Regulation of the ER-bound transcription factor Luman/CREB3 in HEK293 cells

Perinuclear damage from nuclear envelope deterioration elicits stress responses that contribute to LMNA cardiomyopathy

Mutations in the LMNA gene encoding lamins A/C cause an array of tissue-selective diseases, with the heart being the most commonly affected organ. Despite progress in understanding the perturbations emanating from LMNA mutations, an integrative understanding of the pathogenesis underlying cardiac dysfunction remains elusive. Using a novel conditional deletion model capable of translatome profiling, we observed that cardiomyocyte-specific Lmna deletion in adult mice led to rapid cardiomyopathy with pathological remodeling. Before cardiac dysfunction, Lmna-deleted cardiomyocytes displayed nuclear abnormalities, Golgi dilation/fragmentation, and CREB3-mediated stress activation. Translatome profiling identified MED25 activation, a transcriptional cofactor that regulates Golgi stress. Autophagy is disrupted in the hearts of these mice, which can be recapitulated by disrupting the Golgi. Systemic administration of modulators of autophagy or ER stress significantly delayed cardiac dysfunction and prolonged survival. These studies support a hypothesis wherein stress responses emanating from the perinuclear space contribute to the LMNA cardiomyopathy development.

Molecular characterization of wild-type and HSAN2B-linked FAM134B

BACKGROUND

Family with sequence similarity 134, member B (FAM134B), also known as Reticulophagy regulator 1 (RETREG1), is an ER-phagy receptor involved in ER homeostasis. Congenital mutations in the FAM134B gene have been reported to be associated with hereditary sensory and autonomic neuropathy type 2B (HSAN2B); however, the molecular differences between wild-type and HSAN2B-linked FAM134B are not fully understood.

METHODS AND RESULTS

We prepared several human FAM134B constructs, such as the HSAN2B-linked mutant, and compared their features with those of wild-type FAM134B by transfecting these constructs into FAM134B-deficient Neuro2a cells. Although intrinsic FAM134B protein expression in wild-type Neuro2a cells was affected by the supply of amino acids in the culture medium, the expression of each HSAN2B-linked mutant FAM134B protein was hardly affected by serum and amino acid deprivation. On the other hand, the intracellular localization of GFP-tagged HSAN2B-linked mutants, except for P7Gfs133X, overlapped well with ER-localized SP-RFP_KDEL and did not differ from that of GFP-tagged wild-type FAM134B. However, analysis of protein‒protein interactions using the NanoBiT reporter assay revealed the difference between wild-type and C-terminal truncated mutant FAM134B. Furthermore, this NanoBiT assay demonstrated that both wild-type and G216R FAM134B interacted with LC3/GABARAPL1 to the same extent, but the FAM134B construct with mutations near the LC3-interacting region (LIR) did not. Similar to the NanoBiT assay, the C-terminal-truncated FAM134B showed lower ER-phagy activities, as assessed by the cotransfection of GFP-tagged reporters.

CONCLUSIONS

We showed that wild-type and HSAN2B-linked FAM134B have different molecular characteristics by transfecting cells with various types of constructs. Thus, this study provides new insights into the molecular mechanisms underlying HSAN2B as well as the regulation of ER-phagy.

Exposure to brefeldin A induces unusual expression of hybrid- and complex-type free N-glycans in HepG2 cells

