Amyloid precursor protein (APP)
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Amyloid Precursor Protein (APP) Structure and Processing
Amyloid precursor protein (APP) is a conserved transmembrane protein found in both the central and peripheral nervous systems, as well as in other tissues like the liver, pancreas, adipose tissue, and muscle cells 17. APP undergoes complex proteolytic processing through two main pathways: the non-amyloidogenic pathway, which is generally considered beneficial, and the amyloidogenic pathway, which leads to the production of amyloid-beta (Aβ) peptides implicated in Alzheimer’s disease (AD) 1356+2 MORE. The enzymes α-, β-, and γ-secretases are central to these pathways, and their activity determines the types of APP fragments produced 1369.
APP and Alzheimer’s Disease Pathology
APP is best known as the precursor to Aβ peptides, which accumulate in the brains of individuals with AD and form the core of senile plaques, a hallmark of the disease 1356. The neurotoxic Aβ1-42 peptide, in particular, is associated with plaque formation and neurodegeneration . Research has focused on targeting APP processing enzymes to reduce Aβ production as a therapeutic strategy for AD 156. Additionally, genetic variations affecting APP processing may contribute to age-related changes and AD risk .
Physiological Functions of APP and Its Fragments
Beyond its role in AD, APP and its cleavage products have important physiological functions. APP is involved in neuronal development, synapse formation, and maintenance, as well as neuroprotection and brain injury response 1489. Soluble APP fragments, such as sAPPα, have trophic effects and are necessary for maintaining dendritic integrity and synaptic function, especially with aging 89. Loss of APP can lead to reduced dendritic spine density, impaired synaptic plasticity, and altered neural stem cell proliferation and differentiation 489.
APP as a Receptor and Signaling Hub
Recent findings show that APP acts as a receptor for Wnt ligands, such as Wnt3a and Wnt5a, which regulate APP protein levels and trafficking . The interaction with Wnt ligands influences neuronal growth, with loss of APP resulting in abnormal axonal and dendritic development . APP also interacts with proteins involved in the ubiquitin-proteasome system, suggesting a role in regulating protein turnover and synaptic function .
APP in Metabolic and Systemic Diseases
APP is widely expressed outside the brain and influences metabolism in various tissues . Its peptides can have positive or negative effects on metabolic processes, and abnormal APP expression or processing is linked to metabolic diseases like type 2 diabetes, obesity, fatty liver disease, cardiovascular disease, and certain cancers . Modulating APP function or its amyloidogenic processing may offer therapeutic benefits for both AD and metabolic disorders .
Conclusion
APP is a multifunctional protein with critical roles in brain development, synaptic maintenance, and systemic metabolism. While its amyloidogenic processing is central to Alzheimer’s disease pathology, APP and its fragments also have essential physiological functions. Understanding the balance between its beneficial and detrimental effects is key for developing effective therapies for AD and related metabolic diseases 1345+5 MORE.
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