The agrin/perlecan-related protein eyes shut is essential for epithelial lumen formation in the Drosophila retina

An IFT-A protein is required to delimit functionally distinct zones in mechanosensory cilia

BACKGROUND

Conserved intraflagellar transport (IFT) particle proteins and IFT-associated motors are needed to assemble most eukaryotic cilia and flagella. Proteins in an IFT-A subcomplex are generally required for dynein-driven retrograde IFT, from the ciliary tip to the base. We describe novel structural and functional roles for IFT-A proteins in chordotonal organs, insect mechanosensory organs with cilia that are both sensory and motile.

RESULTS

The reduced mechanoreceptor potential A (rempA) locus of Drosophila encodes the IFT-A component IFT140. Chordotonal cilia are shortened in rempA mutants and an IFT-B protein accumulates in the mutant cilia, consistent with a defect in retrograde IFT. A functional REMPA-YFP fusion protein concentrates at the site of the ciliary dilation (CD), a highly structured axonemal inclusion of hitherto unknown composition and function. The CD is absent in rempA mutants, and REMPA-YFP is undetectable in the absence of another IFT-A protein, IFT122. In a mutant lacking the IFT dynein motor, the CD is disorganized and REMPA-YFP is mislocalized. A TRPV ion channel, required to generate sensory potentials and regulate ciliary motility, is normally localized in the cilia, proximal to the CD. This channel spreads into the distal part of the cilia in dynein mutants and is undetectable in rempA mutants.

CONCLUSIONS

IFT-A proteins are located at and required by the ciliary dilation, which separates chordotonal cilia into functionally distinct zones. A requirement for IFT140 in stable TRPV channel expression also suggests that IFT-A proteins may mediate preciliary transport of some membrane proteins.

Limiting lumens: a new role for Cdc42

The formation of a single lumen is a necessary step in the formation of biological tubes. Different tissues have developed diverse ways to form their lumens. In this issue, Jaffe et al. (Jaffe, A.B., N. Kaji, J. Durgan, and A. Hall. 2008. J. Cell Biol. 183:625–633) report the development of an in vitro system for studying lumen formation that is driven by fluid transport, recapitulating intestinal lumen formation. Effective ion and fluid transport requires both cell polarity and proper tissue organization. Surprisingly, polarization of cells in this three-dimensional system does not require Cdc42. Instead, Cdc42 prevents formation of multiple lumens by orienting cell divisions and directing apical membrane biogenesis.

Cdc42 controls spindle orientation to position the apical surface during epithelial morphogenesis

The establishment of apical–basal polarity within a single cell and throughout a growing tissue is a key feature of epithelial morphogenesis. To examine the underlying mechanisms, the human intestinal epithelial cell line Caco-2 was grown in a three-dimensional matrix to generate a cystlike structure, where the apical surface of each epithelial cell faces a fluid-filled central lumen. A discrete apical domain is established as early as the first cell division and between the two daughter cells. During subsequent cell divisions, the apical domain of each daughter cell is maintained at the center of the growing structure through a combination of mitotic spindle orientation and asymmetric abscission. Depletion of Cdc42 does not prevent the establishment of apical–basal polarity in individual cells but rather disrupts spindle orientation, leading to inappropriate positioning of apical surfaces within the cyst. We conclude that Cdc42 regulates epithelial tissue morphogenesis by controlling spindle orientation during cell division.

EYS, encoding an ortholog of Drosophila spacemaker, is mutated in autosomal recessive retinitis pigmentosa

Using a positional cloning approach supported by comparative genomics, we have identified a previously unreported gene, EYS, at the RP25 locus on chromosome 6q12 commonly mutated in autosomal recessive retinitis pigmentosa. Spanning over 2 Mb, this is the largest eye-specific gene identified so far. EYS is independently disrupted in four other mammalian lineages, including that of rodents, but is well conserved from Drosophila to man and is likely to have a role in the modeling of retinal architecture.

