Parkin protects mitochondrial genome integrity and supports mitochondrial DNA repair.

Mutations in the parkin gene are the most common cause of recessive familial Parkinson disease (PD). Parkin has been initially characterized as an ubiquitin E3 ligase, but the pathological relevance of this activity remains uncertain. Recently, an impressive amount of evidence has accumulated that parkin is involved in the maintenance of mitochondrial function and biogenesis. We used a human neuroblastoma cell line as a model to study the influence of endogenous parkin on mitochondrial genomic integrity. Using an unbiased chromatin immunoprecipitation approach, we found that parkin is associated physically with mitochondrial DNA (mtDNA) in proliferating as well as in differentiated SH-SY5Y cells. In vivo, the association of parkin with mtDNA could be confirmed in brain tissue of mouse and human origin. Replication and transcription of mtDNA were enhanced in SH-SY5Y cells over-expressing the parkin gene. The ability of parkin to support mtDNA-metabolism was impaired by pathogenic parkin point mutations. Most importantly, we show that parkin protects mtDNA from oxidative damage and stimulates mtDNA repair. Moreover, higher susceptibility of mtDNA to reactive oxygen species and reduced mtDNA repair capacity was observed in parkin-deleted fibroblasts of a PD patient. Our data indicate a novel role for parkin in directly supporting mitochondrial function and protecting mitochondrial genomic integrity from oxidative stress.

Genome-wide expression profiling and functional network analysis upon neuroectodermal conversion of human mesenchymal stem cells suggest HIF-1 and miR-124a as important regulators.

Tissue-specific stem cells, such as bone-marrow-derived human mesenchymal stem cells (hMSCs), are thought to be lineage restricted and therefore, could only be differentiated into cell types of the tissue of origin. Several recent studies however have suggested that these types of stem cells might be able to break barriers of germ layer commitment and differentiate in vitro into cells with neuroectodermal properties. We reported earlier about efficient conversion of adult hMSCs into a neural stem cell (NSC)-like population (hmNSCs, for human marrow-derived NSC-like cells) with all major properties of NSCs including functional neuronal differentiation capacity. Here we compared the transcriptomes from hMSCs and hmNSCs using a novel strategy by combining classic Affymetrix oligonucleotide microarray profiling with regulatory and protein interaction network analyses to shed light on regulatory protein networks involved in this neuroectodermal conversion process. We found differential regulation of extracellular matrix protein transcripts, up-regulation of distinct neuroectodermal and NSCs marker genes and local chromosomal transcriptional up-regulation at chromosome 4q13.3. In comparison to hMSCs and primary adult hippocampal NSCs, the transcriptome of hmNSCs displayed minor overlap with both other cell populations. Advanced bioinformatics of regulated genes upon neuroectodermal conversion identified transcription factor networks with HIF-1 and microRNA miR-124a as potential major regulators. Together, transgerminal neuroectodermal conversion of hMSCs into NSC-like cells is accompanied by extensive changes of their global gene expression profile, which might be controlled in part by transcription factor networks related to HIF-1 and miR-124a.

Initiation of dopaminergic differentiation of Nurr1(-) mesencephalic precursor cells depends on activation of multiple mitogen-activated protein kinase pathways.

Interleukin-1 (IL-1) plays a pivotal role in terminal dopaminergic differentiation of midbrain-derived neural precursor cells already committed to the mesencephalic dopaminergic phenotype (named mdNPCs for mesencephalic dopaminergic neural precursor cells). Here we characterized the molecular events in long-term expanded rat nuclear receptor related-1(-) (Nurr1(-)) mdNPCs in response to IL-1beta during their terminal dopaminergic specification. We showed that IL-1beta induced a rapid induction of mRNA of dopaminergic key fate-determining transcription factors, such as Nurr1 and Pitx3, and a subsequent increase of tyrosine hydroxylase protein as an early marker for dopaminergic neurons in vitro. These effects of IL-1beta were specific for mdNPCs and were not observed in striatal neural precursor cells (NPCs). Surprisingly, IL-1beta did not activate the NF-kappaB pathway or the transcription factor activating protein 1 (AP-1), but inhibition of nuclear translocation of NF-kappaB by SN50 facilitated IL-1beta-induced Nurr1 expression and dopaminergic differentiation of mdNPCs. Incubation of mdNPCs with IL-1beta led to a rapid phosphorylation of ERK1/2 and p38 mitogen-activated protein (MAP) kinases within 1 to 3 hours, whereas Jun kinase was not phosphorylated in response to IL-1beta. Consistently, inhibition of the ERK1/2 pathway or p38 MAP kinase blocked Nurr1 upregulation and further dopaminergic specification of mdNPCs, but not differentiation into MAP2ab(+) neurons. IL-1 receptor antagonist did not block early dopaminergic differentiation events, suggesting that the effects of IL-1beta are not mediated through activation of IL-1 receptor type I. Our results indicate that induction of terminal dopaminergic specification of Nurr1(-) mdNPCs by IL-1beta depends on activation of the ERK1/2 and p38 MAP kinase pathway.

