Spalt modifies EGFR-mediated induction of chordotonal precursors in the embryonic PNS of Drosophila promoting the development of oenocytes

A function of spalt major as a sequence-specific DNA binding transcription factor mediates repression of knirps in the Drosophila wing imaginal disc

The Spalt transcriptional regulators participate in a variety of cell fate decisions during multicellular development. Vertebrate Spalt proteins have been mostly associated to the organization of heterochromatic regions, but they also contribute regulatory functions through binding to A/T rich motives present in their target genes. The developmental processes in which the Drosophila spalt genes participate are well known through genetic analysis, but the mechanism by which the Spalt proteins regulate transcription are still unknown. Furthermore, despite the prominent changes in gene expression associated to mutations in the spalt genes, the specific DNA sequences they bind are unknow. Here, we analyze a DNA fragment present in the regulatory region of the knirps gene. Spalt proteins are candidate repressors of knirps expression during the formation of the venation pattern in the wing disc, and we identified a minimal conserved 30bp sequence that binds to Spalt major both in vivo and in vitro. This sequence mediates transcriptional repression in the central region of the wing blade, constituting the first confirmed case of a direct regulatory interaction between Spalt major and its target DNA in Drosophila. Interestingly, we also find similar sequences in a set of eight novel candidate Spalt target genes, pointing to a common mechanism of transcriptional repression mediated by Spalt proteins.

The cis-regulatory logic underlying abdominal Hox-mediated repression versus activation of regulatory elements in Drosophila

During development diverse transcription factor inputs are integrated by cis-regulatory modules (CRMs) to yield cell-specific gene expression. Defining how CRMs recruit the appropriate combinations of factors to either activate or repress gene expression remains a challenge. In this study, we compare and contrast the ability of two CRMs within the Drosophila embryo to recruit functional Hox transcription factor complexes. The DCRE CRM recruits Ultrabithorax (Ubx) and Abdominal-A (Abd-A) Hox complexes that include the Extradenticle (Exd) and Homothorax (Hth) transcription factors to repress the Distal-less leg selector gene, whereas the RhoA CRM selectively recruits Abd-A/Exd/Hth complexes to activate rhomboid and stimulate Epidermal Growth Factor secretion in sensory cell precursors. By swapping binding sites between these elements, we found that the RhoA Exd/Hth/Hox site configuration that mediates Abd-A specific activation can convey transcriptional repression by both Ubx and Abd-A when placed into the DCRE. We further show that the orientation and spacing of Hox sites relative to additional binding sites within the RhoA and DCRE is critical to mediate cell- and segment-specific output. These results indicate that the configuration of Exd, Hth, and Hox site within RhoA is neither Abd-A specific nor activation specific. Instead Hox specific output is largely dependent upon the presence of appropriately spaced and oriented binding sites for additional TF inputs. Taken together, these studies provide insight into the cis-regulatory logic used to generate cell-specific outputs via recruiting Hox transcription factor complexes.

A Hox complex activates and potentiates the Epidermal Growth Factor signaling pathway to specify Drosophila oenocytes

Hox transcription factors specify distinct cell types along the anterior-posterior axis of metazoans by regulating target genes that modulate signaling pathways. A well-established example is the induction of Epidermal Growth Factor (EGF) signaling by an Abdominal-A (Abd-A) Hox complex during the specification of Drosophila hepatocyte-like cells (oenocytes). Previous studies revealed that Abd-A is non-cell autonomously required to promote oenocyte fate by directly activating a gene (rhomboid) that triggers EGF secretion from sensory organ precursor (SOP) cells. Neighboring cells that receive the EGF signal initiate a largely unknown pathway to promote oenocyte fate. Here, we show that Abd-A also plays a cell autonomous role in inducing oenocyte fate by activating the expression of the Pointed-P1 (PntP1) ETS transcription factor downstream of EGF signaling. Genetic studies demonstrate that both PntP1 and PntP2 are required for oenocyte specification. Moreover, we found that PntP1 contains a conserved enhancer (PntP1OE) that is activated in oenocyte precursor cells by EGF signaling via direct regulation by the Pnt transcription factors as well as a transcription factor complex consisting of Abd-A, Extradenticle, and Homothorax. Our findings demonstrate that the same Abd-A Hox complex required for sending the EGF signal from SOP cells, enhances the competency of receiving cells to select oenocyte cell fate by up-regulating PntP1. Since PntP1 is a downstream effector of EGF signaling, these findings provide insight into how a Hox factor can both trigger and potentiate the EGF signal to promote an essential cell fate along the body plan.

The development and functions of oenocytes

Oenocytes have intrigued insect physiologists since the nineteenth century. Many years of careful but mostly descriptive research on these cells highlights their diverse sizes, numbers, and anatomical distributions across Insecta. Contemporary molecular genetic studies in Drosophila melanogaster and Tribolium castaneum support the hypothesis that oenocytes are of ectodermal origin. They also suggest that, in both short and long germ-band species, oenocytes are induced from a Spalt major/Engrailed ectodermal zone by MAPK signaling. Recent glimpses into some of the physiological functions of oenocytes indicate that they involve fatty acid and hydrocarbon metabolism. Genetic studies in D. melanogaster have shown that larval oenocytes synthesize very-long-chain fatty acids required for tracheal waterproofing and that adult oenocytes produce cuticular hydrocarbons required for desiccation resistance and pheromonal communication. Exciting areas of future research include the evolution of oenocytes and their cross talk with other tissues involved in lipid metabolism such as the fat body.

