Cellular Senescence Is Induced by the Environmental Neurotoxin Paraquat and Contributes to Neuropathology Linked to Parkinson's Disease

Abstract

Exposure to the herbicide paraquat (PQ) is associated with an increased risk of idiopathic Parkinson's disease (PD). Therapies based on PQ's presumed mechanisms of action have not, however, yielded effective disease therapies. Cellular senescence is an anticancer mechanism that arrests proliferation of replication-competent cells and results in a pro-inflammatory senescence-associated secretory phenotype (SASP) capable of damaging neighboring tissues. Here, we demonstrate that senescent cell markers are preferentially present within astrocytes in PD brain tissues. Additionally, PQ was found to induce astrocytic senescence and an SASP in vitro and in vivo, and senescent cell depletion in the latter protects against PQ-induced neuropathology. Our data suggest that exposure to certain environmental toxins promotes accumulation of senescent cells in the aging brain, which can contribute to dopaminergic neurodegeneration. Therapies that target senescent cells may constitute a strategy for treatment of sporadic PD, for which environmental exposure is a major risk factor.

Keywords: aging; antagonistic pleiotropy; neurodegeneration; senescence-associated secretory phenotype; tumor suppression.

Figure 1. Senescence Markers in PD SNpc versus Age-Matched Controls

(A) RNA isolated from autopsied SNpc tissues from PD versus controls (n = 5) were analyzed for p16^INK4a and SASP factor (IL-6, IL-1α, IL-8, and MMP-3) mRNA levels by qPCR. Transcripts were normalized to actin and are shown as fold change over control levels; *p < 0.05 and ***p < 0.005. (B) Comparison of lamin B1 levels in PD versus age-matched control tissues. Representative immunofluorescence images (left side) showing lamin B1 protein levels (green) in GFAP⁺ (red) astrocytes (red arrows) in the PD SNpc (right panels) compared to age-matched controls (left panels). Neighboring GFAP⁻ cells (yellow arrows) retain lamin B1 expression in PD tissue. Quantification of the image data (right side) using mean pixel values (MPVs) showing that compared to controls (black bar; n = 5 individuals), PD tissues (gray bar; n = 5 individuals) contained significantly less Lamin B1 protein in GFAP⁺, but not GFAP⁻, cells; *p < 0.05 (paired t test).

Figure 2. PQ Induces Senescent Phenotypes in Cultured Human Astrocytes

Cultured human astrocytes were treated with 100 or 250 μM PQ (or vehicle) for 24 hr, and senescence markers were evaluated 7 days later. (A) Representative images of BrdU labeling (left), and percentage of BrdU⁺ cells (right). (B) Representative images of SA-β-gal activity (left), and percentage of SA-β-gal⁺ cells (right). (C) qPCR of p16^INK4a mRNA levels. (D) Representative images of immunostaining for p16^INK4a (left), and percentage of p16^INK4a-positive cells (right). (E) Representative images of 53BP1 immunostaining (left), and percentage of cells with >1 53BP1 positive foci (right). Nuclei were counterstained with DAPI (blue) and white arrows denote enlarged cells shown in insets with punctate nuclear foci. (F) Secreted IL-6 levels measured by ELISA in conditioned media from non-senescent (vehicle) and senescent (PQ) astrocytes. For (A)–(D), n = 4, where n = experimental replicates; *p < 0.05, **p < 0.01, and ***p < 0.005 (unpaired t test).

Figure 3. Paraquat Induces Morphological Features of Senescence and Arrests Proliferation in Both Fibroblasts and Astrocytes

(A) Human fibroblasts (top panels) and human astrocytes (bottom panels) respond to PQ (1 mM and 300 μM, respectively) by decreasing the percentage of Ki67⁺ cells (green) compared to vehicle-treated controls. Representative images (left) and quantification (right) are shown. The decreased percentage of Ki67⁺ cells is similar to the decrease observed when cells are exposed to 10 Gy X-ray (IR, middle panels). For fibroblasts, both IR and 1 mM PQ significantly reduced Ki67 positivity; ***p < 0.00001 (1 mM PQ was not significantly different from IR [p > 0.07]; unpaired t test); n = 4 experiments. For astrocytes, IR and 300 μM PQ significantly reduced Ki67 positivity; **p < 0.01 (300 μM PQ was not significantly different from IR [p > 0.8]; unpaired t test); n = 2 experiments. (B) Human fibroblasts (top) and human astrocytes (bottom) respond to PQ with a significant increase in nuclear size compared to vehicle-treated controls. Representative images (left) and quantification (right) are shown. Increased nuclear size is similar to the increase obtained with IR exposure. For fibroblasts both conditions (IR and 1mM PQ) are significantly greater than control; ***p < 0.0001 (1 mM PQ is not significantly different from IR [p > 0.30]; unpaired t test); n = 6 experiments. For astrocytes, both conditions (IR; 300 μM PQ) are significantly greater than control; *p < 0.02 (300 μM PQ is not significantly different from IR [p > 0.5]; unpaired t test); n = 5 experiments. (C) Human fibroblasts respond to PQ (500 μM and 1 mM) with a significant loss of lamin B1 staining in the nuclear lamina relative to vehicle-treated controls. Representative images (left) and quantification (right) are shown. Decreased lamin B1 staining is similar to that obtained with IR. All conditions (IR; 500 μM and 1 mM PQ) are significantly different from control; **p < 0.01 (neither PQ condition differed significantly from IR [p > 0.25]; unpaired t test); n = 2 experiments. (D) Human fibroblasts (1 mM PQ, top), astrocytes derived from iPSCs (40 μM PQ, middle), and murine primary astrocytes from p16-3MR neonates (40 μM PQ, bottom), display obvious increase in SA-β-gal activity, but cell death occurred when human and mouse astrocytes were exposed to higher concentrations of PQ (500 and 250 μM, respectively).

