Arc/Arg3.1 regulates an endosomal pathway essential for activity-dependent β-amyloid generation

The Role of p38 MAPK and Its Substrates in Neuronal Plasticity and Neurodegenerative Disease

A significant amount of evidence suggests that the p38-mitogen-activated protein kinase (MAPK) signalling cascade plays a crucial role in synaptic plasticity and in neurodegenerative diseases. In this review we will discuss the cellular localisation and activation of p38 MAPK and the recent advances on the molecular and cellular mechanisms of its substrates: MAPKAPK 2 (MK2) and tau protein. In particular we will focus our attention on the understanding of the p38 MAPK-MK2 and p38 MAPK-tau activation axis in controlling neuroinflammation, actin remodelling and tau hyperphosphorylation, processes that are thought to be involved in normal ageing as well as in neurodegenerative diseases. We will also give some insight into how elucidating the precise role of p38 MAPK-MK2 and p38 MAPK-tau signalling cascades may help to identify novel therapeutic targets to slow down the symptoms observed in neurodegenerative diseases such as Alzheimer's and Parkinson's disease.

Alzheimer culprits: cellular crossroads and interplay

Alzheimer's disease (AD) is the primary cause of dementia in the elderly and one of the major health problems worldwide. Since its first description by Alois Alzheimer in 1907, noticeable but insufficient scientific comprehension of this complex pathology has been achieved. All the research that has been pursued takes origin from the identification of the pathological hallmarks in the forms of amyloid-β (Aβ) deposits (plaques), and aggregated hyperphosphorylated tau protein filaments (named neurofibrillary tangles). Since this discovery, many hypotheses have been proposed to explain the origin of the pathology. The "amyloid cascade hypothesis" is the most accredited theory. The mechanism suggested to be one of the initial causes of AD is an imbalance between the production and the clearance of Aβ peptides. Therefore, Amyloid Precursor Protein (APP) synthesis, trafficking and metabolism producing either the toxic Aβ peptide via the amyloidogenic pathway or the sAPPα fragment via the non amyloidogenic pathway have become appealing subjects of study. Being able to reduce the formation of the toxic Aβ peptides is obviously an immediate approach in the trial to prevent AD. The following review summarizes the most relevant discoveries in the field of the last decades.

Novel GαS-protein signaling associated with membrane-tethered amyloid precursor protein intracellular domain

Numerous physiological functions, including a role as a cell surface receptor, have been ascribed to Alzheimer's disease-associated amyloid precursor protein (APP). However, detailed analysis of intracellular signaling mediated by APP in neurons has been lacking. Here, we characterized intrinsic signaling associated with membrane-bound APP C-terminal fragments, which are generated following APP ectodomain release by α- or β-secretase cleavage. We found that accumulation of APP C-terminal fragments or expression of membrane-tethered APP intracellular domain results in adenylate cyclase-dependent activation of PKA (protein kinase A) and inhibition of GSK3β signaling cascades, and enhancement of axodendritic arborization in rat immortalized hippocampal neurons, mouse primary cortical neurons, and mouse neuroblastoma. We discovered an interaction between BBXXB motif of APP intracellular domain and the heterotrimeric G-protein subunit Gα(S), and demonstrate that Gα(S) coupling to adenylate cyclase mediates membrane-tethered APP intracellular domain-induced neurite outgrowth. Our study provides clear evidence that APP intracellular domain can have a nontranscriptional role in regulating neurite outgrowth through its membrane association. The novel functional coupling of membrane-bound APP C-terminal fragments with Gα(S) signaling identified in this study could impact several brain functions such as synaptic plasticity and memory formation.

Calsyntenin-1 mediates axonal transport of the amyloid precursor protein and regulates Aβ production

Understanding the mechanisms that control processing of the amyloid precursor protein (APP) to produce amyloid-β (Aβ) peptide represents a key area of Alzheimer's disease research. Here, we show that siRNA-mediated loss of calsyntenin-1 in cultured neurons alters APP processing to increase production of Aβ. We also show that calsyntenin-1 is reduced in Alzheimer's disease brains and that the extent of this reduction correlates with increased Aβ levels. Calsyntenin-1 is a ligand for kinesin-1 light chains and APP is transported through axons on kinesin-1 molecular motors. Defects in axonal transport are an early pathological feature in Alzheimer's disease and defective APP transport is known to increase Aβ production. We show that calsyntenin-1 and APP are co-transported through axons and that siRNA-induced loss of calsyntenin-1 markedly disrupts axonal transport of APP. Thus, perturbation to axonal transport of APP on calsyntenin-1 containing carriers induces alterations to APP processing that increase production of Aβ. Together, our findings suggest that disruption of calsyntenin-1-associated axonal transport of APP is a pathogenic mechanism in Alzheimer's disease.

Alzheimer mechanisms and therapeutic strategies

There are still no effective treatments to prevent, halt, or reverse Alzheimer's disease, but research advances over the past three decades could change this gloomy picture. Genetic studies demonstrate that the disease has multiple causes. Interdisciplinary approaches combining biochemistry, molecular and cell biology, and transgenic modeling have revealed some of its molecular mechanisms. Progress in chemistry, radiology, and systems biology is beginning to provide useful biomarkers, and the emergence of personalized medicine is poised to transform pharmaceutical development and clinical trials. However, investigative and drug development efforts should be diversified to fully address the multifactoriality of the disease.