PP1 interactomes as a means of characterizing protein functions

A maioria das funções celulares, incluindo expressão de genes, crescimento e proliferação celulares, metabolismo, morfologia, motilidade, comunicação intercelular e apoptose, é regulada por interações proteína-proteína (IPP). A célula responde a uma variedade de estímulos, como tal a expressão de proteínas é um processo dinâmico e os complexos formados são constituídos transitoriamente mudando de acordo com o seu ciclo funcional, adicionalmente, muitas proteínas são expressas de uma forma dependente do tipo de célula. Em qualquer instante a célula pode conter cerca de centenas de milhares de IPPs binárias, e encontrar os companheiros de interação de uma proteína é um meio de inferir a sua função. Alterações em redes de IPP podem também fornecer informações acerca de mecanismos de doença. O método de identificação binário mais frequentemente usado é o sistema Dois Hibrido de Levedura, adaptado para rastreio em larga escala. Esta metodologia foi aqui usada para identificar os interactomas específicos de isoforma da Proteína Fosfatase 1 (PP1), em cérebro humano. A PP1 é uma proteína fosfatase de Ser/Thr envolvida numa grande variedade de vias e eventos celulares. É uma proteína conservada codificada por três genes, que originam as isoformas α, β, e γ, com a última a originar γ1 e γ2 por splicing alternativo. As diferentes isoformas da PP1 são reguladas pelos companheiros de interação – proteínas que interagem com a PP1 (PIPs). A natureza modular dos complexos da PP1, bem como a sua associação combinacional, gera um largo reportório de complexos reguladores e papéis em circuitos de sinalização celular. Os interactomas da PP1 específicos de isofoma, em cérebro, foram aqui descritos, com um total de 263 interações identificadas e integradas com os dados recolhidos de várias bases de dados de IPPs. Adicionalmente, duas PIPs foram selecionadas para uma caracterização mais aprofundada da interação: Taperina e Sinfilina-1A. A Taperina é uma proteína ainda pouco descrita, descoberta recentemente como sendo uma PIP. A sua interação com as diferentes isoformas da PP1 e localização celulares foram analisadas. Foi descoberto que a Taperina é clivada e que está presente no citoplasma, membrana e núcleo e que aumenta os níveis de PP1, em células HeLa. Na membrana ela co-localiza com a PP1 e a actina e uma forma mutada da Taperina, no motivo de ligação à PP1, está enriquecida no núcleo, juntamente com a actina. Mais, foi descoberto que a Taperina é expressa em testículo e localiza-se na região acrossómica da cabeça do espermatozoide, uma estrutura onde a PP1 e a actina estão também presentes. A Sinfilina-1A, uma isoforma da Sinfilina-1, é uma proteína com tendência para agregar e tóxica, envolvida na doença de Parkinson. Foi mostrado que a Sinfilina-1A liga às isoformas da PP1, por co-transformação em levedura, e que mutação do seu motivo de ligação à PP1 diminuiu significativamente a interação, num ensaio de overlay. Quando sobre-expressa em células Cos-7, a Sinfilina-1A formou corpos de inclusão onde a PP1 estava presente, no entanto a forma mutada da Sinfilina-1A também foi capaz de agregar, indicando que a formação de inclusões não foi dependente de ligação à PP1. Este trabalho dá uma nova perspetiva dos interactomas da PP1, incluindo a identificação de dezenas de companheiros de ligação específicos de isoforma, e enfatiza a importância das PIPs, não apenas na compreensão das funções celulares da PP1 mas também, como alvos de intervenção terapêutica.

