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An Emerging Role for the Mammalian Target of Rapamycin in “Pathological” Protein Translation: Relevance to Cocaine Addiction

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An Emerging Role for the Mammalian Target of Rapamycin in “Pathological” Protein Translation: Relevance to Cocaine Addiction View metadata, citation and similar papers at core.ac.uk brought to you by CORE REVIEW ARTICLE published: 06 February 2012provided by PubMed Central doi: 10.3389/fphar.2012.00013 An emerging role for the mammalian target of rapamycin in “pathological” protein translation: relevance to cocaine addiction Christopher V. Dayas*, Doug W. Smith and Peter R. Dunkley School of Biomedical Sciences and Pharmacy, Centre for Translational Neuroscience and Mental Health Research, Hunter Medical Research Institute, University of Newcastle, Callaghan, NSW, Australia Edited by: Complex neuroadaptations within key nodes of the brain’s “reward circuitry” are thought Andrew Lawrence, Florey to underpin long-term vulnerability to relapse. A more comprehensive understanding of Neuroscience Institutes, Australia the molecular and cellular signaling events that subserve relapse vulnerability may lead to Reviewed by: Jason B. Wu, Cedars-Sinai Medical pharmacological treatments that could improve treatment outcomes for psychostimulant- Center, USA addicted individuals. Recent advances in this regard include findings that drug-induced Jacqueline McGinty, University of perturbations to neurotrophin, metabotropic glutamate receptor, and dopamine receptor South Carolina, USA signaling pathways perpetuate plasticity impairments at excitatory glutamatergic synapses *Correspondence: on ventral tegmental area and nucleus accumbens neurons. In the context of addiction, Christopher V. Dayas, School of Biomedical Sciences and Pharmacy, much previous work, in terms of downstream effectors to these receptor systems, has The University of Newcastle, centered on the extracellular-regulated MAP kinase signaling pathway. The purpose of the Callaghan, NSW 2308, Australia. present review is to highlight the evidence of an emerging role for another downstream e-mail: christopher.dayas@newcastle. effector of these addiction-relevant receptor systems – the mammalian target of rapamycin edu.au complex 1 (mTORC1). mTORC1 functions to regulate synaptic protein translation and is a potential critical link in our understanding of the neurobiological processes that drive addic- tion and relapse behavior. The precise cellular and molecular changes that are regulated by mTORC1 and contribute to relapse vulnerability are only just coming to light. Therefore, we aim to highlight evidence that mTORC1 signaling may be dysregulated by drug expo- sure and that these changes may contribute to aberrant translation of synaptic proteins that appear critical to increased relapse vulnerability, including AMPARs. The importance of understanding the role of this signaling pathway in the development of addiction vulnera- bility is underscored by the fact that the mTORC1 inhibitor rapamycin reduces drug-seeking in pre-clinical models and preliminary evidence indicating that rapamycin suppresses drug craving in humans. Keywords: mTOR, addiction, relapse, cocaine, mGluR, BDNF, plasticity, drug INTRODUCTION and molecular correlates of addiction are the ventral tegmental Drug addiction is a complex neuropsychiatric condition with the area (VTA) A10 dopamine neurons and the nucleus accumbens risk of relapse after quitting considered a fundamental obstacle (NAC; Kauer and Malenka, 2007; Kalivas and O’Brien, 2008; to long-term treatment management (O’Brien, 2005; Kalivas and Luscher and Malenka, 2011). Although other brain regions, such O’Brien, 2008). Despite significant recent efforts, our understand- as the prefrontal cortex (PFC), amygdala, and dorsal striatum, ing of the neurobiological processes that lead to addiction, and are also known to play important roles in addiction, our under- subsequently protracted relapse risk, is inadequate to the point standing of the pre- and post-synaptic neuroadaptations that that there has been limited translation of pre-clinical findings into occur within the VTA and NAC is significantly more detailed pharmacological treatments for this condition. Critically, further than these other major nodes in the brain “relapse circuit” (Kali- research is required to unravel the complex neuroadpatations that vas and O’Brien, 2008). Furthermore, the NAC and VTA have cause psychostimulant addiction, in particular the molecular and been ascribed an important role in reward learning (Stuber cellular signaling events that underpin protracted vulnerability et al., 2008; Tsai et al., 2009) and drug-induced synaptic plas- to relapse. This review will focus on recent findings relevant to ticity deficits in these brain regions that occur in response to translational efforts to identify targets to treat psychostimulant chronic