The serine-threonine protein kinase PAK4 is dispensable in zebrafish: identification of a morpholino-generated pseudophenotype.

TALEN-based inactivation of the zebrafish pak4 gene resulted in embryos and adult fish that appear normal and fertile. This is in contrast to our previously published studies which were based on the use of antisense morpholino oligonucleotides (MOs). We have excluded potential explanations such as gene duplication, alternate splicing, cryptic initiation of translation, and translation-independent RNA function. Our conclusion is that pak4 is dispensable in zebrafish, and that even when corroborated by robust controls, such as RNA rescue, MOs may elicit misleading pseudophenotypes that do not correspond to results obtained by genetic mutations, and should thus be used with caution.

Maternal pak4 expression is required for primitive myelopoiesis in zebrafish.

Transcripts of pak4, the zebrafish ortholog of p21-activated kinase 4 (PAK4), are most abundant in the egg and fall to low levels by the end of gastrulation, after which expression is essentially ubiquitous. Translation of maternal mRNA into pak4 protein is first detectable at high stage (3.3hpf). Splice-blocking morpholino oligonucleotides (MOs) were used to prevent zygotic pak4 expression. This had no discernable effect on development through larval stages. In contrast, a translation-blocking MO, alone or in combination with the splice MOs, resulted in a complex lethal phenotype. In addition to disrupted somite development and other morphogenetic abnormalities, the knockdown of maternal pak4 expression led to alterations in regulatory gene expression in the primitive hematopoietic domains, leading to deficiencies in granulocyte and leukocyte lineages. At least some of the effects of pak4 knockdown on gene expression could be mimicked by treatment with actin depolymerization agents, suggesting a mechanistic link between regulation of microfilament dynamics by pak4 and regulation of gene expression in primitive myeloid cell differentiation.

Generation and characterization of a novel neural crest marker allele, Inka1-LacZ, reveals a role for Inka1 in mouse neural tube closure.

Previous studies identified Inka1 as a gene regulated by AP-2alpha in the neural crest required for craniofacial morphogenesis in fish and frog. Here, we extend the analysis of Inka1 function and regulation to the mouse by generating a LacZ knock-in allele. Inka1-LacZ allele expression occurs in the cephalic mesenchyme, heart, and paraxial mesoderm prior to E8.5. Subsequently, expression is observed in the migratory neural crest cells and their derivatives. Consistent with expression of Inka1 in tissues of the developing head during neurulation, a low percentage of Inka1(-/-) mice show exencephaly while the remainder are viable and fertile. Further studies indicate that AP-2alpha is not required for Inka1 expression in the mouse, and suggest that there is no significant genetic interaction between these two factors during embryogenesis. Together, these data demonstrate that while the expression domain of Inka1 is conserved among vertebrates, its function and regulation are not.

Myosin-X is required for cranial neural crest cell migration in Xenopus laevis.

Myosin-X (MyoX) belongs to a large family of unconventional, nonmuscle, actin-dependent motor proteins. We show that MyoX is predominantly expressed in cranial neural crest (CNC) cells in embryos of Xenopus laevis and is required for head and jaw cartilage development. Knockdown of MyoX expression using antisense morpholino oligonucleotides resulted in retarded migration of CNC cells into the pharyngeal arches, leading to subsequent hypoplasia of cartilage and inhibited outgrowth of the CNC-derived trigeminal nerve. In vitro migration assays on fibronectin using explanted CNC cells showed significant inhibition of filopodia formation, cell attachment, spreading and migration, accompanied by disruption of the actin cytoskeleton. These data support the conclusion that MyoX has an essential function in CNC migration in the vertebrate embryo.

Inca: a novel p21-activated kinase-associated protein required for cranial neural crest development.

Inca (induced in neural crest by AP2) is a novel protein discovered in a microarray screen for genes that are upregulated in Xenopus embryos by the transcriptional activator protein Tfap2a. It has no significant similarity to any known protein, but is conserved among vertebrates. In Xenopus, zebrafish and mouse embryos, Inca is expressed predominantly in the premigratory and migrating neural crest (NC). Knockdown experiments in frog and fish using antisense morpholinos reveal essential functions for Inca in a subset of NC cells that form craniofacial cartilage. Cells lacking Inca migrate successfully but fail to condense into skeletal primordia. Overexpression of Inca disrupts cortical actin and prevents formation of actin "purse strings", which are required for wound healing in Xenopus embryos. We show that Inca physically interacts with p21-activated kinase 5 (PAK5), a known regulator of the actin cytoskeleton that is co-expressed with Inca in embryonic ectoderm, including in the NC. These results suggest that Inca and PAK5 cooperate in restructuring cytoskeletal organization and in the regulation of cell adhesion in the early embryo and in NC cells during craniofacial development.

PCNS: a novel protocadherin required for cranial neural crest migration and somite morphogenesis in Xenopus.

