An exploration of possible functional redundancy between APP and APLP2 in neuronal development.

AD is a multifaceted disease with contributions from several genetic and environmental factors. One fundamental pathway involves the clearace of a normal neuronal protein, amyloid precursor protein (APP), to produce toxic fragments called ß-amyloid (Aß). It is therefore reasonable to suggest that treatments aimed at disrupting the production or increasing the turnover of APP could be of use in the treatment of AD. However, before these potential therapies are developed it is necessary to ascertain what effect this proposed disruption of APP would have on the normal function of neurons, especially as it has previously been shown that APP appears to have a role in stabilising neuronal processes (neurites) by attaching them to the surrounding cell scaffolding (extracellular matrix). We postulate that a related protein, amyloid precursor-like protein 2 (APLP2), which is not cleaved to form Aß, can substitute for APP in this role. Both proteins are believed to be similarly involved in the (re)modelling of the brain, but as APLP2 does not give rise to the toxic Aß fragments which lead to AD, if there is redundancy between the normal function(s) of these proteins then the therapeutic disruption of APP would not be expected to result in major unintended side-effects on normal brain structure and function. The project involves the measurement of the APP and APLP2 proteins in the brains of knock-out mice (lacking APP or APLP2) and identifying whether there is any up-regulation in the knockouts a compensatory response. This was not seen in the APLP2 KO. However at a time point (2 day post natal) in APP KO’s there was an increase in the APLP2 protein. Subsequent studies in vitro measured neurite outgrowth and synaptic maintenance in both knockout neurons (APP, APLP2 and double APP and APLP2) and using an siRNA to inhibit protein production in WT neurons. Results showed that without APP, neurites do not grow at the same rate as WT neurites in either the KO or the siRNA knockdown model. Double KO are also shorter compared with WT, but APLP2 KO are not affected. It can be said on the basis of these results that APP is necessary for proper neurite outgrowth, but that APLP2 is not. Synaptic integrity is preserved in the APP and APLP2 single KOs, however it appears to be impaired in the double KO, suggesting APP and APLP2could overlap in function with regard to synaptogenesis. For APP knockdown to be a viable treatment option the level to which it can be knocked down, if at all, without disruption of neurite outgrowth would have to be determined.