drug taking have been hypothesized to impede the learn- addiction. ing of new strategies to overcome addiction (Martin et al., 2006; The brain circuitry that controls drug-seeking and relapse is Kalivas and Volkow, 2011). (We direct readers to several excel- multi-nodal (Koob and Volkow, 2009), however, the two nodes lent recent reviews describing the effects of chronic cocaine self- that have received the most attention with regard to functional administration on synaptic plasticity within the VTA and NAC; www.frontiersin.org February 2012 | Volume 3 | Article 13 | 1 Dayas et al. Role of mTOR in addiction vulnerability Kauer and Malenka, 2007; Kalivas and O’Brien, 2008; Luscher and activity-dependent translation of proteins required for synaptic Malenka, 2011). plasticity (i.e., long-term potentiation; Cammalleri et al., 2003) Given their importance, significant research efforts have been, and long-term depression (LTD; Mameli et al., 2007) by regulat- and continue to be, directed at the VTA and NAC in order ing the translation initiation complex through phosphorylation of to improve our understanding of the molecular signaling path- p70S6K and 4E-BP1. Indeed, there is strong evidence supporting a ways that are responsible for the drug-induced maladaptations. role for mTORC1 in regulating the translation of a number of pro- Advances in these areas have highlighted a role for a number teins necessary for synaptic plasticity, such as Ca2+/Calmodulin- of signaling pathways in the development of addiction (Grimm dependent kinase II alpha (CamKIIα), AMPAR receptor subunits et al., 2003; Lu et al., 2006; Graham et al., 2007; Dietz et al., 2009; (GluRs), microtubule-associated protein (MAP2), post-synaptic Russo et al., 2009; McGinty et al., 2010; Duncan and Lawrence, density protein 95 (PSD-95), glutamate receptor interaction pro- 2011) including molecules downstream of neurotrophin (NTrk), tein (GRIP1),and NMDARs,for example,GRIN1 (Hou and Klann, metabotropic glutamate (mGluR), and dopamine (DR) recep- 2004; Mameli et al., 2007). tors. Drug-induced perturbations in these signaling pathways are Regulation of protein synthesis by downstream effectors of thought to perpetuate plasticity impairments at excitatory glu- mTORC1 generally occurs at the mRNA translation initiation step tamatergic synapses on VTA and NAC neurons. The purpose of and involves a complex array of initiation factors and binding the present review is to highlight the emerging evidence that the proteins working with the ribosomes to ensure that the mRNA mammalian target of rapamycin complex 1 (mTORC1), a down- start codon is efficiently identified for the initiation of translation stream target of these receptor signaling pathways that regulate (Sonenberg and Hinnebusch, 2007, 2009). An important compo- synaptic protein translation, is a potentially critical link in our nent of the translation initiation mechanism is the activation of understanding of the neurobiological processes that drive addic- the mRNA by binding of the eIF4F complex to the mRNA’s 5 cap. tion and relapse behavior (Neasta et al., 2010; Wang et al., 2010; mRNA activation through cap recognition is one way translation Brown et al., 2011). Investigations of mTOR’s role in addiction is regulated and, thus, eIF4F binding constitutes a key regulatory are presently limited to the NAC and VTA, and consequently our step. Regulation of eIF4F is under either direct or indirect con- review will outline relevant molecular and cellular adaptations trol by mTORC1 (Hay and Sonenberg, 2004; Hoeffer and Klann, within these structures. However, where necessary we will draw 2010). For example, mTORC1 phosphorylates 4E-BPs, a family of mechanistic links derived from studies assessing the role of mTOR translational repressor proteins, which leads to their dissociation in synaptic plasticity in other brain regions. We begin by briefly from eIF4E, enabling it to form the eIF4F complex for cap bind- summarizing mTORC1 signaling in the CNS, however, we alert ing and participation in the initiation of translation of a range of readers to a number of important reviews that have recently been mRNAs (Gingras et al., 2004; Richter and Klann, 2009). mTORC1 published on this topic (Klann et al., 2004; Costa-Mattioli et al., can indirectly influence eIF4F activity through S6K protein kinases 2009; Richter and Klann, 2009; Hoeffer and Klann, 2010; Sharma that phosphorylate the ribosomal protein S6 and eIF4E, which in et al., 2010). turn increases mRNA translation (Raught et al., 2004). Therefore, mTORC1-mediated phosphorylation of 4E-BPs and S6Ks leads to AN OVERVIEW OF mTORC1 SIGNALING WITHIN THE BRAIN increased translation of mRNA (Hay and Sonenberg,2004; Richter Mammalian target of rapamycin (mTOR) is a 250-kDa ser- and Klann, 2009). ine/threonine protein kinase. Much of what we know about There is a second mTOR complex known as mTORC2 that mTOR signaling is based
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