Ultrastructural Morphometry Points to a New Role for LAMTOR2 in Regulating the Endo/Lysosomal System

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• The late endosomal adaptor protein LAMTOR2/p14 plays a vital role in tissue homeostasis.
• It regulates key signaling pathways like MAPK and mTOR, controlling cell growth, proliferation, and migration.
• LAMTOR2 is essential for endocytic system architecture and function, including receptor degradation and pathogen defense.

Purpose of the Study:

• To investigate the ultrastructural phenotype of the endo/lysosomal system in LAMTOR2-deficient mouse embryonic fibroblasts.
• To elucidate the role of LAMTOR2 in multivesicular body (MVB) morphology and membrane trafficking.
• To understand the impact of LAMTOR2 deficiency on endo/lysosomal subpopulations and receptor recycling.

Main Methods:

• Quantitative (immuno-)electron microscopy of cryo-fixed samples.
• Analysis of LAMTOR2-deficient and control mouse embryonic fibroblasts.
• Inactivation of mTOR in control cells to compare with LAMTOR2 deficiency effects.

Main Results:

• LAMTOR2 deficiency led to a significant reduction in recycling tubules from maturing MVBs, with vesicles forming a surrounding halo.
• Morphological changes correlated with growth factor presence and were mimicked by mTOR inactivation.
• Proper transferrin receptor trafficking and recycling were dependent on the intact LAMTOR complex.
• A severe imbalance in endo/lysosomal subpopulations was observed, with increased mature MVBs and (autophago)lysosomes.

Conclusions:

• The LAMTOR/Ragulator complex is essential for maintaining endo/lysosomal homeostasis.
• LAMTOR2 contributes to the proper formation of MVB-recycling tubules.
• The complex regulates membrane and cargo recycling from MVBs, impacting overall cellular function.