Non-motor behavioural impairments in parkin-deficient mice

Investigating affective neuropsychiatric symptoms in rodent models of Parkinson's disease

Affective neuropsychiatric disorders such as depression, anxiety and apathy are among the most frequent non-motor symptoms observed in people with Parkinson's disease (PD). These conditions often emerge during the prodromal phase of the disease and are generally considered to result from neurodegenerative processes in meso-corticolimbic structures, occurring in parallel to the loss of nigrostriatal dopaminergic neurons. Depression, anxiety, and apathy are often treated with conventional medications, including selective serotonin reuptake inhibitors, tricyclic antidepressants, and dopaminergic agonists. The ability of these pharmacological interventions to consistently counteract such neuropsychiatric symptoms in PD is still relatively limited and the development of reliable experimental models represents an important tool to identify more effective treatments. This chapter provides information on rodent models of PD utilized to study these affective neuropsychiatric symptoms. Neurotoxin-based and genetic models are discussed, together with the main behavioral tests utilized to identify depression- and anxiety-like behaviors, anhedonia, and apathy. The ability of various therapeutic approaches to counteract the symptoms observed in the various models is also reviewed.

Loss of parkin causes endoplasmic reticulum calcium dyshomeostasis by upregulation of reticulocalbin 1

Increasing evidence suggests that astrocytes play an important role in the progression of Parkinson's disease (PD). Previous studies on our parkin knockout mouse demonstrated a higher accumulation of damaged mitochondria in astrocytes than in surrounding dopaminergic (DA) neurons, suggesting that Parkin plays a crucial role regarding their interaction during PD pathogenesis. In the current study, we examined primary mesencephalic astrocytes and neurons in a direct co-culture system and discovered that the parkin deletion causes an impaired differentiation of mesencephalic neurons. This effect required the parkin mutation in astrocytes as well as in neurons. In Valinomycin-treated parkin-deficient astrocytes, ubiquitination of Mitofusin 2 was abolished, whereas there was no significant degradation of the outer mitochondrial membrane protein Tom70. This result may explain the accumulation of damaged mitochondria in parkin-deficient astrocytes. We examined differential gene expression in the substantia nigra region of our parkin-KO mouse by RNA sequencing and identified an upregulation of the endoplasmic reticulum (ER) Ca²⁺ -binding protein reticulocalbin 1 (RCN1) expression, which was validated using qPCR. Immunostaining of the SN brain region revealed RCN1 expression mainly in astrocytes. Our subcellular fractionation of brain extract has shown that RCN1 is located in the ER and in mitochondria-associated membranes (MAM). Moreover, a loss of Parkin function reduced ATP-stimulated calcium-release in ER mesencephalic astrocytes that could be attenuated by siRNA-mediated RCN1 knockdown. Our results indicate that RCN1 plays an important role in ER-associated calcium dyshomeostasis caused by the loss of Parkin function in mesencephalic astrocytes, thereby highlighting the relevance of astrocyte function in PD pathomechanisms.

How Well Do Rodent Models of Parkinson's Disease Recapitulate Early Non-Motor Phenotypes? A Systematic Review

The prodromal phase of Parkinson's disease (PD) is characterised by many non-motor symptoms, and these have recently been posited to be predictive of later diagnosis. Genetic rodent models can develop non-motor phenotypes, providing tools to identify mechanisms underlying the early development of PD. However, it is not yet clear how reproducible non-motor phenotypes are amongst genetic PD rodent models, whether phenotypes are age-dependent, and the translatability of these phenotypes has yet to be explored. A systematic literature search was conducted on studies using genetic PD rodent models to investigate non-motor phenotypes; cognition, anxiety/depressive-like behaviour, gastrointestinal (GI) function, olfaction, circadian rhythm, cardiovascular and urinary function. In total, 51 genetic models of PD across 150 studies were identified. We found outcomes of most phenotypes were inconclusive due to inadequate studies, assessment at different ages, or variation in experimental and environmental factors. GI dysfunction was the most reproducible phenotype across all genetic rodent models. The mouse model harbouring mutant A53T, and the wild-type hα-syn overexpression (OE) model recapitulated the majority of phenotypes, albeit did not reliably produce concurrent motor deficits and nigral cell loss. Furthermore, animal models displayed different phenotypic profiles, reflecting the distinct genetic risk factors and heterogeneity of disease mechanisms. Currently, the inconsistent phenotypes within rodent models pose a challenge in the translatability and usefulness for further biomechanistic investigations. This review highlights opportunities to improve phenotype reproducibility with an emphasis on phenotypic assay choice and robust experimental design.

Human Induced Pluripotent Stem Cell Phenotyping and Preclinical Modeling of Familial Parkinson's Disease

Parkinson's disease (PD) is primarily idiopathic and a highly heterogenous neurodegenerative disease with patients experiencing a wide array of motor and non-motor symptoms. A major challenge for understanding susceptibility to PD is to determine the genetic and environmental factors that influence the mechanisms underlying the variations in disease-associated traits. The pathological hallmark of PD is the degeneration of dopaminergic neurons in the substantia nigra pars compacta region of the brain and post-mortem Lewy pathology, which leads to the loss of projecting axons innervating the striatum and to impaired motor and cognitive functions. While the cause of PD is still largely unknown, genome-wide association studies provide evidence that numerous polymorphic variants in various genes contribute to sporadic PD, and 10 to 15% of all cases are linked to some form of hereditary mutations, either autosomal dominant or recessive. Among the most common mutations observed in PD patients are in the genes LRRK2, SNCA, GBA1, PINK1, PRKN, and PARK7/DJ-1. In this review, we cover these PD-related mutations, the use of induced pluripotent stem cells as a disease in a dish model, and genetic animal models to better understand the diversity in the pathogenesis and long-term outcomes seen in PD patients.

Genetics, molecular control and clinical relevance of habituation learning

Habituation is the most fundamental form of learning. As a firewall that protects our brain from sensory overload, it is indispensable for cognitive processes. Studies in humans and animal models provide increasing evidence that habituation is affected in autism and related monogenic neurodevelopmental disorders (NDDs). An integrated application of habituation assessment in NDDs and their animal models has unexploited potential for neuroscience and medical care. With the aim to gain mechanistic insights, we systematically retrieved genes that have been demonstrated in the literature to underlie habituation. We identified 258 evolutionarily conserved genes across species, describe the biological processes they converge on, and highlight regulatory pathways and drugs that may alleviate habituation deficits. We also summarize current habituation paradigms and extract the most decisive arguments that support the crucial role of habituation for cognition in health and disease. We conclude that habituation is a conserved, quantitative, cognition- and disease-relevant process that can connect preclinical and clinical work, and hence is a powerful tool to advance research, diagnostics, and treatment of NDDs.

