NOVEL NEUROTROPHIC FACTORS AS POTENTIAL DRUGS FOR NEURODEGENERATIVE DISEASES

Text: Academy Professor Mart Saarma

Director of the Centre of Excellence of Molecular and Integrative Neuroscience Research

Institute of Biotechnology, University of Helsinki, Finland

 

About neurodegenerative diseases

Neurodegenerative diseases (ND), such as Alzheimer’s disease, dementia, Parkinson’s disease (PD), Huntington’s disease and amyotrophic lateral sclerosis (ALS) form an increasing problem for the contemporary society. Already now it is estimated that almost 3-4% of the population of the Western world is suffering from ND and due to the increasing life expectancy and improvement of medical care the number of patients is increasing.

Although serious effort has been made to understand the pathogenic mechanisms and to develop new treatments for the ND current situation is still very difficult. Most importantly, we also lack efficient treatment for the ND. All current drugs are able to alleviate the symptoms, but do not slow down or stop neurodegeneration.

Future therapy should include interventions that slow down or reverse the progression of the neuronal degeneration by preventing the degeneration of neurons, regenerating neuronal axons and stimulating synapse formation, and possibly also increasing the functional activity of the remaining DA neurons.

Parkinson’s disease is the most common cause of movement disorder affecting 1 in 100 people over the age of 60, and the number of affected individuals is also set to rise dramatically. Current descriptions of PD mostly focus on the progressive degeneration of dopaminergic (DA) neurons located in the substantia nigra pars compacta (SNpc). This pathology is thought to underlie several motor symptoms that are reversed by dopamine-replacement therapy using either L-DOPA or dopamine agonists.

Although considerable progress have been made in identifying genes causing the familial PD even the pathogenesis mechanisms of familial PD have remained elusive. Moreover, all available PD treatments are symptomatic and no treatment that could slow down or stop neurodegeneration is currently available. Therefore the focus of our studies is to find novel neuroregenerative therapies.

Neurotrophic factors

 

More than fifty years ago Dr. Rita Levi-Montalcini described naturally occurring proteins that are able to stimulate the growth of neurites and also keep neurons alive. Later together with Dr. Stanley Cohen that biochemically characterized the protein and called it the nerve growth factor, NGF.

Currently there are four classes of proteins that have similar properties to NGF and we call them neurotrophic factors. Since differently from many other cells neurons to not divide they need tropic support to stay alive. This is provided by neurotrophic factors (NTFs) that in addition to survival promoting properties regulate many other aspects of neuronal development and life. In adult organism NTFs are critically important for the maintenance of neurons and during injury and trauma they are able to protect and repair neurons.

Form the point of view of the therapy of PD two classes or neurotrophic factors are the most important. Glial cell line-derived neurotrophic factor GDNF and its homologous protein neurturin are able not only to protect, but also to repair neurons in animal models of PD. Although both proteins work well in animal models of PD they have also several limitations (1).

One of the main problems is that GDNF and NRTN as large proteins (about 30 kDa) are unable to pass the blood-brain-barrier and therefore they should be delivered directly into the brain (Figure 1). In addition, they diffuse very poorly in the brain tissue. Finally GDNF and NRTN are unable to rescue and repair dopaminergic neurons in genetic animal models of PD.

Part of these limitations can be overcome by using the new neurotrophic factor recently discovered in our laboratory. Cerebral dopamine neurotrophic factors (CDNF) (2) is more effective than GDNF in protecting and repairing dopamine neurons in animal models of PD. CDNF and its homologous protein MANF diffuse much better in the brain tissue than GDNF and in addition have a unique property to bind oxidised phospholipids (3).

The latter is the hallmark of neurodegeneration that significantly also contributes to the pathogenesis of PD. Thus, there are four naturally occurring neurotrophic factors that can not only slow down and stop neurodegeneration, but can also repair and regenerate diseased dopaminergic neurons. Without doubt, GDNF and NRTN, but in particular the new neurotrophic factors CDNF and MANF have a potential in the treatment of PD.

Clinical trials

GDNF has been tested in clinical trials for several times. The first successful Phase 1 clinical trial was performed by Dr. Steven Gill’s group at Bristol hospital and by Dr. Don Gash’s group at the University of Kentucky. They delivered recombinant human GDNF protein to the caudate putamen with minipumps and both open-label studies on (5 patients and the other one on 10 patients, respectively) demonstrated that GDNF treatment is safe.

Importantly, GDNF treatment also resulted in significant improvement of the PRDGS motor score and well-being of the patients. In the following Phase 2 trial Amgen company scientists basically repeated the scheme of Dr. Gill, but used a thicker catheter to deliver GDNF protein into the patient’s brain. The results of this study on 34 patients, where 17 patents received placebo where, however disappointing. Unfortunately, no significant clinical benefit of GDNF was observed (4).

Later studies have revealed that the delivery of the growth factors with the minipump needs further development. The use of minipump has advantages - proteins are delivered directly into the brain and the delivery can be regulated.

Another way is to use gene therapy. In this case the gene of the neurotrophic factor is inserted into the gene therapy vector and the vector is injected into the patient’s brain (Figure 1). In this case human brain cells will be transfected with the vector and produce human neurotrophic factor. Ceregene company has used adeno-associated virus serotrype 2 vector to deliver NRTN gene into the PD patient’s brain.

The results of the Phase 1 trial on 12 patients were very promising. Phase 2 trials on 58 patients showed no clear clinical benefit during the first year treatment, but after 1.5 years there was a clear and statistically significant therapeutic effect. Two patients involved in this study unfortunately died in accidents and the brain autopsy demonstrated that NRTN has very poorly diffused in the brain. In any case these results demonstrate that neurotrophic factors have clinical benefit in the treatment of neurodegenerative diseases.

 

Problems and perspectives

The future of the development of neurotrophic factor therapy still needs significant efforts. In addition to protein therapy and gene therapy also cell therapy with cells expressing neurotrophic factors has a great potential. So called encapsulated cell therapy developed by Danish company NsGene has shown great potential (4). Current gene therapy vectors have major limitations as they are continuously active. Future therapy requires regulated vector that can be switched on and off. Luckily these types of vectors are already under development.

The site and the methods of delivery of the therapeutic proteins need significant improvements. In addition to GDNF and NRTN new neurotrophic factors CDNF that our laboratory has discovered has a great therapeutic promise. We are currently making serious efforts to take the new and most promising neurotrophic factor CDNF into the Phase 1 clinical trials for Parkinson’s disease.

 

1. Bespalov, M., Saarma, M. (2007) GDNF receptor complex is an emerging drug target. Trends in Pharmacological Sciences 28 (2), 68-74.

2. Lindholm, P., Voutilainen, M .H., Laurén, J., Peränen, J., Leppänen, V-M., Andressoo, J.-O., Lindahl, M., Janhunen, S.,  Kalkkinen, N., Timmusk, T., Tuominen, R. K. and Saarma, M. (2007) Novel neurotrophic factor CDNF protects and rescues midbrain dopamine neurons in vivo. Nature, 448, 73-77.

3. Voutilainen, M.H., Bäck, S., Pörsti, E., Toppinen, L., Lindgren, L., Lindholm, P., Peränen, J., Saarma, M. and Tuominen, R. K. (2009) Neurotrophic factor MANF is neurorestorative in rat model of Parkinson’s disease. J. Neurosci., 29(30): 9651-9659.

4. Andressoo, J.-O. and Saarma, M. (2008) Signalling mechanisms underlying development and maintenance of dopamine neurons. Current Opinion in Neurobiologt, 18, 297–306.