Since their isolation in the early 1990s, members of the Hedgehog family of intercellular signaling proteins have come to be recognized as key mediators of many fundamental processes in embryonic development. Their activities are central to the growth, patterning, and morphogenesis of many different regions within the body plans of vertebrates and insects, and most likely other invertebrates. In some contexts, Hedgehog signals act as morphogens in the dose-dependent induction of distinct cell fates within a target field, in others as mitogens regulating cell proliferation or as inducing factors controlling the form of a developing organ. These diverse functions of Hedgehog proteins raise many intriguing questions about their mode of operation. How do these proteins move between or across fields of cells? How are their activities modulated and transduced? What are their intracellular targets? In this article we review some well-established paradigms of Hedgehog function inDrosophila and vertebrate development and survey the current understanding of the synthesis, modification, and transduction of Hedgehog proteins. Embryological studies over much of the last century that relied primarily on the physical manipulation of cells within the developing embryo or fragments of the embryo in culture, provided many compelling examples for the primacy of cell–cell interactions in regulating invertebrate and vertebrate development. The subsequent identification of many of the signaling factors that mediate cellular communication has led to two general conclusions. First, although there are many important signals, most of these fall into a few large families of secreted peptide factors: theWnt (Wodarz and Nusse 1998), fibroblast growth factor (Szebenyi and Fallon 1999), TGFsuperfamily (Massague and Chen 2000), plateletderived growth factor (Betsholtz et al. 2001), ephrin (Bruckner and Klein 1998), and Hedgehog families. Second, parallel studies in invertebrate and vertebrate systems have shown that although the final outcome might look quite different (e.g., a fly vs. a mouse), there is a striking conservation in the deployment of members of the same signaling families to regulate development of these seemingly quite different organisms. This review focuses on one of the most intriguing examples of this phenomenon, that of the Hedgehog family. As with many of the advances in our understanding of the genetic regulation of animal development, hedgehog (hh) genes owe their discovery to the pioneering work of Nusslein-Volhard and Wieschaus (1980). In their screen for mutations that disrupt the Drosophila larval body plan, these authors identified several that cause the duplication of denticles (spiky cuticular processes that decorate the anterior half of each body segment) and an accompanying loss of naked cuticle, characteristic of the posterior half of each segment (see Fig. 1). The ensuing appearance of a continuous lawn of denticles projecting from the larval cuticle evidently suggested the spines of a hedgehog to the discoverers, hence the origin of the name of one of these genes. Other loci identified by mutants with this phenotype included armadillo, gooseberry, and wingless (wg). In contrast, animals mutant for the aptly named naked gene showed the converse phenotype, with denticle belts replaced by naked cuticle in every segment. On the basis of these mutant phenotypes, Nusslein-Volhard and Wieschaus (1980) proposed that these so-called segment-polarity genes regulate pattern within each of the segments of the larval body, individual genes acting within distinct subregions of the emerging segmental pattern. The first important breakthrough in unraveling how segment-polarity genes act came in the mid-1980s with the cloning of two members of the class, wingless and engrailed (en). Wg was shown to be the ortholog of the vertebrate proto-oncogene int1 (subsequently renamed Wnt1 and the founder member of the Wnt family of secreted peptide factors; Rijsewijk et al. 1987), whereas the sequence of en revealed that it encodes a homeodomaincontaining transcription factor (Fjose et al. 1985; Poole et al. 1985). Intriguingly, the two genes were found to be expressed in adjacent narrow stripes of cells in each segment (Martinez Arias et al. 1988). A close spatial relationship between Wnt1 and En expression domains was also reported in the primordial midbrain and hindbrain of the vertebrate embryo (McMahon et al. 1992). AnalyWe dedicate this review to the memory of our dear friend and colleague Rosa Beddington, whose encouragement led to our initial collaboration. 3Corresponding authors. E-MAIL p.w.ingham@sheffield.ac.uk; FAX 0114-222-288. E-MAIL amcmahon@biosun.harvard.edu; FAX (617) 496-3763. Article and publication are at http://www.genesdev.org/cgi/doi/10.1101/ gad.938601.

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Hedgehog signaling in animal development: paradigms and principles.

https://www.cell.com/cell/pdf/S0092-8674(02)00722-5.pdf

Coordinated transcription of key pathways in the mouse by the circadian clock.

