Transcription factor control in neuronal maintenance and survival

The Transcription factor (TF) Nurr1 is essential for the development of a group of dopamine neurons that are located in the ventral midbrain (also referred to as mesencephalon). A progressive pathology of these midbrain dopamine (mDA) neurons is evident in Parkinson s disease, the most frequent neurodegenerative movement disorder. In addition Nurr1 is expressed in a number of other CNS areas. In the hippocampus, Nurr1 is co-expressed with its homologues Nur77 and Nor1, collectively named for NR4A TF s. These TF s are rapidly and strongly induced in response to stressful stimuli and an acute NR4A induction has been noted in e. g. hippocampal, cortical and striatal neurons after ischemia, seizures and focal brain injury in rodents. However, the functional roles of the stress-induced NR4A-expression have remained unknown. As Nurr1 is critical for the mDA neuron development and is widely expressed in the adult CNS, we hypothesized that Nurr1 might also play a crucial role in maintaining mature neurons. To address this hypothesis, the functional consequences of gain-of-function or loss-of-function of NR4A proteins were assessed in maturing and adult neurons in vitro and in vivo. The research presented in paper I & II describes the consequences of spatiotemporal ablation of Nurr1 in mDA neurons in mice. Our results revealed that Nurr1 continues to be critical for the maturing mDA neurons and for maintaining a DA phenotype in the mDA neurons of adult mice. In paper III, the function of stress-induced NR4A proteins was characterized in vitro in cultured neurons and in vivo in the hippocampi of mice. The data revealed that stress-induced NR4A promoted neuroprotection in neurons, presumably by up-regulating a subset of neuroprotective genes. The work in paper IV deals with the mechanism by which Nurr1 mediates transcriptional activation. We identified a novel putative Nurr1 coregulator-binding site that might recruit as yet unknown coregulators

α-Synuclein-Induced Down-Regulation of Nurr1 Disrupts GDNF Signaling in Nigral Dopamine Neurons.

Glial cell line-derived neurotrophic factor (GDNF) and its close relative neurturin are currently in clinical trials for neuroprotection in patients with Parkinson disease (PD). However, in animal models of PD, GDNF fails to protect nigral dopamine (DA) neurons against α-synuclein-induced neurodegeneration. Using viral vector delivery of human wild-type α-synuclein to nigral DA neurons in rats, we show that the intracellular response to GDNF is blocked in DA neurons that overexpress α-synuclein. This block is accompanied by reduced expression of the transcription factor Nurr1 and its downstream target, the GDNF receptor Ret. We found that Ret expression was also reduced in nigral DA neurons in PD patients. Conditional knockout of Nurr1 in mice resulted in reduced Ret expression and blockade of the response to GDNF, whereas overexpression of Nurr1 restored signaling, providing protection of nigral DA neurons against α-synuclein toxicity. These results suggest that Nurr1 is a regulator of neurotrophic factor signaling and a key player in the cellular defense against α-synuclein toxicity

Transcription factor Nurr1 maintains fiber integrity and nuclear-encoded mitochondrial gene expression in dopamine neurons

Developmental transcription factors important in early neuron specification and differentiation often remain expressed in the adult brain. However, how these transcription factors function to mantain appropriate neuronal identities in adult neurons and how transcription factor dysregulation may contribute to disease remain largely unknown. The transcription factor Nurr1 has been associated with Parkinson's disease and is essential for the development of ventral midbrain dopamine (DA) neurons. We used conditional Nurr1 gene-targeted mice in which Nurr1 is ablated selectively in mature DA neurons by treatment with tamoxifen. We show that Nurr1 ablation results in a progressive pathology associated with reduced striatal DA, impaired motor behaviors, and dystrophic axons and dendrites. We used laser-microdissected DA neurons for RNA extraction and next-generation mRNA sequencing to identify Nurr1-regulated genes. This analysis revealed that Nurr1 functions mainly in transcriptional activation to regulate a battery of genes expressed in DA neurons. Importantly, nuclear-encoded mitochondrial genes were identified as the major functional category of Nurr1-regulated target genes. These studies indicate that Nurr1 has a key function in sustaining high respiratory function in these cells, and that Nurr1 ablation in mice recapitulates early features of Parkinson's disease

Targeting a Plk1-Controlled Polarity Checkpoint in Therapy-Resistant Glioblastoma-Propagating Cells

The treatment of glioblastoma (GBM) remains challenging in part due to the presence of stem-like tumor-propagating cells that are resistant to standard therapies consisting of radiation and temozolomide. Among the novel and targeted agents under evaluation for the treatment of GBM are BRAF/MAPK inhibitors, but their effects on tumor-propagating cells are unclear. Here, we characterized the behaviors of CD133(+) tumor-propagating cells isolated from primary GBM cell lines. We show that CD133(+) cells exhibited decreased sensitivity to the antiproliferative effects of BRAF/MAPK inhibition compared to CD133(-) cells. Furthermore, CD133(+) cells exhibited an extended G2-M phase and increased polarized asymmetric cell divisions. At the molecular level, we observed that polo-like kinase (PLK) 1 activity was elevated in CD133(+) cells, prompting our investigation of BRAF/PLK1 combination treatment effects in an orthotopic GBM xenograft model. Combined inhibition of BRAF and PLK1 resulted in significantly greater antiproliferative and proapoptotic effects beyond those achieved by monotherapy (P < 0.05). We propose that PLK1 activity controls a polarity checkpoint and compensates for BRAF/MAPK inhibition in CD133(+) cells, suggesting the need for concurrent PLK1 inhibition to improve antitumor activity against a therapy-resistant cell compartment

Targeting a Plk1-Controlled Polarity Checkpoint in Therapy-Resistant Glioblastoma-Propagating Cells

The treatment of glioblastoma (GBM) remains challenging in part due to the presence of stem-like tumor-propagating cells that are resistant to standard therapies consisting of radiation and temozolomide. Among the novel and targeted agents under evaluation for the treatment of GBM are BRAF/MAPK inhibitors, but their effects on tumor-propagating cells are unclear. Here, we characterized the behaviors of CD133+ tumor-propagating cells isolated from primary GBM cell lines. We show that CD133+ cells exhibited decreased sensitivity to the anti-proliferative effects of BRAF/MAPK inhibition compared to CD133− cells. Furthermore, CD133+ cells exhibited an extended G2/M phase and increased polarized asymmetric cell divisions. At the molecular level, we observed that polo-like kinase (PLK) 1 activity was elevated in CD133+ cells, prompting our investigation of BRAF/PLK1 combination treatment effects in an orthotopic GBM xenograft model. Combined inhibition of BRAF and PLK1 resulted in significantly greater anti-proliferative and pro-apoptotic effects beyond those achieved by monotherapy (p<0.05). We propose that PLK1 activity controls a polarity checkpoint and compensates for BRAF/MAPK inhibition in CD133+ cells, suggesting the need for concurrent PLK1 inhibition to improve antitumor activity against a therapy-resistant cell compartment