Krox-20 controls SCIP expression, cell cycle exit and susceptibility to apoptosis in developing myelinating Schwann cells.
The transcription factors Krox-20 and SCIP each play important roles in the differentiation of Schwann cells. However, the genes encoding these two proteins exhibit distinct time courses of expression and yield distinct cellular phenotypes upon mutation. SCIP is expressed prior to the initial appearance of Krox-20, and is transient in both the myelinating and non-myelinating Schwann cell lineages; while in contrast, Krox-20 appears approximately 24 hours after SCIP and then only within the myelinating lineage, where its expression is stably maintained into adulthood. Similarly, differentiation of SCIP-/- Schwann cells appears to transiently stall at the promyelinating stage that precedes myelination, whereas Krox-20(-/-) cells are, by morphological criteria, arrested at this stage. These observations led us to examine SCIP regulation and Schwann cell phenotype in Krox-20 mouse mutants. We find that in Krox-20(-/-) Schwann cells, SCIP expression is converted from transient to sustained. We further observe that both Schwann cell proliferation and apoptosis, which are normal features of SCIP+ cells, are also markedly increased late in postnatal development in Krox-20 mutants relative to wild type, and that the levels of cell division and apoptosis are balanced to yield a stable number of Schwann cells within peripheral nerves. These data demonstrate that the loss of Krox-20 in myelinating Schwann cells arrests differentiation at the promyelinating stage, as assessed by SCIP expression, mitotic activity and susceptibility to apoptosis. (+info
A Meis family protein caudalizes neural cell fates in Xenopus.
A homologue of the Drosophila homothorax (hth) gene, Xenopus Meis3 (XMeis3), was cloned from Xenopus laevis. XMeis3 is expressed in a single stripe of cells in the early neural plate stage. By late neurula, the gene is expressed predominantly in rhombomeres two, three and four, and in the anterior spinal cord. Ectopic expression of RNA encoding XMeis3 protein causes anterior neural truncations with a concomitant expansion of hindbrain and spinal cord. Ectopic XMeis3 expression inhibits anterior neural induction in neuralized animal cap ectoderm explants without perturbing induction of pan-neural markers. In naive animal cap ectoderm, ectopic XMeis3 expression activates transcription of the posteriorly expressed neural markers, but not pan-neural markers. These results suggest that caudalizing proteins, such as XMeis3, can alter A-P patterning in the nervous system in the absence of neural induction. Regionally expressed proteins like XMeis3 could be required to overcome anterior signals and to specify posterior cell fates along the A-P axis. (+info
13-cis-Retinoic acid alters neural crest cells expressing Krox-20 and Pax-2 in macaque embryos.
This study investigates hindbrain and associated neural crest (NCC), otocyst, and pharyngeal arch development in monkey embryos following teratogenic exposure to 13-cis-retinoic acid (cRA). cRA was orally administered (5 mg/kg) to pregnant long-tailed macaques (Macaca fascicularis) between gestational days (GD) 12 and 27. Embryos were surgically collected at desired stages during treatment, analyzed for external morphological changes, and processed for immunohistochemistry. Two transiently expressed nuclear proteins, Krox-20 and Pax-2, were used as markers for the target cellular and anatomical structures. Rhombomere (r) expression patterns of Pax-2 (r4/r6) and Krox-20 (r3/r5) were maintained after cRA treatment, but r4 and r5 were substantially reduced in size. In untreated embryos, Krox-20 immunoreactive NCC derived from r5 migrated caudally around the developing otocyst to contribute to the third pharyngeal arch mesenchyme. In cRA-treated embryos, a subpopulation of NCC rostral to the otocyst also showed Krox-20 immunoreactivity, but there was a substantial reduction in Krox-20 post-otic NCC. Pax-2 immunoreactive NCC migrating from r4 to the second pharyngeal arch were substantially reduced in numbers in treated embryos. Alteration in the otic anlage included delayed invagination, abnormal relationship with the adjacent hindbrain epithelium, and altered expression boundaries for Pax-2. cRA-associated changes in the pharyngeal arch region due to cRA included truncation of the distal portion of the first arch and reduction in the size of the second arch. These alterations in hindbrain, neural crest, otic anlage, and pharyngeal arch morphogenesis could contribute to some of the craniofacial malformations in the macaque fetus associated with exposure to cRA. (+info
Functional consequences of mutations in the early growth response 2 gene (EGR2) correlate with severity of human myelinopathies.
