Multiple roles for FGF-3 during cranial neural development in the chicken

Hindbrain boundaries as niches of neural progenitor and stem cells regulated by the extracellular matrix proteoglycan chondroitin sulphate

The interplay between neural progenitors and stem cells (NPSCs), and their extracellular matrix (ECM) is a crucial regulatory mechanism that determines their behavior. Nonetheless, how the ECM dictates the state of NPSCs remains elusive. The hindbrain is valuable to examine this relationship, as cells in the ventricular surface of hindbrain boundaries (HBs), which arise between any two neighboring rhombomeres, express the NPSC marker Sox2, while being surrounded with the membrane-bound ECM molecule chondroitin sulphate proteoglycan (CSPG), in chick and mouse embryos. CSPG expression was used to isolate HB Sox2+ cells for RNA-sequencing, revealing their distinguished molecular properties as typical NPSCs, which express known and newly identified genes relating to stem cells, cancer, the matrisome and cell cycle. In contrast, the CSPG- non-HB cells, displayed clear neural-differentiation transcriptome. To address whether CSPG is significant for hindbrain development, its expression was manipulated in vivo and in vitro. CSPG manipulations shifted the stem versus differentiation state of HB cells, evident by their behavior and altered gene expression. These results provide further understanding of the uniqueness of hindbrain boundaries as repetitive pools of NPSCs in-between the rapidly growing rhombomeres, which rely on their microenvironment to maintain their undifferentiated state during development.

Shared features in ear and kidney development - implications for oto-renal syndromes

The association between ear and kidney anomalies has long been recognized. However, little is known about the underlying mechanisms. In the last two decades, embryonic development of the inner ear and kidney has been studied extensively. Here, we describe the developmental pathways shared between both organs with particular emphasis on the genes that regulate signalling cross talk and the specification of progenitor cells and specialised cell types. We relate this to the clinical features of oto-renal syndromes and explore links to developmental mechanisms.

Primer on FGF3

Though initially discovered as a proto-oncogene in virally induced mouse mammary tumors, FGF3 is primarily active in prenatal stages, where it is found at various sites at specific times. FGF3 is crucial during development, as its roles include tail formation, inner ear development and hindbrain induction and patterning. FGF3 expression and function are highly conserved in vertebrates, while it also interacts with other FGFs in various developmental processes. Intriguingly, while it is classified as a classical paracrine signaling factor, murine FGF3 was uniquely found to also act in an intracrine manner, depending on alternative translation initiation sites. Corresponding with its conserved role in inner ear morphogenesis, mutations in FGF3 in humans are associated with LAMM syndrome, a disorder that include hearing loss and inner ear malformations. While recent studies indicate of some FGF3 presence in post-natal stages, emerging evidences of its upregulation in various human tumors and cariogenic processes in mouse models, highlights the importance of its close regulation in adult tissues. Altogether, the broad and dynamic expression pattern and regulation of FGF3 in embryonic and adult tissues together with its link to congenital malformations and cancer, calls for further discoveries of its diverse roles in health and disease.

Zbtb16 mediates a switch between Fgf signalling regimes in the developing hindbrain

Developing tissues are sequentially patterned by extracellular signals that are turned on and off at specific times. In the zebrafish hindbrain, fibroblast growth factor (Fgf) signalling has different roles at different developmental stages: in the early hindbrain, transient Fgf3 and Fgf8 signalling from rhombomere 4 is required for correct segmentation, whereas later, neuronal Fgf20 expression confines neurogenesis to specific spatial domains within each rhombomere. How the switch between these two signalling regimes is coordinated is not known. We present evidence that the Zbtb16 transcription factor is required for this transition to happen in an orderly fashion. Zbtb16 expression is high in the early anterior hindbrain, then gradually upregulated posteriorly and confined to neural progenitors. In mutants lacking functional Zbtb16, fgf3 expression fails to be downregulated and persists until a late stage, resulting in excess and more widespread Fgf signalling during neurogenesis. Accordingly, the spatial pattern of neurogenesis is disrupted in Zbtb16 mutants. Our results reveal how the distinct stage-specific roles of Fgf signalling are coordinated in the zebrafish hindbrain.

Pre-mandibular pharyngeal pouches in early non-teleost fish embryos

The vertebrate pharynx is a key embryonic structure with crucial importance for the metameric organization of the head and face. The pharynx is primarily built upon progressive formation of paired pharyngeal pouches that typically develop in post-oral (mandibular, hyoid and branchial) domains. However, in the early embryos of non-teleost fishes, we have previously identified pharyngeal pouch-like outpocketings also in the pre-oral domain of the cranial endoderm. This pre-oral gut (POG) forms by early pouching of the primitive gut cavity, followed by the sequential formation of typical (post-oral) pharyngeal pouches. Here, we tested the pharyngeal nature of the POG by analysing expression patterns of selected core pharyngeal regulatory network genes in bichir and sturgeon embryos. Our comparison revealed generally shared expression patterns, including Shh, Pax9, Tbx1, Eya1, Six1, Ripply3 or Fgf8, between early POG and post-oral pharyngeal pouches. POG thus shares pharyngeal pouch-like morphogenesis and a gene expression profile with pharyngeal pouches and can be regarded as a pre-mandibular pharyngeal pouch. We further suggest that pre-mandibular pharyngeal pouches represent a plesiomorphic vertebrate trait inherited from our ancestor's pharyngeal metameric organization, which is incorporated in the early formation of the pre-chordal plate of vertebrate embryos.

Patterns of senescence and apoptosis during development of branchial arches, epibranchial placodes, and pharyngeal pouches

BACKGROUND

Many developmental processes are coregulated by apoptosis and senescence. However, there is a lack of data on the development of branchial arches, epibranchial placodes, and pharyngeal pouches, which harbor epibranchial signaling centers.

RESULTS

Using immunohistochemical, histochemical, and 3D reconstruction methods, we show that in mice, senescence and apoptosis together may contribute to the invagination of the branchial clefts and the deepening of the cervical sinus floor, in antagonism to the proliferation acting in the evaginating branchial arches. The concomitant apoptotic elimination of lateral line rudiments occurs in the absence of senescence. In the epibranchial placodes, senescence and apoptosis appear to (1) support invagination or at least indentation by immobilizing the margins of the centrally proliferating pit, (2) coregulate the number and fate of Pax8+ precursors, (3) progressively narrow neuroblast delamination sites, and (4) contribute to placode regression. Putative epibranchial signaling centers in the pharyngeal pouches are likely deactivated by rostral senescence and caudal apoptosis.

