Hyperphosphorylation as a defense mechanism to reduce TDP-43 aggregation

Area of Science:

• Neuroscience
• Molecular Biology
• Cell Biology

Background:

• Amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration with ubiquitinated inclusions (FTLD-U) are neurodegenerative diseases.
• These diseases are characterized by the aggregation of TDP-43 (TDP) protein into inclusion bodies.
• The role of TDP-43 hyperphosphorylation in the pathogenesis of these diseases remains unclear.

Purpose of the Study:

• To investigate the role of TDP-43 hyperphosphorylation in the formation and pathogenicity of TDP-43 aggregates.
• To explore the potential of modulating TDP-43 phosphorylation as a therapeutic strategy for ALS and FTLD-U.

Main Methods:

• Established cell and transgenic Drosophila models expressing TDP-43 carboxyl terminal fragments (ND251, ND207).
• Utilized site-directed mutagenesis to create non-phosphorylatable (S5A) and phospho-mimetic (S5D, S5E) TDP-43 variants.
• Assessed aggregate formation, phosphorylation status, ubiquitination, and cellular toxicity using biochemical assays, microscopy, and cell viability tests.

Main Results:

• Hyperphosphorylation and ubiquitination of TDP-43 occurred later than aggregation in cellular models.
• CK2α-mediated phosphorylation reduced TDP-43 aggregation propensity, while inhibition of CK2 increased it.
• Non-phosphorylatable TDP-43 mutants (S5A) showed increased aggregation and toxicity, whereas phospho-mimetic mutants (S5E) exhibited reduced aggregation and alleviated toxicity.
• Transgenic Drosophila expressing non-phosphorylatable TDP-43 formed aggregates, while those expressing phospho-mimetic TDP-43 did not.

Conclusions:

• TDP-43 hyperphosphorylation appears to be a compensatory defense mechanism that inhibits pathogenic aggregation.
• Modulating TDP-43 phosphorylation, specifically enhancing it, may represent a promising therapeutic avenue for ALS and FTLD-U.