ABSTRACT
The phrase "X is a gene for Y" and the preformationist concept of gene action that underlies it are inappropriate for psychiatric disorders such as depression, aggression, sexual orientation, obesity, infidelity, alcoholism, or schizophrenia. Drug addictions are complex, chronic, and mental diseases. Genetic studies of twins and families have suggested that genetic factors might account for 40 to 60% of the overall factors in the risk to the development of drug addictions. In addition, numerous studies aiming to discover genetic variants or candidate genes, including genome-wide linkage scans, candidate gene association studies, gene expression, and genome-wide association studies, have also suggested that multiple genes and genomic regions or markers might play important roles in the development of addictions. A primary behavioral pathology in drug addiction is the overpowering motivational strength and decreased ability to control the desire to obtain drugs. Among the most insidious characteristics of drug addiction is the recurring desire to take drugs even after many years of abstinence. Equally sinister is the compromised ability of addicts to suppress drug seeking in response to that desire even when confronted with seriously adverse consequences. The enduring vulnerability to relapse is a primary feature of the addiction disorder and has been identified as a point were pharmacotherapeutic intervention may be most effectively employed. In order to fashion rationale pharmacotherapy it is necessary to understand the neurobiological underpinnings of craving, relapse, choice, and control, and the last decade has seen significant advances, toward achieving this goal. The fact that the vulnerability to relapse in addicts can persist after years of abstinence implies that addiction is caused by long-lasting changes in brain function as a result of repeated drug use, genetic disposition, and environmental associations made with drugs use. Therefore, understanding neurobiological aspects of drug addiction requires the comprehension of the physiological mechanisms that convey to the enduring neuroplasticity. The goal of this review is to explore how the advances in ge-nomics and proteomics may unleash the understanding of the cellular underpinnings of drug addiction and how the recent advances in functional genomics and proteomics may be expected to improve dramatically the treatment of addictive disorders. Applying genomics and proteomics to drug addiction studies will lead to the identification of genes and their protein products that control the brain reward pathways of the brain and their adaptations to drugs of abuse, as well as variations in these genes and proteins that confer genetic risk for addiction and related disorders. Additionally, this review describes recent findings of addictive drugs-inducing altered changes in gene regulation which produce significant cellular modifications on neuronal function in both human and animal brains as detected in animal models of drug abuse. A major goal of drug abuse research is to identify and understand drug-induced changes in brain function that are common to most if not all drugs of abuse, as well as these may underlie drug dependence and addiction. This work describes recent studies whose purpose is to examine the drugs of abuse effect changes in gene and protein expression that converge in common molecular pathways. One of this recent reports using microarrays analysis to assay brain gene expression in the anterior prefrontal cortex (aPFC) of post mortem brains of 42 cocaine, cannabis and/or phencyclidine human cases compared to 30 individual cases, which were characterized by toxicology and drug abuse history. Another study depicted herewith is focused on how the use of drugs frequently begins and escalates during adolescence, with long-term adverse consequences. The study designed a rodent model of adolescence to mirror cocaine use patterns in teenagers. Microarrays analysis was employed to assay brain gene expression in post mortem PFC of rodents treated with cocaine during adolescence. Results from the study revealed that treatment caused acute alterations in the expression of genes encoding cell adhesion molecules and transcription factors within the PFC. Cocaine alters gene expression patterns and histone modification in the PFC. Furthermore observed decreases in histone metylation, which may indicate a role of chromatin remodeling in the observed changes in gene expression patterns. Chromatin remodeling is an important regulatory mechanism for cocaine-induced neural and behavioral plasticity in the striatum. Most of the gene expression changes induced by cocaine were transient. However, if early cocaine exposure triggered changes in cell structure/adhesion, the impact of those alterations could be long-lasting. It is important to consider that the PFC in humans is involved in a large range of different functions, including working memory, action planning, response inhibition, decision-making, reward processes, and social behavior. Any lasting impact cocaine has on these functions could be detrimental, particularly in adolescents. Findings suggest that exposure to cocaine during adolescence has far-reaching molecular and behavioral consequences in the rat PFC that develop over time and endure long after drug administration has ceased. These neuroadaptations could have serious implications, particularly in the developing brain. However, only a causal relationship between these cocaine-induced molecular and behavioral adaptations can be inferred at this time. Therefore, humans who abused cocaine, cannabis and/or phen-cyclidine share a decrease in transcription of calmoduline-related genes and increased transcription related to lipid/colesterol and Gol-gi/ER function. 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 enhance molecular 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 specific brain structures shown to be vulnerable to the addictive properties of drugs of abuse. These changes represent common molecular features of drug abuse, which may underlie changes in synaptic function and plasticity that could have important ramification for decision-making capabilities in drug addiction. Eventually, all of these discoveries can be exploited for clinical applications as diverse as improved treatments diagnostic tests, and ultimately disease prevention and cure.
