Biology subjects

Fröhlich, F.

Publications and source records attributed to Fröhlich, F..

17 recordsLinked to original sources

bioRxiv2026

Systematic Analysis of GARP- and EARP-Mediated Trafficking Pathways Using Lysosomal Proteomics

Eukaryotic cells utilize tethering complexes for sorting of proteins within the endo-lysosomal system. The GARP complex and the EARP complex both function in endosomal recycling. Both complexes share three common subunits (VPS51, VPS52, and VPS53), with each relying on one specific subunit: VPS54 for GARP and VPS50 for EARP. Mutations in both complexes are linked to various neurodegenerative disorders. While model cargoes have been described, the cargo spectrum of both complexes remains largely elusive. Here, we combined lysosomal proteomics with knockdowns of GARP and EARP to analyze their cargo spectra. Using the lysosome as a terminal reference point for mis-sorted proteins, we show that GARP depletion causes a loss in lysosomal hydrolases and accumulation of non-lysosomal cargo. EARP depletion results in lysosomal accumulation of secreted proteins. Our findings serve as a reference to study GARP and EARP complexes. We demonstrate that a systematic phenotypic analysis of GARP and EARP mutants is necessary to understand the pathological consequences of their dysfunction.

biochemistry↗

bioRxiv2026

Symbolic regression enables coarse-grained model discovery of intracellular signalling dynamics

Cells respond to their environment through protein networks often dysregulated in cancer, making dynamical modelling crucial. Limitations in experimental data and computational resources motivate coarse-graining methods to build low-dimensional descriptions. Yet classical approaches to coarse-grained modelling rely on strong assumptions, leaving it unclear when partial experimental observations support reduced descriptions of system dynamics. Here we show that symbolic regression (SR) provides a data-driven way to test whether, and how compactly, the dynamics of a signalling system coarse-grain over the measured variables, and, when they do, infers mechanistically interpretable models. In synthetic enzyme systems, SR recovers Michaelis-Menten kinetics for the two-step mechanism and under three-step extensions. As data quality is degraded, SR simplifies toward effective kinetic laws while preserving correct theoretical limits. Applied to published time-resolved ERK phosphorylation data, SR identifies compact phospho-ERK rate laws in selected cancer-relevant gene overexpression contexts, yielding interpretable kinetic effects. A sparse neural ODE baseline requires few inputs where SR succeeds, but on average more where it fails, indicating that, where a reduced model is learnable at all, SR failure is associated with more complex dynamics that a simple mathematical model cannot describe. Together, these findings establish symbolic regression as a way to test when a compact coarse-grained description is warranted, generating hypotheses where one holds and motivating potential new measurements where it does not.

systems biology↗

bioRxiv2026

Ire1 membrane stress responses support cell growth upon disruptions in inter-organelle contacts

Disruptions in inter-organelle contacts result in membrane lipid homeostasis defects that ultimately impair cellular function and viability. Yet, essential responses to membrane lipid imbalances remain poorly understood. In this study, we demonstrate that Ire1-dependent Membrane Stress Responses (MSR), distinct from the canonical Unfolded Protein Response (UPR), sustain the growth of yeast cells lacking inter-organelle contacts. Comprehensive lipidomics reveal that the Ire1-mediated MSR compensates for glycerophospholipid synthesis defects by modulating sphingolipid metabolism. Quantitative imaging further indicates that Ire1 mediates these effects, at least in part, by elevating cytoplasmic Ca2+ which in turn stimulates calcineurin activity necessary for cellular homeostasis. Accordingly, inhibition of calcineurin results in severe endoplasmic reticulum stress in yeast cells depleted of inter-organelle contacts. Thus, the Ire1 MSR directs Ca2+-dependent calcineurin activity and lipid metabolism upon disruptions in membrane contact sites to maintain cellular homeostasis. Alterations in inter-organelle contacts are associated with several diseases, including neurodegenerative disorders. Our findings in yeast suggest that evoking the MSR may be a means to ameliorate neuronal degeneration and delay the progression of neurodegenerative disorders.

cell biology↗

bioRxiv2026

Lipid Droplet Targeting Drives Substrate Access during Very Long Chain Fatty Acid Activation by Fat1

