Role of a transcription factor Pax6 in the developing vertebrate olfactory system

Pax6 affects Ras-Raf-ERK1/2 in mouse aging brain

Pax6, a transcription factor and multifunctional protein, changes during aging. It also interacts with regulator proteins involved in cell metabolism and survival signalling pathways including Ras-GAP. Many forms of Ras, Raf and ERK1/2 are known but information on their region-specific expression patterns are unavailable from brain during aging. Therefore, it has been intended to evaluate expressions of Pax6 and forms of Ras, Raf, ERK1/2 in hippocampus, caudate nucleus, amygdale, cerebral cortex, cerebellum and olfactory lobe. Association of Pax6 with Ras, Raf and ERK1/2 was evaluated in co-culture (PC-12, C6-glia, U-87 MG) of neuroglia cell lines. Impacts of Pax6 were evaluated by siRNA mediated knockdown and expression patterns Ras-Raf-Erk1/2. Analysis of activities of Pax6 and impacts of 5'AMP, wild-type and mutant ERK were done by RT-PCR and luciferase reporter assay. Results indicate age-dependent changes of Pax6, Ras, Raf, ERK1/2 in different regions of brain of young and old mice. Erk1/2 shows synergistic activities to Pax6.

Congenital aniridia beyond black eyes: From phenotype and novel genetic mechanisms to innovative therapeutic approaches

Congenital PAX6-aniridia, initially characterized by the absence of the iris, has progressively been shown to be associated with other developmental ocular abnormalities and systemic features making congenital aniridia a complex syndromic disorder rather than a simple isolated disease of the iris. Moreover, foveal hypoplasia is now recognized as a more frequent feature than complete iris hypoplasia and a major visual prognosis determinant, reversing the classical clinical picture of this disease. Conversely, iris malformation is also a feature of various anterior segment dysgenesis disorders caused by PAX6-related developmental genes, adding a level of genetic complexity for accurate molecular diagnosis of aniridia. Therefore, the clinical recognition and differential genetic diagnosis of PAX6-related aniridia has been revealed to be much more challenging than initially thought, and still remains under-investigated. Here, we update specific clinical features of aniridia, with emphasis on their genotype correlations, as well as provide new knowledge regarding the PAX6 gene and its mutational spectrum, and highlight the beneficial utility of clinically implementing targeted Next-Generation Sequencing combined with Whole-Genome Sequencing to increase the genetic diagnostic yield of aniridia. We also present new molecular mechanisms underlying aniridia and aniridia-like phenotypes. Finally, we discuss the appropriate medical and surgical management of aniridic eyes, as well as innovative therapeutic options. Altogether, these combined clinical-genetic approaches will help to accelerate time to diagnosis, provide better determination of the disease prognosis and management, and confirm eligibility for future clinical trials or genetic-specific therapies.

Mapping transgene insertion sites reveals the α-Cre transgene expression in both developing retina and olfactory neurons

The Tg(Pax6-cre,GFP)^2Pgr (α-Cre) mouse is a commonly used Cre line thought to be retinal-specific. Using targeted locus amplification (TLA), we mapped the insertion site of the transgene, and defined primers useful to deduce zygosity. Further analyses revealed four tandem copies of the transgene. The insertion site mapped to clusters of vomeronasal and olfactory receptor genes. Using R26R and Ai14 Cre reporter mice, we confirmed retinal Cre activity, but also detected expression in Gα₀⁺ olfactory neurons. Most α-Cre⁺ olfactory neurons do not express Pax6, implicating the influence of neighboring regulatory elements. RT-PCR and buried food pellet test did not detect any effects of the transgene on flanking genes in the nasal mucosa and retina. Together, these data precisely map α-Cre, show that it does not affect surrounding loci, but reveal previously unanticipated transgene expression in olfactory neurons. The α-Cre mouse can be a valuable tool in both retinal and olfactory research.

The Pax6-α-Cre mouse line used in retinal studies actually contains four transgene insertion within gene clusters of olfactory and vomeronasal receptors, leading to expression in not just retinal, but also olfactory and vomeronasal sensory neurons.

Genetic Regulation of Vertebrate Forebrain Development by Homeobox Genes

Forebrain development in vertebrates is regulated by transcription factors encoded by homeobox, bHLH and forkhead gene families throughout the progressive and overlapping stages of neural induction and patterning, regional specification and generation of neurons and glia from central nervous system (CNS) progenitor cells. Moreover, cell fate decisions, differentiation and migration of these committed CNS progenitors are controlled by the gene regulatory networks that are regulated by various homeodomain-containing transcription factors, including but not limited to those of the Pax (paired), Nkx, Otx (orthodenticle), Gsx/Gsh (genetic screened), and Dlx (distal-less) homeobox gene families. This comprehensive review outlines the integral role of key homeobox transcription factors and their target genes on forebrain development, focused primarily on the telencephalon. Furthermore, links of these transcription factors to human diseases, such as neurodevelopmental disorders and brain tumors are provided.

