We report evidence for a mechanism for the maintenance of long-range conserved synteny across vertebrate genomes. We found the largest mammal-teleost conserved chromosomal segments to be spanned by highly conserved noncoding elements (HCNEs), their developmental regulatory target genes, and phylogenetically and functionally unrelated "bystander" genes. Bystander genes are not specifically under the control of the regulatory elements that drive the target genes and are expressed in patterns that are different from those of the target genes. Reporter insertions distal to zebrafish developmental regulatory genes pax6.1/2, rx3, id1, and fgf8 and miRNA genes mirn9-1 and mirn9-5 recapitulate the expression patterns of these genes even if located inside or beyond bystander genes, suggesting that the regulatory domain of a developmental regulatory gene can extend into and beyond adjacent transcriptional units. We termed these chromosomal segments genomic regulatory blocks (GRBs). After whole genome duplication in teleosts, GRBs, including HCNEs and target genes, were often maintained in both copies, while bystander genes were typically lost from one GRB, strongly suggesting that evolutionary pressure acts to keep the single-copy GRBs of higher vertebrates intact. We show that loss of bystander genes and other mutational events suffered by duplicated GRBs in teleost genomes permits target gene identification and HCNE/target gene assignment. These findings explain the absence of evolutionary breakpoints from large vertebrate chromosomal segments and will aid in the recognition of position effect mutations within human GRBs.

Myelination in Schwann cells is governed by several transcription factors, including the POU proteins Oct6 and Brn2, the high mobility group protein Sox10 and the zinc-finger protein Krox20. How the function of these factors is integrated in the control of myelination has not been established. Previously, we identified an enhancer element controlling Krox20 expression throughout myelination in Schwann cells. In this paper, cell culture experiments were combined with transgenesis to identify transcription factors acting directly upstream of Krox20. The results show that during the promyelin-myelin transition, Krox20 expression is directly activated by Oct6 and Brn2 acting on this enhancer. In addition, the enhancer-dependent synergism between these POU proteins and Sox10 suggests that Krox20 expression requires this combination of factors. These results resolve previous controversy concerning the mechanism of action of Oct6 and Brn2 during myelination and provide an explanation for myelin deficiencies in Waardenberg-Hirschsprung disease patients whereby Sox10 mutations could lead to a loss of Krox20 expression. Keywords: Krox20/Egr2; Oct6/Tst1/SCIP/Pou3f1/Oft-6; Brn2/NOct-3/Pou3f2/Oft-7; myelination; peripheral nervous system; transcriptional regulation EMBO reports (2006) 7, 52-58. doi:10.1038/sj.embor.7400573 INTRODUCTIONThe myelin sheath, which serves to increase nerve conduction velocities, is deposited around axons by specialized cells in the central and peripheral nervous systems of higher vertebrates. In the peripheral nervous system, myelin is synthesized by Schwann cells and, so far, several transcription factors participating in the onset of myelination have been described. A pivotal factor is the zinc-finger transcription factor gene Krox20 (Egr2), the mutation of which in the mouse blocks Schwann cells at the promyelinating stage (Topilko et al, 1994). Consistently, Krox20/Egr2 mutations have also been identified in patients suffering from peripheral neuropathies (Warner et al, 1998). Taken together with cell culture experiments showing that myelin genes are induced by Krox20 (Nagarajan et al, 2001), these data suggest that Krox20 has characteristics of a master regulator of myelination. Previously, our studies into the regulation of Krox20 identified a transcriptional enhancer, designated the myelinating Schwann cell element (MSE), which is under the control of the POU domain transcription factor Oct6 (Ghislain et al, 2002).Oct6 is transiently expressed in Schwann cells, peaking at the promyelinating stage (Jaegle et al, 2003). The analysis of Oct6 lossof-function alleles indicated that mutant Schwann cells show a transient delay in myelination (Jaegle et al, 1996). More recently, the related POU gene, Brn2, expressed in Schwann cells in a manner similar to Oct6, was shown to compensate for the Oct6 mutation, the combined loss of both Oct6 and Brn2 provoking a more severe delay in myelination (Jaegle et al, 2003). Although a role for Oct6 and Brn2 in promoting myelination is established, their mechan...

Onset of myelination in Schwann cells is governed by several transcription factors, including Krox20/Egr2, and mutations affecting Krox20 result in various human hereditary peripheral neuropathies, including congenital hypomyelinating neuropathy (CHN) and Charcot-Marie-Tooth disease (CMT). Similar molecular information is not available on the process of myelin maintenance. We have generated conditional Krox20 mutations in the mouse that allowed us to develop models for CHN and CMT. In the latter case, specific inactivation of Krox20 in adult Schwann cells results in severe demyelination, involving rapid Schwann cell dedifferentiation and increased proliferation, followed by an attempt to remyelinate and a block at the promyelinating stage. These data establish that Krox20 is not only required for the onset of myelination but that it is also crucial for the maintenance of the myelinating state. Furthermore, myelin maintenance appears as a very dynamic process in which Krox20 may constitute a molecular switch between Schwann cell myelination and demyelination programs.

of the Rodent Metabolic Phenotyping core facility of the CRCHUM for metabolic studies in mice; M. Guévremont, J. Morin, and the Cellular Physiology Service core of the CRCHUM for quantification of β cell mass and αLISA assay; C. Tremblay (CRCHUM) for valuable technical assistance; E. Joly for technical advice; and M. Ferdaoussi for fruitful discussions.Address correspondence to: Vincent Poitout, CRCHUM, 900 Saint-Denis Street, Montreal, Quebec, H2X 0A9, Canada. Phone: 514.890.8044; E-mail: vincent.poitout@umontreal.ca.the CHUM (protocols ND-05-035 and 16-048, respectively). Written consent for research was obtained from all donors.

