Genetic rat models of Parkinson's disease

Role of gut microbiota in regulating gastrointestinal dysfunction and motor symptoms in a mouse model of Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disorder characterized primarily by motor and non-motor gastrointestinal (GI) deficits. GI symptoms' including compromised intestinal barrier function often accompanies altered gut microbiota composition and motor deficits in PD. Therefore, in this study, we set to investigate the role of gut microbiota and epithelial barrier dysfunction on motor symptom generation using a rotenone-induced mouse model of PD. We found that while six weeks of 10 mg/kg of chronic rotenone administration by oral gavage resulted in loss of tyrosine hydroxylase (TH) neurons in both germ-free (GF) and conventionally raised (CR) mice, the decrease in motor strength and coordination was observed only in CR mice. Chronic rotenone treatment did not disrupt intestinal permeability in GF mice but resulted in a significant change in gut microbiota composition and an increase in intestinal permeability in CR mice. These results highlight the potential role of gut microbiota in regulating barrier dysfunction and motor deficits in PD.

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.

The development of small molecule angiotensin IV analogs to treat Alzheimer's and Parkinson's diseases

Alzheimer's (AD) and Parkinson's (PD) diseases are neurodegenerative diseases presently without effective drug treatments. AD is characterized by general cognitive impairment, difficulties with memory consolidation and retrieval, and with advanced stages episodes of agitation and anger. AD is increasing in frequency as life expectancy increases. Present FDA approved medications do little to slow disease progression and none address the underlying progressive loss of synaptic connections and neurons. New drug design approaches are needed beyond cholinesterase inhibitors and N-methyl-d-aspartate receptor antagonists. Patients with PD experience the symptomatic triad of bradykinesis, tremor-at-rest, and rigidity with the possibility of additional non-motor symptoms including sleep disturbances, depression, dementia, and autonomic nervous system failure. This review summarizes available information regarding the role of the brain renin-angiotensin system (RAS) in learning and memory and motor functions, with particular emphasis on research results suggesting a link between angiotensin IV (AngIV) interacting with the AT4 receptor subtype. Currently there is controversy over the identity of this AT4 receptor protein. Albiston and colleagues have offered convincing evidence that it is the insulin-regulated aminopeptidase (IRAP). Recently members of our laboratory have presented evidence that the brain AngIV/AT4 receptor system coincides with the brain hepatocyte growth factor/c-Met receptor system. In an effort to resolve this issue we have synthesized a number of small molecule AngIV-based compounds that are metabolically stable, penetrate the blood-brain barrier, and facilitate compromised memory and motor systems. These research efforts are described along with details concerning a recently synthesized molecule, Dihexa that shows promise in overcoming memory and motor dysfunctions by augmenting synaptic connectivity via the formation of new functional synapses.

A Role for the Brain RAS in Alzheimer's and Parkinson's Diseases

The brain renin-angiotensin system (RAS) has available the necessary functional components to produce the active ligands angiotensins II (AngII), angiotensin III, angiotensins (IV), angiotensin (1-7), and angiotensin (3-7). These ligands interact with several receptor proteins including AT1, AT2, AT4, and Mas distributed within the central and peripheral nervous systems as well as local RASs in several organs. This review first describes the enzymatic pathways in place to synthesize these ligands and the binding characteristics of these angiotensin receptor subtypes. We next discuss current hypotheses to explain the disorders of Alzheimer's disease (AD) and Parkinson's disease (PD), as well as research efforts focused on the use of angiotensin converting enzyme (ACE) inhibitors and angiotensin receptor blockers (ARBs), in their treatment. ACE inhibitors and ARBs are showing promise in the treatment of several neurodegenerative pathologies; however, there is a need for the development of analogs capable of penetrating the blood-brain barrier and acting as agonists or antagonists at these receptor sites. AngII and AngIV have been shown to play opposing roles regarding memory acquisition and consolidation in animal models. We discuss the development of efficacious AngIV analogs in the treatment of animal models of AD and PD. These AngIV analogs act via the AT4 receptor subtype which may coincide with the hepatocyte growth factor/c-Met receptor system. Finally, future research directions are described concerning new approaches to the treatment of these two neurological diseases.

Expression of human E46K-mutated α-synuclein in BAC-transgenic rats replicates early-stage Parkinson's disease features and enhances vulnerability to mitochondrial impairment

Parkinson's disease (PD), the second most common neurodegenerative disorder, is etiologically heterogeneous, with most cases thought to arise from a combination of environmental factors and genetic predisposition; about 10% of cases are caused by single gene mutations. While neurotoxin models replicate many of the key behavioral and neurological features, they often have limited relevance to human exposures. Genetic models replicate known disease-causing mutations, but are mostly unsuccessful in reproducing major features of PD. In this study, we created a BAC (bacterial artificial chromosome) transgenic rat model of PD expressing the E46K mutation of α-synuclein, which is pathogenic in humans. The mutant protein was expressed at levels ~2-3-fold above endogenous α-synuclein levels. At 12 months of age, there was no overt damage to the nigrostriatal dopamine system; however, (i) alterations in striatal neurotransmitter metabolism, (ii) accumulation and aggregation of α-synuclein in nigral dopamine neurons, and (iii) evidence of oxidative stress suggest this model replicates several preclinical features of PD. Further, when these animals were exposed to rotenone, a mitochondrial toxin linked to PD, they showed heightened sensitivity, indicating that α-synuclein expression modulates the vulnerability to mitochondrial impairment. We conclude that these animals are well-suited to examination of gene-environment interactions that are relevant to PD.

Importance of the brain Angiotensin system in Parkinson's disease

Parkinson's disease (PD) has become a major health problem affecting 1.5% of the world's population over 65 years of age. As life expectancy has increased so has the occurrence of PD. The primary direct consequence of this disease is the loss of dopaminergic (DA) neurons in the substantia nigra and striatum. As the intensity of motor dysfunction increases, the symptomatic triad of bradykinesia, tremors-at-rest, and rigidity occur. Progressive neurodegeneration may also impact non-DA neurotransmitter systems including cholinergic, noradrenergic, and serotonergic, often leading to the development of depression, sleep disturbances, dementia, and autonomic nervous system failure. L-DOPA is the most efficacious oral delivery treatment for controlling motor symptoms; however, this approach is ineffective regarding nonmotor symptoms. New treatment strategies are needed designed to provide neuroprotection and encourage neurogenesis and synaptogenesis to slow or reverse this disease process. The hepatocyte growth factor (HGF)/c-Met receptor system is a member of the growth factor family and has been shown to protect against degeneration of DA neurons in animal models. Recently, small angiotensin-based blood-brain barrier penetrant mimetics have been developed that activate this HGF/c-Met system. These compounds may offer a new and novel approach to the treatment of Parkinson's disease.