Amyloid-β toxicity modulates tau phosphorylation through the PAX6 signalling pathway

The molecular link between amyloid-β plaques and neurofibrillary tangles, the two pathological hallmarks of Alzheimer's disease, is still unclear. Increasing evidence suggests that amyloid-β peptide activates multiple regulators of cell cycle pathways, including transcription factors CDKs and E2F1, leading to hyperphosphorylation of tau protein. However, the exact pathways downstream of amyloid-β-induced cell cycle imbalance are unknown. Here, we show that PAX6, a transcription factor essential for eye and brain development which is quiescent in adults, is increased in the brains of patients with Alzheimer's disease and in APP transgenic mice, and plays a key role between amyloid-β and tau hyperphosphorylation. Downregulation of PAX6 protects against amyloid-β peptide-induced neuronal death, suggesting that PAX6 is a key executor of the amyloid-β toxicity pathway. Mechanistically, amyloid-β upregulates E2F1, followed by the induction of PAX6 and c-Myb, while Pax6 is a direct target for both E2F1 and its downstream target c-Myb. Furthermore, PAX6 directly regulates transcription of GSK-3β, a kinase involved in tau hyperphosphorylation and neurofibrillary tangles formation, and its phosphorylation of tau at Ser356, Ser396 and Ser404. In conclusion, we show that signalling pathways that include CDK/pRB/E2F1 modulate neuronal death signals by activating downstream transcription factors c-Myb and PAX6, leading to GSK-3β activation and tau pathology, providing novel potential targets for pharmaceutical intervention.

dbCNS: A New Database for Conserved Noncoding Sequences

We developed dbCNS (http://yamasati.nig.ac.jp/dbcns), a new database for conserved noncoding sequences (CNSs). CNSs exist in many eukaryotes and are assumed to be involved in protein expression control. Version 1 of dbCNS, introduced here, includes a powerful and precise CNS identification pipeline for multiple vertebrate genomes. Mutations in CNSs may induce morphological changes and cause genetic diseases. For this reason, many vertebrate CNSs have been identified, with special reference to primate genomes. We integrated ∼6.9 million CNSs from many vertebrate genomes into dbCNS, which allows users to extract CNSs near genes of interest using keyword searches. In addition to CNSs, dbCNS contains published genome sequences of 161 species. With purposeful taxonomic sampling of genomes, users can employ CNSs as queries to reconstruct CNS alignments and phylogenetic trees, to evaluate CNS modifications, acquisitions, and losses, and to roughly identify species with CNSs having accelerated substitution rates. dbCNS also produces links to dbSNP for searching pathogenic single-nucleotide polymorphisms in human CNSs. Thus, dbCNS connects morphological changes with genetic diseases. A test analysis using 38 gnathostome genomes was accomplished within 30 s. dbCNS results can evaluate CNSs identified by other stand-alone programs using genome-scale data.

Paired box 6 inhibits cardiac fibroblast differentiation

Cardiac fibroblast (CF) differentiation plays a crucial role in cardiac fibrosis, which is a specific manifestation distinguishing pathological cardiac hypertrophy from physiological hypertrophy. The DNA-binding activity of paired box 6 (Pax6) has been shown to be oppositely regulated in physiological and pathological hypertrophy; however, it remains unclear whether Pax6 is involved in CF differentiation during cardiac fibrosis. We found that Pax6 is expressed in the heart of and CFs isolated from adult mice. Moreover, angiotensin II (Ang II) induced the downregulation of Pax6 mRNA and protein expression in fibrotic heart tissue and cardiac myofibroblasts. Pax6 knockdown in CFs promoted the expression of the myofibroblast marker α-smooth muscle actin (α-SMA) and the synthesis of the extracellular matrix (ECM) proteins collagen I and fibronectin. Furthermore, we validated the ability of Pax6 to bind to the promoter regions of Cxcl10 and Il1r2 and the intronic region of Tgfb1. Pax6 knockdown in CFs decreased CXC chemokine 10 (CXCL10) and interleukin-1 receptor 2 (IL-1R2) expression and increased transforming growth factor β1 (TGFβ1) expression, mimicking the effects of Ang II. In conclusion, Pax6 exerts an inhibitory effect on CF differentiation and ECM synthesis by transcriptionally activating the expression of the anti-fibrotic factors CXCL10 and IL-1R2 and repressing the expression of the pro-fibrotic factor TGFβ1. Therefore, maintaining Pax6 expression in CFs is essential for preventing CF differentiation, and provides a new therapeutic target for cardiac fibrosis.

A retinal-binding protein mediates olfactory attraction in the migratory locusts

Cellular retinaldehyde-binding protein (CRALBP) is abundantly expressed in retina and its mutations are associated with visual impairments. The functions of CRALBP are less known in extra retinal tissues. Herein, we study the function of CRALBP in modulating olfactory behaviors in gregarious and solitary locusts. The expressions of cralbp mRNA and protein were enriched in locust brains and antennae. RNAi knockdown of cralbp in gregarious locusts decreased their attractive response to gregarious volatiles. RNA-seq and quantitative PCR confirmed that cralbp mRNA and protein expression levels were upregulated and downregulated after octopamine receptor α1 (OctαR1) activation and inhibition, respectively. Gene network analysis revealed that cralbp is the core hub gene in the interactive network among differentially expressed transcripts (DETs) resulting from activating and inhibiting OctαR1. Moreover, cralbp RNAi knockdown inhibited the induction of olfactory attraction by octopamine (OA)-OctαR1 signaling. CRALBP helped to transmit OA signals to mediate olfactory attraction response to guaiacol and veratrole, which are two odorant components in gregarious volatiles. This study suggested that CRALBP may act as a novel effector protein in OctαR1 signaling to mediate olfactory attraction. This study indicated that CRALBP modulates olfactory attraction in extra retina tissues and retinaldehyde metabolism may be crucial for olfactory attraction modulation.

Functional role of pax6 during eye and nervous system development in the annelid Capitella teleta

The transcription factor Pax6 is an important regulator of early animal development. Loss of function mutations of pax6 in a range of animals result in a reduction or complete loss of the eye, a reduction of a subset of neurons, and defects in axon growth. There are no studies focusing on the role of pax6 during development of any lophotrochozoan representative, however, expression of pax6 in the developing eye and nervous system in a number of species suggest that pax6 plays a highly conserved role in eye and nervous system formation. We investigated the functional role of pax6 during development of the marine annelid Capitella teleta. Expression of pax6 transcripts in C. teleta larvae is similar to patterns found in other animals, with distinct subdomains in the brain and ventral nerve cord as well as in the larval and juvenile eye. To perturb pax6 function, two different splice-blocking morpholinos and a translation-blocking morpholino were used. Larvae resulting from microinjections with either splice-blocking morpholino show a reduction of the pax6 transcript. Development of both the larval eyes and the central nervous system architecture are highly disrupted following microinjection of each of the three morpholinos. The less severe phenotype observed when only the homeodomain is disrupted suggests that presence of the paired domain is sufficient for partial function of the Pax6 protein. Preliminary downstream target analysis confirms disruption in expression of some components of the retinal gene regulatory network, as well as disruption of genes involved in nervous system development. Results from this study, taken together with studies from other species, reveal an evolutionarily conserved role for pax6 in eye and neural specification and development.

