Biology subjects

Szczepek, M.

Publications and source records attributed to Szczepek, M..

3 recordsLinked to original sources

bioRxiv2025

Membrane protein condensates polymerize actin and form filopodia

Neuronal morphogenesis is guided by filopodia, dynamically generated plasma membrane protrusions filled with parallel actin filaments. However, how filopodial actin filaments are locally produced, organized, and maintained remains unclear. The transmembrane protein PLPPR3 induces filopodia in neurons and other cells. We find that the intracellular domain (ICD) of PLPPR3 forms liquid condensates, which exhibit strong co-partitioning of actin monomers. These condensates promote actin polymerization within the condensates at the expense of actin monomers in the environment, consistent with thermodynamic coupling of actin partitioning and polymerization, which we recapitulate in a modified polymerization kinetics model. This mechanism requires favorable actin partitioning into the condensate relative to the environment. Using crosslinking mass spectrometry, we identify a WH2-like actin-affinity domain within the PLPPR3 ICD. Deleting this domain lowers actin partitioning in vitro and decreases filopodia formation in vivo. Our findings establish a previously unrecognized mechanism for actin network remodeling, in which condensates act as actin sinks, locally boosting monomer concentrations and facilitating polymerization of actin filaments. One sentence summaryOur study uncovers PLPPR3 condensates to locally enrich actin monomers and promote actin polymerization, driving actin network remodeling and neuronal filopodia formation.

cell biology↗

bioRxiv2021

Structures of active melanocortin-4 receptor--Gs-protein complexes with NDP-α-MSH and setmelanotide

The melanocortin-4 receptor (MC4R), a hypothalamic master regulator of energy homeostasis and appetite, is a G-protein coupled receptor and a prime target for the treatment of obesity. Here, we present cryo-electron microscopy structures of MC4R- Gs-protein complexes with two recently FDA-approved drugs, the peptide agonists NDP--MSH and setmelanotide, with 2.9 [A] and 2.6 [A] resolution. Together with signaling data, the complex structures reveal the agonist-induced origin of transmembrane helix (TM) 6 regulated receptor activation. In both structures, different ligand binding modes of NDP--MSH, a high-affinity variant of the endogenous agonist, and setmelanotide, an anti-obesity drug with biased signaling, underline the key role of TM3 for ligand-specific interactions and of calcium ion as a ligand-adaptable cofactor. The agonist-TM3 interplay subsequently impacts the receptor- Gs-protein interfaces, mainly at intracellular loop 2. These structures reveal mechanistic details of MC4R activation or inhibition and provide important insights into receptor selectivity that will facilitate the development of tailored anti-obesity drugs.

biochemistry↗

bioRxiv2019

Human endogenous retrovirus HERV-K(HML-2) RNA causes neurodegeneration through Toll-like receptors

Although human endogenous retroviruses (HERVs) represent a substantial proportion of the human genome and some HERVs have been suggested to be involved in neurological disorders, little is known about their biological function and pathophysiological relevance. HERV-K(HML-2) comprises evolutionarily young proviruses transcribed in the brain. We report that RNA derived from an HERV-K(HML-2) env gene region binds to the human RNA-sensing Toll-like receptor (TLR) 8, activates human TLR8, as well as murine Tlr7, and causes neurodegeneration through TLR8 and Tlr7 in neurons and microglia. HERV-K(HML-2) RNA introduced extracellularly into the cerebrospinal fluid (CSF) of either C57BL/6 wild-type mice or APPPS1 mice, a mouse model for Alzheimers disease (AD), resulted in neurodegeneration. Tlr7-deficient mice were protected against neurodegenerative effects, but were re-sensitized towards HERV-K(HML-2) RNA when neurons ectopically expressed murine Tlr7 or human TLR8. Accordingly, transcriptome datasets of human brain samples from AD patients revealed a specific correlation of upregulated HERV-K(HML-2) and TLR8 RNA expression. HERV-K(HML-2) RNA was detectable more frequently in CSF from AD individuals compared to controls. Our data establish HERV-K(HML-2) RNA as an endogenous ligand for human TLR8 and murine Tlr7 and imply a functional contribution of specific human endogenous retroviral transcripts to neurodegenerative processes such as AD.

neuroscience↗