Evidence for bidirectional and trans-synaptic parasympathetic and sympathetic propagation of alpha-synuclein in rats

Abstract

The conversion of endogenous alpha-synuclein (asyn) to pathological asyn-enriched aggregates is a hallmark of Parkinson’s disease (PD). These inclusions can be detected in the central and enteric nervous system (ENS). Moreover, gastrointestinal symptoms can appear up to 20 years before the diagnosis of PD. The dual-hit hypothesis posits that pathological asyn aggregation starts in the ENS, and retrogradely spreads to the brain. In this study, we tested this hypothesis by directly injecting preformed asyn fibrils into the duodenum wall of wild-type rats and transgenic rats with excess levels of human asyn. We provide a meticulous characterization of the bacterial artificial chromosome (BAC) transgenic rat model with respect to initial propagation of pathological asyn along the parasympathetic and sympathetic pathways to the brainstem, by performing immunohistochemistry at early time points post-injection. Induced pathology was observed in all key structures along the sympathetic and parasympathetic pathways (ENS, autonomic ganglia, intermediolateral nucleus of the spinal cord (IML), heart, dorsal motor nucleus of the vagus, and locus coeruleus (LC)) and persisted for at least 4 months post-injection. In contrast, asyn propagation was not detected in wild-type rats, nor in vehicle-injected BAC rats. The presence of pathology in the IML, LC, and heart indicate trans-synaptic spread of the pathology. Additionally, the observed asyn inclusions in the stomach and heart may indicate secondary anterograde propagation after initial retrograde spreading. In summary, trans-synaptic propagation of asyn in the BAC rat model is fully compatible with the “body-first hypothesis” of PD etiopathogenesis. To our knowledge, this is the first animal model evidence of asyn propagation to the heart, and the first indication of bidirectional asyn propagation via the vagus nerve, i.e., duodenum-to-brainstem-to-stomach. The BAC rat model could be very valuable for detailed mechanistic studies of the dual-hit hypothesis, and for studies of disease modifying therapies targeting early pathology in the gastrointestinal tract.Electronic supplementary materialThe online version of this article (10.1007/s00401-019-02040-w) contains supplementary material, which is available to authorized users.

Brain-First versus Gut-First Parkinson’s Disease: A Hypothesis

“…In a recent study, we performed injections with preformed asyn fibrils into the duodenum of transgenic rats, which overexpress human full-length wild-type asyn (Fig. 5) [71]. At 2-and 4 months postinjection, we detected very robust propagation of asyn pathology via the vagus nerve to the DMV, and via the sympathetic connectome to the coeliac ganglion and the intermediolateral cell column (IML).…”

Criticality as a measure of developing proteinopathy in engineered human neural networks

“…In a recent publication, Van den Berge et al, (13) showed that alpha-synuclein PFF seeds injected into the duodenum wall of a transgenic rat model induce an alpha-synuclein pathology which propagates transynaptically and bidirectionally through the parasympathetic and sympathetic nervous system to the brain stem in a pattern which recapitulates Braak's hypothesis (6) of the development of a patterned pathological propagation in PD. This finding, together with the seminal demonstration of PFF induced pathology propagating from the gastrointestinal tract to the brains in rats (11), strengthens the growing suspicion of idiopathic PD originating from a yet unidentified pathogen capable of passing the mucosal barrier (84).…”

“…Focusing on the second most common neurodegenerative disease, PD, the implicated proteinopathy mainly consists of misfolded and aggregated forms of alphasynuclein. These intracellular alpha-synuclein inclusions are termed Lewy bodies or Lewy neurites, and can be found propagating throughout central, peripheral and autonomic parts of the nervous system, as well as in multiple organs, as the disease progresses (6,7,(10)(11)(12)(13)(14). Furthermore, the neurodegenerative process characteristic of PD particularly affects and progressively depletes the dopaminergic neurons in the substantia nigra pars compacta (SNpc), a process which is thought to underlie most of the movement-related symptoms (15).…”

“…Based on this, neuroscientific research has narrowed in on two likely pathological mechanisms that could underlie the propagating pattern of neurodegeneration seen in PD (as well as in AD and ALS), namely selective neuronal vulnerability and pathological proteinopathic seeds (2,3,6,33,(36)(37)(38). At this point, research efforts have uncovered several mechanisms of neuron-to-neuron transfer of pathological seeds of alpha-synuclein pre-formed fibrils (PFFs) (39)(40)(41)(42)(43), both in vitro and in vivo (11,13), highlighting its contribution as a source of pathological propagation, however, the functional consequences of this interneuronal spread remain to be elucidated.…”

The Intestinal Barrier in Parkinson’s Disease: Current State of Knowledge

“…van IJzendoorn and P. Derkinderen / Intestinal Barrier in PD S327 lowing their injection into the mouse muscularis layer of the pylori and duodenum but not when the mice were subjected to truncal vagotomy following alpha-synuclein fibril injection. Finally, also in a bacterial artificial chromosome transgenic rat model that overexpressed the human alpha-synuclein encoding gene, gut-to-brain propagation of injected alpha-synuclein fibrils via the vagus nerve was demonstrated [20]. Conceivably, the Braak model requires that the intestinal epithelial barrier should be porous enough to give way to a hitherto unknown neurotropic pathogen to trigger alpha-synuclein misfolding and aggregation in the nearby enteric neurons [21].…”