Signal Transduction
FIG 22.9 Intracellular trafficking of the Notch receptor.
Intracellular trafficking of the Notch receptor has negative and positive effects on its signalling capacity. Receptor endocytosis and transport to early sorting endosomes (Rab5 marker) normally result in partial destruction in multivesicular bodies and then total degradation in the lysosome (Rab7 marker). This effectively removes the receptor from the surface. Alternatively, endocytosis may positively regulate signalling. First, it protects the receptor from destruction and conducting it to recycling endosomes (Rab11 marker) assisted by Deltex-mediated ubiquitylation, which provides a novel sorting signal. From here
the receptor returns to the membrane, perhaps in a modified version (indicated by red asterisks) or at a membrane domain more favourable for ligand binding. Secondly, endocytosis also favours proteolysis of the S2-site by bringing the receptor to the compartment that harbours -secretase (early and late endosomes). Dynamin (D) and Rabs 5, 7, and 11 are marked by blue numerals. Red spots indicate Itch or Nedd, Blue spots indicate Deltex.
The embryonic life of a fruit fly is brief. It hatches as a first instar larva and then moults successively through second and third instar stages. All three
of these instar stages are dedicated to eating. Finally, after a copious
intake of food, it pupates. All this takes about 4 days. It then remains as a pupa for another 5 days, at the end of which it emerges as an adult fly.
Notch and sensory progenitor cells of Drosophila; the importance of endocytosis
Here we discuss the formation of cuticular patterns, the cellular machinery that generates them, and the genetic circuitry that organizes the machinery. All the events described start at the pupal stage, where the imaginal discs, already visible in third-instar larvae, expand and develop into the body parts of the adult fly. This process is known as metamorphosis (Figure 22.11). During this process most of the larval structures are destroyed through apoptosis and replaced by tissue derived from the discs, giving rise to thorax, wings, legs, antennae, and genitalia (though not the digestive system). A full-grown
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FIG 22.10 Domain architecture of proteins that affect Notch signalling.
Numb and Epsin link Sanpodo and Notch with the endocytotic machinery (AP-2 and clathrin), whereas dynamin is required for fission of the endocytotic vesicle. Neuralized, Itch, NEDD4 and Deltex all are E3–ubiquitin ligases that ubiquitylate the Notch receptor and its ligands, thereby providing sorting signals for the endocytotic machinery.
larva is 6 mm long, whereas the adult fruit fly is only half that size. When the adults emerge from the pupae, they are fully formed. They become fertile after 10 h, copulate, the females lay eggs, and the cycle, of roughly 12–14 days, begins again.
Development of mechanoreceptors on thorax and wing
The pair of large wing discs produce both wings and the dorsal site of the thorax (notum) (see Figures 22.11 and 22.12). We focus on the proneural clusters, small patches of cells within these discs. They are detected by expression of neurogenic markers such as the genes achaete, scute, and daughterless (Figure 22.11b), the products of which give epidermal cells the ability to become sensory organ precursors. The cells surrounding the proneural clusters express high levels of Hairy, a bHLH protein that suppresses the neurogenic genes. Hairy belongs to same family as the previously described
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Signal Transduction
Macrochaetes and microchaetes: Precursors of the macrochaetes appear early in the larval period. Due to the long period of intervening growth before metamorphosis, these bristles are large and widely spaced. Precursors of the microchaetes appear later, during the pupal stage, and as a result they are smaller and more closely spaced. Macrochaetes are longer, thicker, and stouter
than microchaetes, and they are arranged into a stereotyped array. Each macrochaete has a
specific axonal projection pattern in the thoracic ganglion that depends upon the position
at which the bristle precursor first emerges.58 The notal macrochaetes of Drosophila have directional sensitivity, their neurons responding to movements of the shaft in a preferred axis. Although they display some regional specificity, microchaetes are variable in number and position and do not appear to have individually defined functions.59
FIG 22.11 Imaginal discs of a third-instar larva and the corresponding body parts of the adult fly.
(a)The imaginal discs harbour cells that form the different body parts of the adult fly during the process of metamorphosis,. Here, only the eye and the wing imaginal discs are indicated (by double-headed arrows). The abdomen arises from histoblast nests. Note that the adult is much smaller than the larva, most of the cells outside the imaginal discs having been removed by apoptosis.
Image adapted with permission from V. Hartenstein, http://flybase.bio.indiana.edu/
(b)The imaginal wing disc contains clusters of cells, proneural clusters, which express the neurogenic genes achaete/scute. These clusters later form the mechanosensory organs. In each selected cluster, just one cell that expresses a particularly high level of achaete/scute retains the neural fate. The others develop as epidermal cells. Image of immunochemical staining adapted from Cubas et al.57
E(spl), Hes, and Hey proteins, but it is regulated differently. These achaete- expressing proneural clusters give rise to the bristle bearing sensory organs (mechanosensors) that adorn the dorsal side of the thorax and the edges of the wings (also known as macroand microchaetes).58,59 Mechanoreceptors are also abundantly present on the abdomen, legs, and eyes, but these arise from other imaginal discs. (These structures should not be confused with the wing hairs, which are much smaller and arise from epidermal cells, not neuronal cells; see page 421.)
Within the proneural clusters, only the single cell, the sensory organ precursor cell (SOP) has the privilege of developing the organ, providing the neuronal glia, sheath, socket, and bristle cell. The other cells are ruled out through lateral inhibition, and make their contribution in the formation of the epidermis (which produces the cuticular exoskeleton) (Figure 22.12).
Mutations that affect the number or the pattern of sensory organs are thus readily observed by changes in the number or the pattern of the sensory bristles. For example, loss of function mutations in the gene achaete, which
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FIG 22.12 Sensory organ development from proneural clusters in the wing imaginal disc.
(a) Proneural clusters in the wing imaginal disc express (among others) the neurogenic genes achaete and scute. Through lateral inhibition, a progressively smaller group of cells maintains expression of these genes, whereas others lose them and take on an epidermal fate. This process eventually singles out just one cell, designated the sensory organ precursor cell (SOP). (b) The sensory organ precursor cells divide and again through lateral inhibition. Only a single cell maintains the neural phenotype (pIIb), the other (pIIa) losing it. Notch then plays a further role in determining cell lineage (shaft vs socket, sheath/neuron vs glia). The process represents a series of binary switches in which Notch determines the outcome. What we describe here applies for the sensory organs with small bristles but the same principle also operates in development of the organs having large bristles. (c) Schematic presentation of the localization of sensory organs bearing small bristles (microchaetes) and large bristles (macrochaetes), on the thorax and wings of Drosophila. Image adapted with permission from V. Hartenstein, http://flybase.bio.indiana.edu/.
codes for an activating bHLH transcription factor that drives differentiation of neurons, lead to flies having a sparse covering of hairs. Conversely, a loss- of-function mutation of the gene suppressor of hair results in flies having an excess of sensory bristles (ectopic production).
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