This study determined the effect of brefeldin A (BFA) on the free N-glycomic profile of HepG2 cells to better understand the effect of blocking intracellular vesicle formation and transport of proteins from the endoplasmic reticulum to the Golgi apparatus. A series of exoglycosidase- and endoglycosidase-assisted analyses clarified the complex nature of altered glycomic profiles. A key feature of BFA-mediated alterations in Gn2-type glycans was the expression of unusual hybrid-, monoantennary- and complex-type free N-glycans (FNGs). BFA-mediated alterations in Gn1-type glycans were characterized by the expression of unusual hybrid- and monoantennary-FNGs, without significant expression of complex-type FNGs. A time course analysis revealed that sialylated hybrid- and complex-type Gn2-type FNGs were generated later than asialo-Gn2-type FNGs, and the expression profiles of Gn2-type FNGs and N-glycans were found to be similar, suggesting that the metabolic flux of FNGs is the same as that of protein-bound N-glycans. Subcellular glycomic analysis revealed that almost all FNGs were detected in the cytoplasmic extracts. Our data suggest that hybrid-, monoantennary- and complex-type Gn2-type FNGs were cleaved from glycoproteins in the cytosol by cytosolic PNGase, and subsequently digested by cytosolic endo-β-N-acetylglucosaminidase (ENGase) to generate Gn1-type FNGs. The substrate specificity of ENGase explains the limited expression of complex Gn1 type FNGs.

Expression analysis and functional characterization of thioredoxin domain-containing protein 11

BACKGROUNDS

The endoplasmic reticulum (ER) is a crucial organelle that regulates both the folding, modification and transport of many proteins and senses certain stimuli inside and outside of cells. ER-associated degradation (ERAD), including SEL1L is a crucial mechanism to maintain homeostasis. In this study, we performed comparative proteome analysis in wild-type (wt) and SEL1L-deficient cells.

METHODS AND RESULTS

We found constitutively high expression of thioredoxin domain-containing protein 11 (TXNDC11) mRNA and protein in our SEL1L-deficient HEK293 cells by RT-PCR and Western blot analysis. The TXNDC11 gene possesses a well-conserved unfolded protein response element (UPRE) around its transcription start site, and ER stress increased TXNDC11 mRNA and luciferase reporter activity via this putative UPRE in HEK293 cells. The amounts of TXNDC11 protein in wild-type and SEL1L-deficient cells with or without thapsigargin (Tg) treatment were parallel to their mRNAs in these cells, which was almost proportional to spliced XBP1 (sXBP1) mRNA expression. The establishment and characterization of TXNDC11-deficient HEK293 cells revealed that the expression of three different ER resident stress sensors, ATF6α, CREB3 and CREB3L2, is regulated by TXNDC11. The rate of disappearance of the three proteins by CHX treatment in wt cells was remarkably different, and the full-length CREB3L2 protein was almost completely degraded within 15 min after CHX treatment. TXNDC11 deficiency increased the expression of each full-length form under resting conditions and delayed their disappearance by CHX treatment. Interestingly, the degree of increase in full-length CREB3/CREB3L2 by TXNDC11 deficiency was apparently higher than that in full-length ATF6α. The increase in these proteins by TXNDC11 deficiency was hardly correlated with the expression of each mRNA. Treatment with ER stress inducers influenced each full-length mature form, and the difference in each full-length form observed in wt and TXNDC11-deficient cells was smaller.

CONCLUSION

This study demonstrated that TXNDC11 is an ER stress-inducible gene regulated by the IRE1-sXBP1 pathway. In addition, TXNDC11 is involved in the regulation of ATF6α, CREB3 and CREB3L2 protein expression, although the contribution to the stability of these proteins is quite variable. Therefore, its further characterization will provide new insights for understanding protein homeostasis in ER physiology and pathology.

Molecular characterization of mouse CREB3 regulatory factor in Neuro2a cells

We performed expression and functional analysis of mouse CREB3 regulatory factor (CREBRF) in Neuro2a cells by constructing several expression vectors. Overexpressed full-length (FL) CREBRF protein was stabilized by MG132; however, the intrinsic CREBRF expression in Neuro2a cells was negligible under all conditions. On the other hand, N- or C-terminal deletion of CREBRF influenced its stability. Cotransfection of CREBRF together with GAL4-tagged FL CREB3 increased luciferase reporter activity, and only the N-terminal region of CREBRF was sufficient to potentiate luciferase activity. Furthermore, this positive effect of CREBRF was also observed in cells expressing GAL4-tagged cleaved CREB3, although CREBRF hardly influenced the protein stability of NanoLuc-tagged cleaved CREB3 or intracellular localization of EGFP-tagged one. In conclusion, this study suggests that CREBRF, a quite unstable proteasome substrate, positively regulates the CREB3 pathway, which is distinct from the canonical ER stress pathway in Neuro2a cells.