The ETS domain transcriptional repressor Anterior open inhibits MAP kinase and Wingless signaling to couple tracheal cell fate with branch identity

Cells at the tips of budding branches in the Drosophila tracheal system generate two morphologically different types of seamless tubes. Terminal cells (TCs) form branched lumenized extensions that mediate gas exchange at target tissues, whereas fusion cells (FCs) form ring-like connections between adjacent tracheal metameres. Each tracheal branch contains a specific set of TCs, FCs, or both, but the mechanisms that select between the two tip cell types in a branch-specific fashion are not clear. Here, we show that the ETS domain transcriptional repressor anterior open (aop) is dispensable for directed tracheal cell migration, but plays a key role in tracheal tip cell fate specification. Whereas aop globally inhibits TC and FC specification, MAPK signaling overcomes this inhibition by triggering degradation of Aop in tip cells. Loss of aop function causes excessive FC and TC specification, indicating that without Aop-mediated inhibition, all tracheal cells are competent to adopt a specialized fate. We demonstrate that Aop plays a dual role by inhibiting both MAPK and Wingless signaling, which induce TC and FC fate, respectively. In addition, the branch-specific choice between the two seamless tube types depends on the tracheal branch identity gene spalt major, which is sufficient to inhibit TC specification. Thus, a single repressor, Aop, integrates two different signals to couple tip cell fate selection with branch identity. The switch from a branching towards an anastomosing tip cell type may have evolved with the acquisition of a main tube that connects separate tracheal primordia to generate a tubular network.

The Spalt transcription factors regulate cell proliferation, survival and epithelial integrity downstream of the Decapentaplegic signalling pathway

The expression of the spalt genes is regulated by the Decapentaplegic signalling pathway in the Drosophila wing. These genes participate in the patterning of the longitudinal wing veins by regulating the expression of vein-specific genes, and in the establishment of cellular affinities in the central region of the wing blade epithelium. The Spalt proteins act as transcription factors, most likely regulating gene expression by repression, but the identity of their target genes in the wing is still unknown. As a preliminary step to unravel the genetic hierarchy controlled by the Spalt proteins, we have analysed their requirements during wing development, and addressed to what extent they mediate all the functions of the Decapentaplegic pathway in this developmental system. We identify additional functions for Spalt in cell division, survival, and maintenance of epithelial integrity. Thus, Spalt activity is required to promote cell proliferation, acting in the G2/M transition of the cell cycle. The contribution of Spalt to cell division is limited to the central region of the wing blade, as they do not mediate the extra growth triggered by Decapentaplegic signalling in the peripheral regions of the wing disc. In addition, Spalt function is required to maintain cell viability in cells exposed to high levels of Decapentaplegic signalling. This aspect of Spalt function is related to the repression of JNK signalling in the spalt domain of expression. Finally, we further characterise the requirements of Spalt to maintain epithelial integrity by regulating cellular affinities between cells located in the central wing region. Our results indicate that Spalt function mediates most of the requirements identified for Decapentaplegic signalling, contributing to establish the cellular qualities that differentiate central versus peripheral territories in the wing blade.

Whole transcriptome analysis of a reversible neurodegenerative process in Drosophila reveals potential neuroprotective genes

Background

Neurodegenerative diseases are progressive and irreversible and they can be initiated by mutations in specific genes. Spalt-like genes (Sall) encode transcription factors expressed in the central nervous system. In humans, SALL mutations are associated with hereditary syndromes characterized by mental retardation, sensorineural deafness and motoneuron problems, among others. Drosophila sall mutants exhibit severe neurodegeneration of the central nervous system at embryonic stage 16, which surprisingly reverts later in development at embryonic stage 17, suggesting a potential to recover from neurodegeneration. We hypothesize that this recovery is mediated by a reorganization of the transcriptome counteracting SALL lost. To identify genes associated to neurodegeneration and neuroprotection, we used mRNA-Seq to compare the transcriptome of Drosophila sall mutant and wild type embryos from neurodegeneration and reversal stages.

Results

Neurodegeneration stage is associated with transcriptional changes in 220 genes, of which only 5% were already described as relevant for neurodegeneration. Genes related to the groups of Redox, Lifespan/Aging and Mitochondrial diseases are significantly represented at this stage. By contrast, neurodegeneration reversal stage is associated with significant changes in 480 genes, including 424 not previously associated with neuroprotection. Immune response and Salt stress are the most represented groups at this stage.

Conclusions

We identify new genes associated to neurodegeneration and neuroprotection by using an mRNA-Seq approach. The strong homology between Drosophila and human genes raises the possibility to unveil novel genes involved in neurodegeneration and neuroprotection also in humans.

