Neurons with Cat’s Eyes: A Synthetic Strain of α-Synuclein Fibrils Seeding Neuronal Intranuclear Inclusions

Abstract

The distinct neuropathological features of the different α-Synucleinopathies, as well as the diversity of the α-Synuclein (α-Syn) intracellular inclusion bodies observed in post mortem brain sections, are thought to reflect the strain diversity characterizing invasive α-Syn amyloids. However, this “one strain, one disease” view is still hypothetical, and to date, a possible disease-specific contribution of non-amyloid factors has not been ruled out. In Multiple System Atrophy (MSA), the buildup of α-Syn inclusions in oligodendrocytes seems to result from the terminal storage of α-Syn amyloid aggregates first pre-assembled in neurons. This assembly occurs at the level of neuronal cytoplasmic inclusions, and even earlier, within neuronal intranuclear inclusions (NIIs). Intriguingly, α-Syn NIIs are never observed in α-Synucleinopathies other than MSA, suggesting that these inclusions originate (i) from the unique molecular properties of the α-Syn fibril strains encountered in this disease, or alternatively, (ii) from other factors specifically dysregulated in MSA and driving the intranuclear fibrillization of α-Syn. We report the isolation and structural characterization of a synthetic human α-Syn fibril strain uniquely capable of seeding α-Syn fibrillization inside the nuclear compartment. In primary mouse cortical neurons, this strain provokes the buildup of NIIs with a remarkable morphology reminiscent of cat’s eye marbles (see video abstract). These α-Syn inclusions form giant patterns made of one, two, or three lentiform beams that span the whole intranuclear volume, pushing apart the chromatin. The input fibrils are no longer detectable inside the NIIs, where they become dominated by the aggregation of endogenous α-Syn. In addition to its phosphorylation at S129, α-Syn forming the NIIs acquires an epitope antibody reactivity profile that indicates its organization into fibrils, and is associated with the classical markers of α-Syn pathology p62 and ubiquitin. NIIs are also observed in vivo after intracerebral injection of the fibril strain in mice. Our data thus show that the ability to seed NIIs is a strain property that is integrally encoded in the fibril supramolecular architecture. Upstream alterations of cellular mechanisms are not required. In contrast to the lentiform TDP-43 NIIs, which are observed in certain frontotemporal dementias and which are conditional upon GRN or VCP mutations, our data support the hypothesis that the presence of α-Syn NIIs in MSA is instead purely amyloid-strain-dependent.

Aggregated Alpha-Synuclein Inclusions within the Nucleus Predict Impending Neuronal Cell Death in a Mouse Model of Parkinsonism

“…Cell cultures expressing aSyn tagged with a nuclear localization signal develop pSyn-positive nuclear inclusions, and this aggregation is not caused by the localization signal [ 16 ]. Primary neuron cultures treated with fibrils formed from copper-exposed aSyn showed increased nuclear inclusions, which is consistent with mouse data that indicate that specific aSyn strains have a greater propensity for nuclear aggregation [ 28 , 30 ]. Following neurotoxin exposure, apoptotic cells in culture increase their nuclear aSyn and exhibit nuclear aSyn aggregation [ 31 ].…”

“…Additionally, wildtype mice that received a gastric wall PFF injection displayed nuclear inclusions [ 27 ]. The formation of nuclear inclusions increases when specific aSyn PFF strains are used or if toxin insults, such as MPTP exposure, are combined with PFF injection [ 28 , 29 ]. In vitro models can also display nuclear inclusions.…”

GRK2‐Targeted Knockdown as Therapy for Multiple System Atrophy

“…15 Demonstration remains, therefore, to be provided in a model of fibrillar formation, possibly using specific preformed fibrils inducing a pathology clearly reminiscent of MSA. 16 Our results pave the way for a more comprehensive analysis of the role of GRK2 in MSA. The metabolic role of GRK2 in insulin resistance and associated conditions receives much attention.…”

“…We recently showed that S129‐phosphorylated α‐syn corresponds to the accumulation of phosphorylated α‐syn monomers/oligomers and not to the appearance of the distinctive fibrillar α‐syn aggregates that are present in the brains of MSA or PD patients 15 . Demonstration remains, therefore, to be provided in a model of fibrillar formation, possibly using specific preformed fibrils inducing a pathology clearly reminiscent of MSA 16 …”

Unraveling the Complex Interplay between Alpha-Synuclein and Epigenetic Modification

“…Interestingly, distinct αS species translocate from the nucleus to neuronal processes during neuronal differentiation, which suggests that the maturation process of the nervous system may affect subcellular localization [ 59 ]. With respect to protein conformational change and aggregation, exposure to human fibrotic αS seed facilitates the formation of intranuclear inclusions in mouse primary cortical neurons [ 60 ]. Furthermore, the inoculation of αS preformed fibrils into the stomach wall of wild-type mice resulted in the formation of a small number of nuclear inclusions in the dorsal motor nucleus of the vagal nerve [ 61 ].…”