Apolipoprotein (apo) E4 increases the risk and accelerates the onset of Alzheimer's disease (AD).
However, the underlying mechanisms remain to be determined.
ApoE undergoes proteolytic cleavage in AD brains and in cultured neuronal cells, resulting in the accumulation of carboxyl-terminal truncated fragments of apoE that are neurotoxic.
Human apolipoprotein (apo) E, a 34-kDa protein composed of 299 amino acids, occurs as three major isoforms, apoE2, apoE3, and apoE4. ApoE4 is a major risk or susceptibility factor for Alzheimer's disease (AD).
The apoE4 allele, which is found in 40-65% of cases of sporadic and familial AD, increases the occurrence and lowers the age of onset of the disease.
Biochemical, cell biological, and transgenic animal studies have suggested several potential mechanisms to explain apoE4's contribution to the pathogenesis of AD.
These include the modulation of the deposition and clearance of amyloid β peptides and the formation of plaques, impairment of the antioxidative defense system, dysregulation of neuronal signaling pathways, disruption of cytoskeletal structure and function, and altered phosphorylation of tau and the formation of neurofibrillary tangles (NFTs).
However, the mechanisms of these apoE4-mediated detrimental effects are still largely unknown, and it is not known which are the primary effects and which are subsequent or downstream effects.
The neuropathological hallmarks of AD include extracellular amyloid plaques and intracellular NFTs in the brain.
The plaques consist primarily of amyloid β peptides.
The NFTs are composed largely of the highly phosphorylated microtubule-associated protein tau (p-tau) and, to a lesser extent, of phosphorylated neurofilaments.
Both amyloid plaques and NFTs contain apoE; however, the role of apoE in the pathogenesis of these two lesions is uncertain.
Histopathological and behavioral analyses of transgenic mice expressing different human apoE isoforms in the brain have revealed clear evidence for a dominant adverse effect of apoE4, but the underlying mechanism is unknown.
ApoE is subject to proteolytic cleavage, resulting in bioactive carboxyl-terminal-truncated fragments.
These apoE fragments are generated inside cultured neurons and in AD brains and can interact with p-tau and phosphorylated neurofilaments of high molecular weight, resulting in large, filamentous intracellular inclusions in neuronal cells.
Generation of these toxic apoE fragments is one of the early events in the pathogenesis of AD.
ApoE4 is more susceptible than apoE3 to proteolytic cleavage by a chymotrypsin-like serine protease and that the resulting bioactive carboxyl-terminal truncated fragments induce AD-like neuropathology and behavioral deficits in transgenic mice.
It is tempting to speculate that the neuronal alterations elicited by the expression of the truncated apoE4 in transgenic mice relate to neuronal alterations observed in AD and in transgenic mice expressing full-length apoE4 in neurons.
Neurodegeneration was not caused simply by expressing a truncated form of apoE4 but specifically by expressing apoE4 (D272-299), which contains the lipid-binding region (amino acids 244-272) (Fig. 1).
Fig. 1. Neurodegeneration in the brains of hemizygous transgenic mice expressing apoE4 (D272-299) in neurons. Brain sections from 2- to 4-month-old transgenic mice (C57BLy6J background) expressing high levels of apoE4 (D272-299) (A-E) or apoE4(D241-299) (F and G) at comparable levels were stained with anti-apoE (A-C and F) or hematoxylin and eosin (D, E, and G). Nontransgenic wild-type (Wt) mice (D and E) served as additional controls. Note the formation of truncated apoE4-containing inclusions in cortical (A), CA1 (B), and CA3 (C) neurons and degeneration of neurons in CA1 (D Upper) and CA3 (E Upper) in apoE4(D272-299) mice. Original magnifications:A, C, and F,3600; B, D, E, and G, 3400.
The importance of this region of apoE in neurotoxicity is consistent with previous in vitro observations in which the lipid-binding region of apoE is toxic to cultured neuronal cells.
The amino-terminal 22-kDa thrombin-cleavage fragment (amino acids 1-191) of apoE4 is also neurotoxic in vitro, but this toxicity appears to require relatively high concentrations of the fragment and has not yet been confirmed in vivo.
The apoE fragments generated in AD brains are different from the 22-kDa thrombin-cleavage products, which lack the lipid-binding domain (amino acids 244-272).
Indeed, the lipid-binding domain is present in all of the major apoE fragments generated in AD brains and appears to be essential for apoE fragments to have neurotoxic effects in vivo.
Interestingly, the lipid-binding domain of apoE is also responsible for interaction of apoE with amyloid β peptides.
Thus, it is reasonable to speculate that the apoE fragments released from neurons might also interact with amyloid β and act synergistically to induce neuronal deficits.
Initially, apoE was thought to be synthesized in the brain by astrocytes, oligodendrocytes, activated microglia, and ependymal layer cells, but not by neurons.
However, numerous subsequent studies have demonstrated that CNS neurons can also express apoE, albeit at lower levels than astrocytes.
Both apoE protein and mRNA are found in cortical and hippocampal neurons in humans and in transgenic mice expressing human apoE under the control of the human apoE promoter.
In rats treated with kainic acid, apoE expression is induced in hippocampal neurons that survive excitotoxic stress, as determined by both in situ hybridization and anti-apoE immunohistochemistry.
Furthermore, expression of neuronal apoE can be induced in human brains after cerebral infarction.
ApoE is also expressed in primary cultured human CNS neurons and in many human neuronal cell lines.
CNS injury may induce neuronal expression of apoE to participate in neuronal repair or remodeling or to protect neurons from injury.
However, in neurons expressing apoE4, the proteolytic processing and fragment generation may turn a neuroprotective response into a pathogenic process.
Thus the carboxylterminal-truncated fragments of apoE4 elicit AD-like neurodegeneration and behavioral deficits in transgenic mice.
We hypothesize that apoE4 produced in neurons under diverse pathological conditions is cleaved by chymotrypsin-like serine protease.
The resulting bioactive carboxyl-terminal-truncated fragments, probably together with other AD-related factors (e.g., amyloid β peptides), induce neuropathology and behavioral deficits characteristic of AD.
Consequently, chymotrypsin-like serine protease represents a potential new target for drugs aimed at inhibiting the detrimental effects of apoE4 in AD and other neurological diseases.