The crystal structure of DR6 in complex with the amyloid precursor protein provides insight into death receptor activation

Endothelial DR6 in blood-brain barrier malfunction in Alzheimer's disease

The impairment of the blood-brain barrier (BBB) has been increasingly recognised as a critical element in the early pathogenesis of Alzheimer's disease (AD), prompting a focus on brain endothelial cells (BECs), which serve as the primary constituents of the BBB. Death receptor 6 (DR6) is highly expressed in brain vasculature and acts downstream of the Wnt/β-catenin pathway to promote BBB formation during development. Here, we found that brain endothelial DR6 levels were significantly reduced in a murine model of AD (APPswe/PS1dE9 mice) at the onset of amyloid-β (Aβ) accumulation. Toxic Aβ25-35 oligomer treatment recapitulated the reduced DR6 in cultured BECs. We further showed that suppressing DR6 resulted in BBB malfunction in the presence of Aβ25-35 oligomers. In contrast, overexpressing DR6 increased the level of BBB functional proteins through the activation of the Wnt/β-catenin and JNK pathways. More importantly, DR6 overexpression in BECs was sufficient to rescue BBB dysfunction in vitro. In conclusion, our findings provide new insight into the role of endothelial DR6 in AD pathogenesis, highlighting its potential as a therapeutic target to tackle BBB dysfunction in early-stage AD progression.

Amyloid Precursor Protein: A Regulatory Hub in Alzheimer's Disease

Decades of research have demonstrated an incontrovertible role of amyloid-β (Aβ) in the etiology of Alzheimer's disease (AD). However, the overemphasis on the pathological impacts of Aβ may obscure the role of its metabolic precursor, amyloid precursor protein (APP), as a significant hub in the occurrence and progression of AD. The complicated enzymatic processing, ubiquitous receptor-like properties, and abundant expression of APP in the brain, as well as its close links with systemic metabolism, mitochondrial function and neuroinflammation, imply that APP plays multifaceted roles in AD. In this review, we briefly describe the evolutionarily conserved biological characteristics of APP, including its structure, functions and enzymatic processing. We also discuss the possible involvement of APP and its enzymatic metabolites in AD, both detrimental and beneficial. Finally, we describe pharmacological agents or genetic approaches with the capability to reduce APP expression or inhibit its cellular internalization, which can ameliorate multiple aspects of AD pathologies and halt disease progression. These approaches provide a basis for further drug development to combat this terrible disease.

The Labyrinthine Landscape of APP Processing: State of the Art and Possible Novel Soluble APP-Related Molecular Players in Traumatic Brain Injury and Neurodegeneration

Amyloid Precursor Protein (APP) and its cleavage processes have been widely investigated in the past, in particular in the context of Alzheimer’s Disease (AD). Evidence of an increased expression of APP and its amyloidogenic-related cleavage enzymes, β-secretase 1 (BACE1) and γ-secretase, at the hit axon terminals following Traumatic Brain Injury (TBI), firstly suggested a correlation between TBI and AD. Indeed, mild and severe TBI have been recognised as influential risk factors for different neurodegenerative diseases, including AD. In the present work, we describe the state of the art of APP proteolytic processing, underlining the different roles of its cleavage fragments in both physiological and pathological contexts. Considering the neuroprotective role of the soluble APP alpha (sAPPα) fragment, we hypothesised that sAPPα could modulate the expression of genes of interest for AD and TBI. Hence, we present preliminary experiments addressing sAPPα-mediated regulation of BACE1, Isthmin 2 (ISM2), Tetraspanin-3 (TSPAN3) and the Vascular Endothelial Growth Factor (VEGFA), each discussed from a biological and pharmacological point of view in AD and TBI. We finally propose a neuroprotective interaction network, in which the Receptor for Activated C Kinase 1 (RACK1) and the signalling cascade of PKCβII/nELAV/VEGF play hub roles, suggesting that vasculogenic-targeting therapies could be a feasible approach for vascular-related brain injuries typical of AD and TBI.

Peptidomimetics in cancer targeting

The low efficiency of treatment strategies is one of the main obstacles to developing cancer inhibitors. Up to now, various classes of therapeutics have been developed to inhibit cancer progression. Peptides due to their small size and easy production compared to proteins are highly regarded in designing cancer vaccines and oncogenic pathway inhibitors. Although peptides seem to be a suitable therapeutic option, their short lifespan, instability, and low binding affinity for their target have not been widely applicable against malignant tumors. Given the peptides' disadvantages, a new class of agents called peptidomimetic has been introduced. With advances in physical chemistry and biochemistry, as well as increased knowledge about biomolecule structures, it is now possible to chemically modify peptides to develop efficient peptidomimetics. In recent years, numerous studies have been performed to the evaluation of the effectiveness of peptidomimetics in inhibiting metastasis, angiogenesis, and cancerous cell growth. Here, we offer a comprehensive review of designed peptidomimetics to diagnose and treat cancer.

