Biology subjects

Nuechel, J.

Publications and source records attributed to Nuechel, J..

6 recordsLinked to original sources

bioRxiv2026

Proteolytic Remodeling of Cargo Receptor Networks by RHBDL4 Tunes Secretory Pathway Flux

Cargo receptors are central organizers of the secretory pathway, yet the mechanisms controlling their abundance remain poorly understood. The endoplasmic reticulum (ER)-resident intramembrane protease RHBDL4 promotes substrate turnover via a non-canonical branch of ER-associated degradation and has recently been implicated in regulating secretory pathway components. We previously identified the p24 cargo receptor TMED7 as an RHBDL4 substrate, suggesting that cargo receptor turnover contributes to secretory pathway regulation. Here, quantitative proteomics identify members of the ER-Golgi intermediate compartment (ERGIC) cargo receptor family as endogenous RHBDL4 substrates, demonstrating that RHBDL4 targets multiple cargo receptor families within the early secretory pathway. Accordingly, RHBDL4 modulates multiple ERGIC-dependent transport pathways. In addition, unbiased secretome analysis reveals increased secretion of lysosomal precursor proteins upon RHBDL4 ablation. Mechanistically, we show that this phenotype is mediated, at least in part, by RHBDL4-dependent cleavage of the lysosomal cargo receptor sortilin/SORT1. Together, these findings identify cargo receptors as a major class of RHBDL4 substrates and establish proteolytic remodeling of cargo receptor networks as a mechanism for regulating secretory pathway flux.

cell biology↗

bioRxiv2026

A Golgi-bypass secretion mechanism for proTGFα involving TMED9 and GRASP65

We recently demonstrated that the endoplasmic reticulum export of proTGF is tightly gated, and that proteostasis factors, including the intramembrane protease RHBDL4, can promote its release by an unknown mechanism. While investigating a potential link between RHBDL4-regulated p24/TMED family proteins and proTGF trafficking, we uncovered an RHBDL4-independent pathway in which the cargo receptor TMED9 induces unconventional protein secretion (UcPS) of ectopically expressed proTGF. This pathway selectively requires GRASP65, but not the closely related GRASP55, and depends on components of the autophagic machinery and the ESCRT-associated protein ALIX, consistent with a Golgi-bypass secretion route. Although largely based on overexpression systems, our findings identify a previously unrecognized UcPS mechanism and provide insight into alternative protein trafficking pathways that may be exploited under conditions of proteostasis stress and in pathological contexts such as cancer and inflammation.

cell biology↗

bioRxiv2024

GRASP55 Safeguards Proper Lysosome Function by Controlling Sorting of Lysosomal Enzymes at the Golgi

Lysosomes are multifunctional organelles that play important roles in cellular recycling, signaling, and homeostasis, relying on precise trafficking and activation of lysosomal enzymes. While the Golgi apparatus plays a central role in lysosomal enzyme sorting, the mechanisms linking Golgi function to lysosomal activity remain incompletely understood. Here, we identify the Golgi-resident protein GRASP55, but not its paralog GRASP65, as a key regulator of lysosome function. More specifically, we demonstrate that loss of GRASP55 expression leads to missorting and secretion of lysosomal enzymes, lysosomal dysfunction and bloating. GRASP55 deficiency also disrupts lysosomal mTORC1 signaling, reducing the phosphorylation of its lysosomal substrates, TFEB and TFE3, while sparing its non-lysosomal targets. Mechanistically, GRASP55 interacts with GNPTAB, a critical enzyme required for mannose 6-phosphate (M6P) tagging of lysosomal enzymes, and is necessary for its correct trafficking and stability. These findings reveal an essential role for GRASP55 in Golgi-lysosome communication and lysosomal enzyme trafficking, and suggest that GRASP55/GORASP2 may act as a susceptibility gene for lysosomal storage disorder (LSD)-like conditions. Overall, this work underscores the importance of Golgi-mediated protein sorting in lysosome function and lysosomal mTORC1 signaling, and provides insights into the molecular basis of LSD-related pathologies.

cell biology↗

bioRxiv2023

Spatially and Functionally Distinct mTORC1 Entities Orchestrate the Cellular Response to Amino Acid Availability

