Genetic control of cell fate specification in Drosophila peripheral nervous system.

INTRODUCTION

The Drosophila peripheral nervous system (PNS) comprises four major types of sensory elements: external sensory organs (such as bristles), chordotonal organs (stretch receptors), multiple dendritic neurons, and photoreceptors. Considerable progress has been made in understanding the genetic control of the development of these sensory organs. A number of key genes have been identified that control where, how many, and what types of sensory elements form in a fly (reviewed in 19, 30, 38, 39, 52, 54, 107).

In principle, the fate of a cell might be determined by one or both types of mechanisms: cell lineage or interaction with the environment (including other cells or extracellular factors). Both types of mechanisms are used in the construction of Drosophila PNS. At one extreme, photoreceptor formation relies solely on cellular interactions; cell lineage plays no role (63, 79). At the other extreme, cells that constitute an external sensory (es) organ are clonally related and derived from stereotyped cell lineage (7, 13, 44), though cellular interactions also play important roles (4, 5). As evident from studies summarized below, even though the development of photoreceptors and es organs appear to be quite different superficially, there is considerable overlap in the mechanisms used in their formation.

In this review, we focus on the genetic control of development of ei organs. We compare the development of other sensory organs (and to a lesser extent, CNS) with that of es organs to assess the generality of the notions developed from the study of es organ formation.

PROGRESSIVE DETERMINATION MODEL OF THE GENESIS OF EXTERNAL SENSORY ORGANS

In 1989, based on then available experimental evidence, Ghysen & Dambly-Chaudiere proposed a progressive determination model to account for the formation of es organs (38). This model has stood the experimental tests well. Figure 1 shows a slightly modified version of their original model. Briefly, the first step of es organ development is the turning on of proneural genes in clusters of cells (proneural clusters). The proneural genes are so named because they endow the cells that express them with the potential to form neural precursors. The position of proneural clusters prefigures where es organs will form (26, 90). Within each proneural cluster, the cells compete with each other such that only a subset of cells (often just one) is singled out to develop into an es organ precursor. This singling out process is mediated by cell-cell interaction through the action of neurogenic genes (reviewed in 1, 19, 39). Once a cell is singled out to become an es organ precursor, it starts to express two groups of genes: the neural precursor genes, which are shared by most or all neural precursors and may function in controlling neural differentiation (9, 15, 100); and the neuronal type selector genes, which are expressed in more restricted patterns (e.g. cut is expressed in es organ precursors but not chordotonal (ch) organs, whereas pox-neural (poxn) is expressed in a subset of es organs) and specify neuronal type (10, 12, 29). The es organ precursor then goes through a stereotyped pattern of cell division and produces a fixed number of progeny cells that constitute an es organ (7, 13, 44).

In the following, we use this model as a framework to discuss cell fate specification in PNS.

PRONEURAL GENES

The achaete-scute Complex

Genes of the achaete-scute complex (AS-C), which consists of achaete (ac), scute (sc), asense (ase), and lethal of scute (l'sc), have crucial roles in es organ and CNS development (reviewed in 20, 37). All four genes encode transcriptional regulators of the basic helix-loop-helix (bHLH) type. Of the four genes in AS-C, ac and sc are the major proneural genes for es organ development (35, 36). In loss-of-function ac-sc double mutants, the great majority of es organs fail to form. Conversely, in gain-of-function ac or sc mutants in which ac or sc is misexpressed to ectopic locations, ectopic es organs form (82). ase functions as a proneural gene for a small subset of es organs (28). In contrast to ac and sc, ase has an additional role as a neural precursor gene in controlling neuronal differentiation later in es organ development (15, 32, 55). l'sc is a proneural gene for CNS development (58, 59). The role of AS-C in controlling fly neurogenesis bears a striking resemblance to that of myogenic genes in controlling vertebrate myogenesis (reviewed in 53). The study of AS-C provides a useful example of redundancy of gene functions. The four members of AS-C have somewhat overlapping but distinct loss-of-function phenotypes. However, ectopic expression of any one of the four gene products can induce ectopic es organ formation (15, 32, 49, 82). This similar gain-of-function phenotype probably results from the fact that the four bHLH proteins are sufficiently homologous that they can functionally substitute for one another. These results provide a cautionary note against relying on gain-of-function phenotype alone for identifying a master regulatory gene.

The Regulation of Proneural Gene Activity

Where ac or sc is expressed determines where es organs form. Thus, the problem of how a fly determines where to form its es organs becomes the problem of how ac and sc expression patterns and activities are regulated during development. In the embryo, ac and sc are expressed in cell clusters at stereotyped anterior-posterior (AP) and dorsal-ventral (DV) coordinates in a tartan-like pattern (83, 91). Skeath et al showed that AP and DV position of these proneural clusters is controlled by pair-rule and DV polarity genes, and is later maintained by a subset of segment polarity genes (91). The expression of ac and sc overlaps temporally with the expression of pair-rule and DV polarity genes, suggesting that the latter may control the expression of ac and sc directly. In the imaginal discs, how the ac and sc expression patterns are set up is less well understood because the coordinate system has not yet been well characterized there. However, considerable progress has been made recently (6, 23, 25, 76, 104). In both embryo and imaginal discs, the ac and sc patterns are not only controlled by genes that set up the coordinate systems, but ac and sc can crossregulate each other (68, 83). Further, they are also regulated by neurogenic genes.

The activities of ac and sc are controlled not only at the transcription level but also at the protein level. The achaete or scute bHLH proteins function as transcription regulators by forming DNA-binding homodimers or heterodimers with the ubiquitous bHLH protein daughterless to control downstream genes (16, 71, 101). A negative regulator of ac and sc is the gene extramacrochaete (emc), which encodes an HLH protein without a DNA-binding basic domain. Emc can dimerize with achaete, scute, or daughterless resulting in heterodimers incapable of binding DNA; this thereby prevents the formation of active heterodimers or homodimers (101). The expression pattern of emc is approximately complementary to those of ac and sc although it is controlled independently of ac and sc (27, 102). It appears that the relative level of ac/sc and emc determines the competence of the cells to form neural precursors. Thus, the expression pattern of emc contributes to refining the final pattern of neural precursor distribution.

Is the Concept of "Proneural Gene" Generally Applicable?

PNS

Of the four major types of sensory elements, two (es organs and the majority of multiple dendritic (md) neurons) require ac and sc for their formation, whereas the other two, ch organs and photoreceptors, do not (28, 58). This raises the question of whether other proneural genes exist or whether a different mechanism(s) initiates neurogenesis of ch organs and photoreceptors. Recently, a new bHLH gene, atonal (ato), was found to be the proneural gene for both ch organs and photoreceptors (56, 57). In ato loss-of-function mutants, ch organs as well as photoreceptors (in both compound eyes and ocelli) fail to form. There are interesting qualitative differences between the actions of ato and ac/sc (see below).

ato and AS-C account for the origin of almost the entire PNS. Deletion of both AS-C and ato removes all but two md neurons in each hemisegment; presumably one more proneural gene is yet to be identified (56). Thus, the involvement of proneural genes appears to be a general mechanism used to initiate neural development in…