Frizzled/PCP-Dependent Asymmetric Neuralized Expression Determines R3/R4 Fates in the Drosophila Eye

Abstract

Planar cell polarity (PCP) is a common feature in many epithelia, reflected in cellular organization within the plane of an epithelium. In the Drosophila eye, Frizzled (Fz)/PCP signaling induces cell-fate specification of the R3/R4 photoreceptors through regulation of Notch activation in R4. Except for Dl upregulation in R3, the mechanism of how Fz/PCP signaling regulates Notch in this context is not understood. We demonstrate that the E3-ubiquitin ligase Neuralized (Neur), required for Dl-N signaling, is asymmetrically expressed within the R3/R4 pair. It is required in R3, where it is also upregulated in a Fz/PCP-dependent manner. As is the case for Dl, N activity in R4 further represses neur expression, thus, reinforcing the asymmetry. We demonstrate that Neur asymmetry is instructive in correct R3/R4 specification. Our data indicate that Fz/PCP-dependent Neur expression in R3 ensures the proper directionality of Dl-N signaling during R3/R4 specification.

Ral GTPase promotes asymmetric Notch activation in the Drosophila eye in response to Frizzled/PCP signaling by repressing ligand-independent receptor activation

“…The small effects observed for Ral could result, at least in part, from the Ral + function remaining in the three hypomorphic mutants we used. However, similar weak effects were observed in analogous experiments using a strong neur loss-offunction allele (del Alamo and Mlodzik, 2006 clones of otherwise wild-type eye disc cells had no effect on the pattern of Notch activation; m‫-ץ‬lacZ is still expressed specifically in the polar cell (pre-R4) (Fig. 8A,AЈ).…”

“…In fz -eyes, ommatidia are symmetrical, or their chirality is randomized (wildtype or reversed). Fz activation increases transcription of Delta and neur (Fanto and Mlodzik, 1999;del Alamo and Mlodzik, 2006) and may also repress Notch receptor activation in the equatorial cell (Strutt et al, 2002). To determine whether Ral PG69 activity depends on Fz signaling, we monitored -galactosidase (-gal) in fz -clones with Ral…”

A Screen for Modifiers of Notch Signaling Uncovers Amun, a Protein With a Critical Role in Sensory Organ Development

“…In the developing eye, Notch signaling first promotes formation of a single R8 photoreceptor cell within a group of equivalent proneural cells and then prevents neighboring cells from adopting the R8 fate, thereby restricting the number of R8 photoreceptors to one per ommatidium (Baker and Yu 1997;Baker 2000;Lee et al 2000). Notch signaling is subsequently required for differential specification of the R3/R4 photoreceptor fates (Del Alamo and Mlodzik 2006) and for induction of the R7 photoreceptor, cone cell, and primary pigment cell fates (Cooper and Bray 2000;Tomlinson and Struhl 2001;Tsuda et al 2002;Carthew 2007;Nagaraj and Banerjee 2007).…”

Ral inhibits ligand-independent Notch signaling in Drosophila

“…Frizzled signaling leads to elevated transcription in pre-R3 of the ligand Delta and also the Ubiquitinligase Neuralized. 28,31 Neuralized ubiquitylates Delta, a prerequisite for Delta endocytosis by pre-R3 and activation of Notch in pre-R4. 27 Alternatively or in addition, activated Frizzled relocalizes in a complex with a cortical protein called Disheveled to the R3/R4 interface where the complex prevents Notch receptor activation directly in pre-R3.…”

“…This difference in Fz activity is amplified by subsequent Notch activation in pre-R4. [28][29][30] Fz activation results in increased transcription of Delta and neuralized (neur) [28][29][30][31] and also relocalization of a Fz/ Dsh (Disheveled) complex that represses Notch receptor at the pre-R3 plasma membrane that faces pre-R4. 32 Thus pre-R3 becomes the Delta signaling cell and Notch is activated in pre-R4.…”