Dopaminergic neuron loss in mice due to increased levels of wild-type human α-Synuclein only takes place under conditions of accelerated aging.

Understanding the intricate pathogenic mechanisms behind Parkinson's disease (PD) and its multifactorial nature presents a significant challenge in disease modeling. To address this, we explore genetic models that better capture the disease's complexity. Given that aging is the primary risk factor for PD, this study investigates the impact of aging in conjunction with overexpression of wild-type human α-synuclein (α-Syn) in the dopaminergic system. This is achieved by introducing a novel transgenic mouse strain overexpressing α-Syn under the TH-promoter within the senescence-accelerated SAMP8 (P8) genetic background. Behavioral assessments, conducted at both 10 and 16 months of age, unveil motor impairments exclusive to P8 α-SynTg mice, a phenomenon conspicuously absent in α-SynTg mice. These findings suggest a synergistic interplay between heightened α-Syn levels and the aging process, resulting in motor deficits. These motor disturbances correlate with reduced dopamine (DA) levels, increased DA turnover, synaptic terminal loss, and notably, the depletion of dopaminergic neurons in the substantia nigra and noradrenergic neurons in the locus coeruleus. Furthermore, P8 α-SynTg mice exhibit alterations in gut transit time, mirroring early PD symptoms. In summary, P8 α-SynTg mice effectively replicate parkinsonian phenotypes by combining α-Syn transgene expression with accelerated aging. This model offers valuable insights into the understanding of PD and serves as a valuable platform for further research.

Interaction between Angiotensin Type 1, Type 2, and Mas Receptors to Regulate Adult Neurogenesis in the Brain Ventricular-Subventricular Zone.

The renin-angiotensin system (RAS), and particularly its angiotensin type-2 receptors (AT2), have been classically involved in processes of cell proliferation and maturation during development. However, the potential role of RAS in adult neurogenesis in the ventricular-subventricular zone (V-SVZ) and its aging-related alterations have not been investigated. In the present study, we analyzed the role of major RAS receptors on neurogenesis in the V-SVZ of adult mice and rats. In mice, we showed that the increase in proliferation of cells in this neurogenic niche was induced by activation of AT2 receptors but depended partially on the AT2-dependent antagonism of AT1 receptor expression, which restricted proliferation. Furthermore, we observed a functional dependence of AT2 receptor actions on Mas receptors. In rats, where the levels of the AT1 relative to those of AT2 receptor are much lower, pharmacological inhibition of the AT1 receptor alone was sufficient in increasing AT2 receptor levels and proliferation in the V-SVZ. Our data revealed that interactions between RAS receptors play a major role in the regulation of V-SVZ neurogenesis, particularly in proliferation, generation of neuroblasts, and migration to the olfactory bulb, both in young and aged brains, and suggest potential beneficial effects of RAS modulators on neurogenesis.

Synaptic Regulator α-Synuclein in Dopaminergic Fibers Is Essentially Required for the Maintenance of Subependymal Neural Stem Cells.

Synaptic protein α-synuclein (α-SYN) modulates neurotransmission in a complex and poorly understood manner and aggregates in the cytoplasm of degenerating neurons in Parkinson's disease. Here, we report that α-SYN present in dopaminergic nigral afferents is essential for the normal cycling and maintenance of neural stem cells (NSCs) in the brain subependymal zone of adult male and female mice. We also show that premature senescence of adult NSCs into non-neurogenic astrocytes in mice lacking α-SYN resembles the effects of dopaminergic fiber degeneration resulting from chronic exposure to 1-methyl-4-phenyl-1,2,3,6-tetra-hydropyridine or intranigral inoculation of aggregated toxic α-SYN. Interestingly, NSC loss in α-SYN-deficient mice can be prevented by viral delivery of human α-SYN into their sustantia nigra or by treatment with l-DOPA, suggesting that α-SYN regulates dopamine availability to NSCs. Our data indicate that α-SYN, present in dopaminergic nerve terminals supplying the subependymal zone, acts as a niche component to sustain the neurogenic potential of adult NSCs and identify α-SYN and DA as potential targets to ameliorate neurogenic defects in the aging and diseased brain.SIGNIFICANCE STATEMENT We report an essential role for the protein α-synuclein present in dopaminergic nigral afferents in the regulation of adult neural stem cell maintenance, identifying the first synaptic regulator with an implication in stem cell niche biology. Although the exact role of α-synuclein in neural transmission is not completely clear, our results indicate that it is required for stemness and the preservation of neurogenic potential in concert with dopamine.