Failure of Diphtheria Toxin Model to Induce Parkinson-Like Behavior in Mice

Rodent models of Parkinson’s disease are based on transgenic expression of mutant synuclein, deletion of PD genes, injections of MPTP or rotenone, or seeding of synuclein fibrils. The models show histopathologic features of PD such as Lewi bodies but mostly only subtle in vivo manifestations or systemic toxicity. The models only partly mimic a predominant loss of dopaminergic neurons in the substantia nigra. We therefore generated mice that express the transgenic diphtheria toxin receptor (DTR) specifically in DA neurons by crossing DAT-Cre mice with Rosa26 loxP-STOP-loxP DTR mice. After defining a well-tolerated DTx dose, DAT-DTR and DTR-flfl controls were subjected to non-toxic DTx treatment (5 × 100 pg/g) and subsequent histology and behavioral tests. DAT protein levels were reduced in the midbrain, and tyrosine hydroxylase-positive neurons were reduced in the substantia nigra, whereas the pan-neuronal marker NeuN was not affected. Despite the promising histologic results, there was no difference in motor function tests or open field behavior. These are tests in which double mutant Pink1^−/−SNCA^A53T Parkinson mice show behavioral abnormalities. Higher doses of DTx were toxic in both groups. The data suggest that DTx treatment in mice with Cre/loxP-driven DAT-DTR expression leads to partial ablation of DA-neurons but without PD-reminiscent behavioral correlates.

Neuropsychiatric and Cognitive Deficits in Parkinson's Disease and Their Modeling in Rodents

Parkinson’s disease (PD) is associated with a large burden of non-motor symptoms including olfactory and autonomic dysfunction, as well as neuropsychiatric (depression, anxiety, apathy) and cognitive disorders (executive dysfunctions, memory and learning impairments). Some of these non-motor symptoms may precede the onset of motor symptoms by several years, and they significantly worsen during the course of the disease. The lack of systematic improvement of these non-motor features by dopamine replacement therapy underlines their multifactorial origin, with an involvement of monoaminergic and cholinergic systems, as well as alpha-synuclein pathology in frontal and limbic cortical circuits. Here we describe mood and neuropsychiatric disorders in PD and review their occurrence in rodent models of PD. Altogether, toxin-based rodent models of PD indicate a significant but non-exclusive contribution of mesencephalic dopaminergic loss in anxiety, apathy, and depressive-like behaviors, as well as in learning and memory deficits. Gene-based models display significant deficits in learning and memory, as well as executive functions, highlighting the contribution of alpha-synuclein pathology to these non-motor deficits. Collectively, neuropsychiatric and cognitive deficits are recapitulated to some extent in rodent models, providing partial but nevertheless useful options to understand the pathophysiology of non-motor symptoms and develop therapeutic options for these debilitating symptoms of PD.

Parkin regulates drug-taking behavior in rat model of methamphetamine use disorder

There is no FDA-approved medication for methamphetamine (METH) use disorder. New therapeutic approaches are needed, especially for people who use METH heavily and are at high risk for overdose. This study used genetically engineered rats to evaluate PARKIN as a potential target for METH use disorder. PARKIN knockout, PARKIN-overexpressing, and wild-type young adult male Long Evans rats were trained to self-administer high doses of METH using an extended-access METH self-administration paradigm. Reinforcing/rewarding properties of METH were assessed by quantifying drug-taking behavior and time spent in a METH-paired environment. PARKIN knockout rats self-administered more METH and spent more time in the METH-paired environment than wild-type rats. Wild-type rats overexpressing PARKIN self-administered less METH and spent less time in the METH-paired environment. PARKIN knockout rats overexpressing PARKIN self-administered less METH during the first half of drug self-administration days than PARKIN-deficient rats. The results indicate that rats with PARKIN excess or PARKIN deficit are useful models for studying neural substrates underlying "resilience" or vulnerability to METH use disorder and identify PARKIN as a novel potential drug target to treat heavy use of METH.

Multitasking guardian of mitochondrial quality: Parkin function and Parkinson's disease

The familial form of Parkinson’s disease (PD) is linked to mutations in specific genes. The mutations in parkin are one of the most common causes of early-onset PD. Mitochondrial dysfunction is an emerging active player in the pathology of neurodegenerative diseases, because mitochondria are highly dynamic structures integrated with many cellular functions. Herein, we overview and discuss the role of the parkin protein product, Parkin E3 ubiquitin ligase, in the cellular processes related to mitochondrial function, and how parkin mutations can result in pathology in vitro and in vivo.

Mechanisms for sperm mitochondrial removal in embryos

BACKGROUND

Different animal species have different characteristics regarding the transmission of mitochondrial DNA. While some species have biparental mitochondrial inheritance, others have developed pathways to remove paternal mtDNA. These pathways guarantee the uniparental mitochondrial inheritance, so far well known in mammals, avoiding heteroplasmy, which may have the potential to cause certain mitochondrial diseases in the offspring.

SCOPE OF REVIEW

This review aims to address the main mechanisms that involve mitochondrial degradation in different animal species, as well as to describe what is present in the literature on the mechanisms involved in mitochondrial inheritance.

MAJOR CONCLUSIONS

Two theories are proposed to explain the uniparental inheritance of mtDNA: (i) active degradation, where mechanisms for paternal mitochondrial DNA elimination involve mitochondrial degradation pathway by autophagy and, in some species, may also involve the endocytic degradation pathway; and (ii) passive dilution, where the paternal mitochondria are diluted in the cells of the embryo according to cell division, until becoming undetectable.

GENERAL SIGNIFICANCE

This work brings a wide review of the already published evidence on mitochondrial inheritance in the animal kingdom and the possible mechanisms to mtDNA transmission already described in literature.

Characterization of a Cul9-Parkin double knockout mouse model for Parkinson's disease

Mitochondrial quality control is essential for the long-term survival of postmitotic neurons. The E3 ubiquitin ligase Parkin promotes the degradation of damaged mitochondria via mitophagy and mutations in Parkin are a major cause of early-onset Parkinson’s disease (PD). Surprisingly however, mice deleted for Parkin alone are rather asymptomatic for PD-related pathology, suggesting that other complementary or redundant mitochondrial quality control pathways may exist in neurons. Mitochondrial damage is often accompanied by the release of toxic proteins such as cytochrome c. We have reported that once in the cytosol, cytochrome c is targeted for degradation by the E3 ligase CUL9 in neurons. Here we examined whether CUL9 and Parkin cooperate to promote optimal neuronal survival in vivo. We generated mice deficient for both Cul9 and Parkin and examined them for PD-related phenotypes. Specifically, we conducted assays to examine behavioural deficits (locomotor, sensory, memory and learning) and loss of dopaminergic neurons in both males and females. Our results show that the loss of Cul9 and Parkin together did not enhance the effect of Parkin deficiency alone. These results indicate that while both Parkin and CUL9 participate in mitochondrial quality control, neurons likely have multiple redundant mechanisms to ensure their long-term survival.