Role of metalloproteases in the CD95 signaling pathways

CD95L (also known as FasL or CD178) is a member of the tumor necrosis family (TNF) superfamily. Although this transmembrane ligand has been mainly considered as a potent apoptotic inducer in CD95 (Fas)-expressing cells, more recent studies pointed out its role in the implementation of non-apoptotic signals. Accordingly, this ligand has been associated with the aggravation of inflammation in different auto-immune disorders and in the metastatic occurrence in different cancers. Although it remains to decipher all key factors involved in the ambivalent role of this ligand, accumulating clues suggest that while the membrane bound CD95L triggers apoptosis, its soluble counterpart generated by metalloprotease-driven cleavage is responsible for its non-apoptotic functions. Nonetheless, the metalloproteases (MMPs and ADAMs) involved in the CD95L shedding, the cleavage sites and the different stoichiometries and functions of the soluble CD95L remain to be elucidated. To better understand how soluble CD95L triggers signaling pathways from apoptosis to inflammation or cell migration, we propose herein to summarize the different metalloproteases that have been described to be able to shed CD95L, their cleavage sites and the biological functions associated with the released ligands. Based on these new findings, the development of CD95/CD95L-targeting therapeutics is also discussed.

A Structural and Functional Perspective of Death Receptor 6

As a member of the tumor necrosis factor receptor superfamily (TNFRSF), death receptor 6 (DR6) has a similar structural architecture to other family members. The extracellular region of DR6 contains four cysteine-rich domains, followed by a single-pass transmembrane domain and an intracellular region. Since its discovery, DR6 has become an orphan receptor ubiquitously expressed to transduce unique signaling pathways. Although the free ectodomains of β-amyloid precursor protein (APP) can bind to DR6 to induce apoptotic signals, the natural ligands of DR6 still remain largely unknown. In this review, we focus on recent research progress of structural and functional studies on DR6 for better understanding DR6-mediated signaling and the treatment of DR6-related diseases.

Multi-Arm PEG/Peptidomimetic Conjugate Inhibitors of DR6/APP Interaction Block Hematogenous Tumor Cell Extravasation

The binding of amyloid precursor protein (APP) expressed on tumor cells to death receptor 6 (DR6) could initiate the necroptosis pathway, which leads to necroptotic cell death of vascular endothelial cells (ECs) and results in tumor cells (TCs) extravasation and metastasis. This study reports the first inhibitor of DR6/APP interaction as a novel class of anti-hematogenous metastatic agent. By rationally utilizing three combined strategies including selection based on phage display library, d-retro-inverso modification, and multiple conjugation of screened peptidomimetic with 4-arm PEG, the polymer-peptidomimetic conjugate PEG-tAHP-DRI (tetra-(D-retro-inverso isomer of AHP-12) substitued 4-arm PEG5k ) is obtained as the most promising agent with the strongest binding potency (KD  = 51.12 × 10-9  m) and excellent pharmacokinetic properties. Importantly, PEG-tAHP-DRI provides efficient protection against TC-induced ECs necroptosis both in vitro and in vivo. Moreover, this ligand exhibits prominent anti-hematogenous metastatic activity in serval different metastatic mouse models (B16F10, 4T1, CT26, and spontaneous lung metastasis of 4T1 orthotopic tumor model) and displays no apparent detrimental effects in preliminary safety evaluation. Collectively, this study demonstrates the feasibility of exploiting DR6/APP interaction to regulate hematogenous tumor cells transendothelial migration and provides PEG-tAHP-DRI as a novel and promising inhibitor of DR6/APP interaction for developments of anti-hematogenous metastatic therapies.

Regulation of Death Receptor Signaling by S-Palmitoylation and Detergent-Resistant Membrane Micro Domains-Greasing the Gears of Extrinsic Cell Death Induction, Survival, and Inflammation

Death-receptor-mediated signaling results in either cell death or survival. Such opposite signaling cascades emanate from receptor-associated signaling complexes, which are often formed in different subcellular locations. The proteins involved are frequently post-translationally modified (PTM) by ubiquitination, phosphorylation, or glycosylation to allow proper spatio-temporal regulation/recruitment of these signaling complexes in a defined cellular compartment. During the last couple of years, increasing attention has been paid to the reversible cysteine-centered PTM S-palmitoylation. This PTM regulates the hydrophobicity of soluble and membrane proteins and modulates protein:protein interaction and their interaction with distinct membrane micro-domains (i.e., lipid rafts). We conclude with which functional and mechanistic roles for S-palmitoylation as well as different forms of membrane micro-domains in death-receptor-mediated signal transduction were unraveled in the last two decades.

