SoxE group transcription factor Sox8 promotes astrocytic differentiation of neural stem/precursor cells downstream of Nfia.

The brain consists of three major cell types: neurons and two glial cell types (astrocytes and oligodendrocytes). Although they are generated from common multipotent neural stem/precursor cells (NS/PCs), embryonic NS/PCs cannot generate all of the cell types at the beginning of brain development. NS/PCs first undergo extensive self-renewal to expand their pools, and then acquire the potential to produce neurons, followed by glial cells. Astrocytes are the most frequently found cell type in the central nervous system (CNS), and play important roles in brain development and functions. Although it has been shown that nuclear factor IA (Nfia) is a pivotal transcription factor for conferring gliogenic potential on neurogenic NS/PCs by sequestering DNA methyltransferase 1 (Dnmt1) from astrocyte-specific genes, direct targets of Nfia that participate in astrocytic differentiation have yet to be completely identified. Here we show that SRY-box transcription factor 8 (Sox8) is a direct target gene of Nfia at the initiation of the gliogenic phase. We found that expression of Sox8 augmented leukemia inhibitory factor (LIF)-induced astrocytic differentiation, while Sox8 knockdown inhibited Nfia-enhanced astrocytic differentiation of NS/PCs. In contrast to Nfia, Sox8 did not induce DNA demethylation of an astrocyte-specific marker gene, glial fibrillary acidic protein (Gfap), but instead associated with LIF downstream transcription factor STAT3 through transcriptional coactivator p300, explaining how Sox8 expression further facilitated LIF-induced Gfap expression. Taken together, these results suggest that Sox8 is a crucial Nfia downstream transcription factor for the astrocytic differentiation of NS/PCs in the developing brain.

Synergistic induction of astrocytic differentiation by factors secreted from meninges in the mouse developing brain.

Astrocytes, which support diverse neuronal functions, are generated from multipotent neural stem/precursor cells (NS/PCs) during brain development. Although many astrocyte-inducing factors have been identified and studied in vitro, the regions and/or cells that produce these factors in the developing brain remain elusive. Here, we show that meninges-produced factors induce astrocytic differentiation of NS/PCs. Consistent with the timing when astrocytic differentiation of NS/PCs increases, expression of astrocyte-inducing factors is upregulated. Meningeal secretion-mimicking combinatorial treatment of NS/PCs with bone morphogenetic protein 4, retinoic acid and leukemia inhibitory factor synergistically activate the promoter of a typical astrocytic marker, glial fibrillary acidic protein. Taken together, our data suggest that meninges play an important role in astrocytic differentiation of NS/PCs in the developing brain.

NAD(+)-SIRT1 control of H3K4 trimethylation through circadian deacetylation of MLL1.

The circadian clock controls the transcription of hundreds of genes through specific chromatin-remodeling events. The histone methyltransferase mixed-lineage leukemia 1 (MLL1) coordinates recruitment of CLOCK-BMAL1 activator complexes to chromatin, an event associated with cyclic trimethylation of histone H3 Lys4 (H3K4) at circadian promoters. Remarkably, in mouse liver circadian H3K4 trimethylation is modulated by SIRT1, an NAD(+)-dependent deacetylase involved in clock control. We show that mammalian MLL1 is acetylated at two conserved residues, K1130 and K1133. Notably, MLL1 acetylation is cyclic, controlled by the clock and by SIRT1, and it affects the methyltransferase activity of MLL1. Moreover, H3K4 methylation at clock-controlled-gene promoters is influenced by pharmacological or genetic inactivation of SIRT1. Finally, levels of MLL1 acetylation and H3K4 trimethylation at circadian gene promoters depend on NAD(+) circadian levels. These findings reveal a previously unappreciated regulatory pathway between energy metabolism and histone methylation.

Goofy coordinates the acuity of olfactory signaling.

The basic scheme of odor perception and signaling from olfactory cilia to the brain is well understood. However, factors that affect olfactory acuity of an animal, the threshold sensitivity to odorants, are less well studied. Using signal sequence trap screening of a mouse olfactory epithelium cDNA library, we identified a novel molecule, Goofy, that is essential for olfactory acuity in mice. Goofy encodes an integral membrane protein with specific expression in the olfactory and vomeronasal sensory neurons and predominant localization to the Golgi compartment. Goofy-deficient mice display aberrant olfactory phenotypes, including the impaired trafficking of adenylyl cyclase III, stunted olfactory cilia, and a higher threshold for physiological and behavioral responses to odorants. In addition, the expression of dominant-negative form of cAMP-dependent protein kinase results in shortening of olfactory cilia, implying a possible mechanistic link between cAMP and ciliogenesis in the olfactory sensory neurons. These results demonstrate that Goofy plays an important role in establishing the acuity of olfactory sensory signaling.

The histone methyltransferase MLL1 permits the oscillation of circadian gene expression.