Kv7.2 subunit-containing M-type potassium channels in the lateral habenula are involved in the regulation of working memory in parkinsonian rats

Although the lateral habenula (LHb) is involved in the regulation of multiple brain functions and this region expresses abundant M-type potassium channel (M-channel) subunits Kv7.2 and Kv7.3, the role of M-channels in regulating working memory is unclear, particularly in Parkinson's disease (PD). Here we tested the effects of activation and blockade of LHb M-channels on working memory by the T-maze rewarded alternation test in rats with unilateral 6-hydroxydopamine lesions of the substantia nigra compacta (SNc). The SNc lesion induced working memory impairment, increased the firing rate of LHb neurons, decreased dopamine (DA) level in the ventral medial prefrontal cortex (vmPFC) and reduced the expression of Kv7.2 subunit in the LHb. Intra-LHb injection of M-channel activator retigabine induced enhancement of working memory in SNc sham-lesioned and SNc-lesioned rats; conversely, the injection of M-channel blocker XE-991 impaired working memory in the two groups of rats. However, doses producing significant effects in SNc-lesioned rats were higher than those in SNc sham-lesioned rats. Further, intra-LHb injection of retigabine decreased the firing rate of LHb neurons and increased release of DA and serotonin (5-HT) in the vmPFC, while XE-991 increased the firing rate and decreased DA and 5-HT release in the two groups of rats. Compared with SNc sham-lesioned rats, the duration of M-channel activation and blockade action on the firing rate of the neurons and release of DA and 5-HT was significantly shortened in SNc-lesioned rats, which was consistent with reduced expression of Kv7.2 subunit in the LHb after lesioning the SNc. Collectively, these findings suggest involvement of LHb Kv7.2 subunit-containing M-channels in working memory impairment in SNc-lesioned rats, and that enhanced or impaired working memory after activation or blockade of M-channels in the LHb is related to the changes in the firing activity of LHb neurons and DA and 5-HT release in the vmPFC.

Neuropsychiatric Disorders in Parkinson's Disease: What Do We Know About the Role of Dopaminergic and Non-dopaminergic Systems?

Besides the hallmark motor symptoms (rest tremor, hypokinesia, rigidity, and postural instability), patients with Parkinson’s disease (PD) have non-motor symptoms, namely neuropsychiatric disorders. They are frequent and may influence the other symptoms of the disease. They have also a negative impact on the quality of life of patients and their caregivers. In this article, we will describe the clinical manifestations of the main PD-related behavioral disorders (depression, anxiety disorders, apathy, psychosis, and impulse control disorders). We will also provide an overview of the clinical and preclinical literature regarding the underlying mechanisms with a focus on the role of the dopaminergic and non-dopaminergic systems.

Behavioural phenotypes in the cuprizone model of central nervous system demyelination

The feeding of cuprizone (CPZ) to animals has been extensively used to model the processes of demyelination and remyelination, with many papers adopting a narrative linked to demyelinating conditions like multiple sclerosis (MS), the aetiology of which is unknown. However, no current animal model faithfully replicates the myriad of symptoms seen in the clinical condition of MS. CPZ ingestion causes mitochondrial and endoplasmic reticulum stress and subsequent apoptosis of oligodendrocytes leads to central nervous system demyelination and glial cell activation. Although there are a wide variety of behavioural tests available for characterizing the functional deficits in animal models of disease, including that of CPZ-induced deficits, they have focused on a narrow subset of outcomes such as motor performance, cognition, and anxiety. The literature has not been systematically reviewed in relation to these or other symptoms associated with clinical MS. This paper reviews these tests and makes recommendations as to which are the most important in order to better understand the role of this model in examining aspects of demyelinating diseases like MS.

Sensory neuropathy and nociception in rodent models of Parkinson's disease

Parkinson's disease (PD) often manifests with prodromal pain and sensory losses whose etiologies are not well understood. Multiple genetic and toxicity-based rodent models of PD partly recapitulate the histopathology and motor function deficits. Although far less studied, there is some evidence that rodents, similar to humans, develop sensory manifestations of the disease, which may precede motor disturbances and help to elucidate the underlying mechanisms of PD-associated pain at the molecular and neuron circuit levels. The present Review summarizes nociception and other sensory functions in frequently used rodent PD models within the context of the complex phenotypes. In terms of mechanisms, it appears that the acute loss of dopaminergic neurons in systemic toxicity models (MPTP, rotenone) primarily causes nociceptive hyperexcitability, presumably owing to a loss of inhibitory control, whereas genetic models primarily result in a progressive loss of heat perception, reflecting sensory fiber neuropathies. At the molecular level, neither α-synuclein deposits alone nor failure of mitophagy alone appear to be strong enough to result in axonal or synaptic pathology of nociceptive neurons that manifest at the behavioral level, and peripheral sensory loss may mask central ‘pain’ in behavioral tests. Hence, allostatic combinations or additional challenges and novel behavioral assessments are needed to better evaluate PD-associated sensory neuropathies and pain in rodents.

Summary: Rodent models of Parkinson's disease partially develop prodromal somatosensory and olfactory dysfunctions reminiscent of sensory neuropathies in patients and reveal mechanistic insight, but data are incomplete and fragmented.

Cross-species models of attention-deficit/hyperactivity disorder and autism spectrum disorder: lessons from CNTNAP2, ADGRL3, and PARK2

Animal and cellular models are essential tools for all areas of biological research including neuroscience. Model systems can also be used to investigate the pathophysiology of psychiatric disorders such as attention-deficit/hyperactivity disorder (ADHD) and autism spectrum disorder (ASD). In this review, we provide a summary of animal and cellular models for three genes linked to ADHD and ASD in human patients - CNTNAP2, ADGRL3, and PARK2. We also highlight the strengths and weaknesses of each model system. By bringing together behavioral and neurobiological data, we demonstrate how a cross-species approach can provide integrated insights into gene function and the pathogenesis of ADHD and ASD. The knowledge gained from transgenic models will be essential to discover and validate new treatment targets for these disorders.

Increased anxiety-like behavior following circuit-specific catecholamine denervation in mice

Parkinson's disease (PD) presents with a constellation of non-motor symptoms, notably increased anxiety, which are currently poorly treated and underrepresented in animal models of the disease. Human post-mortem studies report loss of catecholaminergic neurons in the pre-symptomatic phases of PD when anxiety symptoms emerge, and a large literature from rodent and human studies indicate that catecholamines are important mediators of anxiety via their modulatory effects on limbic regions such as the amygdala. On the basis of these observations, we hypothesized that anxiety in PD could result from an early loss of catecholaminergic inputs to the amygdala and/or other limbic structures. To interrogate this hypothesis, we bilaterally injected the neurotoxin 6-OHDA in the mouse basolateral amygdala (BL). This produced a restricted pattern of catecholaminergic (tyrosine-hydroxylase-labeled) denervation in the BL, intercalated cell masses and ventral hippocampus, but not the central amygdala or prefrontal cortex. We found that this circuit-specific lesion did not compromise performance on multiple measures of motor function (home cage, accelerating rotarod, beam balance, pole climbing), but did increase anxiety-like behavior in the elevated plus-maze and light-dark exploration tests. Fear behavior in the pavlovian cued conditioning and passive avoidance assays was, by contrast, unaffected; possibly due to preservation of catecholamine innervation of the central amygdala from the periaqueductal gray. These data provide some of the first evidence implicating loss of catecholaminergic neurotransmission in midbrain-amygdala circuits to increased anxiety-like behavior. Our findings offer an initial step towards identifying the neural substrates for pre-motor anxiety symptoms in PD.