Dissecting the role of Profilin-1 in microglial cell function: the impact on ROS production

Microglial cells are the resident immune cells of central nervous system (CNS) and the major players in neuroinflammation. These cells are also responsible for surveilling the neuronal microenvironment, and upon injury to the CNS they change their morphology and molecular profile and become activated. Activated status is associated with microglia proliferation, migration to injury foci, increased phagocytic capacity, production and release of reactive oxygen species (ROS), cytokines (pro- or anti-inflammatory) and reactive nitrogen species. Microglia activation is crucial for tissue repair in the healthy brain. However, their chronic activation or deregulation might contribute for the pathophysiology of neurodegenerative diseases. A better understanding of the mechanisms underlying microglial cell activation is important for defining targets and develop appropriate therapeutic strategies to control the chronic activation of microglia. It has been observed an increase in profilin (Pfn) mRNA in microglial cells in the rat hippocampus after unilateral ablation of its major extrinsic input, the entorhinal cortex. This observation suggested that Pfn might be involved in microglia activation. Pfn1 is an actin binding protein that controls assembly and disassembly of actin filaments and is important for several cellular processes, including, motility, cell proliferation and survival. Here, we studied the role of Pfn1 in microglial cell function. For that, we used primary cortical microglial cell cultures and microglial cell lines in which we knocked down Pfn1 expression and assessed the activation status of microglia, based on classical activation markers, such as: phagocytosis, glutamate release, reactive oxygen species (ROS), pro- and anti-inflammatory cytokines. We demonstrated that Pfn1 (i) is more active in hypoxia-challenged microglia, (ii) modulates microglia pro- and anti-inflammatory signatures and (iii) plays a critical role in ROS generation in microglia. Altogether, we conclude that Pfn1 is a key protein for microglia homeostasis, playing an essential role in their activation, regardless the polarization into a pro or anti-inflammatory signature.

Chemokines impact in Alzheimer’s disease

Alzheimer’s Disease (AD) is a neurodegenerative disorder neuropathologically characterized by the presence of extracellular senile plaques, intracellular neurofibrillary tangles and synaptic loss. Neuroinflammation has been associated with some neurodegenerative diseases, such as AD. In AD, increased Aβ production and aggregation, have a fundamental role in the activation of the inflammatory process. In turn, this could be fundamental in the early stages of this pathology, regarding the Aβ clearance and brain protection. However, chronic inflammation leads to an increase of the inflammatory mediators, such as cytokines, released by activated microglia, astrocytes, and neurons. The excessive production of these inflammatory components promotes alterations in both amyloid precursor protein (APP) expression and processing, stimulating the increase of Aβ accumulation and abnormal tau phosphorylation. This results in neurotoxic effects, irreversible damage and neuronal loss. Chronic inflammation is a feature of AD however, little is known about the effects of some chemokines on its pathogenesis. Thus, the main aim of this thesis was to study the impact of the interleukin-8 (IL-8) and monocyte chemoattractant protein-1 (MCP-1) on apoptosis, APP and tau. The both studied chemokines resulted in small alterations regarding the cytotoxicity on SH-SY5Y differentiated cells, being a significant increase in apoptosis observed only for the MCP-1 at the highest concentration. For the APP processing no significant differences were obtained, although a tendency to increase at different concentrations and periods was registered for both IL-8 and MCP-1. With respect to tau and other cytoskeleton-associated proteins, it was possible to observe a tendency to increase in the phosphorylated residue (Ser396) at the higher concentrations, as well as alterations on actin and tubulin with an increase on acetylated-α tubulin. This effect can be translated by neuronal architectural and survival alterations. Therefore additional studies could contribute to a better understanding of the way that these chemokines act on AD pathogenesis.

