TMEM165 a new player in proteoglycan synthesis: loss of TMEM165 impairs elongation of chondroitin- and heparan-sulfate glycosaminoglycan chains of proteoglycans and triggers early chondrocyte differentiation and hypertrophy

Insights into molecular and cellular functions of the Golgi calcium/manganese-proton antiporter TMEM165

The Golgi compartment performs a number of crucial roles in the cell. However, the exact molecular mechanisms underlying these actions are not fully defined. Pathogenic mutations in genes encoding Golgi proteins may serve as an important source for expanding our knowledge. For instance, mutations in the gene encoding Transmembrane protein 165 (TMEM165) were discovered as a cause of a new type of congenital disorder of glycosylation (CDG). Comprehensive studies of TMEM165 in different model systems, including mammals, yeast, and fish uncovered the new realm of Mn2+ homeostasis regulation. TMEM165 was shown to act as a Ca2+/Mn2+:H+ antiporter in the medial- and trans-Golgi network, pumping the metal ions into the Golgi lumen and protons outside. Disruption of TMEM165 antiporter activity results in defects in N- and O-glycosylation of proteins and glycosylation of lipids. Impaired glycosylation of TMEM165-CDG arises from a lack of Mn2+ within the Golgi. Nevertheless, Mn2+ insufficiency in the Golgi is compensated by the activity of the ATPase SERCA2. TMEM165 turnover has also been found to be regulated by Mn2+ cytosolic concentration. Besides causing CDG, recent investigations have demonstrated the functional involvement of TMEM165 in several other pathologies including cancer and mental health disorders. This systematic review summarizes the available information on TMEM165 molecular structure, cellular function, and its roles in health and disease.

Porcine nasal septum cartilage-derived decellularized matrix promotes chondrogenic differentiation of human umbilical mesenchymal stem cells without exogenous growth factors

BACKGROUND

In the domain of plastic surgery, nasal cartilage regeneration is of significant importance. The extracellular matrix (ECM) from porcine nasal septum cartilage has shown potential for promoting human cartilage regeneration. Nonetheless, the specific biological inductive factors and their pathways in cartilage tissue engineering remain undefined.

METHODS

The decellularized matrix derived from porcine nasal septum cartilage (PN-DCM) was prepared using a grinding method. Human umbilical cord mesenchymal stem cells (HuMSCs) were cultured on these PN-DCM scaffolds for 4 weeks without exogenous growth factors to evaluate their chondroinductive potential. Subsequently, proteomic analysis was employed to identify potential biological inductive factors within the PN-DCM scaffolds.

RESULTS

Compared to the TGF-β3-cultured pellet model serving as a positive control, the PN-DCM scaffolds promoted significant deposition of a Safranin-O positive matrix and Type II collagen by HuMSCs. Gene expression profiling revealed upregulation of ACAN, COL2A1, and SOX9. Proteomic analysis identified potential chondroinductive factors in the PN-DCM scaffolds, including CYTL1, CTGF, MGP, ITGB1, BMP7, and GDF5, which influence HuMSC differentiation.

CONCLUSION

Our findings have demonstrated that the PN-DCM scaffolds promoted HuMSC differentiation towards a nasal chondrocyte phenotype without the supplementation of exogenous growth factors. This outcome is associated with the chondroinductive factors present within the PN-DCM scaffolds.

A mechanism of lysosomal calcium entry

Lysosomal calcium (Ca2+) release is critical to cell signaling and is mediated by well-known lysosomal Ca2+ channels. Yet, how lysosomes refill their Ca2+ remains hitherto undescribed. Here, from an RNA interference screen in Caenorhabditis elegans, we identify an evolutionarily conserved gene, lci-1, that facilitates lysosomal Ca2+ entry in C. elegans and mammalian cells. We found that its human homolog TMEM165, previously designated as a Ca2+/H+ exchanger, imports Ca2+ pH dependently into lysosomes. Using two-ion mapping and electrophysiology, we show that TMEM165, hereafter referred to as human LCI, acts as a proton-activated, lysosomal Ca2+ importer. Defects in lysosomal Ca2+ channels cause several neurodegenerative diseases, and knowledge of lysosomal Ca2+ importers may provide previously unidentified avenues to explore the physiology of Ca2+ channels.

