Soluble amyloid-β peptides (sAβ), produced by neurons in the CNS, can find their way out of the brain and into the circulation, where peripheral organs clear them from the body. But in the brains of Alzheimer’s patients, a portion of the peptides will never make it out, ending up instead in amyloid fibrils or plaques (see recent ARF live discussion). Factors that determine the peptides' fate are poorly understood, even though they may hold precious clues to potential therapies. A trio of recent papers tries to fill in some of the gaps.
In the October 1 Journal of Neuroscience, researchers led by David Holtzman at the Washington University School of Medicine in St. Louis, Missouri, report that they have developed a microdialysis method that allows them to measure levels of interstitial sAβ in transgenic mouse models of AD. The interstitial fluid (ISF) is the first destination of sAβ released by the cell, and what happens to the peptides there may be key to amyloidogenesis.
First author John Cirrito and colleagues show that sAβ in the ISF is in equilibrium with Aβ trapped by fibrils, plaques, or protein complexes. They base this conclusion on the following evidence: First, the levels of ISF sAβ in young (three months) and old (12-15 months) mice is the same, suggesting that production and clearance rates are similar at all ages; second, when the researchers blocked sAβ production with a potent γ-secretase inhibitor, they found that ISF levels of the peptides dropped faster in the younger mice, suggesting that in older animals Aβ must be diffusing away from fibrils or plaques to replenish the ISF pool. “The increase in half-life, without an increase in steady-state levels, suggests that inhibition of Aβ synthesis reveals a portion of the insoluble Aβ pool that is in dynamic equilibrium with ISF Aβ,” the authors write. This explains previous observations by the same authors that Aβ can be sucked from plaques by antibodies in the circulation (see ARF related news story).
Meanwhile, in the October 6 Neuroscience, William Banks, John Morley, and colleagues at the nearby St. Louis University School of Medicine report that the ability to export Aβ from the brain changes as animals age. Using senescence-accelerated mice (SAM) as a model, Banks measured the efflux of radioactive Aβ after direct injection into the brain. He found that the rate of efflux of mouse Aβ1-42 was highest in normal young mice (two months old), but was reduced by about half in SAMs of the same age. Older SAMs (12 months) had even slightly lower rates. By also injecting unlabelled Aβ to compete with the radioactive form, Banks and colleagues show that the efflux of Aβ1-42 is saturable, indicating that some active process underlies the elimination of the peptide from the brain. The authors found that efflux of mouse Aβ1-40 is not saturable, indicating that it passively diffuses from the brain. Curiously, human forms of the peptide generated opposite results, where transport of the Aβ1-40 variant was saturated.
The authors found statistically different efflux rates in normal and SAM mice only in the case of actively transported Aβ peptides. This seems to indicate that the lower transport rate in SAM mice is due to age-related loss of a vital transportation mechanism.
Aβ may also be cleaved into smaller peptides before being transported from the brain. According to a second report in the October 1 Journal of Neuroscience by Sidney Strickland and colleagues at Rockefeller University in New York, tissue-type plasminogen activator (tPA) and plasmin may be crucial players in this potential clearance mechanism.
First author Jerry Melchor and colleagues found that in mouse models of AD, tPA activity is significantly reduced in the hippocampus and amygdala as compared to levels in wild-type mice. This, the authors show, correlates with increased levels of plasminogen activator inhibitor-1 (PAI-1), which can inactivate tPA.
To test the relationship between tPA/plasmin and Aβ clearance, the authors injected some Aβ into the brains of both normal mice and those lacking tPA or plasmin. In controls, the Aβ was cleared within three days, but in both tPA- and plasmin-negative mice analyzed at the same time, Aβ immunoreactivity was 10-fold higher. Furthermore, in the mutant mice, the Aβ injection provoked activation of microglia and a degeneration of neurons that were not observed in wild type animals.
This paper extends a recent report by this group implicating tPA and tPA blockers in AD pathogenesis and therapy (Melchor et al., 2003). Its experiments provide in-vivo support for earlier in-vitro experiments showing that plasmin cleaves Aβ (Tucker et al., 2000), and they follow a paper suggesting that plasmin is downregulated in AD brain (Ledesma et al., 2000). Plasmin cleavage of Aβ "may be important for slowing the progression of Alzheimer's disease," write the authors, but they caution that tPA has also been shown to be deleterious to neurons under conditions of excitotoxicity. Nevertheless, plasmin now appears ready to be added to the list of proteases that degrade Aβ in vivo (see ARF related news story).
Interestingly, the gene PLAU, which encodes the urokinase variant of plasminogen activator (uPA), is among the candidate risk factor genes for AD currently pursued by several labs. Steve Younkin’s group reported initial data on PLAU last year at a conference in Stockholm and again this year (see Younkin section in ARF conference report), while German and Swiss researchers published similar data this summer in Neurogenetics (Finckh et al., 2003).—Tom Fagan
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- Melchor JP, Pawlak R, Chen Z, Strickland S. The possible role of tissue-type plasminogen activator (tPA) and tPA blockers in the pathogenesis and treatment of Alzheimer's disease. J Mol Neurosci. 2003;20(3):287-9. PubMed.
- Tucker HM, Kihiko M, Caldwell JN, Wright S, Kawarabayashi T, Price D, Walker D, Scheff S, McGillis JP, Rydel RE, Estus S. The plasmin system is induced by and degrades amyloid-beta aggregates. J Neurosci. 2000 Jun 1;20(11):3937-46. PubMed.
- Ledesma MD, Da Silva JS, Crassaerts K, Delacourte A, De Strooper B, Dotti CG. Brain plasmin enhances APP alpha-cleavage and Abeta degradation and is reduced in Alzheimer's disease brains. EMBO Rep. 2000 Dec;1(6):530-5. PubMed.
- Finckh U, van Hadeln K, Müller-Thomsen T, Alberici A, Binetti G, Hock C, Nitsch RM, Stoppe G, Reiss J, Gal A. Association of late-onset Alzheimer disease with a genotype of PLAU, the gene encoding urokinase-type plasminogen activator on chromosome 10q22.2. Neurogenetics. 2003 Aug;4(4):213-7. PubMed.
- Banks WA, Robinson SM, Verma S, Morley JE. Efflux of human and mouse amyloid beta proteins 1-40 and 1-42 from brain: impairment in a mouse model of Alzheimer's disease. Neuroscience. 2003;121(2):487-92. PubMed.
- Melchor JP, Pawlak R, Strickland S. The tissue plasminogen activator-plasminogen proteolytic cascade accelerates amyloid-beta (Abeta) degradation and inhibits Abeta-induced neurodegeneration. J Neurosci. 2003 Oct 1;23(26):8867-71. PubMed.
- Cirrito JR, May PC, O'Dell MA, Taylor JW, Parsadanian M, Cramer JW, Audia JE, Nissen JS, Bales KR, Paul SM, Demattos RB, Holtzman DM. In vivo assessment of brain interstitial fluid with microdialysis reveals plaque-associated changes in amyloid-beta metabolism and half-life. J Neurosci. 2003 Oct 1;23(26):8844-53. PubMed.