A minimum level of ER functionality is required for cell viability. Thus, the functional capacity and timing of cell division and cER inheritance may be coordinated by a checkpoint that ensures a minimum ER functionality before cell division. We report here the identification of an ER surveillance (ERSU) pathway that may function as the gatekeeper for this checkpoint and monitors the functional capacity of the ER during cell division. When ER stress is induced, ERSU causes cytokinesis delay and cER to be retained in the mother cell until it replenishes ER function (). The delay in cytokinesis correlates with, and is likely caused by, altered dynamics of the septin complex. ERSU is independent of UPR signaling, and instead relies upon the MAP kinase Slt2, ensuring that only functional ER is transmitted to daughter cells. Thus, ER stress activates both the ERSU pathway, which controls cell cycle progression, and UPR pathways which re-establishes ER functions. Once the ER capacity is re-established, we expect that the ERSU pathway will be turned off. Thus, although the ER may not be delivered to the daughter cell during ER stress, subsequent daughter cells will receive functional ER. In ERSU deficient cells (for example, slt2Δ cells), this regulation is lost, and mother cells distribute cER into the daughter cells irrespective to functional state of the ER. As a consequence, the level of ER functionality in the mother cell may drop below the minimum requirement causing both cells to undergo cell death. Thus, during ER stress, in slt2Δ cells both the mother and daughter cell are inviable, whereas in WT cells, when the ER is retained in the mother cell, only the daughter cell is inviable. In support of this model, inhibiting ER inheritance in slt2Δ cells through treatment with LatB allowed restoration of mother cell viability (). While we described in this report conditions in which ER stress is highly induced, we believe that the ERSU pathway may also function during the normal cell cycle, serving as a cell cycle “checkpoint”, that assures generation of progeny cells with functional ER.
One intriguing observation is that the ERSU pathway causes stressed cER to be preferentially retained in the mother cell. This type of mother cell retention has also been seen for factors that contribute to cell aging, such as extra-chromosomal rDNA circles (ERCs), maternal nuclear pores and carbonylated proteins (Tessarz et al., 2009
) (Shcheprova et al., 2008
). Such toxic factors accumulate with age and ultimately lead to cell death; they are observed to be retained in the mother cell presumably to increase the lifespan of the newly born daughters (Erjavec et al., 2007; Murray and Szostak, 1983; Sinclair and Guarente, 1997). It has been shown that the retention in the mother cells of nuclear pores and ERCs requires a septin-dependent diffusion barrier within the nuclear envelope (Shcheprova et al., 2008
). Taken together, these data point to a general evolutionary rationale that uses multiple critical mechanisms to assure the asymmetric distribution of elements to the mother cell. At first glance, ERSU seems to act differently from these ageing pathways, because it allows preferential protection of mother cells rather than daughter cells. However, unlike the accumulation of ageing factors, ER stress is reversible. Thus, the ERSU pathway ultimately promotes conservation of offspring by protecting the mother cell and allowing it to generate subsequent generations of daughter cells.
Another intriguing feature of the ERSU pathway is the different behavior of cortical and nuclear ER. We observed that perinuclear ER was inherited normally with the nucleus during ER stress, while cER delivery to the daughter cell was inhibited, suggesting a potential distinction between cortical and perinuclear ER. A recent report using an ER stress reporter indicates that ER stress is not transmitted to daughter cells (Merksamer et al., 2008
). As we have found that perinuclear ER along with the nucleus is transmitted into daughter cell even during ER stress, such observation suggests that perinuclear ER is free of ER stress. It prompts one to ask whether ER stress could somehow be partitioned to the cER, which is retained in the mother cell. Future studies will test such ideas and explore the mechanism and functional implications of this distinction between the two subdomains of the ER.
ERSU controls cell cycle in response to ER stress via Wsc1 and Slt2, distinct from the UPR, CWI and ASR pathways. Currently, we do not know how ER stress is signaled through Wsc1. The mechanism is unlikely to involve gross changes in Wsc1 localization, as we have found that the steady state localization of Wsc1 during ER stress does not change (data not shown). One possibility is that ER stress activates Wsc1 at the cell surface through an unknown mechanism. Alternatively, as Wsc1 transits the ER during its folding process, it might directly detect ER stress and initiate the ERSU pathway. For example, during ER stress, Wsc1 protein might be modified within the ER lumen before exiting from the ER to initiate ERSU. In addition, we do not know how Slt2 kinase activation leads to the cER inheritance delay and septin alteration. Slt2 kinase may directly phosphorylate cER inheritance components or septin subunits. Future studies will be required to uncover the molecular mechanism of Wsc1 and Slt2 activation during ER stress.
In summary, our study has described the discovery of an ER surveillance (ERSU) pathway in yeast. We have mapped a number of the components of the ERSU pathway but anticipate that future studies will provide additional components involved in the pathway. For example, a recent report describes a novel molecular mechanism involving the polarisome, a multi-protein complex that regulates actin cytoskeleton restricting apical growth of S. cerevisiae
(Sheu et al., 2002), that prevents protein aggregates from staying in the daughter cell (Liu et al., 2010
). It may also be possible that the polarisome functions to establish the cER inheritance delay in response to ER stress. In addition, Ptc1, a phosphatase that is thought to negatively regulate Pkc1 and thus ultimately Slt2 kinase (Nanduri and Tartakoff, 2001
), may also function in the ERSU pathway. A recent genetic screen identified Ptc1 is a component with an as yet unknown role in ER inheritance during normal cell growth (Du et al., 2006b). Ptc1 also regulates inheritance of the mitochondria (Roeder et al., 1998
) and the vacuole (Jin et al., 2009
). Therefore, Ptc1 may be a part of a master regulator choreographing different mechanisms that modulate the transmission of organelles and the cytoplasmic components to the daughter cell.
A mechanism of ER surveillance similar to ERSU may exist in mammalian cells. Since the fundamental mechanisms of cytokinesis differ between yeast and mammalian cells, the details of ERSU may differ between the two cell types. However, the failure to properly regulate ER functional capacity in vertebrate cells is increasingly recognized as contributing to the pathophysiology of a number of human diseases, including diabetes and certain cancers. Thus, further understanding of the cellular mechanisms of the ERSU Response that we have reported here, and investigation of the mammalian counterpart may allow for the development of previously unrecognized strategies for therapeutic intervention.