Frederik Tellkamp

The innate immune response is a first line of defense that allows a rapid reaction to pathogens as well as molecules released from damaged cells. It relies on pathogen-sensing receptors, many of which are nucleic acid-binding proteins, including RNA helicases. Plakophilin 1 (PKP1) is a desmosomal protein essential for strong intercellular adhesion and epithelial barrier integrity. Loss-of-function mutations in PKP1 cause ectodermal dysplasia–skin fragility syndrome (EDSFS), which is characterized by skin fragility, chronic inflammation, and recurrent infections. Here, we report that PKP1, whose role in barrier formation is to function as a critical regulator of innate immune responses. PKP1 sequesters a subset of dsRNAs that sense DExD/H-box RNA helicases—DDX1, DDX3X, DDX21, and DHX15. This limits their ability to activate the MDA5-MAVS signaling axis and prevents excessive activation of IRF3- and NFκB-driven gene expression, including the synthesis of proinflammatory cytokines, mainly IFN-β1, IL6, and TNFα. The inhibitory role of PKP1 is erased during innate immune responses by dsRNA cues, inducing proteasomal PKP1 degradation. This releases sequestered helicases, enabling dsRNA sensing and a rapid inflammatory response. Collectively, these findings identify PKP1 as a key gatekeeper that prevents exaggerated inflammation in quiescent keratinocytes, which is supported by excessive inflammation in EDSFS upon PKP1 loss. In healthy keratinocytes with proinflammatory traits, PKP1 is rapidly degraded, allowing the activation of the MDA5-MAVS signaling cascade by helicases to initiate inflammation. Hence, PKP1 combines two complementary functions essential for epidermal immune homeostasis: it provides a physical barrier preventing substance entry and suppresses innate immune responses in keratinocytes.

The innate immune response is a first line of defense that allows a rapid reaction to pathogens as well as molecules released from damaged cells. It relies on pathogen sensing receptors many of which are nucleic acid-binding proteins including RNA helicases. Plakophilin 1 (PKP1) is a desmosomal protein essential for strong intercellular adhesion and epithelial barrier integrity. Loss-of-function mutations in PKP1 cause ectodermal dysplasia-skin fragility syndrome (EDSFS) characterized by skin fragility, chronic inflammation, and recurrent infections. Here, we show that PKP1, extending its role in barrier formation, is a critical regulator of innate immune responses. Mechanistically, PKP1 sequesters a subset of dsRNA sensing DExD/H-box RNA helicases - DDX1, DDX3X, DDX21, and DHX15 - thereby limiting their availability for activation of the MDA5-MAVS signaling axis. The inactivation of the helicases by PKP1 prevents excessive activation of transcription factors IRF3 and NFκB, thus dampening the transcription of proinflammatory cytokines including IFN-β1, IL6, and TNFα. Activation of the innate immune response by a dsRNA challenge induces the proteasomal degradation of PKP1. This releases the helicases and enables dsRNA sensing and a rapid inflammatory response. Thus, upon an acute provocation, keratinocytes circumvent the repressive effect by reducing the PKP1 level, whereas steady state levels limit the inflammatory reaction to avoid exaggerated inflammation as observed upon the general loss of PKP1 in EDSFS. In summary, PKP1 combines two complementary functions essential for epidermal immune homeostasis: it provides a barrier to prevent the entry of harmful substances and it suppresses innate immune responses in keratinocytes.

Skeletal muscle fibers are classified as glycolytic or oxidative, with differing susceptibilities to muscle wasting. However, the intracellular signaling pathways regulating fiber-specific muscle trophism remain unclear because of a lack of experimental models measuring protein synthesis. We developed a mouse model overexpressing a mutated transfer RNA synthetase in muscle fibers, enabling specific protein labeling using an artificial methionine substitute, which can be revealed through click chemistry. This model revealed that denervation increases protein labeling in oxidative muscle fibers through mammalian target of rapamycin complex 1 (mTORC1) activation, while deleting the mTORC1 scaffold protein Raptor reduces labeling in glycolytic fibers. On the other hand, increased muscle activity acutely decreases protein synthesis, accompanied by reduced mTORC1 signaling, glycogen depletion, and adenosine 5′-monophosphate kinase activation. Our findings identify nerve activity as an inhibitory signal for mTORC1-dependent protein synthesis in skeletal muscle, enhancing the understanding of fiber-specific responses to exercise and pathological conditions.

Autophagy provides nutrients during starvation and eliminates detrimental cellular components. However, accumulating evidence indicates that autophagy is not merely a housekeeping process. Here, by combining mouse models of neuron‐specific ATG5 deficiency in either excitatory or inhibitory neurons with quantitative proteomics, high‐content microscopy, and live‐imaging approaches, we show that autophagy protein ATG5 functions in neurons to regulate cAMP‐dependent protein kinase A (PKA)‐mediated phosphorylation of a synapse‐confined proteome. This function of ATG5 is independent of bulk turnover of synaptic proteins and requires the targeting of PKA inhibitory R1 subunits to autophagosomes. Neuronal loss of ATG5 causes synaptic accumulation of PKA‐R1, which sequesters the PKA catalytic subunit and diminishes cAMP/PKA‐dependent phosphorylation of postsynaptic cytoskeletal proteins that mediate AMPAR trafficking. Furthermore, ATG5 deletion in glutamatergic neurons augments AMPAR‐dependent excitatory neurotransmission and causes the appearance of spontaneous recurrent seizures in mice. Our findings identify a novel role of autophagy in regulating PKA signaling at glutamatergic synapses and suggest the PKA as a target for restoration of synaptic function in neurodegenerative conditions with autophagy dysfunction.

The development of modern high-throughput instrumentation and improved core facility infrastructures leads to an accumulation of large amounts of scientific data. However, for a majority of scientists the comprehensive analysis and visualization of their data goes beyond their expertise. To reduce this hurdle, we developed a software suite called Instant Clue that helps scientists to visually analyze data and to gain insights into biological processes from their high-dimensional dataset. Instant Clue combines the power of visual and statistical analytics using a straight forward drag & drop approach making the software highly intuitive. Additionally, it offers a comprehensive portfolio of statistical tools for systematic analysis such as dimensional reduction, (un)-supervised learning, clustering, multi-block (omics) integration and curve fitting. Charts can be combined with high flexibility into a main figure template for direct usage in scientific publications. Even though Instant Clue was developed with the omics-sciences in mind, users can analyze any kind of data from low to high dimensional data sets. The open-source software is available for Windows and Mac OS (http://www.instantclue.uni-koeln.de) and is accompanied by a detailed video tutorial series.