Striatal astrocytes engulf dopaminergic debris in Parkinson's disease: A study in an animal model

Abstract

The role of astrocytes in Parkinson’s disease is still not well understood. This work studied the astrocytic response to the dopaminergic denervation. Rats were injected in the lateral ventricles with 6-hydroxydopamine (25μg), inducing a dopaminergic denervation of the striatum not accompanied by non-selective tissue damage. The dopaminergic debris were found within spheroids (free-spheroids) which retained some proteins of dopaminergic neurons (e.g., tyrosine hydroxylase, the dopamine transporter protein, and APP) but not others (e.g., α-synuclein). Free-spheroids showed the initial (LC3-autophagosomes) but not the late (Lamp1/Lamp2-lysosomes) components of autophagy (incomplete autophagy), preparing their autophagosomes for an external phagocytosis (accumulation of phosphatidylserine). Free-spheroids were penetrated by astrocyte processes (fenestrated-spheroids) which made them immunoreactive for GFAP and S100β, and which had some elements needed to continue the debris degradation (Lamp1/Lamp2). Finally, proteins normally found in neurons (TH, DAT and α-synuclein) were observed within astrocytes 2–5 days after the dopaminergic degeneration, suggesting that the intracellular contents of degenerated cells had been transferred to astrocytes. Taken together, present data suggest phagocytosis as a physiological role of striatal astrocytes, a role which could be critical for cleaning striatal debris during the initial stages of Parkinson’s disease.

Neuroglial transmitophagy and Parkinson's disease

“…In agreement with a previous study (Morales et al, 2017), no accumulation of the cytosolic SNARE protein syntaxin 17 (STX17) was observed in spheroids (see C 1 /C 3 and C 4 /C 6 in Figure 4), which, on the other hand, showed LC3‐puncta (D 1 /D 5 ) but not STX17 immunoreactivity (D 1 /D 5 ). The present study showed that spheroids also present no immunoreactivity for the polyubiquitin chains linked by the Lysine 48 (UbiK48) (E 4 /E 6 in Figure 5), an absence that cannot be attributed to methodological problems because the same slices showed the UbiK48 immunoreactivity expected for this protein in the soma of astrocytes (yellow arrows in E 1 –E 3 ) and of other striatal cells (green arrow in E 1 –E 3 ).…”

“…Similarly to that previously reported (Morales et al, 2017), the puncta shape (A 7 and A 8 in Figure 4) of the microtubule‐associated protein 1 light chain 3 (LC3) immunoreactivity (A 2 and A 6 ) found in the spheroids (A 1 /A 5 ) of rats suggests that these structures may generate autophagosomes (see also Supporting Information Movie S3). In the present study, the YFP‐Mito‐DAn mouse provided similar findings (bottom Figure 4), showing puncta shape LC3 immunoreactivity (E 3 , E 6 , and E 7 ) in DAergic spheroids (E 1 and E 4 ) containing YFP (E 2 and E 5 ).…”

“…According to a previous study (Morales et al, 2017), the administration of 6‐hydroxydopamine (6‐OHDA) in the lateral ventricle degenerated the nearby DAergic terminals of the medial striatum of the rat (Figure 1 top). The same effect was observed here in YFP‐Mito‐DAn mice, which showed saccular structures (spheroids) that contained DAergic cell markers, including the TH and the DAT (Supporting Information Movie S1).…”

“…The immunoreactivity for tyrosine hydroxylase (TH) and dopamine transporter (DAT) of dopaminergic cells, and for the mitochondrial outer membrane protein voltage‐dependent anion channel 1 (VDAC1), mitochondrial respiratory chain complex IV or cytochrome C oxidase (COX4), mitochondrial outer membrane protein TOM22, Parkin, mitochondrial outer membrane protein BCL‐2‐like protein 13 (BCL2L13), autophagy/Beclin‐1 regulator 1 (AMBRA1), LC3‐II, lysosomal‐associated membrane protein 1 (Lamp1), lysosomal‐associated membrane protein 2 (Lamp2), and phosphatidylserine (PS) was evaluated by a densitometric analysis of DAergic spheroids, astrocytic processes, and astrocytic somata. According to previous studies (Morales et al, 2017; Morales, Sanchez, Rodriguez‐Sabate, & Rodriguez, 2015, 2016), the densitometric data were normalized as a percentage of the mean value found in the extracellular medium or in the synaptic boutons of the contralateral striatum. In order to prevent differences due to variations in protocol conditions during the tissue processing and densitometric analysis, all sections were processed simultaneously using the same protocols and chemical reagents, and all microscopic and computer parameters were kept constant throughout the densitometric study.…”

“…Damaged mitochondria could be released to the extracellular medium from where they can activate the neuro‐inflammatory process that normally accelerates the DAn degeneration in PD (Maeda & Fadeel, 2014; Matheoud et al, 2016; Wilkins, Weidling, Ji, & Swerdlow, 2017; Zhang et al, 2010). The authors of this work recently found that proteins of degenerating DAergic terminals accumulate in spheroids (saccular structures of 2–9 μ diameter), and then move to striatal astrocytes ( transautophagy ) (Morales, Sanchez, Rodriguez‐Sabate, & Rodriguez, 2017). The present work was aimed at studying whether damaged mitochondria of degenerating DAergic terminals may also accumulate in spheroids and be transferred to striatal astrocytes for their degradation.…”