Una frase empleada en el argot científico en los primeros años de la era de la genética dictaba que "X es un gen para Y", en donde X representaba a un gen particular del genoma humano y Y correspondía a uno de los complejos trastornos de la conducta humana como la depresión, la agresión, la orientación sexual, la obesidad, la infidelidad, la esquizofrenia y la adicción. Sin embargo, ahora se sabe que la contribución genética a los trastornos psiquiátricos se debe a la acción conjunta de grupos de genes que de manera individual causarían sólo un pequeño impacto incapaz de desencadenar alteraciones conduc-tuales. La contribución de los grupos de genes aunada a un sinnúmero de factores ambientales y sociales es la causa de la amplia variedad de perturbaciones conductuales en el humano. De esta manera, la frase "X es un gen para Y", es inapropiado para los cuadros psiquiátricos. La conducta patológica más importante en la adicción es la búsqueda compulsiva de la droga y la pérdida del control en el deseo de obtenerla. Otra de las graves consecuencias de la adicción es el riesgo de recaídas de los individuos a pesar de tener varios años de abstinencia. Esta última característica ha sido el punto de elección para implementar medidas terapéuticas más eficientes. Para lograr que las terapias sean exitosas es necesario entender los mecanismos neurobiológicos que intervienen en los procesos de adquisición y consolidación del síndrome adictivo. Uno de los puntos que ha llamado la atención es el hecho de que el riesgo de las recaídas puede persistir durante varios años y ha permitido implicar la generación de cambios en la fisiología del cerebro que se mantienen por largos periodos. Así, es de suma relevancia comprender las bases neuro-biológicas de los procesos adictivos que ocasionan cambios en la plasticidad neural. La finalidad de esta revisión es analizar algunos ejemplos representativos de los recientes avances en el campo de las ciencias genó-micas que permiten ampliar el conocimiento de las implicaciones a nivel celular de los procesos adictivos y la importancia que tendrán dichos avances para mejorar la práctica psiquiátrica en general y, de manera específica, el tratamiento de las conductas adictivas. Se describen algunos de los trabajos recientes en los que se ha estudiado la modificación de la expresión génica como consecuencia de la administración de drogas de abuso en diferentes paradigmas de estudio, incluyendo estudios en los que se evalúa la similitud de los efectos ocasionado por tres drogas de abuso diferentes: cocaína, marihuana y fenilciclina. Finalmente se describen las implicaciones moleculares de las modificaciones en la expresión génica de proteínas que participan en diferentes procesos celulares, como el metabolismo del colesterol y los lípidos, las funciones del aparato de Golgi y el retículo endoplásmico, el tráfico intracelular en el citoesqueleto. Todos estos cambios representan modificaciones importantes en la función sináptica y la plasticidad neuronal. Esta información permitirá el desarrollo de aplicaciones clínicas que permitan implementar tratamientos efectivos, métodos de diagnóstico y en última instancia podrá ser de utilidad para prevenir, evitar o curar las adicciones.
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.
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.