Neural hyperplasia induced by RNA interference with m4/malpha gene activity

The E(spl) complex (E(spl)-C) contains three different classes of genes that are downstream of Notch signaling. The bHLH genes mediate the Notch signal by repressing proneural gene activity, for example during the singularization of mechanosensory organ precursor cells (SOPs). Genes of the second class, the E(spl) m4/malpha family, antagonize this process if overexpressed. Here we show that this is based on dominant-negative effects since RNA interference gives neurogenic phenotypes indistinguishable from E(spl)-C mutations. Furthermore, a third member of the m4/malpha gene family, named bbu/tom, behaves differently with respect to RNA expression patterns, its regulation by Notch signaling and loss of function phenotypes.     

Two Genetically and Molecularly Distinct Functions Involved in Early Neurogenesis Reside within the Enhancer of Split Locus of Drosophila Melanogaster

Molecular correlation of the genetic aspects of the function of the neurogenic gene Enhancer of split [E(spl)] has previously been hampered by the densely transcribed nature of the chromosomal region within which it resides. We present data indicating that two distinct molecular species contribute to E(spl) function. Analysis of new E(spl) alleles has allowed us to define two complementing functions within the locus. Subsequent phenotypic analysis of different E(spl) deficiencies combined with P element-transformed constructs has demonstrated that these two functions correspond to: (1) a family of helix-loop-helix (HLH) protein-encoding genes and (2) the single copy gene E(spl) m9/10, whose product shares homology with G-protein beta subunits. The zygotically active E(spl) HLH genes can, at least partially, substitute for one another's functions and their total copy number determines the activity of the locus. E(spl) m9/10 acts synergistically with the E(spl) HLH genes and other neurogenic genes in the process of neurogenesis. The maternal component of E(spl) m9/10 has the most pronounced effect in neurogenesis, while its zygotic component is predominantly required during postembryonic development. The lethality of trans-heterozygotes of null E(spl) deficiency alleles with a strong Delta point mutation is a result of the concomitant reduction in activity of both E(spl) HLH and m9/10 functions. Immunocytochemical localization of the E(spl) m9/10 protein has revealed that it is a ubiquitously distributed nuclear component in embryonic, larval and imaginal tissues.     

bHLH-O proteins are crucial for Drosophila neuroblast self-renewal and mediate Notch-induced overproliferation

Drosophila larval neurogenesis is an excellent system for studying the balance between self-renewal and differentiation of a somatic stem cell (neuroblast). Neuroblasts (NBs) give rise to differentiated neurons and glia via intermediate precursors called GMCs or INPs. We show that E(spl)mγ, E(spl)mβ, E(spl)m8 and Deadpan (Dpn), members of the basic helix-loop-helix-Orange protein family, are expressed in NBs but not in differentiated cells. Double mutation for the E(spl) complex and dpn severely affects the ability of NBs to self-renew, causing premature termination of proliferation. Single mutations produce only minor defects, which points to functional redundancy between E(spl) proteins and Dpn. Expression of E(spl)mγ and m8, but not of dpn, depends on Notch signalling from the GMC/INP daughter to the NB. When Notch is abnormally activated in NB progeny cells, overproliferation defects are seen. We show that this depends on the abnormal induction of E(spl) genes. In fact E(spl) overexpression can partly mimic Notch-induced overproliferation. Therefore, E(spl) and Dpn act together to maintain the NB in a self-renewing state, a process in which they are assisted by Notch, which sustains expression of the E(spl) subset.     

Seven Genes of the Enhancer of Split Complex of Drosophila Melanogaster Encode Helix-Loop-Helix Proteins

Enhancer of split [E(spl)] is one of the neurogenic loci of Drosophila and, as such, is required for normal segregation of neural and epidermal cell progenitors. Genetic observations indicate that the E(spl) locus is in fact a gene complex comprising a cluster of related genes and that other genes of the region are also required for normal early neurogenesis. Three of the genes of the complex were known to encode helix-loop-helix (HLH) proteins and to be transcribed in nearly identical patterns. Here, we show that four other genes in the vicinity also encode HLH proteins and, during neuroblast segregation, three of them are expressed in the same pattern. We show by germ-line transformation that these three genes are also necessary to allow epidermal development of the neuroectodermal cells.     

New tool for an old problem: can RNAi efficiently resolve the issue of genetic redundancy?

RNA interference (RNAi) has become a generally accepted tool for inhibiting gene expression in many laboratory organisms. Nagel et al.,(1) in a recent paper, give an example of how this tool can also be used to address the question of genetic redundancy. Their focus was on the redundancy in Drosophila melanogaster of the Enhancer of split gene complex [E(spl)-C] which comprises seven highly related genes. Their somewhat conflicting findings are probably the typical scenario for most RNAi experiments: some expected results and some surprises.