NFAT signaling in neural development and axon growth

Mitochondrial Ca2+ cycling facilitates activation of the transcription factor NFAT in sensory neurons

Ca(2+)-dependent gene regulation controls many aspects of neuronal plasticity. Significant progress has been made toward understanding the roles of voltage- and ligand-gated Ca(2+) channels in triggering specific transcriptional responses. In contrast, the functional importance of Ca(2+) buffers and Ca(2+) transporters in neuronal gene regulation is less clear despite their critical contribution to the spatiotemporal control of Ca(2+) signals. Here we examined the role of mitochondrial Ca(2+) uptake and release in regulating the Ca(2+)-dependent transcription factor NFAT (nuclear factor of activated T-cells), which has been implicated in synaptic plasticity, axonal growth, and neuronal survival. Intense stimulation of sensory neurons by action potentials or TRPV1 agonists induced rapid activation and nuclear import of NFAT. Nuclear translocation of NFAT was associated with a characteristic prolonged [Ca(2+)](i) elevation (plateau) that resulted from Ca(2+) uptake by, and its subsequent release from, mitochondria. Measurements using a mitochondrial Ca(2+) indicator, mtPericam, showed that this process recruited mitochondria throughout the cell body, including the perinuclear region. [Ca(2+)](i) levels attained during the plateau phase were similar to or higher than those required for NFAT activation (200-300 nm). The elimination of the [Ca(2+)](i) plateau by blocking either mitochondrial Ca(2+) uptake via the uniporter or Ca(2+) release via the mitochondrial Na(+)/Ca(2+) exchanger strongly reduced nuclear import of NFAT. Furthermore, preventing Ca(2+) mobilization via the mitochondrial Na(+)/Ca(2+) exchanger diminished NFAT-mediated transcription. Collectively, these data implicate activity-induced Ca(2+) uptake and prolonged release from mitochondria as a novel regulatory mechanism in neuronal excitation-transcription coupling.

Transcriptional dysregulation in NIPBL and cohesin mutant human cells

Genome-wide studies using cells from patients with Cornelia de Lange Syndrome reveal a role for cohesin in regulating gene expression in human cells.

Cohesin regulates sister chromatid cohesion during the mitotic cell cycle with Nipped-B-Like (NIPBL) facilitating its loading and unloading. In addition to this canonical role, cohesin has also been demonstrated to play a critical role in regulation of gene expression in nondividing cells. Heterozygous mutations in the cohesin regulator NIPBL or cohesin structural components SMC1A and SMC3 result in the multisystem developmental disorder Cornelia de Lange Syndrome (CdLS). Genome-wide assessment of transcription in 16 mutant cell lines from severely affected CdLS probands has identified a unique profile of dysregulated gene expression that was validated in an additional 101 samples and correlates with phenotypic severity. This profile could serve as a diagnostic and classification tool. Cohesin binding analysis demonstrates a preference for intergenic regions suggesting a cis-regulatory function mimicking that of a boundary/insulator interacting protein. However, the binding sites are enriched within the promoter regions of the dysregulated genes and are significantly decreased in CdLS proband, indicating an alternative role of cohesin as a transcription factor.

Appropriate segregation of chromosomes to daughter cells depends upon proper cohesion of sister chromatids during mitosis. The multiprotein cohesin complex and its regulators are key factors in this process. Intriguingly, recent work has shown that the cohesin complex also has other cellular roles, including a role in regulating gene expression. Additionally, mutations in cohesin structural and regulatory components have been linked to human multisystem developmental disorders such as Cornelia de Lange Syndrome (CdLS), but the role cohesin is playing in the pathogenesis of this disorder is unknown. To define the role that cohesin plays in regulating gene expression in human cells, we analyzed gene expression and genome-wide cohesin binding patterns in cells from normal subjects and from CdLS probands with mutations in the cohesin regulator NIPBL or in the cohesin structural component SMC1A. We found a strikingly conserved pattern of gene dysregulation in these different cell lines that correlates with disease severity and a significant correlation between gene dysregulation and cohesin binding around misexpressed genes. The observed pattern of binding and misexpression is consistent with cohesin having a putative role as a boundary/insulator interacting protein or transcription factor, the activity of which is disrupted in CdLS probands.

