Proteoglycan synthesis and Golgi organization in polarized epithelial cells

Abstract

A large number of complex glycosylation mechanisms take place in the Golgi apparatus. In epithelial cells, glycosylated protein molecules are transported to both the apical and the basolateral surface domains. Although the prevailing view is that the Golgi apparatus provides the same lumenal environment for glycosylation of apical and basolateral cargo proteins, there are indications that proteoglycans destined for the two opposite epithelial surfaces are exposed to different conditions in transit through the Golgi apparatus. We will here review data relating proteoglycan and glycoprotein synthesis to characteristics of the apical and basolateral secretory pathways in epithelial cells.

Figure 1.

3′-Phosphoadenosine-5′-phosphosulfate transporter 1 (PAPST1) distribution in the Golgi apparatus of polarized cells. Madin-Darby canine kidney (MDCK II) cells expressing PAPST1–green fluorescent protein (GFP) were grown on polycarbonate filters for 4 days, fixed, and sectioned into 70-nm-thin sections in the cryomicrotome. Prior to examination by transmission electron microscope, the section was immunogold labeled using anti-GFP (Ab6556; Abcam, Cambridge, UK) and protein-A-Gold, followed by staining with uranyl acetate. Gold particles labeling PAPST1-GFP are observed at one side of the Golgi structure (black dots indicated by arrowheads). Scale bar = 500 nm.

Figure 2.

The Golgi apparatus in polarized epithelial cells segregates cargo from common origin in the endoplasmic reticulum (ER)/ERGIC (purple) into the apical (red) and basolateral (blue) pathways. The pathway-specific glycosylation of secreted cargo indicates a partitioning early in the Golgi, before completion of glycosaminoglycan (GAG) synthesis and modification. This implies a segregation of the Golgi itself into domains facilitating apical or basolateral glycosylation and modification. TGN, trans-Golgi network.