NCX1 and NCX3 as potential factors contributing to neurodegeneration and neuroinflammation in the A53T transgenic mouse model of Parkinson's Disease

Na⁺-Ca²⁺ exchanger (NCX) isoforms constitute the major cellular Ca²⁺ extruding system in neurons and microglia. We herein investigated the role of NCX isoforms in the pathophysiology of Parkinson's disease (PD). Their expression and activity were evaluated in neurons and glia of mice expressing the human A53T variant of α-synuclein (A53T mice), an animal model mimicking a familial form of PD. Western blotting revealed that NCX3 expression in the midbrain of 12-month old A53T mice was lower than that of wild type (WT). Conversely, NCX1 expression increased in the striatum. Immunohistochemical studies showed that glial fibrillary acidic protein (GFAP)-positive astroglial cells significantly increased in the substantia nigra pars compacta (SNc) and in the striatum. However, the number and the density of tyrosine hydroxylase (TH)-positive neurons decreased in both brain regions. Interestingly, ionized calcium binding adaptor molecule 1 (IBA-1)-positive microglial cells increased only in the striatum of A53T mice compared to WT. Double immunostaining studies showed that in A53T mice, NCX1 was exclusively co-expressed in IBA-1-positive microglial cells in the striatum, whereas NCX3 was solely co-expressed in TH-positive neurons in SNc. Beam walking and pole tests revealed a reduction in motor performance for A53T mice compared to WT. In vitro experiments in midbrain neurons from A53T and WT mice demonstrated a reduction in NCX3 expression, which was accompanied by mitochondrial overload of Ca²⁺ ions, monitored with confocal microscopy by X-Rhod-1 fluorescent dye. Collectively, in vivo and in vitro findings suggest that the reduction in NCX3 expression and activity in A53T neurons from midbrain may cause mitochondrial dysfunction and neuronal death in this brain area, whereas NCX1 overexpression in microglial cells may promote their proliferation in the striatum.

Characterization of Dopaminergic System in the Striatum of Young Adult Park2-/- Knockout Rats

Mutations in parkin gene (Park2) are linked to early-onset autosomal recessive Parkinson's disease (PD) and young-onset sporadic PD. Park2 knockout (PKO) rodents; however, do not display neurodegeneration of the nigrostriatal pathway, suggesting age-dependent compensatory changes. Our goal was to examine dopaminergic (DAergic) system in the striatum of 2 month-old PKO rats in order to characterize compensatory mechanisms that may have occurred within the system. The striata form wild type (WT) and PKO Long Evans male rats were assessed for the levels of DAergic markers, for monoamine oxidase (MAO) A and B activities and levels, and for the levels of their respective preferred substrates, serotonin (5-HT) and ß-phenylethylamine (ß-PEA). The PKO rats displayed lower activities of MAOs and higher levels of ß-PEA in the striatum than their WT counterparts. Decreased levels of ß-PEA receptor, trace amine-associated receptor 1 (TAAR-1), and postsynaptic DA D2 (D2L) receptor accompanied these alterations. Drug-naive PKO rats displayed normal locomotor activity; however, they displayed decreased locomotor response to a low dose of psychostimulant methamphetamine, suggesting altered DAergic neurotransmission in the striatum when challenged with an indirect agonist. Altogether, our findings suggest that 2 month-old PKO male rats have altered DAergic and trace aminergic signaling.

Parkinson's disease: experimental models and reality

Parkinson's disease (PD) is a chronic, progressive movement disorder of adults and the second most common neurodegenerative disease after Alzheimer's disease. Neuropathologic diagnosis of PD requires moderate-to-marked neuronal loss in the ventrolateral substantia nigra pars compacta and α-synuclein (αS) Lewy body pathology. Nigrostriatal dopaminergic neurodegeneration correlates with the Parkinsonian motor features, but involvement of other peripheral and central nervous system regions leads to a wide range of non-motor features. Nigrostriatal dopaminergic neurodegeneration is shared with other parkinsonian disorders, including some genetic forms of parkinsonism, but many of these disorders do not have Lewy bodies. An ideal animal model for PD, therefore, should exhibit age-dependent and progressive dopaminergic neurodegeneration, motor dysfunction, and abnormal αS pathology. Rodent models of PD using genetic or toxin based strategies have been widely used in the past several decades to investigate the pathogenesis and therapeutics of PD, but few recapitulate all the major clinical and pathologic features of PD. It is likely that new strategies or better understanding of fundamental disease processes may facilitate development of better animal models. In this review, we highlight progress in generating rodent models of PD based on impairments of four major cellular functions: mitochondrial oxidative phosphorylation, autophagy-lysosomal metabolism, ubiquitin-proteasome protein degradation, and endoplasmic reticulum stress/unfolded protein response. We attempt to evaluate how impairment of these major cellular systems contribute to PD and how they can be exploited in rodent models. In addition, we review recent cell biological studies suggesting a link between αS aggregation and impairment of nuclear membrane integrity, as observed during cellular models of apoptosis. We also briefly discuss the role of incompetent phagocytic clearance and how this may be a factor to consider in developing new rodent models of PD.

Orphan G protein-coupled receptors: The role in CNS disorders

There are various types of receptors in the central nervous system (CNS). G protein-coupled receptors (GPCRs) have the highest expression with a wide range of physiological functions. A newer sub group of these receptors namely orphan GPCRs have been discovered. GPR3, GPR6, GPR17, GPR26, GPR37, GPR39, GPR40, GPR50, GPR52, GPR54, GPR55, GPR85, GPR88, GPR103, and GPR139 are the selected orphan GPCRs for this article. Their roles in the central nervous system have not been understood well so far. However, recent studies show that they may have very important functions in the CNS. Hence, in the present study, we reviewed most recent findings regarding the physiological roles of the selected orphan GPCRs in the CNS. After a brief presentation of each receptor, considering the results from genetic and pharmacological manipulation of the receptors, their roles in the pathophysiology of different diseases and disorders including anxiety, depression, schizophrenia, epilepsy, Alzheimer's disease, Parkinson's disease, and substance abuse will be discussed. At present, our knowledge regarding the role of GPCRs in the brain is very limited. However, previous limited studies show that orphan GPCRs have an important place in psychopharmacology and these receptors are potential new targets for the treatment of major CNS diseases.

