Actin cytoskeletal regulation is of critical importance for a number of diverse cellular functions, including cell motility, endocytosis, cell division and transcription. Tight regulation of actin is critical for many aspects of cancer biology and in particular invasion and metastasis. ADF/cofilins are among the most important actin regulatory proteins. Mammals have three highly conserved members, ADF, CFL1 and CFL2, which regulate actin dynamics by severing and depolymerizing actin filaments. Despite a huge literature on the roles of ADF/cofilins in actin treadmilling and cell migration in vitro and in cancer cell behavior during invasion, very little is known about their collective roles in tissue homeostasis. By employing genetic knock-outs of ADF, in conjunction with conditional depletion of CFL1 using a Cre-LoxP system under the control of the keratin 14 promoter, we were able to study the effects of ADF/CFL1 loss in vivo in the mouse epidermis. Furthermore, by generating ADF-null squamous cell carcinoma (SCC) cell lines and by transiently downregulating CFL1 with RNAi, we were able to investigate further the cellular responses after ADF/CFL1 depletion in vitro. Co-depletion of ADF and CFL1 from the mouse epidermis triggered loss of tissue homeostasis characterized by abnormal thickening of the tissue, actin filament accumulation and nuclear deformation. Loss of ADF/CFL1 in cultured malignant keratinocytes also led to aberrant cell morphology accompanied by unrestrained accumulation of actin stress fibers tethered to enlarged focal adhesions. Enhanced SRF/MAL-mediated transcription fuels this uncontrolled actin polymerization which is also mediated by Arp3. Furthermore, these actin filaments are decorated with phospho-myosin light chain, which indicates their contractile nature. As a consequence, the increased intracellular acto-myosin tension results in nuclear deformation, which is promoted by the deregulated actin filaments tethered to the nuclear envelope via the linker of nucleoskeleton and cytoskeleton (LINC) complex. Overall, we describe new conceptual insight into the cellular functions of ADF/cofilins. We show that their activities are essential for the dynamic regulation of contractile actin filaments that, if left unchecked, lead to loss of cellular homeostasis and cell death promoted by loss of nuclear integrity. Additionally, the critical roles of nuclear actin and actin-associated proteins have recently started being appreciated. Thus, for the first time we set out to investigate new functions of cofilins in the nucleus using proteomics, and identify new cofilin binding partners that implicate them in novel cellular pathways, expanding our knowledge on these small actin-binding proteins.

Durant mon travail de thèse, j'ai étudié le rôle central de l'ADF/cofiline, une protéine qui se lie au cytosquelette d'actine, décore spécifiquement les parties ‘âgées' des filaments d'actine, diminue localement par un facteur 5 la rigidité du filament et provoque la fragmentation du filament à l'interface entre les sections nues et décorées. Dans ma première étude (Suarez et al., Current Biology, 2011), j'ai utilisé la microscopie à onde évanescente et une ADF/cofiline fluorescente pour démontrer que l'ADF/cofiline est un marqueur de l'état nucléotidique (ATP, ADP-Pi ou ADP) des sous-unités d'un filament d'actine en cours de polymérisation. De plus, l'ADF/cofiline, en accélérant la dissociation du phosphate inorganique (Pi), limite la taille du cap ATP/ADP-Pi du filament d'actine, sans toutefois le réduire à une taille zéro. Des analyses statistiques sur filaments isolés établissent une corrélation parfaite entre la densité de fixation de l'ADF/cofiline et son efficacité de fragmentation. Paradoxalement, l'efficacité de fragmentation est maximale pour une densité d'ADF/cofiline de 0.5. Ceci est confirmé par des analyses supplémentaires qui montrent que les sites de fragmentation du filament coïncident avec la position des frontières entre zones décorées et zones nues. Les conséquences de ce dernier résultat paradoxal sont l'objet de ma seconde étude (McCullough et al., 2011, Biophysical Journal). En combinant différentes sources d'ADF/cofilines (vertébré et levure) et d'actines (vertébré et levure), nous montrons, sur les quatre couples actine-ADF/cofiline possibles, qu'il existe une très forte corrélation entre (1) l'efficacité de fragmentation (qui dépend de la combinaison entre actine et ADF/cofiline) et (2) la déformation du filament, mesurée à la frontière entre zone décorée et zone nue. Au cours de ma troisième étude (Reymann et al., Molecular Biology of the Cell, 2011), nous montrons que le mécanisme de fragmentation ADF/cofiline-dépendant, établi à l'échelle d'un filament isolé, peut s'appliquer aussi à l'échelle d'une comète d'actine qui comporte un réseau complexe de filaments. Mon travail de thèse a montré que le mode d'action de l'ADF/cofiline se situe à l'intersection entre mécanismes microscopiques et macroscopiques, d'une part, et entre chimie et physique, d'autre part. Les caractéristiques microscopiques des interactions de cette protéine avec un filament d'actine isolé sont fondamentales pour expliquer des évènements macroscopiques, comme la fragmentation de filaments ou de structures complexes. D'autre part, nous avons montré comment les propriétés chimiques de l'ADF/cofiline