|Dr. rer. nat. Tran Tuoc|
1994-1998: Undergraduate study at Natural Science College of Hanoi National University, Vietnam.
2001-2003: Graduate study at University of Hannover, Germany.
2004-2007: Postgraduate study at Max Planck Institute for Biophysical Chemistry, Göttingen, Germany.
1998-2001: Research staff at Institute of Biotechnology, Vietnam Academy of Science and Technology, Vietnam.
2002-2003: Graduate Research Assistant at Institute of Developmental and Applied physiology, University of Hannover, Germany.
2004-2007: Graduate Research Assistant at Max Planck Institute for Biophysical Chemistry, Göttingen, Germany (Supervisor: Prof. Anastassia Stoykova).
2008-2013: Postdoctoral fellow at Max Planck Institute for Biophysical Chemistry, Göttingen, Germany (Supervisor: Prof. Anastassia Stoykova).
4.2013-present: Research group leader at Institute of Neuroanatomy, University medical Göttingen (UMG), Germany (Director: Prof. Dr. Jochen F. Staiger).
Awarded Scholarships & Support
DAAD, DFG, Max Planck Society, EMBO, University Medical Center Göttingen (UMG), DFG Center for Nanoscale Microscopy & Molecular Physiology of the Brain (CNMPB)
Early forebrain patterning entails the correct regional designation of theneuroepithelium, and appropriate specification, generation, and distributionof neural cells during brain development. Specific signaling and transcriptionfactors are known to tightly regulate patterning of the dorsal telencephalon toafford proper structural/functional cortical arealization and morphogenesis.Nevertheless, whether and how changes of the chromatin structure link to thetranscriptional program(s) that control cortical patterning remains elusive. Here,we report that the BAF chromatin remodeling complex regulates thespatiotemporal patterning of the mouse dorsal telencephalon. To determinewhether and how the BAF complex regulates cortical patterning, weconditionally deleted the BAF complex scaffolding subunits BAF155 andBAF170 in the mouse dorsal telencephalic neuroepithelium. Morphologicaland cellular changes in the BAF mutant forebrain were examined usingimmunohistochemistry and in situ hybridization. RNA sequencing, Coimmunoprecipitation,and mass spectrometry were used to investigate themolecular basis of BAF complex involvement in forebrain patterning. We foundthat conditional ablation of BAF complex in the dorsal telencephalonneuroepithelium caused expansion of the cortical hem and medial cortexbeyond their developmental boundaries. Consequently, the hippocampalprimordium is not specified, the mediolateral cortical patterning iscompromised, and the cortical identity is disturbed in the absence of BAFcomplex. The BAF complex was found to interact with the cortical hemsuppressor LHX2. The BAF complex suppresses cortical hem fate to permitproper forebrain patterning. We provide evidence that BAF complex modulatesmediolateral cortical patterning possibly by interacting with the transcriptionfactor LHX2 to drive the LHX2-dependent transcriptional program essential fordorsal telencephalon patterning. Our data suggest a putative mechanisticsynergy between BAF chromatin remodeling complex and LHX2 inregulating forebrain patterning and ontogeny.
Increase in the size of human neocortex―acquired in evolution―accounts for the unique cognitive capacity ofhumans. This expansion reflects the evolutionarily enhanced proliferative ability of basal progenitors (BPs), includingthe basal radial glia and basal intermediate progenitors (bIPs) in mammalian cortex, which may have been acquiredthrough epigenetic alterations in BPs. However, how the epigenome in BPs differs across species is not known.Here, we report that histone H3 acetylation is a key epigenetic regulation in bIP amplification and cortical expansion.Through epigenetic profiling of sorted bIPs, we show that histone H3 lysine 9 acetylation (H3K9ac) is low inmurine bIPs and high in human bIPs. Elevated H3K9ac preferentially increases bIP proliferation, increasing the sizeand folding of the normally smooth mouse neocortex. H3K9ac drives bIP amplification by increasing expressionof the evolutionarily regulated gene, Trnp1, in developing cortex. Our findings demonstrate a previously unknownmechanism that controls cortical architecture.