Mitral cell development in the olfactory bulb of sharks: evidences of a conserved pattern of glutamatergic neurogenesis

In mammals, the development of the olfactory bulb (OB) relies in part on the expression of transcription factors involved in the specifications/differentiation of glutamatergic cells. In a previous study from our group, a high molecular similarity was reported between mammals and cartilaginous fishes regarding the neurogenic mechanisms underlying the development of glutamatergic cells in the telencephalon. However, information about the transcriptional program operating in the development of the glutamatergic system (mainly represented by mitral cells) in the OB is lacking in the catshark Scyliorhinus canicula, a cartilaginous fish. Using immunohistochemistry and in situ hybridization techniques, we have found that, previously to the appearance of the olfactory primordium (OP), proliferating cells expressing Pax6 with molecular hallmarks of progenitor radial glia were located in the ventrolateral pallial ventricular zone. Later in development, when the OP is recognizable, a stream of Pax6-positive cells were observed between the ventricular zone and the OP, where transcription factors involved in mitral cell development in mammals (ScTbr2, ScNeuroD, Tbr1) are expressed. Later in development, these transcription factors became expressed in a layered-like structure where ScVglut1, a marker of mitral cells, is also present. Our data suggest that the transcriptional program related with the specification/differentiation of glutamatergic cells in the telencephalon has been conserved throughout the evolution of vertebrates. These results, in combination with previous studies concerning GABAergic neurogenesis in sharks, have evidenced that the OB of mammals and sharks shares similarities in the timing and molecular programs of development.

mSWI/SNF (BAF) Complexes Are Indispensable for the Neurogenesis and Development of Embryonic Olfactory Epithelium

Neurogenesis is a key developmental event through which neurons are generated from neural stem/progenitor cells. Chromatin remodeling BAF (mSWI/SNF) complexes have been reported to play essential roles in the neurogenesis of the central nervous system. However, whether BAF complexes are required for neuron generation in the olfactory system is unknown. Here, we identified onscBAF and ornBAF complexes, which are specifically present in olfactory neural stem cells (oNSCs) and olfactory receptor neurons (ORNs), respectively. We demonstrated that BAF155 subunit is highly expressed in both oNSCs and ORNs, whereas high expression of BAF170 subunit is observed only in ORNs. We report that conditional deletion of BAF155, a core subunit in both onscBAF and ornBAF complexes, causes impaired proliferation of oNSCs as well as defective maturation and axonogenesis of ORNs in the developing olfactory epithelium (OE), while the high expression of BAF170 is important for maturation of ORNs. Interestingly, in the absence of BAF complexes in BAF155/BAF170 double-conditional knockout mice (dcKO), OE is not specified. Mechanistically, BAF complex is required for normal activation of Pax6-dependent transcriptional activity in stem cells/progenitors of the OE. Our findings unveil a novel mechanism mediated by the mSWI/SNF complex in OE neurogenesis and development.

Cell migration in the developing rodent olfactory system

The components of the nervous system are assembled in development by the process of cell migration. Although the principles of cell migration are conserved throughout the brain, different subsystems may predominantly utilize specific migratory mechanisms, or may display unusual features during migration. Examining these subsystems offers not only the potential for insights into the development of the system, but may also help in understanding disorders arising from aberrant cell migration. The olfactory system is an ancient sensory circuit that is essential for the survival and reproduction of a species. The organization of this circuit displays many evolutionarily conserved features in vertebrates, including molecular mechanisms and complex migratory pathways. In this review, we describe the elaborate migrations that populate each component of the olfactory system in rodents and compare them with those described in the well-studied neocortex. Understanding how the components of the olfactory system are assembled will not only shed light on the etiology of olfactory and sexual disorders, but will also offer insights into how conserved migratory mechanisms may have shaped the evolution of the brain.

PAX6: 25th anniversary and more to learn

The DNA-binding transcription factor PAX6 was cloned 25 years ago by multiple teams pursuing identification of human and mouse eye disease causing genes, cloning vertebrate homologues of pattern-forming regulatory genes identified in Drosophila, or abundant eye-specific transcripts. Since its discovery in 1991, genetic, cellular, molecular and evolutionary studies on Pax6 mushroomed in the mid 1990s leading to the transformative thinking regarding the genetic program orchestrating both early and late stages of eye morphogenesis as well as the origin and evolution of diverse visual systems. Since Pax6 is also expressed outside of the eye, namely in the central nervous system and pancreas, a number of important insights into the development and function of these organs have been amassed. In most recent years, genome-wide technologies utilizing massively parallel DNA sequencing have begun to provide unbiased insights into the regulatory hierarchies of specification, determination and differentiation of ocular cells and neurogenesis in general. This review is focused on major advancements in studies on mammalian eye development driven by studies of Pax6 genes in model organisms and future challenges to harness the technology-driven opportunities to reconstruct, step-by-step, the transition from naïve ectoderm, neuroepithelium and periocular mesenchyme/neural crest cells into the three-dimensional architecture of the eye.

