Arrestins and two receptor kinases are upregulated in Parkinson's disease with dementia

Abstract

Arrestins and G proteins-coupled receptor kinases (GRKs) regulate signaling and trafficking of G protein-coupled receptors. We investigated changes in the expression of arrestins and GRKs in the striatum of patients with Parkinson's disease without (PD) or with dementia (PDD) at postmortem using Western blotting and ribonuclease protection assay. Both PD and PDD groups had similar degree of dopamine depletion in all striatal regions. Arrestin proteins and mRNAs were increased in the PDD group throughout striatum. Protein and mRNA of GRK5, the major subtype in the human striatum, and GRK3 were also upregulated, whereas GRK2 and 6 were mostly unchanged. The PD group had lower concentration of arrestins and GRKs than the PDD group. There was no statistical link between the load of Alzheimer's pathology and the expression of these signaling proteins. Upregulation of arrestins and GRK in PDD may confer resistance to the therapeutic effects of levodopa often observed in these patients. In addition, increased arrestin and GRK concentrations may lead to dementia via perturbation of multiple signaling mechanisms.

Introduction

Parkinson's disease (PD) is an age-related neurodegenerative disorder caused by degeneration of dopaminergic neurons of the substantia nigra that provide dopamine (DA) to the brain, resulting in the depletion of DA in the striatum, the main recipient of dopaminergic input from the substantia nigra. Neurobiological mechanisms responsible for motor symptoms produced by the degeneration of dopaminergic neurons in PD are poorly understood. It is generally believed that dysregulation of DA receptor signaling in the basal ganglia plays a role in generating motor deficits (Guigoni et al., 2005, Joyce et al., 2001, Muriel et al., 1999, Ryoo et al., 1998), but the underlying molecular events are not known.

DA receptors belong to the G protein-coupled receptor (GPCR) superfamily. Upon persistent stimulation many GPCRs undergo desensitization via a two-step process: activation-dependent receptor phosphorylation by G protein-coupled receptor kinases (GRK) followed by the binding of arrestins that precludes further signaling via G proteins by shielding the cytoplasmic surface of receptors (Krupnick and Benovic, 1998). Arrestins promote receptor internalization via interaction with the internalization machinery of coated pits (reviewed in Gurevich and Gurevich, 2003, Gurevich and Gurevich, 2006 and Lefkowitz and Whalen (2004)). Internalized receptor can either be recycled back to the plasma membrane (resensitization) or degraded (receptor down-regulation). In addition to being key players in the GPCR desensitization and trafficking, arrestins are multifunctional signaling molecules translating GPCR activation into modulation of G protein-independent signaling pathways (Beaulieu et al., 2004, Beaulieu et al., 2005, Lefkowitz and Shenoy, 2005) and regulating the expression of specific genes (Kang et al., 2005). Two arrestin subtypes, arrestin2 and arrestin3, are ubiquitous (Attramadal et al., 1992, Bezard et al., 2005, Gurevich et al., 2002, Gurevich et al., 2004). Five out of seven GRK subtypes, GRK2, 3, 4, 5, and 6, are widely expressed in the brain with largely overlapping cellular distribution (Arriza et al., 1992, Benovic and Gomez, 1993, Bezard et al., 2005, Gurevich et al., 2004, Premont et al., 1994). In vitro data demonstrate that most GPCR subtypes can be phosphorylated by several GRKs (Ménard et al., 1996, Richardson and Hosey, 1993) and bind both arrestins equally well (Gurevich et al., 1995, Krupnick and Benovic, 1998, Richardson and Hosey, 1993). Conversely, in vivo studies suggest that receptors may be preferentially phosphorylated by specific GRKs (Gainetdinov et al., 1999, Iaccarino et al., 1998a, Iaccarino et al., 1998b, Koch et al., 1995, Rockman et al., 1996) and interact with specific arrestins (Kohout et al., 2001, Oakley et al., 2000). Importantly, GPCRs phosphorylated by different GRKs may signal differently (Kim et al., 2005, Ren et al., 2005). Thus, signaling via GPCRs may be differentially regulated by the changes in the cellular complement of arrestins and GRKs. Dysregulation of DA receptors following dopaminergic degeneration in PD may be linked to defects in their desensitization, trafficking, and signaling brought about by the alterations in the expression of specific arrestins and/or GRKs.

Approximately 30–40% of PD patients develop dementia as the disease progresses (Emre, 2003 and references therein). Neuropathological basis of dementia in the idiopathic PD remains controversial. Many PD patients have substantial load of Alzheimer's histopathological features (Boller et al., 1980, Braak et al., 1996, Jellinger, 1997, Jellinger et al., 2002). There is evidence that dementia in PD is associated more closely with cortical deposition of the hallmark component of Lewy bodies, α-synuclein, than with Alzheimer's pathology (Aarsland et al., 2005, Hurtig et al., 2000, Mattila et al., 2000), although this conclusion is disputed by some authors (Parkkinen et al., 2005). A strong correlation between frequency of plaques and neurofibrillary tangles with neocortical Lewy bodies has been reported (Apaydin et al., 2002), suggesting common origin and/or cooperativity between the two types of pathology. Although dementia in idiopathic PD is very important for the disease's prognosis and management (e Lau et al., 2005, Jellinger et al., 2002, Poewe, 2005), underlying neurochemical mechanisms remain largely unexplored. Dementia associated with PD is resistant to L-DOPA treatment (Poewe, 2005). Importantly, dementia in PD strongly correlates with declining antiparkinsonian response to dopaminergic medication including direct DA agonists (Apaydin et al., 2002, Bonelli et al., 2004, Joyce et al., 2002), which may reflect specific changes in DA receptor signaling that hamper their responsiveness. It is conceivable that specific alterations in mechanisms governing signaling via DA receptors and/or other GPCRs brought about by concomitant dementia pathology may be responsible. The GPCR regulation machinery is positioned at the intersection of important signaling pathways and is shared by many GPCRs. Therefore, changes in availability and/or cellular complement of arrestins and/or GRKs induced by the activity of specific GPCRs may affect multiple GPCRs and signaling pathways, thereby propagating the pathology.

Here we report profound changes in the concentrations of arrestins and GRKs at postmortem in the basal ganglia of patients with PD and dementia as opposed to control and patients with PD only. Arrestin binding to GRK-phosphorylated GPCRs initiates second round of signaling via multiple mechanisms. Therefore, we investigated changes in expression of other signaling molecules that may be affected by the modifications in arrestin/GRK expression.