The early growth response 2 gene ( EGR2 ) is a Cys2His2zinc finger transcription factor which is thought to play a role in the regulation of peripheral nervous system myelination. This idea is based partly on the phenotype of homozygous Krox20 ( Egr2 ) knockout mice, which display hypomyelination of the PNS and a block of Schwann cells at an early stage of differentiation. Mutations in the human EGR2 gene have recently been associated with the inherited peripheral neuropathies Charcot-Marie-Tooth type 1, Dejerine-Sottas syndrome and congenital hypomyelinating neuropathy. Three of the four EGR2 mutations are dominant and occur within the zinc finger DNA-binding domain. The fourth mutation is recessive and affects the inhibitory domain (R1) that binds the NAB transcriptional co-repressors. A combination of DNA-binding assays and transcriptional analysis was used to determine the functional consequences of these mutations. The zinc finger mutations affect DNA binding and the amount of residual binding directly correlates with disease severity. The R1 domain mutation prevents interaction of EGR2 with the NAB co-repressors and thereby increases transcriptional activity. These data provide insight into the possible disease mechanisms underlying EGR2 mutations and the reason for varying severity and differences in inheritance patterns. (+info
Dynamics of placodal lineage development revealed by targeted transgene expression.
Examination of the expression pattern of the winged-helix transcription factor BF-1 outside of the neural tube in mouse embryos suggests that BF-1 is restricted to a population of cells that gives rise to the ectodermal placodes and their derivatives. Within the sensory cranial nerve ganglia, the expression of BF-1 distinguishes cells that arise from the placodes from those derived from the neural crest. Expression of a lacZ transgene targeted to the BF-1 locus was used to follow the placodal lineage during mouse development. Analysis of placodal development in mice with a targeted deletion of BF-1 reveals that, although BF-1 is required for proliferation of the cells arising from the nasal placode, it is not required for the proliferation, survival, or both, of placode-derived cells in the sensory cranial nerve ganglia. Dev Dyn 1999;215:332-343. (+info
Coordinate augmentation in expression of genes encoding transcription factors and liver secretory proteins in hypo-oncotic states.
BACKGROUND: In the nephrotic syndrome (NS) proteins of intermediate size (40 to 200 kD) are lost into the urine resulting in a decrease in plasma albumin concentration and as a consequence a reduction in plasma colloid osmotic pressure (pi). Plasma pi has also been reported to be reduced in the condition of hereditary analbuminemia. The liver, in an apparent compensatory response, increases synthesis of a group of secreted proteins defending plasma pi. Regulation of several of these proteins, including both positive and negative acute phase proteins, is at the transcriptional level. This is the only known condition in which transcription of both positive and negative acute phase proteins (APPs) are increased simultaneously. The specific transcription factor(s) that might regulate this cascade is not defined. METHODS: RNA was extracted from livers of 5 rats with hereditary analbuminemia (the Nagase analbuminemic rat, NAR), 5 rats with NS induced by adriamycin (Adria), 5 rats with NS caused by passive Heymann nephritis (NS) and 5 control animals. The concentrations of mRNAs encoding four secreted proteins (albumin, transferrin, fibrinogen, and apo A-1), five transcription factors, early growth response factor 1 (EGRF-1), HNF-4, NGFI-C, EGR-3, and Krox20 relative to two housekeeping genes, beta actin and GAPDH were determined simultaneously using kinetic reverse transcriptase polymerase chain methodology (kRT-PCR). RESULTS: The levels of all mRNAs encoding secreted proteins except for albumin (which was reduced in NAR) were increased in NS and NAR and correlated significantly with one another. mRNA encoding EGRF 1 was increased fivefold in NS and NAR, and correlated significantly with mRNAs encoding Apo A-1, transferrin and albumin in the two NS groups. HNF-4 mRNA was increased approximately twofold in both NS groups and correlated with albumin (R = 0.881, P < 0.001), transferrin (R = 0.563, P = 0.012) and apo A-1 (R = 0.644, P = 0. 003). While fibrinogen mRNA correlated with that of each of the other secreted proteins, it did not correlate with either HNF-4 or EGRF-1 mRNA. Krox20, EGR3 and NGF1C were expressed at nearly undetectable levels. CONCLUSIONS: The hepatic response in conditions characterized by reduced plasma pi include increased levels of mRNAs encoding a group of secreted proteins, including the negative APPs albumin, transferrin and apo A-1, and the positive APP fibrinogen. Levels of mRNAs encoding negative APPs and fibrinogen correlate with one another, suggesting that they are coordinately controlled. Both EGRF-1 and HNF-4 may regulate the expression of the negative APPs, which have increased transcription in hypo-oncotic states. (+info
Successive patterns of clonal cell dispersion in relation to neuromeric subdivision in the mouse neuroepithelium.
