Bergamaschini, from the University of Milan, together with Maria Grazia De Simoni, from the Mario Negri Institute for Pharmacological Research, also in Milan, administered the low molecular weight heparin enoxaparin to APP23 mice, which overexpress human amyloid precursor protein. The authors chose enoxaparin because of the high risk of bleeding associated with unfractionated heparin. In addition, heparins are known to bind to a short amino acid stretch in amyloid β (residues 13-16).

Bergamaschini treated 12-month-old APP23 mice with six international units of enoxaparin three times a week for six months. When he examined these animals using a monoclonal antibody to amino acids 17-24 of human Aβ, he found that they had much fewer and smaller amyloid deposits in their neocortex than did untreated mice. The data showed about a threefold reduction in plaque load (number and surface area). Quantification of extracted Aβ1-40 revealed a similar reduction, from an average of 45 pg/mg of Aβ1-40 in saline-treated animals, to 20 pg/mg in animals treated with enoxaparin. This reduction in plaque load correlated with a reduction in the number of activated astrocytes, as judged by morphology, size, and number of GFAP-positive cells surrounding the Aβ deposits. The authors did not correlate these data to changes in cognition, because at one year old these animals do not suffer from significant cognitive impairment.

So how does a heparin help these mice? In-vitro experiments provide some clues. To test if enoxaparin can attenuate Aβ toxicity, Bergamaschini added both molecules, either alone or in combination, to PC12 cells. Adding just Aβ1-40 resulted in a loss of about 90 percent of the cells, while enoxaparin alone had no effect on viability. But the heparin prevented Aβ-induced loss of cell viability in a dose-dependent manner. At the highest concentration used, 10 mM, it protected fully.

But whether this direct protection from Aβ toxicity is relevant to the in-vivo observations is unclear. As the authors point out, it is unknown whether low molecular weight heparins such as enoxaparin, can pass the blood-brain barrier. Other possibilities are that the heparin acts as a peripheral sink, drawing Aβ from the brain (this is not unprecedented, see ARF related news story on peripheral antibodies leaching Aβ from mouse brain), or that it inhibits complement activation, thus reducing inflammatory responses.

Significantly, this drug is used as an anticoagulant in humans. So epidemiological data could reveal a protective effect against dementia.—Tom Fagan.

Reference:
Bergamaschini L, Rossi E, Storini C, Pizzimenti S, Distaso M, Perego C, De Luigi A, Vergani C, and De Simoni MG. Peripheral treatment with enoxaparin, a low molecular weight heparin, reduces plaques and β-amyloid accumulation in a mouse model of Alzheimer’s disease. J. Neurosci. 2004 April 28;24:4181-4186. Abstract