Las adicciones, la genómica y la proteómica / Addictions, genomics and proteomics

ABSTRACT

Drug addiction is a chronically relapsing disorder that has been characterized by (1) compulsion to seek and take the drug, (2) loss of control in limiting intake, and (3) emergence of a negative emotional state (e.g, dysphoria, anxiety, irritability) reflecting a motivational withdrawal syndrome when access to the drug is prevented (defined as Substance Dependence by the Diagnostic and Statistical Manual of Mental Disorders [DSM] of the American Psychiatric Association). Acute exposure to drugs of abuse initiates molecular and cellular alterations in the Central Nervous System that lead to an increased overall vulnerability to addiction with subsequent drug exposures. These drug-induced alterations employ changes in gene transcription that result in the synthesis of new proteins. Therefore, one of the important goals of addiction research is to identify the drug-induced gene expression changes in the specific brain structures related to the addictive properties of various drugs. The molecular and genomic mechanisms by which drugs of abuse induce neuroplastic changes related to addiction remain largely unknown. Several studies have evaluated changes in gene and protein expression profiles in the brain after administration of drugs of abuse. Exposure to psychostimulants induces the activity-dependent gene expression of several transcription activators and repressors. Genomic research strategies have recently transitioned from the search for unknown genes to the identification and evaluation of coordinated gene networks and transcriptional signatures. New opportunities arising from the analysis of these networks include identifying novel relationships between genes and signaling pathways, connecting biological processes with the regulation of gene transcription, and associating genes and gene expression with diseases. The identification of gene networks requires large gene expression data sets with multiple data points. Functional genomics methods, studying the steady-state levels of these mRNA species, such as quantitative RT-PCR (qRT-PCR), whole-genome microarray analysis, and next generation sequencing methods, provide sensitive and high-throughput approaches to quantitatively examining mRNA (and miRNA) species present within the cells of the Nervous System. Functional genomics studies can help to illuminate genes involved in the development of behaviors related to drug abuse and relapse liability, but cannot provide insight into post-translational modifications (e.g., phosphorylation and glycosylation of proteins after translation has occurred) or subcellular localization of the protein product. Therefore, using proteomic techniques presents the opportunity to assess the totality of gene expression, translation, modification, and localization. Unfortunately, the sensitivity of proteomic tools lags behind those of functional genomics. Moreover, examining the mRNA provides a restricted view of primarily the cell body. Indeed, from a systems biology standpoint, analysis of both mRNA and protein levels (as well as miRNA and epigenetic changes) will ultimately provide a more integrated view of the molecular underpinnings of addiction. When applying proteomic technologies to addiction research, an understanding of the power of proteomic analysis is essential. After genetic information is transcribed into mRNA, a template is provided to the cell from which proteins will be synthesized. Neuroproteomic studies offer great promise for increasing understanding of the biochemical basis of addiction. While proteomics is still an evolving field, proteomic approaches have proven useful for elucidating the molecular effects of several drugs of abuse. With a number of ongoing research programs in addiction proteomics and a growing number of investigators taking advantage of these tools, the addiction research field will benefit from a consideration of the capabilities and limitations of proteomic studies. As with other biomedical research fields, drug abuse research is making use of new proteomic capabilities to examine changes in protein expression and modification on a large scale. To obtain the maximum benefit and scientific advancement from these new technologies, a clear understanding of the power and limitations of neuroproteomics is necessary. With the main limitation of neuroproteomic studies being the complexity of the proteome, approaches that focus these studies need to be employed. The salient message is that there is not a single best technical approach for all studies and that the main driver for the choice of proteomic technology and experimental design should be the advancement of the understanding and treatment of drug abuse. An important area that has heretofore received limited attention is the experimental design and interpretation specific to neuro-proteomic studies of drug abuse. These challenges include choice of animal model, ensuring sample quality, the complexity of brain tissue, confirming discovery findings, data analysis strategies, and integration of large data sets with the existing literature. Epigenetics is the study of heritable changes other than those in the DNA sequence and encompasses two major modifications of DNA or chromatin DNA methylation and post-translational modification of histones. In this context, now it is known that regulation of gene expression contribute to the long-term adaptations underlying the effects of drugs of abuse. The precise molecular events that are required for modification of chromatin and that underlie gene repression or activation have not been elucidated. Recent reports have addressed this question and demonstrated that drugs of abuse modify specific methyl-CpG-binding proteins that control histone acetylation and gene expression. Further elucidation of the wide-range of histone modifications and the ensuing consequences on gene expression will be necessarily before the potential for drug development can be realized. It is important to characterize the molecular alterations underlying chromatin remodeling and the regulation of the epigenetics events by drugs of abuse. It is clear that modification in gene expression by drugs of abuse promote cellular changes. This review is intended to provide guidance on recent advances in the field of drug addiction. This review also presents a number of experimental design and sample approaches that have been applied to genomic, proteomic and epigenetic studies of addiction. Coupled with new technologies for data collection, analysis, and reporting, these approaches represent the future of the addiction field and hold the key to unlocking the complex of profile of drug abuse disorders.

