Multi-omic phenotyping of MAPT V337M neurons reveals early changes in axonogenesis and tau phosphorylation

Problem Statement

Tau aggregation is a defining pathological feature of neurodegenerative disorders such as Alzheimer's disease and frontotemporal dementia (FTD). While disease-causing variants in the MAPT gene (encoding tau) are known to correlate with disease progression, the specific molecular mechanisms linking pathological tau to neuronal dysfunction remain poorly understood. A critical gap in current knowledge is the incomplete characterization of tau's normal physiological functions during development and aging, and how these functions are altered by causal disease variants.

Methodology

To address these gaps in an unbiased manner, the researchers employed a multi-omic characterization strategy using induced pluripotent stem cell (iPSC)-derived neurons. The study utilized two distinct genetic models:

1. Disease Model: Neurons harboring the MAPT V337M mutation (a pathogenic variant).
2. Control/Knockdown Model: Neurons with MAPT knockdown to assess the effects of tau deficiency.

The multi-omic approach integrated three high-throughput technologies:

• RNA-seq: To analyze transcriptomic changes.
• ATAC-seq: To assess chromatin accessibility and regulatory landscape changes.
• Phosphoproteomics: To map global protein phosphorylation states.

Additionally, the study incorporated functional assays (direct measurement of axonogenesis), CRISPR-based genetic screens to identify regulators of tau phosphorylation, and pathway-specific investigations (focusing on the p38 MAPK pathway).

Key Contributions & Results

1. Disruption of Axonogenesis and Morphology
Both the MAPT V337M mutation and MAPT knockdown resulted in significant perturbations in gene expression and protein phosphorylation related to axonogenesis and axon morphology. Functional validation confirmed that both conditions led to a measurable decrease in axon length, suggesting that both the presence of the mutant protein and the absence of tau disrupt neuronal structural development.

2. Paradoxical Reduction in Tau Phosphorylation
Contrary to the prevailing hypothesis that pathogenic tau variants lead to hyperphosphorylation, the study found that neurons with the V337M mutation exhibited significantly lower levels of tau phosphorylation compared to wild-type (WT) neurons. This finding challenges the assumption that increased phosphorylation is the sole or primary driver of V337M pathology.

3. Identification of Phosphorylation Regulators via CRISPR Screens
CRISPR-based screens identified specific factors that regulate tau phosphorylation. A key discovery was that factors involved in axonogenesis directly modify tau phosphorylation levels in both WT and V337M neurons, establishing a mechanistic link between structural development and tau post-translational modification.

4. Specific Role of the p38 MAPK Pathway
The study identified the p38 MAPK pathway as a specific modifier of tau phosphorylation in MAPT V337M neurons, distinguishing it from the mechanisms observed in WT neurons. This suggests a mutation-specific signaling dysregulation.

Significance

This study provides a novel mechanistic framework for understanding MAPT variant pathology. By demonstrating that the V337M mutation perturbs axonogenesis pathways and tau phosphorylation in a manner that mimics tau deficiency (rather than just toxic gain-of-function), the authors propose that the disease mechanism involves the disruption of tau's normal developmental functions.

These findings offer a potential explanation for the cognitive changes observed in preclinical carriers of MAPT variants, suggesting that neuronal dysfunction may begin early in development due to altered axonal growth and phosphorylation dynamics, rather than solely as a consequence of late-stage protein aggregation. This shifts the therapeutic focus toward understanding and preserving tau's physiological roles in axonogenesis.