modifient les propriétés physiques locales du filament et conduisent à la fragmentation. L'ADF/cofiline a un rôle central pour l'intégration de mécanismes physico-chimiques, à l'échelle microscopique, afin d'assurer un comportement cohérent à l'échelle de la cellule. / During my thesis, I have studied the pivotal role of ADF/cofilin, a protein that binds to the actin cytoskeleton, specifically decorates ‘old' actin filament parts, decreases by a factor of 5 the local filament rigidity and triggers filament fragmentation at boundaries between decorated and non-decorated filament sections. In my first study (Suarez et al., Current Biology, 2011), I have used evanescent wave microscopy and labeled ADF/cofilin to demonstrate that ADF/cofilin is a marker of the nucleotide state (i.e. ATP, ADP-Pi or ADP) associated with the actin sub-units in actively polymerizing filaments. In addition, because ADF/cofilin accelerates inorganic phosphate (Pi) release, the size of the ATP/ADP-Pi cap is diminished, although it cannot be reduced to zero. Fragmentation events frequency, determined from a thorough analysis of a population of single filaments decorated with labeled ADF/cofilin, is perfectly correlated with the binding density of ADF/cofilin on filaments. However, the maximal severing efficiency is obtained for half ADF/cofilin density. This paradoxical result is confirmed by analysis showing that severing sites are mainly associated with boundaries between decorated and bare actin filament sections. In consequence, in a second paper (McCullough et al., Biophysical Journal, 2011), I have took part in the study of actin filament deformation in relation with severing efficiency. Using different ADF/cofilin (vertebrate and yeast) and actin (vertebrate and yeast), we have shown that filament deformation at the boundary between bare and ADF/cofilin-decorated filament sections (which depends on the ADF/cofilin/actin combination) and severing are highly correlated. During my third study, (Reymann et al., Molecular Biology of the Cell, 2011), we established that stochastic dynamics, discovered at the molecular level for single filaments (or bundles of them), is also relevant to describe the macroscopic fragmentation of a comet tail consisting of hundreds of thousands filaments. I have shown that ADF/cofilin activity is at the crossroad between macroscopic and microscopic systems, on one hand, and physics and chemistry, on the other hand. The characteristics of microscopic interactions of ADF/cofilin with a single filament are fundamental to understand the macroscopic dynamics of a fragmenting comet. In addition, we have established how the binding of ADF/cofilin (chemistry) controls the mechanical properties of the filament (physics) before fragmentation. ADF/cofilin is essential in the integration of physical and chemical mechanisms at the microscopic level, to ensure consistent behavior at the cell scale.

Neospora caninum é um parasita intracelular obrigatório pertencente ao filo Apicomplexa, conhecido por ser uma das principais causas de aborto parasitário em bovinos e por apresentar transmissão transplacentária. Para locomoverem-se e acessarem o conteúdo intracelular de células hospedeiras, organismos apicomplexas fazem uso de um mecanismo não convencional que se utiliza de uma maquinaria celular cujo papel central é exercido pelo motor actina-miosina, auxiliado por proteínas intermediárias e de acoragem, que realiza a propulsão do parasita na direção do movimento. Para o funcionamento dessa maquinaria, é essencial que actina esteja em sua forma filamentosa (actina-F). Porém, actinas de apicomplexas são conhecidas por serem funcional e estruturalmente não convencionais, formando filamentos pequenos e instáveis in vitro, assim como pelo predomínio de grande maioria de actina monomérica (actin-G) nas células in vivo. Desse modo, para formar e manter actina-F a dinâmica de actina desses organismos requer uma regulação precisa, que, em apicomplexas, é conduzida por um arsenal conhecidamente pequeno de proteínas que se ligam a actina (ABPs). Nosso objetivo neste estudo foi identificar e caracterizar ABPs de N. caninum. Para isso, duas ABPs de N. caninum foram estudadas: fator de despolimerização de actina (NcADF) e proteína associada a ciclase (NcCAP); também, foi gerado e caracterizado soro contra região de actina de N. caninum entre aminoácidos 201 e 310 (anti-NcAct201-310). NcADF (correspondente ao acesso NCLIV_012510 em ToxoDB) foi submetida a caracterização molecular e bioquímica. A sua estrutura terciária foi gerada por modelagem molecular baseada em homologia, apresentando folding conservado, porém com F-loop de menor tamanho, quando comparada a ADF/cofilinas canônicas. A forma recombinante de NcADF foi expressa E. coli BL21 por plasmídeos pET32a(+) e pET28a(+) e solubilizada em tampão desnaturante e nativo, respectivamente. NcADF_pET32 foi purificada e utilizada para geração de soro anti-NcADF, que detectou ambas NcADF recombinantes, assim como proteínas endógenas em western blot 1-D e 2-D com peso molecular e pI próximos aos preditos. O soro anti-NcADF também localizou NcADF difusa no citoplasma, com menos intensidade nos polos de taquizoítas de N. caninum extracelulares. NcADF_pET28 foi purificado na forma nativa e utilizado para caracterização funcional para avaliação de seu papel na dinâmica de actina liofilizada