Radial neuronal migration is a key neurodevelopmental event indispensable for proper cortical laminar organization. Cortical neurons mainly use glial fiber guides, cell adhesion dynamics, and cytoskeletal remodeling, among other discrete processes, to radially trek from their birthplace to final layer positions. Dysregulated radial migration can engender cortical mis-lamination, leading to neurodevelopmental disorders. Epigenetic factors, including chromatin remodelers have emerged as formidable regulators of corticogenesis. Notably, the chromatin remodeler BAF complex has been shown to regulate several aspects of cortical histogenesis. Nonetheless, our understanding of how BAF complex regulates neuronal migration is limited. Here, we report that BAF complex is required for neuron migration during cortical development. Ablation of BAF complex in the developing mouse cortex caused alteration in the cortical gene expression program, leading to loss of radial migration-related factors critical for proper cortical layer formation. Of note, BAF complex inactivation in cortex caused defective neuronal polarization resulting in diminished multipolar-to-bipolar transition and eventual disruption of radial migration of cortical neurons. The abnormal radial migration and cortical mis-lamination can be partly rescued by downregulating WNT signaling hyperactivity in the BAF complex mutant cortex. By implication, the BAF complex modulates WNT signaling to establish the gene expression program required for glial fiber-dependent neuronal migration, and cortical lamination. Overall, BAF complex has been identified to be crucial for cortical morphogenesis through instructing multiple aspects of radial neuronal migration in a WNT signaling-dependent manner.
Cortical morphogenesis entails several neurobiological events, including proliferation and differentiation of progenitors, migration of neuroblasts, and neuronal maturation leading to functional neural circuitry. These neurodevelopmental processes are delicately regulated by many factors. Endosomal SNAREs have emerged as formidable modulators of neuronal growth, aside their well-known function in membrane/vesicular trafficking. However, our understanding of their influence on cortex formation is limited. Here, we report that the SNAREs Vti1a and Vti1b (Vti1a/1b) are critical for proper cortical development. Following null mutation of Vti1a/1b in mouse, the late-embryonic mutant cortex appeared dysgenic, and the cortical progenitors therein were depleted beyond normal. Notably, cortical layer 5 (L5) is distinctively disorganized in the absence of Vti1a/1b. The latter defect, coupled with an overt apoptosis of Ctip2-expressing L5 neurons, likely contributed to the substantial loss of corticospinal and callosal projections in the Vti1a/1b-deficient mouse brain. These findings suggest that Vti1a/1b serve key neurodevelopmental functions during cortical histogenesis, which when mechanistically elucidated, can lend clarity to how endosomal SNAREs regulate brain development, or how their dysfunction may have implications for neurological disorders.
The BAF complex plays an important role in the development of a wide range of tissues bymodulating gene expression programs at the chromatin level. However, its role in neuralcrest development has remained unclear. To determine the role of the BAF complex, wedeleted BAF155/BAF170, the core subunits required for the assembly, stability, and functionsof the BAF complex in neural crest cells (NCCs). Neural crest-specific deletion ofBAF155/BAF170 leads to embryonic lethality due to a wide range of developmental defectsincluding craniofacial, pharyngeal arch artery, and OFT defects. RNAseq and transcriptionfactor enrichment analysis revealed that the BAF complex modulates the expression of multiplesignaling pathway genes including Hippo and Notch, essential for the migration, proliferation,and differentiation of the NCCs. Furthermore, we demonstrated that the BAFcomplex is essential for the Brg1-Yap-Tead-dependent transcription of target genes inNCCs. Together, our results demonstrate an important role of the BAF complex in modulatingthe gene regulatory network essential for neural crest development.
