The mechanism of proteoglycan membrane anchorage affects the endocytic pathway of cellular cargo

Abstract

Endocytosis is the process by which the cell is able to take up materials from the extracellular space into the cytosol by enclosing it within its cell membrane. Mechanistically, this is often accomplished by a receptor mediated pathway, which is often controlled by surface molecules known as heparin sulfate proteoglycans (HSPGs). Study of the control of these pathways is therefore of keen interest for the pharmaceutical industry because endocytosis may be used for drug and gene delivery vectors as well as to prevent viruses and other toxins from entering the cell. These proteoglycans may be bound to the cell membrane using a transmembrane core protein such as syndecans or through the use of glycosylphosphatidylinositol (GPI) anchors such as glypicans. It has been proposed that the endocytic pathway followed will vary depending on whether the proteoglycan is membrane bound via a GPI-anchor or using a transmembrane core protein.

The HSPGs have a typical half-life on the surface of the cell membrane ranging from 3-8 hours and afterwards the molecule is either endocytosed or is shed into the extracellular space. If the HSPG is bound with a GPI-anchor the endocytosis will lead immediately to the lysosome for degradation. This pathway is in stark contrast to the HSPGs employing a transmembrane core protein, which undergoes several degradations within prelysosomal compartments steps prior to its final digestion within the lysosome. Immediately after endocytosis the core protein is broken down to its constituent HS chains which are cleaved by endoglycosidases followed by further digestion by endoglycosidases which occurs in a distinct compartment from the previous step where it is finally transferred to the lysosome. The aim of this research will involve observation of the endocytosis of GPI-linked HSPGs to confirm the proposed mechanisms and furthermore to remove the GPI linkage using phospholipases to determine if it is possible to direct cargo into the pathway followed by transmembrane linked HSPGs.