Dissociable effects of PINK1 and chronic stress on neuronal number and microglial morphology: implications for non-motor symptoms in parkinson’s disease

Objectives

Depression and anxiety are associated with chronic stress and recognized as core non-motor symptoms of Parkinson’s disease (PD) [1]. It is not well established whether they stem from genetic neurobiological vulnerability or from chronic stress (CS). Mechanisms have been implicated, their relative contributions—and the extent to which they differentially affect specific brain regions and cellular systems—have not been clearly delineated [2]. Understanding whether genetic susceptibility and environmental stressors exert converging or dissociable effects on the brain is essential for clarifying the pathophysiology of affective symptoms in PD.

Methods

C57BL/6 and PINK1-/- mice were assigned to control, stress, and follow-up groups; stress involved 28 days of rat exposure, tail suspension, and restraint. Following behavioural testing, animals were sacrificed, body and adrenal glands were weighed, and brain tissues were processed for immunofluorescence. Images were acquired using confocal microscopy with z-stack imaging. Neuronal integrity was assessed using NeuN staining in the cortex and hippocampal subregions (CA1–CA3, dentate gyrus), while microglia were evaluated with IBA1 in the cortex, hippocampus, and amygdala. Quantitative image analysis was performed. ImageJ/Fiji macros measured NeuN/IBA1 areas, while microglial soma size and morphology were assessed using particle and skeleton analyses. Linear mixed-effects models were used to account for unequal group sizes and repeated measures. All analyses were conducted blind (p<0.05).

Results

CS induced a stress response, reflected by reduced body weight at weeks 14 and 21 (p=0.001; p=0.033) and increased relative adrenal gland weight (p=0.001). Imaging analyses revealed region-specific neurobiological alterations. PINK1 genotype significantly reduced cortical NeuN-positive neuronal area (F(1,18.4)=4.92, p=0.039; β = −3.90), while stress and follow-up had no significant effects (p>0.05). In contrast, CS significantly reduced neuronal area across hippocampal subregions (CA1: p=0.001; CA2: p<0.001; CA3: p=0.010; dentate gyrus: p=0.035), with the greatest vulnerability observed in CA2. Follow-up had no significant effect (p>0.05).

Microglial alterations were region-specific. In the cortex, PINK1-/- reduced microglial area (p=0.003) and cell numbers (20–50 µm: p=0.017; 51–100 µm: p=0.014), while stress reduced cell numbers in the 51–100 µm range (p=0.023). Morphological analysis showed reduced branch number (p=0.010) and maximum branch length driven by both genotype (p=0.006) and stress (p=0.004), indicating decreased ramification. In the hippocampus, microglial area and counts were unchanged, although follow-up reduced larger microglial populations (p≤0.003) and branch numbers in PINK1-/- mice (p=0.027). Amygdala microglia numbers were reduced in PINK1-/- mice (ps≤0.048), with no morphological changes.

Conclusions

The findings indicate that genetic and environmental influences on the brain are not uniform, rather they exert region-specific and cell type–dependent effects. PINK1 genotype was primarily associated with cortical neuronal loss and microglial structural alterations, whereas stress exposure was linked to neuronal reductions in the hippocampus. In contrast, aging to predominantly affect microglial population dynamics. Results suggest that depression and anxiety in PD cannot be attributed to a single cause; rather, genetic vulnerability, stress, and aging contribute through dissociable yet interacting mechanisms targeting distinct brain systems.

References

1. Kalinderi K, Papaliagkas V, Fidani L. Current genetic data on depression and anxiety in Parkinson’s disease patients. Parkinsonism & Related Disorders. 2024 Jan 1;118:105922.
2. Yapici Eser H, Bora HA, Kuruoğlu A. Depression and Parkinson disease: prevalence, temporal relationship, and determinants. Turkish journal of medical sciences. 2017;47(2):499-503.