Protocadherins (Pcdhs), a major subfamily of cadherins, play an important role in specific intercellular interactions in development. These molecules are characterized by their unique extracellular domain (EC) with more than 5 cadherin-like repeats, a transmembrane domain (TM) and a variable cytoplasmic domain. PCNS (Protocadherin in Neural crest and Somites), a novel Pcdh in Xenopus, is initially expressed in the mesoderm during gastrulation, followed by expression in the cranial neural crest (CNC) and somites. PCNS has 65% amino acid identity to Xenopus paraxial protocadherin (PAPC) and 42-49% amino acid identity to Pcdh 8 in human, mouse, and zebrafish genomes. Overexpression of PCNS resulted in gastrulation failure but conferred little if any specific adhesion on ectodermal cells. Loss of function accomplished independently with two non-overlapping antisense morpholino oligonucleotides resulted in failure of CNC migration, leading to severe defects in the craniofacial skeleton. Somites and axial muscles also failed to undergo normal morphogenesis in these embryos. Thus, PCNS has essential functions in these two important developmental processes in Xenopus.

Msx1 and Msx2 have shared essential functions in neural crest but may be dispensable in epidermis and axis formation in Xenopus.

The homeodomain factors Msx1 and Msx2 are expressed in essentially identical patterns in the epidermis and neural crest of Xenopus embryos during neurula stages. Disruption of Msx1 and Msx2 RNA splicing with antisense morpholino oligonucleotides shows that both factors are also required for expression of the neural crest gene Slug. Loss of Msx1 can be compensated by overexpression of Msx2 and vice versa. Loss of Msx factors also leads to alterations in the expression boundaries for neural and epidermal genes, but does not prevent or reduce expression of epidermal keratin in ventrolateral ectoderm, nor is there a detectable effect on dorsal mesodermal marker gene expression. These results indicate that Msx1 and Msx2 are both essential for neural crest development, but that the two genes have the same function in this tissue. If Msx genes have important functions in epidermis or axial mesoderm induction, these functions must be shared with other regulatory proteins.

Expression of TFAP2beta and TFAP2gamma genes in Xenopus laevis.

The embryonic expression patterns of two additional members of the transcription factor TFAP2 family in Xenopus laevis, TFAP2beta and TFAP2gamma, are described. Both genes share overlapping expression domains with the previously characterized TFAP2alpha in this species, although differences exist. All three genes are expressed in the neural crest (NC) region at late gastrula to early neurula stages. TFAP2alpha and TFAP2gamma are also expressed in outer, epidermal cells, while TFAP2beta is essentially NC-specific. All three are induced by Wnt/beta-catenin -- BMP signals and all bind to a consensus TFAP2 recognition site from an epidermal keratin gene.

Developmental expression of Xenopus fragile X mental retardation-1 gene.

Dysregulation of Fragile X mental retardation-1 (Fmr1) gene expression results in an inherited form of mental retardation known as the Fragile X syndrome (FXS). Fmr1 is a highly conserved gene with a broad yet distinctive expression pattern during vertebrate development. Here, we examined the expression pattern of Fmr1 during Xenopus embryonic development. Zygotic expression of Fmr1 began just prior to gastrulation and gradually increased during subsequent embryonic stages. By in situ hybridization, Fmr1 transcripts were detected by early tailbud stage and showed robust expression in the central nervous system (CNS), eye and pharyngeal arches. By late tailbud stage, Fmr1 expression became stronger in the CNS and craniofacial regions including the ear vesicle and eye. In addition, the notochord expressed high levels of Fmr1 transcripts in the late tailbud stage embryos. In the tadpole brain, the olfactory bulb and cerebellum exhibited strong Fmr1 expression. The developmental expression pattern of Fmr1 is consistent with the wide range of abnormalities observed in FXS. Further, our findings indicate that Xenopus will serve as an excellent model to study the developmental basis of this disease.

Regulatory targets for transcription factor AP2 in Xenopus embryos.

The transcription factor AP2 (TFAP2) has an important role in regulating gene expression in both epidermis and neural crest cells. In order to further characterize these functions we have used a hormone inducible TFAP2alpha fusion protein in a Xenopus animal cap assay to identify downstream targets of this factor. The most common pattern comprised genes predominantly expressed in the epidermis. A second group was expressed at high levels in the neural crest, but all of these were also expressed in the epidermis as well as in other tissues in which TFAP2alpha has not been detected, suggesting modular control involving both TFAP2-dependent and TFAP2-independent components. In addition, a few strongly induced genes did not overlap at all in expression pattern with that of TFAP2alpha in the early embryo, and were also activated precociously in the experimentally manipulated ectoderm, and thus likely represent inappropriate regulatory interactions. A final group was identified that were repressed by TFAP2alpha and were expressed in the neural plate. These results provide further support for the importance of TFAP2alpha in ectoderm development, and also highlight the molecular linkage between the epidermis and neural crest in the Xenopus embryo.

AP-2alpha selectively regulates fragile X mental retardation-1 gene transcription during embryonic development.