CD81 interacting proteins

Fertilization is a multistep and complex process culminating in the merge of gamete membranes, cytoplasmic unity and fusion of genome. CD81 is a tetraspanin protein that participates in sperm-oocyte interaction, being present at the oocyte surface. CD81 has also been implicated in other biological processes, however its specific function and molecular mechanisms of action remain to be elucidated. The interaction between CD81 and its binding partner proteins may underlie the CD81 involvement in a variety of cellular processes and modulate CD81/interactors specific functions. Interestingly, in a Yeast two Hybrid system previously performed in our lab, CD81 has emerged as a putative interactor of the Amyloid Precursor Protein (APP). In the work here described, bioinformatics analyses of CD81 interacting proteins were performed and the retrieved information used to construct a protein-protein interaction network, as well as to perform Gene Ontology enrichment analyses. CD81 expression was further evaluated in CHO, GC-1 and SH-SY5Y cell lines, and in human sperm cells. Additionally, its subcellular localization was analyzed in sperm cells and in the neuronal-like SH-SY5Y cell line. Subsequently, coimmunoprecipitation assays were performed in CHO and SH-SY5Y cells to attempt to prove the physical interaction between CD81 and APP. A functional interaction between these two proteins was accessed thought the analyses of the effects of CD81 overexpression on APP levels. A co-localization analysis of CD81 and some interactors proteins retrieved from the bioinformatics analyses, such as APP, AKT1 and cytoskeleton-related proteins, was also performed in sperm cells and in SH-SY5Y cells. The effects of CD81 in cytoskeleton remodeling was evaluated in SH-SY5Y cells through monitoring the effects of CD81 overexpression in actin and tubulin levels, and analyzing the colocalization between overexpressed CD81 and F-actin. Our results showed that CD81 is expressed in all cell lines tested, and also provided the first evidence of the presence of CD81 in human sperm cells. CD81 immunoreactivity was predominantly detected in the sperm head, including the acrosome membrane, and in the midpiece, where it co-localized with APP, as well as in the post-acrosomal region. Furthermore, CD81 co-localizes with APP in the plasma membrane and in cellular projections in SH-SY5Y cells, where CD81 overexpression has an influence on APP levels, also visible in CHO cells. The analysis of CD81 interacting proteins such as AKT1 and cytoskeletonrelated proteins showed that CD81 is involved in a variety of pathways that may underlie cytoskeleton remodeling events, related to processes such as sperm motility, cell migration and neuritogenesis. These results deepen our understanding on the functions of CD81 and some of its interactors in sperm and neuronal cells.

APP RERMS enriched matrix as an adhesion substrate for neuritic outgrowth

Specific domains can determine protein structural functional relationships. For the Alzheimer’s Amyloid Precursor Protein (APP) several domains have been described, both in its intracellular and extracellular fragments. Many functions have been attributed to APP including an important role in cell adhesion and cell to cell recognition. This places APP at key biological responses, including synaptic transmission. To fulfil these functions, extracellular domains take on added significance. The APP extracellular domain RERMS is in fact a likely candidate to be involved in the aforementioned physiological processes. A multidisciplinary approach was employed to address the role of RERMS. The peptide RERMS was crosslinked to PEG (Polyethylene glycol) and the reaction validated by FTIR (Fourier transform infrared spectrometry). FTIR proved to be the most efficient at validating this reaction because it requires only a drop of sample, and it gives information about the reactions occurred in a mixture. The data obtained consist in an infrared spectra of the sample, where peaks positions give information about the structure of the molecules, and the intensity of peaks is related to the concentration of the molecules. Subsequently substrates of PEG impregnated with RERMS were prepared and SH-SY5Y (human neuroblastoma cell line) cells were plated and differentiated on the latter. Several morphological alterations were clearly evident. The RERMS peptide provoked cells to take on a flatter appearance and the cytoskeletal architecture changed, with the appearance of stress fibres, a clear indicator of actin reorganization. Given that focal adhesions play a key role in determining cellular structure the latter were directly investigated. Focal adhesion kinase (FAK) is one of the most highly expressed proteins in the CNS (central nervous system) during development. It has been described to be crucial for radial migration of neurons. FAK can be localized in growth cones and mediated the response to attractive and repulsive cues during migration. One of the mechanisms by which FAK becomes active is by auto phosphorylation at tyrosine 397. It became clearly evident that in the presence of the RERMS peptide pFAK staining at focal adhesions intensified and more focal adhesions became apparent. Furthermore speckled structures in the nucleus, putatively corresponding to increased expression activity, also increased with RERMS. Taken together these results indicate that the RERMS domain in APP plays a critical role in determining cellular physiological responses. Here is suggested a model by which RERMS domain is recognized by integrins and mediate intracellular responses involving FAK, talin, actin filaments and vinculin. This mechanism probably is responsible for mediating cell adhesion and neurite outgrowth on neurons.