Molecular biology of Alzheimer's disease

Progress toward identification of protease activity involved in proteolysis of apolipoprotein e in human brain

Apolipoprotein E (apoE) genotype is the single most important genetic risk factor for the most common (sporadic) form of Alzheimer's disease (AD). Increasing evidence supports the hypothesis that the presence of the E4 isoform of this cholesterol-binding protein contributes directly to disease risk, age of onset, and severity of the neuropathology. For example, studies in transgenic mice demonstrate that apoE is necessary for the formation of plaques with neuritic pathology. The precise mechanism by which apoE contributes to the disease remains unknown. However, several lines of investigation from a number of laboratories now point to a role for proteolytic fragments of apoE in the formation of both plaques and tangles, the two pathological hallmarks of the disease. In particular, the C-terminal portion of apoE has been implicated in binding to amyloid and is localized to plaques. The N-terminal domain, on the other hand, is neurotoxic in culture and has been localized to, and implicated in the formation of, neurofibrillary tangles. These results suggest that inhibition of apoE proteolysis is a potential therapeutic strategy for AD. Using human brain homogenates, we have determined that proteolysis of apoE is greatest at acidic pH and can be inhibited by compounds targeting aspartic proteases. The feasibility of screening candidate inhibitors is supported by both ELISA and immunoblotting methods. Future studies will use a combination of in vitro and in vivo assays to test the efficacy of the most effective compounds for their ability to inhibit apoE proteolysis in human brain and apoE transgenic mouse brain tissue.

Neurotrophins and ciliary neurotrophic factor: their biology and pathology

Neurotrophins (NTFs) and ciliary neurotrophic factor (CNTF) induce the differentiation of neuronal cells, rescue them from naturally occurring death, and trigger neuronal regeneration. The NTFs bind to two classes of cell surface receptors, whereas CNTF receptor is composed of three subunits. The functions of these polypeptide survival factors with trophic action on nerve cells have recently been approached by the targeted disruption of the CNTF, NTF and their receptor genes by the homologous recombination technique. The embryonic growth and morphogenesis of these gene 'knock-out' mice is normal, but they develop with defects in various subsets of the peripheral nervous system, and the homozygous mutant mice often die during the early postnatal period. Disturbances in the biology of NTFs and CNTF have recently been implicated in the pathogenesis of certain common neurodegenerative disorders, such as Parkinson's disease, motor neurone diseases, and Alzheimer's disease. Intensive research on their pharmaceutical perspective has, therefore, been provoked. All neurotrophins and CNTF can now be synthesized on a large scale as biologically active recombinant proteins, and several alternatives for their local applications to the target tissue have been presented. Their therapeutic potential is discussed.