Deepak R. Thakker

siRNA-mediated knockdown of the serotonin transporter in the adult mouse brain

Selective serotonin reuptake inhibitors (SSRIs) are widely used antidepressant drugs that increase the extracellular levels of serotonin by blocking the reuptake activity of the serotonin transporter (SERT). Although SSRIs elevate brain serotonergic neurotransmission acutely, their full therapeutic effects involve neurochemical adaptations that emerge following chronic drug administration. The adaptive downregulation of SERT has recently been implicated in the therapeutic response of SSRIs. Interestingly, studies using SERT-knockout mice reveal somewhat paradoxical depression-related effects, probably specific to the downregulation of SERT during early development. However, the behavioral significance of SSRI-mediated downregulation of SERT during adulthood is still unknown. We investigated whether somatic gene manipulation, triggered by infusing short interfering RNA (siRNA) into the ventricular system, would enable the downregulation of SERT in the adult mouse brain. Infusing the SERT-targeting siRNA, for 2 weeks, significantly reduced the mRNA levels of SERT in raphe nuclei. Further, a significant, specific and widespread downregulation of SERT-binding sites was achieved in the brain. In contrast, 2-week infusion of the SSRI, citalopram, produced a widespread downregulation of SERT-binding sites, independent of any alterations at the mRNA level. Irrespective of their mechanisms for downregulating SERT in the brain, infusions of SERT-siRNA or citalopram elicited a similar antidepressant-related behavioral response in the forced swim test. These results signify a role for the downregulation of SERT in mediating the antidepressant action of SSRIs in adults. Further, these data demonstrate that siRNA-induced widespread knockdown of gene expression serves as a powerful tool for assessing the function of endogenous genes in the adult brain.

Intracerebroventricular amyloid-β antibodies reduce cerebral amyloid angiopathy and associated micro-hemorrhages in aged Tg2576 mice

Although immunization against amyloid-β (Aβ) holds promise as a disease-modifying therapy for Alzheimer disease (AD), it is associated with an undesirable accumulation of amyloid in the cerebrovasculature [i.e., cerebral amyloid angiopathy (CAA)] and a heightened risk of micro-hemorrhages. The central and peripheral mechanisms postulated to modulate amyloid with anti-Aβ immunotherapy remain largely elusive. Here, we compared the effects of prolonged intracerebroventricular (icv) versus systemic delivery of anti-Aβ antibodies on the behavioral and pathological changes in an aged Tg2576 mouse model of AD. Prolonged icv infusions of anti-Aβ antibodies dose-dependently reduced the parenchymal plaque burden, astrogliosis, and dystrophic neurites at doses 10- to 50-fold lower than used with systemic delivery of the same antibody. Both icv and systemic anti-Aβ antibodies reversed the behavioral impairment in contextual fear conditioning. More importantly, unlike systemically delivered anti-Aβ antibodies that aggravated vascular pathology, icv-infused antibodies globally reduced CAA and associated micro-hemorrhages. We present data suggesting that the divergent effects of icv-delivered anti-Aβ antibodies result from gradually engaging the local (i.e., central) mechanisms for amyloid clearance, distinct from the mechanisms engaged by high doses of anti-Aβ antibodies that circulate in the vasculature following systemic delivery. With robust efficacy in reversing AD-related pathology and an unexpected benefit in reducing CAA and associated micro-hemorrhages, icv-targeted passive immunotherapy offers a promising therapeutic approach for the long-term management of AD.

mGluR7 facilitates extinction of aversive memories and controls amygdala plasticity

Formation and extinction of aversive memories in the mammalian brain are insufficiently understood at the cellular and molecular levels. Using the novel metabotropic glutamate receptor 7 (mGluR7) agonist AMN082, we demonstrate that mGluR7 activation facilitates the extinction of aversive memories in two different amygdala-dependent tasks. Conversely, mGluR7 knockdown using short interfering RNA attenuated the extinction of learned aversion. mGluR7 activation also blocked the acquisition of Pavlovian fear learning and its electrophysiological correlate long-term potentiation in the amygdala. The finding that mGluR7 critically regulates extinction, in addition to acquisition of aversive memories, demonstrates that this receptor may be relevant for the manifestation and treatment of anxiety disorders.