Horizontal Basal Cell-Specific Deletion of Pax6 Impedes Recovery of the Olfactory Neuroepithelium Following Severe Injury

In the mammalian olfactory epithelium (OE), olfactory receptor neurons (ORNs) are continuously regenerated throughout the animal's lifetime. Horizontal basal cells (HBCs) in the OE express the epithelial marker keratin 5 (K5) and the stem cell marker Pax6 and are considered relatively quiescent tissue stem cells in the OE. Pax6 is a key regulator of several developmental processes in the central nervous system and in sensory organs. Although Pax6 is expressed in the OE, its precise role remains unknown, particularly with respect to stem cell-like HBCs. To investigate the function of Pax6 in the developmental and regenerative processes in the OE, we generated conditional Pax6-knockout mice carrying a loxP-floxed Pax6 gene. Homozygous Pax6-floxed mice were crossed with K5-Cre transgenic mice to generate HBC-specific Pax6-knockout (Pax6-cKO) mice. We confirmed that the deletion of Pax6 expression in HBCs was sufficiently achieved in zone 1 of the OE in Pax6-cKO mice 3 days after methimazole-induced severe damage. In this condition, regeneration of the OE was dramatically impaired; both OE thickness and the number of ORNs were significantly decreased in the regenerated OE of Pax6-cKO mice. These results suggest that Pax6 expression is essential for HBCs to differentiate into neuronal cells during the regeneration process following severe injury.

GnRH, anosmia and hypogonadotropic hypogonadism--where are we?

Gonadotropin releasing hormone (GnRH) neurons originate the nasal placode and migrate into the brain during prenatal development. Once within the brain, these cells become integral components of the hypothalamic-pituitary-gonadal axis, essential for reproductive function. Disruption of this system causes hypogonadotropic hypogonadism (HH). HH associated with anosmia is clinically defined as Kallman syndrome (KS). Recent work examining the developing nasal region has shed new light on cellular composition, cell interactions and molecular cues responsible for the development of this system in different species. This review discusses some developmental aspects, animal models and current advancements in our understanding of pathologies affecting GnRH. In addition we discuss how development of neural crest derivatives such as the glia of the olfactory system and craniofacial structures control GnRH development and reproductive function.

Conserved localization of Pax6 and Pax7 transcripts in the brain of representatives of sarcopterygian vertebrates during development supports homologous brain regionalization

Many of the genes involved in brain patterning during development are highly conserved in vertebrates and similarities in their expression patterns help to recognize homologous cell types or brain regions. Among these genes, Pax6 and Pax7 are expressed in regionally restricted patterns in the brain and are essential for its development. In the present immunohistochemical study we analyzed the distribution of Pax6 and Pax7 cells in the brain of six representative species of tetrapods and lungfishes, the closest living relatives of tetrapods, at several developmental stages. The distribution patterns of these transcription factors were largely comparable across species. In all species only Pax6 was expressed in the telencephalon, including the olfactory bulbs, septum, striatum, and amygdaloid complex. In the diencephalon, Pax6 and Pax7 were distinct in the alar and basal parts, mainly in prosomeres 1 and 3. Pax7 specifically labeled cells in the optic tectum (superior colliculus) and Pax6, but not Pax7, cells were found in the tegmentum. Pax6 was found in most granule cells of the cerebellum and Pax7 labeling was detected in cells of the ventricular zone of the rostral alar plate and in migrated cells in the basal plate, including the griseum centrale and the interpeduncular nucleus. Caudally, Pax6 cells formed a column, whereas the ventricular zone of the alar plate expressed Pax7. Since the observed Pax6 and Pax7 expression patterns are largely conserved they can be used to identify subdivisions in the brain across vertebrates that are not clearly discernible with classical techniques.

Glucose modulates Pax6 expression through the JNK/p38 MAP kinase pathway in pancreatic beta-cells

AIM

The paired and homeodomain-containing transcription factor, paired box 6 (Pax6), has shown to play pivotal roles in beta-cell function, including cell survival, insulin biosynthesis and secretion. The present study investigates the signaling events that regulate the modulation of Pax6 expression by glucose and the role of this modulation in cell survival in rat insulinoma-1E (INS-1E) cells.

MAIN METHODS

INS-1E cells were incubated on 1mM (low) or 25 mM (high) glucose overnight. To elucidate the signaling pathways that regulate Pax6 expression, we utilized specific inhibitors. The siRNA transfection of Pax6 into INS-1E cells was performed by electroporation. The mRNA and protein levels were determined by real-time PCR and Western blotting, respectively.

KEY FINDINGS

We found that the mRNA and protein levels of Pax6 were reduced by approximately 4-fold in high, compared to low, glucose-treated cells. Staurosporine, the c-Jun N-terminal kinase (JNK) inhibitor SP600125 and the p38 mitogen-activated protein kinase (p38 MAPK) inhibitor SB203580 significantly increased Pax6 levels in high glucose-treated INS-1E cells compared to their respective controls. However, neither calcium ionophore nor the extracellular signal-regulated kinase (ERK) inhibitor U0126 resulted in any alteration in Pax6 protein expression. Further, a siRNA-mediated knockdown of Pax6 significantly decreased the expression of tumor-suppressor phosphatase with tensin homology (PTEN) while increasing cell viability in low glucose-treated INS-1E cells.

SIGNIFICANCE

This study addresses the signaling events that regulate the glucose-dependent expression of Pax6 and the role of these events in cell survival in pancreatic beta cells.

