Polar transmembrane-based amino acids in presenilin 1 are involved in endoplasmic reticulum localization, Pen2 protein binding, and γ-secretase complex stabilization

The role of ladderlectin in spermatogenesis and ovarian sperm storage in the black rockfish (Sebastes schlegelii)

Ladderlectin, a teleost-specific C-type lectin, has been primarily associated with innate immune defense. However, this study unveils an important role of ladderlectin in the reproductive processes of Sebastes schlegelii. Seven ladderlectin genes (SscLLs) are identified, with SscLL3604 and SscLL3605 exhibiting high testis-specificity expression. Both genes contain a C-type lectin domain (CTLD) and two carbohydrate-binding motifs (QPD and WSD), with SscLL3605 also containing a signal peptide. Notably, SscLL3604 is predominantly cytoplasmic, while SscLL3605 is found both in the cytoplasm and cell membrane. Additionally, SscLLs are primarily localized in Sertoli cells at the mRNA level but also exist in spermatids and spermatozoa at the protein level. Further analysis reveals that SscLLs are present in sperm heads and can bind to ovarian cells, hinting at a pivotal role in long-term sperm storage in ovaries. Knockdown of SscLLs in vitro demonstrates their critical role in maintaining Sertoli cells and Leydig cells within the testis. Finally, inhibition of glycosylation or treatment with antibody of SscLLs leads to an increased incidence of embryonic malformation in S. schlegelii. These findings suggest that ladderlectin may also play an important role in the regulation of reproductive processes, thereby providing an additional adaptive mechanism for the reproduction of viviparous fish.

SARS-CoV-2 spike host cell surface exposure promoted by a COPI sorting inhibitor

Via an insufficient coat protein complex I (COPI) retrieval signal, the majority of SARS-CoV-2 spike (S) is resident in host early secretory organelles and a tiny amount is leaked out in cell surface. Only surface-exposed S can be recognized by B cell receptor (BCR) or anti-S therapeutic monoclonal antibodies (mAbs) that is the trigger step for B cell activation after S mRNA vaccination or infected cell clearance by S mAbs. Now, a drug strategy to promote S host surface exposure is absent. Here, we first combined structural and biochemical analysis to characterize S COPI sorting signals. A potent S COPI sorting inhibitor was then invented, evidently capable of promoting S surface exposure and facilitating infected cell clearance by S antibody-dependent cellular cytotoxicity (ADCC). Importantly, with the inhibitor as a probe, we revealed Omicron BA.1 S is less cell surface exposed than prototypes because of a constellation of S folding mutations, possibly corresponding to its ER chaperone association. Our findings not only suggest COPI is a druggable target against COVID-19, but also highlight SARS-CoV-2 evolution mechanism driven by S folding and trafficking mutations.

Bring it back, bring it back, don't take it away from me - the sorting receptor RER1

The quote "bring it back, bring it back, don't take it away from me" from Queen's Love of my life describes the function of the sorting receptor RER1, a 23 kDa protein with four transmembrane domains (TMDs) that localizes to the intermediate compartment and the cis-Golgi. From there it returns escaped proteins that are not supposed to leave the endoplasmic reticulum (ER) back to it. Unique about RER1 is its ability to recognize its ligands through binding motifs in TMDs. Among its substrates are ER-resident proteins, as well as unassembled subunits of multimeric complexes that are retrieved back into the ER, this way guarding the full assembly of their respective complexes. The basic mechanisms for RER1-dependent retrieval have been already elucidated some years ago in yeast. More recently, several important cargoes of RER1 have been described in mammalian cells, and the in vivo role of RER1 is being unveiled by using mouse models. In this Review, we give an overview of the cell biology of RER1 in different models, discuss its controversial role in the brain and provide an outlook on future directions for RER1 research.

A disease causing ATLASTIN 3 mutation affects multiple endoplasmic reticulum-related pathways

Atlastins (ATLs) are membrane-bound GTPases involved in shaping of the endoplasmic reticulum (ER). Mutations in ATL1 and ATL3 cause spastic paraplegia and hereditary sensory neuropathy. We here show that the sensory neuropathy causing ATL3 Y192C mutation reduces the complexity of the tubular ER-network. ATL3 Y192C delays ER-export by reducing the number of ER exit sites, reduces autophagy, fragments the Golgi and causes malformation of the nucleus. In cultured primary neurons, ATL3 Y192C does not localize to the growing axon, resulting in axon growth deficits. Patient-derived fibroblasts possess a tubular ER with reduced complexity and have a reduced number of autophagosomes. The data suggest that the disease-causing ATL3 Y192C mutation affects multiple ER-related pathways, possibly as a consequence of the distorted ER morphology.