Global down-regulation of gene expression in the brain using RNA interference, with emphasis on monoamine transporters and GPCRs: implications for target characterization in psychiatric and neurological disorders.

RNA interference (RNAi) is a natural mechanism for regulating gene expression, which exists in plants, invertebrates, and mammals. We investigated whether non-viral infusion of short interfering RNA (siRNA) by the intracerebroventricular route would enable a sequence-specific gene knockdown in the mouse brain and whether the knockdown translates into disease-relevant behavioral changes. Initially, we targeted enhanced green fluorescent protein (EGFP) in mice overexpressing EGFP. A selective knockdown of both EGFP protein and mRNA was observed throughout the brain, with lesser down-regulation in regions distal to the infusion site. We then targeted endogenous genes, encoding the dopamine (DAT) and serotonin transporters (SERT). DAT-siRNA infusion in adult mice produced a significant down-regulation of DAT mRNA and protein and elicited hyperlocomotion similar, but delayed, to that produced on infusion of GBR-12909, a potent and selective DAT inhibitor. Similarly, SERT-siRNA infusion resulted in significant knockdown of SERT mRNA and protein and elicited reduced immobility in the forced swim test similar to that obtained on infusion of citalopram, a very selective and potent SSRI. Application of this non-viral RNAi approach may accelerate target validation for neuropsychiatric disorders that involve a complex interplay of gene(s) from various brain regions.

Adult siRNA-induced knockdown of mGlu7 receptors reduces anxiety in the mouse.

Our knowledge regarding the molecular pathophysiology underlying anxiety disorders remains incomplete. Increasing evidence points to a role of glutamate in anxiety. The group III metabotropic glutamate receptors (mGlu4, mGlu6, mGlu7 and mGlu8 receptors) remain the least investigated glutamate receptor subtypes partially due to a delay in the development of specific pharmacological tools. Early work using knockout animals and pharmacological tools aimed at investigating the role of mGlu7 receptor in the pathophysiology of anxiety disorders has yielded exciting yet not always consistent results. To further investigate the role this receptor plays in anxiety-like behaviour, we knocked down mGlu7 receptor mRNA levels in the adult mouse brain using siRNA delivered via an osmotic minipump. This reduced anxiety-like behaviour in the light-dark box coupled with an attenuation of stress-induced hyperthermia (SIH) and a reduction of the acoustic startle response (ASRs) in the fear-potentiated startle paradigm (FPS). These effects on anxiety-like behaviour were independent of any impairment of locomotor activity and surprisingly, no behavioural changes were observed in the forced swim test (FST), which is in contrast to mGlu7 receptor knockout animals. Furthermore, the previously reported epilepsy-prone phenotype seen in mGlu7 receptor knockout animals was not observed following siRNA-induced knockdown of the receptor. These data suggest targeting mGlu7 receptors with selective antagonist drugs may be an effective and safe strategy for the treatment of anxiety disorders.

Centrally Delivered BACE1 Inhibitor Activates Microglia, and Reverses Amyloid Pathology and Cognitive Deficit in Aged Tg2576 Mice.

Multiple small-molecule inhibitors of the β-secretase enzyme (BACE1) are under preclinical or clinical investigation for Alzheimers disease (AD). Prior work has illustrated robust lowering of central amyloid β (Aβ) after acute administration of BACE1 inhibitors. However, very few studies have assessed the overall impact of chronically administered BACE1 inhibitors on brain amyloid burden, neuropathology, and behavioral function in aged preclinical models. We investigated the effects of a potent nonbrain-penetrant BACE1 inhibitor, delivered directly to the brain using intracerebroventricular infusion in an aged transgenic mouse model. Intracerebroventricular infusion of the BACE1 inhibitor (0.3–23.5 μg/d) for 8 weeks, initiated in 17-month-old Tg2576 mice, produced dose-dependent increases in brain inhibitor concentrations (0.2–13 μm). BACE1 inhibition significantly reversed the behavioral deficit in contextual fear conditioning, and reduced brain Aβ levels, plaque burden, and associated pathology (e.g., dystrophic neurites), with maximal effects attained with ∼1 μg/d dose. Strikingly, the BACE1 inhibitor also reversed amyloid pathology below baseline levels (amyloid burden at the start of treatment), without adversely affecting cerebral amyloid angiopathy, microhemorrhages, myelination, or neuromuscular function. Inhibitor-mediated decline in brain amyloid pathology was associated with an increase in microglial ramification. This is the first demonstration of chronically administered BACE1 inhibitor to activate microglia, reverse brain amyloid pathology, and elicit functional improvement in an aged transgenic mouse model. Thus, engagement of novel glial-mediated clearance mechanisms may drive disease-modifying therapeutic benefit with BACE1 inhibition in AD.

