Current Topics in Medicinal Chemistry - Volume 9, Issue 10, 2009

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Parkinson's disease (PD) is characterized by the selective loss of dopamine (DA) neurons in the substantia nigra compacta (SNc). The transcription factor Pitx3 is important for the differentiation and maintenance of midbrain DA neurons during development. There is highly restricted and constitutive expression of Pitx3 in the SNc and ventral tegmental area (VTA) of the midbrain after birth. In addition to its importance during development, Pitx3 also has roles in the long-term survival and maintenance of the midbrain DA neurons. In this review, we discuss the function of Pitx3 throughout the life of midbrain neurons and the contribution of Pitx3 to disease mechanisms.

Parkinson's disease (PD) is a complex neurodegenerative disorder characterised by dopaminergic cell loss in the substantia nigra. In addition, neurodegeneration occurs at a number of extra-nigral locations and involves a variety of non-dopaminergic neurotransmitter systems. Etiopathogenic mechanisms leading to cell death include oxidative stress and free radical generation, mitochondrial dysfunction, glutamate receptor mediated excitotoxicity, inflammation, oligodendrocytic interaction and neurotrophic factors, ubiquitin-proteosome system involvement, autophagy and apoptosis. Each of these is a potential target for novel pharmacotherapies including bioenergetic agents, inhibitors of excitotoxicity, neurotrophic factors, proteosomal enhancers and anti-apoptotic agents. Evidence has also been gained from cell culture and animal models for the potential disease modifying action of currently available dopaminergic therapies. These drugs have undergone clinical evaluation using studies with novel designs including “delayed start” methodology and studies using neuroimaging as a surrogate marker of dopaminergic cell loss. It is estimated from clinical, pathological and imaging studies that at least 50% of dopaminergic neurons are lost before the development of significant motor symptoms with a pre-motor phase of approximately 6-8 years. A number of pre- and post-synaptic neuroplastic homeostatic mechanisms occur during this period to maintain motor function. However these changes have been implicated in the development of motor complications (wearing “off” and dyskinesias). The evidence for treatments of motor complications in PD is discussed as are potential non-dopaminergic therapeutic targets to delay or improve motor complications.

Autophagy is the mechanism through which cells degrade oxidized membranes-organelles and mis/unfolded proteins, in this latter function cooperating with the ubiquitin-proteasome system (UP system). Although autophagy has been known for a long time, its involvement in the pathogenesis of neurodegenerative diseases has been investigated only recently. The most fascinating data are very recent and show an impressive connection between proteins that are mutated in different forms of familial Parkinson's Disease (PD) and the critical role that these proteins play in the physiology of the Autophagy (ATG) pathway. This evidence is supported by neuropathological data showing at the ultrastructural level, the occurrence of an altered ATG in the dopamine (DA) neurons of the Substantia Nigra of patients affected by PD. Accordingly, by using experimental models of PD the involvement of ATG is documented as well. In particular, administration of the DA neurotoxin methamphetamine produces damage to DA-containing cells which is exacerbated and results in neuronal cell death when the ATG pathway is inhibited, thus confirming ATG as a critical pathway for the survival of DA neurons. In the present manuscript, after describing the general molecular and cellular features of ATG, we give a short overview of the most relevant aspects concerning the involvement of ATG in the pathogenesis of PD. We further propose that the ATG and the UP systems might converge in the formation of a so-called “autophagoproteasome” which might represent an early ultrastructure witnessing the presence of an ongoing degeneration within DA cells.

L-DOPA is a di-hydroxy-phenyl, catecholamine precursor, amino acid, initially considered as an inert compound and now the key stone for the treatment of Parkinson's disease (PD) and some hereditary dystonias. L-DOPA, when administered to mammals, is rapidly metabolized to dopamine and 3-OM-DOPA, and its half-life in plasma is roughly 2 hours which has been considered the explanation for some of the L-DOPA related complications in PD. There have been, therefore, sophisticated methods of improving its pharmacokinetics by the association of decarboxylase and COMT inhibitors, slow release preparations and continuous infusions. In addition to its symptomatic effects, the impact of L-DOPA on the natural course of the disease is intriguing. By alleviating motor deficits, L-DOPA may improve health quality and life span in patients with PD, but there are neurotoxic and neurotrophic effects of L-DOPA which may produce long term effects on disease progression. These effects are dependent of the dose, the status of the metabolic pathways involved in catecholamine metabolism, the balance of free radicals and their scavengers and the function of glia. Finally, there is new data suggesting that L-DOPA may be not only a catecholamine precursor but also a neurotransmitter by itself of yet unknown function.