Fine-tuned gene expression is crucial for neurodevelopment. The gene expression program is tightly controlled at different levels, including RNA decay. N6-methyladenosine (m6A) methylation-mediated degradation of RNA is essential for brain development. However, m6A methylation impacts not only RNA stability, but also other RNA metabolism processes. How RNA decay contributes to brain development is largely unknown. Here, we show that Exosc10, a RNA exonuclease subunit of the RNA exosome complex, is indispensable for forebrain development. We report that cortical cells undergo overt apoptosis, culminating in cortical agenesis upon conditional deletion of Exosc10 in mouse cortex. Mechanistically, Exosc10 directly binds and degrades transcripts of the P53 signaling-related genes, such as Aen and Bbc3. Overall, our findings suggest a crucial role for Exosc10 in suppressing the P53 pathway, in which the rapid turnover of the apoptosis effectors Aen and Bbc3 mRNAs is essential for cell survival and normal cortical histogenesis.
Chromatin remodeling factor BAF155 is an important regulator of many biological processes. As a core and scaffold subunit of the BAF (SWI/SNF-like) complex, BAF155 is capable of regulating the stability and function of the BAF complex. The spatiotemporal expression of BAF155 during embryogenesis is essential for various aspects of organogenesis, particularly in the brain development. However, our understanding of the mechanisms that regulate the expression and function of BAF155 is limited. Here, we report that RBM15, a subunit of the m6A methyltransferase complex, interacts with BAF155 mRNA and mediates BAF155 mRNA degradation through the mRNA methylation machinery. Ablation of endogenous RBM15 expression in cultured neuronal cells and in the developing cortex augmented the expression of BAF155. Conversely, RBM15 overexpression decreased BAF155 mRNA and protein levels, and perturbed BAF155 functions in vivo, including repression of BAF155-dependent transcriptional activity and delamination of apical radial glial progenitors as a hallmark of basal radial glial progenitor genesis. Furthermore, we demonstrated that the regulation of BAF155 by RBM15 depends on the activity of the mRNA methylation complex core catalytic subunit METTL3. Altogether, our findings reveal a new regulatory avenue that elucidates how BAF complex subunit stoichiometry and functional modulation are achieved in mammalian cells.
FOXG1 syndrome is a rare neurodevelopmental disorder associated with heterozygous FOXG1 variants or chromosomal microaberrations in 14q12. The study aimed at assessing the scope of structural cerebral anomalies revealed by neuroimaging to delineate the genotype and neuroimaging phenotype associations.
We compiled 34 patients with a heterozygous (likely) pathogenic FOXG1 variant. Qualitative assessment of cerebral anomalies was performed by standardized re‐analysis of all 34 MRI data sets. Statistical analysis of genetic, clinical and neuroimaging data were performed. We quantified clinical and neuroimaging phenotypes using severity scores. Telencephalic phenotypes of adult Foxg1+/− mice were examined using immunohistological stainings followed by quantitative evaluation of structural anomalies.
Characteristic neuroimaging features included corpus callosum anomalies (82%), thickening of the fornix (74%), simplified gyral pattern (56%), enlargement of inner CSF spaces (44%), hypoplasia of basal ganglia (38%), and hypoplasia of frontal lobes (29%). We observed a marked, filiform thinning of the rostrum as recurrent highly typical pattern of corpus callosum anomaly in combination with distinct thickening of the fornix as a characteristic feature. Thickening of the fornices was not reported previously in FOXG1 syndrome. Simplified gyral pattern occurred significantly more frequently in patients with early truncating variants. Higher clinical severity scores were significantly associated with higher neuroimaging severity scores. Modeling of Foxg1 heterozygosity in mouse brain recapitulated the associated abnormal cerebral morphology phenotypes, including the striking enlargement of the fornix.
Combination of specific corpus callosum anomalies with simplified gyral pattern and hyperplasia of the fornices is highly characteristic for FOXG1 syndrome.