Lack of hypoxia-inducible factor-1 alpha impairs midbrain neural precursor cells involving vascular endothelial growth factor signaling.

Oxygen tension is critical for proliferation of human and murine midbrain-derived neural precursor cells (mNPCs). Here, we conditionally inactivated the hypoxia-responsive transcription factor hypoxia-inducible factor-1alpha (HIF-1alpha) in murine NPCs to determine its role in proliferation, survival, and dopaminergic differentiation in vitro as well as survival of murine dopaminergic neurons in vivo. HIF-1alpha conditional knock-out (HIF-1alpha CKO) mNPCs showed midbrain-specific impairment of survival and proliferation. Dopaminergic differentiation of HIF-1alpha CKO mNPCs in vitro was markedly reduced. Expression of vascular endothelial growth factor (VEGF) mRNA was reduced in HIF-1alpha CKO mNPCs, whereas erythropoietin signaling was not affected. Treatment of HIF-1alpha CKO mNPCs with 50 ng/ml VEGF partially recovered proliferation and dopaminergic differentiation in vitro. In substantia nigra (SN) of adult HIF-1alpha CKO mice, protein levels of dopaminergic marker molecules such as tyrosine hydroxylase (TH) and aldehyde dehydrogenase were reduced by 41 and 61%, respectively. The cell survival marker Bcl-2 was reduced by 58% while caspase-3 was activated. Nonbiased stereological cell counts of TH-positive neurons in SN of young adult HIF-1alpha CKO mice revealed a reduction of 31% compared with cre/wt mice (in which the wild-type Hif1a allele is expressed in parallel with the Cre recombinase allele). However, we found no impairment of striatal dopamine concentrations or locomotor behavior. In conclusion, HIF-1alpha seems to be a transcription factor relevant to the development and survival of substantia nigra dopaminergic neurons involving VEGF signaling.

Dopamine D2/D3 receptor stimulation fails to promote dopaminergic neurogenesis of murine and human midbrain-derived neural precursor cells in vitro.

The potential application of neural precursor cells (NPCs) in brain repair of neurodegenerative diseases has placed the factors capable of stimulating neurogenesis under increasing attention. Among these factors are dopamine (DA) D2/D3 receptor agonists, like 7-hydroxy-dipropylaminotetralin (7-OH-DPAT). The purpose of this investigation was to explore proliferating and neurostimulating effects of this drug in murine and human NPCs derived from the fetal midbrain. In both cell types, dopamine D2 and D3 receptors were detected by microarray data analysis and quantitative RT-PCR. Despite D2/D3 receptors expression, treatment with 7-OH-DPAT did not affect proliferation, survival, or neurogenesis of murine and human NPCs. Our data question the relevance of neuroregenerative effects of dopamine agonists for human predopaminergic cells as well as patients with Parkinson's disease.

No dopamine cell loss or changes in cytoskeleton function in transgenic mice expressing physiological levels of wild type or G2019S mutant LRRK2 and in human fibroblasts.