The intermittent oral intake of the dopamine (DA) precursor L-3,4-dihydroxyphenylalanine (L-DOPA) is the classic therapy of Parkinson's disease (PD). In this way, the drug precursor can be metabolised into the active neurotransmitter DA. Although this occurs throughout the brain, the therapeutic relief is believed to be due to restoring extracellular DA levels within the dorsal striatum (more in the putamen than the caudate nucleus) which lacks endogenous DA as a consequence of the disease process. However, differing from physiological DA transmission, this therapeutic pattern leads to abnormal peaks of non-synaptic DA, which are supposed to trigger behavioural sensitisation expressed as abnormal involuntary movements. A similar pattern of abnormal DA stimulation occurs during methamphetamine (METH) intake. In the present review we will provide evidence showing the similarities between METH- and L-DOPAinduced DA stimulation with an intact and denervated striatum respectively. This comparison will encompass various features; the timing, the areas and the amount of extracellular DA levels which reveal surprising homologies. Such an overlapping between L-DOPA in PD and METH will be further analysed to critically assess the commonalities concerning the following points: abnormal receptor stimulation, recruitment of altered transduction pathways, abnormal gene expression, alterations in the phenotype of striatal neurons, and the establishment of behavioural sensitisation which appear as distinct phenomena (i.e. abnormal involuntary movements in PD and drug addiction in METH abuse); nonetheless, this may also lead to common behavioural alterations (METH-like addictive behaviours in PD patients during the course of DA replacement therapy in subsets of PD patients).

Parkinson disease (PD) is an already prevalent neurodegenerative disease that is poised to at least double over the next 25 years. Although best known for its characteristic movement disorder, PD is now appreciated commonly to cause cognitive impairment, including dementia, and behavioral changes. Dementia in patients with PD is consequential and has been associated with reduced quality of life, shortened survival, and increased caregiver distress. Here we review clinical presentation and progression, pathological bases, identification of genetic risk factors, development of small molecule biomarkers, and emerging treatments for cognitive impairment in patients with PD.

Idiopathic Parkinson's Disease (PD) is a progressive neurodegenerative disease characterized by dopaminergic neuronal loss within the substantia nigra. The degeneration of dopamine and other neuronal populations in PD lead to both chronic motor and non-motor disabilities but the mechanisms remain unclear. Molecular genetic studies in familial forms of the disease identified key proteins involved in PD pathogenesis, supporting a major role for (i) protein aggregation and neurotoxic α-synuclein oligomeric species due to an altered protein quality control, (ii) parkin-driven deregulation of the ubiquitin-proteasome system, (iii) oxidative stress and mithocondrial dysfunction, and, finally, (iv) disturbed kinase activity. The elucidation of these new molecular pathways has increased our knowledge of PD pathophysiology, but it remains an open question whether alterations of these pathways lead to different entities of PD or whether they finally converge at a point that is the common pathogenetic denominator of PD. However, the knowledge of validated targets is in its infancy, and thus, traditional target-based drug discovery strategies are of limited use. Alternative approaches are needed, and early attempts were aimed at identifying molecules inhibiting the aggregation of α-synuclein fragments, interfering with the ubiquitin proteasome pathway and reducing oxidative stress. Such discovery strategies have an impact on the configuration of screening cascades for effective translation of drug candidates toward clinical trials. This review examines how these genetic findings provided us with suitable animal models and how the gained insights will contribute to better therapies for PD.

This article describes recent advances in the development and biological evaluation of small molecule mGluR4 positive allosteric modulators (PAMs), and, to a lesser extent, orthosteric agonists. Due to its expression in the basal ganglia, the Family 3 GPCR metabotropic glutamate receptor subtype 4 (mGluR4) has recently garnered a great deal of attention as a putative target for the treatment of Parkinson's disease and a variety of other CNS disorders. Until 2008, with the exception of the prototypical mGluR4 PAM (-)-PHCCC, very few small molecule tools existed to probe the role of selective activation of mGluR4. This review will focus on the explosion of novel mGluR4 PAMs reported in the past year and the further preclinical validation of mGluR4 activation as a potentially groundbreaking treatment for Parkinson's disease.