Edgar Dawkins

Insights into the physiological function of the β-amyloid precursor protein: beyond Alzheimer's disease

The β‐amyloid precursor protein (APP) has been extensively studied for its role as the precursor of the β‐amyloid protein (Aβ) of Alzheimers disease. However, the normal function of APP remains largely unknown. This article reviews studies on the structure, expression and post‐translational processing of APP, as well as studies on the effects of APP in vitro and in vivo. We conclude that the published data provide strong evidence that APP has a trophic function. APP is likely to be involved in neural stem cell development, neuronal survival, neurite outgrowth and neurorepair. However, the mechanisms by which APP exerts its actions remain to be elucidated. The available evidence suggests that APP interacts both intracellularly and extracellularly to regulate various signal transduction mechanisms.

Role of cystatin C in amyloid precursor protein-induced proliferation of neural stem/progenitor cells

Background: The role of the amyloid precursor protein (APP) in neural stem/progenitor cell (NSPC) proliferation is poorly understood. Results: Immunodepletion of cystatin C from NSPC conditioned medium abrogated an effect of APP on NSPC proliferation. Conclusion: Cystatin C mediates APP-induced NSPC proliferation. Significance: The results increase understanding of mechanisms promoting NSPC survival and differentiation. The amyloid precursor protein (APP) is well studied for its role in Alzheimer disease. However, little is known about its normal function. In this study, we examined the role of APP in neural stem/progenitor cell (NSPC) proliferation. NSPCs derived from APP-overexpressing Tg2576 transgenic mice proliferated more rapidly than NSPCs from the corresponding background strain (C57Bl/6xSJL) wild-type mice. In contrast, NSPCs from APP knock-out (APP-KO) mice had reduced proliferation rates when compared with NSPCs from the corresponding background strain (C57Bl/6). A secreted factor, identified as cystatin C, was found to be responsible for this effect. Levels of cystatin C were higher in the Tg2576 conditioned medium and lower in the APP-KO conditioned medium. Furthermore, immunodepletion of cystatin C from the conditioned medium completely removed the ability of the conditioned medium to increase NSPC proliferation. The results demonstrate that APP expression stimulates NSPC proliferation and that this effect is mediated via an increase in cystatin C secretion.

Identifying the primary site of pathogenesis in amyotrophic lateral sclerosis - vulnerability of lower motor neurons to proximal excitotoxicity.

There is a desperate need for targeted therapeutic interventions that slow the progression of amyotrophic lateral sclerosis (ALS). ALS is a disorder with heterogeneous onset, which then leads to common final pathways involving multiple neuronal compartments that span both the central and peripheral nervous system. It is believed that excitotoxic mechanisms might play an important role in motor neuron death in ALS. However, little is known about the mechanisms by which excitotoxicity might lead to the neuromuscular junction degeneration that characterizes ALS, or about the site at which this excitotoxic cascade is initiated. Using a novel compartmentalised model of site-specific excitotoxin exposure in lower motor neurons in vitro, we found that spinal motor neurons are vulnerable to somatodendritic, but not axonal, excitotoxin exposure. Thus, we developed a model of somatodendritic excitotoxicity in vivo using osmotic mini pumps in Thy-1-YFP mice. We demonstrated that in vivo cell body excitotoxin exposure leads to significant motor neuron death and neuromuscular junction (NMJ) retraction. Using confocal real-time live imaging of the gastrocnemius muscle, we found that NMJ remodelling preceded excitotoxin-induced NMJ degeneration. These findings suggest that excitotoxicity in the spinal cord of individuals with ALS might result in a die-forward mechanism of motor neuron death from the cell body outward, leading to initial distal plasticity, followed by subsequent pathology and degeneration.

Synapse Dysfunction of Layer V Pyramidal Neurons Precedes Neurodegeneration in a Mouse Model of TDP-43 Proteinopathies.

TDP-43 is a major protein component of pathological neuronal inclusions that are present in frontotemporal dementia and amyotrophic lateral sclerosis. We report that TDP-43 plays an important role in dendritic spine formation in the cortex. The density of spines on YFP+ pyramidal neurons in both the motor and somatosensory cortex of Thy1-YFP mice, increased significantly from postnatal day 30 (P30), to peak at P60, before being pruned by P90. By comparison, dendritic spine density was significantly reduced in the motor cortex of Thy1-YFP::TDP-43A315T transgenic mice prior to symptom onset (P60), and in the motor and somatosensory cortex at symptom onset (P90). Morphological spine-type analysis revealed that there was a significant impairment in the development of basal mushroom spines in the motor cortex of Thy1-YFP::TDP-43A315T mice compared to Thy1-YFP control. Furthermore, reductions in spine density corresponded to mislocalisation of TDP-43 immunoreactivity and lowered efficacy of synaptic transmission as determined by electrophysiology at P60. We conclude that mutated TDP-43 has a significant pathological effect at the dendritic spine that is associated with attenuated neural transmission.

β-Amyloid Precursor Protein: Function in Stem Cell Development and Alzheimer's Disease Brain

Stem cell therapy may be a suitable approach for the treatment of many neurodegenerative diseases. However, one major impediment to the development of successful cell-based therapies is our limited understanding of the mechanisms that instruct neural stem cell behaviour, such as proliferation and cell fate specification. The β-amyloid precursor protein (APP) of Alzheimers disease (AD) may play an important role in neural stem cell proliferation and differentiation. Our recent work shows that in vitro, APP stimulates neural stem or progenitor cell proliferation and neuronal differentiation. The effect on proliferation is mediated by an autocrine factor that we have identified as cystatin C. As cystatin C expression is also reported to inhibit the development of amyloid pathology in APP transgenic mice, our finding has implications for the possible use of cystatin C for the therapy of AD.

The N‐terminal fragment of the β‐amyloid precursor protein of Alzheimer's disease (N‐APP) binds to phosphoinositide‐rich domains on the surface of hippocampal neurons

The function of the β‐amyloid precursor protein (APP) of Alzheimers disease is poorly understood. The secreted ectodomain fragment of APP (sAPPα) can be readily cleaved to produce a small N‐terminal fragment (N‐APP) that contains heparin‐binding and metal‐binding domains and that has been found to have biological activity. In the present study, we examined whether N‐APP can bind to lipids. We found that N‐APP binds selectively to phosphoinositides (PIPs) but poorly to most other lipids. Phosphatidylinositol 4,5‐bisphosphate (PI(4,5)P2)‐rich microdomains were identified on the extracellular surface of neurons and glia in primary hippocampal cultures. N‐APP bound to neurons and colocalized with PIPs on the cell surface. Furthermore, the binding of N‐APP to neurons increased the level of cell‐surface PI(4,5)P2 and phosphatidylinositol 3,4,5‐trisphosphate. However, PIPs were not the principal cell‐surface binding site for N‐APP, because N‐APP binding to neurons was not inhibited by a short‐acyl‐chain PIP analogue, and N‐APP did not bind to glial cells which also possessed PI(4,5)P2 on the cell surface. The data are explained by a model in which N‐APP binds to two distinct components on neurons, one of which is an unidentified receptor and the second of which is a PIP lipid, which binds more weakly to a distinct site within N‐APP. Our data provide further support for the idea that N‐APP may be an important mediator of APPs biological activity.