Lifting the veils on transmembrane proteins: Potential anticancer targets

Cancer, as a prevalent cause of mortality, poses a substantial global health burden and hinders efforts to enhance life expectancy. Nevertheless, the prognosis of patients with malignant tumors remains discouraging, owing to the lack of specific diagnostic and therapeutic targets. Therefore, the development of early diagnostic indicators and novel therapeutic drugs for the prevention and treatment of cancer is essential. Transmembrane proteins (TMEMs) are a class of proteins that can span the phospholipid bilayer and are stably anchored. They are associated with fibrotic diseases, neurodegenerative diseases, autoimmune diseases, developmental disorders, and cancer. It has been found that the expression levels of TMEMs were elevated or reduced in cancer cells, exerting pro/anticancer effects. These aberrant expression levels have also been linked to the prognostic and clinicopathological features of diverse tumors. In this review, the structures, functions, and roles of TMEMs in cancer were discussed, and the scientific perspectives were described. This review also explored the potential of TMEMs as tumor drug candidates from the perspective of targeted therapies, and the challenges that need to be overcome in a wide range of preclinical and clinical anticancer research were summarized.

Glycobiology in osteoclast differentiation and function

Glycans, either alone or in complex with glycan-binding proteins, are essential structures that can regulate cell biology by mediating protein stability or receptor dimerization under physiological and pathological conditions. Certain glycans are ligands for lectins, which are carbohydrate-specific receptors. Bone is a complex tissue that provides mechanical support for muscles and joints, and the regulation of bone mass in mammals is governed by complex interplay between bone-forming cells, called osteoblasts, and bone-resorbing cells, called osteoclasts. Bone erosion occurs when bone resorption notably exceeds bone formation. Osteoclasts may be activated during cancer, leading to a range of symptoms, including bone pain, fracture, and spinal cord compression. Our understanding of the role of protein glycosylation in cells and tissues involved in osteoclastogenesis suggests that glycosylation-based treatments can be used in the management of diseases. The aims of this review are to clarify the process of bone resorption and investigate the signaling pathways mediated by glycosylation and their roles in osteoclast biology. Moreover, we aim to outline how the lessons learned about these approaches are paving the way for future glycobiology-focused therapeutics.

Metalloglycobiology: The power of metals in regulating glycosylation

The remarkable structural diversity of glycans that is exposed at the cell surface and generated along the secretory pathway is tightly regulated by several factors. The recent identification of human glycosylation diseases related to metal transporter defects opened a completely new field of investigation, referred to herein as "metalloglycobiology", on how metal changes can affect the glycosylation and hence the glycan structures that are produced. Although this field is in its infancy, this review aims to go through the different glycosylation steps/pathways that are metal dependent and that could be impacted by metal homeostasis dysregulations.

Insights into the regulation of cellular Mn2+ homeostasis via TMEM165

Golgi cation homeostasis is known to be crucial for many cellular processes including vesicular fusion events, protein secretion, as well as for the activity of Golgi glycosyltransferases and glycosidases. TMEM165 was identified in 2012 as the first cation transporter related to human glycosylation diseases, namely the Congenital Disorders of Glycosylation (CDG). Interestingly, divalent manganese (Mn) supplementation has been shown to suppress the observed glycosylation defects in TMEM165-deficient cell lines, thus suggesting that TMEM165 is involved in cellular Mn homeostasis. This paper demonstrates that the origin of the Golgi glycosylation defects arises from impaired Golgi Mn homeostasis in TMEM165-depleted cells. We show that Mn supplementation fully rescues the Mn content in the secretory pathway/organelles of TMEM165-depleted cells and hence the glycosylation process. Strong cytosolic and organellar Mn accumulations can also be observed in TMEM165- and SPCA1-depleted cells upon incubation with increasing Mn concentrations, thus demonstrating the crucial involvement of these two proteins in cellular Mn homeostasis. Interestingly, our results show that the cellular Mn homeostasis maintenance in control cells is correlated with the presence of TMEM165 and that the Mn-detoxifying capacities of cells, through the activity of SPCA1, rely on the Mn-induced degradation mechanism of TMEM165. Finally, this paper highlights that TMEM165 is essential in secretory pathway/organelles Mn homeostasis maintenance to ensure both Golgi glycosylation enzyme activities and cytosolic Mn detoxification.