Tau protein isoforms, phosphorylation and role in neurodegenerative disorders

Dietary cholesterol consumption and intestinal cholesterol absorption contribute to plasma cholesterol levels, a risk factor for coronary heart disease. The molecular mechanism of sterol uptake from the lumen of the small intestine is poorly defined. We show that Niemann-Pick C1Like 1(NPC1L1) protein plays a critical role in the absorption of intestinal cholesterol. NPC1L1 expression is enriched in the small intestine and is in the brush border membrane of enterocytes. Although otherwise phenotypically normal, NPC1L1-deficient mice exhibit a substantial reduction in absorbed cholesterol, which is unaffected by dietary supplementation of bile acids. Ezetimibe, a drug that inhibits cholesterol absorption, had no effect in NPC1L1 knockout mice, suggesting that NPC1L1 resides in an ezetimibe-sensitive pathway responsible for intestinal cholesterol absorption.

https://science.sciencemag.org/content/sci/303/5661/1201.full.pdf

Niemann-Pick C1 Like 1 protein is critical for intestinal cholesterol absorption.

Niemann-Pick type C (NP-C) disease, a fatal neurovisceral disorder, is characterized by lysosomal accumulation of low density lipoprotein (LDL)-derived cholesterol. By positional cloning methods, a gene (NPC1) with insertion, deletion, and missense mutations has been identified in NP-C patients. Transfection of NP-C fibroblasts with wild-type NPC1 cDNA resulted in correction of their excessive lysosomal storage of LDL cholesterol, thereby defining the critical role of NPC1 in regulation of intracellular cholesterol trafficking. The 1278-amino acid NPC1 protein has sequence similarity to the morphogen receptor PATCHED and the putative sterol-sensing regions of SREBP cleavage-activating protein (SCAP) and 3-hydroxy-3-methyl-glutaryl coenzyme A (HMG-CoA) reductase.

/pdf/niemann-pick-c1-disease-gene-homology-to-mediators-of-409hu2vbdg.pdf

Niemann-Pick C1 disease gene: homology to mediators of cholesterol homeostasis.

An integrated human-mouse positional candidate approach was used to identify the gene responsible for the phenotypes observed in a mouse model of Niemann-Pick type C (NP-C) disease. The predicted murine NPC1 protein has sequence homology to the putative transmembrane domains of the Hedgehog signaling molecule Patched, to the cholesterol-sensing regions of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase and SREBP cleavage-activating protein (SCAP), and to the NPC1 orthologs identified in human, the nematode Caenorhabditis elegans, and the yeast Saccharomyces cerevisiae. The mouse model may provide an important resource for studying the role of NPC1 in cholesterol homeostasis and neurodegeneration and for assessing the efficacy of new drugs for NP-C disease.

https://science.sciencemag.org/content/sci/277/5323/232.full.pdf

Murine model of Niemann-Pick C disease: mutation in a cholesterol homeostasis gene.

Niemann–Pick disease type C (NP-C) is an autosomal recessive lipidosis linked to chromosome 18q11–12, characterized by lysosomal accumulation of unesterified cholesterol and delayed induction of cholesterol-mediated homeostatic responses. This cellular phenotype is identifiable cytologically by filipin staining and biochemically by measurement of low-density lipoprotein-derived cholesterol esterification. The mutant Chinese hamster ovary cell line (CT60), which displays the NP-C cellular phenotype, was used as the recipient for a complementation assay after somatic cell fusions with normal and NP-C murine cells suggested that this Chinese hamster ovary cell line carries an alteration(s) in the hamster homolog(s) of NP-C. To narrow rapidly the candidate interval for NP-C, three overlapping yeast artificial chromosomes (YACs) spanning the 1 centimorgan human NP-C interval were introduced stably into CT60 cells and analyzed for correction of the cellular phenotype. Only YAC 911D5 complemented the NP-C phenotype, as evidenced by cytological and biochemical analyses, whereas no complementation was obtained from the other two YACs within the interval or from a YAC derived from chromosome 7. Fluorescent in situ hybridization indicated that YAC 911D5 was integrated at a single site per CT60 genome. These data substantially narrow the NP-C critical interval and should greatly simplify the identification of the gene responsible in mouse and man. This is the first demonstration of YAC complementation as a valuable adjunct strategy for positional cloning of a human gene.

https://www.pnas.org/content/pnas/94/14/7378.full.pdf

Christiano Cummings

Papers

Niemann-Pick C1 disease gene: homology to mediators of cholesterol homeostasis.

Murine model of Niemann-Pick C disease: mutation in a cholesterol homeostasis gene.

Substantial narrowing of the Niemann-Pick C candidate interval by yeast artificial chromosome complementation.