Cell polarity is required for asymmetric division, a mechanism to generate cell diversity by distributing fate determinants unequally to daughter cells. The establishment of polarity requires the evolutionarily conserved partitioning-defective (PAR) proteins as well as the actin cytoskeleton. In Caenorhabditis elegans one-cell embryos, the PAR proteins are segregated into an anterior (PAR-3, PAR-6) and a posterior (PAR-1, PAR-2) corticaldomain. The formation of PAR polarity correlates with anterior-posterior differences in the contractile activity of the cortex, known as "contractile polarity". It is thought that regulation of contractile polarity controls the establishment of PAR polarity, but detailed evidence to support this idea is lacking. To investigate how modulation of the actomyosin cytoskeleton affects polarity establishment, the acto-myosin cytoskeleton was perturbed by RNA-mediated interference (RNAi) of two Rho GTPases, CDC-42 and RHO-1. To examine how Rho GTPases are implemented in actin remodeling, it is important to analyze how their activity is controlled and how different activities affect polarity formation. The role of two putative Rho GTPase regulators, the Rho GTPase exchange factor (GEF) ECT-2 and the Rho GTPase activating protein (GAP) K09H11.3 were analyzed with respect to polarity formation. The formation of polarity was analyzed by using GFP-labeled proteins, and several different tracking methods were used to investigate the establishment of contractile and PAR polarity in more detail.This study demonstrates that both RHO-1 and CDC-42 are involved in polarity establishment in C. elegans embryos. But importantly, both act by different mechanisms. RHO-1 organizes the acto-myosin cytoskeleton into a contractile network, and therefore is essential for the formation of contractile polarity. The organization of the acto-myosin cytoskeleton is critical to ensure proper PAR protein distribution. Furthermore, a balance of RHO-1 activity by the GEF ECT-2 and the GAP K09H11.3 appears to be important for cortical contractility, for PAR protein domain size and for mutual exclusion of the PAR proteins. Although CDC-42 was shown to be a universal regulator of the actin cytoskeleton, CDC-42 acts downstream of contractile polarity. CDC-42 is required for linking PAR-6 to the cortex. In the absence of RHO-1 and ECT-2, PAR-6 and CDC-42 are not localized to the anterior cortex. This suggests that RHO-1, by organizing the actomyosin cytoskeleton into a contractile network, regulates the segregation of CDC-42 to the anterior cortex, and concomitantly PAR-6 localization. This study shows that the distribution of PAR is related to cortical activity and supports the model that the actin cytoskeleton plays an important role in polarity establishment.