Fatty acids, the most abundant building blocks for lipid synthesis, require conjugation with Coenzyme A (CoA) for their incorporation into membrane lipids. Of those, very long chain fatty acids (VLCFAs), the precursors of yeast ceramides, are particularly hydrophobic. Yet it remains unresolved where and how the sole VLCFA-CoA synthetase Fat1 activates them. Here we show that lipid droplets (LDs) are both the site of free VLCFAs storage, as well as their place of activation. Free VLCFAs preferentially partition into LDs in molecular simulations and in vitro reconstitutions, whereas Fat1 targets the LD surface in cells via a N-terminal amphipathic helix. Structural predictions identify an essential hydrophobic cavity in Fat1 that connects the active site to the membrane, suggesting that VLCFAs directly shuttle from the LD core into the enzymatic pocket. Our model explains how the organelle targeting of a fatty acid activator evolved to match the biophysical properties and consequently the subcellular localization of its highly hydrophobic substrate.

biochemistry↗

bioRxiv2025

Dynamic Modeling of Cell Cycle Arrest Through Integrated Single-Cell and Mathematical Modelling Approaches

Highly multiplexed imaging assays allow simultaneous quantification of multiple protein and phosphorylation markers, providing a static snapshots of cell types and states. Pseudo-time techniques can transform these static snapshots of unsynchronized cells into dynamic trajectories, enabling the study of dynamic processes such as development trajectories and the cell cycle. Such ordering also enables training of mathematical models on these data, but technical challenges have hitherto made it difficult to integrate multiple experimental conditions, limiting the predictive power and insights these models can generate. In this work, we propose data processing and model training approaches for integrating multiplexed, multi-condition immunofluorescence data with mathematical modelling. We devise training strategies that are applicable to datasets where cells exhibit oscillatory as well as arrested dynamics and use them to train a cell cycle model on a dataset of MCF-10A mammary epithelial exposed to cell-cycle arresting small molecules. We validate the model by investigating predicted growth factor sensitivities and responses to inhibitors of cells at different initial conditions. We anticipate that our framework will generalise to other highly multiplexed measurement techniques such as mass-cytometry, rendering larger bodies of data accessible to dynamic modelling and paving the way to deeper biological insights. Author SummaryAdvanced imaging techniques allow us to see detailed pictures of different proteins and cell changes. By using computational algorithms, we turn these static pictures into dynamic sequences to understand processes like the cell cycle better. However, combining data from different experiments is difficult and limits how well our models can predict outcomes. This study introduces new ways to process data and train models to handle complex data from various conditions. The approach is tested by using data from untreated and treated cells to create a model of the cell cycle. This model was then checked for accuracy by seeing how well it could predict how cells respond to growth factors and drugs from different starting points. In the future, this method could be used with other data types, allowing for more detailed and accurate models of cellular behavior.

systems biology↗

bioRxiv2025

The Myo2 adaptor Ldm1 and its receptor Ldo16 mediate actin-dependent lipid droplet motility

Organelle motility enables strategic cellular reorganizations. In yeast, this process depends on the actin cytoskeleton, type V myosin motor proteins, and organelle-specific myosin adaptor proteins. While the myosin adaptors for most organelles are known, the coupling of myosin to lipid droplets (LDs), the cellular lipid storage organelles, remained enigmatic. Using genome-wide screening, we identified Ldm1 (Lipid Droplet Motility 1/Yer085c) as a myosin adaptor. Ldm1 binds to the globular tail domain of the myosin Myo2 and to the LD surface protein Ldo16 to enable actin-dependent LD motility. Ldo16 has additional roles in LD contact sites to the vacuole and the ER, suggesting a coordination of LD motility and organelle tethering. Ldm1 has a second role in mitochondrial transport and elevated Ldm1 levels rescue defects of the mitochondrial Myo2-adaptors Mmr1/Ypt11. Our work identifies the molecular machinery for LD motility and contributes to a comprehensive understanding of acto-myosin-based cellular reorganization.

cell biology↗

bioRxiv2024

Imaging of MAP kinase dynamics reveals endocytic regulation of pulsatile signalling and network re-wiring in response to targeted therapy in EGFR-mutant non-small cell lung cancer

A better understanding of the signalling mechanisms underlying transitions from drug-sensitive to drug-tolerant states is required to overcome therapy failure. We combined single-cell biosensor imaging with functional perturbations to investigate the regulation of oncogenic signalling in EGFR-mutant lung adenocarcinoma. We find that despite the constant presence of the mutant oncogene, ERK signalling exhibits pulsatile dynamics, with pulse characteristics determined by the endocytic machinery. Analysis of drug-tolerant persisters (DTPs) revealed that, after an initial phase of complete pathway shut-down, signalling was rewired leading to renewed ERK pulses that drive cell cycle progression. FAK- and SRC-regulated adhesion complexes replace mutant EGFR as the driver of reactivated ERK pulses in DTPs, yet they remain controlled by the membrane trafficking machinery. We show that DTPs rely on additional survival pathways including YAP signalling, and that the phosphatase PTPRS represents a key node in therapy resistant cells, coordinating regulation of ERK, the cytoskeleton, and YAP.

cancer biology↗

bioRxiv2024

The structure of the Orm2-containing serine palmitoyltransferase complex reveals distinct inhibitory potentials of yeast Orm proteins.