A survey of ancient conserved non-coding elements in the PAX6 locus reveals a landscape of interdigitated cis-regulatory archipelagos

Biological differences between cell types and developmental processes are characterised by differences in gene expression profiles. Gene-distal enhancers are key components of the regulatory networks that specify the tissue-specific expression patterns driving embryonic development and cell fate decisions, and variations in their sequences are a major contributor to genetic disease and disease susceptibility. Despite advances in the methods for discovery of putative cis-regulatory sequences, characterisation of their spatio-temporal enhancer activities in a mammalian model system remains a major bottle-neck. We employed a strategy that combines gnathostome sequence conservation with transgenic mouse and zebrafish reporter assays to survey the genomic locus of the developmental control gene PAX6 for the presence of novel cis-regulatory elements. Sequence comparison between human and the cartilaginous elephant shark (Callorhinchus milii) revealed several ancient gnathostome conserved non-coding elements (agCNEs) dispersed widely throughout the PAX6 locus, extending the range of the known PAX6 cis-regulatory landscape to contain the full upstream PAX6-RCN1 intergenic region. Our data indicates that ancient conserved regulatory sequences can be tested effectively in transgenic zebrafish even when not conserved in zebrafish themselves. The strategy also allows efficient dissection of compound regulatory regions previously assessed in transgenic mice. Remarkable overlap in expression patterns driven by sets of agCNEs indicates that PAX6 resides in a landscape of multiple tissue-specific regulatory archipelagos.

Developmental, tract-tracing and immunohistochemical study of the peripheral olfactory system in a basal vertebrate: insights on Pax6 neurons migrating along the olfactory nerve

The olfactory system represents an excellent model for studying different aspects of the development of the nervous system ranging from neurogenesis to mechanisms of axon growth and guidance. Important findings in this field come from comparative studies. We have analyzed key events in the development of the olfactory system of the shark Scyliorhinus canicula by combining immunohistochemical and tract-tracing methods. We describe for the first time in a cartilaginous fish an early population of pioneer HuC/D-immunoreactive (ir) neurons that seemed to delaminate from the olfactory pit epithelium and migrate toward the telencephalon before the olfactory nerve was identifiable. A distinct, transient cell population, namely the migratory mass, courses later on in apposition to the developing olfactory nerve. It contains olfactory ensheathing glial (GFAP-ir) cells and HuC/D-ir neurons, some of which course toward an extrabulbar region. We also demonstrate that Pax6-ir cells coursing along the developing olfactory pathways in S. canicula are young migrating (HuC/D and DCX-ir) neurons of the migratory mass that do not form part of the terminal nerve pathway. Evidences that these Pax6 neurons originate in the olfactory epithelium are also reported. As Pax6 neurons in the olfactory epithelium show characteristics of olfactory receptor neurons, and migrating Pax6-ir neurons formed transient corridors along the course of olfactory axons at the entrance of the olfactory bulb, we propose that these neurons could play a role as guideposts for axons of olfactory receptor neurons growing toward the olfactory bulb.

Expression patterns of Pax6 and Pax7 in the adult brain of a urodele amphibian, Pleurodeles waltl

Expression patterns of Pax6, Pax7, and, to a lesser extent, Pax3 genes were analyzed by a combination of immunohistochemical techniques in the central nervous system of adult specimens of the urodele amphibian Pleurodeles waltl. Only Pax6 was found in the telencephalon, specifically the olfactory bulbs, striatum, septum, and lateral and central parts of the amygdala. In the diencephalon, Pax6 and Pax7 were distinct in the alar and basal parts, respectively, of prosomere 3. The distribution of Pax6, Pax7, and Pax3 cells correlated with the three pretectal domains. Pax7 specifically labeled cells in the dorsal mesencephalon, mainly in the optic tectum, and Pax6 cells were the only cells found in the tegmentum. Large populations of Pax7 cells occupied the rostral rhombencephalon, along with lower numbers of Pax6 and Pax3 cells. Pax6 was found in most granule cells of the cerebellum. Pax6 cells also formed a column of scattered neurons in the reticular formation and were found in the octavolateral area. The rhombencephalic ventricular zone of the alar plate expressed Pax7. Dorsal Pax7 cells and ventral Pax6 cells were found along the spinal cord. Our results show that the expression of Pax6 and Pax7 is widely maintained in the brains of adult urodeles, in contrast to the situation in other tetrapods. This discrepancy could be due to the generally pedomorphic features of urodele brains. Although the precise role of these transcription factors in adult brains remains to be determined, our findings support the idea that they may also function in adult urodeles.

Mechanisms of permanent loss of olfactory receptor neurons induced by the herbicide 2,6-dichlorobenzonitrile: effects on stem cells and noninvolvement of acute induction of the inflammatory cytokine IL-6

We explored the mechanisms underlying the differential effects of two olfactory toxicants, the herbicide 2,6-dichlorobenzonitrile (DCBN) and the anti-thyroid drug methimazole (MMZ), on olfactory receptor neuron (ORN) regeneration in mouse olfactory epithelium (OE). DCBN, but not MMZ, induced inflammation-like pathological changes in OE, and DCBN increased interleukin IL-6 levels in nasal-wash fluid to much greater magnitude and duration than did MMZ. At 24h after DCBN injection, the population of horizontal basal cells (HBCs; reserve, normally quiescent OE stem cells) lining the DMM became severely depleted as some of them detached from the basal lamina, and sloughed into the nasal cavity along with the globose basal cells (GBCs; heterogeneous population of stem and progenitor cells), neurons, and sustentacular cells of the neuroepithelium. In contrast, the layer of HBCs remained intact in MMZ-treated mice, as only the mature elements of the neuroepithelium were shed. Despite the respiratory metaplasia accompanying the greater severity of the DCBN lesion, residual HBCs that survived intoxication were activated by the injury and contributed to the metaplastic respiratory epithelium, as shown by tracing their descendants in a K5CreEr(T2)::fl(stop)TdTomato strain of mice in which recombination causes HBCs to express TdTomato in advance of the lesion. But, contrary to published observations with MMZ, the HBCs failed to form ORNs. A role for IL-6 in suppressing ORN regeneration in DCBN-treated mice was rejected by the failure of the anti-inflammatory drug dexamethasone to prevent the subsequent respiratory metaplasia in the DMM, suggesting that other factors lead to HBC neuro-incompetence.

