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This review reflects comments and contributions from Femi Arogundade & Anna Oliveras. Review synthesized by Femi Arogundade & Jonny Coates.

The study investigates the intricate relationship between tau protein aggregation and lipid metabolism in neurodegenerative tauopathies. Utilizing a transgenic rat model and in vitro cell systems, the research reveals that misfolded tau is associated with significant metabolic changes, particularly in the early stages of tauopathy, disrupting lipid homeostasis and potentially accelerating neurodegeneration. The findings highlight the importance of understanding the role of specific lipids, such as lysoPLs and sphingolipids, in tau pathology, suggesting potential therapeutic targets for addressing neurodegenerative diseases. Overall, the study underscores the complex interplay between tau protein, lipid metabolism, and the progression of tauopathies.

Major comments:

• The study employs a comprehensive approach, utilizing both a transgenic rat model and in vitro cell systems, to investigate the intricate interplay between tau protein aggregation and lipid metabolism in neurodegenerative tauopathies. The targeted lipidomic and metabolomic analyses contribute to the robustness of the methodology, allowing for a detailed exploration of metabolic alterations induced by tau pathology. The longitudinal analysis of transgenic and control animals at multiple time points enhances the study's strength, providing insights into the dynamic changes associated with disease progression.

• The identification of specific lipid species, such as LPC, LPE, PC, PG, SM, Cer, and HexCer, in both cerebrospinal fluid (CSF) and brain tissue, adds depth to the lipidomic analysis and strengthens the validity of the results. The association of these lipids with various aspects of tau pathology, including protein fibrillization, membrane reorganization, and inflammation, provides a nuanced understanding of the role of lipid dysregulation in neurodegenerative processes.

• The in vitro cell model system, involving the co-cultivation of human neuroblastoma cells and primary rat glial cells, adds a valuable dimension to the study, allowing for the exploration of extracellular tau-mediated effects on lipid metabolism. The observation of lipid droplet accumulation in microglia and its association with mitochondrial dysfunction provides mechanistic insights into how lipid changes may contribute to neuroinflammation and neurodegeneration.

• Immunohistochemical staining and biochemical analysis revealed increased phosphorylation of tau at specific epitopes, as well as changes in soluble and insoluble forms of tau in different brain regions.

• The study found that CSF more accurately reflected aberrant brain metabolism compared to plasma, suggesting the importance of CSF in monitoring neurological processes.

• The study indicated a connection between the transition of tau into a sarkosyl-insoluble fraction and changes in brain metabolism, emphasizing the interplay between tau pathology and lipid alterations.

• While the study establishes correlations between tau pathology and lipid alterations, establishing causation would require further mechanistic investigations. Additionally, considering the complexity of neurodegenerative diseases, the study appropriately acknowledges the need for further research to fully elucidate the roles of specific lipid classes in tauopathy.

Minor comments:

• Elaborate on the control strategies employed in the tau transgenic model, particularly addressing potential confounding factors that may influence lipid metabolism. Discuss any measures taken to ensure that observed lipid changes are specifically associated with tau pathology and not influenced by other variables.

• Provide information for the rat model - has this been characterized previously as a model for tauopathy? Why did the authors use a spontaneous hypertenisive rat as a genetic background?

• Acknowledge and discuss potential confounding factors that could influence lipidomic profiles, such as diet, age-related changes, or other systemic factors. This discussion will enhance the study's credibility by addressing possible alternative explanations for the observed lipid alterations.

• Details on the preservation methods during sample collection (e.g., use of anticoagulants for blood) might be relevant for ensuring the integrity of the samples.

• The freezing and freeze-drying steps during sample preparation might affect the stability of certain analytes. Were steps taken to minimize potential degradation or alteration of the samples?

• While the use of QC samples is mentioned, details on the criteria for acceptable QC performance and how it was monitored during the analysis could enhance the robustness of the results.

• The study involves the cultivation of BV2 cells and primary rat glial culture. What considerations were taken into account to ensure the relevance of these cell models to the in vivo conditions being studied?

• The concentration of tau protein used for cell stimulation is mentioned. How was this concentration determined, and what evidence supports its physiological relevance?

• The isolation of lipid droplet-associated mitochondria (LDM) is described. Could you provide more information on how the purity and integrity of the isolated mitochondria were assessed?

• The biochemical analysis involves western blotting for tau phosphorylation. Given the complexity of tau phosphorylation patterns, how were the specific phosphorylation sites chosen for analysis, and how does this reflect the overall tau pathology?

• Clarify the choice of specific time points (4, 6, 8, 10, 12, and 14 months) for the age-matched non-Tg animals.

• The discussion touches upon the connection between ceramides and Aβ pathology. How does ceramide accumulation relate to Aβ oligomers, and what evidence supports the idea that Aβ oligomers activate sphingomyelinase (SMase) enzyme, leading to ceramide accumulation and cell death?

• The discussion section would benefit from a brief acknowledgment of any limitations in the study design or methodologies to provide a balanced interpretation of the findings.

Comments on reporting:

• The statistical methods used for data analysis are mentioned, including LOESS correction and pathway analysis. Could the authors elaborate on the specific considerations taken to address potential biases or confounding factors in the data?

Suggestions for future studies:

• Conduct longitudinal studies to delineate the temporal dynamics of lipid changes during different stages of tauopathy. This will help establish a clearer timeline of lipid alterations and their correlation with the progression of tau pathology.

• Explore experimental designs, such as targeted interventions or genetic manipulations, to establish causal relationships between lipid dysregulation and tau aggregation. This could involve modulating specific lipid pathways to observe their impact on tau pathology.

• Investigate the mechanistic details of how extracellular tau interacts with membrane lipids, particularly phospholipids and sphingolipids. Understanding the specific pathways or receptors involved in this interaction could provide insights into potential therapeutic targets.

• Investigate how different cell types within the brain, including neurons, astrocytes, and microglia, respond to lipid changes induced by tau pathology. Understanding cell-specific responses could provide insights into the complex interplay between neurons and glial cells in neurodegenerative processes.

• Evaluate the efficacy of therapeutic interventions targeting lipid metabolism in mitigating tau pathology. This could involve pharmacological agents or lifestyle interventions aimed at restoring lipid homeostasis and, consequently, slowing down or preventing neurodegeneration.

• Expand lipidomic analyses to human samples, including post-mortem brain tissues and cerebrospinal fluid from individuals with tauopathies. This will help validate findings from animal models and strengthen the translational relevance of lipidomic signatures in neurodegenerative diseases.

• Investigate the different pathways involved in the secretion of tau, especially focusing on vesicle-free translocation across the plasma membrane. Understanding the molecular mechanisms behind tau secretion could reveal novel targets for therapeutic intervention.

• Explore the functional consequences of lipid dysregulation on cellular processes such as synaptic transmission, neuronal differentiation, and inflammatory responses. This could provide a more comprehensive understanding of how lipid changes contribute to neurodegeneration.

• Combine lipidomic analyses with other omics approaches, such as genomics, transcriptomics, and proteomics, to gain a holistic understanding of the molecular landscape in tauopathies. Integrating data from multiple omics levels can reveal complex interactions and regulatory networks.

• Explore how genetic factors, including variations in lipid-related genes, influence the susceptibility to tauopathies and the associated lipidomic changes. This could help identify individuals at higher risk and inform personalized therapeutic strategies.

• Implement advanced imaging techniques, such as mass spectrometry imaging, to spatially map lipid changes within specific brain regions. This approach can provide valuable information on the spatial distribution of lipid alterations in relation to tau pathology.

Competing interests

The author declares that they have no competing interests.