Functions of amyloid precursor protein in metabolic diseases

Amyloid precursor protein (APP) is a transmembrane precursor protein that is widely expressed in the central nervous system and peripheral tissues in the liver and pancreas, adipose tissue, and myotubes. APP can be cleaved by proteases in two different ways to produce a variety of short peptides, each with different physiological properties and functions. APP peptides generated by non-amyloidogenic processing can positively influence metabolism, while the peptides produced by amyloidogenic processing have the opposite effects. Here, we summarize the regulatory effects of APP and its cleavage peptides on metabolism in the central nervous system and peripheral tissues. In addition, abnormal expression and function of APP and APP-derived peptides are associated with metabolic diseases, such as type 2 diabetes, obesity, non-alcoholic fatty liver disease, and cardiovascular disease, and cancers. Pharmacological intervention of APP function or reduction of the production of peptides derived from amyloidogenic processing may be effective strategies for the prevention and treatment of Alzheimer's disease, and they may also provide new guidance for the treatment of metabolic diseases.

Systematic elucidation of neuron-astrocyte interaction in models of amyotrophic lateral sclerosis using multi-modal integrated bioinformatics workflow

Cell-to-cell communications are critical determinants of pathophysiological phenotypes, but methodologies for their systematic elucidation are lacking. Herein, we propose an approach for the Systematic Elucidation and Assessment of Regulatory Cell-to-cell Interaction Networks (SEARCHIN) to identify ligand-mediated interactions between distinct cellular compartments. To test this approach, we selected a model of amyotrophic lateral sclerosis (ALS), in which astrocytes expressing mutant superoxide dismutase-1 (mutSOD1) kill wild-type motor neurons (MNs) by an unknown mechanism. Our integrative analysis that combines proteomics and regulatory network analysis infers the interaction between astrocyte-released amyloid precursor protein (APP) and death receptor-6 (DR6) on MNs as the top predicted ligand-receptor pair. The inferred deleterious role of APP and DR6 is confirmed in vitro in models of ALS. Moreover, the DR6 knockdown in MNs of transgenic mutSOD1 mice attenuates the ALS-like phenotype. Our results support the usefulness of integrative, systems biology approach to gain insights into complex neurobiological disease processes as in ALS and posit that the proposed methodology is not restricted to this biological context and could be used in a variety of other non-cell-autonomous communication mechanisms.

Functional roles in cell signaling of adaptor protein TRADD from a structural perspective

TRADD participates in various receptor signaling pathways and plays vital roles in many biological activities, including cell survival and apoptosis, in different cellular contexts. TRADD has two distinct functional domains, a TRAF-binding domain at the N-terminus and a death domain (DD) at the C-terminus. The TRAF binding domain of TRADD folds into an α-β plait topology and is mainly responsible for binding TRAF2, while the TRADD-DD can interact with a variety of DD-containing proteins, including receptors and intracellular signaling molecules. After activation of specific receptors such as TNFR1 and DR3, TRADD can bind to the receptor through DD-DD interaction, creating a membrane-proximal platform for the recruitment of downstream molecules to propagate cellular signals. In this review, we highlight recent advances in the studies of the structural mechanism of TRADD adaptor functions for NF-κB activation and apoptosis induction. We also provide suggestions for future structure research related to TRADD-mediated signaling pathways.

p110δ PI3-Kinase Inhibition Perturbs APP and TNFα Trafficking, Reduces Plaque Burden, Dampens Neuroinflammation, and Prevents Cognitive Decline in an Alzheimer's Disease Mouse Model

