What Is Golgi Homeostasis

In general, organelle acidity is driven by an ATP-mediated proton pump, V(vacuolar)-ATPase, which is balanced by anion influx or cation efflux, with protons leaking back to the cytoplasm through "H⁺ leak channels". Many other energy-consuming pumps and leak channels also exist in the Golgi membrane, which is required to maintain the balance of Cl^-, Ca²⁺, Mn²⁺, and K⁺. These channels include the Golgi pH regulator (GPHR, a chloride channel) and voltage-gated chloride channels ClC-3B in mammalian cells. The Golgi membrane also contains two different Na⁺/H⁺ exchangers (NHE7 and NHE8), of which NHE7 appears to mediate the influx of Na⁺ or K⁺ in exchange for H⁺. These transporters contribute to Golgi resting pH, membrane potential, vacuolar trafficking, and protein sorting in organelles.

The factors required to maintain the unique Golgi environment and the functions that depend on it are complex. Failure to maintain homeostasis can lead to organelle dysfunction and disease.

Fig.1 Golgi resident proteins and membrane trafficking pathway. (Liu, et al., 2021)

Golgi pH Homeostasis and Glycosylation Defects

• Membrane Trafficking and Protein Sorting Defects

Failure to maintain Golgi pH, ionic and redox homeostasis is often associated with defects in membrane trafficking and protein sorting. Monensin, a Na⁺/H⁺ ion cell, was the first compound to be shown to block intra-Golgi and trans-Golgi transport. However, monensin did not inhibit the intracellular transport of Uukuniemi virus membrane glycoproteins (G1 and G2).

The KDEL receptor is an example of a well-characterized pH-dependent protein sorting step that returns escaped ER-resident proteins from the cis-Golgi to the ER. This receptor is a key component of the homeostatic control system that regulates trafficking between the ER and the Golgi, as well as the Golgi itself. The receptor binds the peptide sequence KDEL, resulting in the interaction with two different Golgi-associated isomeric G proteins, which regulate the trafficking machinery through phosphorylation.

Another well-known example is the mannose-6-phosphate receptor, which binds mannose-6-P-tagged lysozymes in the Golgi apparatus and releases them in the low-pH environment of endosomes. In line with this, in some cancer cell lines with problematic lysosomal acidification, ligand-bound receptors fail to unload their ligands in lysosomes and instead accumulate in the endosomal/lysosomal compartment.

• Golgi pH Homeostasis and Glycosylation Defects

Glycosylation is probably the most pH-sensitive process in Golgi function. For example, monensin was shown to prevent the processing of the G protein of Uukuniemi virus into endo-H-resistant and under-sialylated species without affecting membrane trafficking. For the first time, the researchers provide a mechanistic link to these pH-induced changes in glycosylation by using prolonged NH₄Cl treatment in HeLa and LS 174T cells. Inhibition of O-glycan synthesis by NH₄Cl was accompanied by activation of N-acetylgalactosamine transferase 2, β-1,2-N-acetylglucosamine transferase I, and β-1,4-galactosamine transferase 1 mislocalization, while the drug had no effect on the morphology of the Golgi apparatus. However, since most of the enzymes that elongate O-glycans were not addressed in this study, it remains unclear whether enzyme repositioning is the sole cause of the observed glycosylation defects. Later, by using more and more chloroquine, it was demonstrated that as little as 0.2 units of pH increase in the lumen of the Golgi can interfere with mucin-type O-glycosylation and N-linked sugar terminal α-2,3-sialylation without causing any changes in the overall Golgi morphology. All in all, glycosylation in general is very sensitive to changes in the pH of the Golgi lumen and, if altered, may be due to the mislocalization of a selected group of glycosyltransferases. Other known causes are changes in the expression levels of enzymes, but these changes are not strictly related to the sugar profile displayed by the cells.

In conclusion, the maintenance (balance) of the Golgi function depends on the maintenance of specific pH and ion concentrations in the Golgi. The acidic pH of the Golgi apparatus is established and maintained by the ion transport system, which includes the vacuolar H⁺-ATPase (a complex enzyme that pumps protons across the membrane) and the transporter that transports Cl^-, Na⁺, and K⁺ ions across the membrane. Other ions such as Ca²⁺, Mg²⁺, and Mn²⁺ are important for the glycosylation of the Golgi apparatus and maintain high concentrations in the Golgi lumen through several ion channels and transporters. Defects in Golgi balance, such as abnormalities in pH and ion levels, are causative factors for some types of CDG8. Dysequilibrium in the Golgi can be caused by defects in ion transporters and vacuolar H⁺-ATPase complexes. Abnormal pH and ion levels reduce enzyme activity, leading to incorrect localization of glycated components.

Here we describe one of the disorders in the Disorders of Multiple Glycosylation and Other Pathways. CD BioGlyco provides comprehensive and deep insights into disorders of Golgi pH and ion homeostasis. We offer cutting-edge custom glycosylation services and glycan analysis services, including but not limited to Custom Glycosylation of Proteins, Custom Glycosylation of Antibodies, Custom Glycosylation of Cell Membranes, O-Glycan Profiling, and N-Glycan Profiling. If you are interested in our services, please contact us for more details without any hesitation.

Reference:

1. Liu, J.; et al. The role of the Golgi apparatus in disease. International Journal of Molecular Medicine. 2021, 47(4): 1-1.