📄 cell biology

TMEM55B regulates lysosomal acidification and ER-lysosome calcium signaling to promote lipolysis

This study identifies TMEM55B as a critical regulator that, in complex with PDZD8 at ER-lysosome membrane contact sites, coordinates lysosomal acidification and calcium signaling to drive lipolysis and cholesterol metabolism.

Original authors: Mukae, N., Maki, H., Shirane, M.

Published 2026-06-04

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CC BY 4.0

Original authors: Mukae, N., Maki, H., Shirane, M.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine your cells are bustling cities, and inside them, there are specialized delivery trucks and recycling centers. One of the most important recycling centers is the lysosome. Think of the lysosome as a high-tech waste management plant that breaks down old fat and cholesterol so the cell can reuse the materials. For this plant to work, it needs two things: a very acidic environment (like a strong cleaning solution) and a precise system of electrical signals (calcium) to tell it when to start working.

The paper introduces two main characters in this story: PDZD8 and TMEM55B.

PDZD8 is like a master traffic controller. It works at "membrane contact sites," which are basically special docking stations where the cell's main factory (the Endoplasmic Reticulum or ER) connects directly to the recycling plant (the lysosome). We already knew PDZD8 helps manage cholesterol, but we didn't know exactly how it kept the recycling plant running smoothly.

The researchers discovered that TMEM55B is the missing link. Think of TMEM55B as the specialized foreman that PDZD8 hires to run the recycling plant. Here is what this foreman does:

The Acid Pump: The recycling plant needs to be acidic to dissolve waste. TMEM55B works closely with a machine called the v-ATPase, which acts like a pump that fills the plant with acid. The study found that if you remove TMEM55B, the pump slows down, the plant loses its acidity, and it can't break down fat droplets effectively.
The Calcium Signal: The plant also needs a signal to know when to start recycling. This signal is calcium. TMEM55B helps manage the flow of calcium between the factory (ER) and the plant (lysosome). It's like a switch that allows a burst of calcium to jump from the factory to the plant to trigger the "start working" button.
- When TMEM55B is present, this signal flows smoothly, and the plant turns on to digest fat.
- When the researchers removed TMEM55B, this signal got stuck. The factory couldn't send the signal, and the plant didn't know to start breaking down fat. Interestingly, this problem was specific to the recycling plant; the cell's power generators (mitochondria) kept working fine, showing that TMEM55B is a specialist for this specific job.

The Big Picture
In simple terms, this paper shows that PDZD8 and TMEM55B work as a team at the docking station between the factory and the recycling plant. PDZD8 brings in TMEM55B, and TMEM55B ensures the plant has enough acid and the right signals to break down fat and cholesterol. Without this team, the recycling plant goes dark, fat builds up, and the cell's metabolism gets clogged.

The study concludes that this "PDZD8-TMEM55B axis" is the key mechanism that keeps the cell's fat-burning and cholesterol-management systems running efficiently.

Technical Summary: TMEM55B Regulates Lysosomal Acidification and ER-Lysosome Calcium Signaling to Promote Lipolysis

Problem Statement
The lipid transfer protein PDZD8 is known to facilitate cholesterol metabolism by promoting endosomal maturation at membrane contact sites (MCSs) between the endoplasmic reticulum (ER) and endolysosomal compartments. While endosomal maturation is intrinsically linked to lysosomal acidification and calcium signaling, the specific molecular mechanisms connecting PDZD8 to these physiological processes have remained undefined. Furthermore, the identity of proteins that mediate PDZD8's function in regulating lysosomal ion homeostasis and lipid turnover was previously unclear.

Methodology
To address these gaps, the study employed a combination of proteomic screening and functional assays to identify PDZD8-associated proteins. The researchers identified TMEM55B as a lysosomal protein that physically interacts with PDZD8. To characterize the functional role of TMEM55B, the authors utilized expression suppression (depletion) techniques in cellular models. They subsequently assessed the impact of TMEM55B loss on:

Lysosomal acidification status.
Lipid droplet turnover rates.
Calcium ( $Ca^{2+}$ ) dynamics, specifically focusing on lysosomal $Ca^{2+}$ release and reuptake.
ATP-induced $Ca^{2+}$ responses in the ER.
Mitochondrial $Ca^{2+}$ dynamics (as a control for specificity).

Key Contributions and Results
The study establishes TMEM55B as a critical regulator of lysosomal function at ER-lysosome MCSs. The key findings include:

Lysosomal Acidification and Lipolysis: Suppression of TMEM55B expression resulted in reduced lysosomal acidification. Consequently, this impairment led to a significant decrease in lipid droplet turnover, indicating a direct link between TMEM55B-mediated acidification and lipolysis.
Calcium Signaling Dynamics: TMEM55B depletion attenuated both lysosomal $Ca^{2+}$ release and reuptake. This disruption extended to the ER, where diminished ATP-induced $Ca^{2+}$ responses were observed. These data suggest that TMEM55B is essential for maintaining calcium-induced calcium release (CICR) at ER-lysosome contact sites.
Specificity of Action: The study noted that mitochondrial $Ca^{2+}$ dynamics remained unaffected by TMEM55B depletion, highlighting the specific role of TMEM55B in ER-lysosome communication rather than a general disruption of cellular calcium homeostasis.
Interaction Context: The findings align with previous reports suggesting TMEM55B interacts with the v-ATPase complex, reinforcing its proposed role in lysosomal function.

Significance
This work identifies TMEM55B as a novel regulator of lysosomal acidification and calcium dynamics. The paper posits that the PDZD8-TMEM55B axis functions to promote lipolysis and cholesterol metabolism through the coordinated regulation of lysosomal function at ER-lysosome membrane contact sites. By elucidating this molecular axis, the study provides a mechanistic explanation for how PDZD8 influences endosomal maturation and metabolic processes, specifically linking protein interactions at MCSs to ion homeostasis and lipid catabolism.

Technical Summary: TMEM55B Regulates Lysosomal Acidification and ER-Lysosome Calcium Signaling to Promote Lipolysis

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