Metabolism-Based Gene Differences in Neurons Expressing Hyperphosphorylated AT8- Positive (AT8+) Tau in Alzheimer's Disease

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.

Aggregation, Transmission, and Toxicity of the Microtubule-Associated Protein Tau: A Complex Comprehension

The microtubule-associated protein tau is an intrinsically disordered protein containing a few short and transient secondary structures. Tau physiologically associates with microtubules (MTs) for its stabilization and detaches from MTs to regulate its dynamics. Under pathological conditions, tau is abnormally modified, detaches from MTs, and forms protein aggregates in neuronal and glial cells. Tau protein aggregates can be found in a number of devastating neurodegenerative diseases known as "tauopathies", such as Alzheimer's disease (AD), frontotemporal dementia (FTD), corticobasal degeneration (CBD), etc. However, it is still unclear how the tau protein is compacted into ordered protein aggregates, and the toxicity of the aggregates is still debated. Fortunately, there has been considerable progress in the study of tau in recent years, particularly in the understanding of the intercellular transmission of pathological tau species, the structure of tau aggregates, and the conformational change events in the tau polymerization process. In this review, we summarize the concepts of tau protein aggregation and discuss the views on tau protein transmission and toxicity.

Cell-Specific Dysregulation of Iron and Oxygen Homeostasis as a Novel Pathophysiology in PSP

OBJECTIVE

Progressive supranuclear palsy (PSP) is a 4R-tauopathy showing heterogeneous tau cytopathology commencing in the globus pallidus (GP) and the substantia nigra (SN), regions also associated with age-related iron accumulation. Abnormal iron levels have been extensively associated with tau pathology in neurodegenerative brains, however, its role in PSP pathogenesis remains yet unknown. We perform the first cell type-specific evaluation of PSP iron homeostasis and the closely related oxygen homeostasis, in relation to tau pathology in human postmortem PSP brains.

METHODS

In brain regions vulnerable to PSP pathology (GP, SN, and putamen), we visualized iron deposition in tau-affected and unaffected neurons, astroglia, oligodendrocytes, and microglia, using a combination of iron staining with immunolabelling. To further explore molecular pathways underlying our observations, we examined the expression of key iron and oxygen homeostasis mRNA transcripts and proteins.

RESULTS

We found astrocytes as the major cell type accumulating iron in the early affected regions of PSP, highly associated with cellular tau pathology. The same regions are affected by dysregulated expression of alpha and beta hemoglobin and neuroglobin showing contrasting patterns. We discovered changes in iron and oxygen homeostasis-related gene expression associated with aging of the brain, and identified dysregulated expression of rare neurodegeneration with brain iron accumulation (NBIA) genes associated with tau pathology to distinguish PSP from the healthy aging brain.

INTERPRETATION

We present novel aspects of PSP pathophysiology highlighting an overlap with NBIA pathways. Our findings reveal potential novel targets for therapy development and have implications beyond PSP for other iron-associated neurodegenerative diseases. ANN NEUROL 2023;93:431-445.

Functional Diversity of HemN-like Proteins

HemN is a radical S-adenosylmethionine (SAM) enzyme that catalyzes the anaerobic oxidative decarboxylation of coproporphyrinogen III to produce protoporphyrinogen IX, a key intermediate in heme biosynthesis. Proteins homologous to HemN (HemN-like proteins) are widespread in both prokaryotes and eukaryotes. Although these proteins are in most cases annotated as anaerobic coproporphyrinogen III oxidases (CPOs) in the public database, many of them are actually not CPOs but have diverse functions such as methyltransferases, cyclopropanases, heme chaperones, to name a few. This Perspective discusses the recent advances in the understanding of HemN-like proteins, and particular focus is placed on the diverse chemistries and functions of this growing protein family.