Integration of an abdominal Hox complex with Pax2 yields cell-specific EGF secretion from Drosophila sensory precursor cells

Cis-regulatory modules (CRMs) ensure specific developmental outcomes by mediating both proper spatiotemporal gene expression patterns and appropriate transcriptional levels. In Drosophila, the precise transcriptional control of the serine protease rhomboid regulates EGF signaling to specify distinct cell types. Recently, we identified a CRM that activates rhomboid expression and thereby EGF secretion from a subset of abdominal sensory organ precursor cells (SOPs) to induce an appropriate number of lipid-processing cells called oenocytes. Here, we use scanning mutagenesis coupled with reporter assays, biochemistry and genetics to dissect the transcriptional mechanisms regulating SOP-specific rhomboid activation. Our results show that proper spatial activity of the rhomboid CRM is dependent upon direct integration of the abdomen-specific Hox factor Abdominal-A and the SOP-restricted Pax2 factor. In addition, we show that the Extradenticle and Homothorax Hox co-factors are differentially integrated on the rhomboid CRM by abdominal versus thoracic Hox proteins in the presence of Pax2. Last, we show that Abdominal-A uses both Pax2-dependent and Pax2-independent mechanisms to stimulate rhomboid CRM activity to induce proper oenocyte numbers. Thus, these data demonstrate how a CRM integrates Hox and neural transcriptional inputs to regulate the appropriate spatial pattern and levels of EGF secretion to specify an essential cell fate.

Oenocyte development in the red flour beetle Tribolium castaneum

Oenocytes are a specialized cell type required for lipid processing, pheromone secretion, and developmental signaling. Their development has been well characterized in Drosophila melanogaster, but it remains unknown whether the developmental program is conserved in other insect species. In this study, we compare and contrast the specification and development of larval oenocytes between Drosophila and the red flour beetle, Tribolium castaneum. First, we identify several useful reagents to label larval oenocytes, including both a Tribolium GFP enhancer trap line and a simple flurophore-conjugated streptavidin staining method that recognizes oenocytes across insect species. Second, we use these tools to describe oenocyte development in Tribolium embryos, and our findings provide evidence for conserved roles of MAP kinase signaling as well as the Spalt, Engrailed, hepatocyte nuclear factor-4, and ventral veins lacking factors in producing abdominal-specific oenocyte cells. However, Tribolium embryos produce four times as many oenocytes per abdominal segment as Drosophila, and unlike in Drosophila, these cells rapidly downregulate the expression of the Spalt transcription factor. Thus, these results provide new insight into the molecular pathways regulating oenocyte specification across insect species.

Sall1 regulates cortical neurogenesis and laminar fate specification in mice: implications for neural abnormalities in Townes-Brocks syndrome

Progenitor cells in the cerebral cortex undergo dynamic cellular and molecular changes during development. Sall1 is a putative transcription factor that is highly expressed in progenitor cells during development. In humans, the autosomal dominant developmental disorder Townes-Brocks syndrome (TBS) is associated with mutations of the SALL1 gene. TBS is characterized by renal, anal, limb and auditory abnormalities. Although neural deficits have not been recognized as a diagnostic characteristic of the disease, ∼10% of patients exhibit neural or behavioral abnormalities. We demonstrate that, in addition to being expressed in peripheral organs, Sall1 is robustly expressed in progenitor cells of the central nervous system in mice. Both classical- and conditional-knockout mouse studies indicate that the cerebral cortex is particularly sensitive to loss of Sall1. In the absence of Sall1, both the surface area and depth of the cerebral cortex were decreased at embryonic day 18.5 (E18.5). These deficiencies are associated with changes in progenitor cell properties during development. In early cortical progenitor cells, Sall1 promotes proliferative over neurogenic division, whereas, at later developmental stages, Sall1 regulates the production and differentiation of intermediate progenitor cells. Furthermore, Sall1 influences the temporal specification of cortical laminae. These findings present novel insights into the function of Sall1 in the developing mouse cortex and provide avenues for future research into potential neural deficits in individuals with TBS.

Drosophila Sal and Salr are transcriptional repressors

The SALL (Spalt-like) family of zinc-finger transcription factors is conserved in metazoans. In Drosophila Sal (Spalt) and Salr (Spalt-related) control the expression of genes involved in wing and central nervous system development, including cell adhesion and cytoskeletal proteins. In humans, SALL mutations associate with congenital disorders such as the Townes-Brocks and Okihiro syndromes. Human and Drosophila SALL proteins are modified by SUMO (small ubiquitin-related modifier), which influences their subnuclear localization. In the present study, we have analysed the transcriptional activity of Drosophila Sall proteins in cultured cells. We show that both Sal and Salr act as transcriptional repressors in Drosophila cells where they repress transcription through an AT-rich sequence. Furthermore, using the UAS/Gal4 heterologous system, Drosophila Sal and Salr repress transcription in human cells. Under our experimental conditions, only in the case of Salr is the repression activity dependent on the HDAC (histone deacetylase) complex. This complex might interact with the C-terminal zinc fingers of Salr. We describe the differential subcellular localizations of Sal and Salr fragments and identify their repression domains. Surprisingly, both repressors also contain transcription activation domains. In addition, under our experimental conditions SUMOylation has differential effects on Sal and Salr repressor activity. Phylogenetic comparison between nematodes, insects and vertebrates identifies conserved peptide sequences that are presumably critical for SALL protein function.