The abundance of basal progenitors (BPs) - basal radial glia progenitors (bRGs) and basal intermediate progenitors (bIPs), in primate brain has been correlated to the high degree of cortical folding. Here we examined the role of BAF155, a subunit of the chromatin remodeling BAF complex, in generation of cortical progenitor heterogeneity. The conditional deletion of BAF155 led to diminished bIP pool and increased number of bRGs, due to delamination of apical RGs. We found that BAF155 is required for normal activity of neurogenic transcription factor PAX6, thus controlling expression of genes that are involved in bIP specification, cell-cell interaction and establishment of adherens junction. In PAX6-dependent manner, BAF155 regulates the expression of the CDC42 effector protein CEP4, thereby controlling progenitor delamination. Furthermore, BAF155-dependent chromatin remodeling seems to exert a specific role in the genesis of BPs through regulation of human RG-specific genes (such as Foxn4) that possibly acquired evolutionary significance.
During early cortical development, neural stem cells (NSCs) divide symmetrically to expand the progenitor pool, whereas in later stages, NSCs divide asymmetrically to self-renew and produce other cell types. The timely switch from such proliferative to differentiative division critically determines progenitor and neuron numbers. However, the mechanisms that limit proliferative division in late cortical development are not fully understood. Here, we show that the BAF (mSWI/SNF) complexes restrict proliferative competence and promote neuronal differentiation in late corticogenesis. Inactivation of BAF complexes leads to H3K27me3-linked silencing of neuronal differentiation-related genes, with concurrent H3K4me2-mediated activation of proliferation-associated genes via de-repression of Wnt signaling. Notably, the deletion of BAF complexes increased proliferation of neuroepithelial cell-like NSCs, impaired neuronal differentiation and exerted a Wnt-dependent effect on neocortical and hippocampal development. Thus, these results demonstrate that BAF complexes act as both activators and repressors to control global epigenetic and gene expression programs in late corticogenesis.
The generation of individual neurons (neurogenesis) during cortical development occurs in discrete steps that are subtly regulated and orchestrated to ensure normal histogenesis and function of the cortex. Notably, various gene expression programs are known to critically drive many facets of neurogenesis with a high level of specificity during brain development. Typically, precise regulation of gene expression patterns ensures that key events like proliferation and differentiation of neural progenitors, specification of neuronal subtypes, as well as migration and maturation of neurons in the developing cortex occur properly. ATP-dependent chromatin remodeling complexes regulate gene expression through utilization of energy from ATP hydrolysis to reorganize chromatin structure. These chromatin remodeling complexes are characteristically multimeric, with some capable of adopting functionally distinct conformations via subunit reconstitution to perform specific roles in major aspects of cortical neurogenesis. In this review, we highlight the functions of such chromatin remodelers during cortical development. We also bring together various proposed mechanisms by which ATP-dependent chromatin remodelers function individually or in concert, to specifically modulate vital steps in cortical neurogenesis.
The postnatal mammalian olfactory epithelium (OE) represents a major aspect of the peripheral olfactory system. It is a pseudostratified tissue that originates from the olfactory placode and is composed of diverse cells, some of which are specialized receptor neurons capable of transducing odorant stimuli to afford the perception of smell (olfaction). The OE is known to offer a tractable miniature model for studying the systematic generation of neurons and glia that typify neural tissue development. During OE development, stem/progenitor cells that will become olfactory sensory neurons and/or non-neuronal cell types, display fine spatiotemporal expression of neuronal and non-neuronal genes that ensures their proper proliferation, differentiation, survival, and regeneration. Many factors, including transcription and epigenetic factors have been identified as key regulators of the expression of such requisite genes to permit normal OE morphogenesis. Typically, specific interactive regulatory networks established between transcription and epigenetic factors/cofactors orchestrate histogenesis in the embryonic and adult OE. Hence, investigation of these regulatory networks critical for OE development promises to disclose strategies that may be employed in manipulating the stepwise transition of olfactory precursor cells to become fully differentiated and functional neuronal and non-neuronal cell types. Such strategies potentially offer formidable means of replacing injured or degenerated neural cells as therapeutics for nervous system perturbations. This review recapitulates the developmental cellular diversity of the olfactory neuroepithelium and discusses findings on how the precise and cooperative molecular control by transcriptional and epigenetic machinery is indispensable for OE ontogeny.
