Fridolin Kielisch

Differential gene expression rarely explains why certain genes, e.g. associated with tissue-specific manifestations of human disease, exert tissue-specific functions. To understand how gene expression and molecular context act in concert to drive tissue-specific gene function, we focus on Neurodevelopmental disorders (NDDs), for which underlying disease mechanisms are still poorly understood. We developed a computational framework that harmonizes and integrates spatiotemporal transcriptome datasets enabling us to systematically quantify preferential gene expression across tissues and developmental stages. Applying this pipeline to approximately 1,000 NDD-associated genes, we find that only about 10% show brain-preferential expression, with distinct gene clusters instead exhibiting significant upregulation in non-brain tissues. Investigation of these clusters reveals regulatory mechanisms that govern mitochondrial function to meet neurons’ high energetic demands, which are mechanistically distinct from those employed by other energy-intensive tissues such as heart and skeletal muscle. Furthermore, by integrating tissue-agnostic protein-protein interaction data with our expression resource, we identify putative brain-specific interaction partners for 22 uniformly expressed NDD-associated genes. We experimentally validate these interactions for QRICH1 and demonstrate that disease-associated mutations disrupt them, providing a molecular hypothesis for how QRICH1 acquires brain-specific function and how its disruption might cause Ververi-Brady syndrome. Together, our findings highlight the importance of molecular context for brain-specific gene function in the context of neurodevelopmental disease. We provide a framework for dissecting molecular mechanisms of tissue-specific Mendelian-like disorders and for exploring how biological processes are adapted to tissue-specific needs.

Cellular senescence is a state of irreversible cell cycle arrest triggered by telomere erosion, persistent DNA damage or chronic stress. The accumulation of senescent cells disrupts tissue function and contributes to aging and disease. Here, we employ mass spectrometry-based proteomics to systematically interrogate dynamic proteome changes at multiple levels during the progression of replicative cellular senescence. We demonstrate that proteome changes during senescence occur in a coordinated manner, characterized by widespread protein depletion on chromatin. Moreover, components of the cytoplasmic translation machinery are depleted, while mitochondrial proteins display increased insolubility. Autophagic and proteasome activity is compromised in senescent cells along with remodeling of ubiquitin linkages and depletion of ubiquitin E3 ligases. Comparison of the senescent proteome with different pathophysiological cellular states reveals a distinctive senescent signature shaped by changes in the proteostasis network. Collectively, we provide a resource for the exploration of temporally resolved changes in the senescent proteome.

Double-strand breaks (DSBs) are critical lesions in genomic DNA, and their accurate repair is essential for maintaining genome stability. The nuclear actin cytoskeleton has been implicated in homology-directed repair (HDR) of DSBs. However, the underlying mechanism remains poorly understood. Here, we report that Myosin VI (Myo6), an actin-based motor protein, cooperates with F-actin in end resection and DSB mobilization. Our findings reveal that Myo6 directly interacts with both KU70 and the ubiquitin-dependent segregase VCP to facilitate the extraction of the KU70/80 complex from chromatin. This process is supported by F-actin, revealing an interplay between nuclear actin dynamics and the DSB repair machinery. By elucidating the function of Myo6 and its direct interactions with key repair factors, our study provides mechanistic insight into how repair mechanisms rely on nuclear actin to safeguard genome integrity.