The role of membrane trafficking in the processing of amyloid precursor protein and production of amyloid peptides in Alzheimer's disease

Alzheimer's disease (AD) is characterized by progressive accumulation of misfolded proteins, which form senile plaques and neurofibrillary tangles, and the release of inflammatory mediators by innate immune responses. β-Amyloid peptide (Aβ) is derived from sequential processing of the amyloid precursor protein (APP) by membrane-bound proteases, namely the β-secretase, BACE1, and γ-secretase. Membrane trafficking plays a key role in the regulation of APP processing as both APP and the processing secretases traffic along distinct pathways. Genome wide sequencing studies have identified several AD susceptibility genes which regulate membrane trafficking events. To understand the pathogenesis of AD it is critical that the cell biology of APP and Aβ production in neurons is well defined. This review discusses recent advances in unravelling the membrane trafficking events associated with the production of Aβ, and how AD susceptible alleles may perturb the sorting and transport of APP and BACE1. Mechanisms whereby inflammation may influence APP processing are also considered.

Molecular cloning and characterization of presenilin gene in Bombyx mori

Presenilin (PS), the catalytic core of the γ-secretase complex, is considered to be a causative protein of the early‑onset familial form of Alzheimer's disease. Aging is a risk factor for Alzheimer's disease and a number of genetic studies have utilized Bombyx mori (B. mori) as a model, making it possible to use B. mori to investigate Alzheimer's disease. However, the homologous gene of human PS in B. mori has remained to be elucidated. In the present study, the PS homologue gene in B. mori was identified and characterized, and six B. mori presenilin (BmPS) mRNA transcripts were generated by selecting multiple transcription start sites and/or alternative splice sites. The longest mRNA of BmPS (termed BmPS1) contains a 153 nt 5' untranslated region (UTR), a 1,440 nt open reading frame and a 1,063 nt 3' UTR. The predicted protein of BmPS1 consists of 479 amino acid residues and has two highly‑conserved aspartate residues, which form the catalytic core of aspartic proteases. It exhibits a sequence identity of ~44 and 51% with homologues in Homo sapiens and Drosophila melanogaster, respectively. However, the amino acid sequence of the BmPS loop region does not completely match between the two B. mori strains R13Q and Dazao. Genomic analysis revealed that B. mori had a single copy of the BmPS gene, which was composed of 14 exons. A total of four isoforms of BmPS (BmPS‑A, ‑B, ‑C and ‑D) owing to multiple transcriptional start sites and alternative splice sites were identified. The alternative splicing events occurring in the loop region improved the diversity of the BmPS protein and were detectable in all tissues, as determined using reverse transcription quantitative polymerase chain reaction (RT‑qPCR). Furthermore, the expression levels of BmPS in the brain at different developmental stages were detected using RT‑qPCR, and significantly higher expression levels of BmPS were found in the adult stage compared with those in the larval and pupal stages. The present study on BmPS provided insight into the pathogenesis of Alzheimer's disease and mechanisms of silkworm developmental regulation.

G206D Mutation of Presenilin-1 Reduces Pen2 Interaction, Increases Aβ42/Aβ40 Ratio and Elevates ER Ca(2+) Accumulation

Early-onset familial Alzheimer's disease (AD) is most commonly associated with the mutations in presenilin-1 (PS1). PS1 is the catalytic component of the γ-secretase complex, which cleaves amyloid precursor protein to produce amyloid-β (Aβ), the major cause of AD. Presenilin enhancer 2 (Pen2) is critical for activating γ-secretase and exporting PS1 from endoplasmic reticulum (ER). Among all the familial AD-linked PS1 mutations, mutations at the G206 amino acid are the most adjacent position to the Pen2 binding site. Here, we characterized the effect of a familial AD-linked PS1 G206D mutation on the PS1-Pen2 interaction and the accompanied alteration in γ-secretase-dependent and -independent functions. We found that the G206D mutation reduced PS1-Pen2 interaction, but did not abolish γ-secretase formation and PS1 endoproteolysis. For γ-secretase-dependent function, the G206D mutation increased Aβ42 production but not Notch cleavage. For γ-secretase-independent function, this mutation disrupted the ER calcium homeostasis but not lysosomal calcium homeostasis and autophagosome maturation. Impaired ER calcium homeostasis may due to the reduced mutant PS1 level in the ER. Although this mutation did not alter the cell survival under stress, both increased Aβ42 ratio and disturbed ER calcium regulation could be the mechanisms underlying the pathogenesis of the familial AD-linked PS1 G206D mutation.

