Büttner lab

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Selected Publications
Tosal-Castano S, Peselj C, Kohler V, Habernig L, Berglund LL, Ebrahimi M, Vögtle FN, Höög J, Andréasson C, Büttner S.
Snd3 controls nucleus-vacuole junctions in response to glucose signaling. Full text
Cell Rep. 2021

Membrane contact sites facilitate the exchange of metabolites between organelles to support interorganellar communication. The nucleus-vacuole junctions (NVJs) establish physical contact between the perinuclear endoplasmic reticulum (ER) and the vacuole. Although the NVJ tethers are known, how NVJ abundance and composition are controlled in response to metabolic cues remains elusive. Here, we identify the ER protein Snd3 as central factor for NVJ formation. Snd3 interacts with NVJ tethers, supports their targeting to the contacts, and is essential for NVJ formation. Upon glucose exhaustion, Snd3 relocalizes from the ER to NVJs and promotes contact expansion regulated by central glucose signaling pathways. Glucose replenishment induces the rapid dissociation of Snd3 from the NVJs, preceding the slow disassembly of the junctions. In sum, this study identifies a key factor required for formation and regulation of NVJs and provides a paradigm for metabolic control of membrane contact sites.
Berndtsson J, Aufschnaiter A, Rathore S, Marin-Buera L, Dawitz H, Diessl J, Kohler V, Barrientos A, Büttner S, Fontanesi F, Ott M.
Respiratory supercomplexes enhance electron transport by decreasing cytochrome c diffusion distance. Full text
EMBO Rep. 2020

Respiratory chains are crucial for cellular energy conversion and consist of multi-subunit complexes that can assemble into supercomplexes. These structures have been intensively characterized in various organisms, but their physiological roles remain unclear. Here, we elucidate their function by leveraging a high-resolution structural model of yeast respiratory supercomplexes that allowed us to inhibit supercomplex formation by mutation of key residues in the interaction interface. Analyses of a mutant defective in supercomplex formation, which still contains fully functional individual complexes, show that the lack of supercomplex assembly delays the diffusion of cytochrome c between the separated complexes, thus reducing electron transfer efficiency. Consequently, competitive cellular fitness is severely reduced in the absence of supercomplex formation and can be restored by overexpression of cytochrome c. In sum, our results establish how respiratory supercomplexes increase the efficiency of cellular energy conversion, thereby providing an evolutionary advantage for aerobic organisms.
Duan J, Zhao Y, Li H, Habernig L, Gordon MD, Miao X, Engström Y, Büttner S.
Bab2 Functions as an Ecdysone-Responsive Transcriptional Repressor during Drosophila Development. Full text
Cell Rep. 2020

Drosophila development is governed by distinct ecdysone steroid pulses that initiate spatially and temporally defined gene expression programs. The translation of these signals into tissue-specific responses is crucial for metamorphosis, but the mechanisms that confer specificity to systemic ecdysone pulses are far from understood. Here, we identify Bric-à-brac 2 (Bab2) as an ecdysone-responsive transcriptional repressor that controls temporal gene expression during larval to pupal transition. Bab2 is necessary to terminate Salivary gland secretion (Sgs) gene expression, while premature Bab2 expression blocks Sgs genes and causes precocious salivary gland histolysis. The timely expression of bab2 is controlled by the ecdysone-responsive transcription factor Broad, and manipulation of EcR/USP/Broad signaling induces inappropriate Bab2 expression and termination of Sgs gene expression. Bab2 directly binds to Sgs loci in vitro and represses all Sgs genes in vivo. Our work characterizes Bab2 as a temporal regulator of somatic gene expression in response to systemic ecdysone signaling.
Diessl J, Nandy A, Schug C, Habernig L, Büttner S.
Stable and destabilized GFP reporters to monitor calcineurin activity in Saccharomyces cerevisiae. Full text
Microb Cell. 2020

