AICD Overexpression in Neuro 2A Cells Regulates Expression of PTCH1 and TRPC5

Molecular mechanisms of developmental pathways in neurological disorders: a pharmacological and therapeutic review

Developmental signalling pathways such as Wnt/β-catenin, Notch and Sonic hedgehog play a central role in nearly all the stages of neuronal development. The term 'embryonic' might appear to be a misnomer to several people because these pathways are functional during the early stages of embryonic development and adulthood, albeit to a certain degree. Therefore, any aberration in these pathways or their associated components may contribute towards a detrimental outcome in the form of neurological disorders such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and stroke. In the last decade, researchers have extensively studied these pathways to decipher disease-related interactions, which can be used as therapeutic targets to improve outcomes in patients with neurological abnormalities. However, a lot remains to be understood in this domain. Nevertheless, there is strong evidence supporting the fact that embryonic signalling is indeed a crucial mechanism as is manifested by its role in driving memory loss, motor impairments and many other processes after brain trauma. In this review, we explore the key roles of three embryonic pathways in modulating a range of homeostatic processes such as maintaining blood-brain barrier integrity, mitochondrial dynamics and neuroinflammation. In addition, we extensively investigated the effect of these pathways in driving the pathophysiology of a range of disorders such as Alzheimer's, Parkinson's and diabetic neuropathy. The concluding section of the review is dedicated to neurotherapeutics, wherein we identify and list a range of biological molecules and compounds that have shown enormous potential in improving prognosis in patients with these disorders.

Amyloid-β interrupts canonical Sonic hedgehog signaling by distorting primary cilia structure

Background

Primary cilia are small non-motile microtubule and cell membrane protrusions expressed on most vertebrate cells, including cortical and hippocampal neurons. These small organelles serve as sensory structures sampling the extracellular environment and reprogramming the transcriptional machinery in response to environmental change. Primary cilia are decorated with a variety of receptor proteins and are necessary for specific signaling cascades such as the Sonic hedgehog (Shh) pathway. Disrupting cilia structure or function results in a spectrum of diseases collectively referred to as ciliopathies. Common to human ciliopathies is cognitive impairment, a symptom also observed in Alzheimer's disease (AD). One hallmark of AD is accumulation of senile plaques composed of neurotoxic Amyloid-β (Aβ) peptide. The Aβ peptide is generated by the proteolytic cleavage of the amyloid precursor protein (APP). We set out to determine if Aβ affects primary cilia structure and the Shh signaling cascade.

Methods

We utilized in vitro cell-based assays in combination with fluorescent confocal microscopy to address our study goals. Shh signaling and cilia structure was studied using two different cell lines, mouse NIH3T3 and human HeLa cells. To investigate how Aβ levels affect Shh signaling and cilia structure in these cells, we utilized naturally secreted Aβ as well as synthetic Aβ. Effects on Shh signaling were assessed by luciferase activity while cilia structure was analyzed by fluorescent microscopy.

Results

Here, we report that APP localizes to primary cilia and Aβ treatment results in distorted primary cilia structure. In addition, we demonstrate that Aβ treatment interrupts canonical Shh signal transduction.

Conclusions

Overall, our study illustrates that Aβ can alter primary cilia structure suggesting that elevated Aβ levels, like those observed in AD patients, could have similar effects on neuronal primary cilia in the brain. Additionally, our study suggests that Aβ impairs the Shh signaling pathway. Together our findings shed light on two novel targets for future AD therapeutics.

Ryanodine receptors: physiological function and deregulation in Alzheimer disease

