LRRK2 mutation is the most common inherited, autosomal dominant cause of Parkinson's disease (PD) and has also been observed in sporadic cases. Most mutations result in increased LRRK2 kinase activity. LRRK2 is highly expressed in brain regions that receive dense, convergent innervation by dopaminergic and glutamatergic axons, and its levels rise developmentally coincident with glutamatergic synapse formation. The onset and timing of expression suggests strongly that LRRK2 regulates the development, maturation and function of synapses. Several lines of data in mice show that LRRK2-G2019S, the most common LRRK2 mutation, produces an abnormal gain of pathological function that affects synaptic activity, spine morphology, persistent forms of synapse plasticity and behavioral responses to social stress. Effects of the mutation can be detected as early as the second week of postnatal development and can last or have consequences that extend into adulthood and occur in the absence of dopamine loss. These data suggest that the generation of neural circuits that support complex behaviors is modified by LRRK2-G2019S. Whether such alterations impart vulnerability to neurons directly or indirectly, they bring to the forefront the idea that neural circuits within which dopamine neurons eventually degenerate are assembled and utilized in ways that are distinct from circuits that lack this mutation and may contribute to non-motor symptoms observed in humans with PD.

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