The protein phosphatase calcineurin is activated in response to rising intracellular Ca2+ levels and impacts fundamental cellular processes in organisms ranging from yeast to humans. In fungi, calcineurin orchestrates cellular adaptation to diverse environmental challenges and is essential for virulence of pathogenic species. To enable rapid and large-scale assessment of calcineurin activity in living, unperturbed yeast cells, we have generated stable and destabilized GFP transcriptional reporters under the control of a calcineurin-dependent response element (CDRE). Using the reporters, we show that the rapid dynamics of calcineurin activation and deactivation can be followed by flow cytometry and fluorescence microscopy. This system is compatible with live/dead staining that excludes confounding dead cells from the analysis. The reporters provide technology to monitor calcineurin dynamics during stress and ageing and may serve as a drug-screening platform to identify novel antifungal compounds that selectively target calcineurin.
Poveda-Huertes D, Matic S, Marada A, Habernig L, Licheva M, Myketin L, Gilsbach R, Tosal-Castano S, Papinski D, Mulica P, Kretz O, Kücükköse C, Taskin AA, Hein L, Kraft C, Büttner S, Meisinger C, Vögtle FN.
An Early mtUPR: Redistribution of the Nuclear Transcription Factor Rox1 to Mitochondria Protects against Intramitochondrial Proteotoxic Aggregates. Full text
Mol Cell. 2020

The mitochondrial proteome is built mainly by import of nuclear-encoded precursors, which are targeted mostly by cleavable presequences. Presequence processing upon import is essential for proteostasis and survival, but the consequences of dysfunctional protein maturation are unknown. We find that impaired presequence processing causes accumulation of precursors inside mitochondria that form aggregates, which escape degradation and unexpectedly do not cause cell death. Instead, cells survive via activation of a mitochondrial unfolded protein response (mtUPR)-like pathway that is triggered very early after precursor accumulation. In contrast to classical stress pathways, this immediate response maintains mitochondrial protein import, membrane potential, and translation through translocation of the nuclear HMG-box transcription factor Rox1 to mitochondria. Rox1 binds mtDNA and performs a TFAM-like function pivotal for transcription and translation. Induction of early mtUPR provides a reversible stress model to mechanistically dissect the initial steps in mtUPR pathways with the stressTFAM Rox1 as the first line of defense.
Andréasson C, Ott M, Büttner S.
Mitochondria orchestrate proteostatic and metabolic stress responses. Full text
EMBO Rep. 2019

The eukaryotic cell is morphologically and functionally organized as an interconnected network of organelles that responds to stress and aging. Organelles communicate via dedicated signal transduction pathways and the transfer of information in form of metabolites and energy levels. Recent data suggest that the communication between organellar proteostasis systems is a cornerstone of cellular stress responses in eukaryotic cells. Here, we discuss the integration of proteostasis and energy fluxes in the regulation of cellular stress and aging. We emphasize the molecular architecture of the regulatory transcriptional pathways that both sense and control metabolism and proteostasis. A special focus is placed on mechanistic insights gained from the model organism budding yeast in signaling from mitochondria to the nucleus and how this shapes cellular fitness.
Aufschnaiter A, Kohler V, Walter C, Tosal-Castano S, Habernig L, Wolinski H, Keller W, Vögtle FN, Büttner S.
The Enzymatic Core of the Parkinson's Disease-Associated Protein LRRK2 Impairs Mitochondrial Biogenesis in Aging Yeast. Full text
Front Mol Neurosci. 2018