The levels of sphingolipids are crucial determinants of neurodegenerative disorders. Therefore, sphingolipid levels have to be tightly regulated. The Orm protein family and ceramides act as inhibitors in the rate-limiting step of sphingolipid biosynthesis - the condensation of L-serine and palmitoyl-CoA. The two yeast isoforms Orm1 and Orm2 form a complex with the serine palmitoyltransferase (SPT). While the sequences of the Orm proteins are highly similar, however, Orm1 and Orm2 are differentially regulated in yeast cells. The mechanistic details of the differential regulation remain elusive. To elucidate the regulatory mechanism, we determined the cryo-electron microscopy structure of the SPT complex containing Orm2. Through in vitro activity assays and a newly developed plasmid coverage assay combined with targeted lipidomics, we demonstrate that the SPT complex with Orm2 exhibits lower activity in comparison to the complex containing Orm1. This collectively suggests a higher inhibitory potential of Orm2, despite the remarkably similar structures of the Orm1- and Orm2 containing complexes. The high conservation of the SPT from yeast to man implies that the three human ORMDL isoforms could also have different regulatory capacities, which might be essential to understanding their role in sphingolipid-mediated neurodegenerative disorders. HighlightsO_LICryo-EM structures of serine palmitoyltransferase with Orm2 from S. cerevisiae C_LIO_LICeramide-mediated inhibition on SPT activity is conserved amongst SPTs C_LIO_LIThe two yeast Orm isoforms have different regulatory capacity C_LI

biochemistry↗

bioRxiv2023

Structure of the yeast ceramide synthase

Ceramides play a pivotal role as essential lipids, serving as foundational components for complex sphingolipids and potent signaling molecules. Ceramides are the products of the N-acylation of a sphingoid base and a CoA-activated fatty acid. This reaction is catalyzed by the enzymes of the evolutionarily conserved ceramide synthase (CerS) family. Yet, the precise structural details and catalytic mechanisms of CerSs have remained elusive. Here, we employed cryo-EM single particle analysis to unravel the structure of the yeast ceramide synthase complex in both an active and a fumonisin B1 inhibited state. Our findings shed light on the complexs architecture, revealing a dimer of Lip1 subunits bound to the two catalytic subunits, Lag1 and Lac1. Each catalytic subunit forms a hydrophobic crevice that is accessible from both the cytosolic site as well as from the intermembrane space of the endoplasmic reticulum (ER). Within this cavity, we identify amino acids forming the active center and a sphingoid base, one of the substrates of the complex. Together, this suggests a pre-loaded state of the CerS. Additionally, the fumonisin B1 bound structure reveals the inhibitory mechanism by blocking the cytosolic acyl-CoA binding site.

biochemistry↗

bioRxiv2022

Multi range ERK responses shape the proliferative trajectory of single cells following oncogene induced senescence

Oncogene-induced senescence (OIS) is a phenomenon in which aberrant oncogene expression causes non-transformed cells to enter a non-proliferative state. Cells undergoing OIS display phenotypic heterogeneity, with some cells senescing and others remaining proliferative. The causes of the heterogeneity remain poorly understood. We studied the sources of heterogeneity in the responses of human epithelial cells to oncogenic BRAFV600E expression. We found that a narrow expression range of BRAFV600E generated a wide range of activities of its downstream effector ERK. In population-level and single cell assays, ERK activity displayed a non-monotonic relationship to proliferation, with intermediate ERK activities leading to maximal proliferation. We profiled gene expression across a range of ERK activities over time and characterized four distinct ERK response classes, which we propose act in concert to generate the unique ERK-proliferation response. Altogether, our studies mapped the input-output relationships between ERK activity and proliferation providing important insights into how heterogeneity can be generated during OIS.

systems biology↗

bioRxiv2022

Efficient computation of adjoint sensitivities at steady-state in ODE models of biochemical reaction networks