The ATP-dependent BRG1/BRM associated factor (BAF) chromatin remodeling complexes are crucial in regulating gene expression by controlling chromatin dynamics. Over the last decade, it has become increasingly clear that during neural development in mammals, distinct ontogenetic stage-specific BAF complexes derived from combinatorial assembly of their subunits are formed in neural progenitors and post-mitotic neural cells. Proper functioning of the BAF complexes plays critical roles in neural development, including the establishment and maintenance of neural fates and functionality. Indeed, recent human exome sequencing and genome-wide association studies have revealed that mutations in BAF complex subunits are linked to neurodevelopmental disorders such as Coffin-Siris syndrome, Nicolaides-Baraitser syndrome, Kleefstra’s syndrome spectrum, Hirschsprung’s disease, autism spectrum disorder, and schizophrenia. In this review, we focus on the latest insights into the functions of BAF complexes during neural development and the plausible mechanistic basis of how mutations in known BAF subunits are associated with certain neurodevelopmental disorders.
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.
The BAF chromatin remodeling complex plays an essential role in brain development. However its function in postnatal neurogenesis in hippocampus is still unknown. Here, we show that in postnatal dentate gyrus (DG), the BAF170 subunit of the complex is expressed in radial glial-like (RGL) progenitors and in cell types involved in subsequent steps of adult neurogenesis including mature astrocytes. Conditional deletion of BAF170 during cortical late neurogenesis as well as during adult brain neurogenesis depletes the pool of RGL cells in DG, and promotes terminal astrocyte differentiation. These derangements are accompanied by distinct behavioral deficits, as reflected by an impaired accuracy of place responding in the Morris water maze test, during both hidden platform as well as reversal learning. Inducible deletion of BAF170 in DG during adult brain neurogenesis resulted in mild spatial learning deficits, having a more pronounced effect on spatial learning during the reversal test. These findings demonstrate involvement of BAF170-dependent chromatin remodeling in hippocampal neurogenesis and cognition and suggest a specific role of adult neurogenesis in DG in adaptive behavior.
During corticogenesis, genetic programs encoded in progenitor cells at different developmental stages and inherited in postmitotic neurons specify distinct layer and area identities. Transcription factor Zbtb20 has been shown to play a role for hippocampal development but whether it is implicated in mammalian neocortical morphogenesis remains unknown.
Here, we report that during embyogenesis transcription factor Zbtb20 has a dynamic spatio-temporal expression pattern in mitotic cortical progenitors through which it modulates the sequential generation of cortical neuronal layer identities. Zbtb20 knock out mice exhibited enhanced populations of early born L6-L4 neuronal subtypes and a dramatic reduction of the late born L3/L2 neurons. This defect was due to a temporal misbalance in the production of earlier versus later born neurons, leading to a progressive diminishing of the progenitor pool for the generation of L3-L2 neurons. Zbtb20 implements these temporal effects in part by binding to promoter of the orphan nuclear receptor CoupTF1/Nr2f1. In addition to its effects exerted in cortical progenitors, the postmitotic expression of Zbtb20 in L3/L2 neurons starting at birth may contribute to their proper differentiation and migration.
Our findings reveal Zbtb20 as a novel temporal regulator for the generation of layer-specific neuronal identities.
