Aggregated forms of α-synuclein play a crucial role in the pathogenesis of synucleinopathies such as Parkinson’s disease (PD). However, the molecular mechanisms underlying the pathogenic effects of α-synuclein are not completely understood. Here we show that asparagine endopeptidase (AEP) cleaves human α-synuclein, triggers its aggregation and escalates its neurotoxicity, thus leading to dopaminergic neuronal loss and motor impairments in a mouse model. AEP is activated and cleaves human α-synuclein at N103 in an age-dependent manner. AEP is highly activated in human brains with PD, and it fragments α-synuclein, which is found aggregated in Lewy bodies. Overexpression of the AEP-cleaved α-synuclein1–103 fragment in the substantia nigra induces both dopaminergic neuronal loss and movement defects in mice. In contrast, inhibition of AEP-mediated cleavage of α-synuclein (wild type and A53T mutant) diminishes α-synuclein’s pathologic effects. Together, these findings support AEP’s role as a key mediator of α-synuclein-related etiopathological effects in PD.

The gut-brain axis is bidirectional, and gut microbiota influence brain disorders including Alzheimer’s disease (AD). CCAAT/enhancer binding protein β/asparagine endopeptidase (C/EBPβ/AEP) signaling spatiotemporally mediates AD pathologies in the brain via cleaving both β-amyloid precursor protein and Tau. We show that gut dysbiosis occurs in 5xFAD mice, and is associated with escalation of the C/EBPβ/AEP pathway in the gut with age. Unlike that of aged wild-type mice, the microbiota of aged 3xTg mice accelerate AD pathology in young 3xTg mice, accompanied by active C/EBPβ/AEP signaling in the brain. Antibiotic treatment diminishes this signaling and attenuates amyloidogenic processes in 5xFAD, improving cognitive functions. The prebiotic R13 inhibits this pathway and suppresses amyloid aggregates in the gut. R13-induced Lactobacillus salivarius antagonizes the C/EBPβ/AEP axis, mitigating gut leakage and oxidative stress. Our findings support the hypothesis that C/EBPβ/AEP signaling is activated by gut dysbiosis, implicated in AD pathologies in the gut.

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Lewy pathology, composed of α-Synuclein (α-Syn) inclusions, a hallmark of Parkinson's disease (PD), progressively spreads from the enteric nervous system (ENS) to the central nervous system (CNS). However, it remains unclear how this process is regulated at a molecular level. Here we show that δ-secretase (asparagine endopeptidase, AEP) cleaves both α-Syn at N103 and Tau at N368, and mediates their fibrillization and retrograde propagation from the gut to the brain, triggering nigra dopaminergic neuronal loss associated with Lewy bodies and motor dysfunction. α-Syn N103 and Tau N368 robustly interact with each other and are highly elevated in PD patients' gut and brain. Chronic oral administration of the neurotoxin rotenone induces AEP activation and α-Syn N103/ Tau N368 complex formation in the gut, eliciting constipation and dopaminergic neuronal death in an AEP-dependent manner. Preformed fibrils (PFFs) of α-Syn N103/Tau N368 are more neurotoxic and compact, and aggregate more quickly along the vagus nerve than their FL/FL counterparts or the individual fragments' fibrils. Colonic injection of PFFs induces PD pathologies, motor dysfunctions, and cognitive impairments. Thus, δ-secretase plays a crucial role in initiating PD pathology progression from the ENS to the CNS.

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Dopaminergic neurodegeneration in Parkinson's disease (PD) is associated with abnormal dopamine metabolism by MAO-B (monoamine oxidase-B) and intracellular α-Synuclein (α-Syn) aggregates, called the Lewy body. However, the molecular relationship between α-Syn and MAO-B remains unclear. Here, we show that α-Syn directly binds to MAO-B and stimulates its enzymatic activity, which triggers AEP (asparagine endopeptidase; legumain) activation and subsequent α-Syn cleavage at N103, leading to dopaminergic neurodegeneration. Interestingly, the dopamine metabolite, DOPAL, strongly activates AEP, and the N103 fragment of α-Syn binds and activates MAO-B. Accordingly, overexpression of AEP in SNCA transgenic mice elicits α-Syn N103 cleavage and accelerates PD pathogenesis, and inhibition of MAO-B by Rasagiline diminishes α-Syn-mediated PD pathology and motor dysfunction. Moreover, virally mediated expression of α-Syn N103 induces PD pathogenesis in wild-type, but not MAO-B-null mice. Our findings thus support that AEP-mediated cleavage of α-Syn at N103 is required for the association and activation of MAO-B, mediating PD pathogenesis.

The netrin-1/DCC ligand/receptor pair has key roles in central nervous system (CNS) development, mediating axonal, and neuronal navigation. Although expression of netrin-1 and DCC is maintained in the adult brain, little is known about their role in mature neurons. Notably, netrin-1 is highly expressed in the adult substantia nigra, leading us to investigate a role of the netrin-1/DCC pair in adult nigral neuron fate. Here, we show that silencing netrin-1 in the adult substantia nigra of mice induces DCC cleavage and a significant loss of dopamine neurons, resulting in motor deficits. Because loss of adult dopamine neurons and motor impairments are features of Parkinson's disease (PD), we studied the potential impact of netrin-1 in different animal models of PD. We demonstrate that both overexpression of netrin-1 and brain administration of recombinant netrin-1 are neuroprotective and neurorestorative in mouse and rat models of PD. Of interest, we observed that netrin-1 levels are significantly reduced in PD patient brain samples. These results highlight the key role of netrin-1 in adult dopamine neuron fate, and the therapeutic potential of targeting netrin-1 signaling in PD.

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