Loss of PINK1 causes age-dependent decrease of dopamine release and mitochondrial dysfunction

Highlights

• Loss of PINK1 reduces DA release in dorsal striatum in an age-dependent manner.

• The decrease is due to less DA release instead of an alteration of DAT function.

• There is an age-dependent decrease in basal mitochondrial respiration and ATP synthesis.

• Developed an approach to measure mitochondrial respiration by OCR in striatal slices.

Abstract

Mutations and deletions in PTEN-induced kinase 1 (PINK1) cause autosomal recessive Parkinson's disease (PD), the second most common neurodegenerative disorder. PINK1 is a nuclear-genome encoded Ser/Thr kinase in mitochondria. PINK1 deletion was reported to affect dopamine (DA) levels in the striatum and mitochondrial functions but with conflicting results. The role of PINK1 in mitochondrial function and in PD pathogenesis remains to be elucidated thoroughly. In this study, we measured DA release using fast-scan cyclic voltammetry in acute striatal slices from both PINK1 knockout (KO) and wild-type (WT) mice at different ages. We found that single pulse-evoked DA release in the dorsal striatum of PINK1 KO mice was decreased in an age-dependent manner. Furthermore, the decrease was because of less DA release instead of an alteration of DA transporter function or DA terminal degeneration. We also found that PINK1 KO striatal slices had significantly lower basal mitochondria respiration compared with that of WT controls, and this impairment was also age-dependent. These results suggest that the impaired DA release is most likely because of mitochondrial dysfunction and lower ATP production.

Introduction

Parkinson's disease (PD) is the most common neurodegenerative disease, caused by in part a progressive loss of dopamine (DA) neurons within the substantia nigra pars compacta and a subsequent deficiency of the neurotransmitter DA in the striatum (STR) (Dauer and Przedborski, 2003). The occurrence is mostly sporadic, although mutations in twenty genes have been linked to genetic forms of PD (Hewitt and Whitworth, 2017). Homozygous and compound heterozygous mutations in the PINK1 gene are the second most frequent cause of autosomal recessive early-onset Parkinsonism (Valente et al., 2004a, Valente et al., 2004b). PINK1 is a ubiquitously expressed serine/threonine kinase with high levels in the heart, skeletal muscle, and the brain. It also contains a mitochondrial targeting sequence in the N-terminus that directs import of PINK1 into mitochondria, suggesting a role in mitochondrial function. Previous studies have shown that early-onset Parkinsonism–associated mutations have reduced PINK1 kinase activity. Hence, several PINK1 knockout (KO) mouse models were generated to study the pathogenic mechanisms of PD (Akundi et al., 2011, Gispert et al., 2009, Heeman et al., 2011, Kitada et al., 2007).

Although all the PINK1 KO mouse models so far lack overt neurodegeneration, previous studies have shown that PINK1 deletion can alter DA levels in the STR, supporting the notion that synaptic dysfunction occurs before neurodegeneration or behavior deficits. Nevertheless, it is controversial whether DA release is decreased or not. Most of the studies measured DA levels using HPLC which provides little information of phasic DA release and kinetics. Kitada et al. revealed that single pulse (1p)–evoked DA overflow was significantly decreased in PINK1 KO mice at 2–3 months old and attributed this alteration to decreased DA transporter (DAT) function using amperometric recordings (Kitada et al., 2007). On the contrary, a recent finding reported unaltered striatal DA release in 2-month-old PINK1 KO mice using fast-scan cyclic voltammetry (FSCV) recording, and no alteration of DAT functions (Sanchez et al., 2014). The PINK1 KO rats, the only rodent model which exhibited significant motor deficits and substantia nigra pars compacta DA neuron degeneration at 8 months of age paradoxically showed a 2 to 3-fold increase in striatal DA content measured by HPLC (Dave et al., 2014). Because the previous results were not consistent with one another, it is important to know how DA release is altered in PINK1 KO mice, whether the alteration is age-dependent, and whether and how mitochondria are involved in DA release.

Aberrant mitochondrial function has long been implicated in the pathogenesis of PD (Hauser and Hastings, 2013, Moon and Paek, 2015, Perier and Vila, 2012, Van Laar and Berman, 2013) and PINK1 is believed to exert neuroprotection by maintaining mitochondrial integrity (Clark et al., 2006, Narendra and Youle, 2011, Steer et al., 2015). PINK1 KO/KD fibroblasts and neurons display reduced mitochondrial membrane potential (MMP), impairment in respiration, ATP reduction, calcium overload, and heightened reactive oxygen species production (Gandhi et al., 2009, Gandhi et al., 2012, Gautier et al., 2008, Gautier et al., 2012, Heeman et al., 2011, Kostic et al., 2015, Liu et al., 2011, van der Merwe et al., 2017, Wang et al., 2011). Mitochondria isolated from STR or whole brain of aged PINK1 KO mouse show defects in complex I (Gautier et al., 2008, Liu et al., 2011, Morais et al., 2009, Morais et al., 2014). Directly measuring the oxygen consumption rate (OCR) is one of the most popular methods to evaluate mitochondrial function. Previous studies demonstrated an altered mitochondrial respiration rate in isolated mitochondria or primary cultures from PINK1 KO mice, but the results are controversial. One report described that respiratory activity was reduced for both complex I and complex II in isolated mitochondria from STR of PINK1 KO mice at the age of 3–4 months, and increased sensitivity to oxidative stress, but no change for the basal OCR (Gautier et al., 2008). In another study, Gispert found a reduction of respiratory activities for complexes I+III+IV and IV in purified mitochondria from 18-month-old brain tissue of mice (Gispert et al., 2009). Some studies reported increased basal OCR (Akundi et al., 2011, Cooper et al., 2012, Villeneuve et al., 2016), whereas others showed decreased OCR (Liu et al., 2011, Morais et al., 2014). The discrepancy could be either due to different tissues and cells preparation, different measuring methods, or due to loss of physiological condition in isolated mitochondria. The Seahorse Extracellular Flux (XF24) analyzer is a commonly used tool for measuring oxygen consumption in cell cultures or purified mitochondria which offers improved throughput compared with traditional O₂ electrode–based methods (Gerencser et al., 2009). It can also be adapted to study acute brain slices (Fried et al., 2014). In the present study, we have developed a method to measure mitochondrial respiration using XF24 analyzer in acute striatal slices. We found that the basal OCR and ATP production are decreased in the aged PINK1 KO mice. The decrease of mitochondrial respiration is age-dependent, correlated with the age-dependent decreased evoked DA release in the STR. The mitochondrial coupling efficiency was impaired in the PINK1 KO mice from a young age, which might accumulate with age leading to the observed reduced ATP production and reduced DA release later on.