Nonmotor Symptoms in Experimental Models of Parkinson's Disease

Nonmotor symptoms of Parkinson's disease (PD) range from neuropsychiatric, cognitive to sleep and sensory disorders and can arise from the disease process as well as from drug treatment. The clinical heterogeneity of nonmotor symptoms of PD is underpinned by a wide range of neuropathological and molecular pathology, affecting almost the entire range of neurotransmitters present in brain and the periphery. Understanding the neurobiology and pathology of nonmotor symptoms is crucial to the effective treatment of PD and currently a key unmet need. This bench-to-bedside translational concept can only be successful if robust animal models of PD charting the genesis and natural history of nonmotor symptoms can be devised. Toxin-based and transgenic rodent and primate models of PD have given us important clues to the underlying basis of motor symptomatology and in addition, can provide a snapshot of some nonmotor aspects of PD, although the data are far from complete. In this chapter, we discuss some of the nonmotor aspects of the available experimental models of PD and how the development of robust animal models to understand and treat nonmotor symptoms needs to become a research priority.

Nonmotor Parkinson's and Future Directions

Nonmotor symptoms (NMS) of Parkinson's disease (PD) are integral to the condition largely regarded as a motor syndrome. A range of NMS underpin the prodromal stage of Parkinson's and are present with variable frequency, range, and nature across the whole journey of a patient with Parkinson's from the onset of the motor disease to palliative stage. These symptoms also are key determinants of quality of life of the patient as well as the carer. Despite this, recognition management and focused treatment of NMS of PD remain poor. Future would, therefore, need to focus on better definition and management of NMS of PD. This would include development of robust animal models of specific NMS such as cognitive, sleep, and autonomic dysfunctions as well as pain to understand the mechanistic pathways of these symptoms. In turn this will lead to better drug development using a bench to bedside model. Nonmotor clinical subtypes of PD have also been described and, in future, proper biomarkers will consolidate these findings in addition to defining the natural history of the subtypes. Revised versions of established scales and questionnaires will enable the adoption of good clinical practice with recognition of these subtypes in clinic. This will enhance the delivery of true subtype-specific therapies. Drug development should also include nondopaminergic and cell replacement restorative therapies with a nonmotor focus. An additional key area of future research would be the formalizing of true personalized medicine for PD. Personalized medicine pathways should concentrate on the role of exercise, complementary medicine as well as age, body weight, ethnicity on various NMS of PD. Genetics and pharmacogenetic developments in PD will add to the precision of the individualized approach.

Non-motor features of Parkinson disease

Many of the motor symptoms of Parkinson disease (PD) can be preceded, sometimes for several years, by non-motor symptoms that include hyposmia, sleep disorders, depression and constipation. These non-motor features appear across the spectrum of patients with PD, including individuals with genetic causes of PD. The neuroanatomical and neuropharmacological bases of non-motor abnormalities in PD remain largely undefined. Here, we discuss recent advances that have helped to establish the presence, severity and effect on the quality of life of non-motor symptoms in PD, and the neuroanatomical and neuropharmacological mechanisms involved. We also discuss the potential for the non-motor features to define a prodrome that may enable the early diagnosis of PD.

Comparison of the spatial-cognitive functions of dorsomedial striatum and anterior cingulate cortex in mice

Neurons in anterior cingulate cortex (aCC) project to dorsomedial striatum (DMS) as part of a corticostriatal circuit with putative roles in learning and other cognitive functions. In the present study, the spatial-cognitive importance of aCC and DMS was assessed in the hidden-platform version of the Morris water maze (MWM). Brain lesion experiments that focused on areas of connectivity between these regions indicated their involvement in spatial cognition. MWM learning curves were markedly delayed in DMS-lesioned mice in the absence of other major functional impairments, whereas there was a more subtle, but still significant influence of aCC lesions. Lesioned mice displayed impaired abilities to use spatial search strategies, increased thigmotaxic swimming, and decreased searching in the proximity of the escape platform. Additionally, aCC and DMS activity was compared in mice between the early acquisition phase (2 and 3 days of training) and the over-trained high-proficiency phase (after 30 days of training). Neuroplasticity-related expression of the immediate early gene Arc implicated both regions during the goal-directed, early phases of spatial learning. These results suggest the functional involvement of aCC and DMS in processes of spatial cognition that model associative cortex-dependent, human episodic memory abilities.

Sterile Inflammation and Degradation Systems in Heart Failure

In most patients with chronic heart failure (HF), levels of circulating cytokines are elevated and the elevated cytokine levels correlate with the severity of HF and prognosis. Various stresses induce subcellular component abnormalities, such as mitochondrial damage. Damaged mitochondria induce accumulation of reactive oxygen species and apoptogenic proteins, and subcellular inflammation. The vicious cycle of subcellular component abnormalities, inflammatory cell infiltration and neurohumoral activation induces cardiomyocyte injury and death, and cardiac fibrosis, resulting in cardiac dysfunction and HF. Quality control mechanisms at both the protein and organelle levels, such as elimination of apoptogenic proteins and damaged mitochondria, maintain cellular homeostasis. An imbalance between protein synthesis and degradation is likely to result in cellular dysfunction and disease. Three major protein degradation systems have been identified, namely the cysteine protease system, autophagy, and the ubiquitin proteasome system. Autophagy was initially believed to be a non-selective process. However, recent studies have described the process of selective mitochondrial autophagy, known as mitophagy. Elimination of damaged mitochondria by autophagy is important for maintenance of cellular homeostasis. DNA and RNA degradation systems also play a critical role in regulating inflammation and maintaining cellular homeostasis mediated by damaged DNA clearance and post-transcriptional regulation, respectively. This review discusses some recent advances in understanding the role of sterile inflammation and degradation systems in HF.

The Nonmotor Features of Parkinson's Disease

Nonmotor symptoms (NMS) of Parkinson's disease (PD) were recognized by the great James Parkinson himself who mentioned symptoms such as sleep dysfunction, delirium, dementia, and dysautonomia, in his seminal 1817 essay, "An Essay on the Shaking Palsy" (Parkinson, 1817). In spite of the key impact of PD NMS on quality of life, there was little holistic research and awareness till the validation and use of comprehensive tools such as the NMS questionnaire, scale, and the revised version of the unified PD rating scale. Research studies using these tools highlighted the key impact of the burden of NMS on quality of life of PD patients and the need for NMS to be routinely assessed in clinic. We now define PD as a motor and nonmotor disorder, and the natural history includes a long prodromal phase of PD dominated by a range of NMS. The prodromal phase is the subject of much research particularly in relation to neuroprotection and identifying subjects at risk. Use of NMS tools has also validated burden grading of NMS with cutoff values, which can be used as outcome measure in clinical trials. Finally, the complex multineurotransmitter dysfunction that is seen in PD has been shown to manifest clinically as nonmotor subtypes. Recognition of such subtypes is likely to lead to the emergence of personalized and precision medicine in PD.