Cyclin-Dependent Kinase 4 Regulates Adult Neural Stem Cell Proliferation and Differentiation in Response to Insulin.

Insulin is one of the standard components used to culture primary neurospheres. Although it stimulates growth of different types of cells, the effects of insulin on adult neural stem cells (NSCs) have not been well characterized. Here, we reveal that insulin stimulates proliferation, but not survival or self-renewal, of adult NSCs. This effect is mediated by insulin receptor substrate 2 (IRS2) and subsequent activation of the protein kinase B (or Akt), leading to increased activity of the G1-phase cyclin-dependent kinase 4 (Cdk4) and cell cycle progression. Neurospheres isolated from Irs2-deficient mice are reduced in size and fail to expand in culture and this impaired proliferation is rescued by introduction of a constitutively active Cdk4 (Cdk4R24C/R24C ). More interestingly, activation of the IRS2/Akt/Cdk4 signaling pathway by insulin is also necessary for the generation in vitro of neurons and oligodendrocytes from NSCs. Furthermore, the IRS2/Cdk4 pathway is also required for neuritogenesis, an aspect of neuronal maturation that has not been previously linked to regulation of the cell cycle. Differentiation of NSCs usually follows exit from the cell cycle due to increased levels of CDK-inhibitors which prevent activation of CDKs. In contrast, our data indicate that IRS2-mediated Cdk4 activity in response to a mitogen such as insulin promotes terminal differentiation of adult NSCs. Stem Cells 2017;35:2403-2416.

Regulation of the p19(Arf)/p53 pathway by histone acetylation underlies neural stem cell behavior in senescence-prone SAMP8 mice.

Brain aging is associated with increased neurodegeneration and reduced neurogenesis. B1/neural stem cells (B1-NSCs) of the mouse subependymal zone (SEZ) support the ongoing production of olfactory bulb interneurons, but their neurogenic potential is progressively reduced as mice age. Although age-related changes in B1-NSCs may result from increased expression of tumor suppressor proteins, accumulation of DNA damage, metabolic alterations, and microenvironmental or systemic changes, the ultimate causes remain unclear. Senescence-accelerated-prone mice (SAMP8) relative to senescence-accelerated-resistant mice (SAMR1) exhibit signs of hastened senescence and can be used as a model for the study of aging. We have found that the B1-NSC compartment is transiently expanded in young SAMP8 relative to SAMR1 mice, resulting in disturbed cytoarchitecture of the SEZ, B1-NSC hyperproliferation, and higher yields of primary neurospheres. These unusual features are, however, accompanied by premature loss of B1-NSCs. Moreover, SAMP8 neurospheres lack self-renewal and enter p53-dependent senescence after only two passages. Interestingly, in vitro senescence of SAMP8 cells could be prevented by inhibition of histone acetyltransferases and mimicked in SAMR1 cells by inhibition of histone deacetylases (HDAC). Our data indicate that expression of the tumor suppressor p19, but not of p16, is increased in SAMP8 neurospheres, as well as in SAMR1 neurospheres upon HDAC inhibition, and suggest that the SAMP8 phenotype may, at least in part, be due to changes in chromatin status. Interestingly, acute HDAC inhibition in vivo resulted in changes in the SEZ of SAMR1 mice that resembled those found in young SAMP8 mice.

IRS2 signalling is required for the development of a subset of sensory spinal neurons.

