Rita Lo

Role for Runx1 in the Proliferation and Neuronal Differentiation of Selected Progenitor Cells in the Mammalian Nervous System

Neurogenesis requires factors that regulate the decision of dividing progenitors to leave the cell cycle and activate the neuronal differentiation program. It is shown here that the murine runt-related gene Runx1 is expressed in proliferating cells on the basal side of the olfactory epithelium. These include both Mash1+ olfactory receptor neuron (ORN) progenitors and NeuroD+ ORN precursors. Disruption of Runx1 function in vivo does not cause a change in Mash1 expression but leads to a decrease in the number of NeuroD+ neuronal precursors and an increase in differentiated ORNs. These effects result in premature and ectopic ORN differentiation. It is shown further that exogenous Runx1 expression in cultured olfactory neural progenitors causes an expansion of the mitotic cell population. In agreement with these findings, exogenous Runx1 expression also promotes cortical neural progenitor cell proliferation without inhibiting neuronal differentiation. These effects are phenocopied by a chimeric protein containing ETO, the eight twenty one transcriptional repressor, fused to the Runx1 DNA-binding domain, which suggests the involvement of transcription repression mechanisms. Consistent with this possibility, Runx1 represses transcription driven by the promoter of the cell cycle inhibitor p21Cip 1 in cortical progenitors. Together, these findings suggest a previously unrecognized role for Runx1 in coordinating the proliferation and neuronal differentiation of selected populations of neural progenitors.

Regulation of Postnatal Forebrain Amoeboid Microglial Cell Proliferation and Development by the Transcription Factor Runx1

Microglia are the immune cells of the nervous system, where they act as resident macrophages during inflammatory events underlying many neuropathological conditions. Microglia derive from primitive myeloid precursors that colonize the nervous system during embryonic development. In the postnatal brain, microglia are initially mitotic, rounded in shape (amoeboid), and phagocytically active. As brain development proceeds, they gradually undergo a transition to a surveillant nonphagocytic state characterized by a highly branched (ramified) morphology. This ramification process is almost recapitulated in reverse during the process of microglia activation in the adult brain, when surveillant microglia undergo a ramified-to-amoeboid morphological transformation and become phagocytic in response to injury or disease. Little is known about the mechanisms controlling amoeboid microglial cell proliferation, activation, and ramification during brain development, despite the critical role of these processes in the establishment of the adult microglia pool and their relevance to microglia activation in the adult brain. Here we show that the mouse transcription factor Runx1, a key regulator of myeloid cell proliferation and differentiation, is expressed in forebrain amoeboid microglia during the first two postnatal weeks. Runx1 expression is then downregulated in ramified microglia. Runx1 inhibits mouse amoeboid microglia proliferation and promotes progression to the ramified state. We show further that Runx1 expression is upregulated in microglia following nerve injury in the adult mouse nervous system. These findings provide insight into the regulation of postnatal microglia activation and maturation to the ramified state and have implications for microglia biology in the developing and injured brain.

Disrupted development of the cerebral hemispheres in transgenic mice expressing the mammalian Groucho homologue Transducin-like-Enhancer of split 1 in postmitotic neurons

Transducin-like Enhancer of split (TLE) 1 is a mammalian transcriptional corepressor homologous to Drosophila Groucho. In Drosophila, Groucho acts together with bHLH proteins of the Hairy/Enhancer of split (HES) family to negatively regulate neuronal differentiation. Loss of the functions of Groucho or HES proteins results in supernumerary central and peripheral neurons. This suggests that mammalian TLE/Groucho family members may also be involved in the regulation of neuronal differentiation. Consistent with this possibility, TLE1 is expressed in proliferating neural progenitor cells of the central nervous system, but its expression is transiently down-regulated in newly generated postmitotic neurons. Based on these observations, we investigated whether persistent TLE1 expression in postmitotic neurons would perturb the normal course of neuronal development. Transgenic mice were derived in which the human TLE1 gene is regulated by the promoter of the Talpha1 alpha-tubulin gene, which is exclusively expressed in postmitotic neurons. In these mice, constitutive expression of TLE1 inhibits neuronal development in the embryonic forebrain leading to increased apoptosis and neuronal loss in the ventral and dorsal telencephalon. These results provide the first direct evidence that TLE1 is an important negative regulator of postmitotic neuronal differentiation in the mammalian central nervous system.