From cells to organs: building polarized tissue

How do animal cells assemble into tissues and organs? A diverse array of tissue structures and shapes can be formed by organizing groups of cells into different polarized arrangements and by coordinating their polarity in space and time. Conserved design principles underlying this diversity are emerging from studies of model organisms and tissues. We discuss how conserved polarity complexes, signalling networks, transcription factors, membrane-trafficking pathways, mechanisms for forming lumens in tubes and other hollow structures, and transitions between different types of polarity, such as between epithelial and mesenchymal cells, are used in similar and iterative manners to build all tissues.

Identification of a 2 Mb human ortholog of Drosophila eyes shut/spacemaker that is mutated in patients with retinitis pigmentosa

In patients with autosomal-recessive retinitis pigmentosa (arRP), homozygosity mapping was performed for detection of regions harboring genes that might be causative for RP. In one affected sib pair, a shared homozygous region of 5.0 Mb was identified on chromosome 6, within the RP25 locus. One of the genes residing in this interval was the retina-expressed gene EGFL11. Several genes resembling EGFL11 were predicted just centromeric of EGFL11. Extensive long-range RT-PCR, combined with 5'- and 3'- RACE analysis, resulted in the identification of a 10-kb transcript, starting with the annotated exons of EGFL11 and spanning 44 exons and 2 Mb of genomic DNA. The transcript is predicted to encode a 3165-aa extracellular protein containing 28 EGF-like and five laminin A G-like domains. Interestingly, the second part of the protein was found to be the human ortholog of Drosophila eyes shut (eys), also known as spacemaker, a protein essential for photoreceptor morphology. Mutation analysis in the sib pair homozygous at RP25 revealed a nonsense mutation (p.Tyr3156X) segregating with RP. The same mutation was identified homozygously in three arRP siblings of an unrelated family. A frame-shift mutation (pPro2238ProfsX16) was found in an isolated RP patient. In conclusion, we identified a gene, coined eyes shut homolog (EYS), consisting of EGFL11 and the human ortholog of Drosophila eys, which is mutated in patients with arRP. With a size of 2 Mb, it is one of the largest human genes, and it is by far the largest retinal dystrophy gene. The discovery of EYS might shed light on a critical component of photoreceptor morphogenesis.

Identification of novel regulatory factor X (RFX) target genes by comparative genomics in Drosophila species

An RFX-binding site is shown to be conserved in the promoters of a subset of ciliary genes and a subsequent screen for this site in two Drosophila species identified novel RFX target genes that are involved in sensory ciliogenesis.

Background

Regulatory factor X (RFX) transcription factors play a key role in ciliary assembly in nematode, Drosophila and mouse. Using the tremendous advantages of comparative genomics in closely related species, we identified novel genes regulated by dRFX in Drosophila.

Results

We first demonstrate that a subset of known ciliary genes in Caenorhabditis elegans and Drosophila are regulated by dRFX and have a conserved RFX binding site (X-box) in their promoters in two highly divergent Drosophila species. We then designed an X-box consensus sequence and carried out a genome wide computer screen to identify novel genes under RFX control. We found 412 genes that share a conserved X-box upstream of the ATG in both species, with 83 genes presenting a more restricted consensus. We analyzed 25 of these 83 genes, 16 of which are indeed RFX target genes. Two of them have never been described as involved in ciliogenesis. In addition, reporter construct expression analysis revealed that three of the identified genes encode proteins specifically localized in ciliated endings of Drosophila sensory neurons.

Conclusion

Our X-box search strategy led to the identification of novel RFX target genes in Drosophila that are involved in sensory ciliogenesis. We also established a highly valuable Drosophila cilia and basal body dataset. These results demonstrate the accuracy of the X-box screen and will be useful for the identification of candidate genes for human ciliopathies, as several human homologs of RFX target genes are known to be involved in diseases, such as Bardet-Biedl syndrome.