Gamma-secretase subunits associate in intracellular membrane compartments in Arabidopsis thaliana

Gamma-secretase is a multisubunit complex with intramembrane proteolytic activity. In humans it was identified in genetic screens of patients suffering from familial forms of Alzheimer's disease, and since then it was shown to mediate cleavage of more than 80 substrates, including amyloid precursor protein or Notch receptor. Moreover, in animals, γ-secretase was shown to be involved in regulation of a wide range of cellular events, including cell signalling, regulation of endocytosis of membrane proteins, their trafficking, and degradation. Here we show that genes coding for γ-secretase homologues are present in plant genomes. Also, amino acid motifs crucial for γ-secretase activity are conserved in plants. Moreover, all γ-secretase subunits: PS1/PS2, APH-1, PEN-2, and NCT colocalize and interact with each other in Arabidopsis thaliana protoplasts. The intracellular localization of γ-secretase subunits in Arabidopsis protoplasts revealed a distribution in endomembrane system compartments that is consistent with data from animal studies. Together, our data may be considered as a starting point for analysis of γ-secretase in plants.

Mutant ELOVL4 that causes autosomal dominant stargardt-3 macular dystrophy is misrouted to rod outer segment disks

PURPOSE

Autosomal dominant Stargardt macular dystrophy caused by mutations in the Elongation of Very Long Chain fatty acids (ELOVL4) gene results in macular degeneration, leading to early childhood blindness. Transgenic mice and pigs expressing mutant ELOVL4 develop progressive photoreceptor degeneration. The mechanism by which these mutations cause macular degeneration remains unclear, but have been hypothesized to involve the loss of an ER-retention dilysine motif located in the extreme C-terminus. Dominant negative mechanisms and reduction in retinal polyunsaturated fatty acids also have been suggested. To understand the molecular mechanisms involved in disease progression in vivo, we addressed the hypothesis that the disease-linked C-terminal truncation mutant of ELOVL4 exerts a dominant negative effect on wild-type (WT) ELOVL4, altering its subcellular localization and function, which subsequently induces retinal degeneration and loss of vision.

METHODS

We generated transgenic Xenopus laevis that overexpress HA-tagged murine ELOVL4 variants in rod photoreceptors.

RESULTS

Tagged or untagged WT ELOVL4 localized primarily to inner segments. However, the mutant protein lacking the dilysine motif was mislocalized to post-Golgi compartments and outer segment disks. Coexpression of mutant and WT ELOVL4 in rods did not result in mislocalization of the WT protein to outer segments or in the formation of aggregates. Full-length HA-tagged ELOVL4 lacking the dilysine motif (K308R/K310R) necessary for targeting the WT ELOVL4 protein to the endoplasmic reticulum was similarly mislocalized to outer segments.

CONCLUSIONS

We propose that expression and outer segment mislocalization of the disease-linked 5-base-pair deletion mutant ELOVL4 protein alters photoreceptor structure and function, which subsequently results in retinal degeneration, and suggest three possible mechanisms by which mutant ELOVL4 may induce retinal degeneration in STGD3.

Protein quality control by Rer1p in the early secretory pathway: from mechanism to implication in Alzheimer's disease

γ-Secretase-mediated production of amyloid β from the amyloid precursor protein is recognized as a central player in the neuropathogenesis of Alzheimer’s disease (AD). One of the most peculiar features of this enzymatic activity is the fact that it targets transmembrane domains of mostly type I integral membrane proteins and thus manages to proteolyse peptide bonds within the hydrophobic lipid bilayers. In addition, γ-secretase does not exert its activity solely towards amyloid precursor protein, but to an increasing number of membrane proteins, including Notch, cadherins, syndecans, and so on. Because of the requirement of intramembrane proteolysis for a plethora of signaling pathways and cellular processes during embryonic development and organ physiology, this enzyme has drawn a lot of attention in the past 20 years. γ-Secretase is a multimeric transmembrane complex consisting of the catalytic presenilin, nicastrin, presenilin enhancer 2 (PEN2) and anterior-pharynx defective-1 (APH1) subunits. Proper assembly into functional complexes requires quality control mechanisms associated with the early biosynthetic compartments and allows mature complexes to transit to distal compartments where its activity is required. We previously identified Retrieval to ER protein 1 (Rer1p) as the first negative regulator of the stepwise assembly of γ-secretase during endoplasmic reticulum-to-Golgi transport. We review here the state of the art on how Rer1p regulates complex assembly, particularly γ-secretase, and evaluate the therapeutic potential of such regulatory processes in the context of AD.

The very many faces of presenilins and the γ-secretase complex

Presenilin is a central, catalytic component of the γ-secretase complex which conducts intramembrane cleavage of various protein substrates. Although identified and mainly studied through its role in the development of amyloid plaques in Alzheimer disease, γ-secretase has many other important functions. The complex seems to be evolutionary conserved throughout the Metazoa, but recent findings in plants and Dictyostelium discoideum as well as in archeons suggest that its evolution and functions might be much more diversified than previously expected. In this review, a selective survey of the multitude of functions of presenilins and the γ-secretase complex is presented. Following a brief overview of γ-secretase structure, assembly and maturation, three functional aspects are analyzed: (1) the role of γ-secretase in autophagy and phagocytosis; (2) involvement of the complex in signaling related to endocytosis; and (3) control of calcium fluxes by presenilins.