Mutations within the LRRK2 gene have been identified in Parkinson's disease (PD) patients and have been implicated in the dysfunction of several cellular pathways. Here, we explore how pathogenic mutations and the inhibition of LRRK2 kinase activity affect cytoskeleton dynamics in mouse and human cell systems. We generated and characterized a novel transgenic mouse model expressing physiological levels of human wild type and G2019S-mutant LRRK2. No neuronal loss or neurodegeneration was detected in midbrain dopamine neurons at the age of 12 months. Postnatal hippocampal neurons derived from transgenic mice showed no alterations in the seven parameters examined concerning neurite outgrowth sampled automatically on several hundred neurons using high content imaging. Treatment with the kinase inhibitor LRRK2-IN-1 resulted in no significant changes in the neurite outgrowth. In human fibroblasts we analyzed whether pathogenic LRRK2 mutations change cytoskeleton functions such as cell adhesion. To this end we compared the adhesion characteristics of human skin fibroblasts derived from six PD patients carrying one of three different pathogenic LRRK2 mutations and from four age-matched control individuals. The mutant LRRK2 variants as well as the inhibition of LRRK2 kinase activity did not reveal any significant cell adhesion differences in cultured fibroblasts. In summary, our results in both human and mouse cell systems suggest that neither the expression of wild type or mutant LRRK2, nor the inhibition of LRRK2 kinase activity affect neurite complexity and cellular adhesion.

Lack of vascular endothelial growth factor receptor-2/Flk1 signaling does not affect substantia nigra development.

Oxygen tension is critical for proliferation of human and murine midbrain-derived neural precursor cells (mNPCs). Lack of hypoxia-inducible factor-1α (HIF1α) impairs midbrain dopaminergic neurogenesis which could be rescued by vascular endothelial growth factor (VEGF) via VEGFR-2 signaling. Here, we conditionally inactivated the VEGFR-2, encoded by the fetal liver kinase 1 (Flk1) gene, in murine NPCs to determine its role in proliferation and survival in vitro as well as survival of dopaminergic neurons in vivo. Flk1 conditional knock-out (Flk1 CKO) mice showed no general brain phenotype. There was no midbrain-specific impairment of NPC proliferation as seen in HIF1α CKO mice. In the substantia nigra (SN) of adult Flk1 CKO mice, nonbiased stereological cell counts revealed no reduction of TH-positive neurons of Flk1 CKO mice compared with control Cre/wt mice (in which the wild-type Flk1 allele is expressed in parallel with the Cre recombinase allele). In conclusion, VEGF receptor signaling seems not to be relevant to the development and survival of substantia nigra dopaminergic neurons within the hypoxia-HIF1α signaling pathway.

Genetic correction of a LRRK2 mutation in human iPSCs links parkinsonian neurodegeneration to ERK-dependent changes in gene expression.

The LRRK2 mutation G2019S is the most common genetic cause of Parkinson's disease (PD). To better understand the link between mutant LRRK2 and PD pathology, we derived induced pluripotent stem cells from PD patients harboring LRRK2 G2019S and then specifically corrected the mutant LRRK2 allele. We demonstrate that gene correction resulted in phenotypic rescue in differentiated neurons and uncovered expression changes associated with LRRK2 G2019S. We found that LRRK2 G2019S induced dysregulation of CPNE8, MAP7, UHRF2, ANXA1, and CADPS2. Knockdown experiments demonstrated that four of these genes contribute to dopaminergic neurodegeneration. LRRK2 G2019S induced increased extracellular-signal-regulated kinase 1/2 (ERK) phosphorylation. Transcriptional dysregulation of CADPS2, CPNE8, and UHRF2 was dependent on ERK activity. We show that multiple PD-associated phenotypes were ameliorated by inhibition of ERK. Therefore, our results provide mechanistic insight into the pathogenesis induced by mutant LRRK2 and pointers for the development of potential new therapeutics.

Restorative approaches in Parkinson's Disease: which cell type wins the race?

Parkinson's disease (PD) is a progressive neurodegenerative movement disorder and is characterized by a continuous and selective loss of dopaminergic neurons in the midbrain with a subsequent reduction of the neurotransmitter dopamine in the striatum. Strategies to overcome limitations of conventional symptomatic treatment have employed cell-based strategies including transplantation of developing neural tissue or neural stem cells (NSCs) into the degenerated host brain. Still there is a tug of war for determining the ideal cell source for transplantation strategies. ES cells have the widest and most blatant potential to become the winner because they promise to be made in high quantities and to hold large amounts of the desired cell type. Adult and fetal neural stem cells have the capacity to self-renew and they are able to differentiate into all major cell-types of the brain without bearing tumorigenic potential. They can be isolated and expanded in vitro for a long time retaining the potential to differentiate into important neural cell types including dopaminergic neurons. Another source for cell-replacement are bone marrow stromal cells (MSCs). These cells can be converted into a cell type with all major features of NSCs. Efforts are made to improve these cell sources for transplantation or finding new cell sources like induced pluripotent stem cells (iPS). However, novel grounds are broken: bridging transplantations might improve the clinical outcome by restoring the nigro-striatal pathway and recruitment of endogenous stem cells by pharmacological manipulations uses the inherent regenerative potential of the diseased brain. This review discusses recent data on stem cell technology with respect to cell replacement strategies in PD as well as endogenous dopaminergic regeneration.

