UPR Responsive Genes Manf and Xbp1 in Stroke

Abstract

Stroke is a devastating medical condition with no treatment to hasten recovery. Its abrupt nature results in cataclysmic changes in the affected tissues. Resident cells fail to cope with the cellular stress resulting in massive cell death, which cannot be endogenously repaired. A potential strategy to improve stroke outcomes is to boost endogenous pro-survival pathways. The unfolded protein response (UPR), an evolutionarily conserved stress response, provides a promising opportunity to ameliorate the survival of stressed cells. Recent studies from us and others have pointed toward mesencephalic astrocyte-derived neurotrophic factor (MANF) being a UPR responsive gene with an active role in maintaining proteostasis. Its pro-survival effects have been demonstrated in several disease models such as diabetes, neurodegeneration, and stroke. MANF has an ER-signal peptide and an ER-retention signal; it is secreted by ER calcium depletion and exits cells upon cell death. Although its functions remain elusive, conducted experiments suggest that the endogenous MANF in the ER lumen and exogenously administered MANF protein have different mechanisms of action. Here, we will revisit recent and older bodies of literature aiming to delineate the expression profile of MANF. We will focus on its neuroprotective roles in regulating neurogenesis and inflammation upon post-stroke administration. At the same time, we will investigate commonalities and differences with another UPR responsive gene, X-box binding protein 1 (XBP1), which has recently been associated with MANF's function. This will be the first systematic comparison of these two UPR responsive genes aiming at revealing previously uncovered associations between them. Overall, understanding the mode of action of these UPR responsive genes could provide novel approaches to promote cell survival.

Keywords: ARMET; CDNF; ER stress; IRE1; XBP1; mesencephalic astrocyte-derived neurotrophic factor; unfolded protein response.

FIGURE 1

Structure-function relationship of mesencephalic astrocyte-derived neurotrophic factor (MANF). The figure is based on the NMR structures of MANF and C-MANF (Hellman et al., 2011) (PDB codes: 2KVD and 2KVE, respectively. Image created with Chimera 1.16). The C-terminal RTDL-sequence of MANF corresponds to the canonical ER-retrieval signal “KDEL”.

FIGURE 2

Phylogenetic analysis of MANF. (A) Vertebrate MANF orthologs (cream boxes) were obtained by NCBI, as calculated by NCBI’s Eukaryotic Genome Annotation pipeline for the NCBI Gene dataset. Non-vertebrate MANF orthologs were obtained with the NCBI “similar gene” pipeline. For two of the metazoans presented (pink boxes), human protein sequence was blasted in the EnsemblMetazoan database (metazoa.ensembl.org). Protein sequences were aligned using ClustaOWS in Jalview v. 2.10.5 (Clamp et al., 2004). Conserved amino acids in all organisms are highlighted in purple or green for cysteines. Numbers denote the amino acid number. (B) Phylogenetic tree of MANF is based on protein sequence similarity. It was calculated from distance matrices determined from % identity using the neighbor joining algorithm. Each number is a score, and each branch is an additive allowing for comparison of distances in the tree branches. Used sequence IDs: Homo sapiens (T1FAB3), Bos Taurus (Q9N3B0), Rattus norvegicus (A0A0C2MNP5), Mus musculus (A0A0L8FVI8), Danio rerio (B3RIB4), Drosophila grimshawi (B4JT39), Drosophila melanogaster (Q9XZ63), Helobdella robusta (Q3TMX5), Octopus bimaculoides (P55145), Caenorhabditis elegans (P80513), Acropora millepora (F1QDQ5), Hydra vulgaris (B2RZ09), Amphimedon queenslandica (T2MFG7), Trichoplax adhaerens (A0A1 × 7TYG8), and Thelohanellus kitauei (LOC114953444).

FIGURE 3

Secretion and proposed function of MANF. Under normal conditions, MANF is maintained in the endoplasmic reticulum (ER) by KDEL receptor-mediated retrieval and secreted in response to Ca²⁺ depletion. MANF maintains ER homeostasis and proteostasis, as well as enhances cell survival. A CxxC-motif is essential for neuroprotection for both endogenous and exogenous MANF, while the C-terminal RTDL sequence of MANF (corresponding to the canonical KDEL ER-retrieval signal) is not needed for the neuroprotective effect of exogenous MANF (Matlik et al., 2015). It is possible that KDEL and neuroplastin receptors may participate in internalization of extracellular MANF, and interaction with sulfatides may be important for internalization. It should be emphasized the majority of KDEL receptors are in the Golgi, and only a fraction of them can be at the plasma membrane at any given time point. Purple circles, endogenous MANF; green circles, recombinant exogenous MANF.

FIGURE 4

Upregulation of MANF and XBP1 after stroke. “Overexpression confidence” = number of publications showing the upregulation of MANF or XBP1 after stroke (references: Tables 1, 2).