Apolipoprotein (apo) E is likely involved in redistributing cholesterol and phospholipids during compensatorysynaptogenesis in the injured CNS. Three common isoforms of apoE exist in human (E2, E3, and E4). The apoE4 allelefrequency is markedly increased in both late-onset sporadic and familial Alzheimer's disease (AD). ApoE concentration inthe brain of AD subjects follows a gradient: ApoE levels decrease as a function of E2 , E3, E4.
It has been proposed thatthe poor reinnervation capacity reported in AD may be caused by impairment of the apoE/low-density lipoprotein (LDL) receptor activity. To understand further the role of this particular axis in lipid homeostasis in the CNS, we havecharacterized binding, internalization, and degradation of human 125I-LDL to primary cultures of rat astrocytes.
ApoE2 binding was significantly lower than that of the other 125I-apoE isoforms in both cell types. 125I-ApoE4binding was similar to that of 125I-apoE3 in both astrocytes and neurons. On the other hand, 125I-apoE3 binding wassignificantly higher in neurons than in astrocytes. These isoform-specific alterations in apoE-lipoprotein pathway couldexplain some of the differences reported in the pathophysiology of AD subjects carrying different apoE alleles.
An important feature of Alzheimer's disease (AD) is the cerebral deposition of amyloid. The main component of the amyloidis a 39-44-amino acid residue protein called amyloid beta (A beta), which also exists as a normal protein in biological fluids, known as soluble A beta. A major risk factor for late-onset AD is the inheritance of the apolipoprotein (apo) E4 isotype of apoE. How apoE is involved in the pathogenesis of AD is unclear; however, evidence exists for a direct apoE/Abeta interaction.
ApoE copurifies with A beta from AD amyloid plaques and that under certain in vitro conditions apoE promotes a beta-sheet structure in A beta peptides. Currently we document the high affinitybinding of A beta peptides to both human recombinant apoE3 and -E4 with a KD of 20 nM. This interaction is greatly influenced by the conformational state of the A beta peptide used. Furthermore, we show that the fibril modulating effect of apoE is also influenced by the initial secondary structure of the A beta peptide. The preferential binding of apoE to A beta peptides with a beta-sheet conformation can in part explain the copurification of A beta and apoE from AD amyloid plaques.
Prion-related encephalopathies are characterized by the accumulation of an abnormal prion protein isoform (PrPSc) associated whith neuronal degeneration and astrogliosis. The synthetic peptide homologous to PrP fragment 106-126 (PrP 106-126) induced in vitro neuronal apoptosis and glial proliferation. Since clusterin (apolipoprotein J) mRNA levels are increased in prion-related encephalopathies and clusterin immunoreactivity has been located in association with PrPSc in Gerstmann-Straussler-Scheinker brain, the expression of clusterin was determined in neuronal and astroglial cells chronically exposed to PrP 106-126. Although the induction of clusterin has been involved in the apoptotic mechanism in other experimental conditions, its expression was unchanged in PrP 106-126-treated neurons, while a three-fold induction of clusterin mRNA was observed in astrocytes exposed to PrP 106-126. To investigate whether the clusterin up-regulation was simply associated with the astroglial proliferative stimulus of PrP 106-126 or was specifically induced by the peptide, we measured clusterin expression in astrocytes cultured in fetal calf serum-free medium and exposed to PrP 106-126 or fetal calf serum restoration. In this condition the PrP peptide, like fetal calf serum, increased the glial proliferation rate, but only PrP 106-126 doubled clusterin mRNA. The selectivity of this effect indicates that PrPSc is directly involved in the clusterin up-regulation seen in prion-related encephalopathies and is associated with astroglial cells.