Progressive aggregation of alpha-synuclein and selective degeneration of lewy inclusion-bearing neurons in a mouse model of parkinsonism

Abstract

Aggregated alpha-synuclein inclusions are found where cell death occurs in several diseases, including Parkinson's disease, dementia with Lewy bodies, and multiple-system atrophy. However, the relationship between inclusion formation and an individual cell's fate has been difficult to study with conventional techniques. We developed a system that allows for in vivo imaging of the same neurons over months. We show that intracerebral injection of preformed fibrils of recombinant alpha-synuclein can seed aggregation of transgenically expressed and endogenous alpha-synuclein in neurons. Somatic inclusions undergo a stage-like maturation, with progressive compaction coinciding with decreased soluble somatic and nuclear alpha-synuclein. Mature inclusions bear the post-translational hallmarks of human Lewy pathology. Long-term imaging of inclusion-bearing neurons and neighboring neurons without inclusions demonstrates selective degeneration of inclusion-bearing cells. Our results indicate that inclusion formation is tightly correlated with cellular toxicity and that seeding may be a pathologically relevant mechanism of progressive neurodegeneration in many synucleinopathies.

Figure 1. In vivo imaging demonstrates a stage-like, progressive maturation of Syn-GFP inclusions within neurons

A) In vivo imaging in cortex layer II/III of uninjected (left), saline-injected (middle) or PFF-injected (right) animals at 2 mpi demonstrates punctate presynaptic staining within the neuropil and only homogenous somatic Syn-GFP staining within neurons. Scale bar 10 µm. (B) Top: Starting at 2.5 mpi, imaging near the PFF injection site demonstrates frequent neuritic Syn-GFP aggregates in layer II/III, and rare neuritic aggregates in the contralateral hemisphere in the symmetric location to the injection. Schematic representation shows injection location in the right hemisphere (X), extent of cranial window (dashed circle), and location of neuritic aggregates (blue dot). Scale bar 10 µm. Middle: At 3–4 mpi, neurons (green dot in schematic) near the injection site began showing neurons with both disorganized somatic Syn-GFP inclusions and normal homogenous staining. Scale bar 10 µm. Bottom: At 4–13 mpi only mature, organized somatic and neuritic Syn-GFP inclusions are present. Mature somatic inclusions have a stereotyped appearance, often with a single juxtanuclear accumulation and legs that wrap around the nucleus, giving a “spider-like” appearance. The normal homogenous somatic and nuclear Syn-GFP staining is absent in neurons containing mature inclusions. Schematic representation shows location of numerous mature somatic inclusions (red dot) near the injection site and in the contralateral hemisphere. Scale bar 10 µm. (C) Example of two neurons (C₁), one with normal homogeneous staining and one with a mature inclusion, and transect ROIs used to measure fluorescence (C₂) across the cell body. An inhomogeneity index was calculated (see Methods) and used to measure the mean index value (C₃) in neurons from uninjected, saline-injected, and PFF-injected animals with and without inclusions. Neurons from uninjected, saline-injected and PFF-injected animals without inclusions show low levels of inhomogeneity, while inclusion-bearing neurons from PFF-injected animals were significantly more inhomogeneous. N = cells/animals. Histogram of inhomogeneity index values from cells from all three groups (C₄) shows single populations with low inhomogeneity in uninjected and saline-injected animals, while PFF-injected animals show two distinct populations, a low index one nearly identical to saline-injected animals, and a high index population corresponding to neurons with mature somatic inclusions. Scale bar 10 µm.

Figure 2. In vivo FRAP demonstrates progressive compaction of and a low molecular turnover rate within Syn-GFP inclusions

(A) Individual neuron (A₁) with an immature inclusion at 3 mpi demonstrates mixed aggregated and unaggregated somatic Syn-GFP staining pattern. Two ROIs, one within (green circle) and one outside (black circle) the aggregated portion are photobleached simultaneously and sequential images shown before and after FRAP. Bleach pulse occurs just before time 0. Scale bar 5 µm. Similar FRAP experiment of the aggregated portion (red circle) of a mature inclusion (A₂) at >4 mpi. Scale bar 5 µm. Mean fluorescence intensity (A₃) from the three ROIs from the two cells in A_1–2 plotted on the same time scale demonstrates complete recovery back to baseline of the homogeneous staining, but large immobile fractions in the immature and mature inclusions. (B) Individual neuron (B₁) with a mature somatic inclusion at 4 mpi and absent unaggregated Syn-GFP staining in the remaining cytoplasm or nucleus. ROI (red rectangle) shows part of inclusion that was photobleached. Scale bar 7 µm. Mean intensity from this ROI over time (B₂) shows a large immobile fraction at 5–10 min post-bleach, which recovers substantially at 7 days post-bleach. (C) Group data comparing measured immobile fractions from unaggregated somatic Syn-GFP, immature and mature inclusions at 5 min post-bleach, and mature inclusions 7 days post-bleach, showing a significant progressive increase in the immobile fraction from immature to mature inclusions as measured at 5 min. At 7 days post-bleach, however, 49% of the previously immobile fraction recovers, demonstrating a slow turnover of protein within inclusions. N = cells/animals.

Figure 3. Mature somatic inclusions bear the hallmarks of human Lewy pathology

Mature Syn-GFP inclusions are heavily phosphorylated at serine-129 (A_1–4), in an amyloid dye binding configuration (A_1–2) and ubiquitinated (A₃). The green fluorescence from these inclusions is due to the presence of Syn-GFP and not an endogenous autofluorescent species, since they colabel with antibody to GFP (A₄). Similar mature inclusions made only of untagged endogenous mouse alpha-synuclein (A₅) are also present in nearby neurons that do not express Syn-GFP. These inclusions share a similar morphology and modification state to Syn-GFP-positive ones. Syn-GFP levels within mature inclusions (B, arrow) are greatly increased over levels of the protein in nearby neurons without inclusions (arrowhead). Colocalization of Syn-GFP with serine-129 phosphorylated alpha-synuclein (B) is also increased in inclusions versus cells without inclusions. N = cells/animals. Individual neurons from human frontal cortex in cases of Dementia with Lewy Bodies (C) are stained with hematoxylin to outline the nuclei in blue and for serine-129 phospho-synuclein aggregates in brown. Two examples demonstrate wrapping of aggregated alpha-synuclein around the nucleus (C_1–2), similar to that seen in mature mouse inclusions. Two other examples show “legs” of the inclusion that wrap around the nucleus (C_3–4, black arrowhead), also similar to mature mouse inclusions. Scale bar 5 µm. See also Fig. S1&2.

Figure 4. Chronic in vivo imaging demonstrates selective degeneration of inclusion-bearing neurons

Field of neurons (A&B) imaged one week apart demonstrate blood vessels by their negative stain, multiple neurons without inclusions and three mature inclusion-bearing neurons (white square & arrowhead) on Day 0. Scale bar 20 µm. One of these neurons (white square) degenerates by Day 7 and is shown at higher power (C₁). Scale bar C₁ 10 µm, C₂-E₂ 5 µm. Chronic imaging of individual neurons over months shows degeneration of some inclusion-bearing cells (C) with persistence of other cells with (D) and without (E) inclusions. Group data (F) shows selective degeneration of inclusion-bearing neurons over 7 months. Schematic showing the loss of neurons in brain regions affected by Parkinson’s Disease with the presence of Lewy pathology (red circle) in some of the remaining cells (G), and two different models for how this same final state might arise (H). One model posits a positive correlation between Lewy pathology presence and cell death, while the other an inverse correlation.