Elucidation of brefeldin A-induced ER and Golgi stress responses in Neuro2a cells

Brefeldin A (BFA) disrupts the structure of the Golgi apparatus to trigger ER stress signaling pathways. On the other hand, treatment with BFA induces the activation of CREB3, the protein structure of which is similar to that of ATF6. In this study, we established Neuro2a cells in which three different transcription factors, namely, ATF4, ATF3 and CREB3, were deficient using the CRISPR/Cas9 approach, and we investigated the BFA-induced ER and Golgi stress response in these cells. BFA treatment rapidly induced ATF4, ATF3, Herp and GADD153 protein expression in Neuro2a cells. ATF4-deficient Neuro2a cells exhibited significantly decreased mRNA and protein expression of ATF3 and Herp but not GADD153; however, cells deficient in ATF3 exhibited minimal effects on GADD34, GADD153 and Herp expression. The cleavage of CREB3 in Neuro2a cells was triggered by BFA; however, the expression of several ER and Golgi stress-related factors was hardly influenced by the CREB3 deficiency in these Neuro2a cells. This study shows that CREB3 minimally associates with typical ER stress-inducible responses in Neuro2a cells. Therefore, identification and characterization of the downstream transcriptional targets of CREB3 is required to clarify not only Golgi stress response but also its relationship with ER stress signaling pathways.

Comparative Analysis of CREB3 and CREB3L2 Protein Expression in HEK293 Cells

We performed a comparative analysis of two ER-resident CREB3 family proteins, CREB3 and CREB3L2, in HEK293 cells using pharmacological and genome editing approaches and identified several differences between the two. Treatment with brefeldin A (BFA) and monensin induced the cleavage of full-length CREB3 and CREB3L2; however, the level of the full-length CREB3 protein, but not CREB3L2 protein, was not noticeably reduced by the monensin treatment. On the other hand, treatment with tunicamycin (Tm) shifted the molecular weight of the full-length CREB3L2 protein downward but abolished CREB3 protein expression. Thapsigargin (Tg) significantly increased the expression of only full-length CREB3L2 protein concomitant with a slight increase in the level of its cleaved form. Treatment with cycloheximide and MG132 revealed that both endogenous CREB3 and CREB3L2 are proteasome substrates. In addition, kifunensine, an α-mannosidase inhibitor, significantly increased the levels of both full-length forms. Consistent with these findings, cells lacking SEL1L, a crucial ER-associated protein degradation (ERAD) component, showed increased expression of both full-length CREB3 and CREB3L2; however, cycloheximide treatment downregulated full-length CREB3L2 protein expression more rapidly in SEL1L-deficient cells than the full-length CREB3 protein. Finally, we investigated the induction of the expression of several CREB3 and CREB3L2 target genes by Tg and BFA treatments and SEL1L deficiency. In conclusion, this study suggests that both endogenous full-length CREB3 and CREB3L2 are substrates for ER-associated protein degradation but are partially regulated by distinct mechanisms, each of which contributes to unique cellular responses that are distinct from canonical ER signals.

The stability of CREB3/Luman is regulated by protein kinase CK2 phosphorylation

CREB3 (Luman) is a family member of ER resident transcription factors, which are cleaved upon the induction of ER stress. Their N-terminal fragments shuttle into the nucleus where they regulate the transcription of target genes. Here, we found that human CREB3 is phosphorylated within its transcription activation domain on serine 46 by protein kinase CK2. Further analyses revealed that the phosphorylation of this site does neither affect the cleavage by S1P/S2P proteases, nor the nuclear localisation nor the transcriptional activity of CREB3. However, phosphorylation at serine 46 reduced the stability of CREB3.