Proneural and abdominal Hox inputs synergize to promote sensory organ formation in the Drosophila abdomen

The atonal (ato) proneural gene specifies a stereotypic number of sensory organ precursors (SOP) within each body segment of the Drosophila ectoderm. Surprisingly, the broad expression of Ato within the ectoderm results in only a modest increase in SOP formation, suggesting many cells are incompetent to become SOPs. Here, we show that the SOP promoting activity of Ato can be greatly enhanced by three factors: the Senseless (Sens) zinc finger protein, the Abdominal-A (Abd-A) Hox factor, and the epidermal growth factor (EGF) pathway. First, we show that expression of either Ato alone or with Sens induces twice as many SOPs in the abdomen as in the thorax, and do so at the expense of an abdomen-specific cell fate: the larval oenocytes. Second, we demonstrate that Ato stimulates abdominal SOP formation by synergizing with Abd-A to promote EGF ligand (Spitz) secretion and secondary SOP recruitment. However, we also found that Ato and Sens selectively enhance abdominal SOP development in a Spitz-independent manner, suggesting additional genetic interactions between this proneural pathway and Abd-A. Altogether, these experiments reveal that genetic interactions between EGF-signaling, Abd-A, and Sens enhance the SOP-promoting activity of Ato to stimulate region-specific neurogenesis in the Drosophila abdomen.

Netrin-guided accessory cell morphogenesis dictates the dendrite orientation and migration of a Drosophila sensory neuron

Accessory cells, which include glia and other cell types that develop in close association with neurons, have been shown to play key roles in regulating neuron development. However, the underlying molecular and cellular mechanisms remain poorly understood. A particularly intimate association between accessory cells and neurons is found in insect chordotonal organs. We have found that the cap cell, one of two accessory cells of v'ch1, a chordotonal organ in the Drosophila embryo, strongly influences the development of its associated neuron. As it projects a long dorsally directed cellular extension, the cap cell reorients the dendrite of the v'ch1 neuron and tows its cell body dorsally. Cap cell morphogenesis is regulated by Netrin-A, which is produced by epidermal cells at the destination of the cap cell process. In Netrin-A mutant embryos, the cap cell forms an aberrant, ventrally directed process. As the cap cell maintains a close physical connection with the tip of the dendrite, the latter is dragged into an abnormal position and orientation, and the neuron fails to undergo its normal dorsal migration. Misexpression of Netrin-A in oenocytes, secretory cells that lie ventral to the cap cell, leads to aberrant cap cell morphogenesis, suggesting that Netrin-A acts as an instructive cue to direct the growth of the cap cell process. The netrin receptor Frazzled is required for normal cap cell morphogenesis, and mutant rescue experiments indicate that it acts in a cell-autonomous fashion.

Atonal, Senseless, and Abdominal-A regulate rhomboid enhancer activity in abdominal sensory organ precursors

The atonal (ato) proneural gene specifies different numbers of sensory organ precursor (SOP) cells within distinct regions of the Drosophila embryo in an epidermal growth factor-dependent manner through the activation of the rhomboid (rho) protease. How ato activates rho, and why it does so in only a limited number of sensory cells remains unclear. We previously identified a rho enhancer (RhoBAD) that is active within a subset of abdominal SOP cells to induce larval oenocytes and showed that RhoBAD is regulated by an Abdominal-A (Abd-A) Hox complex and the Senseless (Sens) transcription factor. Here, we show that ato is also required for proper RhoBAD activity and oenocyte formation. Transgenic reporter assays reveal RhoBAD contains two conserved regions that drive SOP gene expression: RhoD mediates low levels of expression in both thoracic and abdominal SOP cells, whereas RhoA drives strong expression within abdominal SOP cells. Ato indirectly stimulates both elements and enhances RhoA reporter activity by interfering with the ability of the Sens repressor to bind DNA. As RhoA is also directly regulated by Abd-A, we propose a model for how the Ato and Sens proneural factors are integrated with an abdominal Hox factor to regulate region-specific SOP gene expression.

A survey of well conserved families of C2H2 zinc-finger genes in Daphnia

Background

A recent comparative genomic analysis tentatively identified roughly 40 orthologous groups of C2H2 Zinc-finger proteins that are well conserved in "bilaterians" (i.e. worms, flies, and humans). Here we extend that analysis to include a second arthropod genome from the crustacean, Daphnia pulex.

Results

Most of the 40 orthologous groups of C2H2 zinc-finger proteins are represented by just one or two proteins within each of the previously surveyed species. Likewise, Daphnia were found to possess a similar number of orthologs for all of these small orthology groups. In contrast, the number of Sp/KLF homologs tends to be greater and to vary between species. Like the corresponding mammalian Sp/KLF proteins, most of the Drosophila and Daphnia homologs can be placed into one of three sub-groups: Class I-III. Daphnia were found to have three Class I proteins that roughly correspond to their Drosophila counterparts, dSP1, btd, CG5669, and three Class II proteins that roughly correspond to Luna, CG12029, CG9895. However, Daphnia have four additional KLF-Class II proteins that are most similar to the vertebrate KLF1/2/4 proteins, a subset not found in Drosophila. Two of these four proteins are encoded by genes linked in tandem. Daphnia also have three KLF-Class III members, one more than Drosophila. One of these is a likely Bteb2 homolog, while the other two correspond to Cabot and KLF13, a vertebrate homolog of Cabot.

Conclusion

Consistent with their likely roles as fundamental determinants of bilaterian form and function, most of the 40 groups of C2H2 zinc-finger proteins are conserved in kind and number in Daphnia. However, the KLF family includes several additional genes that are most similar to genes present in vertebrates but missing in Drosophila.

