Signs of Chronic Hypoxia Suggest a Novel Pathophysiological Event in α-Synucleinopathies

Cellular iron deposition patterns predict clinical subtypes of multiple system atrophy

BACKGROUND

Multiple system atrophy (MSA) is a primary oligodendroglial synucleinopathy, characterized by elevated iron burden in early-affected subcortical nuclei. Although neurotoxic effects of brain iron deposition and its relationship with α-synuclein pathology have been demonstrated, the exact role of iron dysregulation in MSA pathogenesis is unknown. Therefore, advancing the understanding of iron dysregulation at the cellular level is critical, especially in relation to α-synuclein cytopathology.

METHODS

Iron burden in subcortical and brainstem regions were histologically mapped in human post-mortem brains of 4 MSA-parkinsonian (MSA-P), 4 MSA-cerebellar (MSA-C), and 1 MSA case with both parkinsonian and cerebellar features. We then performed the first cell type-specific evaluation of pathological iron deposition in α-synuclein-affected and -unaffected cells of the globus pallidus, putamen, and the substantia nigra, regions of highest iron concentration, using a combination of iron staining with immunolabelling. Selective regional and cellular vulnerability patterns of iron deposition were compared between disease subtypes. In 7 MSA cases, expression of key iron- and closely related oxygen-homeostatic genes were examined.

RESULTS

MSA-P and MSA-C showed different patterns of regional iron burden across the pathology-related systems. We identified subcortical microglia to predominantly accumulate iron, which was more distinct in MSA-P. MSA-C showed relatively heterogenous iron accumulation, with greater or similar deposition in astroglia. Iron deposition was also found outside cellular bodies. Cellular iron burden associated with oligodendrocytic, and not neuronal, α-synuclein cytopathology. Gene expression analysis revealed dysregulation of oxygen homeostatic genes, rather than of cellular iron. Importantly, hierarchal cluster analysis revealed the pattern of cellular vulnerability to iron accumulation, distinctly to α-synuclein pathology load in the subtype-related systems, to distinguish MSA subtypes.

CONCLUSIONS

Our comprehensive evaluation of iron deposition in MSA brains identified distinct regional, and for the first time, cellular distribution of iron deposition in MSA-P and MSA-C and revealed cellular vulnerability patterns to iron deposition as a novel neuropathological characteristic that predicts MSA clinical subtypes. Our findings suggest distinct iron-related pathomechanisms in MSA clinical subtypes that are therefore not a consequence of a uniform down-stream pathway to α-synuclein pathology, and inform current efforts in iron chelation therapies at the disease and cellular-specific levels.

Hypoxia Sensing and Responses in Parkinson's Disease

Parkinson's disease (PD) is associated with various deficits in sensing and responding to reductions in oxygen availability (hypoxia). Here we summarize the evidence pointing to a central role of hypoxia in PD, discuss the relation of hypoxia and oxygen dependence with pathological hallmarks of PD, including mitochondrial dysfunction, dopaminergic vulnerability, and alpha-synuclein-related pathology, and highlight the link with cellular and systemic oxygen sensing. We describe cases suggesting that hypoxia may trigger Parkinsonian symptoms but also emphasize that the endogenous systems that protect from hypoxia can be harnessed to protect from PD. Finally, we provide examples of preclinical and clinical research substantiating this potential.

Association Between Orthostatic Hypotension and Dementia in Patients With Parkinson Disease and Multiple System Atrophy

BACKGROUND AND OBJECTIVES

Orthostatic hypotension (OH) increases dementia risk in patients with Parkinson disease (PD), although the underlying mechanisms and whether a similar association between OH and cognitive impairment exists in other synucleinopathies remain unknown. The aim is to evaluate the association between OH and dementia risk in patients with PD, and cognitive impairment risk in patients with multiple system atrophy (MSA), and to explore relevant clinical and neuropathologic factors to understand underlying pathogenic mechanisms.

METHODS

This is a retrospective cohort study. Medical records throughout the entire disease course of consecutive patients with neuropathology-confirmed PD and MSA from the Queen Square Brain Bank were systematically reviewed. Time of onset and severity of OH-related symptoms were documented, and their association with other clinical and neuropathologic variables was evaluated. Dementia risk for patients with PD and cognitive impairment risk for patients with MSA were estimated using multivariable hazard regression.

RESULTS

One hundred thirty-two patients with PD and 137 with MSA were included. Patients with MSA developed OH more frequently, earlier in the disease course and with more severe symptoms. Cumulative dementia prevalence was higher in patients with PD. Multivariable adjusted regression models showed that early OH, but not its symptom severity, increased dementia risk in patients with PD by 14% per year (hazard ratio [HR] = 0.86; 95% CI, 0.80-0.93) and cognitive impairment risk in patients with MSA by 41% per year (HR = 0.59; 95% CI, 0.42-0.83). Early OH was not associated with increased α-synuclein, β-amyloid, tau, Alzheimer, or cerebrovascular pathologies. No significant associations were found between severity of OH symptoms and other clinical or neuropathologic variables.

DISCUSSION

Early OH, but not its symptom severity, increases the risk of cognitive impairment in patients with PD and MSA. OH is not associated with more extensive Lewy, β-amyloid, tau, Alzheimer, or cerebrovascular pathologies. It is likely that OH contributes to cognitive impairment in patients with PD and MSA by hypoxia-induced nonspecific neurodegeneration. Further research should evaluate whether improving brain perfusion by treating OH may modify the risk of dementia in these conditions.