CONCLUSIONS

Our results reveal a plethora of novel patterns of apoptosis and senescence, some overlapping, some complementary, whose functional contributions to the development of the branchial region, including the epibranchial placodes and their signaling centers, can now be tested experimentally.

Genome-wide association studies for egg quality traits in White Leghorn layers using low-pass sequencing and SNP chip data

Low-pass sequencing data have been proposed as an alternative to single nucleotide polymorphism (SNP) chips in genome-wide association studies (GWAS) of several species. However, it has not been used in layer chickens yet. This study aims at comparing the GWAS results of White Leghorn chickens using low-pass sequencing data (1×) and 54 k SNP chip data. Ten commercially relevant egg quality traits including albumen height, shell strength, shell colour, egg weight and yolk weight collected from up to 1,420 White Leghorn chickens were analysed. The results showed that the genomic heritability estimates based on low-pass sequencing data were higher than those based on SNP chip data. Although two GWAS analyses showed similar overall landscape for most traits, low-pass sequencing captured some significant SNPs that were not on the SNP chip. In GWAS analysis using 54 k SNP chip data, after including more individuals (up to 5,700), additional significant SNPs not detected by low-pass sequencing data were found. In conclusion, GWAS using low-pass sequencing data showed similar results to those with SNP chip data and may require much larger sample sizes to show measurable advantages.

Segmentation and patterning of the vertebrate hindbrain

During early development, the hindbrain is sub-divided into rhombomeres that underlie the organisation of neurons and adjacent craniofacial tissues. A gene regulatory network of signals and transcription factors establish and pattern segments with a distinct anteroposterior identity. Initially, the borders of segmental gene expression are imprecise, but then become sharply defined, and specialised boundary cells form. In this Review, we summarise key aspects of the conserved regulatory cascade that underlies the formation of hindbrain segments. We describe how the pattern is sharpened and stabilised through the dynamic regulation of cell identity, acting in parallel with cell segregation. Finally, we discuss evidence that boundary cells have roles in local patterning, and act as a site of neurogenesis within the hindbrain.

Hindbrain induction and patterning during early vertebrate development

The hindbrain is a key relay hub of the central nervous system (CNS), linking the bilaterally symmetric half-sides of lower and upper CNS centers via an extensive network of neural pathways. Dedicated neural assemblies within the hindbrain control many physiological processes, including respiration, blood pressure, motor coordination and different sensations. During early development, the hindbrain forms metameric segmented units known as rhombomeres along the antero-posterior (AP) axis of the nervous system. These compartmentalized units are highly conserved during vertebrate evolution and act as the template for adult brainstem structure and function. TALE and HOX homeodomain family transcription factors play a key role in the initial induction of the hindbrain and its specification into rhombomeric cell fate identities along the AP axis. Signaling pathways, such as canonical-Wnt, FGF and retinoic acid, play multiple roles to initially induce the hindbrain and regulate Hox gene-family expression to control rhombomeric identity. Additional transcription factors including Krox20, Kreisler and others act both upstream and downstream to Hox genes, modulating their expression and protein activity. In this review, we will examine the earliest embryonic signaling pathways that induce the hindbrain and subsequent rhombomeric segmentation via Hox and other gene expression. We will examine how these signaling pathways and transcription factors interact to activate downstream targets that organize the segmented AP pattern of the embryonic vertebrate hindbrain.

A new role of hindbrain boundaries as pools of neural stem/progenitor cells regulated by Sox2

Background

Compartment boundaries are an essential developmental mechanism throughout evolution, designated to act as organizing centers and to regulate and localize differently fated cells. The hindbrain serves as a fascinating example for this phenomenon as its early development is devoted to the formation of repetitive rhombomeres and their well-defined boundaries in all vertebrates. Yet, the actual role of hindbrain boundaries remains unresolved, especially in amniotes.

Results

Here, we report that hindbrain boundaries in the chick embryo consist of a subset of cells expressing the key neural stem cell (NSC) gene Sox2. These cells co-express other neural progenitor markers such as Transitin (the avian Nestin), GFAP, Pax6 and chondroitin sulfate proteoglycan. The majority of the Sox2⁺ cells that reside within the boundary core are slow-dividing, whereas nearer to and within rhombomeres Sox2⁺ cells are largely proliferating. In vivo analyses and cell tracing experiments revealed the contribution of boundary Sox2⁺ cells to neurons in a ventricular-to-mantle manner within the boundaries, as well as their lateral contribution to proliferating Sox2⁺ cells in rhombomeres. The generation of boundary-derived neurospheres from hindbrain cultures confirmed the typical NSC behavior of boundary cells as a multipotent and self-renewing Sox2⁺ cell population. Inhibition of Sox2 in boundaries led to enhanced and aberrant neural differentiation together with inhibition in cell-proliferation, whereas Sox2 mis-expression attenuated neurogenesis, confirming its significant function in hindbrain neuronal organization.

Conclusions

Data obtained in this study deciphers a novel role of hindbrain boundaries as repetitive pools of neural stem/progenitor cells, which provide proliferating progenitors and differentiating neurons in a Sox2-dependent regulation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0277-y) contains supplementary material, which is available to authorized users.

Fgf3 and Fgf16 expression patterns define spatial and temporal domains in the developing chick inner ear

The inner ear is a morphologically complex sensory structure with auditory and vestibular functions. The developing otic epithelium gives rise to neurosensory and non-sensory elements of the adult membranous labyrinth. Extrinsic and intrinsic signals manage the patterning and cell specification of the developing otic epithelium by establishing lineage-restricted compartments defined in turn by differential expression of regulatory genes. FGF3 and FGF16 are excellent candidates to govern these developmental events. Using the chick inner ear, we show that Fgf3 expression is present in the borders of all developing cristae. Strong Fgf16 expression was detected in a portion of the developing vertical and horizontal pouches, whereas the cristae show weaker or undetected Fgf16 expression at different developmental stages. Concerning the rest of the vestibular sensory elements, both the utricular and saccular maculae were Fgf3 positive. Interestingly, strong Fgf16 expression delimited these Fgf16-negative sensory patches. The Fgf3-negative macula neglecta and the Fgf3-positive macula lagena were included within weakly Fgf16-expressing areas. Therefore, different FGF-mediated mechanisms might regulate the specification of the anterior (utricular and saccular) and posterior (neglecta and lagena) maculae. In the developing cochlear duct, dynamic Fgf3 and Fgf16 expression suggests their cooperation in the early specification and later cell differentiation in the hearing system. The requirement of Fgf3 and Fgf16 genes in endolymphatic apparatus development and neurogenesis are discussed. Based on these observations, FGF3 and FGF16 seem to be key signaling pathways that control the inner ear plan by defining epithelial identities within the developing otic epithelium.