Hox and senseless antagonism functions as a molecular switch to regulate EGF secretion in the Drosophila PNS

Hox factors are key regulators of distinct cells, tissues, and organs along the body plan. However, little is known about how Hox factors regulate cell-specific gene expression to pattern diverse tissues. Here, we show an unexpected Hox transcriptional mechanism: the permissive regulation of EGF secretion, and thereby cell specification, by antagonizing the Senseless transcription factor in the peripheral nervous system. rhomboid expression in a subset of sensory cells stimulates EGF secretion to induce hepatocyte-like cell development. We identified a rhomboid enhancer that is active in these cells and show that an abdominal Hox complex directly competes with Senseless for enhancer binding, with the transcriptional outcome dependent upon their relative binding activities. Thus, Hox-Senseless antagonism forms a molecular switch that integrates neural and anterior-posterior positional information. As the vertebrate senseless homolog is essential for neural development as well as hematopoiesis, we propose Hox-Senseless antagonism will broadly control cell fate decisions.

Identity, origin, and migration of peripheral glial cells in the Drosophila embryo

Glial cells are crucial for the proper development and function of the nervous system. In the Drosophila embryo, the glial cells of the peripheral nervous system are generated both by central neuroblasts and sensory organ precursors. Most peripheral glial cells need to migrate along axonal projections of motor and sensory neurons to reach their final positions in the periphery. Here we studied the spatial and temporal pattern, the identity, the migration, and the origin of all peripheral glial cells in the truncal segments of wildtype embryos. The establishment of individual identities among these cells is reflected by the expression of a combinatorial code of molecular markers. This allows the identification of individual cells in various genetic backgrounds. Furthermore, mutant analysis of two of these marker genes, spalt major and castor, reveal their implication in peripheral glial development. Using confocal 4D microscopy to monitor and follow peripheral glia migration in living embryos, we show that the positioning of most of these cells is predetermined with minor variations, and that the order in which cells migrate into the periphery is almost fixed. By studying their lineages, we uncovered the origin of each of the peripheral glial cells and linked them to identified central and peripheral neural stem cells.

Mouse homolog of SALL1, a causative gene for Townes?Brocks syndrome, binds to A/T-rich sequences in pericentric heterochromatin via its C-terminal zinc finger domains

The Spalt (sal) gene family is conserved from Drosophila to humans. Mutations of human SALL1 cause Townes-Brocks syndrome, with features of ear, limb, anal, renal and heart anomalies. Sall1, a murine homolog of SALL1, is essential for kidney formation, and both Sall1 and SALL1 localize to heterochromatin in the nucleus. Here, we present a molecular mechanism for the heterochromatin localization of Sall1. Mutation analyses revealed that the 7th-10th C-terminal double zinc finger motifs were required for the localization. A recombinant protein of the most C-terminal double zinc finger (9th-10th) bound to specific A/T-rich sequences. Furthermore, Sall1 associated with A/T-rich sequences of the major satellite DNA in heterochromatin. Thus Sall1 may bind to A/T-rich sequences of the major satellite DNA via its C-terminal double zinc fingers, thereby mediating its localization to heterochromatin.

Specialized hepatocyte-like cells regulate Drosophila lipid metabolism

Lipid metabolism is essential for growth and generates much of the energy needed during periods of starvation. In Drosophila, fasting larvae release large quantities of lipid from the fat body but it is unclear how and where this is processed. Here we identify the oenocyte as the principal cell type accumulating lipid droplets during starvation. Tissue-specific manipulations of the Slimfast amino-acid channel, the Lsd2 fat-storage regulator and the Brummer lipase indicate that oenocytes act downstream of the fat body. In turn, oenocytes are required for depleting stored lipid from the fat body during fasting. Hence, lipid-metabolic coupling between the fat body and oenocytes is bidirectional. When food is plentiful, oenocytes have critical roles in regulating growth, development and feeding behaviour. In addition, they specifically express many different lipid-metabolizing proteins, including Cyp4g1, an omega-hydroxylase regulating triacylglycerol composition. These findings provide evidence that some lipid-processing functions of the mammalian liver are performed in insects by oenocytes.

Knockdown of spalt function by RNAi causes de-repression of Hox genes and homeotic transformations in the crustacean Artemia franciscana

Hox genes play a central role in the specification of distinct segmental identities in the body of arthropods. The specificity of Hox genes depends on their restricted expression domains, their interaction with specific cofactors and selectivity for particular target genes. spalt genes are associated with the function of Hox genes in diverse species, but the nature of this association varies: in some cases, spalt collaborates with Hox genes to specify segmental identities, in others, it regulates Hox gene expression or acts as their target. Here we study the role of spalt in the branchiopod crustacean Artemia franciscana. We find that Artemia spalt is expressed in the pre-segmental 'growth zone' and in stripes in each of the trunk (thoracic, genital and post-genital) segments that emerge from this zone. Using RNA interference (RNAi), we show that knocking down the expression of spalt has pleiotropic effects, which include thoracic to genital (T-->G), genital to thoracic (G-->T) and post-genital to thoracic (PG-->T) homeotic transformations. These transformations are associated with a stochastic de-repression of Hox genes in the corresponding segments of RNAi-treated animals (AbdB for T-->G and Ubx/AbdA for G-->T and PG-->T transformations). We discuss a possible role of spalt in the maintenance of Hox gene repression in Artemia and in other animals.