Mitochondrial dysfunction is a prominent trait of cellular decline during aging and intimately linked to neuronal degeneration during Parkinson's disease (PD). Various proteins associated with PD have been shown to differentially impact mitochondrial dynamics, quality control and function, including the leucine-rich repeat kinase 2 (LRRK2). Here, we demonstrate that high levels of the enzymatic core of human LRRK2, harboring GTPase as well as kinase activity, decreases mitochondrial mass via an impairment of mitochondrial biogenesis in aging yeast. We link mitochondrial depletion to a global downregulation of mitochondria-related gene transcripts and show that this catalytic core of LRRK2 localizes to mitochondria and selectively compromises respiratory chain complex IV formation. With progressing cellular age, this culminates in dissipation of mitochondrial transmembrane potential, decreased respiratory capacity, ATP depletion and generation of reactive oxygen species. Ultimately, the collapse of the mitochondrial network results in cell death. A point mutation in LRRK2 that increases the intrinsic GTPase activity diminishes mitochondrial impairment and consequently provides cytoprotection. In sum, we report that a downregulation of mitochondrial biogenesis rather than excessive degradation of mitochondria underlies the reduction of mitochondrial abundance induced by the enzymatic core of LRRK2 in aging yeast cells. Thus, our data provide a novel perspective for deciphering the causative mechanisms of LRRK2-associated PD pathology.
Suhm T, Kaimal JM, Dawitz H, Peselj C, Masser AE, Hanzén S, Ambrožič M, Smialowska A, Björck ML, Brzezinski P, Nyström T, Büttner S, Andréasson C, Ott M.
Mitochondrial Translation Efficiency Controls Cytoplasmic Protein Homeostasis. Full text
Cell Metab. 2018

Cellular proteostasis is maintained via the coordinated synthesis, maintenance, and breakdown of proteins in the cytosol and organelles. While biogenesis of the mitochondrial membrane complexes that execute oxidative phosphorylation depends on cytoplasmic translation, it is unknown how translation within mitochondria impacts cytoplasmic proteostasis and nuclear gene expression. Here we have analyzed the effects of mutations in the highly conserved accuracy center of the yeast mitoribosome. Decreased accuracy of mitochondrial translation shortened chronological lifespan, impaired management of cytosolic protein aggregates, and elicited a general transcriptional stress response. In striking contrast, increased accuracy extended lifespan, improved cytosolic aggregate clearance, and suppressed a normally stress-induced, Msn2/4-dependent interorganellar proteostasis transcription program (IPTP) that regulates genes important for mitochondrial proteostasis. Collectively, the data demonstrate that cytosolic protein homeostasis and nuclear stress signaling are controlled by mitochondrial translation efficiency in an inter-connected organelle quality control network that determines cellular lifespan.
Suhm T, Habernig L, Rzepka M, Kaimal JM, Andréasson C, Büttner S, Ott M.
A novel system to monitor mitochondrial translation in yeast. Full text
Microb Cell. 2018

The mitochondrial genome is responsible for the production of a handful of polypeptides that are core subunits of the membrane-bound oxidative phosphorylation system. Until now the mechanistic studies of mitochondrial protein synthesis inside cells have been conducted with inhibition of cytoplasmic protein synthesis to reduce the background of nuclear gene expression with the undesired consequence of major disturbances of cellular signaling cascades. Here we have generated a system that allows direct monitoring of mitochondrial translation in unperturbed cells. A recoded gene for superfolder GFP was inserted into the yeast (Saccharomyces cerevisiae) mitochondrial genome and enabled the detection of translation through fluorescence microscopy and flow cytometry in functional mitochondria. This novel tool allows the investigation of the function and regulation of mitochondrial translation during stress signaling, aging and mitochondrial biogenesis.
Aufschnaiter A, Habernig L, Kohler V, Diessl J, Carmona-Gutierrez D, Eisenberg T, Keller W, Büttner S.
The Coordinated Action of Calcineurin and Cathepsin D Protects Against α-Synuclein Toxicity. Full text
Front Mol Neurosci. 2017

The degeneration of dopaminergic neurons during Parkinson's disease (PD) is intimately linked to malfunction of α-synuclein (αSyn), the main component of the proteinaceous intracellular inclusions characteristic for this pathology. The cytotoxicity of αSyn has been attributed to disturbances in several biological processes conserved from yeast to humans, including Ca2+homeostasis, general lysosomal function and autophagy. However, the precise sequence of events that eventually results in cell death remains unclear. Here, we establish a connection between the major lysosomal protease cathepsin D (CatD) and the Ca2+/calmodulin-dependent phosphatase calcineurin. In a yeast model for PD, high levels of human αSyn triggered cytosolic acidification and reduced vacuolar hydrolytic capacity, finally leading to cell death. This could be counteracted by overexpression of yeast CatD (Pep4), which re-installed pH homeostasis and vacuolar proteolytic function, decreased αSyn oligomers and aggregates, and provided cytoprotection. Interestingly, these beneficial effects of Pep4 were independent of autophagy. Instead, they required functional calcineurin signaling, since deletion of calcineurin strongly reduced both the proteolytic activity of endogenous Pep4 and the cytoprotective capacity of overexpressed Pep4. Calcineurin contributed to proper endosomal targeting of Pep4 to the vacuole and the recycling of the Pep4 sorting receptor Pep1 from prevacuolar compartments back to the trans-Golgi network. Altogether, we demonstrate that stimulation of this novel calcineurin-Pep4 axis reduces αSyn cytotoxicity.

