Abstract
Age-related increases in monoamine oxidase B (MAO-B) may contribute to neurodegeneration associated with Parkinson's disease (PD). The MAO-B inhibitor deprenyl, a long-standing antiparkinsonian therapy, is currently used clinically in concert with the dopamine precursor L-DOPA. Clinical studies suggesting that deprenyl treatment alone is not protective against PD associated mortality were targeted to symptomatic patients. However, dopamine loss is at least 60% by the time PD is symptomatically detectable, therefore lack of effect of MAO-B inhibition in these patients does not negate a role for MAO-B in pre-symptomatic dopaminergic loss. In order to directly evaluate the role of age-related elevations in astroglial MAO-B in the early initiation or progression of PD, we created genetically engineered transgenic mice in which MAO-B levels could be specifically induced within astroglia in adult animals. Elevated astrocytic MAO-B mimicking age related increase resulted in specific, selective and progressive loss of dopaminergic neurons in the substantia nigra (SN), the same subset of neurons primarily impacted in the human condition. This was accompanied by other PD-related alterations including selective decreases in mitochondrial complex I activity and increased mitochondrial oxidative stress. Along with a global astrogliosis, we observed local microglial activation within the SN. These pathologies correlated with decreased locomotor activity. Importantly, these events occurred even in the absence of the PD-inducing neurotoxin MPTP. Our data demonstrates that elevation of murine astrocytic MAO-B by itself can induce several phenotypes of PD, signifying that MAO-B could be directly involved in multiple aspects of disease neuropathology. Mechanistically this may involve increases in membrane permeant H2O2 which can oxidize dopamine within dopaminergic neurons to dopaminochrome which, via interaction with mitochondrial complex I, can result in increased mitochondrial superoxide. Our inducible astrocytic MAO-B transgenic provides a novel model for exploring pathways involved in initiation and progression of several key features associated with PD pathology and for therapeutic drug testing.
Highlights
Monoamine oxidase B (MAO-B) is found in the brain primarily in non-neuronal cells such as astrocytes and radial glia [1,2,3]
Whole brain extracts from induced animals with the highest levels of expression exhibited an approximate 2.5-fold increase in MAOB and lacZ activities versus uninduced controls (Figure 1C); increased MAO-B activity was completely inhibited by deprenyl treatment
We demonstrate that elevations in astrocytic MAO-B levels results in a relatively selective loss of dopaminergic substantia nigra (SN) neurons and the severity of this loss appears to be age-dependent
Summary
Monoamine oxidase B (MAO-B) is found in the brain primarily in non-neuronal cells such as astrocytes and radial glia [1,2,3]. Neurons, which contain significantly lower levels of these protective components, are vulnerable to this mild oxidizing agent [16,17,18]. This suggests that H2O2 produced within astrocytes by MAO-B may be either broken down to H2O within these cells or may diffuse to vulnerable nearby cells such as dopaminergic neurons [19]. MAO-B activity levels have been found to be doubled in the SN in Parkinson’s disease, and to correlate with the percentage of dopaminergic SN cell loss [20]. This suggests that increases in MAO-B levels could play a role in subsequent dopaminergic cell death
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