Dynamical models in the form of systems of ordinary differential equations have become a standard tool in systems biology. Many parameters of such models are usually unknown and have to be inferred from experimental data. Gradient-based optimization has proven to be effective for parameter estimation. However, computing gradients becomes increasingly costly for larger models, which are required for capturing the complex interactions of multiple biochemical pathways. Adjoint sensitivity analysis has been pivotal for working with such large models, but methods tailored for steady-state data are currently not available. We propose a new adjoint method for computing gradients, which is applicable if the experimental data include steady-state measurements. The method is based on a reformulation of the backward integration problem to a system of linear algebraic equations. The evaluation of the proposed method using real-world problems shows a speedup of total simulation time by a factor of up to 4.4. Our results demonstrate that the proposed approach can achieve a substantial improvement in computation time, in particular for large-scale models, where computational efficiency is critical. Author summaryLarge-scale dynamical models are nowadays widely used for the analysis of complex processes and the integration of large-scale data sets. However, computational cost is often a bottleneck. Here, we propose a new gradient computation method that facilitates the parameterization of large-scale models based on steady-state measurements. The method can be combined with existing gradient computation methods for time-course measurements. Accordingly, it is an essential contribution to the environment of computationally efficient approaches for the study of large-scale screening and omics data, but not tailored to biological applications, and, therefore, also useful beyond the field of computational biology.

systems biology↗

bioRxiv2020

Uptake of endogenous serine is important to maintain sphingolipid homeostasis in Saccharomyces cerevisiae

Sphingolipids are abundant and essential molecules in eukaryotes that have crucial functions as signaling molecules and as membrane components. Sphingolipid biosynthesis starts in the endoplasmic reticulum with the condensation of serine and palmitoyl-CoA. Sphingolipid biosynthesis is highly regulated to maintain sphingolipid homeostasis. Even though, serine is an essential component of the sphingolipid biosynthesis pathway, its role in maintaining sphingolipid homeostasis has not been precisely studied. Here we show that serine uptake is an important factor for the regulation of sphingolipid biosynthesis in Saccharomyces cerevisiae. Using genetic experiments, we find the broad-specificity amino acid permease Gnp1 to be important for serine uptake. We confirm these results with serine uptake assays in gnp1{Delta} cells. We further show that uptake of exogenous serine by Gnp1 is important to maintain cellular serine levels and observe a specific connection between serine uptake and the first step of sphingolipid biosynthesis. Using mass spectrometry-based flux analysis, we further observed imported serine as the main source for de novo sphingolipid biosynthesis. Our results demonstrate that yeast cells preferentially use the uptake of exogenous serine to regulate sphingolipid biosynthesis. Our study can also be a starting point to analyze the role of serine uptake in mammalian sphingolipid metabolism. Author SummarySphingolipids (SPs) are membrane lipids globally required for eukaryotic life. In contrast to other lipid classes, SPs cannot be stored in the cell and therefore their levels have to be tightly regulated. Failure to maintain sphingolipid homeostasis can result in pathologies including neurodegeneration, childhood asthma and cancer. However, we are only starting to understand how SP biosynthesis is adjusted according to need. In this study, we use genetic and biochemical methods to show that the uptake of exogenous serine is necessary to maintain SP homeostasis in Saccharomyces cerevisiae. Serine is one of the precursors of long chain bases in cells, the first intermediate of SP metabolism. Our results suggest that the uptake of serine is directly coupled to SP biosynthesis at ER-plasma membrane contact sites. Overall, our study identifies serine uptake as a novel regulatory factor of SP homeostasis. While we use yeast as a discovery tool, these results also provide valuable insights into mammalian SP biology especially under pathological conditions.

genetics↗

bioRxiv2019

Lowe Syndrome-linked endocytic adaptors direct membrane cycling kinetics with OCRL in Dictyostelium discoideum.

Mutations of the inositol 5-phosphatase OCRL cause Lowe Syndrome (LS), characterized by congenital cataract, low IQ and defective kidney proximal tubule resorption. A key subset of LS mutants abolishes OCRLs interactions with endocytic adaptors containing F&H peptide motifs. Converging unbiased methods examining human peptides and the unicellular phagocytic organism Dictyostelium discoideum, reveal that, like OCRL, the Dictyostelium OCRL orthologue Dd5P4 binds two proteins closely related to the F&H proteins APPL1 and Ses1/2 (also referred to as IPIP27A/B). In addition, a novel conserved F&H interactor was identified, GxcU (in Dictyostelium) and the Cdc42-GEF Frabin (in human cells). Examining these proteins in Dictyostelium discoideum, we find that, like OCRL, Dd5P4 acts at well-conserved and physically distinct endocytic stations. Dd5P4 functions in coordination with F&H proteins to control membrane deformation at multiple stages of endocytosis, and suppresses GxcU-mediated activity during fluid-phase micropinocytosis. We also reveal that OCRL/Dd5P4 acts at the contractile vacuole, an exocytic osmoregulatory organelle. We propose F&H peptide-containing proteins may be key modifiers of LS phenotypes.