Neuropathic pain promotes adaptive changes in gene expression in brain networks involved in stress and depression

Neuropathic pain is a complex chronic condition characterized by various sensory, cognitive, and affective symptoms. A large percentage of patients with neuropathic pain are also afflicted with depression and anxiety disorders, a pattern that is also seen in animal models. Furthermore, clinical and preclinical studies indicate that chronic pain corresponds with adaptations in several brain networks involved in mood, motivation, and reward. Chronic stress is also a major risk factor for depression. We investigated whether chronic pain and stress affect similar molecular mechanisms and whether chronic pain can affect gene expression patterns that are involved in depression. Using two mouse models of neuropathic pain and depression [spared nerve injury (SNI) and chronic unpredictable stress (CUS)], we performed next-generation RNA sequencing and pathway analysis to monitor changes in gene expression in the nucleus accumbens (NAc), the medial prefrontal cortex (mPFC), and the periaqueductal gray (PAG). In addition to finding unique transcriptome profiles across these regions, we identified a substantial number of signaling pathway-associated genes with similar changes in expression in both SNI and CUS mice. Many of these genes have been implicated in depression, anxiety, and chronic pain in patients. Our study provides a resource of the changes in gene expression induced by long-term neuropathic pain in three distinct brain regions and reveals molecular connections between pain and chronic stress.

Investigation of diazepam efficacy on anxiety-like behavior in hemiparkinsonian rats

There is growing recognition that anxiety disorders have a greater impact on quality of life in Parkinson's disease than motor symptoms. Yet, little is known about the pathophysiology underlying this non-motor symptom in Parkinson's disease which poses a considerable barrier in developing effective treatment strategies. Here, we administered diazepam to hemiparkinsonian and non-parkinsonian rats and assessed its efficacy in three anxiety behavioral tests. At present, no information about this exists in preclinical research with sparse data in the clinical literature. Moreover, diazepam is an acute anxiolytic which makes this drug a suitable research tool to unmask differences in anxiety-like behavior. Using the unilateral, medial forebrain bundle 6-hydroxydopamine rat model of Parkinson's disease, we noted that hemiparkinsonian rats had more baseline anxiety-like behavior with 60% of them exhibiting high anxiety (HA) behavior in the elevated plus maze. In contrast, 41% of sham-lesioned rats and 8% of naïve rats exhibited HA behavior. Next, we employed the elevated plus maze and noted that diazepam (1.5mg/kg) was anxiolytic in low anxiety (LA) sham-lesioned (p=0.006) and HA sham-lesioned rats (p=0.016). Interestingly, diazepam was anxiolytic for LA hemiparkinsonian rats (p=0.017), but not for HA hemiparkinsonian rats (p=0.174) despite both groups having similar motor impairment and parkinsonian phenotype. Overall, diazepam administration unmasked differences in anxiolytic efficacy between HA hemiparkinsonian rats, LA hemiparkinsonian rats and non-parkinsonian rats. Our data suggests that neuro-circuits involved in anxiety-like behavior may differ within these groups and posits that diazepam may have reduced efficacy in certain individuals with PD anxiety disorders.

Inability to acquire spatial information and deploy spatial search strategies in mice with lesions in dorsomedial striatum

Dorsal striatum has been shown to contribute to spatial learning and memory, but the role of striatal subregions in this important aspect of cognitive functioning remains unclear. Moreover, the spatial-cognitive mechanisms that underlie the involvement of these regions in spatial navigation have scarcely been studied. We therefore compared spatial learning and memory performance in mice with lesions in dorsomedial (DMS) and dorsolateral striatum (DLS) using the hidden-platform version of the Morris water maze (MWM) task. Compared to sham-operated controls, animals with DMS damage were impaired during MWM acquisition training. These mice displayed delayed spatial learning, increased thigmotaxis, and increased search distance to the platform, in the absence of major motor dysfunction, working memory defects or changes in anxiety or exploration. They failed to show a preference for the target quadrant during probe trials, which further indicates that spatial reference memory was impaired in these animals. Search strategy analysis moreover demonstrated that DMS-lesioned mice were unable to deploy cognitively advanced spatial search strategies. Conversely, MWM performance was barely affected in animals with lesions in DLS. In conclusion, our results indicate that DMS and DLS display differential functional involvement in spatial learning and memory. Our results show that DMS, but not DLS, is crucial for the ability of mice to acquire spatial information and their subsequent deployment of spatial search strategies. These data clearly identify DMS as a crucial brain structure for spatial learning and memory, which could explain the occurrence of neurocognitive impairments in brain disorders that affect the dorsal striatum.

Annual research review: Rare genotypes and childhood psychopathology--uncovering diverse developmental mechanisms of ADHD risk

BACKGROUND

Through the increased availability and sophistication of genetic testing, it is now possible to identify causal diagnoses in a growing proportion of children with neurodevelopmental disorders. In addition to developmental delay and intellectual disability, many genetic disorders are associated with high risks of psychopathology, which curtail the wellbeing of affected individuals and their families. Beyond the identification of significant clinical needs, understanding the diverse pathways from rare genetic mutations to cognitive dysfunction and emotional-behavioural disturbance has theoretical and practical utility.

METHODS

We overview (based on a strategic search of the literature) the state-of-the-art on causal mechanisms leading to one of the most common childhood behavioural diagnoses - attention deficit hyperactivity disorder (ADHD) - in the context of specific genetic disorders. We focus on new insights emerging from the mapping of causal pathways from identified genetic differences to neuronal biology, brain abnormalities, cognitive processing differences and ultimately behavioural symptoms of ADHD.

FINDINGS

First, ADHD research in the context of rare genotypes highlights the complexity of multilevel mechanisms contributing to psychopathology risk. Second, comparisons between genetic disorders associated with similar psychopathology risks can elucidate convergent or distinct mechanisms at each level of analysis, which may inform therapeutic interventions and prognosis. Third, genetic disorders provide an unparalleled opportunity to observe dynamic developmental interactions between neurocognitive risk and behavioural symptoms. Fourth, variation in expression of psychopathology risk within each genetic disorder points to putative moderating and protective factors within the genome and the environment.

CONCLUSION

A common imperative emerging within psychopathology research is the need to investigate mechanistically how developmental trajectories converge or diverge between and within genotype-defined groups. Crucially, as genetic predispositions modify interaction dynamics from the outset, longitudinal research is required to understand the multi-level developmental processes that mediate symptom evolution.

The neurobiological basis of cognitive impairment in Parkinson's disease

The recent formalization of clinical criteria for Parkinson's disease with dementia (PDD) codifies many studies on this topic, including those assessing biological correlates. These studies show that the emergence of PDD occurs on the background of severe dopamine deficits with, the main pathological drivers of cognitive decline being a synergistic effect between alpha-synuclein and Alzheimer's disease pathology. The presence of these pathologies correlates with a marked loss of limbic and cortically projecting dopamine, noradrenaline, serotonin, and acetylcholine neurons, although the exact timing of these relationships remains to be determined. Genetic factors, such as triplications in the α-synuclein gene, lead to a clear increased risk of PDD, whereas others, such as parkin mutations, are associated with a reduced risk of PDD. The very recent formalization of clinical criteria for PD with mild cognitive impairment (PD-MCI) allows only speculation on its biological and genetic bases. Critical assessment of animal models shows that chronic low-dose MPTP treatment in primates recapitulates PD-MCI over time, enhancing the current biological concept of PD-MCI as having enhanced dopamine deficiency in frontostriatal pathways as well as involvement of other neurotransmitter systems. Data from other animal models support multiple transmitter involvement in cognitive impairment in PD. Whereas dopamine dysfunction has been highlighted because of its obvious role in PD, the role of the other neurotransmitter systems, neurodegenerative pathologies, and genetic factors in PD-MCI remains to be fully elucidated.