Expression of Xenopus XlSALL4 during limb development and regeneration

The multi-C2H2 zinc-finger domain containing transcriptional regulators of the spalt (SAL) family plays important developmental regulatory roles. In a competitive subtractive hybridization screen of genes expressed in Xenopus laevis hindlimb regeneration blastemas, we identified a SAL family member that, by phylogenetic analysis, falls in the same clade as human SALL4 and have designated it as XlSALL4. Mutations of human SALL4 have been linked to Okihiro syndrome, which includes preaxial (anterior) limb defects. The expression pattern of XlSALL4 transcripts during normal forelimb and hindlimb development and during hindlimb regeneration at the regeneration-competent and regeneration-incompetent stages is temporally and regionally dynamic. We show for the first time that a SAL family member (XlSALL4) is expressed at the right place and time to play a role regulating both digit identity along the anterior/posterior axis and epimorphic limb regeneration.

Photoreceptor differentiation in Drosophila: from immature neurons to functional photoreceptors

How a pool of equipotent cells acquires a multitude of distinct fates is a major question in developmental biology. The study of photoreceptor (PR) cell differentiation in Drosophila has been used to address this question. PR differentiation is a process that extends over a period of 5 days: It begins in the larval eye imaginal disc when PRs are recruited and commit to particular PR fates, and it culminates in the pupal eye disc with the morphogenesis of the rhabdomeres and the initiation of rhodopsin expression. Several models for PR specification agree that the Ras and Notch signaling pathways are important for the specification of different PR subtypes (Freeman [1997] Development 124:261-270; Cooper and Bray [2000] Curr. Biol. 10:1507-1510; Tomlinson and Struhl [2001] Mol. Cell. 7:487-495). In the first part of this review, we briefly describe the different signaling pathways and transcription factors required for the specification and differentiation of the different PR subtypes in the larval eye disc. In the second part, we review the roles of several transcription factors, which are required for the terminal photoreceptor differentiation and rhodopsin expression.

Hearing in Drosophila: development of Johnston's organ and emerging parallels to vertebrate ear development

In this review, I describe recent progress toward understanding the developmental genetics governing formation of the Drosophila auditory apparatus. The Drosophila auditory organ, Johnston's organ, is housed in the antenna. Intriguingly, key genes needed for specification or function of auditory cell types in the Drosophila antenna also are required for normal development or function of the vertebrate ear. These genes include distal-less, spalt and spalt-related, atonal, crinkled, nanchung and inactive, and prestin, and their vertebrate counterparts Dlx, spalt-like (sall), atonal homolog (ath), myosin VIIA, TRPV, and prestin, respectively. In addition, Drosophila auditory neurons recently were shown to serve actuating as well as transducing roles, much like their hair cell counterparts of the vertebrate cochlea. The emerging genetic and physiologic parallels have come as something of a surprise, because conventional wisdom holds that vertebrate and invertebrate hearing organs have separate evolutionary origins. The new findings raise the possibility that auditory organs are more ancient than previously thought and indicate that Drosophila is likely to be a powerful model system in which to gain insights regarding the etiologies of human deafness disorders.

EGF Receptor Signaling Regulates Pulses of Cell Delamination from the Drosophila Ectoderm

Many different intercellular signaling pathways are known but, for most, it is unclear whether they can generate oscillating cell behaviors. Here we use time-lapse analysis of Drosophila embryogenesis to show that oenocytes delaminate from the ectoderm in discrete bursts of three. This pulsatile process has a 1 hour period, occurs without cell division, and requires a localized EGF receptor (EGFR) response. High-threshold EGFR targets are sequentially activated in rings of three cells, prefiguring the temporal pattern of delamination. Surprisingly, widespread misexpression of the relevant activating ligand, Spitz, is compatible with robust delamination pulses. Moreover, although Spitz ligand becomes limiting after only two pulses, artificially prolonging its secretion generates up to six additional cycles, revealing a rhythmic underlying mechanism. These findings illustrate how intercellular signaling and cell movements can generate multiple cycles of a cell behavior, despite individual cells experiencing only one cycle of receptor activation.

EGF Receptor Signaling Triggers Recruitment of Drosophila Sense Organ Precursors by Stimulating Proneural Gene Autoregulation

In Drosophila, commitment of a cell to a sense organ precursor (SOP) fate requires bHLH proneural transcription factor upregulation, a process that depends in most cases on the interplay of proneural gene autoregulation and inhibitory Notch signaling. A subset of SOPs are selected by a recruitment pathway involving EGFR signaling to ectodermal cells expressing the proneural gene atonal. We show that EGFR signaling drives recruitment by directly facilitating atonal autoregulation. Pointed, the transcription factor that mediates EGFR signaling, and Atonal protein itself bind cooperatively to adjacent conserved binding sites in an atonal enhancer. Recruitment is therefore contingent on the combined presence of Atonal protein (providing competence) and EGFR signaling (triggering recruitment). Thus, autoregulation is the nodal control point targeted by signaling. This exemplifies a simple and general mechanism for regulating the transition from competence to cell fate commitment whereby a cell signal directly targets the autoregulation of a selector gene.

Regulation of R7 and R8 differentiation by the spalt genes

Photoreceptor development begins in the larval eye imaginal disc, where eight distinct photoreceptor cells (R1-R8) are sequentially recruited into each of the developing ommatidial clusters. Final photoreceptor differentiation, including rhabdomere formation and rhodopsin expression, is completed during pupal life. During pupation, spalt was previously proposed to promote R7 and R8 terminal differentiation. Here we show that spalt is required for proper R7 differentiation during the third instar larval stage since the expression of several R7 larval markers (prospero, enhancer of split mdelta0.5, and runt) is lost in spalt mutant clones. In R8, spalt is not required for cell specification or differentiation in the larval disc but promotes terminal differentiation during pupation. We show that spalt is necessary for senseless expression in R8 and sufficient to induce ectopic senseless in R1-R6 during pupation. Moreover, misexpression of spalt or senseless is sufficient to induce ectopic rhodopsin 6 expression and partial suppression of rhodopsin 1. We demonstrate that spalt and senseless are part of a genetic network, which regulates rhodopsin 6 and rhodopsin 1. Taken together, our results suggest that while spalt is required for R7 differentiation during larval stages, spalt and senseless promote terminal R8 differentiation during pupal stages, including the regulation of rhodopsin expression.