de coelho. Ensaios de cossedimentação, cinética de polimerização e despolimerização, viscosimentria de baixo cisalhamento (queda de bola), estado estacionário e ligação entre actina-G e NcADF, em conjunto, mostraram que NcADF causa despolimerização de actina-F, realiza sequestro de monômeros de actina e quebra de filamentos. NcCAP foi submetida a caracterização molecular e foi identificada como produto de expressão do gene de acesso NCLIV_054140. NcCAP recombinante foi expressa em pET32a(+) e pET28a(+) predominantemente em corpos de inclusão e foi solubilizada em tampão desnaturante. A forma purificada de pET32_NcCAP, identificada por espectrometria de massas, foi utilizada para imunização e o soro resultante detectou NcCAP recombinante e endógena por western blot 1-D e 2-D, apresentando bandas e spots de peso molecular e pI próximos ao esperado. O soro anti-NcCAP também localizou NcCAP em taquizoítas ii extracelulares de N. caninum difusa no citoplasma e/ou com predomínio na região periplasmática da célula. Por fim, o soro anti-NcAct201-310 foi gerado, sendo capaz de detectar proteínas em sua forma nativa e realizar marcação na região periférica e, possivelmente, nuclear de taquizoítas de N. caninum extracelulares. A caracterização de ABPs de N. caninum feita neste trabalho amplia o conhecimento sobre a conservação dessas proteínas ao longo do filo Apicomplexa. Ademais, representa uma contribuição para o entendimento da dinâmica de actina e, por consequência, futuramente, pode colaborar para a elucidação de mecanismos-chave para a sobrevivência e disseminação dos parasitas pelo seu hospedeiro / Neospora caninum is an obligate intracellular parasite that belongs to the phylum Apicomplexa. It is known as one of the main causes of infectious abortion in cows and for its efficient transplacentary transmission. Apicomplexan organisms use a phylum-specific mechanism of invasion and gliding motility, which use an unusual cellular machinery based on an actin myosin motor assisted by intermediary and anchoring proteins that creates the traction force to impulse the parasite forward. Filamentous actin (F-actin) is essential to the appropriate functioning of this machinery, although apicomplexan unconventional actin forms small and unstable filaments in vitro and is found preponderantly as monomer (G-actin) in cells. Thus, the parasites need actin-binding proteins (ABPs) to strictly regulate actin dynamics and to form and maintain F-actin when it is necessary to the cell. Here, we aimed at identifying and characterising ABPs from N. caninum. Two ABPs were characterised: actin-depolymerising factor (NcADF) and cyclase-associated protein (NcCAP) from N. caninum. In addition, a serum against the actin region between amino acids 201 and 310 (anti-NcAct201-310) was raised. NcADF, which corresponds to identification NCLIV_012510 on ToxoDB, was molecular and biochemically characterised. Firstly, the tertiary structure of NcADF was generated by molecular modelling based on homology. Comparing to canonical ADF/cofilins, NcADF presented a conserved folding, albeit its smaller F-loop. The recombinant form of NcADF was expressed in E. coli BL21 using pET32a(+) and pET28a(+) plasmids and solubilized in denaturing and native buffers, respectively. Polyclonal antibodies were raised in mice against purified NcADF_pET32, which was able to detect both forms of recombinant NcADF as well as proteins in 1-D and 2-D western blot with expected molecular weight and isoelectric point (pI). Additionally, NcADF was localised in extracellular N. caninum tachyzoites as a diffuse pattern on cytoplasm with less intensity in both poles. NcADF_pET28 was successfully purified in native form and used for functional characterisation to evaluate the role of recombinant NcADF on lyophilised rabbit actin dynamics. Together, co-sedimentation, polymerisation and depolymerisation kinetic, low shearing viscometry (falling ball), steady state, and G-actin and NcADF binding assays showed that NcADF was able to depolymerise actin-F, sequester actin monomers, and sever filaments. Moreover, NcCAP (identification NCLIV_054140) was also characterised. Recombinant NcCAP was expressed in pET32a(+) and pET28a(+) plasmids predominantly in inclusion bodies and was solubilised in denaturing buffer. NcCAP_pET32 was purified and identified by mass spectrometry. Then, the polyclonal antibodies against this recombinant protein was generated in mice. It was able to detect recombinant and endogenous NcCAP, presenting bands and spots in 1-D and 2-D western blot with molecular weight and pI quite near to the predicted ones. NcCAP was localised as a diffuse pattern on cytoplasm and/or predominantly on periplasmic regions of extracellular taclyzoites of N. caninum. Finally, the serum containing anti-NcAct201-310 polyclonal antibodies was raised in mice. It detected endogenous proteins mainly in native form and localised them on periplasmic and possibly nuclear region in extracellular N. caninum tachyzoites. The characterisation of N. caninum ABPs iv extends our understanding of these proteins conservation and their function throughout the Apicomplexa phylum. Furthrmore, it represents a contribution to the field towards the comprehention of actin dynamics and in the future might provide information for important mechanisms of dissemination and survival of the parasite at its host