Membrane Biology


Elina Ikonen, M.D. Dr.Med.Sci., Academy Professor (Director), Head
Tomas Blom, Ph.D., Affiliated team leader
Maarit Hölttä-Vuori, Ph.D., University lecturer
Kristiina Kanerva, Ph.D.
Shiqian Li, Ph.D.
Xin Zhou, Ph.D.
Heljä Lång, M.D.
Juho Pirhonen, M.D.
Kecheng Zhou, M.Sc. (team of T. Blom)
Päivi Kleemola, Research assistant
Lauri Vanharanta, Medical student
Anna Uro, Laboratory technician
Katharina Ven, Research assistant

Our group aims to understand how cholesterol functions in the cellular context, both under physiological conditions and in human diseases. To this end, we also develop novel imaging techniques to analyze cholesterol and other lipids in cells. Cholesterol is an essential component of cell membranes, where it dictates important biophysical properties of the bilayer and participates in lipid-protein interactions. These aspects are important for understanding the mechanisms that underlie the adverse effects of cholesterol. Besides cardiovascular diseases, disturbances in cholesterol metabolism are implicated e.g., in several neurodegenerative diseases.

Figure. The principle of third-harmonic generation (THG) microscopy for labelfree imaging of cellular lipid storage. Left: Both linearly polarized (LP) and circularly polarized (CP) light can be used to generate signals from lipid droplets. Right: Representative images of single cells with either triacylglycerol or cholesteryl ester deposition. Cholesteryl ester storage generates a more pronounced THG signal, with individual lipid droplets visualized. Courtesy of Dr. Godofredo Bautista, Tampere University of Technology (for details see Bautista et al., Biophys
J 107:2230). 

Cholesterol metabolism and Alzheimer’s disease (AD) are connected, but the molecular mechanisms involved are not well understood. Amyloid precursor protein (APP) undergoes amyloidogenic processing (beta-cleavage), that predisposes a patient to AD, in a cholesterol-dependent manner. Moreover, increasing evidence suggests that APP itself regulates cholesterol metabolism. We have recently discovered a new link between APP and cholesterol that is relevant both physiologically and in AD: APP regulates the key transcription factor of cholesterol metabolism, sterol-regulatory element binding protein 2 (SREBP2) via its secretory ectodomain fragments. This regulation can be either positive or negative, depending on APP non-amyloidogenic vs. amyloidogenic (alpha vs. beta) processing. The physiologically dominant, APPs-alpha fragment stimulates SREBP2, leading to enhanced cholesterol synthesis and LDL receptor levels. In contrast, APP secretory beta fragment suppresses SREBP2 signaling. In line with this finding, in familial AD patients with increased beta-cleavage, serum cholesterol synthesis markers were decreased and fibroblast LDL-receptor levels were reduced (Wang et al., 2014).

Cells can store excess lipids in cellular lipid droplets. The major storage lipids inside lipid droplets are cholesteryl esters (CE) and triacylglycerols (TAG). Lipid droplets can be visualized in cells without exogenous labels by non-linear imaging techniques. In collaboration with physicists, we have recently discovered that one such technique, polarized third-harmonic generation (THG) microscopy, can differentiate between native TAGand CE-enriched lipid droplets in mammalian cells. This distinction is based on the differential ordering of the two lipid classes (Bautista, Pfisterer et al., 2014). The degree of lipid ordering is thought to play an important role in the mobility and enzymatic processing of lipids in lipid droplets This technique may, in the future, be useful for differentiating lipid storage types in a label-free fashion.

Contact info

Elina Ikonen, MD, PhD, Professor

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