New insights into the pathogenicity of TMEM165 variants using structural modeling based on AlphaFold 2 predictions

TMEM165 is a Golgi protein playing a crucial role in Mn²⁺ transport, and whose mutations in patients are known to cause Congenital Disorders of Glycosylation. Some of those mutations affect the highly-conserved consensus motifs E-φ-G-D-[KR]-[TS] characterizing the CaCA2/UPF0016 family, presumably important for the transport of Mn²⁺ which is essential for the function of many Golgi glycosylation enzymes. Others, like the G>R³⁰⁴ mutation, are far away from these motifs in the sequence. Until recently, the classical membrane protein topology prediction methods were unable to provide a clear picture of the organization of TMEM165 inside the cell membrane, or to explain in a convincing manner the impact of patient and experimentally-generated mutations on the transporter function of TMEM165. In this study, AlphaFold 2 was used to build a TMEM165 model that was then refined by molecular dynamics simulation with membrane lipids and water. This model provides a realistic picture of the 3D protein scaffold formed from a two-fold repeat of three transmembrane helices/domains where the consensus motifs face each other to form a putative acidic cation-binding site at the cytosolic side of the protein. It sheds new light on the impact of mutations on the transporter function of TMEM165, found in patients and studied experimentally in vitro, formerly and within this study. More particularly and very interestingly, this model explains the impact of the G>R³⁰⁴ mutation on TMEM165's function. These findings provide great confidence in the predicted TMEM165 model whose structural features are discussed in the study and compared to other structural and functional TMEM165 homologs from the CaCA2/UPF0016 family and the LysE superfamily.

New insights into the role of the Golgi apparatus in the pathogenesis and therapeutics of human diseases

The Golgi apparatus is an essential cellular organelle that mediates homeostatic functions, including vesicle trafficking and the post-translational modification of macromolecules. Its unique stacked structure and dynamic functions are tightly regulated, and several Golgi proteins play key roles in the functioning of unconventional protein secretory pathways triggered by cellular stress responses. Recently, an increasing number of studies have implicated defects in Golgi functioning in human diseases such as cancer, neurodegenerative, and immunological disorders. Understanding the extraordinary characteristics of Golgi proteins is important for elucidating its associated intracellular signaling mechanisms and has important ramifications for human health. Therefore, analyzing the mechanisms by which the Golgi participates in disease pathogenesis may be useful for developing novel therapeutic strategies. This review articulates the structural features and abnormalities of the Golgi apparatus reported in various diseases and the suspected mechanisms underlying the Golgi-associated pathologies. Furthermore, we review the potential therapeutic strategies based on Golgi function.

Differential Effects of D-Galactose Supplementation on Golgi Glycosylation Defects in TMEM165 Deficiency

