Pathophysiological studies of Parkinson's disease dementia

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Parkinsons disease (PD) is typically characterized as a degenerative movement disorder, clinically manifested with distinct motor disturbances, including slowness of movement, resting tremor, rigidity, and postural instability, resulting from extensive loss of nigrostriatal dopaminergic neurons (nDANs) in the substantia nigra pars compacta (SNc) of midbrain. Besides motor symptoms, PD patients often develop cognitive dysfunctions, which leads to Parkinsons disease dementia (PDD). Approximately 75% of PD patients develop dementia within 10 years of diagnosis, and the prevalence of PDD is 0.3-0.5% in general population older than 65 years. There is no cure for PDD. The exact pathogenic mechanisms of PDD are largely unknown. Levodopa, the most effective drug to treat the motor symptoms in PD, however, does not respond well against cognitive dysfunctions. On the other hand, cholinesterase inhibitors can improve the cognitive functions, but exacerbate the motor symptoms. Therefore, an important step in intervening complexed neurological disorder like PDD, is to elucidate the functional roles of different neural circuits responsible for specific behavioral phenotypes. Accumulative evidence supports an association of dopaminergic dysfunction with PDD. PDD is likely resulted from extensive generation of midbrain DANs beyond the SNc regions in the late stages of PD. Midbrain DANs are heterogenous and can be categorized into different subpopulations based on anatomic locations, gene expression, electrophysiological properties, neuronal morphology, axonal projections, physiological functions, and disease vulnerabilities. Which subpopulations of midbrain DANs contribute to PDD remains to be determined. Recently, we discovered that a subpopulation of aldehyde dehydrogenase 1A1-positive (ALDH1A1+) nigrostriatal dopaminergic neuron (nDAN) located in the ventral SNc display the most profound loss in the postmortem human PD brains. The ALDH1A1+ nDANs account for approximately 70% nDANs in human and mouse brains. While ALDH1A1+ nDANs receive diverse monosynaptic inputs from multiple brain regions, their axons project exclusively to the dorsal striatum. The dorsal striatum is generally known for motor control and processing the implicit motor learning. Correlatively, genetic ablation of ALDH1A1+ nDANs in rodents caused severe impairments in motor skill learning in conjunction with a modest reduction of walking speed. However, those ALDH1A1+ nDAN-ablated mice did not develop any cognitive deficiency (unpublished data), suggesting an involvement of other midbrain DAN subpopulations, especially the ones located in ventral tegmental area (VTA), in the formation of explicit memory. The advancement of gene profiling in individual neurons allows to genetically define DAN subtypes in different SNc and VTA subregions. Using intersectional genetic labeling strategy, a recent study found that a cluster of vesicular glutamate transporter 2-positive (VGLT2+) DANs in the ventral VTA project predominantly to the entorhinal (ENT) and prefrontal cortices (PFC). Interestingly, ENT atrophy is particularly associated with PDD. By contrast, VTA DANs only sparsely project to the hippocampal formation. Instead, hippocampus receives the most dopamine inputs from the afferent fibers of locus coeruleus. Therefore, following our recently established workflow in defining the connectivity and functionality of ALDH1A1+ nDANs in implicit motor learning, we will investigate the synaptic inputs and physiological functions of VTA-VGLT2+ DAN subpopulations in declarative memory formation. The knowledge gained from this study will provide cell type and circuit specific mechanisms of PDD and lay the foundation for designing new therapeutic interventions for treatment of cognitive impairments in PDD. Specific Aims To unravel the contribution of different midbrain DAN subpopulations, especially the VTA-VGLUT2+ DAN subtypes in the formation of declarative memory, we propose the following three specific aims: Aim 1. To investigate the physiological function of VTA-VGLUT2+ DANs in declarative memory formationIn our previous study, we have shown that ALDH1A1+ nDANs are essential for the implicit motor skill learning, but not the explicit cognitive functions. In this proposal, we will test the hypothesis that VTA-VGLUT2+ DANs contribute to the formation of declarative memory. Using intersectional genetic strategy, we will genetically ablate VTA-VGLUT2+ DANs in mouse brains and examine the cognitive functions of the affected mice with a battery of social behavioral, spatial learning and memory, and fear/anxiety tests. Aim 2. To elucidate the underlying transmitter and circuit mechanisms of VTA-VGLUT2+ DANs in declarative memory formationIf the VTA-VGLUT2+ DAN-ablated mice show any specific cognitive abnormalities (investigated in Aim 1), we will further investigate how the VTA-VGLUT2+ DANs execute this function at the transmitter and circuit levels. The VGLUT2+ DANs can release both dopamine and glutamate, and may integrate various presynaptic inputs and undergo either tonic or phasic firing patterns to relay the signals to the downstream neurons in the ENT and PFC12. We will apply series of molecular genetics, chemical genetics and optogenetics tools to identify which neuron transmitters and specific presynaptic inputs are critical for accomplishing the expected behavioral tasks. The alterations of neural activity and transmitter release will be further evaluated by live imaging in behaving mice using fiber photometry and genetically encoded calcium and dopamine sensors. This aim allows us to identify specific presynaptic inputs and transmitter release essential for the memory formation and provides potential therapeutic targets to rescue the memory deficiency. Aim 3. To explore potential therapeutic procedures to mitigate cognitive impairments in the VTA-VGLUT2+ DAN-ablated miceSince levodopa alone is ineffective in treating PDD, to alleviate the cognitive deficiency we will treat the affected mice with levodopa in combination with other neurotransmitter receptor agonists or antagonists as well as neuromodulation. The results from Aim 2 will provide the specific targets for formulating the combinatory treatment paradigms. Taken collectively, this proposed study will unravel the specific cell type and circuit mechanisms of midbrain dopamine transmission in declarative memory formation and provides new insights to address the unmet need of PDD treatment.