Pax6 interactions with chromatin and identification of its novel direct target genes in lens and forebrain

Pax6 encodes a specific DNA-binding transcription factor that regulates the development of multiple organs, including the eye, brain and pancreas. Previous studies have shown that Pax6 regulates the entire process of ocular lens development. In the developing forebrain, Pax6 is expressed in ventricular zone precursor cells and in specific populations of neurons; absence of Pax6 results in disrupted cell proliferation and cell fate specification in telencephalon. In the pancreas, Pax6 is essential for the differentiation of α-, β- and δ-islet cells. To elucidate molecular roles of Pax6, chromatin immunoprecipitation experiments combined with high-density oligonucleotide array hybridizations (ChIP-chip) were performed using three distinct sources of chromatin (lens, forebrain and β-cells). ChIP-chip studies, performed as biological triplicates, identified a total of 5,260 promoters occupied by Pax6. 1,001 (133) of these promoter regions were shared between at least two (three) distinct chromatin sources, respectively. In lens chromatin, 2,335 promoters were bound by Pax6. RNA expression profiling from Pax6^+/− lenses combined with in vivo Pax6-binding data yielded 76 putative Pax6-direct targets, including the Gaa, Isl1, Kif1b, Mtmr2, Pcsk1n, and Snca genes. RNA and ChIP data were validated for all these genes. In lens cells, reporter assays established Kib1b and Snca as Pax6 activated and repressed genes, respectively. In situ hybridization revealed reduced expression of these genes in E14 cerebral cortex. Moreover, we examined differentially expressed transcripts between E9.5 wild type and Pax6^−/− lens placodes that suggested Efnb2, Fat4, Has2, Nav1, and Trpm3 as novel Pax6-direct targets. Collectively, the present studies, through the identification of Pax6-direct target genes, provide novel insights into the molecular mechanisms of Pax6 gene control during mouse embryonic development. In addition, the present data demonstrate that Pax6 interacts preferentially with promoter regions in a tissue-specific fashion. Nevertheless, nearly 20% of the regions identified are accessible to Pax6 in multiple tissues.

Guidelines for genetic study of aniridia

INTRODUCTION

Aniridia is a panocular disorder which occurs in 1/50,000 to 1/100,000 live births and can appear either in isolated form or in the context of a syndrome. Isolated aniridia is inherited as an autosomal dominant condition and is caused by mutations of the PAX6 gene. A variety of techniques and methodologies within molecular genetics and cytogenetics are used to study these mutations.

OBJECTIVE

To identify the different aspects of this disease and to provide a guide for proper genetic diagnosis leading to improved clinical management of the disease.

DEVELOPMENT

Aniridia is an autosomal dominant disease that primarily affects the iris, though it can impact most of the ocular structures. The disease is mainly caused by mutations in the PAX6 gene located on chromosome 11p13 which encodes a transcription factor that is involved in the development of the eye. Genetic analysis of aniridia is complex and requires the use of both molecular genetics and cytogenetics techniques. These procedures are indicated in all cases of aniridia. It is important bear certain clinical and technical aspects in mind prior to starting analysis or providing genetic counseling for patients and their families.

CONCLUSIONS

The use of molecular genetic techniques in the genetic diagnosis of aniridia enables patients and their families to receive better clinical management.

Dynamic expression of Pax6 in the shark olfactory system: evidence for the presence of Pax6 cells along the olfactory nerve pathway

Pax6 is involved in the control of neuronal specification, migration, and differentiation in the olfactory epithelium and in the generation of different interneuron subtypes in the olfactory bulb. Whether these roles are conserved during evolution is not known. Cartilaginous fish are extremely useful models for assessing the ancestral condition of brain organization because of their phylogenetic position. To shed light on the evolution of development of the olfactory system in vertebrates and on the involvement of Pax6 in this process, we analyzed by in situ hybridization and immunohistochemistry the expression pattern of Pax6 in the developing olfactory system in a basal vertebrate, the lesser spotted dogfish Scyliorhinus canicula. This small shark is becoming an important fish model in studies of vertebrate development. We report Pax6 expression in cells of the olfactory epithelium and olfactory bulb, and present the first evidence in vertebrates of strings of Pax6-expressing cells extending along the developing olfactory nerve. The results indicate the olfactory epithelium as the origin of these cells. These data are compatible with a role for Pax6 in the development of the olfactory epithelium and fibers, and provide a basis for future investigations into the mechanisms that regulate development of the olfactory system throughout evolution.

The orchestration of mammalian tissue morphogenesis through a series of coherent feed-forward loops

Tissue morphogenesis requires intricate temporal and spatial control of gene expression that is executed through specific gene regulatory networks (GRNs). GRNs are comprised from individual subcircuits of different levels of complexity. An important question is to elucidate the mutual relationship between those genes encoding DNA-binding factors that trigger the subcircuit with those that play major "later" roles during terminal differentiation via expression of specific genes that constitute the phenotype of individual tissues. The ocular lens is a classical model system to study tissue morphogenesis. Pax6 is essential for both lens placode formation and subsequent stages of lens morphogenesis, whereas c-Maf controls terminal differentiation of lens fibers, including regulation of crystallins, key lens structural proteins required for its transparency and refraction. Here, we show that Pax6 directly regulates c-Maf expression during lens development. A 1.3-kb c-Maf promoter with a 1.6-kb upstream enhancer (CR1) recapitulated the endogenous c-Maf expression pattern in lens and retinal pigmented epithelium. ChIP assays revealed binding of Pax6 and c-Maf to multiple regions of the c-Maf locus in lens chromatin. To predict functional Pax6-binding sites, nine novel variants of Pax6 DNA-binding motifs were identified and characterized. Two of these motifs predicted a pair of Pax6-binding sites in the CR1. Mutagenesis of these Pax6-binding sites inactivated transgenic expression in the lens but not in retinal pigmented epithelium. These data establish a novel regulatory role for Pax6 during lens development, link together the Pax6/c-Maf/crystallin regulatory network, and suggest a novel type of GRN subcircuit that controls a major part of embryonic lens development.

