The N-terminal region of Nurr1 (a.a 1-31) is essential for its efficient degradation by the ubiquitin proteasome pathway

Lysine-independent ubiquitination and degradation of REV-ERBα involves a bi-functional degradation control sequence at its N-terminus

REV-ERBα and REV-ERBβ proteins play crucial roles in linking the circadian system to overt daily rhythms in mammalian physiology and behavior. In most tissues, REV-ERBα protein robustly cycles such that it is detected only within a tight interval of 4-6 hours each day, suggesting both its synthesis and degradation are tightly controlled. Several ubiquitin ligases are known to drive REV-ERBα degradation, but how they interact with REV-ERBα and which lysine residues they ubiquitinate to promote degradation are unknown. In this study, we attempted to identify both ubiquitin-ligase-binding and ubiquitination sites within REV-ERBα required for its degradation. Surprisingly, mutating all lysine residues, the common sites for ubiquitin conjugation, in REV-ERBα to arginines (K20R), did very little to impair its degradation in cells. K20R were degraded much faster by co-expression of two E3 ligases, SIAH2 or SPSB4, suggesting possible N-terminal ubiquitination. To explore this, we examined if small deletions at the N-terminus of REV-ERBα would alter its degradation. Interestingly, deletion of amino acid (AA) residues 2 to 9 (delAA2-9) clearly resulted in a less stable REV-ERBα. We found that it was the length (i.e. 8 AA), and not the specific sequence, that confers stability in this region. Simultaneously, we also mapped the interaction site of the E3 ligase SPSB4 to this same region, specifically requiring AA4-9 of REV-ERBα. Thus, the first 9 AA of REV-ERBα has two opposing roles in regulating REV-ERBα turnover. Further, deleting eight additional AAs (delAA2-17) from the N-terminus strongly prevents REV-ERBα degradation. Combined, these results suggest that complex interactions within the first 25AAs potentially act as an endogenous 'switch' that allows REV-ERBα to exist in a stabilized conformation in order to accumulate at one time of day, but then rapidly shifts to a destabilized form, to enhance its removal at the end of its daily cycle.

Mechanisms of NURR1 Regulation: Consequences for Its Biological Activity and Involvement in Pathology

NURR1 (Nuclear receptor-related 1 protein or NR4A2) is a nuclear protein receptor transcription factor with an essential role in the development, regulation, and maintenance of dopaminergic neurons and mediates the response to stressful stimuli during the perinatal period in mammalian brain development. The dysregulation of NURR1 activity may play a role in various diseases, including the onset and progression of neurodegenerative diseases, and several other pathologies. NURR1 is regulated by multiple mechanisms, among which phosphorylation by kinases or SUMOylation are the best characterized. Both post-translational modifications can regulate the activity of NURR1, affecting its stability and transcriptional activity. Other non-post-translational regulatory mechanisms include changes in its subcellular distribution or interaction with other protein partners by heterodimerization, also affecting its transcription activity. Here, we summarize the currently known regulatory mechanisms of NURR1 and provide a brief overview of its participation in pathological alterations.

α-Synuclein Induces the GSK-3-Mediated Phosphorylation and Degradation of NURR1 and Loss of Dopaminergic Hallmarks

In Parkinson’s disease, the dysfunction of the dopaminergic nigrostriatal tract involves the loss of function of dopaminergic neurons of the substantia nigra pars compacta followed by death of these neurons. The functional recovery of these neurons requires a deep knowledge of the molecules that maintain the dopaminergic phenotype during adulthood and the mechanisms that subvert their activity. Previous studies have shown that transcription factor NURR1, involved in differentiation and maintenance of the dopaminergic phenotype, is downregulated by α-synuclein (α-SYN). In this study, we provide a mechanistic explanation to this finding by connecting α-SYN-induced activation of glycogen synthase kinase-3 (GSK-3) with NURR1 phosphorylation followed by proteasomal degradation. The use of sequential deletion mutants and single point mutants of NURR1 allowed the identification of a domain comprising amino acids 123-PSSPPTPSTPS-134 that is targeted by GSK-3 and leads to subsequent ubiquitination and proteasome degradation. This study provides a detailed analysis of the regulation of NURR1 stability by phosphorylation in synucleinopathies such as Parkinson’s disease.

