| Abstract: | Disruption of neurotransmitter vesicle dynamics (transport, capacity, release) has been implicated in a variety of neurodegenerative and neuropsychiatric conditions. Here, we report a novel mouse model of enhanced vesicular function via bacterial artificial chromosome (BAC)-mediated overexpression of the vesicular monoamine transporter 2 (VMAT2; Slc18a2). A twofold increase in vesicular transport enhances the vesicular capacity for dopamine (56%), dopamine vesicle volume (33%), and basal tissue dopamine levels (21%) in the mouse striatum. The elevated vesicular capacity leads to an increase in stimulated dopamine release (84%) and extracellular dopamine levels (44%). VMAT2-overexpressing mice show improved outcomes on anxiety and depressive-like behaviors and increased basal locomotor activity (41%). Finally, these mice exhibit significant protection from neurotoxic insult by the dopaminergic toxicant 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), as measured by reduced dopamine terminal damage and substantia nigra pars compacta cell loss. The increased release of dopamine and neuroprotection from MPTP toxicity in the VMAT2-overexpressing mice suggest that interventions aimed at enhancing vesicular capacity may be of therapeutic benefit in Parkinson disease.Faulty monoamine neurotransmission is characteristic of many disorders, including Parkinson disease, depression, dystonia, attention deficit hyperactivity disorder, schizophrenia, addiction, and Huntington disease (1–7). Several strategies have been used to enhance monoamine signaling: administration of precursors to increase synthesis, inhibition of enzymes to prevent metabolism/degradation, inhibition of plasma membrane transporters to increase synaptic lifespan, and administration of receptor agonists to directly activate postsynaptic targets. However, these therapies fail to preserve many, if not all, of the critical aspects of chemical neurotransmission: normal transmitter synthesis, activity-dependent transmitter release and receptor activation, and receptor recovery following signal termination both by transmitter uptake and metabolism. Thus, these approaches often produce deleterious side effects or lose efficacy over time.Increasing the neurotransmitter content in the synaptic vesicle may represent a therapeutic approach capable of increasing the release of monoamines without the aforementioned adverse effects. The vesicular monoamine transporter 2 (VMAT2, SLC18A2) is responsible for the packaging of neurotransmitter into vesicles for subsequent release from monoaminergic neurons. VMAT2 is an H+-ATPase antiporter, which uses the vesicular electrochemical gradient to drive the packaging of cytosolic transmitter into small synaptic and dense core vesicles (8–10). VMAT2 is also essential for survival of dopamine neurons as cytosolic dopamine is neurotoxic (11, 12). By sequestering intracellular dopamine into vesicles, VMAT2 prevents cytosolic dopamine accumulation and its subsequent conversion to neurotoxic species (13–17). Thus, VMAT2 serves two primary functions: to mediate monoamine neurotransmission and to counteract intracellular toxicity.Previous data clearly show that disruption of VMAT2 function produces adverse effects. Pharmacological VMAT2 inhibition by reserpine or tetrabenazine results in monoamine depletion and negative behavioral consequences, including akinesia and depressive behaviors (18–20). Genetic reduction of VMAT2 in mice also causes depletion of dopamine, norepinephrine, and serotonin and progressive neurodegeneration in multiple monoaminergic regions (21–25). Similarly, a VMAT2 mutation in humans that dramatically reduces vesicular function was recently linked to an infantile parkinsonism-like condition with symptoms caused by deficits in all of the monoamines (26). SNPs in the VMAT2 gene have also been associated with neurocognitive function in relatives of patients with schizophrenia and even in posttraumatic stress disorder (27, 28). Interestingly, haplotypes within the VMAT2 promoter region that increase VMAT2 expression have been associated with a decreased risk of Parkinson disease (29, 30). Although the detrimental effects of reduced VMAT2 function are recognized, our understanding of the potential benefits of increased VMAT2 function in vivo has been limited to a Drosophila model (31–33). Thus, we generated VMAT2-overexpressing mice using a bacterial artificial chromosome (BAC) to determine whether increased vesicular packaging could provide an elevation of monoamine output in a mammalian system. We report here that VMAT2-overexpressing mice (VMAT2-HI) have increased vesicle capacity, increased synaptic dopamine release, improved outcomes on anxiety-like and depressive-like behaviors, and reduced vulnerability to toxic insult by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). These data demonstrate that the manipulation of vesicular capacity is capable of providing a sustained enhancement of the dopamine system and suggest that the vesicle is a viable therapeutic target for numerous monoamine-deficient disorders. |