Early steps in inner ear development: induction and morphogenesis of the otic placode

Various cellular replacement therapies using in vitro generated cells to replace damaged tissue have been proposed as strategies to alleviate hearing loss. All such therapies must involve a complete understanding of the earliest steps in inner ear development; its induction as a thickened plate of cells in the non-neural, surface ectoderm of the embryo, to its internalization as an otocyst embedded in the head mesenchyme of the embryo. Such knowledge informs researchers addressing the feasibility of the proposed strategy and present alternatives if needed. In this review we describe the mechanisms of inner ear induction, concentrating on the factors that steer the fate of ectoderm into precursors of the inner ear. Induction then leads to inner ear morphogenesis and we describe the cellular changes that occur as the inner ear is converted from a superficial placode to an internalized otocyst, and how they are coordinated with a particular emphasis on how the signaling environment surrounding the inner ear influences these processes.

Mesodermal Fgf10b cooperates with other fibroblast growth factors during induction of otic and epibranchial placodes in zebrafish

BACKGROUND

Vertebrate otic and epibranchial placodes develop in close proximity in response to localized fibroblast growth factor (Fgf) signaling. Although less is known about epibranchial induction, the process of otic induction in highly conserved, with important roles for Fgf3 and Fgf8 reported in all species examined. Fgf10 is also critical for otic induction in mouse, but the only zebrafish ortholog examined to date, fgf10a, is not expressed early enough to play such a role. A second zebrafish ortholog, fgf10b, has not been previously examined.

RESULTS

We find that zebrafish fgf10b is expressed at tailbud stage in paraxial cephalic mesoderm beneath prospective epibranchial tissue, lateral to the developing otic placode. Knockdown of fgf10b does not affect initial otic induction but impairs subsequent accumulation of otic cells. Formation of epibranchial placodes and ganglia are also moderately impaired. Combinatorial disruption of fgf10b and fgf3 exacerbates the deficiency of otic cells and eliminates epibranchial induction entirely. Disruption of fgf10b and fgf24 also strongly reduces, but does not eliminate, epibranchial induction.

CONCLUSIONS

fgf10b participates in a late phase of otic induction and, in combination with fgf3, is especially critical for epibranchial induction.

Hox proteins drive cell segregation and non-autonomous apical remodelling during hindbrain segmentation

Hox genes encode a conserved family of homeodomain transcription factors regulating development along the major body axis. During embryogenesis, Hox proteins are expressed in segment-specific patterns and control numerous different segment-specific cell fates. It has been unclear, however, whether Hox proteins drive the epithelial cell segregation mechanism that is thought to initiate the segmentation process. Here, we investigate the role of vertebrate Hox proteins during the partitioning of the developing hindbrain into lineage-restricted units called rhombomeres. Loss-of-function mutants and ectopic expression assays reveal that Hoxb4 and its paralogue Hoxd4 are necessary and sufficient for cell segregation, and for the most caudal rhombomere boundary (r6/r7). Hox4 proteins regulate Eph/ephrins and other cell-surface proteins, and can function in a non-cell-autonomous manner to induce apical cell enlargement on both sides of their expression border. Similarly, other Hox proteins expressed at more rostral rhombomere interfaces can also regulate Eph/ephrins, induce apical remodelling and drive cell segregation in ectopic expression assays. However, Krox20, a key segmentation factor expressed in odd rhombomeres (r3 and r5), can largely override Hox proteins at the level of regulation of a cell surface target, Epha4. This study suggests that most, if not all, Hox proteins share a common potential to induce cell segregation but in some contexts this is masked or modulated by other transcription factors.

A novel role for Pax6 in the segmental organization of the hindbrain

Complex patterns and networks of genes coordinate rhombomeric identities, hindbrain segmentation and neuronal differentiation and are responsible for later brainstem functions. Pax6 is a highly conserved transcription factor crucial for neuronal development, yet little is known regarding its early roles during hindbrain segmentation. We show that Pax6 expression is highly dynamic in rhombomeres, suggesting an early function in the hindbrain. Utilization of multiple gain- and loss-of-function approaches in chick and mice revealed that loss of Pax6 disrupts the sharp expression borders of Krox20, Kreisler, Hoxa2, Hoxb1 and EphA and leads to their expansion into adjacent territories, whereas excess Pax6 reduces these expression domains. A mutual negative cross-talk between Pax6 and Krox20 allows these genes to be co-expressed in the hindbrain through regulation of the Krox20-repressor gene Nab1 by Pax6. Rhombomere boundaries are also distorted upon Pax6 manipulations, suggesting a mechanism by which Pax6 acts to set hindbrain segmentation. Finally, FGF signaling acts upstream of the Pax6-Krox20 network to regulate Pax6 segmental expression. This study unravels a novel role for Pax6 in the segmental organization of the early hindbrain and provides new evidence for its significance in regional organization along the central nervous system.

Induction of the inner ear: stepwise specification of otic fate from multipotent progenitors

Despite its complexity in the adult, during development the inner ear arises from a simple epithelium, the otic placode. Placode specification is a multistep process that involves the integration of various signalling pathways and downstream transcription factors in time and space. Here we review the molecular events that successively commit multipotent ectodermal precursors to the otic lineage. The first step in this hierarchy is the specification of sensory progenitor cells, which can contribute to all sensory placodes, followed by the induction of a common otic-epibranchial field and finally the establishment the otic territory. In recent years, some of the molecular components that control this process have been identified, and begin to reveal complex interactions. Future studies will need to unravel how this information is integrated and encoded in the genome. This will form the blueprint for stem cell differentiation towards otic fates and generate a predictive gene regulatory network that models the earliest steps of otic specification.

Expression of hindbrain boundary markers is regulated by FGF3

Compartment boundaries act as organizing centers that segregate adjacent areas into domains of gene expression and regulation, and control their distinct fates via the secretion of signalling factors. During hindbrain development, a specialized cell-population forms boundaries between rhombomeres. These boundary cells demonstrate unique morphological properties and express multiple genes that differs them from intra-rhombomeric cells. Yet, little is known regarding the mechanisms that controls the expression or function of these boundary markers.Multiple components of the FGF signaling system, including ligands, receptors, downstream effectors as well as proteoglycans are shown to localize to boundary cells in the chick hindbrain. These patterns raise the possibility that FGF signaling plays a role in regulating boundary properties. We provide evidence to the role of FGF signaling, particularly the boundary-derived FGF3, in regulating the expression of multiple markers at hindbrain boundaries. These findings enable further characterization of the unique boundary-cell population, and expose a new function for FGFs as regulators of boundary-gene expression in the chick hindbrain.