The transcription factor Nurr1 is upregulated in amyotrophic lateral sclerosis patients and SOD1-G93A mice

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that affects both lower and upper motor neurons (MNs) in the central nervous system. ALS etiology is highly multifactorial and multifarious, and an effective treatment is still lacking. Neuroinflammation is a hallmark of ALS and could be targeted to develop new therapeutic approaches. Interestingly, the transcription factor Nurr1 has been demonstrated to have an important role in the inflammatory process in several neurological disorders, such as Parkinson's disease and multiple sclerosis. In the present paper, we demonstrate for the first time that Nurr1 expression levels are upregulated in the peripheral blood of ALS patients. Moreover, we investigated Nurr1 function in the SOD1-G93A mouse model of ALS. Nurr1 was strongly upregulated in the spinal cord during the asymptomatic and early symptomatic phases of the disease, where it promoted the expression of brain-derived neurotrophic factor mRNA and the repression of NFκB pro-inflammatory targets, such as inducible nitric oxide synthase. Therefore, we hypothesize that Nurr1 is activated in an early phase of the disease as a protective endogenous anti-inflammatory mechanism, although not sufficient to reverse disease progression. On the basis of these observations, Nurr1 could represent a potential biomarker for ALS and a promising target for future therapies.

The Critical Role of Nurr1 as a Mediator and Therapeutic Target in Alzheimer's Disease-related Pathogenesis

Several studies have revealed that the transcription factor nuclear receptor related 1 (Nurr1) plays several roles not only in the regulation of gene expression related to dopamine synthesis, but also in alternative splicing, and miRNA targeting. Moreover, it regulates cognitive functions and protects against inflammation-induced neuronal death. In particular, the role of Nurr1 in the pathogenesis of Parkinson's disease (PD) has been well investigated; for example, it has been shown that it restores behavioral and histological impairments in PD models. Although many studies have evaluated the connection between Nurr1 and PD pathogenesis, the role of Nurr1 in Alzheimer's disease (AD) remain to be studied. There have been several studies describing Nurr1 protein expression in the AD brain. However, only a few studies have examined the role of Nurr1 in the context of AD. Therefore, in this review, we highlight the overall effects of Nurr1 under the neuropathologic conditions related to AD. Furthermore, we suggest the possibility of using Nurr1 as a therapeutic target for AD or other neurodegenerative disorders.

Ubiquitination of nuclear receptors

Nuclear receptors (NRs) are cellular proteins, which upon ligand activation, act to exert regulatory control over transcription and subsequent expression. Organized via systemic classification into seven subfamilies, NRs partake in modulating a vast expanse of physiological functions essential for maintenance of life. NRs display particular characteristics towards ubiquitination, the process of addition of specific ubiquitin tags at appropriate locations. Orchestrated through groups of enzymes harboring a diverse array of specialized structural components, the ubiquitination process emphatically alters the fate or downstream effects of NRs. Such influence is especially prominent in transcriptional processes such as promoter clearing for optimization and degradation pathways eliminating or recycling targeted proteins. Ultimately, the ubiquitination of NRs carries significant implications in terms of generating pathological clinical manifestations. Increasing evidence from studies involving patients and disease models suggests a role for ubiquitinated NRs in virtually every organ system. This supports the broad repertoire of roles that NRs play in the body, including modulatory conductors, facilitators, responders to external agents, and critical constituents for pharmacological or biological interventions. This review aims to cover relevant background and mechanisms of NRs and ubiquitination, with a focus towards elucidating subsequent pathophysiology and therapeutics in clinical disorders encompassing such ubiquitinated NRs.

Efficient Generation of Dopamine Neurons by Synthetic Transcription Factor mRNAs

Generation of functional dopamine (DA) neurons is an essential step for the development of effective cell therapy for Parkinson's disease (PD). The generation of DA neurons can be accomplished by overexpression of DA-inducible genes using virus- or DNA-based gene delivery methods. However, these gene delivery methods often cause chromosomal anomalies. In contrast, mRNA-based gene delivery avoids this problem and therefore is considered safe to use in the development of cell-based therapy. Thus, we used mRNA-based gene delivery method to generate safe DA neurons. In this study, we generated transformation-free DA neurons by transfection of mRNA encoding DA-inducible genes Nurr1 and FoxA2. The delivery of mRNA encoding dopaminergic fate inducing genes proved sufficient to induce naive rat forebrain precursor cells to differentiate into neurons exhibiting the biochemical, electrophysiological, and functional properties of DA neurons in vitro. Additionally, the generation efficiency of DA neurons was improved by the addition of small molecules, db-cAMP, and the adjustment of transfection timing. The successful generation of DA neurons using an mRNA-based method offers the possibility of developing clinical-grade cell sources for neuronal cell replacement treatment for PD.

