Dual function of Sec16B: Endoplasmic reticulum-derived protein secretion and peroxisome biogenesis in mammalian cells

Intestinal SEC16B modulates obesity by regulating chylomicron metabolism

OBJECTIVE

Genome-wide association studies (GWAS) have identified genetic variants in SEC16 homolog B (SEC16B) locus to be associated with obesity and body mass index (BMI) in various populations. SEC16B encodes a scaffold protein located at endoplasmic reticulum (ER) exit sites that is implicated to participate in the trafficking of COPII vesicles in mammalian cells. However, the function of SEC16B in vivo, especially in lipid metabolism, has not been investigated.

METHODS

We generated Sec16b intestinal knockout (IKO) mice and assessed the impact of its deficiency on high-fat diet (HFD) induced obesity and lipid absorption in both male and female mice. We examined lipid absorption in vivo by acute oil challenge and fasting/HFD refeeding. Biochemical analyses and imaging studies were performed to understand the underlying mechanisms.

RESULTS

Our results showed that Sec16b intestinal knockout (IKO) mice, especially female mice, were protected from HFD-induced obesity. Loss of Sec16b in intestine dramatically reduced postprandial serum triglyceride output upon intragastric lipid load or during overnight fasting and HFD refeeding. Further studies showed that intestinal Sec16b deficiency impaired apoB lipidation and chylomicron secretion.

CONCLUSIONS

Our studies demonstrated that intestinal SEC16B is required for dietary lipid absorption in mice. These results revealed that SEC16B plays important roles in chylomicron metabolism, which may shed light on the association between variants in SEC16B and obesity in human.

Peroxisome biogenesis, membrane contact sites, and quality control

Peroxisomes are conserved organelles of eukaryotic cells with important roles in cellular metabolism, human health, redox homeostasis, as well as intracellular metabolite transfer and signaling. We review here the current status of the different co-existing modes of biogenesis of peroxisomal membrane proteins demonstrating the fascinating adaptability in their targeting and sorting pathways. While earlier studies focused on peroxisomes as autonomous organelles, the necessity of the ER and potentially even mitochondria as sources of peroxisomal membrane proteins and lipids has come to light in recent years. Additionally, the intimate physical juxtaposition of peroxisomes with other organelles has transitioned from being viewed as random encounters to a growing appreciation of the expanding roles of such inter-organellar membrane contact sites in metabolic and regulatory functions. Peroxisomal quality control mechanisms have also come of age with a variety of mechanisms operating both during biogenesis and in the cellular response to environmental cues.

Involvement of regucalcin gene promoter region-related protein-p117, a transcription factor, in human obesity

Regucalcin gene promoter region-related protein-p117 (RGPR-p117; gene symbol, rgpr-117) was identified in 2001 as a novel transcription factor that specifically binds to a nuclear factor I consensus motif, TTGGC(N)₆CC in the promoter region of the regucalcin (rgn) gene. The human RGPR-p117 gene consists of 26 exons spanning ~4.1 kbp and is localized on chromosome 1q25.2. The nuclear translocation of cytoplasm RGPR-p117 is mediated via the protein kinase C-dependent signaling pathway. Overexpression of RGPR-p117 enhances the transcription activity of rgn, and a protective effect on cell death by inhibition of gene expression levels of caspase-3, caspase-8 and FADD proteins that possess the TTGGC motif in the promoter region of those genes was revealed. RGPR-p117 has a crucial role as a transcription factor. Notably, RGPR-p117 was shown to localize in the plasma membranes, mitochondria and microsomes (endoplasmic reticulum; ER). RGPR-p117, which is located in the ER, was also shown to have a role as an ER export factor implicated in the transports of proteins and lipids. As a result of this finding, it was proposed in 2007 that RGPR-p117 is renamed SEC 16 homolog B, endoplasmic reticulum export factor (SEC16B). Recently, there is increasing evidence that RGPR-p117/SEC16B may be involved in human obesity. Thus, the current review presents data regarding the involvement of RGPR-p117 in human obesity.

