Dhand in Hand with Gli3

HIGHLIGHTS

GENE MAPPING mapping studies. This is why Singer the paternal meiotic contribution, and colleagues have embarked on a can be routinely generated in project to construct a meiotic map of zebrafish. Singer et al.generated 94 the male zebrafish (Danio rerio), using such embryos from a father who was Inequality of the sexes the genetic trickery that only these fish heterozygous for many polymorphic can offer. The result — that male markers. The haploid embryos were zebrafish recombine about three times genotyped at 185 of these markers, It’s no secret that there are differences less frequently than females — can be which were then placed on the male in how males and females behave, and put to good use in designing experi- meiotic map. The result was unam- it’s no less of a secret among geneticists ments. biguous: across the 25 chromosomes, that these differences often extend all The map that the authors have the male map is strikingly condensed the way to their chromosomes. When constructed is based on the recombi- (especially at the centromere), com- it comes to the rate at which homolo- nation frequency that occurs during pared with a previously generated gous chromosomes recombine during male meiosis. This is the equivalent of female meiotic map made with the meiosis, for instance, one sex can looking at how well genetic markers same markers; the sex-average map outdo the other severalfold. The rela- have been reshuffled on single sperm lies almost exactly in-between. tive recombination rates in the two derived from a parent that is het- So, what can we do now that we sexes aren’t always easy to quantify — erozygous at many loci. Although couldn’t do before? As zebrafish genetic maps are often averaged across such a direct experiment is beyond continues to raise its profile as an males and females — but such knowl- our current means, androgenetic experimental model for vertebrates, edge has useful implications for gene- haploid embryos, which contain only efficient gene mapping is becoming

DEVELOPMENTAL BIOLOGY asymmetry, te Welscher and colleagues looked at a transcription factor, dHand, because it is required to establish Shh signalling and because its initially uniform dHand in hand with Gli3 expression becomes restricted to the posterior of the limb bud just before Shh activation — but how? The authors How vertebrate limbs are formed is reasoned that answering this question considered one of the better understood would provide information about the Limb bud processes in development. It is generally earliest events in limb bud patterning. accepted that the early patterning of a Shh is repressed in the anterior of the Anterior vertebrate limb requires a feedback loop limb bud by another transcription factor, between sonic hedgehog (Shh) and Gli3. The authors found that, unlike in fibroblast growth factor (Fgf) signals. But wild-type mice, dHand is expressed in the the limb bud is already patterned before anterior mesenchyme of Gli3-deficient Gli3 this feedback loop is established, and the limbs, which suggests that dHand is molecules responsible for this have until repressed by Gli3. In the posterior limb now remained unknown. New work by bud mesenchyme, Gli3 is in turn repressed Alx4 te Welscher and colleagues shows that the by dHand. earlier patterning results from a mutual How do these early events relate to the antagonism between two transcription Shh/Fgf-mediated patterning? As both factors — dHand and Gli3 — and that this Gremlin and the Fgfs are expressed before dHand interaction participates in positioning the Shh, the authors looked at their expression Shh/Fgf signalling feedback loop. in Gli3-deficient mice. It turns out that During limb development, Shh signalling Gli3 is required to repress Gremlin and from the posterior mesenchyme regulates Fgf4 in the anterior limb bud. The authors Shh Fgfs Fgf expression in the apical ectodermal argue that the anterior expansion of ridge, whereas the Fgfs maintain Shh Gremlin and Fgf4 in the absence of Gli3 Gremlin expression in the zone of polarizing could result from upregulating dHand in activity. But Gremlin, a molecule that the anterior. Their hypothesis was Posterior relays the Shh signal, and the Fgfs have confirmed by the anterior upregulation of asymmetric expression even before Shh Gremlin expression when dHand was comes on, suggesting that the limb bud is ectopically expressed. prepatterned. To investigate this Interestingly, the authors show that Alx4,

