sAPPβ and sAPPα increase structural complexity and E/I input ratio in primary hippocampal neurons and alter Ca2+ homeostasis and CREB1-signaling

One major pathophysiological hallmark of Alzheimer's disease (AD) is senile plaques composed of amyloid beta (A beta). In the amyloidogenic pathway, cleavage of the amyloid precursor protein (APP) is shifted towards A beta production and soluble APP beta (sAPP beta) levels. AD is known to impair synaptic function; however, much less is known about the physiological functions of sAPP beta. The neurotrophic properties of sAPP alpha, derived from the non-amyloidogenic pathway of APP cleavage, are well-established, whereas only a few, conflicting studies on sAPP beta exist. The intracellular pathways of sAPP beta are largely unknown. Since sAPP beta is generated alongside A beta by beta-secretase (BACE1) cleavage, we tested the hypothesis that sAPP beta effects differ from sAPP alpha effects as a neurotrophic factor. We therefore performed a head-to-head comparison of both mammalian recombinant peptides in developing primary hippocampal neurons (PHN). We found that sAPP alpha significantly increases axon length (p = 0.0002) and that both sAPP alpha and sAPPB increase neurite number (p < 0.0001) of PHN at 7 days in culture (DIV7) but not at DIV4. Moreover, both sAPP alpha- and sAPP beta-treated neurons showed a higher neuritic complexity in Sholl analysis. The number of glutamatergic synapses (p < 0.0001), as well as layer thickness of postsynaptic densities (PSDs), were significantly increased, and GABAergic synapses decreased upon sAPP overexpression in PHN. Furthermore, we showed that sAPP alpha enhances ERK and CREB1 phosphorylation upon glutamate stimulation at DIV7, but not DIV4 or DIV14. These neurotrophic effects are further associated with increased glutamate sensitivity and CREB1-signaling. Finally, we found that sAPP alpha levels are significantly reduced in brain homogenates of AD patients compared to control subjects. Taken together, our data indicate critical stage dependent roles of sAPPs in the developing glutamatergic system in vitro, which might help to understand deleterious consequences of altered APP shedding in AD patients, beyond A beta pathophysiology.