Directed differentiation of postnatal hippocampal neural stem cells generates nuclear receptor related‑1 protein‑ and tyrosine hydroxylase‑expressing cells

Parkinson's disease (PD) is a severe neurodegenerative disorder. Although the detailed underlying molecular mechanism remains to be elucidated, the major pathological feature of PD is the loss of dopaminergic (DA) neurons of the substantia nigra. The use of donor stem cells to replace DA neurons may be a key breakthrough in the treatment of PD. In the present study, the growth kinetics of hippocampal neural stem cells (Hip-NSCs) isolated from postnatal mice and cultured in vitro were observed, specifically the generation of cells expressing DA neuronal markers nuclear receptor related-1 protein (Nurr1) and tyrosine hydroxylase (TH). It was revealed that Hip-NSCs differentiated primarily into astrocytes when cultured in serum-containing medium. However, in low serum conditions, the number of βIII tubulin-positive neurons increased markedly. The proportion of Nurr1-positive cells and TH-positive neurons, significantly increased with increasing duration of directed differentiation of Hip-NSCs (P=0.0187 and 0.0254, respectively). The results of the present study reveal that Hip-NSCs may be induced to differentiate in vitro into neurons expressing Nurr1 and TH, known to be critical regulators of DA neuronal fate. Additionally, their expression may be necessary to facilitate neuronal maturation in vitro. These data suggest that Hip-NSCs may serve as a source of DA neurons for cell therapy in patients diagnosed with PD.

The Schizophrenia-Related Protein Dysbindin-1A Is Degraded and Facilitates NF-Kappa B Activity in the Nucleus

Dystrobrevin-binding protein 1 (DTNBP1), a gene encoding dysbindin-1, has been identified as a susceptibility gene for schizophrenia. Functioning with partners in synapses or the cytoplasm, this gene regulates neurite outgrowth and neurotransmitter release. Loss of dysbindin-1 affects schizophrenia pathology. Dysbindin-1 is also found in the nucleus, however, the characteristics of dysbindin in the nucleus are not fully understood. Here, we found that dysbindin-1A is degraded in the nucleus via the ubiquitin-proteasome system and that amino acids 2-41 at the N-terminus are required for this process. By interacting with p65, dysbindin-1A promotes the transcriptional activity of NF-kappa B in the nucleus and positively regulates MMP-9 expression. Taken together, the data obtained in this study demonstrate that dysbindin-1A protein levels are highly regulated in the nucleus and that dysbindin-1A regulates transcription factor NF-kappa B activity to promote the expression of MMP-9 and TNF-α.

Intrinsically disordered segments affect protein half-life in the cell and during evolution

Precise control of protein turnover is essential for cellular homeostasis. The ubiquitin-proteasome system is well established as a major regulator of protein degradation, but an understanding of how inherent structural features influence the lifetimes of proteins is lacking. We report that yeast, mouse, and human proteins with terminal or internal intrinsically disordered segments have significantly shorter half-lives than proteins without these features. The lengths of the disordered segments that affect protein half-life are compatible with the structure of the proteasome. Divergence in terminal and internal disordered segments in yeast proteins originating from gene duplication leads to significantly altered half-life. Many paralogs that are affected by such changes participate in signaling, where altered protein half-life will directly impact cellular processes and function. Thus, natural variation in the length and position of disordered segments may affect protein half-life and could serve as an underappreciated source of genetic variation with important phenotypic consequences.

Nur transcription factors in stress and addiction

The Nur transcription factors Nur77 (NGFI-B, NR4A1), Nurr1 (NR4A2), and Nor-1 (NR4A3) are a sub-family of orphan members of the nuclear receptor superfamily. These transcription factors are products of immediate early genes, whose expression is rapidly and transiently induced in the central nervous system by several types of stimuli. Nur factors are present throughout the hypothalamus-pituitary-adrenal (HPA) axis where are prominently induced in response to stress. Drugs of abuse and stress also induce the expression of Nur factors in nuclei of the motivation/reward circuit of the brain, indicating their participation in the process of drug addiction and in non-hypothalamic responses to stress. Repeated use of addictive drugs and chronic stress induce long-lasting dysregulation of the brain motivation/reward circuit due to reprogramming of gene expression and enduring alterations in neuronal function. Here, we review the data supporting that Nur transcription factors are key players in the molecular basis of the dysregulation of neuronal circuits involved in chronic stress and addiction.

Molecular Pathogenesis and Pathophysiology of Parkinson’s Disease: New Targets for New Therapies

Parkinson’s disease (PD) is a complex chronic neurodegenerative disease of unknown etiology. A conceptual framework for all chronic diseases involves a series of channels or pathways (aging, genetic, environment, oxidative stress, mitochondrial damage, protein aggregation, etc.) and their interactions. Those channels with specificities may explain the ‘developmental’ program that through transcriptional reprogramming results in stressed dopamine neurons that eventually become dysfunctional or die, giving rise to the clinical manifestations of PD. In Chapter 2 we review the molecular mechanisms of those channels that may be implicated in the pathogenesis of PD and the pathophysiology of the disease based on the anatomo‐physiological complexity of the basal ganglia. This illustrates that understanding the molecular mechanisms of a disease may not be enough, or we have to reach an adequate system level to understand the disease process. Finally, we suggest that common therapies used for the treatment of other chronic diseases may be useful for the treatment (or help to advance the understanding) of PD, as well as new targets for new therapies that may be useful in the prevention of, or to stop the progression of, PD and other synucleinopathies.