Insulin and insulin-like growth factor-I play important roles in the development and maintenance of neurons and glial cells of the nervous system. Both factors activate tyrosine kinase receptors, which signal through adapter proteins of the insulin receptor substrate (IRS) family. Although insulin and insulin-like growth factor-I receptors are expressed in dorsal root ganglia (DRG), the function of IRS-mediated signalling in these structures has not been studied. Here we address the role of IRS2-mediated signalling in murine DRG. Studies in cultured DRG neurons from different embryonic stages indicated that a subset of nerve growth factor-responsive neurons is also dependent on insulin for survival at very early time points. Consistent with this, increased apoptosis during gangliogenesis resulted in a partial loss of trkA-positive neurons in DRG of Irs2 mutant embryos. Analyses in adult Irs2(-/-) mice revealed that unmyelinated fibre afferents, which express calcitonin gene-related peptide/substance P and isolectin B4, as well as some myelinated afferents to the skin were affected by the mutation. The diminished innervation of glabrous skin in adult Irs2(-/-) mice correlated with longer paw withdrawal latencies in the hot-plate assay. Collectively, these findings indicate that IRS2 signalling is required for the proper development of spinal sensory neurons involved in the perception of pain.

Perivascular nerve fiber α-synuclein regulates contractility of mouse aorta: a link to autonomic dysfunction in Parkinson's disease.

Parkinson's disease and other neurodegenerative disorders associated to changes in alpha-synuclein often result in autonomic dysfunction, most of the time accompanied by abundant expression of this synaptic protein in peripheral autonomic neurons. Given that expression of alpha-synuclein in vascular elements has been previously reported, the present study was undertaken to determine whether alpha-synuclein directly participates in the regulation of vascular responsiveness. We detected by immunohistochemistry perivascular nerve fibers containing alpha-synuclein in the aorta of mice while aortic endothelial cells and muscular fibers themselves did not exhibit detectable levels of this protein. To assess the effect of alpha-synuclein on vascular reactivity, aortic ring preparations obtained from alpha-synuclein-deficient knockout mice and from transgenic mice overexpressing human wild-type alpha-synuclein under the control of the tyrosine hydroxylase-promoter were mounted and equilibrated in organ baths for isometric tension recording. Lack of alpha-synuclein did not modify the relaxant responses to the endothelium-dependent (acetylcholine) and -independent (sodium nitroprusside) vasodilators, but resulted in a greater than normal norepinephrine-induced vasoconstriction along with a lowered response to dopamine, suggesting potential presynaptic changes in dopamine and norepinephrine releases in knockout mice. Overexpression of alpha-synuclein in TH-positive fibers resulted in complex abnormal responses, characterized by lowered acetylcholine-induced relaxation and lowered norepinephrin-induced contraction. Taken together, our data show for the first time that alpha-synuclein is present in sympathetic fibers supplying the murine aorta and provide evidence that changes in alpha-synuclein levels in perivascular fibers play a physiological role in the regulation of vascular function.

Vulnerability of peripheral catecholaminergic neurons to MPTP is not regulated by alpha-synuclein.

Although generally considered a prototypical movement disorder, Parkinson's disease is commonly associated with a broad-spectrum of non-motor symptoms, including autonomic dysfunctions caused by significant alterations in catecholaminergic neurons of the peripheral sympathetic nervous system. Here we present evidence that alpha-synuclein is highly expressed by sympathetic ganglion neurons throughout embryonic and postnatal life and that it is found in tyrosine hydroxylase-positive sympathetic fibers innervating the heart of adult mice. However, mice deficient in alpha-synuclein do not exhibit any apparent alterations in sympathetic development. Sympathetic neurons isolated from mouse embryos and early postnatal mice are sensitive to the parkinsonian drug MPTP/MPP(+) and intoxication requires entry of the neurotoxin through the noradrenaline transporter. Furthermore, recovery of noradrenaline from cardiac sympathetic fibers is reduced in adult mice treated with MPTP systemically. However, MPP(+)-induced sympathetic neuron loss in vitro or MPTP-induced cardiac noradrenaline depletion in vivo is not modified in mice lacking alpha-synuclein. This is in clear contrast with the observation that dopaminergic neurons of the central nervous system are significantly less vulnerable to MPTP/MPP(+) in the absence of alpha-synuclein, suggesting different actions of this molecule in central and peripheral catecholaminergic neurons.