Jacques Thélu

Localisation of members of the notch system and the differentiation of vibrissa hair follicles: receptors, ligands, and fringe modulators.

Hair vibrissa follicle morphogenesis involves several cell segregation phases, in the dermis as well as in the epidermis. The expression of Notch‐related genes, which are well established mediators of multiple cell segregation events in Drosophila development, was studied by in situ hybridisation during embryonic mouse vibrissa follicle morphogenesis and the first adult hair cycle. The results show that two receptors, Notch1 and ‐2, three ligands, Delta1, Serrate1, and ‐2, and the three Fringe regulators, Lunatic, Manic, and Radical, are expressed in different locations and morphogenetic stages. First, the appearance of hair vibrissa primordia involves the expression of complementary patterns of Notch2, Delta1, and Lunatic Fringe in the dermis and of Notch1, Serrate2, and Lunatic Fringe in the epidermis. Second, this expression pattern is no longer found after stage 3 in the dermis. Meanwhile, in the epidermis, the expression of Notch1, Serrate2, and Lunatic Fringe before the formation of the placode may be involved in determining two populations of epidermal cells in the developing follicle. Third, complementary expression patterns for Notch1, Manic, and Lunatic Fringe, as well as Serrate1 and ‐2 as previously shown (Powell et al., 1998 ), are progressively established from stage 4 of embryonic development both in the outer root sheath and in the hair matrix. These patterns are consistent with the one found in the adult anagen phase. During the hair vibrissa cycle, Notch1 and Manic Fringe display temporal and spatial changes of expression, suggesting that they may intervene as modulators of trichocyte activities. Dev Dyn 2000;218:426–437.

CHOXC-8 and CHOXD-13 Expression in Embryonic Chick Skin and Cutaneous Appendage Specification

We studied the expression of two distantly clustered Hox genes which could, respectively, be involved in specification of dorsal feather‐ and foot scale‐forming skin in the chick embryo: cHoxc‐8, a median paralog, and cHoxd‐13, located at the 5′ extremity of the HoxD cluster. The cHoxc‐8 transcripts are present at embryonic day 3.5 (E3.5)in the somitic cells, which give rise to the dorsal dermis by E5, and at E6.5–8.5 in the dorsal dermal and epidermal cells during the first stages of feather morphogenesis. The cHoxd‐13 transcripts are present at E4.5–9.5 in the autopodial mesenchyme and at E10.5–12.5 in the plantar dermis during the initiation of reticulate scale morphogenesis. Both the cHoxc‐8 and cHoxd‐13 transcripts are no longer detectable after the anlagen stage of cutaneous appendage morphogenesis. Furthermore, heterotopic dermal–epidermal recombinations of dorsal, plantar, and apteric tissues revealed that the epidermal ability or inability to form feathers is already established by the time of skin formation. Retinoic acid (RA) treatment at E11 induces after 12 hr an inhibition of cHoxd‐13 expression in the plantar dermis, followed by the formation of feather filaments on the reticulate scales. When E7.5 dorsal explants are treated with RA for 6 days, they form scale‐like structures where the Hox transcripts are no more detectable. Protein analysis revealed that the plantar filaments, made up of feather β‐keratins, corresponded to a homeotic transformation, whereas the scale‐like structures, composed also of feather β‐keratins, were teratoid. These results strengthen the hypothesis that different homeobox genes play a significant role in specifying the regional identity of the different epidermal territories. Dev. Dyn. 1997;210: 274–287.© 1997 Wiley‐Liss, Inc.

EVIDENCE FOR EXPRESSION OF A RAS-LIKE AND A STAGE SPECIFIC GTP BINDING HOMOLOGOUS PROTEIN BY PLASMODIUM FALCIPARUM

Plasmodium falciparum, the parasite responsible for the most severe form of malaria, undergoes an asexual multiplication in man and a sexual one in mosquito. The asexual cycle can be reproduced in vitro. The present work reports the isolation of a small guanosine triphosphate-binding protein in Plasmodium falciparum extracts. This protein, a 21,000 M(r) Ras-like molecule, was revealed by western blotting in each stage of the intraerythrocytic asexual life cycle. Conversely, a 46,000 M(r) G alpha subunit of a heterotrimeric GTP-binding protein was found to be expressed during a short period from mature schizonts to free merozoites. In order to provide additional evidence for the presence of these GTP-binding proteins in Plasmodium falciparum cultures and also to determine the kinetics, we tested two toxins that are involved in the cellular signalling transduction. We observed that pertussis toxin increases P. falciparum growth, whereas cholera toxin induces crisis forms, and subsequent parasite death within the following 24 h.