Perturbed Endoplasmic Reticulum (ER) calcium (Ca2+) homeostasis emerges as a central player in Alzheimer disease (AD). Accordingly, different studies have reported alterations of the expression and the function of Ryanodine Receptors (RyR) in human AD-affected brains, in cells expressing familial AD-linked mutations on the β amyloid precursor protein (βAPP) and presenilins (the catalytic core in γ-secretase complexes cleaving the βAPP, thereby generating amyloid β (Aβ) peptides), as well as in the brain of various transgenic AD mice models. Data converge to suggest that RyR expression and function alteration are associated to AD pathogenesis through the control of: i) βAPP processing and Aβ peptide production, ii) neuronal death; iii) synaptic function; and iv) memory and learning abilities. In this review, we document the network of evidences suggesting that RyR could play a complex dual "compensatory/protective versus pathogenic" role contributing to the setting of histopathological lesions and synaptic deficits that are associated with the disease stages. We also discuss the possible mechanisms underlying RyR expression and function alterations in AD. Finally, we review recent publications showing that drug-targeting blockade of RyR and genetic manipulation of RyR reduces Aβ production, stabilizes synaptic transmission, and prevents learning and memory deficits in various AD mouse models. Chemically-designed RyR "modulators" could therefore be envisioned as new therapeutic compounds able to delay or block the progression of AD.

The amyloid precursor protein (APP) triplicated gene impairs neuronal precursor differentiation and neurite development through two different domains in the Ts65Dn mouse model for Down syndrome

Intellectual disability in Down syndrome (DS) appears to be related to severe proliferation impairment during brain development. Recent evidence shows that it is not only cellular proliferation that is heavily compromised in DS, but also cell fate specification and dendritic maturation. The amyloid precursor protein (APP), a gene that is triplicated in DS, plays a key role in normal brain development by influencing neural precursor cell proliferation, cell fate specification, and neuronal maturation. APP influences these processes via two separate domains, the APP intracellular domain (AICD) and the soluble secreted APP. We recently found that the proliferation impairment of neuronal precursors (NPCs) from the Ts65Dn mouse model for DS was caused by derangement of the Shh pathway due to overexpression of patched1(Ptch1), its inhibitory regulator. Ptch1 overexpression was related to increased levels within the APP/AICD system. The overall goal of this study was to determine whether APP contributes to neurogenesis impairment in DS by influencing in addition to proliferation, cell fate specification, and neurite development. We found that normalization of APP expression restored the reduced neuronogenesis, the increased astrogliogenesis, and the reduced neurite length of trisomic NPCs, indicating that APP overexpression underpins all aspects of neurogenesis impairment. Moreover, we found that two different domains of APP impair neuronal differentiation and maturation in trisomic NPCs. The APP/AICD system regulates neuronogenesis and neurite length through the Shh pathway, whereas the APP/secreted AP system promotes astrogliogenesis through an IL-6-associated signaling cascade. These results provide novel insight into the mechanisms underlying brain development alterations in DS.

Nuclear signalling by membrane protein intracellular domains: the AICD enigma

Alzheimer's disease (AD) is a neurodegenerative illness and the leading cause of dementia in the elderly. The accumulation of amyloid-β peptide (Aβ) is a well-known pathological hallmark associated with the disease. However, Aβ is only one of several metabolites produced by β- and γ-secretase actions on the transmembrane protein, the amyloid precursor protein (APP). A proteolytic fragment termed the APP intracellular domain (AICD) is also produced. By analogy with the Notch signalling pathway, AICD has been proposed as a transcriptional regulator although its mechanism of action and the complement of genes regulated remain controversial. This review will focus on the contributions that studies of APP processing have brought to the understanding of a novel nuclear signalling pathway that may contribute to the pathology of AD and may provide new therapeutic opportunities.

The amyloid precursor protein intracellular domain alters gene expression and induces neuron-specific apoptosis

Although amyloid precursor protein (APP) plays a central role in Alzheimer's disease, the physiological functions of this protein have yet to be fully elucidated. As previously reported, we established an embryonic carcinoma P19 cell line expressing the intracellular domain of APP (AICD). While neurons were differentiated from these cell lines with retinoic acid treatment, expression of AICD induced neuron-specific apoptosis. As the first step to identify the genes involved in this process, we evaluated AICD-induced changes in gene expression through cell death. The levels of expression of 41,256 transcripts were monitored by DNA microarray analysis. The expression of 277 genes showed up-regulation by more than 10-fold in the presence of AICD. Conversely, the expression of 341 genes showed down-regulation to less than one-tenth of the original level. Reverse transcription-polymerase chain reaction of 17 selected genes showed excellent agreement with the microarray results. These results suggest that AICD induces dynamic changes in gene expression, which may be closely correlated with AICD-induced neuron-specific apoptosis.