Iddo Magen

Genetic mouse models of Parkinson's disease The state of the art.

The identification of several mutations causing familial forms of Parkinsons disease (PD) has led to the creation of multiple lines of mice expressing similar genetic alterations. These models present a unique opportunity for understanding pathophysiological mechanisms leading to PD in a mammalian brain and provide models that are suitable for the preclinical testing of new therapies. Different lines of mice recapitulate the symptoms and pathological features of PD to various extents. This chapter examines their respective advantages and highlights some of the key findings that have already emerged from the analysis of these new models of PD.

A Progressive Mouse Model of Parkinson’s Disease: The Thy1-aSyn (“Line 61”) Mice

Identification of mutations that cause rare familial forms of Parkinson’s disease (PD) and subsequent studies of genetic risk factors for sporadic PD have led to an improved understanding of the pathological mechanisms that may cause nonfamilial PD. In particular, genetic and pathological studies strongly suggest that alpha-synuclein, albeit very rarely mutated in PD patients, plays a critical role in the vast majority of individuals with the sporadic form of the disease. We have extensively characterized a mouse model over-expressing full-length, human, wild-type alpha-synuclein under the Thy-1 promoter. We have also shown that this model reproduces many features of sporadic PD, including progressive changes in dopamine release and striatal content, alpha-synuclein pathology, deficits in motor and nonmotor functions that are affected in pre-manifest and manifest phases of PD, inflammation, and biochemical and molecular changes similar to those observed in PD. Preclinical studies have already demonstrated improvement with promising new drugs in this model, which provides an opportunity to test novel neuroprotective strategies during different phases of the disorder using endpoint measures with high power to detect drug effects.

Elevated tonic extracellular dopamine concentration and altered dopamine modulation of synaptic activity precede dopamine loss in the striatum of mice overexpressing human α-synuclein.

Overexpression or mutation of α‐synuclein (α‐Syn), a protein associated with presynaptic vesicles, causes familial forms of Parkinsons disease in humans and is also associated with sporadic forms of the disease. We used in vivo microdialysis, tissue content analysis, behavioral assessment, and whole‐cell patch clamp recordings from striatal medium‐sized spiny neurons (MSSNs) in slices to examine dopamine transmission and dopaminergic modulation of corticostriatal synaptic function in mice overexpressing human wild‐type α‐Syn under the Thy1 promoter (α‐Syn mice). Tonic striatal extracellular dopamine and 3‐methoxytyramine levels were elevated in α‐Syn mice at 6 months of age, prior to any reduction in total striatal tissue content, and were accompanied by an increase in open‐field activity. Dopamine clearance and amphetamine‐induced dopamine efflux were unchanged. The frequency of MSSN spontaneous excitatory postsynaptic currents (sEPSCs) was lower in α‐Syn mice. Amphetamine reduced sEPSC frequency in wild types (WTs) but produced no effect in α‐Syn mice. Furthermore, whereas quinpirole reduced and sulpiride increased sEPSC frequency in WT mice, they produced the opposite effects in α‐Syn mice. These observations indicate that overexpression of α‐Syn alters dopamine efflux and D2 receptor modulation of corticostriatal glutamate release at a young age. At 14 months of age, the α‐Syn mice presented with significantly lower striatal tissue dopamine and tyrosine hydroxylase content relative to WT littermates, accompanied by an L‐DOPA‐reversible sensory motor deficit. Together, these data further validate this transgenic mouse line as a slowly progressing model of Parkinsons disease and provide evidence for early dopamine synaptic dysfunction prior to loss of striatal dopamine.

Cognitive deficits in a mouse model of pre-manifest Parkinson’s disease

Early cognitive deficits are increasingly recognized in patients with Parkinson’s disease (PD), and represent an unmet need for the treatment of PD. These early deficits have been difficult to model in mice, and their mechanisms are poorly understood. α‐Synuclein is linked to both familial and sporadic forms of PD, and is believed to accumulate in brains of patients with PD before cell loss. Mice expressing human wild‐type α‐synuclein under the Thy1 promoter (Thy1‐aSyn mice) exhibit broad overexpression of α‐synuclein throughout the brain and dynamic alterations in dopamine release several months before striatal dopamine loss. We now show that these mice exhibit deficits in cholinergic systems involved in cognition, and cognitive deficits in domains affected in early PD. Together with an increase in extracellular dopamine and a decrease in cortical acetylcholine at 4–6 months of age, Thy1‐aSyn mice made fewer spontaneous alternations in the Y‐maze and showed deficits in tests of novel object recognition (NOR), object–place recognition, and operant reversal learning, as compared with age‐matched wild‐type littermates. These data indicate that cognitive impairments that resemble early PD manifestations are reproduced by α‐synuclein overexpression in a murine genetic model of PD. With high power to detect drug effects, these anomalies provide a novel platform for testing improved treatments for these pervasive cognitive deficits.

Mice overexpressing wild-type human alpha-synuclein display alterations in colonic myenteric ganglia and defecation

Background  Prevalent non‐motor symptoms of Parkinson’s disease (PD) include gastrointestinal motor impairments and advanced stage PD displays pathological aggregates of α‐synuclein in colonic enteric neurons. We previously showed that 12 months old mice overexpressing human wild type (WT) α‐synuclein under the Thy1 promoter (Thy1‐aSyn) displayed colonic motor dysfunction. We investigated functional gut alterations at earlier ages and histological correlates.

