Using in vitro cell culture systems and in vivo models applied to β-catenin reporters, this study uncovered the adult rodent midbrain Aq-ventricular region as a novel Wnt-responsive niche. MPTP-induced DA neuron death promoted a remarkable astrocyte-dependent remodeling and Wnt/β-catenin signaling activation in Nurr1+ postmitotic DA precursors, in surviving and repairing SNpc-DA neurons, a process correlated with a robust time-dependent DA neurorescue. By contrast, the changing properties of midbrain-Aq microenvironment with age impact in DA neurogenic potential of Aq-mNPCs via loss of astrocytic Wnt1 and failure of Wnt/β-catenin signaling activation both inside and outside the niche, in turn associated to failure to recover from MPTP insult. Importantly, aged mNPCs still retain their neurogenic and DA differentiation potential when Wnt/β-catenin signaling is restored via “astrocyte rejuvenation”-induced Wnt1 expression or under Wnt/β-catenin activation regimens, such as GSK-3β antagonism, leading to DA neuron formation. Together, these findings suggest that disruption of a key neurodevelopmental signaling pathway with age may predispose to loss of mDA plasticity via inhibition of Wnt/β-catenin signaling as a prelude for PD development and vulnerability. These results may indicate the potential to restore mDA neuron functionality by activating Wnt/β-catenin signaling in endogenous Wnt-responsive sources, through either pharmacological/cellular approaches aimed at activating/recruiting endogenous progenitors and rescuing the imperiled/diseased DA neurons [44, 47].
A lack of appropriate niche environmental signals is recognized to restrict the neurogenic potential of multipotent progenitors isolated from PD-related brain regions [29, 32, 43-46, 64]. Studying the PVRs throughout the whole ventricular axis of the young intact mouse brain, Hermann et al.  reported that proliferative cell populations are restricted to the PVR-LV. However, in Aq-PVRs, the PSA-Ncam+-like cells observed in vivo strongly correlated with the number of neurosphere-forming cells isolated, in vitro, suggesting that a quiescent subtype of PSA-Ncam+ cells might be the source of mNPCs endowed with neurogenic and DA differentiation potential [45, 46]. Here, the profound morphological remodeling of the Aq-PVRs and β-catenin signaling activation upon DA neuron injury may suggest a high degree of plasticity of this caudal midbrain ventricular region upon lesion of DA nigrostriatal system, while loss of plasticity was observed with age. Interestingly, such upregulated Wnt/β-catenin response in Aq-PVRs, close to the SNpc DA cell bodies, as opposed to the transient β-catenin downregulation observed in SVZ [22, 39] adjacent to striatal DA terminals [29-31], denotes a region-specific response to DA neurodegeneration. The positive correlation between NPC proliferation and β-catenin signaling in the SVZ [22, 39, 58] as opposed to the Aq-PVRs [45, 59] may be linked to factors/mechanisms restraining the proliferative potential within the Aq-PVRs, in vivo, as indicated by the robust proliferation observed after growth factor  or herein with GSK-3β antagonist infusion. Within the SVZ, the intimate contact of NPCs with surrounding glia ( and references herein), coupled to the vast array of growth/neurotrophic factors, neurotransmitters (in particular dopamine), morphogens, cytokines, and Wnt/β-catenin signaling components, contributes to the NPC homeostatic regulation via complex cell-cell interactions and signaling cascades [22, 29-33, 37, 66-69], suggesting that similar mechanisms may be at play here. Different lines of evidences point to age-dependent dysregulation of Wnt signaling as causal factor in aging-induced impairment of both SVZ and SGZ niches. In the SVZ, aging and MPTP antagonize Wnt/β-catenin signaling leading to neurogenic impairment via crosstalk with inflammatory pathways at least in part mediated by upregulation of microglial proinflammatory mediator-induced downregulation of Wnt/β-catenin signaling [22, 33, 68, 69], with potential implications for mNPC neurogenic impairment herein observed [68, 69]. In SGZ, decreased Wnt3 release from aged hippocampal astrocytes regulates the age-associated decline of adult neurogenesis . Importantly, the endogenous Wnt antagonist Dkk1 increases with age, resulting in the suppression of adult neurogenesis and proliferation, whereas Dkk1 knockout mice show increased Wnt signaling, leading to enhanced neurogenesis and improved spatial memory . Our findings add to this scenario the midbrain ventricular region and both pre-DA and DA neurons as candidate targets that respond to SNpc lesion with an intrinsic repair program against injury [57, 60, 68, 69]. In this connection, the activation of astrocyte compartment and increased Wnt/β-catenin signaling herein observed in Nurr1+/TH− precursors of Aq-PVRs appear of specific interest in the light of the pivotal roles of Wnt1 and β-catenin in the development of these neurons [6-10, 13]. β-Catenin maintains adherent junctions and cell polarity of progenitor cells, as well as the integrity of radial glia which provide scaffolds for newly generated DA neurons to migrate on toward their final destinations [61, 62, 70]. Radial glia-like progenitors express Wnt1 , while deletion of Wnt1 induces a severe loss of radial glia-like cells and DA neurons [6, 7, 10, 13, 61, 62]. Wnt signaling via β-catenin promotes the differentiation of Nurr1+/TH− DA precursors. Then, when the degradation of β-catenin is inhibited with specific GSK-3β antagonists, the size of DA neurons increases, through conversion of precursors expressing Nurr1 into TH+ . By contrast, removal of β-catenin in DA progenitors reduces the progression from committed DA progenitors to DA neuron [6, 7, 10, 61, 62]. Of specific mention, the Wnt/β-catenin-dependent  Nurr1 expression is required for the maintenance, the survival, and protection of adult mDA neurons . Conversely, active GSK-3β overexpression is critically involved in neuronal death and depletion of the neurogenic niches, whereas GSK-3β inhibitors can in part mitigate these effects [20, 21, 72-75]. Here, with age, astrocyte dysfunction with loss of endogenous Wnts leads to upregulated GSK-3β levels, engendering (a) disruption of Aq-PVR architecture and function with failure of Wnt/β-catenin/Nurr1 signaling activation and neurogenic impairment; (b) increased vulnerability of mDA neurons likely responsible for reduced of both DA survival and repair.
The observation that β-catenin signaling is active in discrete DA neuronal populations of the intact brain is of interest in the light of its recognized role in neuronal synapse regulation and remodeling as well as in DA neuron maintenance and neuroprotection, as part of a neuron-astrocyte crosstalk [17, 20-22, 25, 68, 69]. Here, stabilization of β-catenin reinstated nigrostriatal plasticity and motor coordination in aged MPTP, whereas ablation of β-catenin in DA neurons alters motor and reward-associated memories and affects striatal mRNA levels for several markers known to regulate synaptic plasticity and DA neurotransmission .