Organelle Membrane Extensions in Mammalian Cells

Abstract

Organelles within eukaryotic cells are not isolated static compartments, instead being morphologically diverse and highly dynamic in order to respond to cellular needs and carry out their diverse and cooperative functions. One phenomenon exemplifying this plasticity, and increasingly gaining attention, is the extension and retraction of thin tubules from organelle membranes. While these protrusions have been observed in morphological studies for decades, their formation, properties and functions are only beginning to be understood. In this review, we provide an overview of what is known and still to be discovered about organelle membrane protrusions in mammalian cells, focusing on the best-characterised examples of these membrane extensions arising from peroxisomes (ubiquitous organelles involved in lipid metabolism and reactive oxygen species homeostasis) and mitochondria. We summarise the current knowledge on the diversity of peroxisomal/mitochondrial membrane extensions, as well as the molecular mechanisms by which they extend and retract, necessitating dynamic membrane remodelling, pulling forces and lipid flow. We also propose broad cellular functions for these membrane extensions in inter-organelle communication, organelle biogenesis, metabolism and protection, and finally present a mathematical model that suggests that extending protrusions is the most efficient way for an organelle to explore its surroundings.

Keywords: membrane dynamics; membrane protrusion; mitochondria; nanotubule; organelle interaction; organelles; peroxisomes.

Figure 1

Examples of peroxisomal and mitochondrial membrane extensions (A) Electron micrograph of a mitochondrion (blue) in a rat hippocampal neuron displaying a tubulovesicular protrusion (red arrowheads). Bar = 200 nm. Image taken from [17]. (B) Electron micrograph of mitochondria (blue) in human skeletal muscle connected by membrane extensions (red arrows), generating a network. Image taken from [16]. (C) Electron micrograph showing peroxisomes (darkly stained from the reaction of catalase with 3,3′ diaminobenzidine tetrahydrochloride [DAB]) in regenerating rat liver). The upper panel shows a protrusion emanating from a peroxisome body; the lower panel shows constriction (black arrows) of a tubule prior to fission. Bars = 500 nm. Image taken from [2] with permission from Rockefeller University Press. ©1987 Yamamoto & Fahimi. Originally published in J. Cell. Biol. https://doi.org/10.1083/jcb.105.2.713. (D) Electron micrograph of a peroxisome (green) in an MFF-deficient (dMFF) skin fibroblast, where a block in peroxisome fission leads to highly elongated membrane extensions arising from the spherical peroxisome body. Note the peroxisome body is closely associated with the ER (yellow), presumably for membrane lipid transfer to support elongation. Bar = 200 nm. (E) Immunofluorescence showing hyper-elongated peroxisomal membrane extensions in a dMFF cell. Peroxisomes were stained with antibodies against PEX14 (peroxisomal membrane marker, red) and catalase (peroxisomal matrix marker, green). The arrowhead indicates a potential tubule branch point. Bar = 20 μm. Image adapted from [18]. (F) Immunofluorescence of a peroxisomal protrusion in a PEX5-deficient fibroblast, induced by overexpression of a peroxisomal-targeted version of the motor protein MIRO1 (green). The protrusion runs along microtubule tracks, stained with anti-tubulin (red, indicated by arrowheads). Bar = 5 µm. Image taken from [12]. (G) Stills from live-cell imaging of a COS-7 cell expressing the peroxisomal membrane-shaping protein PEX11β-EGFP and stained with Mitotracker Red. A protrusion from a peroxisome (PO, green) can be seen to come into close contact with a mitochondrion (MITO, red). Bar = 5 µm. Image taken from [19]. (H) Stills from live-cell imaging of a cotyledon cell from an Arabidopsis mutant exhibiting a high frequency of peroxules, expressing YFP-PTS1 (peroxisomal matrix marker) and mito-GFP (mitochondrial marker). A peroxisome (px, yellow/orange), associated with a chloroplast (c, blue), extends a protrusion that contacts a mitochondrion (m, green). Image taken from [20].

Figure 2

Dynamics of peroxisomal membrane protrusions. Time-lapse of MIRO1-expressing PEX5-deficient fibroblasts labelled with a peroxisomal membrane marker (green). (A) Membrane protrusions form from enlarged (non-functional) peroxisomal membranes (ghosts), elongate and retract. Bars = 20 μm (overview), 5 μm (magnification). (B) Kymograph of peroxisome elongation observed in (A). Note the high velocity during tubule retraction. Bars = 20 s (vertical), 5 μm (horizontal). (C) Schematic of protrusion forming from an ER-tethered peroxisome (PO) due to MIRO1-mediated motor forces acting along the microtubule cytoskeleton. Images taken from [12].

Figure 3

Overview of the possible functions of peroxisomal and mitochondrial membrane extensions. Schematic showing the potential roles of peroxisomal/mitochondrial membranes extensions in mediating organelle biogenesis, protection/metabolism and inter-organelle communication.

Figure 4

Mathematical model of organelle searching. (A) Representation of the peroxisome (PO) in the model, with a spherical body and an optional piece-wise extension (consisting of vertices connected by straight segments) that represent the protrusion. (B) Mean search time as a function of the protrusion length, showing that a protrusion can significantly decrease the organelle search time. Note that for simplicity, the cell was assumed to be a sphere of radius 1 μm. Error bars show the standard error of the mean. See Appendix A for a detailed explanation of the model generation.