Fam20C regulates protein secretion by Cab45 phosphorylation

Gingival proteomics reveals the role of TGF beta and YAP/TAZ signaling in Raine syndrome fibrosis

Raine syndrome (RNS) is a rare autosomal recessive osteosclerotic dysplasia. RNS is caused by loss-of-function disease-causative variants of the FAM20C gene that encodes a kinase that phosphorylates most of the secreted proteins found in the body fluids and extracellular matrix. The most common RNS clinical features are generalized osteosclerosis, facial dysmorphism, intracerebral calcifications and respiratory defects. In non-lethal RNS forms, oral traits include a well-studied hypoplastic amelogenesis imperfecta (AI) and a much less characterized gingival phenotype. We used immunomorphological, biochemical, and siRNA approaches to analyze gingival tissues and primary cultures of gingival fibroblasts of two unrelated, previously reported RNS patients. We showed that fibrosis, pathological gingival calcifications and increased expression of various profibrotic and pro-osteogenic proteins such as POSTN, SPARC and VIM were common findings. Proteomic analysis of differentially expressed proteins demonstrated that proteins involved in extracellular matrix (ECM) regulation and related to the TGFβ/SMAD signaling pathway were increased. Functional analyses confirmed the upregulation of TGFβ/SMAD signaling and subsequently uncovered the involvement of two closely related transcription cofactors important in fibrogenesis, Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ). Knocking down of FAM20C confirmed the TGFβ-YAP/TAZ interplay indicating that a profibrotic loop enabled gingival fibrosis in RNS patients. In summary, our in vivo and in vitro data provide a detailed description of the RNS gingival phenotype. They show that gingival fibrosis and calcifications are associated with, and most likely caused by excessed ECM production and disorganization. They furthermore uncover the contribution of increased TGFβ-YAP/TAZ signaling in the pathogenesis of the gingival fibrosis.

Quantification of Protein Exit at the Trans-Golgi Network

With one-third of all newly synthesized proteins entering the secretory pathway, correct protein sorting is essential for cellular homeostasis. In the last three decades, researchers have developed numerous biochemical, genetic, and cell biological approaches to study protein export and sorting from the trans-Golgi network (TGN). However, accurately quantifying protein transport from one compartment to the next in the secretory pathway has been challenging. The Retention Using Selective Hooks (RUSH) system is a method that allows monitoring trafficking of a protein of interest in real time, similar to a pulse-chase experiment but without the need of radiolabeling. Accurate calculations, however, are necessary and currently lacking. Here, we combine the RUSH system with live cell imaging to quantify and calculate half lives. We exemplify our approach using a soluble secreted protein (LyzC). This system will benefit membrane trafficking researchers by adding numbers to protein export and comparing the export kinetics of different cargoes and variating conditions.

Cab45 deficiency leads to the mistargeting of progranulin and prosaposin and aberrant lysosomal positioning

The trans-Golgi Network (TGN) sorts molecular "addresses" and sends newly synthesized proteins to their destination via vesicular transport carriers. Despite the functional significance of packaging processes at the TGN, the sorting of soluble proteins remains poorly understood. Recent research has shown that the Golgi resident protein Cab45 is a significant regulator of secretory cargo sorting at the TGN. Cab45 oligomerizes upon transient Ca2+ influx, recruits soluble cargo molecules (clients), and packs them in sphingomyelin-rich transport carriers. However, the identity of client molecules packed into Cab45 vesicles is scarce. Therefore, we used a precise and highly efficient secretome analysis technology called hiSPECs. Intriguingly, we observed that Cab45 deficient cells manifest hypersecretion of lysosomal hydrolases. Specifically, Cab45 deficient cells secrete the unprocessed precursors of prosaposin (PSAP) and progranulin (PGRN). In addition, lysosomes in these cells show an aberrant perinuclear accumulation suggesting a new role of Cab45 in lysosomal positioning. This work uncovers a yet unknown function of Cab45 in regulating lysosomal function.

Regulation of secretory pathway kinase or kinase-like proteins in human cancers

Secretory pathway kinase or kinase-like proteins (SPKKPs) are effective in the lumen of the endoplasmic reticulum (ER), Golgi apparatus (GA), and extracellular space. These proteins are involved in secretory signaling pathways and are distinctive from typical protein kinases. Various reports have shown that SPKKPs regulate the tumorigenesis and progression of human cancer via the phosphorylation of various substrates, which is essential in physiological and pathological processes. Emerging evidence has revealed that the expression of SPKKPs in human cancers is regulated by multiple factors. This review summarizes the current understanding of the contribution of SPKKPs in tumorigenesis and the progression of immunity. With the epidemic trend of immunotherapy, targeting SPKKPs may be a novel approach to anticancer therapy. This study briefly discusses the recent advances regarding SPKKPs.

