Cell type-specific plasticity of striatal projection neurons in parkinsonism and L-DOPA-induced dyskinesia

The striatum is widely viewed as the fulcrum of pathophysiology in Parkinson’s disease (PD) and L-DOPA-induced dyskinesia (LID). In these disease states, the balance in activity of striatal direct pathway spiny projection neurons (dSPNs) and indirect pathway spiny projection neurons (iSPNs) is disrupted, leading to aberrant action selection. However, it is unclear whether countervailing mechanisms are engaged in these states. Here we report that iSPN intrinsic excitability and excitatory corticostriatal synaptic connectivity were lower in PD models than normal; ​L-DOPA treatment restored these properties. Conversely, dSPN intrinsic excitability was elevated in tissue from PD models and suppressed in LID models. Although the synaptic connectivity of dSPNs did not change in PD models, it fell with ​L-DOPA treatment. In neither case, however, was the strength of corticostriatal connections globally scaled. Thus, SPNs manifested homeostatic adaptations in intrinsic excitability and in the number but not strength of excitatory corticostriatal synapses

Striatal adaptations in experimental parkinsonism and L-DOPA-induced dyskinesia

Parkinson’s disease (PD) is a neurodegenerative disorder, characterized by the loss of dopamine (DA) producing neurons in the substantia nigra pars compacta (SNc), resulting in typical motor symptoms. DA replacement with L-DOPA is the standard therapy for PD. However, with treatment duration many patients face the severe treatment complication of L-DOPA-induced dyskinesia (LID), constituting in abnormal involuntary movements (AIMs). The etiology of PD and LID is largely unknown, but both pathophysiological states are linked to DA. How neurons in a DA-receptive brain region adapt to the pathophysiological states of PD and LID is the topic of this thesis’ work. The striatum is the “hub” into the basal ganglia network and implicated in movement control. Striatal spiny projection neurons (SPNs) divide into two subpopulations, forming the so-called direct and indirect pathway of the basal ganglia. Due to the expression of different DA receptors, direct and indirect pathway SPNs (dSPNs and iSPNs, respectively) are oppositely modulated by DA. D1 receptor (D1R) stimulation in the DA-denervated, parkinsonian striatum leads to a supersensitive activation of ERK1/2 in dSPNs. This aberrant signaling activation is widely believed to be a core mechanism leading to the development of LID. In the first study we investigated which signaling pathways participate in this D1R-induced ERK1/2 activation. We found a distinct and complex interaction between PKA- and Ca2+-dependent pathways, which is critically modulated by mGluR5. In the second study we further investigated the antidsykinetic profile of mGluR5 antagonist treatment, finding that the choice of animal model influences the outcome of antidyskinetic therapy testing. Striatal adaptations, sensitive to beneficial mGluR5 inhibition, appear not to be represented in only partially DA-denervated animals. In the last study we investigated possible homeostatic mechanisms in SPNs during PD and LID. We found that both iSPNs and dSPNs display potential homeostatic adaptations of excitability that are likely to counteract the loss of DA signaling and balance perturbations in firing activity. The changes were oppositely directed in iSPNs and dSPNs, reflecting the bidirectional modulation by DA. In contrast, PD-associated dendritic atrophy was found in both subpopulations and is independent of DAergic signaling. Synaptic adaptations in SPNs in PD and LID appeared not to follow homeostatic ruling. Specifically, we found that SPNs do not exhibit synaptic scaling, but rather selective elimination of spines. The failure to preserve the pattern of weighted synaptic inputs suggests that SPNs may not be able to appropriately regulate basal ganglia related behavior in PD and LID. Taken together, the results of this thesis reveal new molecular and physiological adaptations of SPNs in experimental models of PD and LID. Identifying if they are compensatory or maladaptive is difficult, but the more our understanding proceeds the better we can refine preclinical animal models and define potential treatment options for PD and LID

Alterations of striatal indirect pathway neurons precede motor deficits in two mouse models of Huntington's disease

Striatal neurons forming the indirect pathway (iSPNs) are particularly vulnerable in Huntington's disease (HD). In this study we set out to investigate morphological and physiological alterations of iSPNs in two mouse models of HD with relatively slow disease progression (long CAG repeat R6/2 and zQ175-KI). Both were crossed with a transgenic mouse line expressing eGFP in iSPNs. Using the open-field and rotarod tests, we first defined two time points in relation to the occurrence of motor deficits in each model. Then, we investigated electrophysiological and morphological properties of iSPNs at both ages. Both HD models exhibited increased iSPN excitability already before the onset of motor deficits, associated with a reduced number of primary dendrites and decreased function of Kir- and voltage-gated potassium channels. Alterations that specifically occurred at symptomatic ages included increased calcium release by back-propagating action potentials in proximal dendrites, due to enhanced engagement of intracellular calcium stores. Moreover, motorically impaired mice of both HD models showed a reduction in iSPN spine density and progressive formation of huntingtin (Htt) aggregates in the striatum. Our study therefore reports iSPN-specific alterations relative to the development of a motor phenotype in two different mouse models of HD. While some alterations occur early and are partly non-progressive, others potentially provide a pathophysiological marker of an overt disease state

Non‐Apoptotic Caspase‐3 Activation Mediates Early Synaptic Dysfunction of Indirect Pathway Neurons in the Parkinsonian Striatum