Glycosome biogenesis in trypanosomes and the de novo dilemma

Trypanosomatid parasites, including Trypanosoma and Leishmania, are the causative agents of lethal diseases threatening millions of people around the world. These organisms compartmentalize glycolysis in essential, specialized peroxisomes called glycosomes. Peroxisome proliferation can occur through growth and division of existing organelles and de novo biogenesis from the endoplasmic reticulum. The level that each pathway contributes is debated. Current evidence supports the concerted contribution of both mechanisms in an equilibrium that can vary depending on environmental conditions and metabolic requirements of the cell. Homologs of a number of peroxins, the proteins involved in peroxisome biogenesis and matrix protein import, have been identified in T. brucei. Based on these findings, it is widely accepted that glycosomes proliferate through growth and division of existing organelles; however, to our knowledge, a de novo mechanism of biogenesis has not been directly demonstrated. Here, we review recent findings that provide support for the existence of an endoplasmic reticulum (ER)-derived de novo pathway of glycosome biogenesis in T. brucei. Two studies recently identified PEX13.1, a peroxin involved in matrix protein import, in the ER of procyclic form T. brucei. In other eukaryotes, peroxins including PEX13 have been found in the ER of cells undergoing de novo biogenesis of peroxisomes. In addition, PEX16 and PEX19 have been characterized in T. brucei, both of which are important for de novo biogenesis in other eukaryotes. Because glycosomes are rapidly remodeled via autophagy during life cycle differentiation, de novo biogenesis could provide a method of restoring glycosome populations following turnover. Together, the findings we summarize provide support for the hypothesis that glycosome proliferation occurs through growth and division of pre-existing organelles and de novo biogenesis of new organelles from the ER and that the level each mechanism contributes is influenced by glucose availability.

Localization of a Trypanosome Peroxin to the Endoplasmic Reticulum

Trypanosoma brucei is the causative agent of diseases that affect 30,000–50,000 people annually. Trypanosoma brucei harbors unique organelles named glycosomes that are essential to parasite survival, which requires growth under fluctuating environmental conditions. The mechanisms that govern the biogenesis of these organelles are poorly understood. Glycosomes are evolutionarily related to peroxisomes, which can proliferate de novo from the endoplasmic reticulum or through the growth and division of existing organelles depending on the organism and environmental conditions. The effect of environment on glycosome biogenesis is unknown. Here, we demonstrate that the glycosome membrane protein, TbPex13.1, is localized to glycosomes when cells are cultured under high glucose conditions and to the endoplasmic reticulum in low glucose conditions. This localization in low glucose was dependent on the presence of a C‐terminal tripeptide sequence. Our findings suggest that glycosome biogenesis is influenced by extracellular glucose levels and adds to the growing body of evidence that de novo glycosome biogenesis occurs in trypanosomes. Because the movement of peroxisomal membrane proteins is a hallmark of ER‐dependent peroxisome biogenesis, TbPex13.1 may be a useful marker for the study such processes in trypanosomes.

Targeting and insertion of peroxisomal membrane proteins: ER trafficking versus direct delivery to peroxisomes

The importance of peroxisomes is highlighted by severe inherited human disorders linked to impaired peroxisomal biogenesis. Besides the simple architecture of these ubiquitous and dynamic organelles, their biogenesis is surprisingly complex and involves specialized proteins, termed peroxins, which mediate targeting and insertion of peroxisomal membrane proteins (PMPs) into the peroxisomal bilayer, and the import of soluble proteins into the protein-dense matrix of the organelle. The long-standing paradigm that all peroxisomal proteins are imported directly into preexisting peroxisomes has been challenged by the detection of PMPs inside the endoplasmic reticulum (ER). New models propose that the ER originates peroxisomal biogenesis by mediating PMP trafficking to the peroxisomes via budding vesicles. However, the relative contribution of this ER-derived pathway to the total peroxisome population in vivo, and the detailed mechanisms of ER entry and exit of PMPs are controversially discussed. This review aims to summarize present knowledge about how PMPs are targeted to the ER, instead of being inserted directly into preexisting peroxisomes. Moreover, molecular mechanisms that facilitate bilayer insertion of PMPs among different species are discussed.

Identification and functional characterization of Trypanosoma brucei peroxin 16

Protozoan parasites of the family Trypanosomatidae infect humans as well as livestock causing devastating diseases like sleeping sickness, Chagas disease, and Leishmaniasis. These parasites compartmentalize glycolytic enzymes within unique organelles, the glycosomes. Glycosomes represent a subclass of peroxisomes and they are essential for the parasite survival. Hence, disruption of glycosome biogenesis is an attractive drug target for these Neglected Tropical Diseases (NTDs). Peroxin 16 (PEX16) plays an essential role in peroxisomal membrane protein targeting and de novo biogenesis of peroxisomes from endoplasmic reticulum (ER). We identified trypanosomal PEX16 based on specific sequence characteristics and demonstrate that it is an integral glycosomal membrane protein of procyclic and bloodstream form trypanosomes. RNAi mediated partial knockdown of Trypanosoma brucei PEX16 in bloodstream form trypanosomes led to severe ATP depletion, motility defects and cell death. Microscopic and biochemical analysis revealed drastic reduction in glycosome number and mislocalization of the glycosomal matrix enzymes to the cytosol. Asymmetry of the localization of the remaining glycosomes was observed with a severe depletion in the posterior part. The results demonstrate that trypanosomal PEX16 is essential for glycosome biogenesis and thereby, provides a potential drug target for sleeping sickness and related diseases.