ARTICLE IN BRIEF
In animal models of Parkinson disease, investigators were able to unravel clues why glial cell-derived neurotrophic factor (GDNF) is not neuroprotective; they found that intracellular response to GDNF is blocked in the dopamine neurons that overexpress alpha-synuclein. At the same time, they found that a transcription factor called Nurr1 and its downstream target, the GDNF receptor Ret, also shows reduced expression.
The use of glial cell-derivedneurotrophic factor (GDNF) has consistently failed toprotect nigral dopamine neurons against alpha-synuclein-induced neurodegeneration in animal models of Parkinson'sdisease (PD). Now, as clinical trials using GDNF are under way in people with PD,investigations are unraveling the reason why.
Anders Björklund, MD, PhD, professor of histology at Wallenberg Neuroscience Center at Lund University in Lund, Sweden, and his colleagues reported in a Dec. 5, 2012, paper in Science Translational Medicine that the intracellular response to GDNF is blocked in the dopamine neurons that overexpress alpha-synuclein. At the same time, they found that a transcription factor called Nurr1 and its downstream target, the GDNF receptor Ret, also shows reduced expression. Knowing that Ret expression is reduced in nigral dopamine neurons in PD patients, they designed an animal study to figure out what is going on in this pathway.
THE ANIMAL EXPERIMENT
To do so, the research team used a recombinant adeno-associated vector (AAV) to overexpress human wild-type alpha-synuclein in nigral dopamine neurons of rat brains. They injected the vector into the substantia nigra of adult rats; control rats received a similar injection of an AAV expressing a gene encoding green fluorescent protein (GF) instead of the gene encoding alpha-synuclein. They observed that the intracellular signaling response induced by GDNF was almost completely blocked in the alpha-synuclein affected dopamine neurons. The blockade, said Dr. Björklund, was accompanied by reduced expression of Nurr1 and reduced Ret expression. They realized that the down-regulation of Nurr1 was critical in modulating the ability of the nigral dopamine neurons to respond to GDNF.
By overexpressing Nurr1 they were able to restore signaling and protect the nigral dopamine neurons against alpha-synuclein toxicity.
Ret is a GDNF receptor and virtually disappears in cells with a high load of alpha-synuclein, Dr. Björklund explained. “When alpha-synuclein enters the nucleus it affects the expression of Nurr1 and, as a consequence, Ret is expressed in low levels and it makes the cell insensitive to GDNF.”
Alpha-synuclein is jeopardizing the cell, putting it under stress and affecting its survival, Dr. Björklund said. “The cell needs GDNF. We believe that overexpressing Nurr1, or increasing Nurr1 function, can combat the neurotoxicity of alpha-synuclein. Our finding suggests that Nurr1 regulates neurotrophic factor signaling and could be a good target to protect dopamine neurons from dying in PD,” he said.
The investigators made two additional observations from their experiments. “First, retrograde transport of GDNF from striatum to nigra was markedly reduced (by about 75%) in the alpha-synuclein–overexpressing rat DA [dopaminergic] neurons, and this defect was reversed by Nurr1,” they wrote. “Second, enhanced expression of Nurr1, in the absence of any exogenous GDNF, was able to induce a GDNF-like trophic response, seen as an activation of both the Ret receptor and its downstream signaling pathways…Consistent with these observations, we show that enhanced expression of Nurr1 is sufficient to protect the nigral DA neurons — both the cell bodies and the axonal projections in the striatum — against alpha-synuclein–induced toxicity. This suggests that Nurr1 can act on its own to activate the cell's neuroprotective machinery in a way that mimics the response induced by GDNF. This intriguing similarity suggests the possibility that Nurr1 may act by restoring the ability of the alpha-synuclein–affected neurons to access and respond to endogenous GDNF.”
Nurr1 is the focus of a number of drug development strategies because it is a nuclear receptor with properties that make them ideal drug targets. Nuclear receptors cross through the blood-brain barrier and into brain cells and have specific actions. This has attracted the attention of both the pharmaceutical industry and biotech companies and there are now several compounds in testing that activate Nurr1 function.
Dr. Björklund and his colleagues are now testing some of these compounds.
EXPERTS WEIGH IN
“This is a very important paper as it provides new insights into alpha-synuclein toxicity and may explain why clinical trials of GDNF in PD have largely failed,” said Joseph Jankovic, MD, professor of neurology and distinguished chair in movement disorders and director of the Parkinson's Disease Center and Movement Disorders Clinic at Baylor College of Medicine.
“This research also provides support for the role of Nurr1 not only in development and maintenance of dopaminergic neurons but also in neurotrophic factor signaling. It's possible that in the future the effects of trophic factors, such as GDNF or neurturin, can be enhanced by co-administration of a Nurr1 activator, such as the one we are currently studying in our Parkinson's Disease Research Laboratory. This Nurr1 activator, however, may also act on its own, independent of exogenous or endogenous neurotrophic factors.”
Jeffrey H. Kordower, PhD, professor of neurosurgery and director of the Research Center for Brain Repair at Rush Medical College, said that scientists involved with trophic factors had for years known that GDNF did not work in the alpha-synuclein models and “ignored this fact and used other models. But we all knew it was a problem.”
“This paper explains why GDNF doesn't work in alpha-synuclein models and explains how we can make it work. It is a major advance in the field,” he said. “Not only can we now understand the full breadth of trophic factors but this provides a target using Nurr1 in gene therapy trials,” said Dr. Kordower, whose own work has shown that Nurr1 is decreased in PD.
MORE ON TRIALS WITH GDNF
Glial cell-derived neurotropic factor (GDNF) is part of a family of substances that are now being developed as potential therapeutic growth factors. One of these sister molecules is neurturin. Ceregene, Inc., a San Diego-based company, is now conducting a phase 2b study to evaluate the safety and efficacy of using a viral vector to deliver a gene that makes neurturin into the substantia nigra and in the putamen in PD patients. The multicenter study, which plans to enroll 52 patients, will randomize half of the patients to gene therapy and the others to sham surgery.
A phase 1 study completed in 2006 with a dozen patients found that it was safe and well tolerated; the findings were published in May 2008 in the Lancet Neurology. The company then conducted a larger study with 58 PD patients, reporting in the December 2010 edition of the Lancet Neurology that there was no difference between those who received the viral vector with the active gene and those who did not. They measured motor coordination, gait, tremor and stiffness. They reported a modest improvement in other motor tests and in time spent freezing and being able to move. There was also a difference in some quality of life measures. Findings from these studies led to significant modifications in the technique now being used, including where the viral vectors are infused and in the dosing of the gene therapy.