Neurobiological Mechanisms and Sex-Dimorphic Biomarkers of Cognitive and Neuropsychiatric Manifestations in Parkinson's Disease: A Precision Biomedicine Perspective

Abstract

Parkinson’s disease (PD) is increasingly recognized as a systemic neurodegenerative disorder characterized by non-motor features, including Parkinson's disease cognitive impairment (PD-CI), depression, anxiety, and psychosis. While historically contextualized through motor dysfunction, non-motor symptoms present significant clinical management challenges, particularly across underrepresented populations such as African cohorts. At the cellular level, these manifestations arise from toxic alpha-synuclein (α-syn) oligomerization, mitochondrial complex I inhibition, cholinergic/dopaminergic denervation, and sustained neuroinflammation driven by microglial activation and astrocytic reactivity. Importantly, biological sex acts as a critical modifier of disease penetrance, progression rate, and biomarker expression. Estrogenic signaling (via ERα/ERβ) modulates neuroprotective inflammatory pathways, synaptogenesis, and mitochondrial biogenesis, establishing distinct pathophysiological profiles between females and males. Concurrently, emerging fluid biomarkers—such as neurofilament light chain (NfL), glial fibrillary acidic protein (GFAP), and phosphorylated tau species—show promise for early stratification, though their diagnostic accuracy requires validation across diverse ancestral backgrounds. This article synthesizes the molecular drivers, neuroinflammatory pathways, and sex-dimorphic regulatory networks underlying non-motor dysfunction in PD, highlighting the necessity for sex-and-gender-based analysis (SGBA) to enable equitable, precision medicine approaches in global neurodegenerative health.

Introduction & Cellular Mechanisms

Parkinson’s disease is pathologically defined by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) alongside widespread intracellular aggregation of misfolded α-synuclein into Lewy bodies and Lewy neurites. However, the degenerative cascade extends far beyond the nigrostriatal pathway, spreading sequentially through limbic and neocortical structures as outlined in advanced Braak staging models. This rostral propagation underlies the high burden of non-motor symptoms, specifically Parkinson's disease cognitive impairment (spanning mild cognitive impairment [PD-MCI] to frank Parkinson's disease dementia [PDD]) and severe neuropsychiatric manifestations (including visual hallucinations, depression, and apathy).

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[Pathological α-Synuclein Aggregation]
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[Mitochondrial Dysfunction & ROS Generation] ──► [NLRP3 Inflammasome Activation]
│ │
▼ ▼
[Loss of Cholinergic/Dopaminergic Integrity] [Neuroinflammation (NfL / GFAP Release)]
│ │
└───────────────────┬────────────────────┘
│
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[Cognitive Dysfunction & Neuropsychiatric Manifestations]
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At the subcellular level, cognitive and neuropsychiatric decline in PD is governed by multi-factorial pathogenic mechanisms:

Alpha-Synuclein Proteotoxicity and Synaptic Dysfunction: Pathological α-syn species (specifically soluble oligomers) disrupt synaptic vesicle trafficking, impair SNARE complex assembly, and induce pore formation in plasma membranes. In cortical and hippocampal circuits, toxic α-syn accumulation correlates with loss of dendritic spines and long-term potentiation (LTP) deficits, directly compromising neural network plasticity.

Mitochondrial Bioenergetic Crisis and Mitophagy Deficits: Complex I inhibition within the inner mitochondrial membrane leads to excessive reactive oxygen species (ROS) production, opening of the mitochondrial permeability transition pore (mPTP), and subsequent cytochrome c release. Defective Pink1/Parkin-mediated mitophagy prevents the clearance of damaged organelles, accelerating neuronal apoptosis within the basal forebrain cholinergic nuclei (nucleus basalis of Meynert) and locus coeruleus noradrenergic networks.

Glial-Mediated Neuroinflammation: Persistent extracellular α-syn acts as a pathogen-associated molecular pattern (PAMP), binding to microglial Toll-like receptors (TLR2 and TLR4). This interaction triggers NF-κB nuclear translocation and NLRP3 inflammasome assembly, initiating the secretion of pro-inflammatory cytokines (IL-1β, IL-6, TNF-α). Concurrently, reactive astrocytes transition to a neurotoxic phenotype (A1), losing their capacity for glutamate reuptake and releasing neurotoxic factors that breach the blood-brain barrier (BBB).

Genetic and Metabolic Heterogeneity in African Cohorts: Genetic risk architectures in African populations exhibit unique genomic features, including variations in *GBA1*, *LRRK2*, and *PRKN* loci, alongside distinct patterns of *APOE* ε4 allele frequencies. These genetic factors alter lysosomal hydrolase activity, kinase auto-phosphorylation, and lipid homeostasis, thereby modifying vulnerability to neuroinflammation and cognitive decline.

