer segment of the dendrite. The elicited signal is then transferred to the locomotor central pattern generator circuit in the central nervous system. Multiple ChOs have been described in the adult fly. These are located near the joints of the adult fly appendages (legs, wings and halters) and in the thorax and abdomen. In addition, several hundreds of ChOs collectively form the Johnston's organ in the adult antenna that transduce acoustic to mechanical energy. In contrast to the extensive knowledge about the development of ChOs in embryonic stages, very little is known about the morphology of these organs during larval stages. Moreover, with the exception of femoral ChOs and Johnston's organ, our knowledge about the development and structure of ChOs in the adult fly is very fragmentary. Here we describe a method for staining and visualizing ChOs in third instar larvae and pupae. This method can be applied together with genetic tools to better characterize the morphology and understand the development of the various ChOs in the fly.

Spatial regulation of cell adhesion in the Drosophila wing is mediated by Delilah, a potent activator of ßPS integrin expression.

In spite of our conceptual view of how differential gene expression is used to define different cell identities, we still do not understand how different cell identities are translated into actual cell properties. The example discussed here is that of the fly wing, which is composed of two main cell types: vein and intervein cells. These two cell types differ in many features, including their adhesive properties. One of the major differences is that intervein cells express integrins, which are required for the attachment of the two wing layers to each other, whereas vein cells are devoid of integrin expression. The major signaling pathways that divide the wing to vein and intervein domains have been characterized. However, the genetic programs that execute these two alternative differentiation programs are still very roughly drawn. Here we identify the bHLH protein Delilah (Dei) as a mediator between signaling pathways that specify intervein cell-fate and one of the most significant realizators of this fate, ßPS integrin. Dei's expression is restricted to intervein territories where it acts as a potent activator of ßPS integrin expression. In the absence of normal Dei activity the level of ßPS integrin is reduced, leading to a failure of adhesion between the dorsal and ventral wing layers and a consequent formation of wing blisters. The effect of Dei on ßPS expression is not restricted to the wing, suggesting that Dei functions as a general genetic switch, which is turned on wherever a sticky cell-identity is determined and integrin-based adhesion is required.