Without Nurr1, a gene at the root of a rare form of familial Parkinson disease (PD), the brain’s inflammatory response spirals out of control and damages neurons. This finding may apply to sporadic PD and other neurodegenerative diseases, according to the authors of a paper in today’s issue of Cell. They find that Nurr1 is not only important in dopaminergic neurons, but also acts in glia to check inflammation.
The transcription factor Nurr1 is known primarily for its role in the development and maintenance of dopaminergic neurons, which degenerate in people with Parkinson’s. In a few families with inherited PD, Nurr1 mutations result in less of the protein being produced. Clinically, the Nurr1 disease closely mirrors other forms of PD; age of onset ranges from 45 to 67.
First author Kaoru Saijo, principal investigator Christopher Glass, both of the University of California, San Diego, and their coauthors set out to investigate the role of Nurr1 in inflammation in the brain. Saijo and Glass, both immunologists, collaborated closely with neuroscientists in the lab of Fred Gage at the Salk Institute in La Jolla, California. Nurr1-deficient mice die young, so the scientists instead used short hairpin RNA (shRNA), delivered by lentivirus, to knock down Nurr1 expression in the substantia nigra of wild-type animals. Two days after shRNA treatment, the scientists injected the same area with lipopolysaccharide (LPS), a common component of the bacterial cell membrane that induces a strong immune response in animals.
Normally it takes a few weeks for the LPS-induced immune response to damage dopaminergic neurons, but in the Nurr1-depleted mice, the effect was evident seven days after LPS treatment. These animals had fewer dopaminergic neurons, identified because they express tyrosine hydroxylase (TH), than mice that received control shRNA. The Nurr1-deficient mice also expressed higher levels of inflammation markers, such as TNFα, than control animals. In addition, the scientists overexpressed α-synuclein, another protein associated with familial PD, to cause inflammation; in the absence of Nurr1, the immune response was much stronger and damaged dopaminergic neurons. Saijo and colleagues concluded that Nurr1 protects neurons from an otherwise exaggerated immune response.
Nurr1 could be shielding neurons from inflammation by acting as a protector in the neurons themselves, or by preventing the microglia or astrocytes surrounding them from releasing neurotoxic factors. The authors used tissue culture to examine these possibilities. To assay the effect of Nurr1 deficiency in neurons, they knocked down its expression in N2A neuroblastoma cells. This did not render the neurons extra-sensitive to LPS, suggesting that it is the glia that cause neuron death. Conditioned media from microglia first treated with Nurr1 shRNA and then stimulated with LPS killed nearly all the dopaminergic neurons in mixed cultures, leaving behind other types of neurons and astrocytes. This implies that Nurr1 normally acts in microglia to dampen their inflammatory activation. Since those cultures contained astrocytes, the scientists wondered if astrocytes could amplify the microglia’s neurotoxic effect. They found that when they conditioned media first with Nurr1-deficient microglia, then with Nurr1-depleted astrocytes, the media was more toxic to pure neuroblastoma cultures than media conditioned with either cell type alone. This suggested to the authors that astrocytes amplify the toxicity initiated by microglia.
Then, the scientists delved into the details of Nurr1 activity. “We basically deconvoluted signaling and molecular pathways that enable Nurr1 to play its role,” Glass said. Nurr1 can act as a transcriptional activator or as a co-repressor. It behaves as a constitutively active activator by binding specific DNA sequences and turning on genes for dopaminergic neuron maintenance. SUMOylation of the protein likely turns Nurr1 into a repressor, the authors suggest. In this form it recruits the co-repressor CoREST and its associated partners to knock the transcription factor NF-κB off the DNA, thus silencing the inflammatory genes NF-κB activates.
The scientists propose a model in which something in the brain—perhaps infection, perhaps the natural wear and tear of aging—triggers an inflammatory response in the microglia. They release cytokines that induce astrocytes to activate as well, and both release toxic factors, such as nitric oxide and reactive oxygen species, that can injure neurons. Normally, Nurr1 then steps in to dampen the immune response and restore the cells to their usual quiescent state. But without Nurr1, that negative feedback cannot occur. Instead the glia continue to harm neurons, neural death induces further inflammation, and the system becomes locked into an intensifying cycle of destruction.
This study is “superb,” said Robert Nussbaum of the University of California, San Francisco, because the authors combine biochemistry, cell culture, and in vivo techniques to support their model. “They never make a claim based on a single kind of experiment; they go at it in two or three ways,” he said. Nussbaum noted that Nurr1 was recently shown to regulate α-synuclein levels as well (Yang and Latchman, 2008), so loss of the protein is “a double whammy.”
Nurr1 deficiencies are rare, but inflammation has been implicated in other forms of PD, so inflammation in excess may be a common amplifier of this and other neurodegenerative diseases. An overblown immune response is common in chronic disease, Glass said, and scientists must consider the whole system, not just the neurons, when developing therapies for neurodegenerative disease. “Don’t forget about the glial effect,” Saijo said. “Nurr1 function is not only important in the neuron site…if you don’t have the protein in the glial site, the glia cannot control the inflammation.” That makes inflammation an appealing target for drug development, the authors suggested.—Amber Dance
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