PPAR基因在脊椎动物发育过程中的功能研究

过氧化物酶体增殖剂激活受体(peroxisome proliferstor activated receptor,PPAR)是核激素受体家族中的配体激活受体,在不同的物种中已经发现了它的3种亚型,即PPARα、PPARβ(也有称δ)和PPARγ。通过结合到相应的激活剂上,这些受体在一些重要的代谢途径中刺激靶基因的表达。本文是就PPAR在一些脊椎动物胚胎发育过程中的功能方面作一综述。PPAR的功能表现在脂肪组织、脑组织、胎盘和皮肤的分化方面。     

PPAR expression and function during vertebrate development

The peroxisome proliferator activated receptors (PPARs) are ligand activated receptors which belong to the nuclear hormone receptor family. As with other members of this superfamily, it is thought that the ability of PPAR to bind to a ligand was acquired during metazoan evolution. Three different PPAR isotypes (PPARalpha, PPARbeta, also called 6, and PPARgamma) have been identified in various species. Upon binding to an activator, these receptors stimulate the expression of target genes implicated in important metabolic pathways. The present article is a review of PPAR expression and involvement in some aspects of Xenopus laevis and rodent embryonic development. PPARalpha and beta are ubiquitously expressed in Xenopus early embryos but become more tissue restricted later in development. In rodents, PPARalpha, PPARbeta and PPARgamma show specific time- and tissue-dependent patterns of expression during fetal development and in the adult animals. PPARs are implicated in several aspects of tissue differentiation and rodent development, such as differentiation of the adipose tissue, brain, placenta and skin. Particular attention is given to studies undertaken by us and others on the implication of PPARalpha and beta in rodent epidermal differentiation.     

The role of Hox genes during vertebrate limb development

The potential role of Hox genes during vertebrate limb development was brought into focus by gene expression analyses in mice (P Dolle, JC Izpisua-Belmonte, H Falkenstein, A Renucci, D Duboule, Nature 1989, 342:767-772), at a time when limb growth and patterning were thought to depend upon two distinct and rather independent systems of coordinates; one for the anterior-to-posterior axis and the other for the proximal-to-distal axis (see D Duboule, P Dolle, EMBO J 1989, 8:1497-1505). Over the past years, the function and regulation of these genes have been addressed using both gain-of-function and loss-of-function approaches in chick and mice. The use of multiple mutations either in cis-configuration in trans-configuration or in cis/trans configurations, has confirmed that Hox genes are essential for proper limb development, where they participate in both the growth and organization of the structures. Even though their molecular mechanisms of action remain somewhat elusive, the results of these extensive genetic analyses confirm that, during the development of the limbs, the various axes cannot be considered in isolation from each other and that a more holistic view of limb development should prevail over a simple cartesian, chess grid-like approach of these complex structures. With this in mind, the functional input of Hox genes during limb growth and development can now be re-assessed.     

Regulation of GATA gene expression during vertebrate development

GATA factors regulate critical events in hematopoietic lineages (GATA-1/2/3), the heart and gut (GATA-4/5/6) and various other tissues. Transgenic approaches have revealed that GATA genes are regulated in a modular fashion by sets of enhancers that govern distinct temporal and/or spatial facets of the overall expression patterns. Efforts are underway to resolve how these GATA gene enhancers are themselves regulated in order to elucidate the genetic and molecular hierarchies that govern GATA expression in particular developmental contexts. These enhancers also afford a raft of tools that can be used to selectively perturb and probe various developmental events in transgenic animals.     

Myosin light-chain expression during avian muscle development

Monoclonal antibodies to adult chicken myosin light chains were generated and used to quantitate the types of myosin light-chain (MLC) isoforms expressed during development of the pectoralis major (PM), anterior latissimus dorsi (ALD), and medial adductor (MA) muscles of the chicken. These are muscles which, in the adult, are composed predominantly of fast, slow, and a mixture of fiber types, respectively. Three distinct phases of MLC expression characterized the development of the PM and MA muscles. The first identifiable pase occurred during the period of 5-7 d of incubation in ovo. Extracts of muscles from the pectoral region (which included the presumptive PM muscle) contained only fast MLC isoforms. This period of exclusive fast light-chain synthesis was followed by a phase (8- 12 d of incubation in ovo) in which coexpression of both fast and slow MLC isoforms was apparent in both PM and MA muscles. During the period, the composition of both fast and slow MLC isoforms in the PM and MA muscles was identical. Beginning at day 12 in ovo, the ALD was also subjected to immunochemical analyses. The proportion of fast and slow MLCs in this muscle at day 12 was similar to that present in the other muscles studied. The third development phase of MLC expression began at approximately 12 d of incubation in ovo and encompassed the transition in MLC composition to the isoform patterns incubation in ovo and encompassed the transition in MLC composition to the isoform patterns typical of adult muscle. During this period, the relative proportion of slow MLC rose in both the MA and ALD and fell in the PM. By day 16, the third fast light chain, LC(3f), was apparent in extracts of both the PM and MA. These results show that there is a developmental progression in the expression of MLC in the two avian muscles studied from day 5 in ovo; first, only fast MLCs are accumulated, then both fast and slow MLC isoforms are expressed. Only during the latter third of development in ovo is the final MLC isoform pattern characteristic of a particular muscle type expressed.     

