Overview: Growth and development of Drosophila
Most cells in the body have the same genotype, but behave differently from one another. Many of these differences depend on signals in the extracellular environment. Studies of pattern formation in Drosophila and in other organisms show that animals share a common set of signaling pathways that together regulate a wide array of cell fates. Most widely important are the Notch, TGF-beta, Wnt, Receptor Tyrosine Kinase and Hedgehog pathways. None of these pathways is consistently tied to any particular cellular response, but instead can have distinct effects in each developmental field. In the Positional Information model, the combination of signaling pathways each cell receives can be thought of as a ‘code’ that determines cellular behaviour only when interpreted by the receiving cell, usually through transcription of specific target genes in each tissue.
Photoreceptor neurons
The determination of neural precursor cells in the fly eye has helped understand how many neural precursor cells are selected from the epithelia in which they originate. Neural precursor cells are defined by the sustained expression of ‘proneural’ bHLH transcription factors. Neural specification is opposed by activity of the Notch signaling pathway. A particular photoreceptor neuron class, called R8 cells, is specified by the proneural gene atonal. In the adult fly, R8 cells detect blue or green light.
The logic of R8 specification resembles that of lysogeny in bacteriophage Lambda. The Lambda Repressor gene (cI) is transcribed from separate promoters for establishment and maintenance of lysogeny. Interrupting Repressor-dependent maintenance results in lysis. Neural determination seems to have similar basic logic, involving separate transcriptional regulation for initiation and maintenance. Notch signaling interrupts maintenance, so preventing neural cell fate specification. It is possible that variations in dynamics, such as when initiation ceases and maintenance begins, contribute to the diverse patterns of neurogenesis that occur in different tissues and organisms.
In the case of the R8 cells, transcription of atonal is initiated by a transient wave of signaling though the Hh and Dpp pathways (Dpp is a member of the TGF-beta superfamily). Only about one cell in ten maintains atonal expression after Hh/Dpp signaling passes, because Notch activity blocks atonal autoregulation in others. Once lost, atonal expression cannot re-start, since Hh and Dpp are no longer active. Initiation and maintenance of atonal transcription depend on separate transcriptional enhancers. The exact pattern of neurogenesis therefore depends on where Notch signaling is active, but on when atonal autoregulation begins, and when Hh/Dpp signaling ceases. Another issue we currently seek to understand is how atonal initiation occurs in response to Hh and Dpp signaling, and its relationship to the class of ‘Eye Specification’ genes that includes Eyeless/Pax-6.
Regulating cell proliferation in vivo
What is the relationship between cell proliferation and patterning? Cell proliferation and growth are regulated spatially and temporally, like differentiation is, and the generation and growth of tissue is a prerequisite for patterning of cell fates. A great deal is known about signals that control proliferation in tissue culture, but it has not been so clear what is limiting for proliferation in vivo. We have now identified the main extracellular signals that spatially regulate the cell cycle during fly eye development. These include Hh, Dpp, N, and EGFR. Thus, the same signaling pathways that control cell fates, also control cell proliferation, and there may not be separate signals devoted to proliferation. Dependence on the same signaling pathways leads to many correlations between patterning and cell proliferation, but proliferation and fate specification also occur in patterns distinct from one another, due to signals acting in different combinations.
Cell cycle targets
The fly eye includes examples of induced G1/S progression, G1 arrest, and regulated G2/M. We know that the cell cycle genes string/cdc25 and dacapo (a homolog of p21 and p27) are transcribed in response to EGFR signaling to mediate these cell cycle events, but do not know all the other targets. Some cell cycle targets may be specific for particular tissues, just as targets for patterning are, because Hh, Dpp, N, RTK’s and Wnts have distinct cell cycle effects on different tissues. It is predicted that there are many genes whose products regulate the core cell cycle apparatus, each important for distinct tissues. Identifying some of these genes, and their mechanisms of action, is one of our current interests.
