Human epidermal stem cells express higher levels of ␤1 integrins and are more adhesive than keratinocytes that are destined to differentiate. To investigate whether high ␤1 integrin expression and adhesiveness are essential for maintaining keratinocytes in the stem cell compartment, we introduced a dominant-negative ␤1 integrin mutant, CD8␤1, into cultured human keratinocytes, thereby interfering with ␤1 integrin function. Surface ␤1 integrin levels, adhesiveness, and mitogen-activated protein (MAP) kinase activation on fibronectin were reduced, and exit from the stem cell compartment was stimulated. Adhesiveness and proliferative potential were restored by overexpressing wild-type ␤1 integrin or by constitutive MAP kinase activation. Conversely, a dominant-negative MAP kinase kinase 1 mutant decreased adhesiveness and stem cell number in the absence of CD8␤1. MAP kinase activation by ␣6␤4-mediated adhesion and mitogens was normal in CD8␤1 cells, and constitutive MAP kinase activation did not affect adhesion and proliferation of control keratinocytes. We conclude that ␤1 integrins and MAP kinase cooperate to maintain the epidermal stem cell compartment in vitro.

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Niemann-Pick type C (NPC) disease is a fatal autosomal-recessive neurodegenerative disorder characterized by the inappropriate accumulation of unesterified cholesterol in aberrant organelles. The disease is due to mutations in either of two genes, NPC1, which encodes a transmembrane protein related to the Hedgehog receptor Patched, and NPC2, which encodes a secreted cholesterol-binding protein. Npc1 mutant mice can be partially rescued by treatment with specific steroids. We have created a Drosophila NPC model by mutating dnpc1a, one of two Drosophila genes related to mammalian NPC1. Cells throughout the bodies of dnpc1a mutants accumulated sterol in a punctate pattern, as in individuals with NPC1 mutations. The mutants developed only to the first larval stage and were unable to molt. Molting after the normal first instar period was restored to various degrees by feeding the mutants the steroid molting hormone 20-hydroxyecdysone, or the precursors of ecdysone biosynthesis, cholesterol and 7-dehydrocholesterol. dnpc1ais normally highly expressed in the ecdysone-producing ring gland. Ring gland-specific expression of dnpc1a in otherwise mutant flies allowed development to adulthood, suggesting that the lack of ecdysone in the mutants is the cause of death. We propose that dnpc1a mutants have sterols trapped in aberrant organelles, leading to a shortage of sterol in the endoplasmic reticulum and/or mitochondria of ring gland cells, and,consequently, inadequate ecdysone synthesis.

How developmental signaling proteins traverse tissue during animal development, through or around tightly packed cells, remains an incompletely resolved mystery. Signaling protein movement is regulated to create gradients, control amounts, impose barriers, or provide direction. Signaling can be controlled by the rate of signal production, modification, active transport, trapping along the path, or by the properties of the receptor apparatus. Signals may move by diffusion outside cells, attached to migrating cells, attached to carrier molecules, through cells by transcytosis, along cell extensions, or in released membrane packets. Recent findings about the movement of Hedgehog, Wingless (Wnt), and TGF-β signaling proteins have helped to clarify the molecular mechanisms used to ensure that developmental signals carry only good news.