Parkinson's disease: animal models and dopaminergic cell vulnerability

Parkinson's disease (PD) is a neurodegenerative disorder that affects about 1.5% of the global population over 65 years of age. A hallmark feature of PD is the degeneration of the dopamine (DA) neurons in the substantia nigra pars compacta (SNc) and the consequent striatal DA deficiency. Yet, the pathogenesis of PD remains unclear. Despite tremendous growth in recent years in our knowledge of the molecular basis of PD and the molecular pathways of cell death, important questions remain, such as: (1) why are SNc cells especially vulnerable; (2) which mechanisms underlie progressive SNc cell loss; and (3) what do Lewy bodies or α-synuclein reveal about disease progression. Understanding the variable vulnerability of the dopaminergic neurons from the midbrain and the mechanisms whereby pathology becomes widespread are some of the primary objectives of research in PD. Animal models are the best tools to study the pathogenesis of PD. The identification of PD-related genes has led to the development of genetic PD models as an alternative to the classical toxin-based ones, but does the dopaminergic neuronal loss in actual animal models adequately recapitulate that of the human disease? The selection of a particular animal model is very important for the specific goals of the different experiments. In this review, we provide a summary of our current knowledge about the different in vivo models of PD that are used in relation to the vulnerability of the dopaminergic neurons in the midbrain in the pathogenesis of PD.

Behavioral phenotyping of Parkin-deficient mice: looking for early preclinical features of Parkinson's disease

There is considerable evidence showing that the neurodegenerative processes that lead to sporadic Parkinson's disease (PD) begin many years before the appearance of the characteristic motor symptoms. Neuropsychiatric, sensorial and cognitive deficits are recognized as early non-motor manifestations of PD, and are not attenuated by the current anti-parkinsonian therapy. Although loss-of-function mutations in the parkin gene cause early-onset familial PD, Parkin-deficient mice do not display spontaneous degeneration of the nigrostriatal pathway or enhanced vulnerability to dopaminergic neurotoxins such as 6-OHDA and MPTP. Here, we employed adult homozygous C57BL/6 mice with parkin gene deletion on exon 3 (parkin^−/−) to further investigate the relevance of Parkin in the regulation of non-motor features, namely olfactory, emotional, cognitive and hippocampal synaptic plasticity. Parkin^−/− mice displayed normal performance on behavioral tests evaluating olfaction (olfactory discrimination), anxiety (elevated plus-maze), depressive-like behavior (forced swimming and tail suspension) and motor function (rotarod, grasping strength and pole). However, parkin^−/− mice displayed a poor performance in the open field habituation, object location and modified Y-maze tasks suggestive of procedural and short-term spatial memory deficits. These behavioral impairments were accompanied by impaired hippocampal long-term potentiation (LTP). These findings indicate that the genetic deletion of parkin causes deficiencies in hippocampal synaptic plasticity, resulting in memory deficits with no major olfactory, emotional or motor impairments. Therefore, parkin^−/− mice may represent a promising animal model to study the early stages of PD and for testing new therapeutic strategies to restore learning and memory and synaptic plasticity impairments in PD.

Parkin loss of function contributes to RTP801 elevation and neurodegeneration in Parkinson's disease

Mutations in the PARK2 gene are associated with an autosomal recessive form of juvenile parkinsonism (AR-JP). These mutations affect parkin solubility and impair its E3 ligase activity, leading to a toxic accumulation of proteins within susceptible neurons that results in a slow but progressive neuronal degeneration and cell death. Here, we report that RTP801/REDD1, a pro-apoptotic negative regulator of survival kinases mTOR and Akt, is one of such parkin substrates. We observed that parkin knockdown elevated RTP801 in sympathetic neurons and neuronal PC12 cells, whereas ectopic parkin enhanced RTP801 poly-ubiquitination and proteasomal degradation. In parkin knockout mouse brains and in human fibroblasts from AR-JP patients with parkin mutations, RTP801 levels were elevated. Moreover, in human postmortem PD brains with mutated parkin, nigral neurons were highly positive for RTP801. Further consistent with the idea that RTP801 is a substrate for parkin, the two endogenous proteins interacted in reciprocal co-immunoprecipitates of cell lysates. A potential physiological role for parkin-mediated RTP801 degradation is indicated by observations that parkin protects neuronal cells from death caused by RTP801 overexpression by mediating its degradation, whereas parkin knockdown exacerbates such death. Similarly, parkin knockdown enhanced RTP801 induction in neuronal cells exposed to the Parkinson's disease mimetic 6-hydroxydopamine and increased sensitivity to this toxin. This response to parkin loss of function appeared to be mediated by RTP801 as it was abolished by RTP801 knockdown. Taken together these results indicate that RTP801 is a novel parkin substrate that may contribute to neurodegeneration caused by loss of parkin expression or activity.

Viral vector-based models of Parkinson's disease

In order to study the molecular pathways of Parkinson's disease (PD) and to develop novel therapeutic strategies, scientific investigators rely on animal models. The identification of PD-associated genes has led to the development of genetic PD models as an alternative to toxin-based models. Viral vector-mediated loco-regional gene delivery provides an attractive way to express transgenes in the central nervous system. Several vector systems based on various viruses have been developed. In this chapter, we give an overview of the different viral vector systems used for targeting the CNS. Further, we describe the different viral vector-based PD models currently available based on overexpression strategies for autosomal dominant genes such as α-synuclein and LRRK2, and knockout or knockdown strategies for autosomal recessive genes, such as parkin, DJ-1, and PINK1. Models based on overexpression of α-synuclein are the most prevalent and extensively studied, and therefore the main focus of this chapter. Many efforts have been made to increase the expression levels of α-synuclein in the dopaminergic neurons. The best α-synuclein models currently available have been developed from a combined approach using newer AAV serotypes and optimized vector constructs, production, and purification methods. These third-generation α-synuclein models show improved face and predictive validity, and therefore offer the possibility to reliably test novel therapeutics.

Mitochondrial acetylation and genetic models of Parkinson's disease

Parkinson's disease (PD) is frequent at old age, leading to atrophy of specific neurons and to early death. Lifespan and healthy aging of organisms depend on growth factor/nutrient signaling and on bioenergetics via mitochondria, all of which regulate downstream nuclear functions through FOXO and SIR proteins. Mammalian SIRtuins include the mitochondrial deacetylase SIRT3, and recently mitochondrial lysine acetylation (AcLys) was found to initiate mitochondrial degradation by autophagy. This mitophagy process is closely regulated by PINK1 and Parkin, two interacting proteins which relocalize to mitochondria with deficient proton gradients, and whose mutations cause autosomal recessive variants of PD. Strong generalized deacetylation of mitochondrial proteins and altered SIRT3 levels occur in rodent models of PD before the onset of toxic aggregate formation. We propose that the development of site-specific AcLys-antibodies and their characterization in patients will have medical value.