Transcription profiling of adult and fetal human neuroprogenitors identifies divergent paths to maintain the neuroprogenitor cell state.

Global gene expression profiling was performed using RNA from adult human hippocampus-derived neuroprogenitor cells (NPCs) and multipotent frontal cortical fetal NPCs compared with adult human mesenchymal stem cells (hMSCs) as a multipotent adult stem cell control, and adult human hippocampal tissue, to define a gene expression pattern that is specific for human NPCs. The results were compared with data from various databases. Hierarchical cluster analysis of all neuroectodermal cell/tissue types revealed a strong relationship of adult hippocampal NPCs with various white matter tissues, whereas fetal NPCs strongly correlate with fetal brain tissue. However, adult and fetal NPCs share the expression of a variety of genes known to be related to signal transduction, cell metabolism and neuroectodermal tissue. In contrast, adult NPCs and hMSCs overlap in the expression of genes mainly involved in extracellular matrix biology. We present for the first time a detailed transcriptome analysis of human adult NPCs suggesting a relationship between hippocampal NPCs and white matter-derived precursor cells. We further provide a framework for standardized comparative gene expression analysis of human brain-derived NPCs with other stem cell populations or differentiated tissues. Disclosure of potential conflicts of interest is found at the end of this article.

Epigenetic conversion of human adult bone mesodermal stromal cells into neuroectodermal cell types for replacement therapy of neurodegenerative disorders.

Tissue-specific stem cells, such as bone marrow-derived mesodermal stromal cells (MSCs), are thought to be lineage restricted and, therefore, could only be differentiated into cell types of the tissue of origin. Several recent studies, however, suggest that these types of stem cells might be able to break barriers of germ layer commitment and differentiate in vitro and/or in vivo into cells of different tissues, such as neuroectodermal cell types. Recently, protocols for high-yield generation of undifferentiated neural stem cell (NSC)-like cells from MSCs of primate and human origin were reported. Undifferentiated NSCs are commonly used and are more suitable for neurotransplantation compared with fully differentiated neural cells, as differentiated neural cells are well known to poorly survive detachment and subsequent transplantation procedures. These human MSC-derived NSC-like cells (MSC-NSCs) grow in neurosphere-like structures and express high levels of early neuroectodermal markers, but lose characteristics of MSCs. In the presence of selected growth factors, human MSC-NSCs can be differentiated into the three main neural phenotypes: astroglia, oligodendroglia and neurons. Compared with direct differentiation of human MSCs into mature neural cells, the conversion step seems to be essential to generate mature functional neuroectodermal cells. This review describes the techniques for the conversion of human MSCs into NSCs and summarises the data on epigenetic conversion of human MSCs into immature neuroectodermal cells. These cells provide a powerful tool for investigating the molecular mechanisms of neural differentiation, and might serve as an autologous cell source to treat acute and chronic neurodegenerative diseases.

Multipotent neural stem cells from the adult tegmentum with dopaminergic potential develop essential properties of functional neurons.

