RESUMO
Suppressor of cancer cell invasion (SCAI) acts as a transcriptional repressor of serum response factor (SRF)-mediated gene expression by binding to megakaryoblastic leukemia (MKL)/myocardin-related transcription factor (MRTF), which is an SRF transcriptional coactivator. Growing evidence suggests that SCAI is a negative regulator of neuronal morphology, whereas MKL2/MRTFB is a positive regulator. The mRNA expression of SCAI is downregulated during brain development, suggesting that a reduction in SCAI contributes to the reduced suppression of SRF-mediated gene induction, thus increasing dendritic complexity and developing neuronal circuits. In the present study, we hypothesized that brain-derived neurotrophic factor (BDNF), which is important for neuronal plasticity and development, might alter SCAI mRNA levels. We therefore investigated the effects of BDNF on SCAI mRNA levels in primary cultured cortical neurons. Furthermore, because alternative splicing generates several SCAI variants in the brain, we measured SCAI variant mRNA after BDNF stimulation. Both SCAI variant 1 and total SCAI mRNA expression levels were downregulated by BDNF. Moreover, the extracellular signal-regulated protein kinase/mitogen-activated protein kinase (ERK/MAPK) pathway was involved in the BDNF-mediated decrease in SCAI mRNA expression. Our findings provide insights into the molecular mechanism underlying a neurotrophic factor switch for the repressive transcriptional complex that includes SCAI.
Assuntos
Fator Neurotrófico Derivado do Encéfalo , Neurônios , Humanos , Fator Neurotrófico Derivado do Encéfalo/genética , Fator Neurotrófico Derivado do Encéfalo/metabolismo , Fator Neurotrófico Derivado do Encéfalo/farmacologia , RNA Mensageiro/genética , RNA Mensageiro/metabolismo , Neurônios/metabolismo , Regulação da Expressão Gênica , MAP Quinases Reguladas por Sinal Extracelular/metabolismo , Invasividade Neoplásica , Células CultivadasRESUMO
Suppressor of cancer cell invasion (SCAI) is a suppressor of myocardin-related transcription factor (MRTF)-mediated transcription and cancer cell invasion. However, roles of SCAI in the brain and neuronal cells are not fully resolved. In this study, we initially investigated the distribution of Scai mRNA in the developing rat brain and in neurons. We found that, although Scai mRNA levels decreased during brain development, it was highly expressed in several brain regions and in neurons but not astrocytes. Subsequently, in addition to Scai variant 1, we identified novel rat Scai variants 2 and 3 and characterized their functions in Neuro-2a cells. The novel Scai variants 2 and 3 contain unique exons that possess stop codons and therefore encode shorter proteins compared with the full-length Scai variant 1. SCAI variants 2 and 3 possess a nuclear localization signal, but do not have an MRTF-binding site. Immunostaining of green fluorescent protein (GFP)-tagged SCAI variants revealed a nuclear localization of variant 1, whereas localization of variants 2 and 3 was throughout the cytoplasm and nucleus, suggesting that other nuclear localization signals, which act in Neuro-2a cells, exist in SCAI. All three SCAI variants suppressed the neuron-like morphological change of Neuro-2a cells induced by a Rho effector, constitutively active mDia; however, the suppressive effects of variants 2 and 3 were weaker than that of full-length SCAI variant 1, indicating that the SCAI-mediated change toward a neuronal morphology appeared to be consistent with their nuclear localization. These findings indicate that generation of multiple SCAI splice variants fines-tune neuronal morphology.
Assuntos
Astrócitos/metabolismo , Encéfalo/metabolismo , Neurônios/metabolismo , Splicing de RNA , Fatores de Transcrição/genética , Animais , Animais Recém-Nascidos , Encéfalo/citologia , Encéfalo/crescimento & desenvolvimento , Linhagem Celular Tumoral , Células Cultivadas , Feminino , Perfilação da Expressão Gênica , Regulação da Expressão Gênica no Desenvolvimento , Masculino , Camundongos , Células NIH 3T3 , Isoformas de Proteínas/genética , Isoformas de Proteínas/metabolismo , Ratos Sprague-Dawley , Fatores de Transcrição/metabolismoRESUMO
Phosphatase and actin regulator 3/nuclear scaffold-associated protein phosphatase 1-inhibiting protein (Phactr3/Scapinin) is an actin- and protein phosphatase 1 (PP1)-binding protein known to negatively regulate axon elongation. In this study, we examined the expression pattern of Phactr3/Scapinin in several tissues and investigated the effect of Phactr3/Scapinin on dendritic morphology of cortical neurons. Results showed that Phactr3/Scapinin expression was up-regulated in the developing brain and enriched in neurons and in the postsynaptic density fraction, but not in astrocytes. Overexpression of wild type or mutant Phactr3/Scapinin, which lacked actin-binding activity, resulted in increased dendritic complexity and percentage of spines with a mushroom or stubby shape, as well as a decrease in spine density. However, overexpression of mutant Phactr3/Scapinin that lacked PP1-binding activity did not. Taken together, these findings suggest that Phactr3/Scapinin expression is neuronal and might contribute to synaptic formation via distinct actin- and PP1-binding domains involved in dendritic and axonal morphology, respectively.
Assuntos
Dendritos/metabolismo , Proteínas dos Microfilamentos/química , Proteínas dos Microfilamentos/metabolismo , Proteínas Associadas à Matriz Nuclear/química , Proteínas Associadas à Matriz Nuclear/metabolismo , Matriz Nuclear/metabolismo , Proteínas Nucleares/química , Proteínas Nucleares/metabolismo , Proteína Fosfatase 1/metabolismo , Animais , Córtex Cerebral/citologia , Feminino , Masculino , Camundongos Endogâmicos C57BL , Mutação/genética , Ligação Proteica , Domínios Proteicos , RNA Mensageiro/genética , RNA Mensageiro/metabolismo , Ratos Sprague-Dawley , Frações Subcelulares/metabolismo , Regulação para Cima/genéticaRESUMO
Dynamic changes in neuronal morphology and transcriptional regulation play crucial roles in the neuronal network and function. Accumulating evidence suggests that the megakaryoblastic leukemia (MKL) family members, which function not only as actin-binding proteins but also as serum response factor (SRF) transcriptional coactivators, regulate neuronal morphology. However, the extracellular ligands and signaling pathways, which activate MKL-mediated morphological changes in neurons, remain unresolved. Here, we demonstrate that in addition to MKL1, MKL2, highly enriched in the forebrain, strongly contributes to the dendritic complexity, and this process is triggered by stimulation with activin, a member of the transforming growth factor ß (TGF-ß) superfamily. Activin promoted dendritic complexity in a SRF- and MKL-dependent manner without drastically affecting MKL localization and protein levels. In contrast, activin promoted the nuclear export of suppressor of cancer cell invasion (SCAI), which is a corepressor for SRF and MKL. Furthermore, overexpression of SCAI blocked activin-induced SRF transcriptional responses and dendritic complexity. Collectively, these results strongly suggest that activin-SCAI-MKL signaling is a novel pathway that regulates the dendritic morphology of rat cortical neurons by excluding SCAI from the nucleus and activating MKL/SRF-mediated gene expression.