The multi-subunit chromatin-remodeling SWI/SNF (known as BAF for Brg/Brm-associated factor) complexes play essential roles in development. Studies have shown that the loss of individual BAF subunits often affects local chromatin structure and specific transcriptional programs. However, we do not fully understand how BAF complexes function in development because no animal mutant had been engineered to lack entire multi-subunit BAF complexes. Importantly, we recently reported that double conditional knock-out (dcKO) of the BAF155 and BAF170 core subunits in mice abolished the presence of the other BAF subunits in the developing cortex. The generated dcKO mutant provides a novel and powerful tool for investigating how entire BAF complexes affect cortical development. Using this model, we found that BAF complexes globally control the key heterochromatin marks, H3K27me2 and -3, by directly modulating the enzymatic activity of the H3K27 demethylases, Utx and Jmjd3. Here, we present further insights into how the scaffolding ability of the BAF155 and BAF170 core subunits maintains the stability of BAF complexes in the forebrain and throughout the embryo during development. Furthermore, we show that the loss of BAF complexes in the above-described model up-regulates H3K27me3 and impairs forebrain development and embryogenesis. These findings improve our understanding of epigenetic mechanisms and their modulation by the chromatin-remodeling SWI/SNF complexes that control embryonic development.
BAF (Brg/Brm-associated factors) complexes play important roles in development and are linked to chromatin plasticity at selected genomic loci. Nevertheless, a full understanding of their role in development and chromatin remodeling has been hindered by the absence of mutants completely lacking BAF complexes. Here, we report that the loss of BAF155/BAF170 in double-conditional knock-out (dcKO) mice eliminates all known BAF subunits, resulting in an overall reduction in active chromatin marks (H3K9Ac), a global increase in repressive marks (H3K27me2/3), and down-regulation of gene expression. We demonstrate that BAF complexes interact with H3K27 demethylases (JMJD3, UTX) and potentiate their activity. Importantly BAF complexes are indispensable for forebrain development, including proliferation, differentiation and cell survival of neural progenitor cells. Our findings reveal a molecular mechanism mediated by BAF complexes that controls global transcriptional program and chromatin state in development.
ATP-dependent BAF chromatin remodeling complexes play an essential role in the maintenance of the gene expression program by regulating the structure of chromatin. There is increasing evidence that BAF complexes based on the alternative ATPase subunits, Brg1 and Brm, control the differentiation of neural stem cells (NSCs) to generate distinct neural cell types and modulate trans-differentiation between cell types. The BAF complexes have dedicated functions at different stages of neural differentiation that appear to arise by combinatorial assembly of their subunits. Furthermore, the differentiation of NSCs is regulated by the tight interactions between the BAF chromatin remodeling complex and the transcriptional machinery. Here, we review recent insights into the functional interaction between BAF complexes and various transcription factors (TFs) in neural differentiation and cellular reprogramming
The mammalian neocortex is a sheet of cells covering the cerebrum that provides the structural basis for the perception of sensory inputs, motor output responses, cognitive function, and mental capacity of primates. Recent discoveries promote the concept that increased cortical surface size and thickness in phylogenetically advanced species is a result of an increased generation of neurons, a process that underlie higher cognitive and intellectual performance in higher primates and humans. Here, we review some of the advances in the field, focusing on the diversity of neocortical progenitors in different species, and cellular mechanisms of neurogenesis. We discuss recent views on intrinsic and extrinsic molecular determinants, including the role of epigenetic chromatin modifiers and microRNA, in the control of neuronal output in developing cortex and in the establishment of normal cortical architecture.
The multi-subunit chromatin remodeling BAF complex controls different developmental processes. Using cortex-specific conditional knockout and overexpression mouse models, we have recently reported that BAF170, a subunit of the vertebrate BAF chromatin remodeling complex, interacts with transcription factor (TF) Pax6 to control cortical size and volume. The mechanistic basis includes suppression of the expression of Pax6 target genes, which are required for genesis of cortical intermediate progenitors (IPs) and specification of late neuronal subtype identity. In addition, we showed that a dynamic competition between BAF170 and BAF155 subunits within the BAF complex during progression of neurogenesis is a primary event in modulating the size of the mammalian cortex. Here, we present additional insights into the interaction between the BAF complex and TF Pax6 in the genesis of IPs of the developing cortex. Furthermore, we show that such competition between BAF170 and BAF155 is involved as well in the determination of the size of the embryonic body. Our results add new insights into a cell-intrinsic mechanism, mediated by the chromatin remodeling BAF complex, that controls vertebrate body shape and size.