Amino acid (AA) availability is a robust determinant of cell growth, through controlling mTORC1 activity 1. According to the predominant model in the field, AA sufficiency drives the recruitment and activation of mTORC1 on the lysosomal surface by the heterodimeric Rag GTPases, from where it coordinates the majority of cellular processes (reviewed in 2,3). Importantly, however, 15 years after its initial discovery, the teleonomy of the proposed lysosomal regulation of mTORC1, and where mTORC1 acts on its effector proteins remain enigmatic 4. Here, by using multiple pharmacological and genetic means to perturb the lysosomal AA sensing and protein recycling machineries, we describe the spatial separation of mTORC1 regulation and downstream functions in mammalian cells, with lysosomal and non-lysosomal mTORC1 phosphorylating distinct substrates in response to different AA sources. Moreover, we reveal that a fraction of mTOR localizes at lysosomes due to basal lysosomal proteolysis that locally supplies new AAs, even in cells grown in the presence of extracellular nutrients, whereas cytoplasmic mTORC1 is regulated by exogenous AAs. Overall, our study substantially expands our knowledge about the topology of mTORC1 regulation by AAs, and hints at the existence of distinct, Rag- and lysosome-independent mechanisms that control its activity at other subcellular locations. Given the importance of mTORC1 signalling and AA sensing for human ageing and disease 2, our findings will likely open new directions toward the identification of function-specific mTORC1 regulators, and suggest new targets for drug discovery against conditions with dysregulated mTORC1 activity in the future.

cell biology↗

bioRxiv2022

Malonyl-CoA is an ancient physiological ATP-competitive mTORC1 inhibitor

Cell growth is regulated primarily by the mammalian/mechanistic Target of Rapamycin Complex 1 (mTORC1) that functions both as a nutrient sensor and a master controller of virtually all biosynthetic pathways 1. This ensures that cells are metabolically active only when conditions are optimal for growth. Notably, although mTORC1 is known to regulate fatty acid (FA) biosynthesis, how and whether the cellular lipid biosynthetic capacity signals back to fine-tune mTORC1 activity remains poorly understood. Here, we show that mTORC1 senses the capacity of a cell to synthesize FAs by detecting the levels of malonyl-CoA, an intermediate of this biosynthetic pathway. We find that, in both yeast and mammalian cells, this regulation is very direct, with malonyl-CoA binding to the mTOR catalytic pocket and acting as a specific ATP-competitive inhibitor. When ACC1 (acetyl-CoA carboxylase 1) is hyperactive or FASN (fatty acid synthase) is downregulated/inhibited, elevated malonyl-CoA levels are channelled to proximal mTOR molecules that form direct protein-protein interactions with ACC1 and FASN. Our findings represent a conserved, unique, homeostatic mechanism whereby impaired FA biogenesis leads to reduced mTORC1 activity to coordinatively link this metabolic pathway to the overall cellular biosynthetic output. Moreover, they reveal the first-described example of a physiological metabolite that directly inhibits the activity of a signalling kinase by competing with ATP for binding.

cell biology↗

bioRxiv2021

Mitochondrial membrane proteins and VPS35 orchestrate selective removal of mtDNA

Integrity of mitochondrial DNA (mtDNA), encoding several subunits of the respiratory chain, is essential to maintain mitochondrial fitness. Mitochondria, as a central hub for metabolism, are affected in a wide variety of human diseases but also during normal ageing, where mtDNA integrity is compromised. Mitochondrial quality control mechanisms work at different levels, and mitophagy and its variants are critical to remove dysfunctional mitochondria together with mtDNA to maintain cellular homeostasis. Understanding the mechanisms governing a selective turnover of mutation-bearing mtDNA without affecting the entire mitochondrial pool is fundamental to design therapeutic strategies against mtDNA diseases and ageing. Here we show that mtDNA depletion after expressing a dominant negative version of the mitochondrial helicase Twinkle, or by chemical means, is due to an exacerbated mtDNA turnover. Targeting of nucleoids is controlled by Twinkle which, together with the mitochondrial transmembrane proteins ATAD3 and SAMM50, orchestrate mitochondrial membrane remodeling to form extrusions. mtDNA removal depends on autophagy and requires the vesicular trafficking protein VPS35 which binds to Twinkle-enriched mitochondrial subcompartments upon mtDNA damage. Stimulation of autophagy by rapamycin selectively removes mtDNA deletions which accumulated during muscle regeneration in vivo, but without affecting mtDNA copy number. With these results we unveil a new complex mechanism specifically targeting and removing mutant mtDNA which occurs outside the mitochondrial network. We reveal the molecular targets involved in a process with multiple potential benefits against human mtDNA related diseases, either inherited, acquired or due to normal ageing.

cell biology↗