Transcriptional condensates at super-enhancers are thought to concentrate BRD4, Mediator, and RNA polymerase II (Pol II) to promote gene activation, yet their compositional organization and regulation remain poorly understood. We developed a high-throughput live-cell phenomics platform based on endogenous fluorescent tagging of BRD4, MED14 (Mediator), and POLR2A (Pol II) to systematically quantify transcriptional condensate states across >1,000 chemical perturbations. Contrary to prevailing models of largely co-occupied assemblies, we find compositionally heterogenous condensate populations. In particular, BRD4-only spots emerged as a prominent class that is depleted of Mediator and Pol II, enriched at chromatin, and resistant to transcription initiation inhibition. Mechanistically, compound screening coupled to mechanism-of-action analysis identifies histone acetylation as a dominant regulatory axis for BRD4-only spots: Bromodomain and Extra-Terminal motif (BET) and histone acetyltransferase inhibition selectively deplete BRD4-only condensates, while histone deacetylase inhibition expands them. Together, these findings support a model in which acetylation-dependent BRD4 condensates define a distinct chromatin-associated regulatory state that is separable from canonical transcriptionally engaged condensates. More broadly, our work establishes condensate composition as a quantitative phenotype and provides a scalable framework for systematically dissecting the regulation of condensates across perturbations, cell types, and disease contexts.

Gene duplication underlies evolutionary innovation, yet many paralogues remain highly similar, raising questions about their functional divergence and physiological relevance. The spliceosomal Sm core protein SNRPB and its mammalian-specific paralogue SNRPN share over 90% sequence identity, but their distinct expression patterns - SNRPB being ubiquitous and SNRPN confined to the brain - suggest specialized functions. Why mammals have two different spliceosomes has remained obscure. Here, we generated isogenic human cell lines expressing ectopically either SNRPB or SNRPN exclusively and found that SNRPN stabilizes transcripts involved in energy metabolism and mitochondrial function, leading to increased mitochondrial abundance and oxygen consumption. Despite similar spliceosomal interactomes, SNRPN more strongly associates with the PRMT5 methylosome complex and exhibits dynamic arginine methylation in its C-terminal region that is sensitive to translation inhibition and amino acid availability. The SNRPN-dependent transcriptome responds to translation inhibition by stabilizing long, intron-rich genes involved in amino acid and energy metabolism. Our findings reveal a nutrient-sensitive, methylation-dependent mechanism that differentiates the two paralogues. This suggests that SNRPN functions as a metabolic-specialized spliceosomal subunit thereby providing tissue-specific adaptation of RNA processing in mammals.

Neuronal aggregates of Tau are a hallmark of Alzheimer’s disease (AD), but more than half of the patients exhibit additional TDP-43 inclusions, while some have co-aggregates of the two proteins. The presence of such co-aggregates is associated with increased disease severity, although whether there is a causal relationship remains unclear. Here, we demonstrate that Tau and TDP-43 mutually promote each other’s condensation through direct interaction in vitro, forming irregularly-shaped or multiphasic co-condensates with lower TDP-43 mobility, but higher Tau mobility. While Tau promotes TDP-43 aggregation in vitro, TDP-43 suppresses formation of Tau fibrils and instead causes formation of oligomeric Tau and Tau/TDP-43 species. These co-assemblies hinder Tau seeding in a biosensor assay specific for proteopathic Tau seeds. Consistent with these data, insoluble material extracted from AD patient brains with Tau/TDP-43 co-aggregates exhibits reduced Tau seeding compared to AD patient brains with Tau aggregates only. In contrast, patient-derived extracts from AD patient brains with Tau/TDP-43 co-aggregates are highly potent in seeding new TDP-43 aggregates in a TDP-43 reporter cell line. Our results suggest that direct interaction between TDP-43 and Tau may suppress Tau pathology, while promoting TDP-43 pathology in Alzheimer’s disease patients.

Neuronal Tau aggregates are a hallmark of Alzheimer’s disease (AD), but more than half of the patients exhibit additional TDP-43 inclusions and some have co-aggregates of both proteins. The presence of Tau/TDP-43 co-pathology is associated with increased disease severity, although the causal relationship remains unclear. Here we demonstrate that Tau and TDP-43 mutually promote each other’s condensation through direct interaction in vitro , forming irregularly shaped or multiphasic co-condensates with lower TDP-43 mobility, but higher Tau dynamics. While Tau promotes TDP-43 aggregation in vitro , TDP-43 suppresses formation of Tau fibrils and instead causes formation of oligomeric Tau and Tau/TDP-43 species. These co-assemblies hinder Tau seeding in a biosensor assay specific for proteopathic Tau seeds. Consistent with this data, SarkoSpin extracts from AD brains with Tau/TDP-43 co-pathology exhibit reduced Tau seeding compared to Tau-only AD brains. In contrast, patient-derived extracts from AD brains with Tau/TDP-43 co-pathology are highly potent in seeding TDP-43 neoaggregates in a TDP-43 reporter cell line. Our results suggest that direct interaction of TDP-43 and Tau may suppress Tau pathology, while promoting TDP-43 pathology. Graphical