O mecanismo pelo qual uma célula responde a algum dano no seu material genético é extremamente importante. Isto ocorre pela rápida ativação da maquinaria de reparo de danos no DNA, a qual é composta por uma rede intrincada de sinalização proteica, culminando no reparo do DNA; porém se o dano for irreparável ocorre ativação de mecanismos de morte celular. RhoA,e Rac1 pertencem a família das pequenas proteínas sinalizadoras Rho GTPases, as quais atuam como interruptores moleculares ciclando entre estado ativo (ligada a GTP) e inativo (ligada a GDP). Os componentes desta família estão relacionados ao controle dos mais diversos processos celulares como, por exemplo, remodelamento do citoesqueleto, migração, adesão, endocitose, progressão do ciclo celular e oncogênese. No entanto, apesar das proteínas Rho GTPases estarem envolvidas em um amplo espectro de atividades biológicas, há poucas informações sobre seu papel na manutenção da integridade genômica quando células são submetidas a algum agente genotóxico. Para investigar o envolvimento das GTPases RhoA e Rac1 nas respostas de células submetidas a radiação gama, foram gerados, a partir de células de carcinoma de cervix humano - HeLa, sublinhagens clonais mutantes de RhoA e Rac1 expressando exogenamente RhoA constitutivamente ativa (HeLa-RhoA V14), RhoA dominante negativa (HeLa-RhoA N19), Rac1 constitutivamente ativa (HeLa-Rac1 V12) e Rac1 dominante negativa (HeLa-Rac N17). Após estas linhagens celulares serem expostas a diferentes doses de radiação gama, observamos que ambas GTPases, RhoA e Rac1, são ativadas em resposta aos efeitos da radiação. Além disso, a modulação da atividade destas enzimas, através das mutações, levou a uma alteração das respostas celulares frente aos danos no DNA, como uma redução da capacidade de reparar quebras simples e duplas nas fitas do DNA. Por outro lado, a deficiência de RhoA ou Rac1 GTPase levou a uma redução da ativação de Chk1 e Chk2 ou da fosforilação da histona H2AX, respectivamente, prejudicando os mecanismos de detecção de danos no DNA e levando as células a permanecerem mais tempo nos pontos de checagem G1/S e/ou G2/M do ciclo celular. Esses fatores contribuíram de modo expressivo para a redução da proliferação e sobrevivência celular levando as células à morte. Por fim, ensaios celulares de reparo de danos de um DNA exógeno através de mecanismos de Recombinação Homóloga (HR) e Recombinação Não-Homóloga de extremidades (NHEJ), demonstraram que a inibição da atividade de RhoA reduz significativamente a eficiência de ambas vias de reparo. Desta maneira, este trabalho demonstra e reforça a existência de mais um viés de atuação das pequenas GTPases RhoA e Rac1, agora em células HeLa, nas respostas celulares aos danos induzidos por exposição a radiação gama, modulando a sobrevivência, proliferação e indiretamente modulando resposta ao reparo do DNA através da via de Recombinação Homóloga e Não-Homóloga / The mechanism by which a cell responds to DNA damage is extremely important. This occurs by a quick activation of the DNA damage repair machinery, which consists of an intricate protein signaling network culminating in DNA repair. But if the damages are irreparable occurs there is activation of cell death mechanisms. RhoA and Rac1 belong to family of small Rho GTPases, signaling proteins that act as molecular switches cycling between the active state (GTP-bound) and inactive state (GDP-bound). Members of this family are implicated in the control of diverse cellular process such as cytoskeletal remodeling, migration, adhesion, endocytosis, cell cycle progression, and oncogenesis. However, despite Rho proteins are involved in a broad spectrum of biological activities, there is just a few information about their roles in the maintenance of genomic integrity, that is, when the cells are subjected to some kinf of genotoxic agent. To investigate the involvement of the GTPases RhoA and Rac1 in cellular responses to gamma radiation, we generated from human cervix carcinoma cells - HeLa, clonal sublines of RhoA and Rac1 mutants, exogenous and stably expressing the constitutively active RhoA (HeLa-RhoA V14), the dominant negative RhoA (HeLa-RhoA N19), the constitutively active Rac1 (HeLa-Rac1 V12) and the dominant negative Rac1 (HeLa-Rac1 N17). After all these cell lines have been exposed to different doses of gamma radiation, we found that both GTPases, RhoA and Rac1, are activated in response to the radiation effects. Furthermore, the modulation of two enzymes activity, by using the mutant clones, led to a change in cellular responses to the DNA damage, as the reduction in the capacity of repairing DNA single and double strand breaksr. On the other hand, the deficiency of RhoA or Rac1 GTPase led to a reduction of Chk1 and Chk2 activation, or on the phosphorylation of histone H2AX, respectively, hindering the mechanisms of DNA damage detection and arresting cells in the G1/S and/or G2/M checkpoints of cell cycle. These factors significantly contributed to the reduction of cell proliferation and survival, leading cells to death. Finally, cellular assays of DNA damage repair of exogenous DNA by Homologous Recombination (HR) and Non-Homologous End Joining (NHEJ), demonstrated that RhoA inhibition significantly reduced the repair efficiency of both pathways. Thus, this work demonstrates and reinforces the existence of other biological functions of small GTPases RhoA and Rac1 in HeLa cells, by regulating cellular responses to DNA damage induced by exposure to gamma radiation, modulating the survival, proliferation and indirectly modulating the response to DNA damage repair pathway through the Homologous Recombination and Non-Homologous Recombination

Cell polarity is required for asymmetric division, a mechanism to generate cell diversity by distributing fate determinants unequally to daughter cells. The establishment of polarity requires the evolutionarily conserved partitioning-defective (PAR) proteins as well as the actin cytoskeleton. In Caenorhabditis elegans one-cell embryos, the PAR proteins are segregated into an anterior (PAR-3, PAR-6) and a posterior (PAR-1, PAR-2) corticaldomain. The formation of PAR polarity correlates with anterior-posterior differences in the contractile activity of the cortex, known as "contractile polarity". It is thought that regulation of contractile polarity controls the establishment of PAR polarity, but detailed evidence to support this idea is lacking. To investigate how modulation of the actomyosin cytoskeleton affects polarity establishment, the acto-myosin cytoskeleton was perturbed by RNA-mediated interference (RNAi) of two Rho GTPases, CDC-42 and RHO-1. To examine how Rho GTPases are implemented in actin remodeling, it is important to analyze how their activity is controlled and how different activities affect polarity formation. The role of two putative Rho GTPase regulators, the Rho GTPase exchange factor (GEF) ECT-2 and the Rho GTPase activating protein (GAP) K09H11.3 were analyzed with respect to polarity formation. The formation of polarity was analyzed by using GFP-labeled proteins, and several different tracking methods were used to investigate the establishment of contractile and PAR polarity in more detail.This study demonstrates that both RHO-1 and CDC-42 are involved in polarity establishment in C. elegans embryos. But importantly, both act by different mechanisms. RHO-1 organizes the acto-myosin cytoskeleton into a contractile network, and therefore is essential for the formation of contractile polarity. The organization of the acto-myosin cytoskeleton is critical to ensure proper PAR protein distribution. Furthermore, a balance of RHO-1 activity by the GEF ECT-2 and the GAP K09H11.3 appears to be important for cortical contractility, for PAR protein domain size and for mutual exclusion of the PAR proteins. Although CDC-42 was shown to be a universal regulator of the actin cytoskeleton, CDC-42 acts downstream of contractile polarity. CDC-42 is required for linking PAR-6 to the cortex. In the absence of RHO-1 and ECT-2, PAR-6 and CDC-42 are not localized to the anterior cortex. This suggests that RHO-1, by organizing the actomyosin cytoskeleton into a contractile network, regulates the segregation of CDC-42 to the anterior cortex, and concomitantly PAR-6 localization. This study shows that the distribution of PAR is related to cortical activity and supports the model that the actin cytoskeleton plays an important role in polarity establishment.

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