GARP dysfunction results in COPI displacement, depletion of Golgi v-SNAREs and calcium homeostasis proteins

Golgi-associated retrograde protein (GARP) is an evolutionary conserved heterotetrameric protein complex that tethers endosome-derived vesicles and is vital for Golgi glycosylation. Microscopy and proteomic approaches were employed to investigate defects in Golgi physiology in RPE1 cells depleted for the GARP complex. Both cis and trans-Golgi compartments were significantly enlarged in GARP-knock-out (KO) cells. Proteomic analysis of Golgi-enriched membranes revealed significant depletion of a subset of Golgi residents, including Ca2+ binding proteins, enzymes, and SNAREs. Validation of proteomics studies revealed that SDF4 and ATP2C1, related to Golgi calcium homeostasis, as well as intra-Golgi v-SNAREs GOSR1 and BET1L, were significantly depleted in GARP-KO cells. Finding that GARP-KO is more deleterious to Golgi physiology than deletion of GARP-sensitive v-SNAREs, prompted a detailed investigation of COPI trafficking machinery. We discovered that in GARP-KO cells COPI is significantly displaced from the Golgi and partially relocalized to the ER-Golgi intermediate compartment (ERGIC). Moreover, COPI accessory proteins GOLPH3, ARFGAP1, GBF1, and BIG1 are also relocated to off-Golgi compartments. We propose that the dysregulation of COPI machinery, along with the depletion of Golgi v-SNAREs and alteration of Golgi Ca2+ homeostasis, are the major driving factors for the depletion of Golgi resident proteins, structural alterations, and glycosylation defects in GARP deficient cells.

Liquid-liquid phase separation facilitates the biogenesis of secretory storage granules

Insulin is synthesized by pancreatic β-cells and stored into secretory granules (SGs). SGs fuse with the plasma membrane in response to a stimulus and deliver insulin to the bloodstream. The mechanism of how proinsulin and its processing enzymes are sorted and targeted from the trans-Golgi network (TGN) to SGs remains mysterious. No cargo receptor for proinsulin has been identified. Here, we show that chromogranin (CG) proteins undergo liquid-liquid phase separation (LLPS) at a mildly acidic pH in the lumen of the TGN, and recruit clients like proinsulin to the condensates. Client selectivity is sequence-independent but based on the concentration of the client molecules in the TGN. We propose that the TGN provides the milieu for converting CGs into a "cargo sponge" leading to partitioning of client molecules, thus facilitating receptor-independent client sorting. These findings provide a new receptor-independent sorting model in β-cells and many other cell types and therefore represent an innovation in the field of membrane trafficking.

Nutrient Regulation of Pancreatic Islet β-Cell Secretory Capacity and Insulin Production

Pancreatic islet β-cells exhibit tremendous plasticity for secretory adaptations that coordinate insulin production and release with nutritional demands. This essential feature of the β-cell can allow for compensatory changes that increase secretory output to overcome insulin resistance early in Type 2 diabetes (T2D). Nutrient-stimulated increases in proinsulin biosynthesis may initiate this β-cell adaptive compensation; however, the molecular regulators of secretory expansion that accommodate the increased biosynthetic burden of packaging and producing additional insulin granules, such as enhanced ER and Golgi functions, remain poorly defined. As these adaptive mechanisms fail and T2D progresses, the β-cell succumbs to metabolic defects resulting in alterations to glucose metabolism and a decline in nutrient-regulated secretory functions, including impaired proinsulin processing and a deficit in mature insulin-containing secretory granules. In this review, we will discuss how the adaptative plasticity of the pancreatic islet β-cell's secretory program allows insulin production to be carefully matched with nutrient availability and peripheral cues for insulin signaling. Furthermore, we will highlight potential defects in the secretory pathway that limit or delay insulin granule biosynthesis, which may contribute to the decline in β-cell function during the pathogenesis of T2D.

Golgi Metal Ion Homeostasis in Human Health and Diseases

The Golgi apparatus is a membrane organelle located in the center of the protein processing and trafficking pathway. It consists of sub-compartments with distinct biochemical compositions and functions. Main functions of the Golgi, including membrane trafficking, protein glycosylation, and sorting, require a well-maintained stable microenvironment in the sub-compartments of the Golgi, along with metal ion homeostasis. Metal ions, such as Ca²⁺, Mn²⁺, Zn²⁺, and Cu²⁺, are important cofactors of many Golgi resident glycosylation enzymes. The homeostasis of metal ions in the secretory pathway, which is required for proper function and stress response of the Golgi, is tightly regulated and maintained by transporters. Mutations in the transporters cause human diseases. Here we provide a review specifically focusing on the transporters that maintain Golgi metal ion homeostasis under physiological conditions and their alterations in diseases.

Proteolytic processing of secretory pathway kinase Fam20C by site-1 protease promotes biomineralization

Family with sequence similarity 20C (Fam20C), the major protein kinase in the secretory pathway, generates the vast majority of the secreted phosphoproteome. However, the regulatory mechanisms of Fam20C transport, secretion, and function remain largely unexplored. Here, we show that Fam20C exists as a type II transmembrane protein within the secretory compartments, with its N-terminal signal peptide-like region serving as a membrane anchor for Golgi retention. The secretion and kinase activity of Fam20C are governed by site-1 protease (S1P), a key regulator of cholesterol homeostasis. We find that only mature Fam20C processed by S1P functions in osteoblast differentiation and mineralization. Together, our findings reveal a unique mechanism for Fam20C secretion and activation via proteolytic regulation, providing a molecular link between biomineralization and lipid metabolism.