Fragile X syndrome (FXS) is almost always caused by silencing of the FMR1 gene. The defects observed in FXS indicate that the normal FMR1 gene has a range of functions and plays a particularly prominent role during development. However, the mechanisms regulating FMR1 expression in vivo are not known. Here, we have tested the role of the transcription factor AP-2alpha in regulating Fmr1 expression. Chromatin immunoprecipitation showed that AP-2alpha associates with the Fmr1 promoter in vivo. Furthermore, Fmr1 transcript levels are reduced >4-fold in homozygous null AP-2alpha mutant mice at embryonic day 18.5 when compared with normal littermates. Notably, AP-2alpha exhibits a strong gene dosage effect, with heterozygous mice showing approximately 2-fold reduction in Fmr1 levels. Examination of conditional AP-2alpha mutant mice indicates that this transcription factor plays a major role in regulating Fmr1 expression in embryos, but not in adults. We further investigated the role of AP-2alpha in the developmental regulation of Fmr1 expression using the Xenopus animal cap assay. Over-expression of a dominant-negative AP-2alpha in Xenopus embryos led to reduced Fmr1 levels. Moreover, exogenous wild-type AP-2alpha rescued Fmr1 expression in embryos where endogenous AP-2alpha had been suppressed. We conclude that AP-2alpha associates with the Fmr1 promoter in vivo and selectively regulates Fmr1 transcription during embryonic development.

Specification of the otic placode depends on Sox9 function in Xenopus.

The vertebrate inner ear develops from a thickening of the embryonic ectoderm, adjacent to the hindbrain, known as the otic placode. All components of the inner ear derive from the embryonic otic placode. Sox proteins form a large class of transcriptional regulators implicated in the control of a variety of developmental processes. One member of this family, Sox9, is expressed in the developing inner ear, but little is known about the early function of Sox9 in this tissue. We report the functional analysis of Sox9 during development of Xenopus inner ear. Sox9 otic expression is initiated shortly after gastrulation in the sensory layer of the ectoderm, in a bilateral patch of cells immediately adjacent to the cranial neural crest. In the otic placode, Sox9 colocalizes with Pax8 one of the earliest gene expressed in response to otic placode inducing signals. Depletion of Sox9 protein in whole embryos using morpholino antisense oligonucleotides causes a dramatic loss of the early otic placode markers Pax8 and Tbx2. Later in embryogenesis, Sox9 morpholino-injected embryos lack a morphologically recognizable otic vesicle and fail to express late otic markers (Tbx2, Bmp4, Otx2 and Wnt3a) that normally exhibit regionalized expression pattern throughout the otocyst. Using a hormone inducible inhibitory mutant of Sox9, we demonstrate that Sox9 function is required for otic placode specification but not for its subsequent patterning. We propose that Sox9 is one of the key regulators of inner ear specification in Xenopus.

Induction of neural crest in Xenopus by transcription factor AP2alpha.

We report experiments with Xenopus laevis, using both intact embryos and ectodermal explants, showing that the transcription factor AP2alpha is positively regulated by bone morphogenetic protein (BMP) and Wnt signaling, and that this activation is an essential step in the induction of neural crest (NC). Ectopic expression of AP2alpha is sufficient to activate high-level expression of NC-specific genes such as Slug and Sox9, which can occur as isolated domains within the neural plate as well as by expansion of endogenous NC territories. AP2alpha also has the property of inducing NC in isolated ectoderm in which Wnt signaling is provided but BMP signaling is minimized by overexpression of chordin. Like other NC regulatory factors, activation of AP2alpha requires some attenuation of endogenous BMP signaling; however, this process occurs at a lower threshold for AP2alpha. Furthermore, AP2alpha expression domains are larger than for other NC factors. Loss-of-function experiments with antisense AP2alpha morpholino oligonucleotides result in severe reduction in the NC territory. These results support a central role for AP2alpha in NC induction. We propose a model in which AP2alpha expression, along with inactivation of NC inhibitory factors such as Dlx3, establish a feedback loop comprising AP2alpha, Sox9, and Slug, leading to and maintaining NC specification.

Transcription factor AP-2 is an essential and direct regulator of epidermal development in Xenopus.

Expression of the Xenopus homolog of the mammalian transcription factor AP-2alpha (XAP-2) is activated throughout the animal hemisphere shortly after the midblastula transition, and becomes restricted to prospective epidermis by the end of gastrulation, under the control of BMP signal modulation. Elevated expression in the future neural crest region begins at this time. Ectopic expression of XAP-2 can restore transcription of epidermal genes in neuralized ectoderm, both in ectodermal explants and in the intact embryo. Likewise, loss of XAP-2 function, accomplished by injection of antisense oligonucleotides or by overexpression of antimorphic XAP-2 derivatives, leads to loss of epidermal and gain of neural gene expression. These treatments also result in gastrulation failure. Thus, AP-2 is a critical regulator of ectodermal determination that is required for normal epidermal development and morphogenesis in the frog embryo.