The classical view of the molecular clock is based on interlocked transcriptional-translational feedback loops. Because a substantial fraction of the mammalian genome is expressed in a circadian manner, chromatin remodeling has been proposed to be crucial in clock function. Here we show that Lys4 (K4) trimethylation of histone H3 is rhythmic and follows the same profile as previously described H3 acetylation on circadian promoters. MLL1, a mammalian homolog of Drosophila trithorax, is an H3K4-specific methyltransferase implicated in transcriptional control. We demonstrate that MLL1 is essential for circadian transcription and cyclic H3K4 trimethylation. MLL1 is in a complex with CLOCK-BMAL1 and contributes to its rhythmic recruitment to circadian promoters and to H3 acetylation. Yet MLL1 fails to interact with CLOCKΔ19, providing an explanation for this mutation's dominant negative phenotype. Our results favor a scenario in which H3K4 trimethylation by MLL1 is required to establish a permissive chromatin state for circadian transcription.

Amino acids involved in conformational dynamics and G protein coupling of an odorant receptor: targeting gain-of-function mutation.

Thousands of different odorants are recognized and discriminated by odorant receptors (ORs) in the guanine nucleotide-binding protein (G protein)-coupled seven-transmembrane receptor family. Odorant-bound ORs stimulate Gs-type G proteins, Galphaolf, which in turn activates cAMP-mediated signaling pathway in olfactory sensory neurons. To better understand the molecular basis for OR activation and G protein coupling, we analyzed the effects of a series of site-directed mutations of mouse ORs, on function. Mutations of conserved amino acid residues in an intracellular loop or the C-terminus resulted in loss of activity without impairing ligand-binding activity, indicating that these residues are involved in Galphas/olf coupling. Moreover, mutation of the serine in KAFSTC, the OR-specific sequence motif, resulted in a dramatic increase in odorant responsiveness, suggesting that the motif is involved in a conformational change of the receptor that regulates G protein coupling efficiency. Our results provide insights into how ORs switch from an inactive to an active state, as well as where and how activated ORs interact with G proteins.

Odorant receptor map in the mouse olfactory bulb: in vivo sensitivity and specificity of receptor-defined glomeruli.

Odorant identity is represented in the olfactory bulb (OB) by the glomerular activity pattern, which reflects a combination of activated odorant receptors (ORs) in the olfactory epithelium. To elucidate this neuronal circuit at the molecular level, we established a functional OR identification strategy based on glomerular activity by combining in vivo Ca(2+) imaging, retrograde dye labeling, and single-cell RT-PCR. Spatial and functional mapping of OR-defined glomeruli revealed that the glomerular positional relationship varied considerably between individual animals, resulting in different OR maps in the OB. Notably, OR-defined glomeruli exhibited different ligand spectra and far higher sensitivity compared to the in vitro pharmacological properties of corresponding ORs. Moreover, we found that the olfactory mucus was an important factor in the regulation of in vivo odorant responsiveness. Our results provide a methodology to examine in vivo glomerular responses at the receptor level and further help address the long-standing issues of olfactory sensitivity and specificity under physiological conditions.

Structural determinants for membrane trafficking and G protein selectivity of a mouse olfactory receptor.

The G protein-coupled olfactory receptor (OR) superfamily plays a critical role in recognizing a broad range of odorants. Each OR appears to recognize odorants based on similarities in molecular structures such that mOR-EG, a mouse OR, binds eugenol, vanillin, and some other structurally related odorants. Only a few ORs, however, have been characterized functionally due to the difficulties in expressing ORs in heterologous cells. In this report, we demonstrate roles of the N- and C-terminal domains as key elements in the functional expression and signal transducing activity of an OR. Disruption of the N-terminal glycosylation site of the mOR-EG completely impaired its membrane trafficking to the cell surface. Functional expression of the mOR-EG was greatly enhanced by addition of extra N-terminal glycosylation sequences. Addition of a C-terminal epitope-tag or C-terminal truncation significantly reduced the odorant-response activity, although the receptors were properly targeted to the plasma membrane. Analysis of a series of truncated ORs revealed a region in the C-terminus that was crucial for the receptor activity. Replacement of the C-terminal portion of the mOR-EG with that of rhodopsin disrupted the coupling to G(alphas) but not to G(alpha15), demonstrating that the C-terminus is involved in regulating G protein specificity. These results suggest that glycosylation of the N-terminal portion is critical for OR expression and membrane trafficking, while the C-terminal portion plays a role in defining proper conformation, which, in turn, specifies the G protein selectivity of the OR. This information helps clarify the mechanisms that regulate membrane trafficking and G protein interaction of the OR superfamily.

Odorant response assays for a heterologously expressed olfactory receptor.

Odorant responsiveness of a mouse olfactory receptor, mOR-EG, was investigated in various heterologous cells using a variety of detection methods. Odorant-induced Ca(2+) response was observed in HEK293 cells that coexpressed mOR-EG and the promiscuous G protein, G alpha 15. Without G alpha 15, a robust increase in cAMP level was observed upon odorant-stimulation in various mammalian cells. A luciferase reporter gene assay using zif268 promoter was adopted to amplify the cAMP signals. In Xenopus laevis oocytes, odorant-stimulated currents were recorded when mOR-EG cRNA was co-injected with either G alpha 15 or cAMP-dependent channel. These results suggest that odorant responsiveness can be monitored via a signaling pathway mediated by endogenous G alphas or transfected G alpha 15 in heterologous cell systems. Various functional assays for a heterologously expressed olfactory receptor reported in this study, are potentially useful for high-throughput ligand screening and functional analyses of hundreds of olfactory receptors.