Alzheimer's disease (AD) is associated with the cleavage of the amyloid precursor protein (APP) to produce the toxic amyloid-β (Aβ) peptide. Accumulation of Aβ, together with the concomitant inflammatory response, ultimately leads to neuronal death and cognitive decline. Despite AD progression being underpinned by both neuronal and immunological components, therapeutic strategies based on dual targeting of these systems remains unexplored. Here, we report that inactivation of the p110δ isoform of phosphoinositide 3-kinase (PI3K) reduces anterograde axonal trafficking of APP in hippocampal neurons and dampens secretion of the inflammatory cytokine tumor necrosis factor-alpha by microglial cells in the familial AD APP_swe/PS1_ΔE9 (APP/PS1) mouse model. Moreover, APP/PS1 mice with kinase-inactive PI3Kδ (δ^D910A) had reduced Aβ peptides levels and plaques in the brain and an abrogated inflammatory response compared with APP/PS1 littermates. Mechanistic investigations reveal that PI3Kδ inhibition decreases the axonal transport of APP by eliciting the formation of highly elongated tubular-shaped APP-containing carriers, reducing the levels of secreted Aβ peptide. Importantly, APP/PS1/δ^D910A mice exhibited no spatial learning or memory deficits. Our data highlight inhibition of PI3Kδ as a new approach to protect against AD pathology due to its dual action of dampening microglial-dependent neuroinflammation and reducing plaque burden by inhibition of neuronal APP trafficking and processing.SIGNIFICANCE STATEMENT During Alzheimer's disease (AD), the accumulation of the toxic amyloid-β (Aβ) peptide in plaques is associated with a chronic excessive inflammatory response. Uncovering new drug targets that simultaneously reduce both Aβ plaque load and neuroinflammation holds therapeutic promise. Using a combination of genetic and pharmacological approaches, we found that the p110δ isoform of phosphoinositide 3-kinase (PI3K) is involved in anterograde trafficking of the amyloid precursor protein in neurons and in the secretion of tumor necrosis factor-alpha from microglial cells. Genetic inactivation of PI3Kδ reduces Aβ plaque deposition and abrogates the inflammatory response, resulting in a complete rescue of the life span and spatial memory performance. We conclude that inhibiting PI3Kδ represents a novel therapeutic approach to ameliorate AD pathology by dampening plaque accumulation and microglial-dependent neuroinflammation.

The TNF Family of Ligands and Receptors: Communication Modules in the Immune System and Beyond

The tumor necrosis factor (TNF) and TNF receptor (TNFR) superfamilies (TNFSF/TNFRSF) include 19 ligands and 29 receptors that play important roles in the modulation of cellular functions. The communication pathways mediated by TNFSF/TNFRSF are essential for numerous developmental, homeostatic, and stimulus-responsive processes in vivo. TNFSF/TNFRSF members regulate cellular differentiation, survival, and programmed death, but their most critical functions pertain to the immune system. Both innate and adaptive immune cells are controlled by TNFSF/TNFRSF members in a manner that is crucial for the coordination of various mechanisms driving either co-stimulation or co-inhibition of the immune response. Dysregulation of these same signaling pathways has been implicated in inflammatory and autoimmune diseases, highlighting the importance of their tight regulation. Investigation of the control of TNFSF/TNFRSF activities has led to the development of therapeutics with the potential to reduce chronic inflammation or promote anti-tumor immunity. The study of TNFSF/TNFRSF proteins has exploded over the last 30 yr, but there remains a need to better understand the fundamental mechanisms underlying the molecular pathways they mediate to design more effective anti-inflammatory and anti-cancer therapies.

Physiological functions of non-apoptotic caspase activity in the nervous system

Caspases are cysteine proteases that play important and well-defined roles in apoptosis and inflammation. Increasing evidence point to alternative functions of caspases where restricted and localized caspase activation within neurons allows for a variety of non-apoptotic and non-inflammatory processes required for brain development and function. In this review, we highlight sublethal caspase functions in axon and dendrite pruning, neurite outgrowth and dendrite branches formation, as well as in long-term depression and synaptic plasticity. Importantly, as non-apoptotic activity of caspases is often confined in space and time in neurons, we also discuss the mechanisms that restrict caspase activity in order to maintain the neuronal networks in a healthy and functional state.

HIV-1 counteracts an innate restriction by amyloid precursor protein resulting in neurodegeneration

While beta-amyloid (Aβ), a classic hallmark of Alzheimer's disease (AD) and dementia, has long been known to be elevated in the human immunodeficiency virus type 1 (HIV-1)-infected brain, why and how Aβ is produced, along with its contribution to HIV-associated neurocognitive disorder (HAND) remains ill-defined. Here, we reveal that the membrane-associated amyloid precursor protein (APP) is highly expressed in macrophages and microglia, and acts as an innate restriction against HIV-1. APP binds the HIV-1 Gag polyprotein, retains it in lipid rafts and blocks HIV-1 virion production and spread. To escape this restriction, Gag promotes secretase-dependent cleavage of APP, resulting in the overproduction of toxic Aβ isoforms. This Gag-mediated Aβ production results in increased degeneration of primary cortical neurons, and can be prevented by γ-secretase inhibitor treatment. Interfering with HIV-1's evasion of APP-mediated restriction also suppresses HIV-1 spread, offering a potential strategy to both treat infection and prevent HAND.