Emerging use of non-viral RNA interference in the brain

Psychiatric and neurological disorders are among the most complex, poorly understood and debilitating diseases in medicine. Abrogating gene function using knockout animals is one of the primary means of examining the pathophysiological significance of a given gene product and has been used successfully in models of neuropsychiatric disorders. However, the developmental compensations that may potentially arise from such approaches are problematic and difficult to assess. The recent discovery of RNAi (RNA interference), as a highly efficient method for gene knockdown, has opened up the possibility for its application in examining the potential role of genes in adult brain function and/or disorders. Recent efforts have focused on applying RNAi-based knockdown to understand the genes implicated in neuropsychiatric disorders. We have developed a method of gene knockdown involving chronic infusion of siRNA (short interfering RNA) using osmotic minipumps. We have silenced a number of genes including those for the serotonin and dopamine transporter. Such tailoring of tools that deliver RNAi in the brain will significantly aid in our understanding of the complex pathophysiology of neuropsychiatric disorders where there is an immensely unmet medical need.

Widespread brain distribution and activity following i.c.v. infusion of anti‐β‐secretase (BACE1) in nonhuman primates

The potential for therapeutic antibody treatment of neurological diseases is limited by poor penetration across the blood–brain barrier. I.c.v. delivery is a promising route to the brain; however, it is unclear how efficiently antibodies delivered i.c.v. penetrate the cerebrospinal spinal fluid (CSF)‐brain barrier and distribute throughout the brain parenchyma.

WIDESPREAD BRAIN DISTRIBUTION AND ROBUST BACE1 INHIBITION FOLLOWING INTRACEREBROVENTRICULAR INFUSION OF ANTI-BACE1 IN NON-HUMAN PRIMATES

Background: The Alzheimer’s Disease Cooperative Study was founded in 1991 in response to an NIA RFA. The primary aim of the ADCS is to ‘advance research in the development of interventions that might be useful for treating, delaying, or preventing AD’. Toward that goal the ADCS has designed, managed, and analyzed over 20 large, multi-site AD clinical trials. The psychometric, clinical, neuroimaging, and biological data from these trials have been collected using a web-based electronic data capture (EDC) system developed by the ADCS Informatics Core. This system serves as a central hub used to coordinate the data management, quality assurance, and monitoring activities of thousands of affiliates across several continents. Methods: As the volume, complexity, and sources of data collected in the conduct of a clinical trial have increased, traditional data transfer and processing methods and tools have struggled to keep up. The use of data-intensive approaches to study enrollment, randomization, and monitoring, which require just-in-time data, have only exacerbated these challenges. One-time, end-of-study data transfers with cursory quality controls have been supplanted by the need for near real-time data flows that are subjected to a comprehensive battery of quality checks and must be rapidly integrated into the main study database. In response to these challenges, the ADCS has developed a web-based data exchange and processing platform that allows data managers to configure, manage, and monitor each data flow for a given study. Results: This poster has the following aims:1. Describe the ADCS EDC’s data exchange and processing capabilities.2. Discuss how these capabilities were used to support real-time, periodic and sporadic data flows to facilitate the conduct of recent ADCS clinical trials.3. Outline opportunities for future improvement. Conclusions: The ADCS EDC system provides a sophisticated data exchange and processing framework that accelerates data management and analysis activities at every stage of a clinical study’s lifecycle.