Notch signalling stabilises boundary formation at the midbrain-hindbrain organiser

The midbrain-hindbrain interface gives rise to a boundary of particular importance in CNS development as it forms a local signalling centre, the proper functioning of which is essential for the formation of tectum and cerebellum. Positioning of the mid-hindbrain boundary (MHB) within the neuroepithelium is dependent on the interface of Otx2 and Gbx2 expression domains, yet in the absence of either or both of these genes, organiser genes are still expressed, suggesting that other, as yet unknown mechanisms are also involved in MHB establishment. Here, we present evidence for a role for Notch signalling in stabilising cell lineage restriction and regulating organiser gene expression at the MHB. Experimental interference with Notch signalling in the chick embryo disrupts MHB formation, including downregulation of the organiser signal Fgf8. Ectopic activation of Notch signalling in cells of the anterior hindbrain results in an exclusion of those cells from rhombomeres 1 and 2, and in a simultaneous clustering along the anterior and posterior boundaries of this area, suggesting that Notch signalling influences cell sorting. These cells ectopically express the boundary marker Fgf3. In agreement with a role for Notch signalling in cell sorting, anterior hindbrain cells with activated Notch signalling segregate from normal cells in an aggregation assay. Finally, misexpression of the Notch modulator Lfng or the Notch ligand Ser1 across the MHB leads to a shift in boundary position and loss of restriction of Fgf8 to the MHB. We propose that differential Notch signalling stabilises the MHB through regulating cell sorting and specifying boundary cell fate.

Analysis of expression and function of FGF-MAPK signaling components in the hindbrain reveals a central role for FGF3 in the regulation of Krox20, mediated by Pea3

The development of the vertebrate hindbrain requires multiple coordinated signals which act via several pathways. One such signal is Fibroblast Growth Factor (FGF), which is necessary for the patterning of a major transcription factor in the hindbrain, Krox20. However, in the chick, it is still not known which specific FGF ligand is responsible for the regulation of Krox20 and how the signal is dispatched. The most characterized signaling pathway which FGF acts through in the nervous system is the MAPK/Erk1/2 pathway. Nevertheless, a detailed analysis of the hindbrain distribution of various components of this pathway has not been fully described. In this study we present a comprehensive atlas of the FGF ligands, receptors and members of the MAPK/Erk1/2 signaling components in subsequent stages of avian hindbrain development. Moreover, we show that FGF is a major signaling pathway that contributes to the activation of ERK1/2 and expression of the downstream targets Pea3 and Erm. Central to this study, we provide multiple evidence that FGF3 is required for the upregulation of Pea3 that in turn is necessary for Krox20 distribution in rhombomeres 3 and 5. These results show for the first time that Pea3 mediates the FGF3 signal to regulate the hindbrain expression of Krox20.

From shared lineage to distinct functions: the development of the inner ear and epibranchial placodes

The inner ear and the epibranchial ganglia constitute much of the sensory system in the caudal vertebrate head. The inner ear consists of mechanosensory hair cells, their neurons, and structures necessary for sound and balance sensation. The epibranchial ganglia are knots of neurons that innervate and relay sensory signals from several visceral organs and the taste buds. Their development was once thought to be independent, in line with their independent functions. However, recent studies indicate that both systems arise from a morphologically distinct common precursor domain: the posterior placodal area. This review summarises recent studies into the induction, morphogenesis and innervation of these systems and discusses lineage restriction and cell specification in the context of their common origin.

FGF signaling controls caudal hindbrain specification through Ras-ERK1/2 pathway

Background

During early steps of embryonic development the hindbrain undergoes a regionalization process along the anterior-posterior (AP) axis that leads to a metameric organization in a series of rhombomeres (r). Refinement of the AP identities within the hindbrain requires the establishment of local signaling centers, which emit signals that pattern territories in their vicinity. Previous results demonstrated that the transcription factor vHnf1 confers caudal identity to the hindbrain inducing Krox20 in r5 and MafB/Kreisler in r5 and r6, through FGF signaling [1].

Results

We show that in the chick hindbrain, Fgf3 is transcriptionally activated as early as 30 min after mvHnf1 electroporation, suggesting that it is a direct target of this transcription factor. We also analyzed the expression profiles of FGF activity readouts, such as MKP3 and Pea3, and showed that both are expressed within the hindbrain at early stages of embryonic development. In addition, MKP3 is induced upon overexpression of mFgf3 or mvHnf1 in the hindbrain, confirming vHnf1 is upstream FGF signaling. Finally, we addressed the question of which of the FGF-responding intracellular pathways were active and involved in the regulation of Krox20 and MafB in the hindbrain. While Ras-ERK1/2 activity is necessary for MKP3, Krox20 and MafB induction, PI3K-Akt is not involved in that process.

Conclusion

Based on these observations we propose that vHnf1 acts directly through FGF3, and promotes caudal hindbrain identity by activating MafB and Krox20 via the Ras-ERK1/2 intracellular pathway.

Molecular and tissue interactions governing induction of cranial ectodermal placodes

Whereas neural crest cells are the source of the peripheral nervous system in the trunk of vertebrates, the "ectodermal placodes," together with neural crest, form the peripheral nervous system of the head. Cranial ectodermal placodes are thickenings in the ectoderm that subsequently ingress or invaginate to make important contributions to cranial ganglia, including epibranchial and trigeminal ganglia, and sensory structures, the ear, nose, lens, and adenohypophysis. Recent studies have uncovered a number of molecular signals mediating induction and differentiation of placodal cells. Here, we described recent advances in understanding the tissue interactions and signals underlying induction and neurogenesis of placodes, with emphasis on the trigeminal and epibranchial. Important roles of Fibroblast Growth Factors, Platelet Derived Growth Factors, Sonic Hedgehog, TGFbeta superfamily members, and Wnts are discussed.

Boundary cells regulate a switch in the expression of FGF3 in hindbrain rhombomeres

Background

During formation of the vertebrate central nervous system, the hindbrain is organized into segmental units, called rhombomeres (r). These cell-lineage restricted segments are separated by a subpopulation of cells known as boundary cells. Boundary cells display distinct molecular and cellular properties such as an elongated shape, enriched extracellular matrix components and a reduced proliferation rate compared to intra-rhombomeric cells. However, little is known regarding their functions and the mechanisms that regulate their formation.

