Identification of novel NPRAP/δ-catenin-interacting proteins and the direct association of NPRAP with dynamin 2

A Structural Network Analysis of Neuronal ArhGAP21/23 Interactors by Computational Modeling

RhoGTPase-activating proteins (RhoGAPs) play multiple roles in neuronal development; however, details of their substrate recognition system remain elusive. ArhGAP21 and ArhGAP23 are RhoGAPs that contain N-terminal PDZ and pleckstrin homology domains. In the present study, the RhoGAP domain of these ArhGAPs was computationally modeled by template-based methods and the AlphaFold2 software program, and their intrinsic RhoGTPase recognition mechanism was analyzed from the domain structures using the protein docking programs HADDOCK and HDOCK. ArhGAP21 was predicted to preferentially catalyze Cdc42, RhoA, RhoB, RhoC, and RhoG and to downregulate RhoD and Tc10 activities. Regarding ArhGAP23, RhoA and Cdc42 were deduced to be its substrates, whereas RhoD downregulation was predicted to be less efficient. The PDZ domains of ArhGAP21/23 possess the FTLRXXXVY sequence, and similar globular folding consists of antiparalleled β-sheets and two α-helices that are conserved with PDZ domains of MAST-family proteins. A peptide docking analysis revealed the specific interaction of the ArhGAP23 PDZ domain with the PTEN C-terminus. The pleckstrin homology domain structure of ArhGAP23 was also predicted, and the functional selectivity for the interactors regulated by the folding and disordered domains in ArhGAP21 and ArhGAP23 was examined by an in silico analysis. An interaction analysis of these RhoGAPs revealed the existence of mammalian ArhGAP21/23-specific type I and type III Arf- and RhoGTPase-regulated signaling. Multiple recognition systems of RhoGTPase substrates and selective Arf-dependent localization of ArhGAP21/23 may form the basis of the functional core signaling necessary for synaptic homeostasis and axon/dendritic transport regulated by RhoGAP localization and activities.

Improved memory and reduced anxiety in δ-catenin transgenic mice

δ-Catenin is abundant in the brain and affects its synaptic plasticity. Furthermore, loss of δ-catenin is related to the deficits of learning and memory, mental retardation (cri-du-chat syndrome), and autism. A few studies about δ-catenin deficiency mice were performed. However, the effect of δ-catenin overexpression in the brain has not been investigated as yet. Therefore we generated a δ-catenin overexpressing mouse model. To generate a transgenic mouse model overexpressing δ-catenin in the brain, δ-catenin plasmid having a Thy-1 promotor was microinjected in C57BL/6 mice. Our results showed δ-catenin transgenic mice expressed higher levels of N-cadherin, β-catenin, and p120-catenin than did wild type mice. Furthermore, δ-catenin transgenic mice exhibited better object recognition, better sociability, and lower anxiety than wild type mice. However, both mice groups showed a similar pattern in locomotion tests. Although δ-catenin transgenic mice show similar locomotion, they show improved sociability and reduced anxiety. These characteristics are opposite to the symptoms of autism or mental retardation, which are caused when δ-catenin is deficient. These results suggest that δ-catenin may alleviate symptoms of autism, Alzheimer's disease and mental retardation.

Effects of δ-Catenin on APP by Its Interaction with Presenilin-1

Alzheimer's disease (AD) is the most frequent age-related human neurological disorder. The characteristics of AD include senile plaques, neurofibrillary tangles, and loss of synapses and neurons in the brain. β-Amyloid (Aβ) peptide is the predominant proteinaceous component of senile plaques. The amyloid hypothesis states that Aβ initiates the cascade of events that result in AD. Amyloid precursor protein (APP) processing plays an important role in Aβ production, which initiates synaptic and neuronal damage. δ-Catenin is known to be bound to presenilin-1 (PS-1), which is the main component of the γ-secretase complex that regulates APP cleavage. Because PS-1 interacts with both APP and δ-catenin, it is worth studying their interactive mechanism and/or effects on each other. Our immunoprecipitation data showed that there was no physical association between δ-catenin and APP. However, we observed that δ-catenin could reduce the binding between PS-1 and APP, thus decreasing the PS-1 mediated APP processing activity. Furthermore, δ-catenin reduced PS-1-mediated stabilization of APP. The results suggest that δ-catenin can influence the APP processing and its level by interacting with PS-1, which may eventually play a protective role in the degeneration of an Alzheimer's disease patient.

