Organelle Homeostasis: Biogenesis and Degradation of Endoplasmic Reticulum
Eukaryotic cells differ strikingly in the abundance and architecture of their organelles. Moreover, cells rapidly adjust organelle composition, size and shape to changing conditions, thereby maintaining homeostasis during growth, differentiation, stress and disease. These adaptive responses are mediated by the regulated synthesis of new organelle components and the selective degradation of damaged or redundant ones. The mechanisms underlying organelle homeostasis are of fundamental importance to cell function, and uncovering their molecular nature is a fascinating challenge.
We want to elucidate how cells maintain homeostasis of their endoplasmic reticulum (ER). When the ER cannot process the load of proteins it needs to fold and modify, misfolded proteins accumulate and cause ER stress. To overcome this problem, cells activate the unfolded protein response (UPR), which causes massive expansion of the ER to increase its capacity and induces ER-associated degradation (ERAD) to degrade misfolded proteins. Furthermore, autophagy (cellular self-eating) is turned on. A special type of autophagy, called ER-phagy, selectively targets the ER, likely to destroy terminally misfolded and aggregated proteins. In addition, ER-phagy may control organelle size by mediating ER contraction. Thus, the UPR, ERAD and ER-phagy appear to work together to expand or shrink the ER as needed and relieve ER stress by refolding or degrading damaged proteins.
One particular focus of our lab is ER-phagy. ER-phagy can occur by a macroautophagic mechanism involving autophagosomes (macro-ER-phagy) and a microautophagic mechanism involving direct uptake of substrates at the lysosomal membrane (micro-ER-phagy). Remarkably, micro-ER-phagy does not require the known autophagy genes needed for other types of organelle-selective autophagy but relies on molecular machinery that remains to be discovered. We aim to identify this machinery through yeast genetics, unravel the biochemical mechanisms by which ER-phagy achieves destruction of select portions of the ER and understand the physiological roles of ER-phagy in both yeast and mammalian cells.
ER-phagy. ER stress triggers formation of ER whorls underneath the plasma membrane (A). These whorl then make contact with the lysosomal membrane (B). Finally, they are taken up into the lumen of the yeast lysosome (C). C, cytoplasm; Ly, lysosome; PM, plasma membrane.
Schuck S, Gallagher CM and Walter P. ER-phagy mediates selective degradation of endoplasmic reticulum independently of the core autophagy machinery. Journal of Cell Science, 2014. (Abstract)
Rubio C, Pincus D, Korennykh A, Schuck S, El-Samad H and Walter P. Homeostatic adaptation to endoplasmic reticulum stress depends on Ire1 kinase activity. Journal of Cell Biology, 2011. (Abstract)
Schuck S, Prinz WA, Thorn KS, Voss C and Walter P. Membrane expansion alleviates endoplasmic reticulum stress independently of the unfolded protein response. Journal of Cell Biology, 2009. (Abstract)
Lingwood D, Schuck S, Ferguson C, Gerl MJ and Simons K. Morphological homeostasis by autophagy. Autophagy, 2009. (Abstract)
ER expansion during stress.
(A) Untreated yeast cells expressing Sec63-GFP to highlight the nuclear envelope (n) and the cortical ER (c).
(B) Yeast cells exposed to ER stress have a vastly expanded cortical ER, which extends into the cell interior (arrows). Scale bar, 2 µm.
This movie illustrates our current high-resolution, antropomorphic model of ER-phagy. At the start, a yeast cell with patchy peripheral ER is shown. Upon ER stress, the peripheral ER expands, sends distress signals and forms ER whorls. The yeast lysosome appears, equipped with a boat and a fishing rod. Skillfully, it catches a whorl and gobbles it up. The ER-phagy job done and ER stress resolved, the sated lysosome falls asleep. (Artwork by Karen Moreira)