Transcription factors FOXG1 and Groucho/TLE promote glioblastoma growth

Glioblastoma (GBM) is the most common and deadly malignant brain cancer, with a median survival of less than two years. GBM displays a cellular complexity that includes brain tumour-initiating cells (BTICs), which are considered as potential key targets for GBM therapies. Here we show that the transcription factors FOXG1 and Groucho/TLE are expressed in poorly differentiated astroglial cells in human GBM specimens and in primary cultures of GBM-derived BTICs, where they form a complex. FOXG1 knockdown in BTICs causes downregulation of neural stem/progenitor and proliferation markers, increased replicative senescence, upregulation of astroglial differentiation genes, and decreased BTIC-initiated tumour growth upon intracranial transplantation into host mice. These effects are phenocopied by Groucho/TLE knockdown or dominant-inhibition of the FOXG1:Groucho/TLE complex. These results provide evidence that transcriptional programs regulated by FOXG1 and Groucho/TLE are important for BTIC-initiated brain tumour growth, implicating FOXG1 and Groucho/TLE in GBM tumorigenesis.

Cofactor-Activated Phosphorylation Is Required for Inhibition of Cortical Neuron Differentiation by Groucho/TLE1

Background Transcriptional co-repressors of the Groucho/transducin-like Enhancer of split (Gro/TLE) family regulate the expression of a variety of genes and are involved in numerous developmental processes in both invertebrate and vertebrate species. More specifically, Gro/TLE1 participates in mechanisms that inhibit/delay the differentiation of cerebral cortex neural progenitor cells into neurons during mammalian forebrain development. The anti-neurogenic function of Gro/TLE1 depends on the formation of protein complexes with specific DNA-binding transcription factors that engage Gro/TLE1 through WRP(W/Y) sequences. Interaction with those transcription partners results in Gro/TLE1 recruitment to selected DNA sites and causes increased Gro/TLE1 phosphorylation. The physiological significance of the latter event, termed “cofactor-activated phosphorylation,” had not been determined. Therefore, this study aimed at clarifying the role of cofactor-activated phosphorylation in the anti-neurogenic function of Gro/TLE1. Methods and Principal Findings A combination of site-directed mutagenesis, mass spectrometry, biochemistry, primary cell culture, and immunocytochemical assays was utilized to characterize point mutations of Ser-286, a residue that is phosphorylated in vivo and is located within the serine/proline-rich (SP) domain of Gro/TLE1. Mutation of Ser-286 to alanine or glutamic acid does not perturb the interaction of Gro/TLE1 with DNA-binding partners, including the basic helix-loop-helix transcription factor Hes1, a prototypical anti-neurogenic WRP(W/Y) motif protein. Ser-286 mutations do not prevent the recruitment of Gro/TLE1 to DNA, but they impair cofactor-activated phosphorylation and weaken the interaction of Gro/TLE1 with chromatin. These effects are correlated with an impairment of the anti-neurogenic activity of Gro/TLE1. Similar results were obtained when mutations of Ser-289 and Ser-298, which are also located within the SP domain of Gro/TLE1, were analyzed. Conclusion Based on the positive correlation between Gro/TLE1 cofactor-activated phosphorylation and ability to inhibit cortical neuron differentiation, we propose that hyperphosphorylation induced by cofactor binding plays a positive role in the regulation of Gro/TLE1 anti-neurogenic activity.

Prolyl isomerase Pin1 and protein kinase HIPK2 cooperate to promote cortical neurogenesis by suppressing Groucho/TLE:Hes1-mediated inhibition of neuronal differentiation.

The Groucho/transducin-like Enhancer of split 1 (Gro/TLE1):Hes1 transcriptional repression complex acts in cerebral cortical neural progenitor cells to inhibit neuronal differentiation. The molecular mechanisms that regulate the anti-neurogenic function of the Gro/TLE1:Hes1 complex during cortical neurogenesis remain to be defined. Here we show that prolyl isomerase Pin1 (peptidyl-prolyl cis-trans isomerase NIMA-interacting 1) and homeodomain-interacting protein kinase 2 (HIPK2) are expressed in cortical neural progenitor cells and form a complex that interacts with the Gro/TLE1:Hes1 complex. This association depends on the enzymatic activities of both HIPK2 and Pin1, as well as on the association of Gro/TLE1 with Hes1, but is independent of the previously described Hes1-activated phosphorylation of Gro/TLE1. Interaction with the Pin1:HIPK2 complex results in Gro/TLE1 hyperphosphorylation and weakens both the transcriptional repression activity and the anti-neurogenic function of the Gro/TLE1:Hes1 complex. These results provide evidence that HIPK2 and Pin1 work together to promote cortical neurogenesis, at least in part, by suppressing Gro/TLE1:Hes1-mediated inhibition of neuronal differentiation.