RESUMEN

La adicción a las drogas es una enfermedad mental que se caracteriza por ocasionar graves implicaciones sociales, económicas y de salud de los individuos que la padecen. La exposición aguda a las drogas de abuso provoca alteraciones moleculares y celulares en el Sistema Nervioso Central que ocasionan una vulnerabilidad para sufrir adicción a subsecuentes exposiciones a sustancias de abuso diferentes. Las alteraciones inducidas por las drogas producen cambios en la transcripción de genes que resultan en la síntesis de nuevas proteínas. Uno de los objetivos importantes en la investigación en el campo de las adicciones es identificar los cambios en la expresión de genes inducidos por las drogas en estructuras específicas del cerebro que están relacionadas con las propiedades adictivas de diferentes sustancias. El campo de la genómica y la proteómica, aplicada al estudio de las adicciones, tiene como objetivo identificar a los genes y las proteínas candidatos involucrados en la regulación de los procesos adictivos. Se han logrado progresos considerables en la identificación de genes y proteínas que regulan las conductas complejas presentes en los procesos adictivos en modelos de animales y modelos de estudio en humanos con material obtenido post-mortem. Estos descubrimientos se han sumado a los esfuerzos por identificar los circuitos neurales implicados en las manifestaciones conductuales relacionadas con las adicciones. También han permitido la identificación de genes candidatos que podrán ser blancos de futuras estrategias terapéuticas desarrolladas para tratar los procesos adictivos. Los estudios de genómica funcional han permitido identificar algunos de los genes involucrados en el desarrollo de las conductas adictivas, pero no tienen la capacidad de proporcionar información sobre las modificaciones post-traduccionales ni de la localización sub-celular de las proteínas para las que codifican los genes. Por lo tanto, la incorporación de estudios proteómicos ofrece la oportunidad de lograr evaluar, en su totalidad, la expresión, la traducción, las modificaciones y la localización de los genes y sus productos de expresión. Para obtener los máximos beneficios y avances con el empleo de estas nuevas tecnologías, deben comprenderse en su totalidad los alcances y limitaciones de la neuroproteómica. En este sentido, se debe tener especial cuidado en la elección del modelo de estudio, asegurar la calidad de la muestra, la complejidad de la estructura en estudio, confirmar los resultados obtenidos, las estrategias de análisis de resultados y la integración de los datos obtenidos con los ya reportados en la literatura científica. Los estudios recientes sobre los mecanismos moleculares que controlan los cambios inducidos por las drogas de abuso sobre la función transcipcional, la conducta y la plasticidad sináptica han identificado el importante papel que desempeña la remodelación de cromatina en la regulación y estabilidad de los programas genéticos neuronales mediados por las drogas y la subsecuente manifestación de las conductas adictivas. Se han identificado alteraciones epigenéticas sobre el genoma, tales como metilación del DNA y modificaciones en la función de las proteínas histonas. Estos importantes mecanismos se ven afectados como una respuesta neurobiológica a la administración de sustancias de abuso. Esta revisión pretende mostrar algunos de los avances recientes en el campo de las adicciones, presentando una breve descripción de los hallazgos que emplean aproximaciones genómicas, proteómicas y epigenéticas. Las implicaciones de estos estudios moleculares ponen de manifiesto nuevos conocimientos sobre el probable desarrollo de intervenciones terapéuticas en el futuro.