α-Synuclein is a soluble, cytosolic protein with a yet not fully identified function and the ability to adopt one or more pathological conformations that have been identified in cellular inclusions in a group of neurodegenerative conditions designated as synucleinopathies [
11,
50,
51,
59]. Most commonly associated with these diseases are Parkinson’s disease (PD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA). What causes α-synuclein to adopt a pathological conformation in these diseases is largely unknown. While a number of familial mutations can cause PD, including missense mutations in α-synuclein and those resulting in elevated protein levels of wild-type α-synuclein, synucleinopathies are mostly regarded as sporadic diseases [
1,
12,
25,
27,
29,
35,
37,
39,
48,
65]. A growing body of evidence shows that in an aggregated state, α-synuclein has prion-like properties. “Classical” prions are infectious conformers of the prion protein (PrP) which cause transmissible spongiform encephalopathies, such as scrapie in sheep, chronic wasting disease in cervids, bovine spongiform encephalopathy (BSE) in cattle, or Creutzfeldt–Jakob disease (CJD) in humans [
13,
26,
40]. Dependent on the prion strain and the host, PrP prions can cause CNS disease after natural or accidental horizontal transmission via several entryways, including the intracerebral, intraperitoneal, intravenous, or oral route [
32]. Reminiscent of PrP prions, intracerebral challenge of different animal models with synthetic or patient-derived pathological α-synuclein has revealed that CNS pathology propagates in a stereotypic manner by interneuronal transmission [
7,
31,
60]. Moreover, distinct strains of pathological α-synuclein are associated with specific diseases, and strain characteristics persist upon repeated passaging in one or between different animal models [
62,
64]. Recently, we showed that, similar to what has long been known for prions, intraperitoneal and intraglossal challenge of Tg(M83
+/−:
Gfap-luc
+/−) mice, which express the A53T mutant of human α-synuclein and luciferase, with α-synuclein fibrils results in neuroinflammation and CNS disease after neuroinvasion of pathological α-synuclein from the periphery [
4,
5,
10,
17,
22,
24]. Similar to our observations, challenge of TgM83
+/− mice with α-synuclein fibrils via intrasciatic, intraperitoneal, or intramuscular injection, or with brain homogenate of patients with multiple system atrophy (MSA) via intraglossal, intraperitoneal, or intramuscular injection was reported to result in neurological disease [
2,
3,
17,
44,
45,
63]. Because prions can also be transmitted orally or by blood, for instance, bovine spongiform encephalopathy (BSE) prions have been transmitted from infected cattle to humans by the consumption of tainted beef products, and the resulting variant Creutzfeldt–Jakob disease (CJD) prions between humans by blood transfusion, we wondered whether α-synuclein prions could be transmitted via these two routes as well [
20,
21]. To address this question, we challenged TgM83
+/− mice expressing the A53T mutant of human α-synuclein with recombinant fibrils of human, wild-type α-synuclein, or bovine serum albumin (BSA) as a negative control by the oral and intravenous route [
17]. Whereas none of the BSA-injected control mice developed signs of disease or neuropathology, all of the mice challenged intravenously and up to 50% of the mice challenged orally with α-synuclein fibrils developed neurological disease with α-synuclein pathology in their CNS. Our results unmistakably show that a single challenge with α-synuclein fibrils by the oral, intravenous, or intraperitoneal route can be sufficient for α-synuclein fibrils to invade the CNS and to cause neuropathology and disease in TgM83
+/− mice.