Prostaglandin J2 promotes O-GlcNAcylation raising APP processing by α- and β-secretases: relevance to Alzheimer's disease

Regulation of the amyloid precursor protein (APP) processing by α- and β-secretases is of special interest to Alzheimer's disease (AD), as these proteases prevent or mediate amyloid beta formation, respectively. Neuroinflammation is also implicated in AD. Our data demonstrate that the endogenous mediator of inflammation prostaglandin J2 (PGJ2) promotes full-length APP (FL-APP) processing by α- and β-secretases. The decrease in FL-APP was independent of proteasomal, lysosomal, calpain, caspase, and γ-secretase activities. Moreover, PGJ2-treatment promoted cleavage of secreted APP, specifically sAPPα and sAPPβ, generated by α and β-secretase, respectively. Notably, PGJ2-treatment induced caspase-dependent cleavage of sAPPβ. Mechanistically, PGJ2-treatment selectively diminished mature (O- and N-glycosylated) but not immature (N-glycosylated only) FL-APP. PGJ2-treatment also increased the overall levels of protein O-GlcNAcylation, which occurs within the nucleocytoplasmic compartment. It is known that APP undergoes O-GlcNAcylation and that the latter protects proteins from proteasomal degradation. Our results suggest that by increasing protein O-GlcNAcylation levels, PGJ2 renders mature APP less prone to proteasomal degradation, thus shunting APP toward processing by α- and β-secretases.

Not just amyloid: physiological functions of the amyloid precursor protein family

Amyloid precursor protein (APP) gives rise to the amyloid-β peptide and thus has a key role in the pathogenesis of Alzheimer disease. By contrast, the physiological functions of APP and the closely related APP-like proteins (APLPs) remain less well understood. Studying these physiological functions has been challenging and has required a careful long-term strategy, including the analysis of different App-knockout and Aplp-knockout mice. In this Review, we summarize these findings, focusing on the in vivo roles of APP family members and their processing products for CNS development, synapse formation and function, brain injury and neuroprotection, as well as ageing. In addition, we discuss the implications of APP physiology for therapeutic approaches.

Death Receptor 6 Promotes Wallerian Degeneration in Peripheral Axons

Axon degeneration during development is required to sculpt a functional nervous system and is also a hallmark of pathological insult, such as injury [1, 2]. Despite similar morphological characteristics, very little overlap in molecular mechanisms has been reported between pathological and developmental degeneration [3-5]. In the peripheral nervous system (PNS), developmental axon pruning relies on receptor-mediated extrinsic degeneration mechanisms to determine which axons are maintained or degenerated [5-7]. Receptors have not been implicated in Wallerian axon degeneration; instead, axon autonomous, intrinsic mechanisms are thought to be the primary driver for this type of axon disintegration [8-10]. Here we survey the role of neuronally expressed, paralogous tumor necrosis factor receptor super family (TNFRSF) members in Wallerian degeneration. We find that an orphan receptor, death receptor 6 (DR6), is required to drive axon degeneration after axotomy in sympathetic and sensory neurons cultured in microfluidic devices. We sought to validate these in vitro findings in vivo using a transected sciatic nerve model. Consistent with the in vitro findings, DR6^-/- animals displayed preserved axons up to 4 weeks after injury. In contrast to phenotypes observed in Wld^s and Sarm1^-/- mice, preserved axons in DR6^-/- animals display profound myelin remodeling. This indicates that deterioration of axons and myelin after axotomy are mechanistically distinct processes. Finally, we find that JNK signaling after injury requires DR6, suggesting a link between this novel extrinsic pathway and the axon autonomous, intrinsic pathways that have become established for Wallerian degeneration.

Physiological role for amyloid precursor protein in adult experience-dependent plasticity

Changes in neural circuits after experience-dependent plasticity are brought about by the formation of new circuits via axonal growth and pruning. Here, using a combination of electrophysiology, adeno-associated virus-delivered fluorescent proteins, analysis of mutant mice, and two-photon microscopy, we follow long-range horizontally projecting axons in primary somatosensory cortex before and after selective whisker plucking. Whisker plucking induces axonal growth and pruning of horizontal projecting axons from neurons located in the surrounding intact whisker representations. We report that amyloid precursor protein is crucial for axonal pruning and contributes in a cell autonomous way.