an imperative. The high rates of DEVELOPMENTAL BIOLOGY recombination of females can be exploited for fine-mapping mutations. Conversely, if a more approxi- Sharpening the mate map is desired, or if there is a need to preserve alleles in cis, then males, with their smaller map, focus on Crumbs would be preferred. Why there is such discrepancy in Thousands of people blinded by inherited retinal recombination between the sexes we dystrophies harbour mutations in CRB1 — don’t know, but this shouldn’t stop us which encodes a protein that is homologous to a from using it to our advantage. regulator of Drosophila epithelial polarity called Besides, where else can you get away Crumbs. It has therefore been proposed that with saying that males and females retinal epithelial cell disruption underlies the are suited for different tasks? pathology of certain human retinal disorders — Tanita Casci such as retinitis pigmentosa and Leber congenital References and links amaurosis — that are caused by CRB1 mutations. ORIGINAL RESEARCH PAPER Singer, A. et al. Sex-specific recombination rates in zebrafish. Now, Pellikka et al. and Izaddoost et al. report in Genetics 160, 649–657 (2002) Nature that Crumbs has two specific roles in In wild-type adult fly retinas, rhabdomeres extend into long rod-like WEB SITE elaborating the architecture of photoreceptor structures (left) but shorten by 50% of their normal length in the absence John Postlethwait’s lab: of Crumbs (right). Reproduced with permission from Pellikka, M. et al. http://www.neuro.uoregon.edu/postle/mydoc.html cells (PRCs) that, surprisingly, occur well after © (2002) Macmillan Magazines Ltd. early cell polarization. As these functions of Crumbs might be conserved in mammals, these findings shed new light on the cellular defects restricted to the stalk and inner segment, that underlie human blindness caused by CRB1 respectively. Both groups report a role for mutations. Crumbs in rhabdomere elongation, and Pellikka To overcome an early lethal phenotype and to et al. show that the extracellular domain of maintain its function in the earlier stages of Crumbs has a role in stalk formation as well. another transcription factor epithelial morphogenesis, both groups used First, they showed that loss of Crumbs shortened that is also expressed in anterior mosaic analysis to remove Crumbs specifically in the stalks in flies, whereas overexpressing its mesenchyme and participates in the developing retina of Drosophila. It was also extracellular domain during PRC development restricting Shh signalling, is not overexpressed in fly PRCs. Previous studies had extended them. They also show that required for regulating dHand shown that Crumbs organizes the zonula beta[[H]]spectrin physically associates with (or Gli3) expression. In fact, adherens (ZA) — a band of ‘glue’ that encircles Crumbs and contributes to stalk elongation, because Alx4 expression is each epithelial cell and attaches the cells together possibly by negatively regulating endocytosis. reduced in Gli3-deficient mice it in sheets. The studies reported here confirmed Together, the studies suggest that, in developing must lie genetically downstream that Crumbs maintains ZA integrity in PRCs PRCs, Crumbs binds through an intracellular of Gli3. during the rapid expansion of the rhabdomere, an domain to protein partners to position the ZA. This study provides important insight into the very early organelle that is packed with photosensitive Once the ZA is in place, it then interacts with patterning events in the membranes. During PRC differentiation, different protein partners on the inner and outer vertebrate limb, but before we rhabdomeres (called the outer segment in surfaces of the cells to recruit membranes into have the complete picture of vertebrates) elongate dramatically, and are the extending stalk. limb development, more factors supported by structures called stalks (or inner Although the eyes of flies and humans are will need to be incorporated segments in vertebrates). Both groups noticed strikingly different in their morphologies and into the already complex that loss of Crumbs in PRCs causes the abnormal phototransduction cascades, these studies network of interactions. positioning of the ZA and a striking shortening of highlight similarities in photoreceptor Magdalena Skipper rhabdomeres (see figure) but no significant loss of morphogenesis and provide new models and epithelial polarity. Izaddoost et al. showed that, approaches for uncovering the molecular when they overexpressed the intracellular interactions that might be involved in certain References and links ORIGINAL RESEARCH PAPER domains of both Crumbs and CRB1 in Drosophila forms of human blindness. te Welscher, P. et al. Mutual genetic PRCs, the ZA was strikingly mislocalized. This Natalie DeWitt, antagonism involving GLI3 and dHAND Senior Editor, Nature prepatterns the vertebrate limb bud suggested that Crumbs has a role in ZA formation mesenchyme prior to SHH signalling. that is distinct from its role in establishing References and links Genes Dev. 16, 421–426 (2002) epithelial polarity, which requires different ORIGINAL RESEARCH PAPERS FURTHER READING Capdevila, J. & Pellikka, M. et al. Crumbs, the Drosophila homologue of human Izpisua Belmonte, J. C. Patterning structural domains of the protein and uses CRB1/RP12, is essential for photoreceptor morphogenesis. Nature 17 mechanisms controlling vertebrate limb different protein partners. February 2002 (DOI 10.1038/nature721) | Izaddoost, S. et al. development. Annu. Rev. Cell Dev. Biol. 17, When Pellikka et al. compared developing Drosophila Crumbs is a positional cue in photoreceptor adherens 87–132 (2001) junctions and rhabdomeres. Nature 17 February 2002 (DOI WEB SITE PRCs in fly and mouse retinas, they found that, 10.1038/nature720) Encyclopedia of Life Sciences: once distinct rhabdomere (outer segment) and WEB SITES http://www.els.net Kwang-Wook Choi’s lab: Vertebrate embryo: limb development stalk (inner segment) membranes are http://www.bcm.tmc.edu/db/db_fac-choi.html established, Crumbs and CRB1 become Ulrich Tepass’ lab: http://www.zoo.utoronto.ca/utepass/tepasshome