Gene regulation by RNA interference (RNAi) is a conserved process driven by double-stranded RNA (dsRNA). It responds to exogenous cues and drives endogenous gene regulation. In Caenorhabditis elegans , RNAi can be inherited from parents to offspring. While a number of factors have been implicated in this inheritance process, we do not understand how and when they function. Using a new inheritance assay, we establish a hierarchy amongst previously identified inheritance factors. We show that the nuclear Argonaute protein HRDE-1 is required for RNAi establishment in parents and offspring, but not for the inheritance process. In contrast, the cytoplasmic Argonaute protein WAGO-3 is the only factor essential for inheritance, via sperm and oocyte, while not affecting establishment in either parent or offspring. We propose a cycle in which nuclear and cytoplasmic Argonaute proteins interact to generate both a silencing response and a cytoplasmic factor that transmits the silencing between parent and offspring, WAGO-3. Finally, we implicate the RNA helicase ZNFX-1 as a factor that allows the inherited WAGO-3 protein to trigger silencing in the offspring.

Germ cell specification is driven, in many species, by germplasm: a collection of phase-separation-based structures in the embryo, formed by RNA and proteins that derive from the cytoplasm of the oocytes. How the formation of germplasm is regulated, especially in vertebrates, remains unclear. In this study, we show that two multi-Tudor proteins, Tdrd6a and Tdrd6c, together are necessary for the stability of germplasm in zebrafish, while the related Balbiani body in the oocyte is largely unaffected. Combined lack of maternal Tdrd6a and Tdrd6c still allows the initial germplasm formation but causes its dispersal during the first hours of development. This results in the absence of primordial germ cells during later development and in sterility of the resulting adult animals. Furthermore, our study suggests that the Prion-like domain of Tdrd6c is relevant for Tdrd6c self-interaction as well as for its interaction with Bucky ball, the organizer of germplasm in zebrafish and that these dynamics are modulated by Tdrd6c Tudor domains. The identification of Tdrd6a and Tdrd6c as required for germplasm stability is an important step in our understanding of how this phase-separated structure is controlled during development. Author Summary: We show that maternal Tdrd6a and Tdrd6c proteins interact with Bucky Ball and are essential for embryonic germplasm stability, germ cell specification and fertility in zebrafish.

Gene regulation by RNA interference (RNAi) is a conserved process driven by double-stranded RNA (dsRNA). It responds to exogenous cues and drives endogenous gene regulation. In Caenorhabditis elegans , RNAi can be inherited from parents to offspring. While a number of factors have been implicated in this inheritance process, we do not understand how and when they function. Using a new inheritance assay, we establish a hierarchy amongst previously identified inheritance factors. The nuclear argonaute protein HRDE-1 was required for RNAi establishment in parents and offspring, but not for the inheritance process. In contrast, the cytoplasmic argonaute protein WAGO-3 was the only factor essential for inheritance, via sperm and oocyte, while not affecting establishment in either parent or offspring. We propose a cycle between nuclear and cytoplasmic argonaute proteins, where nuclear activity drives most of the silencing and cytoplasmic activity ensures inheritance. Finally, we implicate the RNA helicase ZNFX-1 as a factor that controls the entry of exogenous versus endogenous small RNAs into this cycle, ensuring a proper balance between gene silencing and activity.