Behavioral and neurotransmitter abnormalities in mice deficient for Parkin, DJ-1 and superoxide dismutase

Parkinson’s disease (PD) is a progressive neurodegenerative disease characterized by loss of neurons in the substantia nigra that project to the striatum and release dopamine. The cause of PD remains uncertain, however, evidence implicates mitochondrial dysfunction and oxidative stress. Although most cases of PD are sporadic, 5-10% of cases are caused by inherited mutations. Loss-of-function mutations in Parkin and DJ-1 were the first to be linked to recessively inherited Parkinsonism. Surprisingly, mice bearing similar loss-of-function mutations in Parkin and DJ-1 do not show age-dependent loss of nigral dopaminergic neurons or depletion of dopamine in the striatum. Although the normal cellular functions of Parkin and DJ-1 are not fully understood, we hypothesized that loss-of-function mutations in Parkin and DJ-1 render cells more sensitive to mitochondrial dysfunction and oxidative stress. To test this hypothesis, we crossed mice deficient for Parkin and DJ-1 with mice deficient for the mitochondrial antioxidant protein Mn-superoxide dismutase (SOD2) or the cytosolic antioxidant protein Cu-Zn-superoxide dismutase (SOD1). Aged Parkin^-/-DJ-1^-/- and Mn-superoxide dismutase triple deficient mice have enhanced performance on the rotorod behavior test. Cu/Zn-superoxide dismutase triple deficient mice have elevated levels of dopamine in the striatum in the absence of nigral cell loss. Our studies demonstrate that on a Parkin/DJ-1 null background, mice that are also deficient for major antioxidant proteins do not have progressive loss of dopaminergic neurons but have behavioral and striatal dopamine abnormalities.

Surprising behavioral and neurochemical enhancements in mice with combined mutations linked to Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disorder behind Alzheimer's disease. There are currently no therapies proven to halt or slow the progressive neuronal cell loss in PD. A better understanding of the molecular and cellular causes of PD is needed to develop disease-modifying therapies. PD is an age-dependent disease that causes the progressive death of dopamine-producing neurons in the brain. Loss of substantia nigra dopaminergic neurons results in locomotor symptoms such as slowness of movement, tremor, rigidity and postural instability. Abnormalities in other neurotransmitters, such as serotonin, may also be involved in both the motor and non-motor symptoms of PD. Most cases of PD are sporadic but many families show a Mendelian pattern of inherited Parkinsonism and causative mutations have been identified in genes such as Parkin, DJ-1, PINK1, alpha-synuclein and leucine rich repeat kinase 2 (LRRK2). Although the definitive causes of idiopathic PD remain uncertain, the activity of the antioxidant enzyme glutathione peroxidase 1 (Gpx1) is reduced in PD brains and has been shown to be a key determinant of vulnerability to dopaminergic neuron loss in PD animal models. Furthermore, Gpx1 activity decreases with age in human substantia nigra but not rodent substantia nigra. Therefore, we crossed mice deficient for both Parkin and DJ-1 with mice deficient for Gpx1 to test the hypothesis that loss-of-function mutations in Parkin and DJ-1 cause PD by increasing vulnerability to Gpx1 deficiency. Surprisingly, mice lacking Parkin, DJ-1 and Gpx1 have increased striatal dopamine levels in the absence of nigral cell loss compared to wild type, Gpx1(-/-), and Parkin(-/-)DJ-1(-/-) mutant mice. Additionally, Parkin(-/-)DJ-1(-/-) mice exhibit improved rotarod performance and have increased serotonin in the striatum and hippocampus. Stereological analysis indicated that the increased serotonin levels were not due to increased serotonergic projections. The results of our behavioral, neurochemical and immunohistochemical analyses reveal that PD-linked mutations in Parkin and DJ-1 cause dysregulation of neurotransmitter systems beyond the nigrostriatal dopaminergic circuit and that loss-of-function mutations in Parkin and DJ-1 lead to adaptive changes in dopamine and serotonin especially in the context of Gpx1 deficiency.

Parkin-knockout mice did not display increased vulnerability to intranasal administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)

The loss of nigral dopaminergic neurons in Parkinson's disease (PD) is believed to result from interactions between genetic susceptibility and environmental factors. Although loss-of-function mutations in the parkin gene cause early-onset familial PD, the hybrid 129Sv-C57BL/6 parkin-deficient mice did not display spontaneous degeneration of the nigrostriatal pathway or enhanced vulnerability to neurotoxicity induced by 6-hydroxydopamine (6-OHDA) or intraperitoneal 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication. We aimed to re-evaluate the role of parkin in a pure C57BL/6 background after an acute intranasal (i.n.) MPTP administration, a new route of toxin delivery to the brain that mimics environmental exposure to neurotoxins. We found that the deficiency of parkin gene modifies the D-amphetamine-induced locomotion in saline-treated animals. Intranasal MPTP induced Parkinsonism in parkin⁺/⁺ mice, through depletion of striatal dopamine, decreased number of dopaminergic neurons in the substantia nigra, and decreased D-amphetamine-induced hyperlocomotion. Additionally, the deletion of the parkin gene in a pure C57BL/6 background did not lead to increased vulnerability to i.n. MPTP-induced neurotoxicity. Moreover, the i.n. MPTP induced nigral astrogliosis predominantly in the pars reticulata in wild type and parkin⁻/⁻ mice. Taken together, these results showed that the absence of parkin did not modify the vulnerability of nigrostriatal dopaminergic pathway after i.n. MPTP intoxication, suggesting that independently of mouse strain, the endogenous parkin is not required for protection of this system. These findings also suggest that the development of familial parkin-linked PD is not associated with exposure to environmental factors that specifically affects the dopaminergic system.

The role of innate and adaptive immunity in Parkinson's disease

In recent years, inflammation has become implicated as a major pathogenic factor in the onset and progression of Parkinson's disease. Understanding the precise role for inflammation in PD will likely lead to understanding of how sporadic disease arises. In vivo evidence for inflammation in PD includes microglial activation, increased expression of inflammatory genes in the periphery and in the central nervous system (CNS), infiltration of peripheral immune cells into the CNS, and altered composition and phenotype of peripheral immune cells. These findings are recapitulated in various animal models of PD and are reviewed herein. Furthermore, we examine the potential relevance of PD-linked genetic mutations to altered immune function and the extent to which environmental exposures that recapitulate these phenotypes, which may lead to sporadic PD through similar mechanisms. Given the implications of immune system involvement on disease progression, we conclude by reviewing the evidence supporting the potential efficacy of immunomodulatory therapies in PD prevention or treatment. There is a clear need for additional research to clarify the role of immunity and inflammation in this chronic, neurodegenerative disease.