Different membrane‐bound forms of acetylcholinesterase are present at the cell surface of hepatocytes

Abstract

In the present study we have determinated the acetylcholinesterase molecular forms present in rat liver hepatocytes; we have also studied the association of acetylcholinesterase with the cell surface of the hepatocytes. Subcellular fractionation indicated that rough endoplasmic reticulum and plasma-membrane-enriched fractions contains G4 and G1 acetylcholinesterase forms bound to membranes. Hepatocytes incubated with phosphatidylinositol-specific phospholipase C released about 70% of the surface acetylcholinesterase. Sedimentation analysis showed that all the solubilized acetylcholinesterase activity comes exclusively from a G2 dimer. The G4 hydrophobic form of acetylcholinesterase accounts for the additional cell-surface activity. The existence of these two forms of acetylcholinesterase on the surface of hepatocytes was further established by analyzing the phosphatidylinositol-specific phospholipase C sensitivity of the acetylcholinesterase molecular forms present in isolated rat liver plasma membranes.Acetylcholinesterase is the enzyme responsible of the hydrolysis of the neurotransmitter acetylcholine [l, 21. The enzyme exists in multiple molecular forms which can be separated by velocity sedimentation. The acetylcholinesterase molecular forms can be classified as globular (monomers GI, dimers G1 and tetramers G4) and asymmetric forms (containing one, two or three tetramers plus a collagen-like tail: A4, As and AI2) [I -41. Asymmetric forms of acetylcholinesterase are associated with the extracellular matrix by heparan sulfate proteoglycans [5,6]. Globular acetylcholinesterases have been identified as both soluble and membrane-bound forms and some of them appear to have hydrophobic domains that attach the enzyme to the plasma membrane [3, 71. The G2 forms of acetylcholinesterase are apparently anchored to the membrane through a glycolipid moiety which contains phosphatidylinositol (PtdIns) covalently linked to the C-terminal [8 -1 I]; the G4 acetylcholinesterase form contains a non-catalytic hydrophobic 20-kDa polypeptide subunit [12]. Futerman et al. [I31 have communicated that a G2 dimer of acetylcholinesterase was present in rat liver homogenates. Here we show that isolated hepatocytes and plasma membrane fractions contain both GZ and G4 forms of acetylcholinesterase. In particular, the dimer was mainly localized in the cell surface from where it can be entirely solubilized by treatment with PtdIns-specific phospholipase C.Correspondence to E. Brandan, Molecular Neurobiology Unit,

Changes in liver and plasma acetylcholinesterase in rats with cirrhosis induced by bile duct ligation

“…6C-D. Similar to previous reports, we found that AChE was present in hepatocytes 5,32 and in the resident liver macrophages, the Kupffer cells. 4,8 Histochemical analysis of AChE activity confirmed both the pattern and the intensity of AChE staining (not shown).…”

“…Several authors have previously described the distribution of cholinesterases in the liver of rat, [3][4][5][6] mouse, 7 rabbit, 8,9 and chick. 10 However, no significant conclusion has been reached about the potential physiological role of AChE in the liver and whether liver is an important source for serum AChE.…”

Muscular dystrophy alters the processing of light acetylcholinesterase but not butyrylcholinesterase forms in liver ofLama2dy mice

“…As in mouse heart , liver AChE dimers and monomers possess GPI domains for membrane anchorage, and this coincides with previous observations made in rat hepatocytes (Perelman and Brandan, 1989;Perelman et al, 1990). The GPI residues are responsible for the amphiphilic properties of the light AChE forms.…”

“…As regards the physiological role of ChEs in liver, their identification in intestine epithelial cells (Sine et al, 1992;L'Hermite et al, 1996), meninges (Ummenhofer et al, 1998), bronchial cells (Taisne et al, 1997), and hepatocytes (Perelman and Brandan, 1989;Perelman et al, 1990) strongly suggests that these enzymes may function in epithelial cells as a hydrolytic filter for the withdrawal of blood-circulating acetylcholine (through AChE activity) and other aliphatic or aromatic esters (through BuChE activity). In addition, the occurrence of GPI-linked AChE dimers and monomers in mouse liver and rat hepatocytes (Perelman et al, 1990), the abundance of GPI-anchored proteins in caveolae-rich membrane domains (Masserini et al, 1999), and the identification of caveolin-1, the structural component of caveolae, on the surface membrane of hepatocytes and sinusoidal plasma membrane (Pol et al, 1999), raise the interesting possibility that AChE forms are concentrated in caveolae in the sinusoidal space.…”

Identification of a Novel Type of Alternatively Spliced Exon from the Acetylcholinesterase Gene of Bungarus fasciatus

“…Several lines of evidence indicate that the Bungarus AChE gene does not possess an H exon: (a) we could not find any GPI-anchored AChE dimers, which would be generated from AChE H subunits, in Bungarus tissues (this is particularly significant in the liver, because this organ is rich in GPI-anchored AChE in rat) (18,35,36); (b) the genomic region separating the last common exon from the T exon does not contain any sequence that might encode an H-peptide; and (c) COS cells transfected with a construct, AChE gT , in which this region was included produced only AChE T subunits, showing that it does not contain any alternative exon. In the case of human AChE, a similar construction led to the production of both H and T subunits (37).…”