I. The Molecular Basis of the Unfolded Protein Response (UPR):

Intrinsic Capacities of Molecular Sensors of the Unfolded Protein Response to Sense Alternate Forms of Endoplasmic Reticulum Stress.
DuRose, J., Tam, A.B., and M. Niwa.
Molecular Biology of the Cell 17, 3095-3107, (2006).

The unfolded protein response (UPR) regulates the protein-folding capacity of the endoplasmic reticulum (ER) according to cellular demand. In mammalian cells, three ER transmembrane components, IRE1, PERK, and ATF6, initiate distinct UPR signaling branches. We show that these UPR components display distinct sensitivities toward different forms of ER stress. ER stress induced by ER Ca2+ release in particular revealed fundamental differences in the properties of UPR signaling branches. Compared with the rapid response of both IRE1 and PERK to ER stress induced by thapsigargin, an ER Ca2+ ATPase inhibitor, the response of ATF6 was markedly delayed. These studies are the first side-by-side comparisons of UPR signaling branch activation and reveal intrinsic features of UPR stress sensor activation in response to alternate forms of ER stress. As such, they provide initial groundwork toward understanding how ER stress sensors can confer different responses and how optimal UPR responses are achieved in physiological settings.

II. The Role of the UPR in Cellular Change and Differentiation

B- and T-cell Development Both Involve Activity of the Unfolded Protein Response Pathway.
Brunsing, R., Omori, S.A. , Weber, F., Bicknell, A., Friend, L. , Rickert, R., M. Niwa.
Journal of Biological Chemistry, 283,17954-17961, (2008).

The unfolded protein response (UPR) signaling pathway regulates the functional capacity of the endoplasmic reticulum for protein folding. Beyond a role for UPR signaling during terminal differentiation of mature B cells to antibody-secreting plasma cells, the status or importance of UPR signaling during hematopoiesis has not been explored, due in part to difficulties in isolating sufficient quantities of cells at developmentally intermediate stages required for biochemical analysis. Following reconstitution of irradiated mice with hematopoietic cells carrying a fluorescent UPR reporter construct, we found that IRE1 nuclease activity for XBP1 splicing is active at early stages of T- and B-lymphocyte differentiation: in bone marrow pro-B cells and in CD4+CD8+ double positive thymic T cells. IRE1 was not active in B cells at later stages. In T cells, IRE activity was not detected in the more mature CD4+ T-cell population but was active in the CD8+ cytotoxic T-cell population. Multiple signals are likely to be involved in activating IRE1 during lymphocyte differentiation, including rearrangement of antigen receptor genes. Our results show that reporter-transduced hematopoietic stem cells provide a quick and easy means to identify UPR signaling component activation in physiological settings.

III. ER Inheritance and the Cell Cycle

A Novel Role in Cytokinesis Reveals a Housekeeping Function for the Unfolded Protein Response.
Bicknell, A., Babour, A., Federovitch, C.M., and M. Niwa.
The Journal of Cell Biology, 177, 1017-1027. (2007).

The unfolded protein response (UPR) pathway helps cells cope with endoplasmic reticulum (ER) stress by activating genes that increase the ER's functional capabilities. We have identified a novel role for the UPR pathway in facilitating budding yeast cytokinesis. Although other cell cycle events are unaffected by conditions that disrupt ER function, cytokinesis is sensitive to these conditions. Moreover, efficient cytokinesis requires the UPR pathway even during unstressed growth conditions. UPR-deficient cells are defective in cytokinesis, and cytokinesis mutants activate the UPR. The UPR likely achieves its role in cytokinesis by sensing changes in ER load and making according changes in ER capacity. We propose that cytokinesis is one of many cellular events that require a subtle increase in ER function and that the UPR pathway has a previously uncharacterized housekeeping role in maintaining ER plasticity during normal cell growth.