Results

Hindbrain boundary cells express several signaling molecules, such as FGF3, which at earlier developmental stages is transiently expressed in specific rhombomeres. We show that chick embryos that lack boundary cells due to overexpression of truncated EphA4 receptor in the hindbrain have continued segmental expression of FGF3 at stages when it is normally restricted to hindbrain boundaries. Furthermore, surgical ablation of the boundary between r3 and r4, or blocking of the contact of r4 with boundary cells, results in sustained FGF3 expression in this segment.

Conclusion

These findings suggest that boundary cells are required for the downregulation of segmental FGF3, presumably mediated by a soluble factor(s) that emanates from boundaries. We propose that this new function of boundary cells enables a switch in gene expression that may be required for stage-specific functions of FGF3 in the developing hindbrain.

Inhibition of BMPs by follistatin is required for FGF3 expression and segmental patterning of the hindbrain

A network of molecular interactions is required in the developing vertebrate hindbrain for the formation and anterior-posterior patterning of the rhombomeres. FGF signaling is required in this network to upregulate the expression of the Krox20 and Kreisler segmentation genes, but little is known of how FGF gene expression is regulated in the hindbrain. We show that the dynamic expression of FGF3 in chick hindbrain segments and boundaries is similar to that of the BMP antagonist, follistatin. Consistent with a regulatory relationship between BMP signaling and FGF3 expression, we find that an increase in BMP activity due to blocking of follistatin translation by morpholino antisense oligonucleotides or overexpression of BMP results in strong inhibition of FGF3 expression. Conversely, addition of follistatin leads to an increase in the level of FGF3 expression. Furthermore, the segmental inhibition of BMP activity by follistatin is required for the expression of Krox20, Hoxb1 and EphA4 in the hindbrain. In addition, we show that the maintenance of FGF3 gene expression requires FGF activity, suggestive of an autoregulatory loop. These results reveal an antagonistic relationship between BMP activity and FGF3 expression that is required for correct segmental gene expression in the chick hindbrain, in which follistatin enables FGF3 expression by inhibiting BMP activity.

The emergence of ectomesenchyme

In the head, neural crest cells generate ectomesenchymal derivatives: cartilage, bone, and connective tissue. Indeed, these cells generate much of the cranial skeleton. There have, however, been few studies of how this lineage is established. Here, we show that neural crest cells stop expressing early neural crest markers upon entering the pharyngeal arches and switch to become ectomesenchymal. By contrast, those neural crest cells that do not enter the arches persist in their expression of early neural crest markers. We further show that fibroblast growth factor (FGF) signaling is involved in directing neural crest cells to become ectomesenchymal. If neural crest cells are rendered insensitive to FGFs, they persist in their expression of early neural crest markers, even after entering the pharyngeal arches. However, our results further suggest that, although FGF signaling is required for the realization of the ectomesenchymal lineages, other cues from the pharyngeal epithelia are also likely to be involved.

Molecular basis for skeletal variation: insights from developmental genetic studies in mice

Skeletal variations are common in humans, and potentially are caused by genetic as well as environmental factors. We here review molecular principles in skeletal development to develop a knowledge base of possible alterations that could explain variations in skeletal element number, shape or size. Environmental agents that induce variations, such as teratogens, likely interact with the molecular pathways that regulate skeletal development.

Expression of chick Fgf19 and mouse Fgf15 orthologs is regulated in the developing brain by Fgf8 and Shh

Fibroblast growth factors (Fgfs) constitute a family of signaling molecules that play essential roles in development. We have studied the expression pattern of mouse Fgf15 in the developing brain. Fgf19 is another member of the FGF family that has been suggested as the chick and human ortholog of mouse and rat Fgf15. Here, we compare the expression pattern during neural development of chick Fgf19 with mouse Fgf15. Unlike Fgf15, Fgf19 presents an expression in the isthmic alar plate, diencephalic and mesencephalic parabasal plates, hindbrain basal plate, as well as in the zona limitans intrathalamica (zli). Moreover, we explored the regulation between Fgf19 and the signaling molecules of the isthmic and zli organizers: Fgf8 and Shh, respectively. Considering the possibility that Fgf19 plays a similar role in humans and chicks, this finding could explain the significant diencephalic phenotypic differences between humans and mice in models and diseases where the Shh pathway is affected.

Differential requirements for FGF3, FGF8 and FGF10 during inner ear development

FGF signaling is required during multiple stages of inner ear development in many different vertebrates, where it is involved in induction of the otic placode, in formation and morphogenesis of the otic vesicle as well as for cellular differentiation within the sensory epithelia. In this study we have looked to define the redundant and conserved roles of FGF3, FGF8 and FGF10 during the development of the murine and avian inner ear. In the mouse, hindbrain-derived FGF10 ectopically induces FGF8 and rescues otic vesicle formation in Fgf3 and Fgf10 homozygous double mutants. Conditional inactivation of Fgf8 after induction of the placode does not interfere with otic vesicle formation and morphogenesis but affects cellular differentiation in the inner ear. In contrast, inactivation of Fgf8 during induction of the placode in a homozygous Fgf3 null background leads to a reduced size otic vesicle or the complete absence of otic tissue. This latter phenotype is more severe than the one observed in mutants carrying null mutations for both Fgf3 and Fgf10 that develop microvesicles. However, FGF3 and FGF10 are redundantly required for morphogenesis of the otic vesicle and the formation of semicircular ducts. In the chicken embryo, misexpression of Fgf3 in the hindbrain induces ectopic otic vesicles in vivo. On the other hand, Fgf3 expression in the hindbrain or pharyngeal endoderm is required for formation of the otic vesicle from the otic placode. Together these results provide important insights into how the spatial and temporal expression of various FGFs controls different steps of inner ear formation during vertebrate development.

