Upstream regulatory factor (USF) and sterol regulatory element binding protein (SREBP) play key roles in the transcriptional regulation of the fatty acid synthase (FAS) gene by feeding and insulin. Due to the dual binding specificity of SREBP, as well as the presence of multiple consensus sites for these transcription factors in the FAS promoter, their physiologically relevant functional binding sites have been controversial. Here, in order to determine the occupancy of the putative USF and SREBP binding sites, we examined their protein-DNA interactions in living animals by using formaldehyde cross-linking and immunoprecipitation of chromatin and tested the function of these elements by employing mice transgenic for a reporter gene driven by various 5 deletions as well as site-specific mutations of the FAS promoter. We show that the ؊332 and ؊65 E-boxes are bound by USF in both fasted and refed mice, while the ؊150 SRE is bound by SREBP-1 only in refed mice. We also found that mutation of either the ؊150 SRE or the ؊65 E-box abolishes the feeding-induced activation of the FAS promoter in transgenic mice. Furthermore, in vivo occupancy of the FAS promoter by SREBP in the fed state can be prevented by mutation not only of the ؊150 SRE but, unexpectedly, of the ؊65 E-box as well. We conclude that the FAS promoter is activated during refeeding via the induced binding of SREBP to the ؊150 SRE and that USF binding to the ؊65 E-box is also required for SREBP binding and activation of the FAS promoter.

The expression of the ␤-amyloid precursor protein (APP), which plays a key role in the development of Alzheimer's disease, is regulated by a variety of cellular mediators in a cell-dependent manner. In the present study, we present evidence that thyroid hormones negatively regulate the expression of the APP gene in neuroblastoma cells. Transient transfection studies using plasmids that contain progressive deletions of the 5 region of the gene demonstrate that triiodothyronine (T3), the more active form of the thyroid hormones, represses APP promoter activity by a mechanism that requires binding of the nuclear T3 receptor (TR) to a specific sequence located in the first exon. The unliganded receptor increases promoter activity, and T3 reverses that activity to basal levels. The repressive effect of T3 does not exhibit TR isoform specificity, and it is equally mediated by TR␣ and TR␤. Gel mobility shift assays using in vitro synthesized nuclear receptors and nuclear extracts led to the identification of a negative thyroid hormone response element, at nucleotide position ؉80/ ؉96, that preferentially binds heterodimers of TR with the retinoid X receptor. Insertion of sequences containing this element confers negative regulation by T3 to a heterologous TK promoter, thus indicating the functionality of the element.

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A critical feature of vertebrate neural precursors is the to-and-fro displacement of their nuclei as cell cycle progresses, thus giving rise to a pseudostratified epithelium. This nuclear behavior, referred to as interkinetic nuclear migration (INM), is translated into the disposition of the cell somas at different orthogonal levels depending on the cell cycle stage they are. The finding that important regulators of neurogenesis, such as the proneural and neurogenic genes, undergo cyclic changes of expression and function in coordination with the cell cycle and the INM, and that the neurogenic process correlates with a particular window of the cell cycle, in coincidence with the apical localization in the neuroepithelium of neural precursors, is a novel concept that facilitates our understanding of the neurogenic process in vertebrates. As such, recent data support the notion that the three-dimensional structure of the neuroepithelium is crucial for proper neuronal production. In this review, we describe current knowledge of the molecular mechanisms involved in the differential expression and function of the proneural and neurogenic gene products along the cell cycle, and we discuss important consequences for vertebrate neurogenesis derived from this observation. KEY WORDS: proneural gene, neurogenic gene, lateral inhibition, mRNA stabilityDuring the initial stages of development, the vertebrate embryo undergoes a dorsal invagination of the neuroectoderm to form the neural tube. This structure, which subsequently will generate the brain and spinal cord as well as the neural crest derivatives, is initially a monostratified epithelium with its apical side forming the lumenal surface. As development proceeds neural precursors divide vigorously in an unsynchronized manner, increasing dramatically its cellular density and acquiring a highly packed, pseudostratified disposition characterized by the presence of their nuclei at different levels depending on the cell cycle stage they are (Fig. 1). A hallmark of the neural precursors is therefore the to-and-fro displacement of the nucleus during the cell cycle, a process that is referred to as interkinetic nuclear migration (INM) (Sauer, 1935;Sauer and Walker, 1959;Sidman et al., 1959;Fujita, 1962;Takahashi et al., 1993;Hayes and Nowakowski, 2000). This nuclear movement spans the entire apical-basal axis of the cell, with the nucleus migrating to the basal side during the first gap (G1) phase of the cell cycle, staying at the basal side Int. J. Dev. Biol. 53: 895-908 (2009) doi: 10.1387/ijdb.082721ml Abbreviations used in this paper: Adcyap, adenylate cyclase-activating polypeptide; APC, anaphase-promoting complex; ARE, AU-rich element; asc, achaete scute; ascl, asc-like; ato, atonal; atoh, ato homolog; bHLH, basic helix-loophelix; CDK, cyclin-dependent kinase; CKI, CDK inhibitor; csnk, casein kinase; Dll1, Delta-like 1; ELAV, embryonic lethal abnormal vision; elavl, ELAV-like; emc, extra macrochaetae; E(spl), enhancer of split; Esr, enhancer of split related;...

The b-amyloid protein, component of the senile plaques found in Alzheimer brains is proteolytically derived from the b-amyloid precursor protein (APP), a larger membraneassociated protein that is expressed in both neural and nonneural cells. Overexpression of APP might be one of the mechanisms that more directly contributes to the development of Alzheimer's disease. The APP gene expression is regulated by a number of cellular mediators including nerve growth factor (NGF) and other ligands of tyrosine kinase receptors. We have previously described that NGF increases APP mRNA levels in PC12 cells. However, the molecular mechanisms and the precise signalling pathways that mediate its regulation are not yet well understood. In the present study we present evidence that NGF, and to a lesser extent ®broblast growth factor and epidermal growth factor, stimulate APP promoter activity in PC12 cells. This induction is mediated by DNA sequences located between the nucleotides 2 307 and 2 15, and involves activation of the Ras±MAP kinase signalling pathway. In contrast, we have also found that NGFinduced secretion of soluble fragments of APP into the culture medium is mediated by a Ras independent mechanism.