Increased cortical size is essential to the enhanced intellectual capacity of primates during mammalian evolution. The mechanisms that control cortical size are largely unknown. Here, we show that mammalian BAF170, a subunit of the chromatin remodeling complex mSWI/SNF, is an intrinsic factor that controls cortical size. We find that conditional deletion of BAF170promotes indirect neurogenesis by increasing the pool of intermediate progenitors (IPs) and results in an enlarged cortex, whereas cortex-specific BAF170 over-expression results the opposite phenotype. Mechanistically, BAF170 competes with BAF155 subunit in the BAF complex, affecting euchromatin structure and thereby modulating the binding efficiency of the Pax6/REST-corepressor complex to Pax6 target genes that regulate the generation of IPs and late cortical progenitors. Our findings reveal a molecular mechanism mediated by the mSWI/SNF chromatin-remodeling complex that controls cortical architecture.
The ubiquitin-proteosome system (UPS) is a non-lysosomal proteolysis system involved in the degradation of irrelevant/misfolded intracellular proteins. The protein substrates of this system are tagged by ubiquitin in sequential reactions that target them for proteasome-dependent destruction. In the developing central nervous system, ubiquitin-mediated proteolysis has recently emerged as an important player in the regulation of neural progenitor proliferation, cell specification, neuronal differentiation, maturation, and migration. E3 ubiquitin ligases are crucial components in the UPS because they provide the specificity that determines which substrates are targeted for ubiquitin-dependent proteolysis. In this review, we discuss the molecular mechanisms of the UPS, focusing primarily on the roles of E3 ligases and their substrates in sequential steps of neurogenesis.
The transcription factor Pax6 has been implicated in neocortical neurogenesis in vertebrates, including humans. Analyses of the role of Pax6 in layer formation and cognitive abilities have been hampered by perinatal lethality of Pax6 mutants. Here, we generated viable mutants exhibiting timed, restricted inactivation of Pax6 during early and late cortical neurogenesis using Emx1-Cre and hGFAP-Cre lines, respectively. The disruption of Pax6 at the onset of neurogenesis using Emx1-Cre line resulted in premature cell cycle exit of early progenitors, increase of early born neuronal subsets located in the marginal zone and lower layers, and a nearly complete absence of upper layer neurons, especially in the rostral cortex. Furthermore, progenitors, which accumulated in the enlarged germinal neuroepithelium at the pallial/subpallial border in the Pax6 mutants, produced an excess of oligodendrocytes. The inactivation of Pax6 after generation of the lower neuronal layers using hGFAP-Cre line did not affect specification or numbers of late-born neurons, indicating that the severe reduction of upper layer neurons in Pax6 deficiency is mostly attributable to a depletion of the progenitor pool, available for late neurogenesis. We further show that Pax6(fl/fl);Emx1-Cre mutants exhibited deficiencies in sensorimotor information integration, and both hippocampus-dependent short-term and neocortex-dependent long-term memory recall. Because a majority of the morphological and behavior disabilities of the Pax6 mutant mice parallel abnormalities reported for aniridia patients, a condition caused by PAX6 haploinsufficiency, the Pax6 conditional mutant mice generated here represent a valuable genetic tool to understand how the developmental cortical disruption can lead to a human behavior abnormality.