A spatial and temporal map of FGF/Erk1/2 activity and response repertoires in the early chick embryo

During early vertebrate development Fibroblast Growth Factor (FGF) signalling is required for multiple activities including specification of mesodermal, neural and heart tissue, as well as gastrulation movements and regulation of differentiation and pattern onset in the extending body axis. A current challenge is to understand how FGF signalling generates such diverse outcomes. A key FGF downstream pathway is the Ras-MAPK/Erk1/2 cascade, which culminates in the phosphorylation of target proteins, such as the Ets family of transcription factors. To begin to assess specificity downstream of FGF in the chick embryo we have characterised the patterns of Fgfr1-4 expression and Erk1/2 activation, as well as expression of the Erk1/2 specific phosphatase, Mkp3 and of three Ets factor genes (Erm, Pea3 and Er81) from early blastula to the 10 somite stage. We identify new sites of Fgfr expression and show that nearly all regions of Erk1/2 activity are within Fgfr expression domains and require FGF signalling. Differences in intensity, duration, distribution and sub-cellular localisation of activated Erk1/2 are observed in distinct cell populations within the embryo and during wound healing. With few exceptions, a tight correspondence between Erk1/2 activation and Mkp3 expression is found, while specific combinations of Ets factors are associated with distinct regions of Erk1/2 activation. These findings provide a comprehensive spatial and temporal map of FGF/Erk1/2 activity during early chick development and identify region and tissue specific differences in expression of Fgfrs as well as Erk1/2 phosphorylation and transcriptional targets which help to define response specificity.

Fgf-dependent otic induction requires competence provided by Foxi1 and Dlx3b

Background

The inner ear arises from a specialized set of cells, the otic placode, that forms at the lateral edge of the neural plate adjacent to the hindbrain. Previous studies indicated that fibroblast growth factors (Fgfs) are required for otic induction; in zebrafish, loss of both Fgf3 and Fgf8 results in total ablation of otic tissue. Furthermore, gain-of-function studies suggested that Fgf signaling is not only necessary but also sufficient for otic induction, although the amount of induced ectopic otic tissue reported after misexpression of fgf3 or fgf8 varies among different studies. We previously suggested that Foxi1 and Dlx3b may provide competence to form the ear because loss of both foxi1 and dlx3b results in ablation of all otic tissue even in the presence of a fully functional Fgf signaling pathway.

Results

Using a transgenic line that allows us to misexpress fgf8 under the control of the zebrafish temperature-inducible hsp70 promoter, we readdressed the role of Fgf signaling and otic competence during placode induction. We find that misexpression of fgf8 fails to induce formation of ectopic otic vesicles outside of the endogenous ear field and has different consequences depending upon the developmental stage. Overexpression of fgf8 from 1-cell to midgastrula stages leads to formation of no or small otic vesicles, respectively. Overexpression of fgf8 at these stages never leads to ectopic expression of foxi1 or dlx3b, contrary to previous studies that indicated that foxi1 is activated by Fgf signaling. Consistent with our results we find that pharmacological inhibition of Fgf signaling has no effect on foxi1 or dlx3b expression, but instead, Bmp signaling activates foxi1, directly and dlx3b, indirectly. In contrast to early activation of fgf8, fgf8 overexpression at the end of gastrulation, when otic induction begins, leads to much larger otic vesicles. We further show that application of a low dose of retinoic acid that does not perturb patterning of the anterior neural plate leads to expansion of foxi1 and to a massive Fgf-dependent otic induction.

Conclusion

These results provide further support for the hypothesis that Foxi1 and Dlx3b provide competence for cells to respond to Fgf and form an otic placode.

Epibranchial and otic placodes are induced by a common Fgf signal, but their subsequent development is independent

The epibranchial placodes are cranial, ectodermal thickenings that give rise to sensory neurons of the peripheral nervous system. Despite their importance in the developing animal, the signals responsible for their induction remain unknown. Using the placodal marker, sox3, we have shown that the same Fgf signaling required for otic vesicle development is required for the development of the epibranchial placodes. Loss of both Fgf3 and Fgf8 is sufficient to block placode development. We further show that epibranchial sox3 expression is unaffected in mutants in which no otic placode forms, where dlx3b and dlx4b are knocked down, or deleted along with sox9a. However, the forkhead factor, Foxi1, is required for both otic and epibranchial placode development. Thus, both the otic and epibranchial placodes form in a common region of ectoderm under the influence of Fgfs, but these two structures subsequently develop independently. Although previous studies have investigated the signals that trigger neurogenesis from the epibranchial placodes, this represents the first demonstration of the signaling events that underlie the formation of the placodes themselves, and therefore, the process that determines which ectodermal cells will adopt a neural fate.

Patterning the developing diencephalon

The diencephalon is the embryonic precursor to the caudal forebrain. The major diencephalic derivative is the thalamus, which functions as a relay station between the cortex and lower nervous system structures. Although the diencephalon has been recognized as a vital brain region, our understanding of its development remains superficial. In this review, we discuss recent progresses in understanding one essential aspect of diencephalic development, diencephalic patterning. Signaling centers identified in the zona limitans intrathalamica and along the dorsal and ventral midlines have emerged as essential organizers in diencephalic patterning. The cumulative data reveal that the diencephalon shares some developmental principles with more caudal brain regions, whereas other mechanisms are unique to this region.

Distinct roles for hindbrain and paraxial mesoderm in the induction and patterning of the inner ear revealed by a study of vitamin-A-deficient quail

The hindbrain and cranial paraxial mesoderm have been implicated in the induction and patterning of the inner ear, but the precise role of the two tissues in these processes is still not clear. We have addressed these questions using the vitamin-A-deficient (VAD) quail model, in which VAD embryos lack the posterior half of the hindbrain that normally lies next to the inner ear. Using a battery of molecular markers, we show that the anlagen of the inner ear, the otic placode, is induced in VAD embryos in the absence of the posterior hindbrain. By performing grafting and ablation experiments in chick embryos, we also show that cranial paraxial mesoderm which normally lies beneath the presumptive otic placode is necessary for otic placode induction and that paraxial mesoderm from other locations cannot induce the otic placode. Two members of the fibroblast growth factor family, FGF3 and FGF19, continue to be expressed in this mesodermal population in VAD embryos, and these may be responsible for otic placode induction in the absence of the posterior hindbrain. Although the posterior hindbrain is not required for otic placode induction in VAD embryos, the subsequent patterning of the inner ear is severely disrupted. Several regional markers of the inner ear, such as Pax2, EphA4, SOHo1 and Wnt3a, are incorrectly expressed in VAD otocysts, and the sensory patches and vestibulo-acoustic ganglia are either greatly reduced or absent. Exogenous application of retinoic acid prior to 30 h of development is able rescue the VAD phenotype. By performing such rescue experiments before and after 30 h of development, we show that the inner ear defects of VAD embryos correlate with the absence of the posterior hindbrain. These results show that induction and patterning of the inner ear are governed by separate developmental processes that can be experimentally uncoupled from each other.

