Molecular Cell Biology
The Molecular Cell Biology division, headed by Prof. Dr. Joost Holthuis, investigates how cells assemble membranes from a structurally diverse repertoire of lipids, thereby creating the most advanced biomolecular systems in nature.
Organellar lipid codes
The identity and function of cellular organelles critically relies on information encoded in their lipid bilayers. In the Holthuis lab, we study the functional implications of, and compensatory cellular responses to disease-induced imbalances in organellar lipid codes, with a main focus on aberrant distributions of sphingolipids. In collaboration with clinical researchers, we identified mutations in sphingomyelin synthase SMS2 as the underlying cause of a rare form of skeletal dysplasia. Pathogenic SMS2 variants retain enzymatic activity but fail to exit the endoplasmatic reticulum (ER), mislocalize to the cis-Golgi, or display an aberrant ER-Golgi cycling. Consequently, cells harboring pathogenic variants of SMS2 produce sphingomyelin in the wrong place. This, in turn, leads to a disrupted sphingomyelin/cholesterol gradient in membranes of the secretory pathway and perturbations in various cellular signaling lipid pools. Using disease-relevant cell models and lipid biosensors, we aim to decipher organellar lipid codes and unravel the mechanism by which pathogenic SMS2 variants interfere with normal bone formation.
Membrane asymmetry
A fundamental feature of cellular membranes is an asymmetric lipid distribution between the bilayer leaflets, with sphingolipids enriched in the exoplasmic leaflet and aminophospholipids concentrated in the cytosolic leaflet. This out-of-equilibrium arrangement is maintained by ATP-driven transporters that move lipids between leaflets against their concentration gradients. Although membrane asymmetry has been linked to many physiological processes, it is often not membrane asymmetry per se but rather its controlled subversion that mediates vital cellular responses. Damaged lysosomes, for instance, display a transient release of sphingomyelin asymmetry as part of a mechanism that mediates their repair via an inverse membrane budding and fission process. Moreover, the release of lipid asymmetry by Ca2+-activated scramblases plays a crucial role in the repair of cells damaged by bacterial toxins. To investigate the direct consequences of a scrambled lipid distribution on the biophysics, plasticity and function of cellular membranes, we use cells equipped with drug-inducible scramblases and chemical compounds designed to trigger lipid scrambling.
Tumor suppressor lipids
Ceramides are central intermediates of sphingolipid metabolism that can activate a variety of tumor suppressive cellular programs, including apoptosis. We previously showed that ceramides can act directly on mitochondria to trigger apoptosis and identified voltage-dependent anion channels (VDACs) as putative effectors of ceramide-mediated cell death. VDAC residues involved in ceramide binding are also required for mobilizing hexokinase type-I to mitochondria, a potential checkpoint in apoptosis. Collectively, our data support a model in which ceramides function as modulators of VDAC-based platforms to control mitochondrial recruitment of pro- and anti-apoptotic machinery. To challenge this model, we exploit switchable ceramide transfer proteins and mitochondrial-specific release of photocaged ceramides in combination with live cell imaging and functional studies. Understanding the molecular principles by which ceramides commit cells to death may facilitate the development of novel strategies to enhance their anti-tumor potential for therapeutic treatment.