Transcription factor Pax6 exerts a prominent rostrolateral(high) to caudomedial(low) expression gradient in the cortical progenitors and have been implicated in regulation of area identity in the mammalian cortex. Herein, we analyzed the role of Pax6 in molecular arealization and development of thalamocortical connections in the juvenile cortex-specific conditional Pax6 knock-out mice (Pax6cKO). Using a set of molecular markers of positional identity (Id2, Cadherin6, COUP-TF1, RZRbeta, and EphA7), we show that, in the juvenile Pax6cKO, the relative size of caudal cortical areas (putative visual and somatosensory) are mildly enlarged, whereas the rostral domain (putative motor) is severely reduced. Despite the rostral shift of graded expression of areal markers, the distribution of area-specific thalamocortical and corticofugal projections appear normal in the Pax6cKO. This indicates that change of the size of cortical areas is not accompanied by a change in cortical identity. We show furthermore that, despite a severe depletion of supragranular cortical layers and accumulation of cells along the pallial-subpallial boundary, thalamocortical fibers establish a periphery-related pattern of the somatosensory cortex in normal position in Pax6cKO. Our findings indicate that Pax6 expression gradients in cortical progenitors do not directly impart thalamocortical or corticofugal areal identity.
The transcription factor Pax6 is an important developmental regulator. Spatiotemporal control of Pax6 expression during embryogenesis is crucial for regulating distinct aspects of cortical development. Here, we report that Trim11, a member of the TRIM/RBCC protein family of E3 ubiquitin ligases, interacts with Pax6 and mediates Pax6 degradation via the ubiquitin-proteasome system. Trim11 overexpression decreases endogenous Pax6 protein levels and represses Pax6 functions, including Pax6-dependent transactivation and neurogenesis. Abrogation of endogenous Trim11 expression in the developing cortex increases the level of insoluble forms of Pax6 and enhances apoptosis. We provide evidence that the B30.2 domain of Trim11 is essential for the clearance of insoluble cell proteins. Furthermore, we show that the expression of Trim11 is directly regulated by Pax6 in developing cortex in vivo. Our findings indicate that an autoregulatory feedback loop between Trim11 and Pax6 maintains a balance between the levels of Pax6 and Trim11 proteins in cortical progenitors, having an essential role for the Pax6-dependent neurogenesis.
Although the transcription factor Pax6 plays an essential role in neurogenesis, layer formation and arealization in the developing mammalian cortex, the mechanisms by which it accomplishes these regulatory functions are largely unknown. Pax6 and the ETS family transcription factor Er81, which is presumed to play a role in the specification of a sublineage of layer 5 projection neurons, are expressed with a prominent rostrolateral-high to caudomedial-low gradient in cortical progenitors. In the absence of functional Pax6, progenitors do not express Er81 and the rostrolateral cortex lacks Er81-positive layer 5 neurons. In this study, we investigated the transcriptional regulation of Er81 and provide evidence that Er81 is a direct target of Pax6.
We identified and analyzed the regulatory function of an evolutionarily conserved upstream DNA sequence in the putative mouse Er81 promoter. Three potential Pax6 binding sites were identified in this region. We found that the presence of one of these sites is necessary and sufficient for full activation of the Er81 promoter in Pax6-transfected HeLa cells, while other still unknown factors appear to contribute to Er81 promoter activity in cortical progenitors and neuronal cells. The results suggest that endogenous Pax6, which is expressed at the highest level in progenitors of the rostrolateral cortex, exerts region-specific control of Er81 activity, thus specifying a subpopulation of layer 5 projection neurons.
We conclude that the genetic interplay between the transcription factors, Pax6 and Er81, is responsible, in part, for the regional specification of a distinct sublineage of layer 5 projection neurons.
During development, Pax6 is expressed in a rostrolateral-high to caudomedial-low gradient in the majority of the cortical radial glial progenitors and endows them with neurogenic properties. Using a Cre/loxP-based approach, we studied the effect of conditional activation of two Pax6 isoforms, Pax6 and Pax6-5a, on the corticogenesis of transgenic mice. We found that activation of either Pax6 or Pax6-5a inhibits progenitor proliferation in the developing cortex. Upon activation of transgenic Pax6, specific progenitor pools with distinct endogenous Pax6 expression levels at different developmental stages show defects in cell cycle progression and in the acquisition of apoptotic or neuronal cell fate. The results provide new evidence for the complex role of Pax6 in mammalian corticogenesis.