Alzheimer's disease (AD) is characterized by a progressive phenotypic downregulation of markers within cholinergic basal forebrain (CBF) neurons, frank CBF cell loss and reduced cortical choline acetyltransferase activity associated with cognitive decline. Delaying CBF neurodegeneration or minimizing its consequences is the mechanism of action for most currently available drug treatments for cognitive dysfunction in AD. Growing evidence suggests that imbalances in the expression of NGF, its precursor proNGF and the high (TrkA) and low (p75 NTR ) affinity NGF receptors are crucial factors underlying CBF dysfunction in AD. Drugs that maintain a homeostatic balance between TrkA and p75 NTR may slow the onset of AD. A NGF gene therapy trial reduced cognitive decline and stimulated cholinergic fiber growth in humans with mild AD. Drugs treating the multiple pathologies and clinical symptoms in AD (e.g., M1 cholinoceptor and/or galaninergic drugs) should be considered for a more comprehensive treatment approach for cholinergic dysfunction.

Disease Research (CNDR). Written informed consent was obtained from all subjects. The cases used in this study are summarized in Supplemental Table 3. RNA-seq data. All original RNA-seq data were previously deposited in the NCBI's Gene Expression Omnibus database (GEO GSE101689).

Background Endocytic dysfunction and neurotrophin signaling deficits may underlie the selective vulnerability of hippocampal neurons during the progression of Alzheimer’s disease (AD), although there is little direct in vivo and biochemical evidence to support this hypothesis. Methods Microarray analysis of hippocampal CA1 pyramidal neurons acquired via laser capture microdissection (LCM) was performed using postmortem brain tissue. Validation was achieved using real-time quantitative PCR (qPCR) and immunoblot analysis. Mechanistic studies were performed using human fibroblasts subjected to overexpression with viral vectors or knockdown via siRNA. Results Expression levels of genes regulating early endosomes (rab5) and late endosomes (rab7) are selectively up regulated in homogeneous populations of CA1 neurons from individuals with mild cognitive impairment (MCI) and AD. The levels of these genes are selectively increased as antemortem measures of cognition decline during AD progression. Hippocampal qPCR and immunoblot analyses confirmed increased levels of these transcripts and their respective protein products. Elevation of select rab GTPases regulating endocytosis paralleled the down regulation of genes encoding the neurotrophin receptors TrkB and TrkC. Overexpression of rab5 in cells suppressed TrkB expression, whereas knockdown of TrkB expression did not alter rab5 levels, suggesting that TrkB down regulation is a consequence of endosomal dysfunction associated with elevated rab5 levels in early AD. Conclusions These data support the hypothesis that neuronal endosomal dysfunction is associated with preclinical AD. Increased endocytic pathway activity, driven by elevated rab GTPase expression, may result in long term deficits in hippocampal neurotrophic signaling and represent a key pathogenic mechanism underlying AD progression.

An additional copy of the β-amyloid precursor protein (APP) gene causes early-onset Alzheimer’s disease (AD) in trisomy 21 (DS). Endosome dysfunction develops very early in DS and AD and has been implicated in the mechanism of neurodegeneration. Here, we show that morphological and functional endocytic abnormalities in fibroblasts from individuals with DS are reversed by lowering the expression of APP or β-APP-cleaving enzyme 1 (BACE-1) using short hairpin RNA constructs. By contrast, endosomal pathology can be induced in normal disomic (2N) fibroblasts by overexpressing APP or the C-terminal APP fragment generated by BACE-1 (βCTF), all of which elevate the levels of βCTFs. Expression of a mutant form of APP that cannot undergo β-cleavage had no effect on endosomes. Pharmacological inhibition of APP γ-secretase, which markedly reduced Aβ production but raised βCTF levels, also induced AD-like endosome dysfunction in 2N fibroblasts and worsened this pathology in DS fibroblasts. These findings strongly implicate APP and the βCTF of APP, and exclude Aβ and the αCTF, as the cause of endocytic pathway dysfunction in DS and AD, underscoring the potential multifaceted value of BACE-1 inhibition in AD therapeutics.