Temporal and spatial expression of genes for cartilage extracellular matrix proteins during avian mandibular arch development

We have examined the temporal expression of genes for extracellular matrix proteins (type I collagen, type II collagen, and the cartilage specific proteoglycan core protein) during the development of the avian mandibular arch. We detected low levels of type II collagen mRNA in the mandibular arch as early as stage 15. Type II collagen mRNA remained low but increased slightly as development progressed from stage 15 to stage 25. More dramatic increases occurred after stage 25 coincident with overt chondrogenesis. In contrast, mRNA for the core protein of cartilage specific proteoglycan was not detected prior to the onset of chondrogenesis, appeared at stage 25, and increased thereafter. Type I collagen mRNA was also present as early as stage 15 and dramatically increased after stage 28/29, coincident with initiation of osteogenesis. Using in situ hybridization, we found that type II collagen mRNA became detectable in the center of the mandible around stage 24/25 coincident with the initiation of chondrogenesis. At later stages (26-32) type II collagen mRNA was localized in the cartilaginous rudiment. The pattern of hybridization observed with the proteoglycan core protein probe at later stages of development was essentially identical to that observed with the type II collagen probe. In contrast, the probe for the alpha 1 (I) collagen mRNA was localized over the perichondrium, over differentiated bone, and in areas within the mandibular arch where bone formation had been initiated.     

Differential expression of the cellular src gene during vertebrate development

Cellular genes that are homologous to the transforming genes of certain RNA tumor viruses are suspected to play a functional role during normal developmental processes. To investigate this further, we are studying the expression of the cellular homolog of the Rous sarcoma virus transforming gene (c-src) during embryogenesis of fish, frog, and chicken by quantitative determination of the activity of the c-src encoded protein kinase (pp60^c-src). The kinase activity from embryos of fish, frog, and chicken displays the same enzymatic characteristics as the kinase from adult animals: It phosphorylates only tyrosine residues in protein substrates, and its activity is relatively insensitive to inhibition by the diadenosine nucleotide Ap4A. During the course of development, the varying kinase activity level reflects differential expression of the c-src gene product. The kinase activity is low during early development, increases dramatically during organogenesis, and decreases thereafter to the level found in adult animals. The kinase activity displays an organ specificity, with brain showing the highest activity in embryos as well as in adults. Muscle, however, shows high activities during organogenesis, but no or barely detectable activity in adult animals. Our data suggest, therefore, that the c-src gene product plays more of a role in differentiation than in proliferation processes during embryogenesis, and that it may act as a pleiotropic effector.     

The expression patterns of heat shock genes and proteins and their role during vertebrate's development

Highly evolutionary conserved heat shock proteins (HSPs) act as molecular chaperones in regulation of cellular homeostasis and promoting survival. Generally they are induced by a variety of stressors whose effect could be disastrous on the organism, but they are also widely constitutively expressed in the absence of stress. Varied HSP expressions seem to be very essential in the critical steps of embryonic and extra-embryonic structures formation and may correspond to cell movements, proliferation, morphogenesis and apoptosis, which occur during embryonic development. While our knowledge of detailed HSP expression patterns is in constant progress, their functions during embryonic development are not yet fully understood. In the paper, we review available data on HSP expression and discuss their role during vertebrate development.     

Drosophila melanogaster genes for U1 snRNA variants and their expression during development.

We have cloned and characterized a complete set of seven U1-related sequences from Drosophila melanogaster. These sequences are located at the three cytogenetic loci 21D, 82E, and 95C. Three of these sequences have been previously studied: one U1 gene at 21D which encodes the prototype U1 sequence (U1a), one U1 gene at 82E which encodes a U1 variant with a single nucleotide substitution (U1b), and a pseudogene at 82E. The four previously uncharacterized genes are another U1b gene at 82E, two additional U1a genes at 95C, and a U1 gene at 95C which encodes a new variant (U1c) with a distinct single nucleotide change relative to U1a. Three blocks of 5' flanking sequence similarity are common to all six full length genes. Using specific primer extension assays, we have observed that the U1b RNA is expressed in Drosophila Kc cells and is associated with snRNP proteins, suggesting that the U1b-containing snRNP particles are able to participate in the process of pre-mRNA splicing. We have also examined the expression throughout Drosophila development of the two U1 variants relative to the prototype sequence. The U1c variant is undetectable by our methods, while the U1b variant exhibits a primarily embryonic pattern reminiscent of the expression of certain U1 variants in sea urchin, Xenopus, and mouse.     

Homeobox genes and development of the vertebrate CNS

The discovery of homeobox genes in vertebrates may allow analysis of a basic problem in developmental neurobiology: how regional differences in CNS organization are specified during development. This view is based on the roles defined for homologous genes in Drosophila development, and is supported by studies of the patterns of homeobox gene expression in vertebrate embryos. Homeobox genes comprise a multigene family, members of which are expressed in different spatially restricted domains along the anterior-posterior axis of the CNS. These observations are consistent with homeobox genes having roles in the positional specification of CNS organization, and experimental tests of this should be forthcoming shortly.     

Human tRNAGlu genes: their copy number and organisation

The tRNAGlu gene copy number, determined by genomic blot analysis of human placental DNA, is approximately thirteen. These studies, using several probes and DNA digested with several restriction enzymes singly or in combination, show that most of these tRNAGlu genes are flanked by DNA of very similar sequence for at least 5 kb. This conclusion is supported by the close similarity of the restriction maps of two lambda Charon-4A recombinants of human genomic DNA containing two different tRNAGlu genes.