cell biology↗

bioRxiv2019

Transcranial Alternating Current Stimulation (tACS) Entrains Alpha Oscillations by Preferential Phase Synchronization of Fast-Spiking Cortical Neurons to Stimulation Waveform

Modeling studies predict that transcranial alternating current stimulation (tACS) entrains brain oscillations, yet direct examination has been lacking or potentially contaminated by stimulation artefact. Here we first demonstrate how the posterior parietal cortex drives primary visual cortex and thalamic LP in the alpha-band in head-fixed awake ferrets. The spike-field synchrony is maximum within alpha frequency, and more prominent for narrow-spiking neurons than broad-spiking ones. Guided by a validated model of electric field distribution, we produced electric fields comparable to those in humans and primates (< 0.5 mV/mm). We found evidence to support the model-driven predictions of how tACS entrains neural oscillations as explained by the triangular Arnold tongue pattern. In agreement with the stronger spike-field coupling of narrow-spiking cells, tACS more strongly entrained this cell population. Our findings provide the first in vivo evidence of how tACS with electric field amplitudes used in human studies entrains neuronal oscillators.

neuroscience↗

bioRxiv2018

A Systematic Approach to Identify Recycling Endocytic Cargo Depending on the GARP Complex

Proteins and lipids of the plasma membrane underlie constant remodeling via a combination of the secretory- and the endocytic pathway. In the yeast endocytic pathway, cargo is sorted for recycling to the plasma membrane or degradation in vacuoles. In a previous paper we have shown a role for the GARP complex in sphingolipid sorting and homeostasis (Frohlich et al. 2015). However, the majority of cargo sorted in a GARP dependent process remain largely unknown. Here we use auxin induced degradation of GARP combined with mass spectrometry based vacuolar proteomics and lipidomics to show that recycling of two specific groups of proteins, the amino-phospholipid flippases and cell wall synthesis proteins depends on a functional GARP complex. Our results suggest that mis-sorting of flippases and remodeling of the lipid composition are the first occurring defects in GARP mutants. Our assay can be adapted to systematically map cargo of the entire endocytic pathway.

cell biology↗

bioRxiv2018

MIN1PIPE: A Miniscope 1-photon-based Calcium Imaging Signal Extraction Pipeline

In vivo calcium imaging using 1-photon based miniscope and microendoscopic lens enables studies of neural activities in freely behaving animals. However, the high and fluctuating background, the inevitable movements and distortions of imaging field, and the extensive spatial overlaps of fluorescent signals emitted from imaged neurons inherent in this 1-photon imaging method present major challenges for extracting neuronal signals reliably and automatically from the raw imaging data. Here we develop a unifying algorithm called MINiscope 1-photon imaging PIPEline (MIN1PIPE) that contains several standalone modules and can handle a wide range of imaging conditions and qualities with minimal parameter tuning, and automatically and accurately isolate spatially localized neural signals. We quantitatively compare MIN1PIPE with other existing partial methods using both synthetic and real datasets obtained from different animal models, and show that MIN1PIPE has a superior performance both in terms of efficiency and precision in analyzing noisy miniscope calcium imaging data.

neuroscience↗

bioRxiv2017

Mechanistic hierarchical population model identifies latent causes of cell-to-cell variability

All biological systems exhibit cell-to-cell variability, and this variability often has functional implications. To gain a thorough understanding of biological processes, the latent causes and underlying mechanisms of this variability must be elucidated. Cell populations comprising multiple distinct subpopulations are commonplace in biology, yet no current methods allow the sources of variability between and within individual subpopulations to be identified. This limits the analysis of single-cell data, for example provided by flow cytometry and microscopy. In this study, we present a data-driven modeling framework for the analysis of populations comprising heterogeneous subpopulations. Our approach combines mixture modeling with frameworks for distribution approximation, facilitating the integration of multiple single-cell datasets and the detection of causal differences between and within subpopulations. The computational efficiency of our framework allows hundreds of competing hypotheses to be compared, giving unprecedented depth of a study. We demonstrated the ability of our method to capture multiple levels of heterogeneity in the analyzes of simulated data and data from highly heterogeneous sensory neurons involved in pain initiation. Our approach identified the sources of cell-to-cell variability and revealed mechanisms that underlie the modulation of nerve growth factor-induced Erk1/2 signaling by extracellular scaffolds.

systems biology↗