NGF and BDNF signaling control amyloidogenic route and Aβ production in hippocampal neurons

Abstract

Here, we report that interruption of NGF or BDNF signaling in hippocampal neurons rapidly activates the amyloidogenic pathway and causes neuronal apoptotic death. These events are associated with an early intracellular accumulation of PS1 N-terminal catalytic subunits and of APP C-terminal fragments and a progressive accumulation of intra-and extracellular A␤ aggregates partly released into the culture medium. The released pool of A␤ induces an increase of APP and PS1 holoprotein levels, creating a feedforward toxic loop that might also cause the death of healthy neurons. These events are mimicked by exogenously added A␤ and are prevented by exposure to ␤-and ␥-secretase inhibitors and by antibodies directed against A␤ peptides. The same cultured neurons deprived of serum die, but APP and PS1 overexpression does not occur, A␤ production is undetectable, and cell death is not inhibited by anti-A␤ antibodies, suggesting that hippocampal amyloidogenesis is not a simple consequence of an apoptotic trigger but is due to interruption of neurotrophic signaling. Hippocampal neurons are among the most severely affected cells in AD (1). BDNF and NGF carry out a variety of actions on these neurons and are involved in the clinical and pathophysiological signs of AD (2, 3). In AD, a reduction in neurotrophin levels occurs in some areas of the CNS, and neurotrophic factors have begun to be used in clinical trials to prevent or to reduce neuronal cell loss (4) or to improve hippocampal neurogenesis in adult and aged male rats (5). In a previous study of NGF-differentiated PC12 cells, we reported a close correlation between NGF deprivation and activation of the amyloidogenic route (6), thus broadening to this clonal line the link between amyloidogenesis and cell death formerly reported in cultured cerebellar granule cells (7,8). Although this clonal cell line has contributed to elucidating a large number of neuronal properties, because of its clonal neoplastic origin, it might not provide information specifically connected to the pathological events actually occurring in the neurons of the adult brain. Therefore, to clarify the molecular events found in NGF-differentiated PC12 cells, we resorted to primary hippocampal neurons and also tried to assess whether another neurotrophin, such as BDNF, shares the same antiamyloidogenic activity previously found with NGF.In the present study, we confirm and largely extend findings obtained in NGF-differentiated PC12 cells demonstrating that interruption of the NGF (or BDNF) signal induces death through an intra-and extracellular accumulation of A␤ aggregates and activation of a feed-forward toxic loop that also causes the death of healthy neurons. These events are associated with the formation of varicosities along neurites and with an accumulation of APP C-terminal fragments in neurons undergoing death. Furthermore, A␤ peptides released in medium induce an increase of APP and PS1 holoprotein levels, and all these events are prevented by ␥-and ␤-secretase inhibitors or by antibody direct...

Does the term ‘trophic’ actually mean anti-amyloidogenic? The case of NGF

“…76 Together, these results also seem to open a new paradigm in the field of NGF intracellular signal transduction, because the same TrkA transmembrane receptor may switch from pro-survival to pro-apoptotic action, in the presence or absence of its physiological cognate ligand. In addition, the evidence that NGF receptors, directly or indirectly, interact with Ab peptide(s) and PS1 in apoptotic neurons and that analogous effects are induced by synthetic Ab exogenously added peptide 53,68 corroborates the ongoing hypothesis of a cellular interplay between the NGF receptors (TrkA and p75) and the amyloidogenic APP processing machinery (Figure 3). 77 …”

“…Thus, even in NGF-deprived hippocampal neurons, we observed that (1) the released Ab extracellular peptides created a neurotoxic loop, because the extent of ensuing cell death was much greater than that expected on the potential number of NGF-responsive target neurons previously found; (2) b-or g-secretase inhibitors -or mixture of both -and Ab antibody, 4G8, largely prevented not only the Ab intra-and extra-production but also neuronal death. 53 The temporal interplay between Ab overproduction and tau protein post-translational modifications was also established from the observation that two specific amino-acidic sites 42 ). (a) CGNs at 6 DIV (upper wells) were transferred in sister cultures kept in S-K25 whereas the correspondent co-cultured neurons (bottom wells) were switched to apoptotic S-K5 medium.…”

Nerve growth factor as a paradigm of neurotrophins related to Alzheimer's disease

“…We observed that primary hippocampal neurons after 48 h of NGF exposure, die following NGF withdrawal, with an increase of APP processing and an intra and extracellular Ab accumulation (Matrone et al, 2008b; Fig. 1).…”

“…We observe that an early, transient and site-specific GSK-3b-mediated tau hyperphosphorylation (3-6 h after NGF withdrawal)-at two AD-relevant pathological epitopes (Ser262 and Thr231)-is temporally and causally related with an activation of the endogenous amyloidogenic pathway in NGF-deprived hippocampal primary neurons (Matrone et al, 2008b). The specific changes of tau phosphorylation state, as well as apoptotic death, are blocked by 4G8 and 6E10 Ab antibodies or by specific b and/or c secretases inhibitors, suggesting that such events are induced by endogenously overproduced Ab.…”

Combination of Aβ clearance and neurotrophic factors as a potential treatment for Alzheimer’s disease

“…This is supported by the finding that the AD11 mouse model, which expresses the recombinant monoclonal antibody αD11 that specifically neutralizes NGF thus causing NGF deprivation, shows ADlike pathology including Aβ accumulation and hippocampusdependent memory deficits [63,64] . Based on the report that activation of the amyloidogenic route by NGF deprivation induces apoptotic death in PC12 cells [65] , Matrone and colleagues further found that APP and presenilin 1 N-terminus (which is the active component endowed with γ-secretase activity) levels were increased in hippocampal neurons that were previously exposed to NGF or BDNF for 48 h followed by deprivation by anti-NGF (or -BDNF) antibodies [66] . These findings suggest that Aβ and NTF deficits cause a feedforward loop that accelerates the toxicity of Aβ sufficient to cause neuronal death.…”