Sending out molecules from the TGN

The sorting of secreted cargo proteins and their export from the trans-Golgi network (TGN) remains an enigma in the field of membrane trafficking; although the sorting mechanisms of many transmembrane proteins have been well described. The sorting of secreted proteins at the TGN is crucial for the release of signaling factors, as well as extracellular matrix proteins. These proteins are required for cell-cell communication and integrity of an organism. Missecretion of these factors can cause diseases such as neurological disorders, autoimmune disease, or cancer. The major open question is how soluble proteins that are not associated with the membrane are packed into TGN derived transport carriers to facilitate their transport to the plasma membrane. Recent investigations have identified novel types of protein and lipid machinery that facilitate the packing of these molecules into a TGN derived vesicle. In addition, novel research has uncovered an exciting link between cargo sorting and export in which TGN structure and dynamics, as well as TGN/endoplasmic reticulum contact sites, play a significant role. Here, we have reviewed the progress made in our understanding of these processes.

The PKD-Dependent Biogenesis of TGN-to-Plasma Membrane Transport Carriers

Membrane trafficking is essential for processing and transport of proteins and lipids and to establish cell compartmentation and tissue organization. Cells respond to their needs and control the quantity and quality of protein secretion accordingly. In this review, we focus on a particular membrane trafficking route from the trans-Golgi network (TGN) to the cell surface: protein kinase D (PKD)-dependent pathway for constitutive secretion mediated by carriers of the TGN to the cell surface (CARTS). Recent findings highlight the importance of lipid signaling by organelle membrane contact sites (MCSs) in this pathway. Finally, we discuss our current understanding of multiple signaling pathways for membrane trafficking regulation mediated by PKD, G protein-coupled receptors (GPCRs), growth factors, metabolites, and mechanosensors.

PAM: diverse roles in neuroendocrine cells, cardiomyocytes, and green algae

Our understanding of the ways in which peptides are used for communication in the nervous and endocrine systems began with the identification of oxytocin, vasopressin, and insulin, each of which is stored in electron-dense granules, ready for release in response to an appropriate stimulus. For each of these peptides, entry of its newly synthesized precursor into the ER lumen is followed by transport through the secretory pathway, exposing the precursor to a sequence of environments and enzymes that produce the bioactive products stored in mature granules. A final step in the biosynthesis of many peptides is C-terminal amidation by peptidylglycine α-amidating monooxygenase (PAM), an ascorbate- and copper-dependent membrane enzyme that enters secretory granules along with its soluble substrates. Biochemical and cell biological studies elucidated the highly conserved mechanism for amidated peptide production and raised many questions about PAM trafficking and the effects of PAM on cytoskeletal organization and gene expression. Phylogenetic studies and the discovery of active PAM in the ciliary membranes of Chlamydomonas reinhardtii, a green alga lacking secretory granules, suggested that a PAM-like enzyme was present in the last eukaryotic common ancestor. While the catalytic features of human and C. reinhardtii PAM are strikingly similar, the trafficking of PAM in C. reinhardtii and neuroendocrine cells and secretion of its amidated products differ. A comparison of PAM function in neuroendocrine cells, atrial myocytes, and C. reinhardtii reveals multiple ways in which altered trafficking allows PAM to accomplish different tasks in different species and cell types.

The ABCs of the atypical Fam20 secretory pathway kinases

The study of extracellular phosphorylation was initiated in late 19th century when the secreted milk protein, casein, and egg-yolk protein, phosvitin, were shown to be phosphorylated. However, it took more than a century to identify Fam20C, which phosphorylates both casein and phosvitin under physiological conditions. This kinase, along with its family members Fam20A and Fam20B, defined a new family with altered amino acid sequences highly atypical from the canonical 540 kinases comprising the kinome. Fam20B is a glycan kinase that phosphorylates xylose residues and triggers peptidoglycan biosynthesis, a role conserved from sponges to human. The protein kinase, Fam20C, conserved from nematodes to humans, phosphorylates well over 100 substrates in the secretory pathway with overall functions postulated to encompass endoplasmic reticulum homeostasis, nutrition, cardiac function, coagulation, and biomineralization. The preferred phosphorylation motif of Fam20C is SxE/pS, and structural studies revealed that related member Fam20A allosterically activates Fam20C by forming a heterodimeric/tetrameric complex. Fam20A, a pseudokinase, is observed only in vertebrates. Loss-of-function genetic alterations in the Fam20 family lead to human diseases such as amelogenesis imperfecta, nephrocalcinosis, lethal and nonlethal forms of Raine syndrome with major skeletal defects, and altered phosphate homeostasis. Together, these three members of the Fam20 family modulate a diverse network of secretory pathway components playing crucial roles in health and disease. The overarching theme of this review is to highlight the progress that has been made in the emerging field of extracellular phosphorylation and the key roles secretory pathway kinases play in an ever-expanding number of cellular processes.