Abstract:Cells activate protective mechanisms to overcome stressful conditions that threaten cellular homeostasis, including imbalances in calcium, redox, and nutrient levels. Endoplasmic reticulum (ER) stress activates an intracellular signaling pathway, known as the unfolded protein response (UPR), to mitigate such circumstances and protect cells. Although ER stress is sometimes a negative regulator of autophagy, UPR induced by ER stress typically activates autophagy, a self-degradative pathway that further supports … Show more
“…In our dataset, genes and pathways related to apoptosis regulation and autophagy signaling were also dysregulated in astrocytes, oligodendrocytes, and in the neuronal clusters with reduced proportions in PSP. Although the relationship between the UPR and autophagy has been well studied in neurons, its role in regulating cell autonomous and non-cell autonomous functions in other cell types has not been well studied [81][82][83].…”
Progressive supranuclear palsy (PSP) is a sporadic neurodegenerative tauopathy variably affecting brainstem and cortical structures and characterized by tau inclusions in neurons and glia. The precise mechanism whereby these protein aggregates lead to cell death remains unclear. To investigate the contribution of these different cellular abnormalities to PSP pathogenesis, we performed single-nucleus RNA sequencing and analyzed 45,559 high quality nuclei targeting the subthalamic nucleus and adjacent structures from human post-mortem PSP brains with varying degrees of pathology compared to controls. Cell-type specific differential expression and pathway analysis identified both common and discrete changes in numerous pathways previously implicated in PSP and other neurodegenerative disorders. This included EIF2 signaling, an adaptive pathway activated in response to diverse stressors, which was the top activated pathway in vulnerable cell types. Using immunohistochemistry, we found that activated eIF2α was positively correlated with tau pathology burden in vulnerable brain regions. Multiplex immunofluorescence localized activated eIF2α positivity to hyperphosphorylated tau (p-tau) positive neurons and ALDH1L1-positive astrocytes, supporting the increased transcriptomic EIF2 activation observed in these vulnerable cell types. In conclusion, these data provide insights into cell-type-specific pathological changes in PSP and support the hypothesis that failure of adaptive stress pathways play a mechanistic role in the pathogenesis and progression of PSP.
“…In our dataset, genes and pathways related to apoptosis regulation and autophagy signaling were also dysregulated in astrocytes, oligodendrocytes, and in the neuronal clusters with reduced proportions in PSP. Although the relationship between the UPR and autophagy has been well studied in neurons, its role in regulating cell autonomous and non-cell autonomous functions in other cell types has not been well studied [81][82][83].…”
Progressive supranuclear palsy (PSP) is a sporadic neurodegenerative tauopathy variably affecting brainstem and cortical structures and characterized by tau inclusions in neurons and glia. The precise mechanism whereby these protein aggregates lead to cell death remains unclear. To investigate the contribution of these different cellular abnormalities to PSP pathogenesis, we performed single-nucleus RNA sequencing and analyzed 45,559 high quality nuclei targeting the subthalamic nucleus and adjacent structures from human post-mortem PSP brains with varying degrees of pathology compared to controls. Cell-type specific differential expression and pathway analysis identified both common and discrete changes in numerous pathways previously implicated in PSP and other neurodegenerative disorders. This included EIF2 signaling, an adaptive pathway activated in response to diverse stressors, which was the top activated pathway in vulnerable cell types. Using immunohistochemistry, we found that activated eIF2α was positively correlated with tau pathology burden in vulnerable brain regions. Multiplex immunofluorescence localized activated eIF2α positivity to hyperphosphorylated tau (p-tau) positive neurons and ALDH1L1-positive astrocytes, supporting the increased transcriptomic EIF2 activation observed in these vulnerable cell types. In conclusion, these data provide insights into cell-type-specific pathological changes in PSP and support the hypothesis that failure of adaptive stress pathways play a mechanistic role in the pathogenesis and progression of PSP.
“… 54 Both ERS and autophagy are crucial determinants of cell fate and require tight regulation. 55 Fig. 2 UPR-mediated autophagy and partial mechanisms.…”
“…As a result, it induces host cellular innate oxidative stress and proinflammatory immune responses leading to apoptotic cell death ( 14 ). Additionally, there is a crosstalk between ER stress and autophagy, a process that is known as reticulophagy or ER-phagy where ER stress elicits a selective form of autophagy, aka ER-phagy, that transpires ER-associated degradation (ERAD) leading to degradation of ER-residing proteins or ER destruction ( 31 ). Early studies showed that induction of reticulophagy by SARS-CoV-2 is essential for viral infection and replication, and ORF3a elicits reticulophagy response and then disrupts ER homeostasis to induce ER stress and inflammatory responses during SARS-CoV-2 infection ( 25 – 27 ).…”
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) ORF3a protein plays a vital role in viral pathogenesis and coronavirus disease 2019 (COVID-19). Like the spike protein, ORF3a mutates frequently, and certain variants are associated with the severity of COVID-19. Given the clinical significance and functional implications of ORF3a mutations, we conducted a comprehensive mutagenesis study targeting various known functional elements and revealed two distinctive types of ORF3a proteins based on their subcellular localizations: ORF3a proteins primarily localize on the lysosomal membrane (L-ORF3a) and those present in the endoplasmic reticulum (E-ORF3a). The objective of this study was to contrast the functional and mechanistic distinctions between these two types of ORF3a proteins. We examined six distinct ORF3a mutants and assessed their effects on cellular oxidative stress, nuclear factor kappa B-induced cytokine production, and cell death. Mechanistically, we explored ORF3a-induced ER stress, autophagy, and interactions with relevant cellular proteins. Our findings indicate that ORF3a proteins induce cytopathic effects through a similar mechanism, irrespective of their subcellular location. However, E-ORF3a proteins elicit more pronounced cytopathic effects despite their lower abundance and minimal impact on ER stress and autophagy when compared to L-ORF3a proteins. This discrepancy is attributed to ER-associated degradation since ORF3a proteins bind to a ubiquitin E3 ligase TRIM59. Inhibition of the 26S proteasome partially restores the protein levels of E-ORF3a and cellular ER stress response. This suggests that even a small quantity of ORF3a can lead to significant cytopathic effects due to the delicate nature of ER. Our study underscores the intricate interplay of dynamic cellular signaling within these two subcellular compartments in response to ORF3a.
IMPORTANCE
The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic has tragically claimed millions of lives through coronavirus disease 2019 (COVID-19), and there remains a critical gap in our understanding of the precise molecular mechanisms responsible for the associated fatality. One key viral factor of interest is the SARS-CoV-2 ORF3a protein, which has been identified as a potent inducer of host cellular proinflammatory responses capable of triggering the catastrophic cytokine storm, a primary contributor to COVID-19-related deaths. Moreover, ORF3a, much like the spike protein, exhibits a propensity for frequent mutations, with certain variants linked to the severity of COVID-19. Our previous research unveiled two distinct types of ORF3a mutant proteins, categorized by their subcellular localizations, setting the stage for a comparative investigation into the functional and mechanistic disparities between these two types of ORF3a variants. Given the clinical significance and functional implications of the natural ORF3a mutations, the findings of this study promise to provide invaluable insights into the potential roles undertaken by these mutant ORF3a proteins in the pathogenesis of COVID-19.
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