Cell cycle withdrawal by post-mitotic cells
One particular cell cycle event captures the attention as being little understood. This is the permanent cell cycle withdrawal of terminally differentiated cells, such as post-mitotic neurons. Very few mutations have been discovered that allow post-mitotic cells to go on dividing. This is an indication that there are probably multiple, robust barriers to continued proliferation by post-mitotic cells, which cannot be knocked out by mutations in single genes. We found that simultaneously mutating two cell cycle regulators, Retinoblastoma protein and Dacapo, allows some neurons to divide. In addition to further genetic studies to explore the mechanisms of permanent cell cycle withdrawal, it may prove possible to identify genes through microarray and systems analysis of transcription during cell cycle withdrawal.
A new tumor-suppressor in flies
One interesting new gene is known only as su(comp)3R-1. Homozygous mutant cells have a very strong, hyperplastic growth phenotype, and severely affect differentiation in addition. The combination of these effects gives clones of mutant cells the appearance of ‘tumors’, although they do not seem to be metastatic. Dual affects on proliferation and differentiation by a single mutation suggests a gene product is affected whose normal function is to coordinate the two.
Cell competition
In order to explore un-patterned proliferation that occurs during growth of many organs, we have studied the ‘cell competition’ that occurs when cells with different growth rates are intermingled. Genetic mosaics that place cells in competition within tissues may model features of tissue repair and tumor development, as well as reveal mechanisms of growth regulation. It is reasonable to think that cell competition may occur at the boundary between tumor cells and their genetically normal neighbors. There is also evidence that liver regeneration by transplanted fetal cells occurs by a cell competition-like process. Our studies show that cell competition requires programmed cell death of out-competed cells, and that this depends on specific genes that are not required for other kinds of programmed cell death, some of which we have identified. In addition, cell competition requires the Warts/Hippo pathway, a newly-recognized pathway of negative growth regulators whose mammalian homologs are implicated in cancer. In principle, manipulating engulfment might be useful to extend the circumstances under which cells can be replaced.
Recent publications
Pei Z. and Baker N.E.(2008) Competition between Delta and the Abruptex domain of Notch. BMC Dev Biol., 2008 Jan 21;8(1):4 [Abstract]
Wei Li and Nicholas Baker.(2007) The Active Role of Corpse Engulfment Pathways During Cell Competition. FLY,volume 1 | issue 5 September/October 2007 Pages: 274 - 278 [Abstract]
Li, W. and Baker, N.E. (2007) Cell competition depends on cell engulfment to induce apoptosis of neighboring cells. Cell,2007 Jun 15;129(6):1215-25.[Abstract]
Tyler DM, Li W, Zhuo N, Pellock B, Baker NE.(2007) Genes affecting cell competition in Drosophila. Genetics. 2007 Feb;175(2):643-57 [Abstract]
Firth, L.C., and Baker, N.E. (2007). Spitz from the retina regulates genes transcribed in the Second Mitotic Wave, peripodial epithelium, glia and plasmatocytes of the Drosophila eye imaginal disc. Dev. Biol.,2007 Jul 15;307(2):521-38 [Abstract]
Tyler, D. and Baker, N.E. (2007). expanded and fat regulate growth and differentiation in the Drosophila eye through multiple signaling pathways. Dev. Biol., 305, 187-201.[Abstract]
Yang, L. and Baker, N.E. (2006) Notch activity opposes Ras-induced differentiation during the Second Mitotic Wave of the developing Drosophila eye. BMC Dev Biol 6, 8. [Abstract]
Firth, L.C., Li, W., Zhang, H., and Baker, N.E. (2006) Analyses of RAS regulation of eye development in Drosophila melanogaster. Meth. Enzymol.,407(56), 711-721.[Abstract]
Firth, L.C. and Baker, N.E. (2005) Extracellular signals responsible for spatially regulated proliferation in the differentiating Drosophila eye. Dev Cell 8, 541-551. [Abstract]
Baker, N.E. (2004) Atonal points the way- protein-protein interactions and developmental biology. Dev Cell 7, 632-634. [Abstract]