The role of amyloid precursor proteins : studies using neurons generated from wild-type and mutant embryonic stem cells

Alzheimer′s Disease (AD) is the most common neurodegenerative disorder in elderly people. It is
becoming an ever larger problem due to prolonged life-expectancies in developed countries. One of the
central pathological hallmarks of the disease are deposits of aggregated amyloid β (Aβ), a peptide
proteolytically derived from the amyloid precursor protein (APP). Although the contribution of APP to
the pathology of AD has been at the center of most AD research for about two decades, the
physiological role of this membrane protein still remains elusive. APP belongs to a conserved gene
family including the amyloid precursor-like proteins 1 and 2 (APLP1 and APLP2). While mice lacking
individual genes do not display any overt phenotype, double null mutations for app and aplp2 result in
early postnatal lethality, at the time when neuronal activity begins to be essential. These findings
suggest functional redundancy of APP and APLP2, which is further supported by their similar pattern of
expression. In order to address the role of these genes during neuronal differentiation, we made use of a
newly established method allowing the generation of uniform populations of neurons starting with
cultured mouse embryonic stem (ES) cells. This protocol leads to the generation of an essentially pure
population of neural progenitors defined as Pax6-positive radial glial cells. As known from recent cell
lineage analyses in vivo, these progenitors give rise to essentially all pyramidal, glutamatergic neurons
in the cerebral cortex and in the hippocampus. In this work, we made use of ES cells lacking both app
and aplp2 and compared some of the biochemical properties of neurons derived from such cells with
those obtained from wild-type ES cells. In the absence of app and aplp2, we detected a marked
reduction in the expression of one of the vesicular transporters for glutamate, VGLUT2. In line with
this, we found that the reduction in transporter levels is functionally significant, as less glutamate was
released from these neurons following depolarizing stimuli. With regard to possible mechanisms linking
the transcription of vglut2 and APP, we blocked γ-secretase activity in wild-type neurons and observed
that inhibition of the generation of a soluble intracellular domain of APP (AICD) leads to a decrease in
the transcription of vglut2.
The present work thus suggests a link between APP and the development of glutamatergic synapses.
This may provide an explanation for the early postnatal lethality of animals lacking both app and aplp2.
Dysfunction of APP processing might also provide a mechanistic link explaining some of the synaptic
alterations thought to occur in AD.