The potential of neurotrophic factors for the treatment of Parkinson's disease

Parkinson’s disease (PD) is the second most common neurodegenerative disorder, with an incidence of 1.5 - 2% in the population over 60 years of age, which increases significantly with advancing age (for reviews see de Lau & Breteler, 2006; Toulouse & Sullivan, 2008). Since the Western world is experiencing significant increases in life expectancy, the incidence of PD is steadily escalating. The financial and economical burden to society of the treatment and care of PD patients is substantial and increasing. Thus, research on the causes of this debilitating disease is critical, as is the development of new treatments. PD is caused by progressive degeneration of the nigrostriatal (A9) dopaminergic pathway, which projects from the substantia nigra in the midbrain to the caudate-putamen (striatum) in the forebrain (Braak et al., 2003; Fearnley & Lees, 1991; Hoehn & Yahr, 1967; Olanow & Tatton, 1999). The resulting loss of dopamine neurotransmission in the striatum causes the cardinal symptoms of the disease: tremor at rest, rigidity and bradykinesia. One of the pathological hallmarks of PD is the appearance of intracellular protein aggregates called Lewy bodies, which are found in the substantia nigra and other brain areas (for reviews see Forno, 1996; Gibb & Lees, 1988). Lewy bodies are abnormal aggregates composed of α-synuclein, ubiquitin and other proteins. Approximately 5% of PD cases are caused by heritable genetic mutations, of which at least twelve have been identified (for review see Toulouse & Sullivan, 2008). The remaining cases are sporadic and of unknown origin, although many theories have been proposed to explain the cause of dopaminergic neuronal death which occurs in PD, such as environmental toxins, mitochondrial dysfunction with resulting oxidative stress, and inflammatory mechanisms (for reviews see Dauer & Przedborski, 2003; Dawson & Dawson, 2003; Fahn & Cohen, 1992; Long-Smith et al., 2009). At present there is no effective long-term therapy for PD. The most commonly-used treatment is administration of the dopamine precursor, levodopa, which replaces lost dopamine in the denervated striatum and relieves motor symptoms. Levodopa is generally administered in conjunction with an inhibitor of peripheral decarboxylase (carbide or benserazide), which has the effect of enhancing the central activity of levodopa and decreasing peripheral side-effects. Levodopa is successful in treating PD symptoms; however, it does not stop the ongoing neurodegeneration. Furthermore, about 50% of patients develop complications within the first five years of treatment, primarily severe motor fluctuations and dyskinesias (Freed et al., 2001; Hagell et al., 2002). Other current drug treatments include inhibitors of the dopamine breakdown enzymes catechol-O-methyl-transferase (tolcapone or entacapone) or monoamine oxidase–B (selegiline and rasagiline), and dopamine receptor agonists (bromocriptine, pergolide, pramipexole, ropinirole and others). Surgical methods involving ablation of deep brain structures such as the thalamus or pallidum, or deep brain stimulation of the subthalamic nucleus or pallidum, have also been used with good success, but these procedures are not widely-available or applicable for all patients. In summary, none of the current treatments provide safe and long-lasting relief from the symptoms and none have any effect on the progression of the disease. Much of the current research is aimed at developing new and novel therapies that will slow, halt or reverse the neurodegenerative process, rather than simply treating the symptoms of the disease. These include the use of antioxidants, anti-apoptotic agents, cell-based therapies and neuroprotective factors. Neurotrophic factors are a class of proteins that have the potential to be used as neuroprotectants in PD therapy