Fgf19 regulated by Hh signaling is required for zebrafish forebrain development

Fibroblast growth factor (Fgf) signaling plays important roles in brain development. Fgf3 and Fgf8 are crucial for the formation of the forebrain and hindbrain. Fgf8 is also required for the midbrain to form. Here, we identified zebrafish Fgf19 and examined its roles in brain development by knocking down Fgf19 function. We found that Fgf19 expressed in the forebrain, midbrain and hindbrain was involved in cell proliferation and cell survival during embryonic brain development. Fgf19 was also essential for development of the ventral telencephalon and diencephalon. Regional specification is linked to cell type specification. Fgf19 was also essential for the specification of gamma-aminobutyric acid (GABA)ergic interneurons and oligodendrocytes generated in the ventral telencephalon and diencephalon. The cross talk between Fgf and Hh signaling is critical for brain development. In the forebrain, Fgf19 expression was down-regulated on inhibition of Hh but not of Fgf3/Fgf8, and overexpression of Fgf19 rescued partially the phenotype on inhibition of Hh. The present findings indicate that Fgf19 signaling is crucial for forebrain development by interacting with Hh and provide new insights into the roles of Fgf signaling in brain development.

Endoderm-derived Fgf3 is necessary and sufficient for inducing neurogenesis in the epibranchial placodes in zebrafish

In vertebrates, epibranchial placodes are transient ectodermal thickenings that contribute sensory neurons to the epibranchial ganglia. These ganglia innervate internal organs and transmit information on heart rate, blood pressure and visceral distension from the periphery to the central nervous system. Despite their importance, the molecular mechanisms that govern the induction and neurogenesis of the epibranchial placodes are only now being elucidated. In this study, we demonstrate that endoderm is required for neurogenesis of the zebrafish epibranchial placodes. Mosaic analyses confirm that endoderm is the source of the neurogenic signal. Using a morpholino knockdown approach, we find that fgf3 is required for the majority of placode cells to undergo neurogenesis. Tissue transplants demonstrate that fgf3 activity is specifically required in the endodermal pouches. Furthermore, ectopic fgf3 expression is sufficient for inducing phox2a-positive neurons in wild-type and endoderm-deficient embryos. Surprisingly, ectodermal foxi1 expression, a marker for the epibranchial placode precursors, is present in both endoderm-deficient embryos and fgf3 morphants, indicating that neither endoderm nor Fgf3 is required for initial placode induction. Based on these findings, we propose a model for epibranchial placode development in which Fgf3 is a major endodermal determinant required for epibranchial placode neurogenesis.

Rhombomere boundaries are Wnt signaling centers that regulate metameric patterning in the zebrafish hindbrain

The vertebrate hindbrain develops from a series of segments (rhombomeres) distributed along the anteroposterior axis. We are studying the roles of Wnt and Delta-Notch signaling in maintaining rhombomere boundaries as organizing centers in the zebrafish hindbrain. Several wnt genes (wnt1, wnt3a, wnt8b, and wnt10b) show elevated expression at rhombomere boundaries, whereas several delta genes (dlA, dlB, and dlD) are expressed in transverse stripes flanking rhombomere boundaries. Partial disruption of Wnt signaling by knockdown of multiple wnt genes, or the Wnt mediator tcf3b, ablates boundaries and associated cell types. Expression of dlA is chaotic, and cell types associated with rhombomere centers are disorganized. Similar patterning defects are observed in segmentation mutants spiel-ohne-grenzen (spg) and valentino (val), which fail to form rhombomere boundaries due to faulty interactions between adjacent rhombomeres. Stripes of wnt expression are variably disrupted, with corresponding disturbances in metameric patterning. Mutations in dlA or mind bomb (mib) disrupt Delta-Notch signaling and cause a wide range of patterning defects in the hindbrain. Stripes of wnt1 are initially normal but subsequently dissipate, and metameric patterning becomes increasingly disorganized. Driving wnt1 expression using a heat-shock construct partially rescues metameric patterning in mib mutants. Thus, rhombomere boundaries act as Wnt signaling centers required for precise metameric patterning, and Delta signals from flanking cells provide feedback to maintain wnt expression at boundaries. Similar feedback mechanisms operate in the Drosophila wing disc and vertebrate limb bud, suggesting coaptation of a conserved signaling module that spatially organizes cells in complex organ systems.

vHnf1regulates specification of caudal rhombomere identity in the chick hindbrain

The homeobox-containing gene variant hepatocyte nuclear factor-1 (vHnf1) has recently been shown to be involved in zebrafish caudal hindbrain specification, notably in the activation of MafB and Kro x 20 expression. We have explored this regulatory network in the chick by in ovo electroporation in the neural tube. We show that mis-expression of vHnf1 confers caudal identity to more anterior regions of the hindbrain. Ectopic expression of mvHnf1 leads to ectopic activation of MafB and Kro x 20, and downregulation of Hoxb1 in rhombomere 4. Unexpectedly, mvhnf1 strongly upregulates Fgf3 expression throughout the hindbrain, in both a cell-autonomous and a non-cell-autonomous manner. Blockade of FGF signaling correlates with a selective loss of MafB and Kro x 20 expression, without affecting the expression of vHnf1, Fgf3, or Hoxb1. Based on these observations, we propose that in chick, as in zebrafish, vHnf1 acts with FGF to promote caudal hindbrain identity by activating MafB and Kro x 20 expression. However, our data suggest differences in the vHnf1 downstream cascade in different vertebrates.

Fgf15 is required for proper morphogenesis of the mouse cardiac outflow tract

Evidence in animal models indicates that signaling networks functioning in the developing pharyngeal arches regulate stereotyped processes critical for proper development of the aortic arch and cardiac outflow tract. Here, we describe the phenotype of mice lacking fibroblast growth factor 15 (Fgf15), which encodes a secreted signaling molecule expressed within the developing pharyngeal arches. Homozygous Fgf15 mutants present heart defects consistent with malalignment of the aorta and pulmonary trunk. These defects correlate with early morphological defects of the outflow tract due to aberrant behavior of the cardiac neural crest. We demonstrate that Fgf15 expression within the pharyngeal arches is unaltered by a loss of Tbx1, a key regulator of pharyngeal arch development implicated in DiGeorge syndrome. In addition, Fgf15 and Tbx1 do not interact genetically, suggesting that Fgf15 operates through a pathway independent of Tbx1. These studies reveal a novel role of Fgf15 during development of the cardiac outflow tract.

