Variegated transfer of recombinant glycosylphosphatidylinositol-anchored CD4 among cultured cells: correlation of flow cytometric and microscopic observations

(Patho)Physiology of Glycosylphosphatidylinositol-Anchored Proteins II: Intercellular Transfer of Matter (Inheritance?) That Matters

Glycosylphosphatidylinositol (GPI)-anchored proteins (APs) are anchored at the outer leaflet of the plasma membrane (PM) bilayer by covalent linkage to a typical glycolipid and expressed in all eukaryotic organisms so far studied. Lipolytic release from PMs into extracellular compartments and intercellular transfer are regarded as the main (patho)physiological roles exerted by GPI-APs. The intercellular transfer of GPI-APs relies on the complete GPI anchor and is mediated by extracellular vesicles such as microvesicles and exosomes and lipid-free homo- or heteromeric aggregates, and lipoprotein-like particles such as prostasomes and surfactant-like particles, or lipid-containing micelle-like complexes. In mammalian organisms, non-vesicular transfer is controlled by the distance between donor and acceptor cells/tissues; intrinsic conditions such as age, metabolic state, and stress; extrinsic factors such as GPI-binding proteins; hormones such as insulin; and drugs such as anti-diabetic sulfonylureas. It proceeds either "directly" upon close neighborhood or contact of donor and acceptor cells or "indirectly" as a consequence of the induced lipolytic release of GPI-APs from PMs. Those displace from the serum GPI-binding proteins GPI-APs, which have retained the complete anchor, and become assembled in aggregates or micelle-like complexes. Importantly, intercellular transfer of GPI-APs has been shown to induce specific phenotypes such as stimulation of lipid and glycogen synthesis, in cultured human adipocytes, blood cells, and induced pluripotent stem cells. As a consequence, intercellular transfer of GPI-APs should be regarded as non-genetic inheritance of (acquired) features between somatic cells which is based on the biogenesis and transmission of matter such as GPI-APs and "membrane landscapes", rather than the replication and transmission of information such as DNA. Its operation in mammalian organisms remains to be clarified.

Intercellular transfer of the cellular prion protein

The cellular prion protein (PrP(C)) is a glycosylphosphatidylinositol (GPI)-anchored protein. We investigated whether PrP(C) can move from one cell to another cell in a cell model. Little PrP(C) transfer was detected when a PrP(C) expressing human neuroblastoma cell line was cultured with the human erythroleukemia cells IA lacking PrP(C). Efficient transfer of PrP(C) was detected with the presence of phorbol 12-myristate 13-acetate, an activator of protein kinase C. Maximum PrP(C) transfer was observed when both donor and recipient cells were activated. Furthermore, PrP(C) transfer required the GPI anchor and direct cell to cell contact. However, intercellular protein transfer is not limited to PrP(C), another GPI-anchored protein, CD90, also transfers from the donor cells to acceptor cells after cellular activation. Therefore, this transfer process is GPI-anchor and cellular activation dependent. These findings suggest that the intercellular transfer of GPI-anchored proteins is a regulated process, and may have implications for the pathogenesis of prion disease.

Monocytes confer CD14 antigenicity on activated lymphocytes

In the present study, exposure of human peripheral blood mononuclear cells (PBMC) to phorbol 12-myristate 13-acetate (PMA) was found to elicit the expression of CD14 on lymphocytes. Less than 3% of the lymphocytes present among freshly isolated PBMC were stained with 63D3 anti-CD14 monoclonal antibody (mAb). Within two days of exposure of PBMC to PMA, up to 30% of the lymphocytes reacted with the 63D3 anti-CD14 mAb, though not with the LeuM3 and My4 anti-CD14 mAbs. The appearance of CD14 on lymphocytes was also elicited by exposure of PBMC to phytohemagglutinin (PHA), concanavalin A (Con A), or agarose-bound phytohemagglutinin but not by exposure to lipopolysaccharide, interferon-alpha, or interleukin-2. Purified lymphocyte preparations did not acquire CD14 following stimulation with PMA. Monocytes lost their reactivity with CD14 mAbs (63D3, LeuM3, and My4) within a few hours after exposure to PMA. The level of soluble CD14 was higher in supernatant fluids of cultures of untreated PBMC than of PMA-stimulated PBMC. The addition of PMA to cultures of T cells and monocytes separated by Millipore filters lead to the expression of CD14 on the lymphocytes. The present study indicates that activation of lymphocytes in the presence of monocytes leads to the appearance of CD14 on lymphocytes, and raises the possibility that the expression of CD14 on lymphocytes may result from the transfer of CD14 molecules from monocytes to lymphocytes.