Figure 4. Lower Dosages of PQ Also Induce Cellular Senescence

(A) Images and quantification percentage of SA-β-gal⁺ cells of human astrocytes exposed to 10 or 50 μM PQ for 48; also shown are results from exposure to 150 mM H₂O₂ for 2 hr and 50 nM MG132 for 48 hr; *p < 0.05, **p < 0.01, and ***p < 0.005. (B) Images and quantification of cell density based on DAPI staining after exposure to 10 or 50 μM PQ for 48 hr; *p < 0.05 and **p < 0.01. For (A) and (B), n = 4, where n = experimental replicates (unpaired t test).

Figure 5. Factors Secreted by Senescent Human Astrocytes Have Detrimental Effects

(A) Conditioned media (CM) from PQ-treated human astrocytes was added to DAergic neurons, and cell viability was assessed via an MTT assay; n = 3 experiments. (B) CM from PQ-treated human astrocytes was added to neural progenitor cells (NPCs), and proliferation of NPCs was assessed by BrdU positivity. Representative images (left) and quantification (right) are shown; n = 3 experiments. (C) NPCs migration in the presence of CM from vehicle-treated (black bars) versus PQ-treated (250 μM) (gray bars) astrocytes assayed using the scratch test; images show “cleared areas” of NPCs at 0 hr (far left panels), 24 hr (middle panels), and 48 hr (right panels) post-scratch. Black lines (perpendicular to scratches) indicate the width of the uninvaded area used for the quantification (right side). The percent scratch width >24 and 48 hr was calculated; n = 5 scratches. The experiment was run twice with 2 different sets of CM (2 scratch assays). *p < 0.05 (unpaired t test).

Figure 6. PQ-Induced Senescence Is Prevented by Senescent Cell Ablation

p16-3MR mice were treated with (1) saline, (2) PQ, (3) GCV alone, or (4) PQ plus GCV and assessed for senescence-associated markers. (A) Striatal RNA analyzed by qPCR for p16^INK4a and IL-6 mRNA levels. (B) SNpc immunofluorescence of Lamin B1 (green) of DAPI-stained nuclei (blue) within a field of TH⁺ (magenta)-stained neurites. GFAP⁺ (red) astrocytes are indicated by yellow arrows. A representative image from each of the 4 conditions (left) and corresponding quantification of nuclear Lamin B1 fluorescence (right). Top left inlays show lamin B1 staining within astrocytes. (C) HMGB1 immunofluorescence (green) counterstained as in (B); shown are representative images (left) and quantification of nuclear HMGB1 fluorescence (right). (D) Representative images (left) of lamin B1 immunofluorescence (green) in astrocytes versus non-astrocytic neighbors and quantification (right). Neighboring GFAP⁻ cells marked with yellow arrows. In all experiments, n = 4 mice for saline, PQ plus GCV, and GCV; and n = 3 mice for PQ. *p < 0.05 (unpaired t test).

Figure 7. Depletion of Senescent Cells Abrogates PQ-Mediated Neuropathological Phenotypes

(A) Stereological SN TH⁺ cell counts were performed on p16-3MR mice treated with (1) saline, (2) PQ, (3) GCV alone, or (4) PQ plus GCV. Representative images (left) and quantification (right) are shown. Data are expressed as total SN TH⁺ cell numbers per animal; cell counts were verified by Nissl staining (not shown). (B) Neurogenesis assessed by quantitation of BrdU⁺ cells in the subventricular zone (SVZ) (Peng et al., 2008). Representative sections showing BrdU⁺ cells in the SVZ (left); quantitation of BrdU⁺ cells in the SVZ (right). Values are reported as BrdU⁺ cells per field averaged from 5 fields. (C) Prior to sacrifice, locomotor activity was monitored using the cylinder test as a measurement of spontaneous rearing rate. Still images demonstrating the behavioral phenotype (left) and quantification (right) are shown. For all experiments, n = 6 per condition; *p < 0.05 (unpaired t test).