FGF8 initiates inner ear induction in chick and mouse

In both chick and mouse, the otic placode, the rudiment of the inner ear, is induced by at least two signals, one from the cephalic paraxial mesoderm and the other from the neural ectoderm. In chick, the mesodermal signal, FGF19, induces neural ectoderm to express additional signals, including WNT8c and FGF3, resulting in induction of the otic placode. In mouse, mesodermal Fgf10 acting redundantly with neural Fgf3 is required for induction of the placode. To determine how the mesodermal inducers of the otic placode are localized, we took advantage of the unique strengths of the two model organisms. We show that endoderm is necessary for otic induction in the chick and that Fgf8, expressed in the chick endoderm subjacent to Fgf19, is both sufficient and necessary for the expression of Fgf19 in the mesoderm. In the mouse, Fgf8 is also expressed in endoderm as well as in other germ layers in the periotic placode region. We show that otic induction fails in embryos null for Fgf3 and hypomorphic for Fgf8 and expression of mesodermal Fgf10 is reduced. Thus, Fgf8 plays a critical upstream role in an FGF signaling cascade required for otic induction in chick and mouse.

The floor plate: multiple cells, multiple signals

One of the key organizers in the CNS is the floor plate - a group of cells that is responsible for instructing neural cells to acquire distinctive fates, and that has an important role in establishing the elaborate neuronal networks that underlie the function of the brain and spinal cord. In recent years, considerable controversy has arisen over the mechanism by which floor plate cells form. Here, we describe recent evidence that indicates that discrete populations of floor plate cells, with characteristic molecular properties, form in different regions of the neuraxis, and we discuss data that imply that the mode of floor plate induction varies along the anteroposterior axis.

Genetic interactions underlying otic placode induction and formation

The formation of the otic placode is a complex process requiring multiple inductive signals. In zebrafish, fgf3 and fgf8, dlx3b and dlx4b, and foxi1 have been identified as the earliest-acting genes in this process. fgf3 and fgf8 are required as inductive signals, whereas dlx3b, dlx4b, and foxi1 appear to act directly within otic primordia. We have investigated potential interactions among these genes. Depletion of either dlx3b and dlx4b or foxi1 leads to a delay of pax2a expression in the otic primordia and reduction of the otic vesicle. Depletion of both foxi1 and dlx3b results in a complete ablation of otic placode formation. A strong synergistic interaction is also observed among foxi1, fgf3, and fgf8, and a weaker interaction among dlx3b, fgf3, and fgf8. Misexpression of foxi1 can induce expression of pax8, an early marker for the otic primordia, in embryos treated with an inhibitor of fibroblast growth factor (FGF) signaling. Conversely, morpholino knockdown of foxi1 blocks ectopic pax8 expression and otic vesicle formation induced by misexpression of fgf3 and/or fgf8. The observed genetic interactions suggest a model in which foxi1 and dlx3b/dlx4b act in independent pathways together with distinct phases of FGF signaling to promote otic placode induction and development.

Somite polarity and segmental patterning of the peripheral nervous system

The analysis of the outgrowth pattern of spinal axons in the chick embryo has shown that somites are polarized into anterior and posterior halves. This polarity dictates the segmental development of the peripheral nervous system: migrating neural crest cells and outgrowing spinal axons traverse exclusively the anterior halves of the somite-derived sclerotomes, ensuring a proper register between spinal axons, their ganglia and the segmented vertebral column. Much progress has been made recently in understanding the molecular basis for somite polarization, and its linkage with Notch/Delta, Wnt and Fgf signalling. Contact-repulsive molecules expressed by posterior half-sclerotome cells provide critical guidance cues for axons and neural crest cells along the anterior-posterior axis. Diffusible repellents from surrounding tissues, particularly the dermomyotome and notochord, orient outgrowing spinal axons in the dorso-ventral axis ('surround repulsion'). Repulsive forces therefore guide axons in three dimensions. Although several molecular systems have been identified that may guide neural crest cells and axons in the sclerotome, it remains unclear whether these operate together with considerable overall redundancy, or whether any one system predominates in vivo.

SOX7 and GATA-4 are competitive activators of Fgf-3 transcription

Fgf-3 is expressed in a dynamic and complex spatiotemporal pattern during mouse development. Previous studies identified GATA-4 as a transcription factor that binds the key regulatory element PS4A of the Fgf-3 promoter and stimulates transcription. Here we show that members of the SOX family of transcription factors also bind PS4A and differentially modulate transcription. At least five SOX genes, Sox2, Sox6, Sox7, Sox13, and Sox17, were expressed in F9 cells, and of these, Sox7 and Sox17 were dramatically induced in parallel with Fgf-3 following differentiation into parietal endoderm-like cells with retinoic acid and dibutyryl cAMP. Complexes could be detected on PS4A with SOX2, SOX7, and SOX17 by using nuclear extracts from differentiated F9 cells. However, only Sox7 expression markedly activated the Fgf-3 promoter in these cells. By contrast, SOX2 was a poor activator of Fgf-3 transcription, and when Sox2 was coexpressed with Gata4, it negatively modulated the strong activation mediated by GATA-4. More detailed analyses showed that SOX7 competes with GATA-4 for PS4A occupancy and to activate the Fgf-3 promoter. In situ hybridization analysis showed that Sox7 is co-expressed with Fgf-3 and Gata4 in the parietal endoderm of E7.5 mouse embryos. In culture, GATA-4-deficient embryonal stem cells were shown to express Fgf-3 upon differentiation into embryoid bodies, although at lower levels than were found in wild type embryonal stem cells. This Fgf-3 expression was virtually abolished when Sox7 expression was suppressed by RNA interference. These results show that SOX7 is a potent activator of Fgf-3 transcription.

Meat‐Adaptive Genes and the Evolution of Slower Aging in Humans

The chimpanzee life span is shorter than that of humans, which is consistent with a faster schedule of aging. We consider aspects of diet that may have selected for genes that allowed the evolution of longer human life spans with slower aging. Diet has changed remarkably during human evolution. All direct human ancestors are believed to have been largely herbivorous. Chimpanzees eat more meat than other great apes, but in captivity are sensitive to hypercholesterolemia and vascular disease. We argue that this dietary shift to increased regular consumption of fatty animal tissues in the course of hominid evolution was mediated by selection for "meat-adaptive" genes. This selection conferred resistance to disease risks associated with meat eating also increased life expectancy. One candidate gene is apolipoprotein E (apoE), with the E3 allele evolved in the genus Homo that reduces the risks for Alzheimer's and vascular disease, as well as influencing inflammation, infection, and neuronal growth. Other evolved genes mediate lipid metabolism and host defense. The timing of the evolution of apoE and other candidates for meat-adaptive genes is discussed in relation to key events in human evolution.