The vertebrate hindbrain is subject to a transient segmentation process leading to the formation of seven or eight metameric territories termed rhombomeres (r). This segmentation provides the basis for the subsequent establishment of hindbrain neuronal organization and participates in the patterning of the neural crest involved in craniofacial development. The zinc-finger gene Krox20 is expressed in r3 and r5, and encodes a transcription factor that plays a key role in hindbrain segmentation, coordinating segment formation, specification of odd-and even-numbered rhombomeres, and cell segregation between adjacent segments, through the regulation of numerous downstream genes. In order to further elucidate the genetic network underlying hindbrain segmentation, we have undertaken the analysis of the cis-regulatory sequences governing Krox20 expression. We have found that the control of Krox20 transcription relies on three very long-range (200 kb) enhancer elements (A, B and C) that are conserved between chick, mouse and human genomes. Elements B and C are activated at the earliest stage of Krox20 expression in r5 and r3-r5, respectively, and do not require the Krox20 protein. These elements are likely to function as initiators of Krox20 expression. Element B contains a binding site for the transcription factor vHNF1, the mutation of which abolishes its activity, suggesting that vHNF1 is a direct initiator of Krox20 expression in r5. Element A contains Krox20-binding sites, which are required, together with the Krox20 protein, for its activity. This element therefore allows the establishment of a direct positive autoregulatory loop, which takes the relay of the initiator elements and maintains Krox20 expression. Together, our studies provide a basis for a model of the molecular mechanisms controlling Krox20 expression in the developing hindbrain and neural crest.

Background: FFAR1/GPR40 is a potential target to enhance insulin secretion in type 2 diabetes, yet knowledge of the pharmacobiology of GPR40 remains incomplete. Results: GPR40 functions via both G protein-mediated and ␤-arrestin-mediated mechanisms; endogenous and synthetic ligands differentially engage these pathways to promote insulin secretion. Conclusion: GPR40 is subject to functionally relevant biased agonism. Significance: Biased agonism at GPR40 could be exploited for therapeutic purposes.

Neural crest patterning constitutes an important element in the control of the morphogenesis of craniofacial structures. Krox20, a transcription factor gene that plays a critical role in the development of the segmented hindbrain, is expressed in rhombomeres (r) 3 and 5 and in a stream of neural crest cells migrating from r5 toward the third branchial arch. We have investigated the basis of the specific neural crest expression ofKrox20 and identified a cis-acting enhancer element (NCE) located 26 kb upstream of the gene that is conserved between mouse, man and chick and can recapitulate the Krox20 neural crest pattern in transgenic mice. Functional dissection of the enhancer revealed the presence of two conserved Krox20 binding sites mediating direct Krox20 autoregulation in the neural crest. In addition, the enhancer included another essential element containing conserved binding sites for high mobility group (HMG) box proteins and which responded to factors expressed throughout the neural crest. Consistent with this the NCE was strongly activated in vitro by Sox10, a crest-specific HMG box protein, in synergism with Krox20, and the inactivation of Sox10prevented the maintenance of Krox20 expression in the migrating neural crest. These results suggest that the dependency of the enhancer on both crest- (Sox10) and r5- (Krox20) specific factors limits its activity to the r5-derived neural crest. This organisation also suggests a mechanism for the transfer and maintenance of rhombomere-specific gene expression from the hindbrain neuroepithelium to the emerging neural crest and may be of more general significance for neural crest patterning.

SUMMARYThe mechanisms underlying the generation of neural cell diversity are the subject of intense investigation, which has highlighted the involvement of different signalling molecules including Shh, BMP and Wnt. By contrast, relatively little is known about FGF in this process. In this report we identify an FGF-receptor-dependent pathway in zebrafish hindbrain neural progenitors that give rise to somatic motoneurons, oligodendrocyte progenitors and differentiating astroglia. Using a combination of chemical and genetic approaches to conditionally inactivate FGF-receptor signalling, we investigate the role of this pathway. We show that FGF-receptor signalling is not essential for the survival or maintenance of hindbrain neural progenitors but controls their fate by coordinately regulating key transcription factors. First, by cooperating with Shh, FGF-receptor signalling controls the expression of olig2, a patterning gene essential for the specification of somatic motoneurons and oligodendrocytes. Second, FGF-receptor signalling controls the development of both oligodendrocyte progenitors and astroglia through the regulation of sox9, a gliogenic transcription factor the function of which we show to be conserved in the zebrafish hindbrain. Overall, for the first time in vivo, our results reveal a mechanism of FGF in the control of neural cell diversity.