SEC23B is required for the maintenance of murine professional secretory tissues

Abstract

In eukaryotic cells, newly synthesized secretory proteins require COPII (coat protein complex II) to exit the endoplasmic reticulum (ER). COPII contains five core components: SAR1, SEC23, SEC24, SEC13, and SEC31. SEC23 is a GTPase-activating protein that activates the SAR1 GTPase and also plays a role in cargo recognition. Missense mutations in the human COPII paralogues SEC23A and SEC23B result in craniolenticulosutural dysplasia and congenital dyserythropoietic anemia type II, respectively. We now report that mice completely deficient for SEC23B are born with no apparent anemia phenotype, but die shortly after birth, with degeneration of professional secretory tissues. In SEC23B-deficient embryonic pancreas, defects occur in exocrine and endocrine tissues shortly after differentiation. Pancreatic acini are completely devoid of zymogen granules, and the ER is severely distended. Similar ultrastructural alterations are also observed in salivary glands, but not in liver. Accumulation of proteins in the ER lumen activates the proapoptotic pathway of the unfolded protein response, suggesting a central role for apoptosis in the degeneration of these tissues in SEC23B-deficient embryos. Although maintenance of the secretory pathway should be required by all cells, our findings reveal a surprising tissue-specific dependence on SEC23B for the ER exit of highly abundant cargo, with high levels of SEC23B expression observed in professional secretory tissues. The disparate phenotypes in mouse and human could result from residual SEC23B function associated with the hypomorphic mutations observed in humans, or alternatively, might be explained by a species-specific shift in function between the closely related SEC23 paralogues. mammalian embryo abnormalities | vesicular transport protein | genetics | secretory granules | pancreatitis I n eukaryotic cells, secreted proteins and proteins that are targeted to the plasma membrane and internal organelles are synthesized in the endoplasmic reticulum (ER) and sorted through the secretory pathway. This process has been extensively studied, particularly in budding yeast (1). Proteins destined to traffic from the ER to the Golgi are packaged into COPII (coat protein complex II)-coated vesicles (2-4). COPII is composed of at least five proteins, a small GTPase SAR1 and two cytosolic protein complexes, SEC23-SEC24 and SEC13-SEC31 (5). The GTP-bound form of SAR1 binds to the ER membrane and recruits the SEC23-SEC24 heterodimer to form the "prebudding complex," which in turn recruits the outer coat composed of SEC13-SEC31 heterotetramers to complete the COPII coat structure (6).The COPII complex captures cargo into vesicles and mediates vesicle budding from the ER. Cargo recognition appears to be mediated primarily by the SEC24 subunit, which recognizes divergent export signals located in the cytosolic domain of cargo proteins (7,8). SEC23 and SAR1 also play a role in the recognition of at least a subset of cargos (9, 10). SEC23 is a GTPaseactivating protein (GAP) that activates the SAR1...

Transcriptional divergence and conservation of human and mouse erythropoiesis

“…Although the exact basis for the pathophysiology of this disease is not understood, erythroid cells are thought to be particularly sensitive to mutations in SEC23B due to the lack of expression of SEC23A, which can compensate for the loss of SEC23B in other tissues (10). In contrast, mutations in Sec23b fail to result in erythroid abnormalities in mouse models (11). Using our comparative gene expression framework, we found that SEC23B and Sec23b expression is comparable during erythroid differentiation in humans and mice.…”

“…However, SEC23A expression is down-regulated in human, whereas Sec23a is upregulated in terminal mouse erythropoiesis, indicating that this paralogous gene may be able to provide compensatory function in the mouse Sec23b knockout, suggesting a potential explanation for the lack of hematologic abnormalities in the Sec23b knockout mouse model (Fig. 2C) (11). To support these findings, we found robust protein expression of Sec23a during all stages of terminal erythropoiesis in mice, whereas Sec23b was greatly reduced at the final stages of differentiation, supporting the concept of compensation in the Sec23b knockout animals (Fig.…”

“…For example, humans express a fetal hemoglobin not found in the mouse, and their globin gene regulation pattern differs significantly (5)(6)(7). Mutations that impair erythropoiesis in humans are often not faithfully recapitulated in mouse models (8)(9)(10)(11). Although specific examples of such differences have been documented, no studies have examined the extent to which mouse erythropoiesis recapitulates the global molecular features of human erythropoiesis.…”

Characteristic phenotypes associated with congenital dyserythropoietic anemia (type II) manifest at different stages of erythropoiesis

“…Intriguingly, a SEC23B knockout mouse model did not reproduce the human disease phenotype suggesting species differences in the roles or regulation of the SEC23 proteins between humans and mice. 28 In this study, we use an in vitro erythroid cell culture system to expand and differentiate erythroblasts derived from peripheral blood mononuclear cells of 3 CDAII patients to monitor the appearance of characteristic phenotypes associated with this disease during erythropoiesis. This enabled us to establish a temporal profile by which the phenotypes observed in CDAII patient erythroblasts and erythrocytes in vivo manifest during terminal erythroid differentiation.…”

Clinical aspects and pathogenesis of congenital dyserythropoietic anemias: from morphology to molecular approach

“…However, no significant differences in red blood cell count, hemoglobin or hematocrit levels were observed in blood collected from wild-type (WT) and Sec23b gt/gt neonates. 78 The disparate phenotypes in mouse and human could result from residual SEC23B function associated with the hypomorphic mutations observed in humans or, alternatively, might be explained by a speciesspecific shift in function between the closely related SEC23 paralogs. 78 These data demonstrate that Sec23b deficient humans and mice exhibit disparate phenotypes, apparently restricted to CDA II in humans and a prominent neonatal pancreatic insufficiency in mice.…”