STAGE- AND TISSUE-SPECIFIC EXPRESSION OF A β-1,4-GALACTOSYLTRANSFERASE IN THE EMBRYONIC EPIDERMIS

SummaryCharges in oligosaccharide structures of glycoconjugates have been observed, and are postulated to have key roles in embryonic development and differentiation. N-Acetylglucosamine (GlcNAc) β-1,4-galactosyltransferase (β4GalT) AKI showed different expression patterns in time and space, and different enzymatic activity from the other known family members. The epidermis of mouse embryo included a high level of AKI activities, which transferred galactose (Gal) to endogenous glycoprotein (molecular weight 130 kDa) (GP130). The maximum activity was for 13.5-d postcoitum embryos. Specific antibody against AKI inhibited 81% of GlcNAc β4GalT activities, which indicates that AKI represents the major part of the embryonic epidermis enzymes. AKI shows 2.2 times higher galactosyltransferase activity toward Gal-acceptor glucose with α-lactalbumin (α-LA) than toward GlcNAc without α-LA. AKI is also expressed in mouse melanoma and leukemia cell lines and in human basal cell carcinoma specimens. The GP130 Gal acceptor once galactosylated by AKI may be directly involved in epidermal differentiation and oncogenesis.

Analysis of NUAK1 and NUAK2 expression during early chick development reveals specific patterns in the developing head

Several human diseases are associated with the NUAK1 and NUAK2 genes. These genes encode kinases, members of the AMPK-related kinases (ARK) gene family. Both NUAK1 and NUAK2 are known targets of the serine threonine kinase LKB1, a tumor suppressor involved in regulating cell polarity. While much is known about their functions in disease, their expression pattern in normal development has not been extensively studied. Here, we present the expression patterns for NUAK1 and NUAK2 in the chick during early-stage embryogenesis, until day 3 (Hamburger and Hamilton stage HH20). Several embryonic structures, in particular the nascent head, showed distinct expression levels. NUAK1 expression was first detected at stage HH6 in the rostral neural folds. It was then expressed (HH7-11) throughout the encephalalon, predominantly in the telencephalon and mesencephalon. NUAK1 expression was also detected in the splanchnic endoderm area at HH8-10, and in the vitellin vein derived from this area, but not in the heart. NUAK2 expression was first detected at stage HH6 in the neural folds. It was then found throughout the encephalon at stage HH20. Particular attention was paid in this study to the dorsal ectoderm at stages HH7 and HH8, where a local deficit or accumulation of NUAK2 mRNA were found to correlate with the direction of curvature of the neural plate. This is the first description of NUAK1 and NUAK2 expression patterns in the chick during early development; it reveals non-identical expression profiles for both genes in neural development.

LKB1 signaling in cephalic neural crest cells is essential for vertebrate head development.

Head development in vertebrates proceeds through a series of elaborate patterning mechanisms and cell-cell interactions involving cephalic neural crest cells (CNCC). These cells undergo extensive migration along stereotypical paths after their separation from the dorsal margins of the neural tube and they give rise to most of the craniofacial skeleton. Here, we report that the silencing of the LKB1 tumor suppressor affects the delamination of pre-migratory CNCC from the neural primordium as well as their polarization and survival, thus resulting in severe facial and brain defects. We further show that LKB1-mediated effects on the development of CNCC involve the sequential activation of the AMP-activated protein kinase (AMPK), the Rho-dependent kinase (ROCK) and the actin-based motor protein myosin II. Collectively, these results establish that the complex morphogenetic processes governing head formation critically depends on the activation of the LKB1 signaling network in CNCC.