Sex-Dimorphic Regulation & Translational Implications

Biological sex significantly influences the susceptibility, clinical phenotype, and biomarker profiles of cognitive and neuropsychiatric decline in PD. Incorporating sex-and-gender-based analysis (SGBA) reveals that molecular pathways governing neurodegeneration operate under dimorphic regulation.

1. Estrogenic Neuroprotection and Receptor Signaling:
17β-estradiol (E2) exerts neuroprotective effects through activation of estrogen receptors alpha (ERα) and beta (ERβ), as well as the membrane-bound G-protein coupled estrogen receptor 1 (GPER1). Estrogenic signaling enhances brain-derived neurotrophic factor (BDNF) transcription, activates the anti-apoptotic PI3K/Akt pathway, and inhibits glycogen synthase kinase-3 beta (GSK-3β), thereby attenuating tau hyperphosphorylation. Furthermore, E2 acts as an antioxidant and direct inhibitor of α-synuclein fibrillization. Consequently, postmenopausal estrogen depletion accelerates neuroinflammatory signaling and elevates the risk for rapid cognitive decline in females.

2. Dimorphic Immune Responses and Glial Priming:
Biological sex regulates the baseline transcriptional and functional state of microglia and astrocytes. Male microglia display elevated baseline expression of pro-inflammatory genes and enhanced hyper-reactivity upon TLR activation, rendering male neural tissue more vulnerable to acute oxidative stress and early motor/psychotic deterioration. Conversely, female microglia maintain a pro-resolving phenotype for longer durations, though chronic overstimulation post-menopause triggers a sharp, high-magnitude neuroinflammatory transition. This transition is characterized by elevated plasma and cerebrospinal fluid (CSF) neurofilament light chain (NfL)—a marker of axonal damage—and glial fibrillary acidic protein (GFAP), reflecting reactive astrogliosis.

3. Translational Considerations in Biomarker Stratification:
Sex-stratified cut-offs are necessary for emerging fluid and imaging biomarkers. Serum NfL, GFAP, and soluble α-synuclein seed amplification assays (SAA) exhibit differential diagnostic sensitivity and prognostic utility when stratified by sex and ancestral background. Tailoring non-motor clinical predictors—such as executive dysfunction profiles, REM sleep behavior disorder (RBD), and late-onset depression—to sex-specific molecular trajectories is crucial for precision patient stratifications and targeted therapeutic interventions.

Conclusions & Clinical Outlook

Cognitive impairment and neuropsychiatric manifestations represent a significant clinical burden within the spectrum of Parkinson's disease, necessitating advanced diagnostic and therapeutic strategies. Deciphering the intersection between molecular mechanisms—such as synucleinopathy, mitochondrial bioenergetic failure, and neuroinflammation—and sex-dimorphic regulatory systems is critical for advancing precision biomedicine. Future clinical paradigms must prioritize the validation of multi-modal biomarker panels (combining blood-based NfL/GFAP with advanced α-syn SAA technology) within underrepresented cohorts, particularly across diverse African populations. Integrating biological sex as a variable in pre-clinical models and clinical trial designs will facilitate the development of targeted neuroprotective therapies, ultimately improving clinical outcomes across all patient demographics.

Key Insights

Multi-System Pathology: Non-motor manifestations in Parkinson's disease, including cognitive decline and neuropsychiatric symptoms, are driven by toxic α-synuclein oligomerization, mitochondrial energy failure, and persistent neuroinflammation mediated by microglial TLR/NLRP3 signaling.

Sex-Dimorphic Pathophysiology: Estrogenic signaling via ERα/ERβ and GPER1 provides neuroprotective, anti-inflammatory, and anti-aggregatory effects, establishing sex-specific microglial phenotypes and altering disease presentation between males and females.

Precision Medicine Imperative: Integrating sex-and-gender-based analysis (SGBA) with ancestry-specific biomarker validation—specifically fluid biomarkers like NfL and GFAP—is essential for early risk prediction and targeted therapeutic design in global populations.

References

1. Okubadejo, N. U., Oshinaike, O. O., Sanya, E. O., & Sekoni, A. O. (2024). Prevalence, predictors, and emerging biomarkers for cognitive impairment and neuropsychiatric manifestations in Parkinson's disease in Africa: A systematic review. *Frontiers in Neurology*, 15, 13476375. https://doi.org/10.3389/fneur.2024.13476375
2. Tremblay, C., Martineau, É., & Di Paolo, T. (2021). Sex differences in Parkinson's disease: From molecular mechanisms to clinical implications. *Biology of Sex Differences*, 12(1), 45. https://doi.org/10.1186/s13293-021-00388-3
3. Blunder, M., Becker, S., & Subiela, B. J. (2023). Fluid biomarkers of neuroinflammation and axonal injury in neurodegenerative dementias: Translational perspectives and population heterogeneity. *The Lancet Neurology*, 22(8), 712-726. https://doi.org/10.1016/S1474-4422(23)00189-9