Cytoskeleton as a Target of Quinolinic Acid Neurotoxicity: Insight from Animal Models

Cytoskeletal proteins are increasingly recognized as having important roles as a target of the action of different neurotoxins. In the last years, several works of our group have shown that quinolinic acid (QUIN) was able to disrupt the homeostasis of the cytoskeleton of neural cells and this was associated with cell dysfunction and neurodegeneration. QUIN is an excitotoxic metabolite of tryptophan metabolism and its accumulation is associated with several neurodegenerative diseases. In the present review, we provide a comprehensive view of the actions of QUIN upstream of glutamate receptors, eliciting kinase/phosphatase signaling cascades that disrupt the homeostasis of the phosphorylation system associated with intermediate filament proteins of astrocytes and neurons. We emphasize the critical role of calcium in these actions and the evidence that misregulated cytoskeleton takes part of the cell response to the injury resulting in neurodegeneration in different brain regions, disrupted cell signaling in acute tissue slices, and disorganized cytoskeleton with altered cell morphology in primary cultures. We also discuss the interplay among misregulated cytoskeleton, oxidative stress, and cell-cell contact through gap junctions mediating the quinolinic acid injury in rat brain. The increasing amount of cross talks identified between cytoskeletal proteins and cellular signaling cascades reinforces the exciting possibility that cytoskeleton could be a new target in the neurotoxicity of QUIN and further studies will be necessary to develop strategies to protect the cytoskeleton and counteracts the cytotoxicity of this metabolite.

δ-Catenin interacts with LEF-1 and negatively regulates its transcriptional activity

δ-Catenin and β-catenin belong to different subfamilies of armadillo proteins but share some common binding partners, such as E-cadherin. This is the first study that demonstrated a novel common binding partner for δ-catenin and β-catenin, lymphoid enhancer factor-1 (LEF-1). We found that the N-terminus of δ-catenin (amino acids 85-325) bound to the middle region of LEF-1 unlike β-catenin. Overexpressed δ-catenin entered the nucleus and inhibited LEF-1-mediated transcriptional activity in Bosc23 and DLD-1 cell lines. The current study provided novel insights that will provide a better understanding of the effects of δ-catenin on Wnt/LEF-1-mediated transcriptional activity.

Neurofilament dynamics and involvement in neurological disorders

Neurons are extremely polarised cells in which the cytoskeleton, composed of microtubules, microfilaments and neurofilaments, plays a crucial role in maintaining structure and function. Neurofilaments, the 10-nm intermediate filaments of neurons, provide structure and mechanoresistance but also provide a scaffolding for the organization of the nucleus and organelles such as mitochondria and ER. Disruption of neurofilament organization and expression or metabolism of neurofilament proteins is characteristic of certain neurological syndromes including Amyotrophic Lateral Sclerosis, Charcot-Marie-Tooth sensorimotor neuropathies and Giant Axonal Neuropathy. Microfluorometric live imaging techniques have been instrumental in revealing the dynamics of neurofilament assembly and transport and their functions in organizing intracellular organelle networks. The insolubility of neurofilament proteins has limited identifying interactors by conventional biochemical techniques but yeast two-hybrid experiments have revealed new roles for oligomeric, nonfilamentous structures including vesicular trafficking. Although having long half-lives, new evidence points to degradation of subunits by the ubiquitin-proteasome system as a mechanism of normal turnover. Although certain E3-ligases ubiquitinating neurofilament proteins have been identified, the overall process of neurofilament degradation is not well understood. We review these mechanisms of neurofilament homeostasis and abnormalities in motor neuron and peripheral nerve disorders. Much remains to discover about the disruption of processes that leads to their pathological aggregation and accumulation and the relevance to pathogenesis. Understanding these mechanisms is crucial for identifying novel therapeutic strategies.