Interaction and Antagonistic Roles of NF-κB and Hes6 in the Regulation of Cortical Neurogenesis

ABSTRACT The involvement of nuclear factor kappa B (NF-κB) in several processes in the postnatal and adult brain, ranging from neuronal survival to synaptogenesis and plasticity, has been documented. In contrast, little is known about the functions of NF-κB during embryonic brain development. It is shown here that NF-κB is selectively activated in neocortical neural progenitor cells in the developing mouse telencephalon. Blockade of NF-κB activity leads to premature cortical neuronal differentiation and depletion of the progenitor cell pool. Conversely, NF-κB activation causes decreased cortical neurogenesis and expansion of the progenitor cell compartment. These effects are antagonized by the proneuronal transcription factor Hes6, which physically and functionally interacts with RelA-containing NF-κB complexes in cortical progenitor cells. In turn, NF-κB exerts an inhibitory effect on the ability of Hes6 to promote cortical neuronal differentiation. These results reveal previously uncharacterized functions and modes of regulation for NF-κB and Hes6 during cortical neurogenesis.

Establishment of motor neuron-V3 interneuron progenitor domain boundary in ventral spinal cord requires Groucho-mediated transcriptional corepression.

Background Dorsoventral patterning of the developing spinal cord is important for the correct generation of spinal neuronal types. This process relies in part on cross-repressive interactions between specific transcription factors whose expression is regulated by Sonic hedgehog. Groucho/transducin-like Enhancer of split (TLE) proteins are transcriptional corepressors suggested to be recruited by at least certain Sonic hedgehog-controlled transcription factors to mediate the formation of spatially distinct progenitor domains within the ventral spinal cord. The aim of this study was to characterize the involvement of TLE in mechanisms regulating the establishment of the boundary between the most ventral spinal cord progenitor domains, termed pMN and p3. Because the pMN domain gives rise to somatic motor neurons while the p3 domain generates V3 interneurons, we also examined the involvement of TLE in the acquisition of these neuronal fates. Methodology and Principal Findings A combination of in vivo loss- and gain-of-function studies in the developing chick spinal cord was performed to characterize the role of TLE in ventral progenitor domain formation. It is shown here that TLE overexpression causes increased numbers of p3 progenitors and promotes the V3 interneuron fate while suppressing the motor neuron fate. Conversely, dominant-inhibition of TLE increases the numbers of pMN progenitors and postmitotic motor neurons. Conclusion Based on these results, we propose that TLE is important to promote the formation of the p3 domain and subsequent generation of V3 interneurons.

Phenolic 1,3‐diketones attenuate lipopolysaccharide‐induced inflammatory response by an alternative magnesium‐mediated mechanism

Toll‐like receptor 4 (TLR4) plays a key role in the induction of inflammatory responses both in peripheral organs and the CNS. Curcumin exerts anti‐inflammatory functions by interfering with LPS‐induced dimerization of TLR4–myeloid differentiation protein‐2 (MD‐2) complex and suppressing pro‐inflammatory mediator release. However, the inhibitory mechanism of curcumin remains to be defined.

Regulation of glioblastoma stem-like cell self-renewal by FOXG1 and TLE

Solomon Snyder Dept. of Neuroscience, Johns Hopkins School of Medicine, United States E-mail address: jdemelo@jhmi.edu (J. de Melo). The LIM homeodomain transcription factor Lhx2 is an essential factor for the development of the mammalian retina. Lhx2 knockout mice are embryonic lethal and display complete anophthalmia with arrest of ocular development occurring at the optic vesicle stage. We demonstrate that Lhx2 is expressed in mitotically active retinal progenitors during embryonic development. Expression of Lhx2 becomes restricted to Müller glia (MG) and a subset of amacrine cells in the post-natal retina within which Lhx2 expression remains throughout adulthood. Utilizing a conditional Lhx2 knockout mouse we have demonstrated that loss of Lhx2 at approximately E10 results in mitotic arrest and microphthalmia. Deletion of Lhx2 in Muller glial precursors, however, resulted in a failure in MG formation resulting in laminar disruption and rosette formation in the outer nuclear layer (ONL). In vivo electroporation experiments were performed to generate retinas in which Lhx2 was deleted in a mosaic fashion. Electroporation of plasmids expressing Cre recombinase into floxed Lhx2 mice replicated the MG phenotype seen in the conditional knockout mice. Electroporation of the pro-glial bHLH transcription factor Hes5 promoted formation of MG; however, co-electroporation of Hes5 with cre into floxed Lhx2 mice blocked the enhanced MG formation. Furthermore, co-electroporation of Lhx2 with Hes5 into wild-type retinas also blocked the formation of MG. While loss of Lhx2 function resulted in failed MG development, electroporation of Lhx2 alone did not promote the formation of MG but instead resulted in the generation of wide-field amacrine cells. Intriguingly, generation of wide field amacrine cells was enhanced by co-electroporating Lhx2 with the bHLH transcription factor Neurog2. Taken together these results suggest a model where Lhx2 maintains the retinal progenitor state and is instructive for the generation of wide-field amacrine cells. However, upon commitment to a MG fate Lhx2 is absolutely required for the differentiation of MG.