Glycosylation is a ubiquitous and universal cellular process in all domains of life. In eukaryotes, many glycosylation pathways occur simultaneously onto proteins and lipids for generating a complex diversity of glycan structures. In humans, severe genetic diseases called Congenital Disorders of Glycosylation (CDG), resulting from glycosylation defects, demonstrate the functional relevance of these processes. No real cure exists so far, but oral administration of specific monosaccharides to bypass the metabolic defects has been used in few CDG, then constituting the simplest and safest treatments. Oral D-Galactose (Gal) therapy was seen as a promising tailored treatment for specific CDG and peculiarly for TMEM165-CDG patients. TMEM165 deficiency not only affects the N-glycosylation process but all the other Golgi-related glycosylation types, then contributing to the singularity of this defect. Our previous results established a link between TMEM165 deficiency and altered Golgi manganese (Mn²⁺) homeostasis. Besides the fascinating power of MnCl₂ supplementation to rescue N-glycosylation in TMEM165-deficient cells, D-Gal supplementation has also been shown to be promising in suppressing the observed N-glycosylation defects. Its effect on the other Golgi glycosylation types, most especially O-glycosylation and glycosaminoglycan (GAG) synthesis, was however unknown. In the present study, we demonstrate the differential impact of D-Gal or MnCl₂ supplementation effects on the Golgi glycosylation defects caused by TMEM165 deficiency. Whereas MnCl₂ supplementation unambiguously fully rescues the N- and O-linked as well as GAG glycosylations in TMEM165-deficient cells, D-Gal supplementation only rescues the N-linked glycosylation, without any effects on the other Golgi-related glycosylation types. According to these results, we would recommend the use of MnCl₂ for TMEM165-CDG therapy.

Spatiotemporal diversity and regulation of glycosaminoglycans in cell homeostasis and human disease

Glycosaminoglycans (GAGs) are long, linear polysaccharides that are ubiquitously expressed on the cell surface and in the extracellular matrix of all animal cells. These complex carbohydrates play important roles in many cellular processes and have been implicated in many disease states, including cancer, inflammation, and genetic disorders. GAGs are among the most complex molecules in biology with enormous information content and extensive structural and functional heterogeneity. GAG biosynthesis is a nontemplate-driven process facilitated by a large group of biosynthetic enzymes that have been extensively characterized over the past few decades. Interestingly, the expression of the enzymes and the consequent structure and function of the polysaccharide chains can vary temporally and spatially during development and under certain pathophysiological conditions, suggesting their assembly is tightly regulated in cells. Due to their many key roles in cell homeostasis and disease, there is much interest in targeting the assembly and function of GAGs as a therapeutic approach. Recent advances in genomics and GAG analytical techniques have pushed the field and generated new perspectives on the regulation of mammalian glycosylation. This review highlights the spatiotemporal diversity of GAGs and the mechanisms guiding their assembly and function in human biology and disease.

GALNT8 suppresses breast cancer cell metastasis potential by regulating EGFR O-GalNAcylation

Breast cancer represents the most lethal malignancy that threatens the health of females. Metastasis is the fatal hallmark of breast cancer, and current effective therapeutic targets of metastasis are still lacking. Aberrant O-GalNAcylation, which is attributed to alteration of polypeptide N-acetylgalactosaminyl transferases (GALNTs), has been implicated in cancer metastasis. However, GALNTs that drive metastasis in breast cancer and their underlying mechanisms are largely unclear. In the present study, a negative correlation between GALNT8 and the prognosis of breast cancer patients was observed in multiple groups of Gene Expression Omnibus (GEO) datasets. We then constructed a stable GALNT8 knockdown MCF7 cell line and performed transcriptome analysis using RNA sequencing, which revealed that the expression of multiple migration-related genes was changed. GALNT8 was identified as a regulator of epithelial-mesenchymal transition (EMT) markers, including E-cadherin, N-cadherin, ZO-1 and vimentin. Moreover, loss- and gain-of-function GALNT8 assays demonstrated that this glycosyltransferase inhibited the metastatic potential of breast cancer cells. Interestingly, the O-GalNAcylation of EGFR, which is the key factor related to the metastasis cascade, was impacted by GALNT8. Furthermore, our results suggested that the GALNT8-mediated O-GalNAcylation led to the suppression of the EGFR signaling pathway and metastatic potential in breast cancer cells. These results suggested that GALNT8 acts as a tumor suppressor, represses tumor metastasis and inhibits the EMT process through the EGFR signaling pathway. This finding may provide insight into the mechanism by which aberrant O-glycosylation modulates breast cancer metastasis.