Aging-associated modulation in the expression of Pax6 in mouse brain

Symptoms like mental retardation, depression, and anxiety have been observed during aging. Almost similar phenotypes have been evident in patients having haploinsufficiency or mutations in Pax6, a transcriptional regulator. Since Pax6 regulates axon guidance, differentiation of neurons from glia, and neuronal migration, it has been considered as a marker of newly generated neurons. The immunohistochemical analysis of Pax6 positive cells and expression pattern of Pax6 in olfactory lobe, hippocampus, and cerebellum of aging mouse brain have been investigated. The number of Pax6 positive cells and level of Pax6 were reduced progressively in olfactory lobe, cerebellum, and hippocampus from postnatal day-zero (P0) to old age mice. Pax6 positive cells were significantly lower in dentate gyrus, CA1, CA2, and CA3 regions of hippocampus, in mitral cell (MiCe), and internal plexiform (InPl) layers of olfactory lobe, and in granular cell (GrLa), and Purkinje's cell (PuCe) layers of cerebellum from P0 to old age. Thus, modulation in the expression of Pax6 and reduction in Pax6 positive cells show direct association of Pax6 with aging-related neuronal dystrophy.

Pax6 regulates craniofacial form through its control of an essential cephalic ectodermal patterning center

Normal patterning and morphogenesis of the complex skeletal structures of the skull requires an exquisite, reciprocal cross-talk between the embryonic cephalic epithelia and mesenchyme. The mesenchyme associated with the jaws and the optic and olfactory capsules is derived from a Hox-negative cranial neural crest (CNC) population that acts much as an equivalence group in its interactions with specific local cephalic epithelial signals. Craniofacial pattern and morphogenesis is therefore controlled in large part through the regulation of these local cephalic epithelial signals. Here, we demonstrate that Pax6 is essential to the formation and maturation of the complex cephalic ectodermal patterning centers that govern the development and morphogenesis of the upper jaws and associated nasal capsules. Previous examinations of the craniofacial skeletal defects associated with Pax6 mutations have suggested that they arise from an optic-associated blockage in the migration of a specific subpopulation of midbrain CNC to the lateral frontonasal processes. We have addressed an alternative explanation for the craniofacial skeletal defects. We show that in Pax6(SeyN/SeyN) mutants regional CNC is present by E9.25 while there is already specific disruption in the early ontogenetic elaboration of cephalic ectodermal expression, associated with the nascent lambdoidal junction, of secreted signaling factors (including Fgf8 and Bmp4) and transcription factors (including Six1 and Dlx5) essential for upper jaw and/or nasal capsular development. Pax6 therefore regulates craniofacial form, at stages when CNC has just arrived in the frontonasal region, through its control of surface cephalic ectodermal competence to form an essential craniofacial patterning center.

Pax6 is required at the telencephalic pallial-subpallial boundary for the generation of neuronal diversity in the postnatal limbic system

During embryogenesis, the pallial-subpallial boundary (PSB) divides the two main progenitor domains in the telencephalon: the pallium, the major source of excitatory neurons, and the subpallium, the major source of inhibitory neurons. The PSB is formed at the molecular interface between the pallial (high Pax6+) and subpallial (high Gsx2+) ventricular zone (VZ) compartments. Initially, the PSB contains cells that express both Pax6 and Gsx2, but during later stages of development this boundary is largely refined into two separate compartments. In this study we examined the developmental mechanisms underlying PSB boundary formation and the postnatal consequences of conditional loss of Pax6 function at the PSB on neuronal fate in the amygdala and olfactory bulb, two targets of PSB-derived migratory populations. Our cell fate and time-lapse imaging analyses reveal that the sorting of Pax6+ and Gsx2+ progenitors during embryogenesis is the result of a combination of changes in gene expression and cell movements. Interestingly, we find that in addition to giving rise to inhibitory neurons in the amygdala and olfactory bulb, Gsx2+ progenitors generate a subpopulation of amygdala excitatory neurons. Consistent with this finding, targeted conditional ablation of Pax6 in Gsx2+ progenitors results in discrete local embryonic patterning defects that are linked to changes in the generation of subsets of postnatal excitatory and inhibitory neurons in the amygdala and inhibitory neurons in the olfactory bulb. Thus, in PSB progenitors, Pax6 plays an important role in the generation of multiple subtypes of neurons that contribute to the amygdala and olfactory bulb.

The Pax6b homeodomain is dispensable for pancreatic endocrine cell differentiation in zebrafish

Pax6 is a well conserved transcription factor that contains two DNA-binding domains, a paired domain and a homeodomain, and plays a key role in the development of eye, brain, and pancreas in vertebrates. The recent identification of the zebrafish sunrise mutant, harboring a mutation in the pax6b homeobox and presenting eye abnormalities but no obvious pancreatic defects, raised a question about the role of pax6b in zebrafish pancreas. We show here that pax6b does play an essential role in pancreatic endocrine cell differentiation, as revealed by the phenotype of a novel zebrafish pax6b null mutant and of embryos injected with pax6b morpholinos. Pax6b-depleted embryos have almost no beta cells, a strongly reduced number of delta cells, and a significant increase of epsilon cells. Through the use of various morpholinos targeting intron-exon junctions, pax6b RNA splicing was perturbed at several sites, leading either to retention of intronic sequences or to deletion of exonic sequences in the pax6b transcript. By this strategy, we show that deletion of the Pax6b homeodomain in zebrafish embryos does not disturb pancreas development, whereas lens formation is strongly affected. These data thus provide the explanation for the lack of pancreatic defects in the sunrise pax6b mutants. In addition, partial reduction of Pax6b function in zebrafish embryos performed by injection of small amounts of pax6b morpholinos caused a clear rise in alpha cell number and in glucagon expression, emphasizing the importance of the fine tuning of the Pax6b level to its biological activity.