Non‐apoptotic caspase‐3 activation is critically involved in dendritic spine loss and synaptic dysfunction in Alzheimer’s disease. It is, however, not known whether caspase‐3 plays similar roles in other pathologies. Using a mouse model of clinically manifest Parkinson’s disease, we provide the first evidence that caspase‐3 is transiently activated in the striatum shortly after the degeneration of nigrostriatal dopaminergic projections. This caspase‐3 activation concurs with a rapid loss of dendritic spines and deficits in synaptic long‐term depression (LTD) in striatal projection neurons forming the indirect pathway. Interestingly, systemic treatment with a caspase inhibitor prevents both the spine pruning and the deficit of indirect pathway LTD without interfering with the ongoing dopaminergic degeneration. Taken together, our data identify transient and non‐apoptotic caspase activation as a critical event in the early plastic changes of indirect pathway neurons following dopamine denervation

PDZD8 is not the 'functional ortholog' of Mmm1, it is a paralog

Authors of a recent paper demonstrate that, like ERMES (ER-mitochondria encounter structure) in fungal cells, PDZD8 (PDZ domain containing 8) tethers mitochondria to the ER in mammalian cells. However, identifying PDZD8 as a "functional ortholog" of yeast Mmm1 (maintenance of mitochondrial morphology protein 1) is at odds with the phylogenetic data. PDZD8 and Mmm1 are paralogs, not orthologs, which affects the interpretation of the data with respect to the evolution of ER-mitochondria tethering. Our phylogenetic analyses show that PDZD8 co-occurs with ERMES components in lineages closely related to animals solidifying its identity as a paralog of Mmm1. Additionally, we identify two related paralogs, one specific to flagellated fungi, and one present only in unicellular relatives of animals. These results point to a complex evolutionary history of ER-mitochondria tethering involving multiple gene gains and losses in the lineage leading to animals and fungi

Chemogenetic stimulation of striatal projection neurons modulates responses to Parkinson's disease therapy

Parkinson's disease (PD) patients experience loss of normal motor function (hypokinesia), but can develop uncontrollable movements known as dyskinesia upon treatment with L-DOPA. Poverty or excess of movement in PD has been attributed to overactivity of striatal projection neurons forming either the indirect (iSPNs) or the direct (dSPNs) pathway, respectively. Here, we investigated the two pathways' contribution to different motor features using SPN type-specific chemogenetic stimulation in rodent models of PD (PD mice) and L-DOPA-induced dyskinesia (LID mice). Using the activatory Gq-coupled human M3 muscarinic receptor (hM3Dq), we found that chemogenetic stimulation of dSPNs mimicked, while stimulation of iSPNs abolished the therapeutic action of L-DOPA in PD mice. In LID mice, hM3Dq stimulation of dSPNs exacerbated dyskinetic responses to L-DOPA, while stimulation of iSPNs inhibited these responses. In the absence of L-DOPA, only chemogenetic stimulation of dSPNs mediated through the Gs-coupled modified rat muscarinic M3 receptor (rM3Ds) induced appreciable dyskinesia in PD mice. Combining D2 receptor agonist treatment with rM3Ds-dSPN stimulation reproduced all symptoms of LID. These results demonstrate that dSPNs and iSPNs oppositely modulate both therapeutic and dyskinetic responses to dopamine replacement therapy in PD. We also show that chemogenetic stimulation of different signaling pathways in dSPNs leads to markedly different motor outcomes. Our findings have important implications for the design of effective antiparkinsonian and antidyskinetic drug therapies

Mechanisms of Dopamine D1 Receptor-Mediated ERK1/2 Activation in the Parkinsonian Striatum and Their Modulation by Metabotropic Glutamate Receptor Type 5.

In animal models of Parkinson's disease, striatal overactivation of ERK1/2 via dopamine (DA) D1 receptors is the hallmark of a supersensitive molecular response associated with dyskinetic behaviors. Here we investigate the pathways involved in D1 receptor-dependent ERK1/2 activation using acute striatal slices from rodents with unilateral 6-hydroxydopamine (6-OHDA) lesions. Application of the dopamine D1-like receptor agonist SKF38393 induced ERK1/2 phosphorylation and downstream signaling in the DA-denervated but not the intact striatum. This response was mediated through a canonical D1R/PKA/MEK1/2 pathway and independent of ionotropic glutamate receptors but blocked by antagonists of L-type calcium channels. Coapplication of an antagonist of metabotropic glutamate receptor type 5 (mGluR5) or its downstream signaling molecules (PLC, PKC, IP3 receptors) markedly attenuated SKF38393-induced ERK1/2 activation. The role of striatal mGluR5 in D1-dependent ERK1/2 activation was confirmed in vivo in 6-OHDA-lesioned animals treated systemically with SKF38393. In one experiment, local infusion of the mGluR5 antagonist MTEP in the DA-denervated rat striatum attenuated the activation of ERK1/2 signaling by SKF38393. In another experiment, 6-OHDA lesions were applied to transgenic mice with a cell-specific knockdown of mGluR5 in D1 receptor-expressing neurons. These mice showed a blunted striatal ERK1/2 activation in response to SFK38393 treatment. Our results reveal that D1-dependent ERK1/2 activation in the DA-denervated striatum depends on a complex interaction between PKA- and Ca(2+)-dependent signaling pathways that is critically modulated by striatal mGluR5