Membrane morphology and homeostasis: An electron microscopy approach

Abstract

Biological membranes allow life as we know it to exist. Primarily assembled from lipids and proteins, their morphology and function can greatly vary between species or even within the same cell. They create physical boundaries, separating different cells from each other and forming functional compartments within the cells. The composition and properties of biological membranes must be continuously attuned to their vast variety functions, and this is achieved within the cell by essential homeostatic processes. This thesis studies membranes from two different perspectives. The first focuses on membranes as a means of transport of biological material between compartments, namely between the nucleus and the cytoplasm. The standard view is that nuclear pore complexes act as the main gateways for passive or active transportation of molecules across the nuclear envelope. We have now shown that nuclear membranes are able to form buds enclosing biological material, thus suggesting a hitherto unappreciated second route for transport in and out of the nucleus. While these nuclear envelope buds (NEBs) are present in unperturbed cells, their frequency increases significantly in response to a variety of stress conditions. We also found that NEBs are evolutionarily conserved and were found in all organisms studied: a protozoan protist, two yeast species, a nematode, and a human cell line. The second part of this thesis focuses on an important molecular pathway that contributes to membrane homeostasis. Challenging cultured cells with an excess of saturated fatty acids, such as palmitic acid (PA), causes membrane rigidification and endoplasmic reticulum (ER) stress, accompanied by defects in several biological processes, increased accumulation of reactive oxygen species (ROS), and apoptosis/cell death. The protein AdipoR2 acts to counter membrane rigidification and therefore increases the tolerance of cells towards excess saturated fatty acids (SFAs). We used electron microscopy to describe, at unprecedented resolution, specific membrane defects caused by an excess amount of PA in control cells and cells lacking a functional AdipoR2. We found that the ER, mitochondrial membranes, and nuclear envelope are all impacted by PA treatment, with the effects exacerbated in cells lacking AdipoR2. In particular, the ER of PA-challenged cells often presented a unique morphology resembling either straight lines or in some cases spiral-like structures. A conclusion from this work is that AdipoR2 is required to prevent severe deformation of cellular membranes, especially when cells are challenged with exogenous SFAs.