NFAT-3 is a transcriptional repressor of the growth-associated protein 43 during neuronal maturation

Transcription is essential for neurite and axon outgrowth during development. Recent work points to the involvement of nuclear factor of activated T cells (NFAT) in the regulation of genes important for axon growth and guidance. However, NFAT has not been reported to directly control the transcription of axon outgrowth-related genes. To identify transcriptional targets, we performed an in silico promoter analysis and found a putative NFAT site within the GAP-43 promoter. Using in vitro and in vivo experiments, we demonstrated that NFAT-3 regulates GAP-43, but unexpectedly, does not promote but represses the expression of GAP-43 in neurons and in the developing brain. Specifically, in neuron-like PC-12 cells and in cultured cortical neurons, the overexpression of NFAT-3 represses GAP-43 activation mediated by neurotrophin signaling. Using chromatin immunoprecipitation assays, we also show that prior to neurotrophin activation, endogenous NFAT-3 occupies the GAP-43 promoter in PC-12 cells, in cultured neurons, and in the mouse brain. Finally, we observe that NFAT-3 is required to repress the physiological expression of GAP-43 and other pro-axon outgrowth genes in specific developmental windows in the mouse brain. Taken together, our data reveal an unexpected role for NFAT-3 as a direct transcriptional repressor of GAP-43 expression and suggest a more general role for NFAT-3 in the control of the neuronal outgrowth program.

Dopaminergic signals in primary motor cortex

Brainstem monoamine areas such as the ventral tegmental area (VTA) send dopaminergic projections to the cerebral cortex that are widely distributed across different cortical regions. Whereas the projection to prefrontal areas (PFC) has been studied in detail, little is known about dopaminergic projections to primary motor cortex (M1). These projections have been anatomically characterized in rat and primate M1. Primates have even denser dopaminergic projections to M1 than rats. The physiological role, the effects of dopaminergic input on the activity of M1 circuits, and the behavioral function of this projection are unknown. This review explores the existing anatomical, electrophysiological and behavioral evidence on dopaminergic projections to M1 and speculates about its functional role. The projection may explain basic features of motor learning and memory phenomena. It is of clinical interest because of its potential for augmenting motor recovery after a brain lesion as well as for understanding the symptomatology of patients with Parkinson's disease. Therefore, targeted investigations are necessary.

Transcriptional responses to intermittent hypoxia

Recurrent apneas are characterized by transient repetitive cessations of breathing (two breaths duration or longer) resulting in periodic decreases in arterial blood PO2 or chronic intermittent hypoxia (IH). Patients with recurrent apneas and experimental animals exposed to chronic IH exhibit cardio-respiratory morbidities. The purpose of this article is to highlight the current information on the transcriptional mechanisms associated with chronic IH. Studies on rodents and cell cultures have shown that IH activates a variety of transcription factors including the hypoxia-inducible factor-1 (HIF-1), c-fos (immediate early gene), nuclear factor of activated T-cells (NFAT), and nuclear factor kB (NF-kB). The signaling pathways associated with transcriptional activation associated with IH differ from continuous hypoxia (CH). Compared to same duration and intensity of CH, IH is more potent in activating HIF-1 and c-fos and also results in long-lasting accumulation of HIF-1alpha and c-fos mRNA, a phenomenon that was not seen with CH. IH-evoked transcriptional activation by HIF-1, c-fos as well as the resulting activator protein-1 (AP-1) requires reactive oxygen species (ROS)-mediated signaling and involves complex feed forward interactions between HIF-1 and ROS. Chronic IH-evoked cardio-respiratory responses are absent in Hif-1alpha+/- mice, and hypertension elicited by chronic IH is absent in mice lacking NFAT3c. These studies indicate that cardiorespiratory responses to chronic IH depend on complex interactions between various transcription factors resulting in alterations in several down stream genes and their protein products.

Alternative splicing and expression of human and mouse NFAT genes

Four members of the nuclear factor of activated T cells (NFAT) family (NFATC1, NFATC2, NFATC3, and NFATC4) are Ca(2+)-regulated transcription factors that regulate several processes in vertebrates, including the development and function of the immune, cardiovascular, musculoskeletal, and nervous systems. Here we describe the structures and alternative splicing of the human and mouse NFAT genes, including novel splice variants for NFATC1, NFATC2, NFATC3, and NFATC4, and show the expression of different NFAT mRNAs in various mouse and human tissues and brain regions by RT-PCR. Our results show that alternatively spliced NFAT mRNAs are expressed differentially and could contribute to the diversity of functions of the NFAT proteins. Since NFAT family members are Ca(2+)-regulated and have critical roles in neuronal gene transcription in response to electrical activity, we describe the expression of NFATC1, NFATC2, NFATC3, and NFATC4 mRNAs in the adult mouse brain and in the adult human hippocampus using in situ hybridization and show that all NFAT mRNAs are expressed in the neurons of the mouse brain with specific patterns for each NFAT.