RESUMO
In eukaryotes, the switch between alternative developmental pathways is mainly attributed to a switch in transcriptional programs. A major mode in this switch is the transition between histone deacetylation and acetylation. In budding yeast, early meiosis-specific genes (EMGs) are repressed in the mitotic cell cycle by active deacetylation of their histones. Transcriptional activation of these genes in response to the meiotic signals (i.e., glucose and nitrogen depletion) requires histone acetylation. Here we follow how this regulated switch is accomplished, demonstrating the existence of two parallel mechanisms. (i) We demonstrate that depletion of glucose and nitrogen leads to a transient replacement of the histone deacetylase (HDAC) complex on the promoters of EMG by the transcriptional activator Ime1. The occupancy by either component occurs independently of the presence or absence of the other. Removal of the HDAC complex depends on the protein kinase Rim15, whose activity in the presence of nutrients is inhibited by protein kinase A phosphorylation. (ii) In the absence of glucose, HDAC loses its ability to repress transcription, even if this repression complex is directly bound to a promoter. We show that this relief of repression depends on Ime1, as well as on the kinase activity of Rim11, a glycogen synthase kinase 3beta homolog that phosphorylates Ime1. We further show that the glucose signal is transmitted through Rim11. In cells expressing the constitutive active rim11-3SA allele, HDAC repression in glucose medium is impaired.
Assuntos
Glucose/metabolismo , Histonas/metabolismo , Nitrogênio/metabolismo , Proteínas de Saccharomyces cerevisiae/metabolismo , Saccharomyces cerevisiae/genética , Saccharomyces cerevisiae/metabolismo , Acetilação , Sequência de Bases , Sítios de Ligação/genética , DNA Fúngico/genética , Proteínas de Ligação a DNA/genética , Proteínas de Ligação a DNA/metabolismo , Regulação Fúngica da Expressão Gênica , Genes Fúngicos , Histona Desacetilases/genética , Histona Desacetilases/metabolismo , Histonas/química , Histonas/genética , Peptídeos e Proteínas de Sinalização Intracelular , Meiose/genética , Modelos Biológicos , Proteínas Nucleares/química , Proteínas Nucleares/genética , Proteínas Nucleares/metabolismo , Fosforilação , Regiões Promotoras Genéticas , Proteínas Quinases/genética , Proteínas Quinases/metabolismo , Proteínas Serina-Treonina Quinases/genética , Proteínas Serina-Treonina Quinases/metabolismo , Proteínas Repressoras/genética , Proteínas Repressoras/metabolismo , Saccharomyces cerevisiae/citologia , Proteínas de Saccharomyces cerevisiae/química , Proteínas de Saccharomyces cerevisiae/genética , Fatores de Transcrição/química , Fatores de Transcrição/genética , Fatores de Transcrição/metabolismo , Tirosina/químicaRESUMO
Meiotically arrested mammalian oocytes are stimulated to resume meiosis by LH. This response, which can be reversed by elevation of intraoocyte cAMP levels, is associated with interruption of gap junctional communication (GJC) within the ovarian follicle. In the present study, we examined the hypothesis that disruption of GJC within the ovarian follicle is sufficient for induction of oocyte maturation. For this purpose, we incubated rat follicle-enclosed oocytes with carbenoxolone (CBX), a known blocker of gap junctions. We found that this selective disruptor of GJC promoted maturation of almost all the follicle-enclosed oocytes after 5 h of incubation; this response was also obtained by a transient (2 h) exposure to this agent. CBX-induced oocyte maturation was accompanied by a substantial decrease in intraoocyte concentrations of cAMP that was not associated with elevated activity of type 3A phosphodiesterase (PDE3A). The effect of CBX on reinitiation of meiosis was blocked by isobutylmethylxanthine, a phosphodiesterase inhibitor. Unlike LH, CBX did not activate MAPK in the follicular cells, and inhibition of the MAPK signaling pathway by means of UO126 did not prevent the resumption of meiosis. Injection of CBX into the ovarian bursa of intact animals stimulated maturation in 30% of the oocytes, whereas no maturation was observed in the contralateral ovary injected with PBS. We conclude that, because experimentally induced breakdown of communication within the ovarian follicle is associated with a drop in intraoocyte cAMP concentrations and results in resumption of meiosis, this could be the physiological mechanism employed by LH to stimulate oocyte maturation.
Assuntos
Junções Comunicantes/fisiologia , Oócitos/fisiologia , Folículo Ovariano/metabolismo , 3',5'-AMP Cíclico Fosfodiesterases/metabolismo , Animais , Western Blotting , Butadienos/farmacologia , Carbenoxolona/farmacologia , Comunicação Celular , AMP Cíclico/metabolismo , Nucleotídeo Cíclico Fosfodiesterase do Tipo 3 , Feminino , Proteínas Ativadoras de GTPase/química , Células da Granulosa/metabolismo , Hormônio Luteinizante/metabolismo , Meiose , Modelos Estatísticos , Nitrilas/farmacologia , Oócitos/metabolismo , Folículo Ovariano/citologia , Ovário/metabolismo , Inibidores de Fosfodiesterase/farmacologia , Ratos , Fatores de TempoRESUMO
The source of the inhibitory levels of cAMP that maintain oocytes meiotically arrested is under controversy. A model for regulation of the meiotic division that suggests the transfer of a somatic follicular cells-derived cAMP into the oocyte via gap junctions was first proposed by us in 1978. Later studies provide strong evidence that established gap-junctional communication within the ovarian follicle is indispensable for maintenance of meiotic arrest. On the other hand, other recent reports suggest that oocytes generate their own inhibitory cAMP by a G protein-coupled receptor-activated Gs. These studies as well as other recent reports related to this topic are thoroughly discussed in this chapter.