The beta subunit of voltage-gated Ca2+ channels interacts with and regulates the activity of a novel isoform of Pax6

Ca(2+) channel beta subunits (Ca(v)betas) are essential for regulating the surface expression and gating of high voltage-activated Ca(2+) channels through their interaction with Ca(2+) channel alpha(1) subunits. In efforts to uncover new interacting partners and new functions for Ca(v)beta, we identified a new splicing isoform of Pax6, a transcription factor crucial for the development of the eye, nose, brain, and pancreas. Pax6 contains two DNA binding domains (paired domain and homeodomain), a glycine-rich linker connecting these two domains and a C-terminal proline-, serine-, and threonine-rich transactivation domain. The protein sequence and function of Pax6 are highly conserved from invertebrate to human. The newly isolated isoform, named Pax6(S), retains the paired domain, linker, and homeodomain of Pax6, but its C terminus is composed of a truncated classic proline, serine, and threonine domain and a unique S tail. Pax6(S) shows a similar level of transcriptional activity in vitro as does Pax6, but only in primates is the protein sequence highly conserved. Its spatial-temporal expression profiles are also different from those of Pax6. These divergences suggest a noncanonical role of Pax6(S) during development. The interaction between Pax6(S) and Ca(v)beta is mainly endowed by the S tail. Co-expression of Pax6(S) with a Ca(2+) channel complex containing the beta(3) subunit in Xenopus oocytes does not affect channel properties. Conversely, however, beta(3) is able to suppress the transcriptional activity of Pax6(S). Furthermore, in the presence of Pax6(S), beta(3) is translocated from the cytoplasm to the nucleus. These results suggest that full-length Ca(v)beta may act directly as a transcription regulator independent of its role in regulating Ca(2+) channel activity.

Genetics of human iris colour and patterns

The presence of melanin pigment within the iris is responsible for the visual impression of human eye colouration with complex patterns also evident in this tissue, including Fuchs' crypts, nevi, Wolfflin nodules and contraction furrows. The genetic basis underlying the determination and inheritance of these traits has been the subject of debate and research from the very beginning of quantitative trait studies in humans. Although segregation of blue-brown eye colour has been described using a simple Mendelian dominant-recessive gene model this is too simplistic, and a new molecular genetic perspective is needed to fully understand the biological complexities of this process as a polygenic trait. Nevertheless, it has been estimated that 74% of the variance in human eye colour can be explained by one interval on chromosome 15 that contains the OCA2 gene. Fine mapping of this region has identified a single base change rs12913832 T/C within intron 86 of the upstream HERC2 locus that explains almost all of this association with blue-brown eye colour. A model is presented whereby this SNP, serving as a target site for the SWI/SNF family member HLTF, acts as part of a highly evolutionary conserved regulatory element required for OCA2 gene activation through chromatin remodelling. Major candidate genes possibly effecting iris patterns are also discussed, including MITF and PAX6.

Fibroblast growth factor signaling in development of the cerebral cortex

Despite substantial and exciting recent progress in our understanding of developmental processes in the cerebral cortex, there is still much to be learned about the molecular and cellular mechanisms that account for formation of the cortical structures, and in turn, how the regulation of these mechanisms is linked to cortical functions and behaviors in animals and humans. Fibroblast growth factors (FGFs) are a classic family of growth factors that are important in neural development and whose structures and signaling have been well-studied molecularly and biochemically. Recent advances have revealed their diverse but specific functions in patterning and neurogenesis during cortical development, as evidenced by multiple experimental approaches using in vivo models. Importantly, changes in FGF signaling during development have been shown to influence structure and function of the cerebral cortex as well as animal behavior, and have been implicated in disorders of nervous system function and intellectual development in humans. For example, disturbance of FGF pathways during development has been implicated in the pathogenesis of autism spectrum disorders. Experimental models with altered cortical structure due to perturbations of FGF signaling present a unique opportunity whereby molecular and cellular mechanisms that underlie cortical function and animal behavior can be directly studied and linked to each other.

Subtype-specific reduction of olfactory bulb interneurons in Pax6 heterozygous mutant mice

Interneurons in the olfactory bulb (OB) play essential roles in the processing of olfactory information. They are classified into several subpopulations by the expression of different neurochemical markers. Here we focused on a transcription factor Pax6, and examined its expression and function in distinct subtypes of OB interneurons. We identified Pax6 expression in specific subtypes of interneurons in the external plexiform layer (EPL). The number of these interneuron subtypes was dramatically decreased in Pax6 heterozygous mutant mice. These results indicate that Pax6 is required for differentiation and/or maintenance of EPL interneurons in the adult mouse OB.

Concise review: Pax6 transcription factor contributes to both embryonic and adult neurogenesis as a multifunctional regulator

Pax6 is a highly conserved transcription factor among vertebrates and is important in various developmental processes in the central nervous system (CNS), including patterning of the neural tube, migration of neurons, and formation of neural circuits. In this review, we focus on the role of Pax6 in embryonic and postnatal neurogenesis, namely, production of new neurons from neural stem/progenitor cells, because Pax6 is intensely expressed in these cells from the initial stage of CNS development and in neurogenic niches (the subgranular zone of the hippocampal dentate gyrus and the subventricular zone of the lateral ventricle) throughout life. Pax6 is a multifunctional player regulating proliferation and differentiation through the control of expression of different downstream molecules in a highly context-dependent manner.