We made use of the laacz procedure of single-cell labelling to visualize clones labelled before neuromere formation, in 12.5-day mouse embryos. This allowed us to deduce two successive phases of cell dispersion in the formation of the rhombencephalon: an initial anterior-posterior (AP) cell dispersion, followed by an asymmetrical dorsoventral (DV) cell distribution during which AP cell dispersion occurs in territories smaller than one rhombomere. We conclude that the general arrest of AP cell dispersion precedes the onset of morphological segmentation and is not imposed by the interface between adjacent rhombomeres. This demonstrates a major change in the mode of epithelial growth that precedes or accompanies the formation of neuromeres. We also deduced that the period of DV cell dispersion in the neuroepithelium is followed by a coherent growth phase. These results suggest a cell organization on a Cartesian grid, the coordinates of which correspond to the AP and DV axis of the neural tube. A similar sequence of AP cell dispersion followed by an arrest of AP cell dispersion, a preferential DV cell dispersion and then by a coherent neuroepithelial growth, is also observed in the spinal cord and mesencephalon. This demonstrates that a similar cascade of cell events occurs in these different domains of the CNS. In the prosencephalon, differences in spatial constraints may explain the variability in the orientation of cell clusters. Genetic and clonal patterning in the AP and DV dimensions follow the same spatial sequence. An interesting possibility is that these successive patterns of cell growth facilitate the acquisition of positional information. (+info
Essential role of CREB family proteins during Xenopus embryogenesis.
The leucine zipper transcription factors cAMP response element binding protein (CREB), cAMP response element modulatory protein (CREM) and activating transcription factor 1 (ATF1) bind to the cAMP response element (CRE) with the palindromic consensus sequence TGACGTCA. Their transcriptional activities are dependent on serine phosphorylation induced by various extracellular signals such as hormones, growth factors and neurotransmitters. Here we show that CREB is the predominant CRE-binding protein in Xenopus embryos and that it plays an essential role during early development. The importance of CREB for morphogenetic processes was assessed by injection of RNA encoding a dominant-negative form of CREB that is fused to a truncated progesterone receptor ligand binding domain. In this fusion protein, a dominant-negative function can be induced by application of the synthetic steroid RU486 at given developmental stages. The inhibition of CREB at blastula and early gastrula stages leads to severe posterior defects of the embryos reflected by strong spina bifida, whereas the inhibition of CREB at the beginning of neurulation resulted in stunted embryos with microcephaly. In these embryos, initial induction of neural and mesodermal tissues is not dependent on CREB function, as genes such as Otx2, Krox20, Shh and MyoD are still expressed in injected embryos. But the expression domains of Otx2 and MyoD were found to be distorted reflecting the abnormal development in both neural and somitic derivatives. In summary, our data show that CREB is essential during several developmental stages of Xenopus embryogenesis. (+info