Loss of the Sall3 gene leads to palate deficiency, abnormalities in cranial nerves, and perinatal lethality

Members of the Spalt gene family encode putative transcription factors characterized by seven to nine C2H2 zinc finger motifs. Four genes have been identified in mice--Spalt1 to Spalt4 (Sall1 to Sall4). Spalt homologues are widely expressed in neural and mesodermal tissues during early embryogenesis. Sall3 is normally expressed in mice from embryonic day 7 (E7) in the neural ectoderm and primitive streak and subsequently in the brain, peripheral nerves, spinal cord, limb buds, palate, heart, and otic vesicles. We have generated a targeted disruption of Sall3 in mice. Homozygous mutant animals die on the first postnatal day and fail to feed. Examination of the oral structures of these animals revealed that abnormalities were present in the palate and epiglottis from E16.5. In E10.5 embryos, deficiencies in cranial nerves that normally innervate oral structures, particularly the glossopharyngeal nerve (IX), were observed. These studies indicate that Sall3 is required for the development of nerves that are derived from the hindbrain and for the formation of adjacent branchial arch derivatives.

A hidden program in Drosophila peripheral neurogenesis revealed: fundamental principles underlying sensory organ diversity

How is cell fate diversity reliably achieved during development? Insect sensory organs have been a favorable model system for investigating this question for over 100 years. They are constructed using defined cell lineages that generate a maximum of cell diversity with a minimum number of cell divisions, and display tremendous variety in their morphologies, constituent cell types, and functions. An unexpected realization of the past 5 years is that very diverse sensory organs in Drosophila are produced by astonishingly similar cell lineages, and that their diversity can be largely attributed to only a small repertoire of developmental processes. These include changes in terminal cell differentiation, cell death, cell proliferation, cell recruitment, cell-cell interactions, and asymmetric segregation of cell fate determinants during mitosis. We propose that most Drosophila sensory organs are built from an archetypal lineage, and we speculate about how this stereotyped pattern of cell divisions may have been built during evolution.

Profiling Cu,Zn-superoxide dismutase expression in Drosophila melanogaster--a critical regulatory role for intron/exon sequence within the coding domain

Cu,Zn-superoxide dismutase (SOD1) represents along with catalase the first coordinated line of defense against ROS and is found in all aerobic organisms. The dissection of the regulatory controls that drive the expression of SOD1 may provide further insight into the functional significance of this enzyme. The aim of this study was to elucidate temporal and spatial patterns of SOD1 expression, as well as to identify gene domains that govern its expression. Immunostaining analysis was used to delineate marked tissue and stage-specific expression patterns during the course of development and aging. By and large, there were no significant alterations in SOD1 mRNA and protein levels in response to the stress that accompanies aging, nor in response to different environmental insults, such as heat and hyperoxia. Expression of SOD1 seems to be largely determined by intrinsic factors. By histochemical analysis of transgenics carrying various sod1-reporter gene fusions, it was also possible to identify sequence domains, governing SOD1 expression. In particular a 1140 base pair region, composed of the single sod1 intron along with exon 2, was found to be essential for permitting spatial and temporal expression patterns that approximate normal endogenous expression.

Sall1, a causative gene for Townes–Brocks syndrome, enhances the canonical Wnt signaling by localizing to heterochromatin

The Spalt (sal) gene family plays an important role in regulating developmental processes of many organisms. Mutations of human SALL1 cause the autosomal dominant disorder, Townes-Brocks syndrome (TBS), and result in ear, limb, anal, renal, and heart anomalies. Targeted deletion of mouse Sall1 results in kidney agenesis or severe dysgenesis. Molecular mechanisms of Sall1, however, have remained largely unknown. Here we report that Sall1 synergistically activates canonical Wnt signaling. The transcriptional activity of Sall1 is related to its nuclear localization to punctate nuclear foci (pericentromeric heterochromatin), but not to its localization or association with beta-catenin, the nuclear component of Wnt signaling. In contrast, the RNA interference of Sall1 reduces reporter activities of canonical Wnt signaling. The N-terminal truncated Sall1, produced by mutations often found in TBS, disturbs localization of native Sall1 to heterochromatin, and also down-regulates the synergistic transcriptional enhancement for Wnt signal by native Sall1. Thus, we propose a new mechanism for Wnt signaling activation, that is the heterochromatin localization of Sall1.

