Nuclear envelope localization of PIG-B is essential for GPI-anchor synthesis in Drosophila

PIGB maintains nuclear lamina organization in skeletal muscle of Drosophila

The nuclear lamina (NL) plays various roles and participates in nuclear integrity, chromatin organization, and transcriptional regulation. Lamin proteins, the main components of the NL, form a homogeneous meshwork structure under the nuclear envelope. Lamins are essential, but it is unknown whether their homogeneous distribution is important for nuclear function. Here, we found that PIGB, an enzyme involved in glycosylphosphatidylinositol (GPI) synthesis, is responsible for the homogeneous lamin meshwork in Drosophila. Loss of PIGB resulted in heterogeneous distributions of B-type lamin and lamin-binding proteins in larval muscles. These phenotypes were rescued by expression of PIGB lacking GPI synthesis activity. The PIGB mutant exhibited changes in lamina-associated domains that are large heterochromatic genomic regions in the NL, reduction of nuclear stiffness, and deformation of muscle fibers. These results suggest that PIGB maintains the homogeneous meshwork of the NL, which may be essential for chromatin distribution and nuclear mechanical properties.

Subunits of the GPI transamidase complex localize to the endoplasmic reticulum and nuclear envelope in Drosophila

A total of 10-20% of plasma membrane proteins are anchored by glycosylphosphatidylinositol (GPI). GPI is attached to proteins by GPI transamidase (GPI-T), which contains five subunits named PIGK, PIGS, PIGT, PIGU, and GPAA1. We previously reported that PIGT localizes near the nucleus in Drosophila. However, localizations of the other four subunits remain unknown. Here, we show that a catalytic subunit of GPI-T, PIGK, mainly localizes to the endoplasmic reticulum (ER), while the other four subunits localize to the nuclear envelope (NE) and ER. The NE/ER localization ratio of PIGS differs between cell types and developmental stages. Our results suggest that GPI-T catalyzes GPI attachment in the ER and the other four subunits may have other unknown functions in the NE.

Investigating Developmental and Epileptic Encephalopathy Using Drosophila melanogaster

Developmental and epileptic encephalopathies (DEEs) are the spectrum of severe epilepsies characterized by early-onset, refractory seizures occurring in the context of developmental regression or plateauing. Early infantile epileptic encephalopathy (EIEE) is one of the earliest forms of DEE, manifesting as frequent epileptic spasms and characteristic electroencephalogram findings in early infancy. In recent years, next-generation sequencing approaches have identified a number of monogenic determinants underlying DEE. In the case of EIEE, 85 genes have been registered in Online Mendelian Inheritance in Man as causative genes. Model organisms are indispensable tools for understanding the in vivo roles of the newly identified causative genes. In this review, we first present an overview of epilepsy and its genetic etiology, especially focusing on EIEE and then briefly summarize epilepsy research using animal and patient-derived induced pluripotent stem cell (iPSC) models. The Drosophila model, which is characterized by easy gene manipulation, a short generation time, low cost and fewer ethical restrictions when designing experiments, is optimal for understanding the genetics of DEE. We therefore highlight studies with Drosophila models for EIEE and discuss the future development of their practical use.

Identification of new transmembrane proteins concentrated at the nuclear envelope using organellar proteomics of mesenchymal cells

The double membrane nuclear envelope (NE), which is contiguous with the ER, contains nuclear pore complexes (NPCs) – the channels for nucleocytoplasmic transport, and the nuclear lamina (NL) – a scaffold for NE and chromatin organization. Since numerous human diseases linked to NE proteins occur in mesenchyme-derived cells, we used proteomics to characterize NE and other subcellular fractions isolated from mesenchymal stem cells and from adipocytes and myocytes. Based on spectral abundance, we calculated enrichment scores for proteins in the NE fractions. We demonstrated by quantitative immunofluorescence microscopy that five little-characterized proteins with high enrichment scores are substantially concentrated at the NE, with Itprip exposed at the outer nuclear membrane, Smpd4 enriched at the NPC, and Mfsd10, Tmx4, and Arl6ip6 likely residing in the inner nuclear membrane. These proteins provide new focal points for studying the functions of the NE. Moreover, our datasets provide a resource for evaluating additional potential NE proteins.

Lamin is essential for nuclear localization of the GPI synthesis enzyme PIG-B and GPI-anchored protein production in Drosophila

Membrane lipid biosynthesis is a complex process that occurs in various intracellular compartments. In Drosophila, phosphatidylinositol glycan-B (PIG-B), which catalyzes addition of the third mannose in glycosylphosphatidylinositol (GPI), localizes to the nuclear envelope (NE). Although this NE localization is essential for Drosophila development, the underlying molecular mechanism remains unknown. To elucidate this mechanism, we identified PIG-B-interacting proteins by performing immunoprecipitation followed by proteomic analysis. We then examined which of these proteins are required for the NE localization of PIG-B. Knockdown of Lamin Dm0, a B-type lamin, led to mislocalization of PIG-B from the NE to the endoplasmic reticulum. Lamin Dm0 associated with PIG-B at the inner nuclear membrane, a process that required the tail domain of Lamin Dm0. Furthermore, GPI moieties were distributed abnormally in the Lamin Dm0 mutant. These data indicate that Lamin Dm0 is involved in the NE localization of PIG-B and is required for proper GPI-anchor modification of proteins.

Highlighted Article: Lamin plays a role in post-translational modification of plasma membrane proteins by tethering the GPI modification enzyme PIG-B to the inner nuclear membrane.

Golgi stress response and organelle zones

Organelles have been studied traditionally as single units, but a novel concept is now emerging: each organelle has distinct functional zones that regulate specific functions. The Golgi apparatus seems to have various zones, including zones for: glycosylphosphatidylinositol-anchored proteins; proteoglycan, mucin and lipid glycosylation; transport of cholesterol and ceramides; protein degradation (Golgi membrane-associated degradation); and signalling for apoptosis. The capacity for these specific functions and the size of the corresponding zones appear to be tightly regulated by the Golgi stress response to accommodate cellular demands. For instance, the proteoglycan and mucin zones seem to be separately augmented during the differentiation of chondrocytes and goblet cells, respectively. The mammalian Golgi stress response consists of several response pathways. The TFE3 pathway regulates the general function of the Golgi, such as structural maintenance, N-glycosylation and vesicular transport, whereas the proteoglycan pathway increases the expression of glycosylation enzymes for proteoglycans. The CREB3 and HSP47 pathways regulate pro- and anti-apoptotic functions, respectively. These observations indicate that the Golgi is a dynamic organelle, the capacity of which is upregulated according to cellular needs.

Endoplasmic Reticulum Export of GPI-Anchored Proteins

Protein export from the endoplasmic reticulum (ER) is an essential process in all eukaryotes driven by the cytosolic coat complex COPII, which forms vesicles at ER exit sites for transport of correctly assembled secretory cargo to the Golgi apparatus. The COPII machinery must adapt to the existing wide variety of different types of cargo proteins and to different cellular needs for cargo secretion. The study of the ER export of glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs), a special glycolipid-linked class of cell surface proteins, is contributing to address these key issues. Due to their special biophysical properties, GPI-APs use a specialized COPII machinery to be exported from the ER and their processing and maturation has been recently shown to actively regulate COPII function. In this review, we discuss the regulatory mechanisms by which GPI-APs are assembled and selectively exported from the ER.