Breathing disorders in neurodegenerative diseases

Neurodegenerative disorders are a diverse group of conditions caused by progressive degeneration of neurons resulting in cognitive, motor, sensory, and autonomic dysfunction, leading to severe disability and death. Pulmonary dysfunction is relatively common in these conditions, may be present early in the disease, and is less well recognized and treated than other symptoms. There are variable disorders of upper and lower airways, central control of ventilation, strength of respiratory muscles, and breathing during sleep which further impact daily activities and quality of life and have the potential to injure vulnerable neurons. Laryngopharyngeal dysfunction affects speech, swallowing, and clearance of secretions, increases the risk of aspiration pneumonia, and can cause stridor and sudden death. In Parkinson's disease, L-Dopa benefits some pulmonary symptoms but there are limited pharmacological treatment options for pulmonary dysfunction. Targeted treatments include strengthening of respiratory muscles, positive airway pressure in sleep and techniques to improve cough efficacy. Well-designed clinical trials are needed to evaluate the long-term benefits of these interventions. Challenges for the future include earlier identification of pulmonary dysfunction in the clinic, institution of the most effective treatments (based on clinical trials that measure long-term meaningful outcomes) and the development of neuroprotective treatment.

Fatal attraction - The role of hypoxia when alpha-synuclein gets intimate with mitochondria

Alpha-synuclein aggregation and mitochondrial dysfunction are main pathological hallmarks of Parkinson's disease (PD) and several other neurodegenerative diseases, collectively known as synucleinopathies. However, increasing evidence suggests that they may not be sufficient to cause PD. Here we propose the role of hypoxia as a missing link that connects the complex interplay between alpha-synuclein biochemistry and pathology, mitochondrial dysfunctions and neurodegeneration in PD. We review the partly conflicting literature on alpha-synuclein binding to membranes and mitochondria and its impact on mitochondrial functions. From there, we focus on adverse changes in cellular environments, revolving around hypoxic stress, that may trigger or facilitate PD progression. Inter-dependent structural re-arrangements of mitochondrial membranes, including increased cytoplasmic exposure of mitochondrial cardiolipins and changes in alpha-synuclein localization and conformation are discussed consequences of such conditions. Enhancing cellular resilience could be an integral part of future combination-based therapies of PD. This may be achieved by boosting the capacity of cellular and specifically mitochondrial processes to regulate and adapt to altered proteostasis, redox, and inflammatory conditions and by inducing protective molecular and tissue re-modelling.

Hypoxia, Acidification and Inflammation: Partners in Crime in Parkinson’s Disease Pathogenesis?

Like in other neurodegenerative diseases, protein aggregation, mitochondrial dysfunction, oxidative stress and neuroinflammation are hallmarks of Parkinson’s disease (PD). Differentiating characteristics of PD include the central role of α-synuclein in the aggregation pathology, a distinct vulnerability of the striato-nigral system with the related motor symptoms, as well as specific mitochondrial deficits. Which molecular alterations cause neurodegeneration and drive PD pathogenesis is poorly understood. Here, we summarize evidence of the involvement of three interdependent factors in PD and suggest that their interplay is likely a trigger and/or aggravator of PD-related neurodegeneration: hypoxia, acidification and inflammation. We aim to integrate the existing knowledge on the well-established role of inflammation and immunity, the emerging interest in the contribution of hypoxic insults and the rather neglected effects of brain acidification in PD pathogenesis. Their tight association as an important aspect of the disease merits detailed investigation. Consequences of related injuries are discussed in the context of aging and the interaction of different brain cell types, in particular with regard to potential consequences on the vulnerability of dopaminergic neurons in the substantia nigra. A special focus is put on the identification of current knowledge gaps and we emphasize the importance of related insights from other research fields, such as cancer research and immunometabolism, for neurodegeneration research. The highlighted interplay of hypoxia, acidification and inflammation is likely also of relevance for other neurodegenerative diseases, despite disease-specific biochemical and metabolic alterations.

Nocturnal stridor in multiple system atrophy: Video-polysomnography and clinical features

INTRODUCTION

Nocturnal stridor, a life-threatening condition linked to respiratory failure and sudden death during sleep, is a serious issue in patients with multiple system atrophy (MSA). However, little is known about polysomnographic findings and clinical features of MSA patients with nocturnal stridor. Hence, we investigated video-polysomnography (VPSG) findings and clinical features associated with nocturnal stridor in patients with MSA.

METHODS

We retrospectively analyzed the clinical data of patients with MSA (n = 49) who underwent overnight VPSG for the evaluation of sleep-disordered breathing. The presence of nocturnal stridor was confirmed based on overnight VPSG findings. Clinical data, including VPSG findings and clinical features, were compared between MSA patients with and without nocturnal stridor.

RESULTS

Nocturnal stridor was present in 31 (63.3%) patients with MSA. Patients with stridor showed significantly higher apnea-hypopnea, respiratory disturbance, and oxygen desaturation indices than those without stridor (P = 0.024, P = 0.049, and P = 0.006, respectively). Patients with stridor had more severe axial motor features, more impaired activities of daily living, and longer disease duration than those without stridor (P = 0.012, P = 0.036, and P = 0.003, respectively). However, there were no significant between-group differences in sex, age at disease onset, MSA subtype, parkinsonian features, cerebellar ataxia, residual urine volume, or systolic and diastolic blood pressure change.

CONCLUSIONS

MSA with nocturnal stridor is related to higher apnea indices in conjunction with higher O₂ desaturation index, more severe axial motor features, more impaired activities of daily living, and longer disease duration.