Neurogenesis in the adult brain occurs within the two principal neurogenic regions: the hippocampus and the subventricular zone of the lateral ventricles. The occurrence of adult neurogenesis in non-neurogenic regions, including the midbrain, remains controversial, but isolation of neural stem cells (NSCs) from several parts of the adult brain, including the substantia nigra, has been reported. Nevertheless, it is unclear whether adult NSCs do have the capacity to produce functional dopaminergic neurons, the cell type lost in Parkinson's disease. Here, we describe the isolation, expansion, and in vitro characterization of adult mouse tegmental NSCs (tNSCs) and their differentiation into functional nerve cells, including dopaminergic neurons. These tNSCs showed neurosphere formation and expressed high levels of early neuroectodermal markers, such as the proneural genes NeuroD1, Neurog2, and Olig2, the NSC markers Nestin and Musashi1, and the proliferation markers Ki67 and BrdU (5-bromo-2-deoxyuridine). The cells showed typical propidium iodide-fluorescence-activated cell sorting analysis of slowly dividing cells. In the presence of selected growth factors, tNSCs differentiated into astroglia, oligodendroglia, and neurons expressing markers for cholinergic, GABAergic, and glutamatergic cells. Electrophysiological analyses revealed functional properties of mature nerve cells, such as tetrodotoxin-sensitive sodium channels, action potentials, as well as currents induced by GABA (gamma-aminobutyric acid), glutamate, and NMDA (N-methyl-D-aspartate). Clonal analysis demonstrated that individual NSCs retain the capacity to generate both glia and neurons. After a multistep differentiation protocol using co-culture conditions with PA6 stromal cells, a small number of cells acquired morphological and functional properties of dopaminergic neurons in culture. Here, we demonstrate the existence of adult tNSCs with functional neurogenic and dopaminergic potential, a prerequisite for future endogenous cell replacement strategies in Parkinson's disease.

Mesodermal cell types induce neurogenesis from adult human hippocampal progenitor cells.

Neurogenesis in the adult human brain occurs within two principle neurogenic regions, the hippocampus and the subventricular zone (SVZ) of the lateral ventricles. Recent reports demonstrated the isolation of human neuroprogenitor cells (NPCs) from these regions, but due to limited tissue availability the knowledge of their phenotype and differentiation behavior is restricted. Here we characterize the phenotype and differentiation capacity of human adult hippocampal NPCs (hNPCs), derived from patients who underwent epilepsy surgery, on various feeder cells including fetal mixed cortical cultures, mouse embryonic fibroblasts (MEFs) and PA6 stromal cells. Isolated hNPCs were cultured in clonal density by transferring the cells to serum-free media supplemented with FGF-2 and EGF in 3% atmospheric oxygen. These hNPCs showed neurosphere formation, expressed high levels of early neuroectodermal markers, such as the proneural genes NeuroD1 and Olig2, the NSC markers Nestin and Musashi1, the proliferation marker Ki67 and significant activity of telomerase. The phenotype was CD15low/-, CD34-, CD45- and CD133-. After removal of mitogens and plating them on poly D-lysine, they spontaneously differentiated into a neuronal (MAP2ab+), astroglial (GFAP+), and oligodendroglial (GalC+) phenotype. Differentiated hNPCs showed functional properties of neurons, such as sodium channels, action potentials and production of the neurotransmitters glutamate and GABA. Co-culture of hNPCs with fetal cortical cultures, MEFs and PA6 cells increased neurogenesis of hNPCs in vitro, while only MEFs and PA6 cells also led to a morphological and functional neurogenic maturation. Together we provide a first detailed characterization of the phenotype and differentiation potential of human adult hNPCs in vitro. Our findings reinforce the emerging view that the differentiation capacity of adult hNPCs is critically influenced by non-neuronal mesodermal feeder cells.

Efficient generation of neural stem cell-like cells from adult human bone marrow stromal cells.

Clonogenic neural stem cells (NSCs) are self-renewing cells that maintain the capacity to differentiate into brain-specific cell types, and may also replace or repair diseased brain tissue. NSCs can be directly isolated from fetal or adult nervous tissue, or derived from embryonic stem cells. Here, we describe the efficient conversion of human adult bone marrow stromal cells (hMSC) into a neural stem cell-like population (hmNSC, for human marrow-derived NSC-like cells). These cells grow in neurosphere-like structures, express high levels of early neuroectodermal markers, such as the proneural genes NeuroD1, Neurog2, MSl1 as well as otx1 and nestin, but lose the characteristics of mesodermal stromal cells. In the presence of selected growth factors, hmNSCs can be differentiated into the three main neural phenotypes: astroglia, oligodendroglia and neurons. Clonal analysis demonstrates that individual hmNSCs are multipotent and retain the capacity to generate both glia and neurons. Our cell culture system provides a powerful tool for investigating the molecular mechanisms of neural differentiation in adult human NSCs. hmNSCs may therefore ultimately help to treat acute and chronic neurodegenerative diseases.