Quantitative Trait Loci Affecting Starvation Resistance in Drosophila melanogaster

The ability to withstand periods of scarce food resources is an important fitness trait. Starvation resistance is a quantitative trait controlled by multiple interacting genes and exhibits considerable genetic variation in natural populations. This genetic variation could be maintained in the face of strong selection due to a trade-off in resource allocation between reproductive activity and individual survival. Knowledge of the genes affecting starvation tolerance and the subset of genes that affect variation in starvation resistance in natural populations would enable us to evaluate this hypothesis from a quantitative genetic perspective. We screened 933 co-isogenic P-element insertion lines to identify candidate genes affecting starvation tolerance. A total of 383 P-element insertions induced highly significant and often sex-specific mutational variance in starvation resistance. We also used deficiency complementation mapping followed by complementation to mutations to identify 12 genes contributing to variation in starvation resistance between two wild-type strains. The genes we identified are involved in oogenesis, metabolism, and feeding behaviors, indicating a possible link to reproduction and survival. However, we also found genes with cell fate specification and cell proliferation phenotypes, which implies that resource allocation during development and at the cellular level may also influence the phenotypic response to starvation.

Towards a molecular understanding of Drosophila hearing

The Drosophila auditory system is presented as a powerful new genetic model system for understanding the molecular aspects of development and physiology of hearing organs. The fly's ear resides in the antenna, with Johnston's organ serving as the mechanoreceptor. New approaches using electrophysiology and laser vibrometry have provided useful tools to apply to the study of mutations that disrupt hearing. The fundamental developmental processes that generate the peripheral nervous system are fairly well understood, although specific variations of these processes for chordotonal organs (CHO) and especially for Johnston's organ require more scrutiny. In contrast, even the fundamental physiologic workings of mechanosensitive systems are still poorly understood, but rapid recent progress is beginning to shed light. The identification and analysis of mutations that affect auditory function are summarized here, and prospects for the role of the Drosophila auditory system in understanding both insect and vertebrate hearing are discussed.

Bristles induce bracts via the EGFR pathway on Drosophila legs

A long-standing mystery in Drosophila has been: how do certain bristles induce adjacent cells to make bracts (a type of thick hair) on their proximal side? The apparent answer, based on loss- and gain-of-function studies, is that they emit a signal that neighbors then transduce via the epidermal growth factor receptor pathway. Suppressing this pathway removes bracts, while hyperactivating it evokes bracts indiscriminately on distal leg segments. Misexpression of the diffusible ligand Spitz (but not its membrane-bound precursor) elicits extra bracts at normal sites. What remains unclear is how a secreted signal can have effects in one specific direction.

Hox Genes: it's all a matter of context

Recent studies provide compelling new evidence that Hox gene effects depend on fine-structure spatial and temporal information. Further, in a specific cell type, only one or a few downstream genes may mediate Hox morphogenetic functions. If this is generally true, it will have important implications for how Hox regulatory networks operate and evolve.

Two-step process for photoreceptor formation in Drosophila

The formation of photoreceptor cells (PRCs) in Drosophila serves as a paradigm for understanding neuronal determination and differentiation. During larval stages, a precise series of sequential inductive processes leads to the recruitment of eight distinct PRCs (R1-R8). But, final photoreceptor differentiation, including rhabdomere morphogenesis and opsin expression, is completed four days later, during pupal development. It is thought that photoreceptor cell fate is irreversibly established during larval development, when each photoreceptor expresses a particular set of transcriptional regulators and sends its projection to different layers of the optic lobes. Here, we show that the spalt (sal) gene complex encodes two transcription factors that are required late in pupation for photoreceptor differentiation. In the absence of the sal complex, rhabdomere morphology and expression of opsin genes in the inner PRCs R7 and R8 are changed to become identical to those of outer R1-R6 PRCs. However, these cells maintain their normal projections to the medulla part of the optic lobe, and not to the lamina where outer PRCs project. These data indicate that photoreceptor differentiation occurs as a two-step process. First, during larval development, the photoreceptor neurons become committed and send their axonal projections to their targets in the brain. Second, terminal differentiation is executed during pupal development and the photoreceptors adopt their final cellular properties.

Insect oenocytes: a model system for studying cell-fate specification by Hox genes

During insect development, morphological differences between segments are controlled by the Hox gene family of transcription factors. Recent evidence also suggests that variation in the regulatory elements of these genes and their downstream targets underlies the evolution of several segment-specific morphological traits. This review introduces a new model system, the larval oenocyte, for studying the evolution of fate specification by Hox genes at single-cell resolution. Oenocytes are found in a wide range of insects, including species using both the short and the long germ modes of development. Recent progress in our understanding of the genetics and cell biology of oenocyte development in the fruitfly Drosophila melanogaster is discussed. In the D. melanogaster embryo, the formation of this cell type is restricted to the first 7 abdominal segments and is under Hox gene control. Oenocytes delaminate from the dorsal ectoderm of A1-A7 in response to an induction that involves the epidermal growth factor receptor (EGFR) signalling pathway. Although the receptor itself is required in the presumptive oenocytes, its ligand Spitz (Spi) is secreted by a neighbouring chordotonal organ precursor (COP). Thus, in dorsal regions, local signalling from this component of the developing peripheral nervous system induces the formation of oenocytes. In contrast, in lateral regions of the ectoderm, Spi signal from a different COP induces the formation of secondary COPs in a homeogenetic manner. This dorsoventral difference in the fate induced by Spi ligand is controlled by a prepattern in the responding ectoderm that requires the Spalt (Sal) transcription factor. Sal protein is expressed in the dorsal but not lateral ectoderm and acts as a competence modifier to bias the response to Spi ligand in favour of the oenocyte fate. We discuss a recently proposed model that integrates the roles of Sal and the EGFR pathway in oenocyte/chordotonal organ induction. This model should provide a useful starting point for future comparative studies of these ectodermal derivatives in other insects.