Activity-dependent neuroprotective protein (ADNP) exhibits striking sexual dichotomy impacting on autistic and Alzheimer’s pathologies

Activity-dependent neuroprotective protein (ADNP) is a most frequent autism spectrum disorder (ASD)-associated gene and the only protein significantly decreasing in the serum of Alzheimers disease (AD) patients. Is ADNP associated with ASD being more prevalent in boys and AD more prevalent in women? Our results revealed sex-related learning/memory differences in mice, reflecting hippocampal expression changes in ADNP and ADNP-controlled AD/ASD risk genes. Hippocampal ADNP transcript content was doubled in male vs female mice, with females showing equal expression to ADNP haploinsufficient (ADNP+/−) males and no significant genotype-associated reduction. Increased male ADNP expression was replicated in human postmortem hippocampal samples. The hippocampal transcript for apolipoprotein E (the major risk gene for AD) was doubled in female mice compared with males, and further doubled in the ADNP+/− females, contrasting a decrease in ADNP+/− males. Previously, overexpression of the eukaryotic translation initiation factor 4E (eIF4E) led to ASD-like phenotype in mice. Here, we identified binding sites on ADNP for eIF4E and co-immunoprecipitation. Furthermore, hippocampal eIF4E expression was specifically increased in young ADNP+/− male mice. Behaviorally, ADNP+/− male mice exhibited deficiencies in object recognition and social memory compared with ADNP+/+ mice, while ADNP+/− females were partially spared. Contrasting males, which preferred novel over familiar mice, ADNP+/+ females showed no preference to novel mice and ADNP+/− females did not prefer mice over object. ADNP expression, positioned as a master regulator of key ASD and AD risk genes, introduces a novel concept of hippocampal gene-regulated sexual dimorphism and an ADNP+/− animal model for translational psychiatry.

Microtubule-stabilizing peptides and small molecules protecting axonal transport and brain function: Focus on davunetide (NAP)

This review focuses on the therapeutic effects and mechanisms of action of NAP (davunetide), an eight amino acid snippet derived from activity-dependent neuroprotective protein (ADNP) which was discovered in our laboratory. We have recently described the effects of NAP in neurodegenerative disorders, and we now review the beneficial effects of NAP and other microtubule-stabilizing agents on impairments in axonal transport. Experiments in animal models of microtubule-deficiency including tauopathy (spanning from drosophila to mammals) showed protection of axonal transport by microtubule-stabilizers and NAP, which was coupled to motor and cognitive protection. Clinical trials with NAP (davunetide) are reviewed paving the path to future developments.

Intranasal NAP (davunetide) decreases tau hyperphosphorylation and moderately improves behavioral deficits in mice overexpressing α-synuclein

Genome‐wide association studies have identified strong associations between the risk of developing Parkinsons disease (PD) and polymorphisms in the genes encoding α‐synuclein and the microtubule‐associated protein tau. However, the contribution of tau and its phosphorylated form (p‐tau) to α‐synuclein‐induced pathology and neuronal dysfunction remains controversial. We have assessed the effects of NAP (davunetide), an eight‐amino acid peptide that decreases tau hyperphosphorylation, in mice overexpressing wild‐type human α‐synuclein (Thy1‐aSyn mice), a model that recapitulates aspects of PD. We found that the p‐tau/tau level increased in a subcortical tissue block that includes the striatum and brain stem, and in the cerebellum of the Thy1‐aSyn mice compared to nontransgenic controls. Intermittent intranasal NAP administration at 2 μg/mouse per day, 5 days a week, for 24 weeks, starting at 4 weeks of age, significantly decreased the ratio of p‐tau/tau levels in the subcortical region while a higher dose of 15 μg/mouse per day induced a decrease in p‐tau/tau levels in the cerebellum. Both NAP doses reduced hyperactivity, improved habituation to a novel environment, and reduced olfactory deficits in the Thy1‐aSyn mice, but neither dose improved the severe deficits of motor coordination observed on the challenging beam and pole, contrasting with previous data obtained with continuous daily administration of the drug. The data reveal novel effects of NAP on brain p‐tau/tau and behavioral outcomes in this model of synucleinopathy and suggest that sustained exposure to NAP may be necessary for maximal benefits.

Davunetide: Peptide Therapeutic in Neurological Disorders

This review focuses on the therapeutic effects and mechanisms of action of NAP (davunetide), an eight amino acid snippet derived from activity-dependent neuroprotective protein (ADNP) which was discovered in the laboratory of Prof. Illana Gozes. The effects of NAP and its related peptides in models of neurodegenerative diseases and other neurological disorders will be described here in details. Possible mechanisms of NAP actions include anti-inflammatory effect, antioxidant activity, inhibition of protein aggregation and interaction with microtubules. In line with the fact that all of these features are characteristic to most neurological/neurodegenerative disorders, NAP was found to have beneficial effects on the behavioral manifestations associated with these disorders.

Mouse Models of Cognitive Deficits Due to Alpha-Synuclein Pathology

Synucleopathies are neurodegenerative disorders characterized by abnormal accumulation of alpha-synuclein, most often in neurons. Familial forms are due to mutations or multiplications of the gene encoding for alpha-synuclein but most synucleopathies occur sporadically. They include Parkinsons disease (PD) and dementia with Lewy Bodies (DLB), which are both linked to cognitive decline. In DLB, dementia dominates the symptoms whereas in PD, subtle cognitive deficits are frequent and may appear even before motor symptoms, but only a fraction of patients develop severe dementia-type cognitive deficits. Several lines of mice were developed to model human synucleopathies by over-expressing the wild type or the mutated human alpha-synuclein under a variety of promoters. In addition, mice lacking alpha-synuclein have been used to determine the role of this protein in cognitive function. This chapter will review cognitive alterations observed in these models and discuss how they may help understand the various forms and stages of cognitive deficits observed in patients with synucleopathies.