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Our funding

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Membran Contact Sites during aging

From function to molecular architecture & abundance
A prominent mechanism to establish interorganellar connectivity is direct physical contact between organelles via so-called membrane contact sites (MCS). Virtually all organelles within a cell are connected by MCS, and such physical interaction facilitates interorganellar communication and the integration of compartmentalized processes by exchange of metabolites, lipids and ions. Moreover, MCS promote the formation of lipid rafts and are associated with misfolded and aggregating proteins. So far, the impact of MCS on cellular aging and, vice versa, the impact of age on these contact sites, remains largely unexplored.
We are interested in MCS dynamics, molecular architecture and abundance in response to aging and altered metabolic regimes and the function of MCS in proteostasis and cellular fitness over time. In addition, we study how different nutritional regimes, ranging from caloric restriction, nitrogen starvation and phosphate restriction to fermentative vs respiratory carbon sources, affect interorganellar connectivity and organellar function. Our aim is to provide insights into yet unexplored functions of these MCS in cellular homeostasis during aging.

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Interorganellar connectivity & quality control systems

Intracellular communication to maintain proteostasis
Functional decline during cellular aging is modulated by integrated metabolic and proteostatic subsystems within a cell, and misfolded or aggregated proteins accumulate during and accelerate aging. Lifespan-controlling pathways often influence cellular proteostasis, and efficient maintenance of the proteome is crucial to sustain viability during aging. Cytosolic proteostasis as well as organelle-specific proteostasis subsystems include branches for synthesis, folding, maintenance, and maintenance, but also for degradation, e.g. via autophagy or the ubiquitin-proteasome system. As every eukaryotic cell is organized as a dynamic network of interconnected subsystems, a proper cellular response to any kind of stress, including aging, necessitates efficient intracellular communication between cooperating organelles. We are interested in the crosstalk between different organellar proteostasis subsystems and how their functionality and regulation is modulated by dedicated signal transduction pathways and the physical contacts between organelles.
A special focus is on proteotoxic stress, both at the cellular and organellar level, and compromised protein and organelle degradation via autophagy. We are using post-mitotic yeast cells to shed light on the interrelation between those processes and their functional deterioration with progressing age. In a national consortium (groups Nyström, Höög, Ott, Andréasson, Büttner), we aim to map how interconnected protein quality control changes genetically, functionally, and structurally during aging and thus affects lifespan, a collaborative project funded by the Knut and Alice Wallenberg foundation.

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Modelling proteotoxicity and neurodegeneration

Yeast and fly models for human disease
Using yeast and fly models, we study how the accumulation of abnormal and proteotoxic proteins in the course of aging contributes to the progressive decay of different subsystems involved in the maintenance of cellular proteostasis, including autophagy. The misfolding and aggregation of proteotoxic proteins characterizes various age-associated human diseases, specifically neurodegeneration, and a functional decline of autophagy is linked to cellular dysfunction during aging
As compensatory feedback systems are in place, a subtle functional decline of any proteostasis network component might not lead to cellular demise per se. Instead, this might render aged cells vulnerable to additional stress, for instance increased levels of aggregation-prone proteins, environmental stress or changes in membrane properties. Physiochemical parameters of biological membranes, such as fluidity, curvature, and charge density, are intimately connected to the aggregation behaviour of various proteotoxic proteins. We study how different aspects of membrane biology affect protein aggregation and cellular aggregate handling, also in respect to contact between organelles.