Preserving cell shape under environmental stress

Maintaining cell shape and tone is crucial for the function and survival of cells and tissues. Mechanotransduction relies on the transformation of minuscule mechanical forces into high-fidelity electrical responses. When mechanoreceptors are stimulated, mechanically sensitive cation channels open and produce an inward transduction current that depolarizes the cell. For this process to operate effectively, the transduction machinery has to retain integrity and remain unfailingly independent of environmental changes. This is particularly challenging for poikilothermic organisms, where changes in temperature in the environment may impact the function of mechanoreceptor neurons. Thus, we wondered how insects whose habitat might quickly vary over several tens of degrees of temperature manage to maintain highly effective mechanical senses. We screened for Drosophila mutants with defective mechanical responses at elevated ambient temperatures, and identified a gene, spam, whose role is to protect the mechanosensory organ from massive cellular deformation caused by heat-induced osmotic imbalance. Here we show that Spam protein forms an extracellular shield that guards mechanosensory neurons from environmental insult. Remarkably, heterologously expressed Spam protein also endowed other cells with superb defence against physically and chemically induced deformation. We studied the mechanical impact of Spam coating and show that spam-coated cells are up to ten times stiffer than uncoated controls. Together, these results help explain how poikilothermic organisms preserve the architecture of critical cells during environmental stress, and illustrate an elegant and simple solution to such challenge.

Regulation of cell polarity during epithelial morphogenesis

Epithelial cells have an apical surface facing a lumen or outside of the organism, and a basolateral surface facing other cells and extracellular matrix. The identity of the apical surface is determined by phosphatidylinositol 4,5-bisphosphate, while phosphatidylinositol 3,4,5-trisphophosphate determines the identity of the basolateral surface. The Par3/Par6/atypical protein kinase C complex, as well as the Crumbs and Scribble complexes, controls epithelial polarity. Par4 and AMP kinase regulate polarity during conditions of energy depletion. Lumens are formed in hollow cysts and tubules by fusions of apical vesicles, such as the vacuolar apical compartment, with the plasma membrane. The polarity of individual cells is oriented and coordinated with other cells by interactions with the extracellular matrix.

Genetic control of single lumen formation in the zebrafish gut

Most organs consist of networks of interconnected tubes that serve as conduits to transport fluid and cells and act as physiological barriers between compartments. Biological tubes are assembled through very diverse developmental processes that generate structures of different shapes and sizes. Nevertheless, all biological tubes invariably possess one single lumen. The mechanisms responsible for single lumen specification are not known. Here we show that zebrafish mutants for the MODY5 and familial GCKD gene tcf2 (also known as vhnf1) fail to specify a single lumen in their gut tube and instead develop multiple lumens. We show that Tcf2 controls single lumen formation by regulating claudin15 and Na+/K+-ATPase expression. Our in vivo and in vitro results indicate that Claudin15 functions in paracellular ion transport to specify single lumen formation. This work shows that single lumen formation is genetically controlled and appears to be driven by the accumulation of fluid.

Mechanotransduction and auditory transduction in Drosophila

Insects are utterly reliant on sensory mechanotransduction, the process of converting physical stimuli into neuronal receptor potentials. The senses of proprioception, touch, and hearing are involved in almost every aspect of an adult insect's complex behavioral repertoire and are mediated by a diverse array of specialized sensilla and sensory neurons. The physiology and morphology of several of these have been described in detail; genetic approaches in Drosophila, combining behavioral screens and sensory electrophysiology with forward and reverse genetic techniques, have now revealed specific proteins involved in their differentiation and operation. These include three different TRP superfamily ion channels that are required for transduction in tactile bristles, chordotonal stretch receptors, and polymodal nociceptors. Transduction also depends on the normal differentiation and mechanical integrity of the modified cilia that form the neuronal sensory endings, the accessory structures that transmit stimuli to them and, in bristles, a specialized receptor lymph and transepithelial potential. Flies hear near-field sounds with a vibration-sensitive, antennal chordotonal organ. Biomechanical analyses of wild-type antennae reveal non-linear, active mechanical properties that increase their sensitivity to weak stimuli. The effects of mechanosensory and ciliary mutations on antennal mechanics show that the sensory cilia are the active motor elements and indicate distinct roles for TRPN and TRPV channels in auditory transduction and amplification.