Multi-compartmental modeling of SORLA’s influence on amyloidogenic processing in Alzheimer’s disease

BACKGROUND: Proteolytic breakdown of the amyloid precursor protein (APP) by secretases is a complex cellular process that results in formation of neurotoxic Aβ peptides, causative of neurodegeneration in Alzheimer’s disease (AD). Processing involves monomeric and dimeric forms of APP that traffic through distinct cellular compartments where the various secretases reside. Amyloidogenic processing is also influenced by modifiers such as sorting receptor-related protein (SORLA), an inhibitor of APP breakdown and major AD risk factor. RESULTS: In this study, we developed a multi-compartment model to simulate the complexity of APP processing in neurons and to accurately describe the effects of SORLA on these processes. Based on dose–response data, our study concludes that SORLA specifically impairs processing of APP dimers, the preferred secretase substrate. In addition, SORLA alters the dynamic behavior of β-secretase, the enzyme responsible for the initial step in the amyloidogenic processing cascade. CONCLUSIONS: Our multi-compartment model represents a major conceptual advance over single-compartment models previously used to simulate APP processing; and it identified APP dimers and β-secretase as the two distinct targets of the inhibitory action of SORLA in Alzheimer’s disease

Soluble Alpha-APP (sAPPalpha) Regulates CDK5 Expression and Activity in Neurons

A growing body of evidence suggests a role for soluble alpha-amyloid precursor protein (sAPPalpha) in pathomechanisms of Alzheimer disease (AD). This cleavage product of APP was identified to have neurotrophic properties. However, it remained enigmatic what proteins, targeted by sAPPalpha, might be involved in such neuroprotective actions. Here, we used high-resolution two- dimensional polyacrylamide gel electrophoresis to analyze proteome changes downstream of sAPPalpha in neurons. We present evidence that sAPPalpha regulates expression and activity of CDK5, a kinase that plays an important role in AD pathology. We also identified the cytoprotective chaperone ORP150 to be induced by sAPPalpha as part of this protective response. Finally, we present functional evidence that the sAPPalpha receptor SORLA is essential to mediate such molecular functions of sAPPalpha in neurons

Major transcriptome re-organisation and abrupt changes in signalling, cell cycle and chromatin regulation at neural differentiation <em>in vivo</em>

Here, we exploit the spatial separation of temporal events of neural differentiation in the elongating chick body axis to provide the first analysis of transcriptome change in progressively more differentiated neural cell populations in vivo. Microarray data, validated against direct RNA sequencing, identified: (1) a gene cohort characteristic of the multi-potent stem zone epiblast, which contains neuro-mesodermal progenitors that progressively generate the spinal cord; (2) a major transcriptome reorganisation as cells then adopt a neural fate; and (3) increasing diversity as neural patterning and neuron production begin. Focussing on the transition from multi-potent to neural state cells, we capture changes in major signalling pathways, uncover novel Wnt and Notch signalling dynamics, and implicate new pathways (mevalonate pathway/steroid biogenesis and TGF beta). This analysis further predicts changes in cellular processes, cell cycle, RNA-processing and protein turnover as cells acquire neural fate. We show that these changes are conserved across species and provide biological evidence for reduced proteasome efficiency and a novel lengthening of S phase. This latter step may provide time for epigenetic events to mediate large-scale transcriptome re-organisation; consistent with this, we uncover simultaneous downregulation of major chromatin modifiers as the neural programme is established. We further demonstrate that transcription of one such gene, HDAC1, is dependent on FGF signalling, making a novel link between signals that control neural differentiation and transcription of a core regulator of chromatin organisation. Our work implicates new signalling pathways and dynamics, cellular processes and epigenetic modifiers in neural differentiation in vivo, identifying multiple new potential cellular and molecular mechanisms that direct differentiation

Nuclear positioning and a non-uniform metabolism exert epigenetic control of gene expression in the wing disc

In my PhD project, I used the pseudostratified epithelium of the wing imaginal disc of Drosophila melanogaster as a model system to study the significance of cellular metabolism and nuclear positioning for tissue development in vivo. The wing disc is a larval precursor tissue that mainly gives rise to the adult wing. It is surrounded by the larval hemolymph, providing metabolites, such as lipoprotein-bound lipids. As a read-out for the metabolic state of the wing disc, I investigated the disc epigenome as most histone modifying and demodifying enzymes require metabolic co-factors or substrates. In this study, I identified a nonuniform distribution of histone acetylation in the wing disc with high acetylation levels specifically in nuclei facing the tissue surface. This rim pattern is not defined by cell cycle or cellular identity but by the position of the nucleus within the tissue. Accordingly, genetic ablation of the disc rim causes the formation of a new rim with high histone acetylation in the remaining, now outward facing, tissue region. High acetylation levels in nuclei in the disc rim correlate with hemolymph-exposure as indicated by accumulation of lipoproteins on outwardfacing membranes. The acetylation pattern persists during wing disc development suggesting a continuous regulatory role in tissue growth. The non-uniform distribution of histone acetylation is achieved by local activity of the acetyltransferase nejire in the rim of the disc. Spatially restricted nejire activity in the rim is not determined by non-uniform intrinsic activity or expression of the enzyme but by local availability of its metabolic substrate acetyl-CoA. Required acetyl-CoA is specifically generated in outward-facing nuclei due to increased levels of nuclear acetyl-CoA synthase which converts acetate to acetyl-CoA. Metabolically, this rim acetylation pattern depends on acetate derived from fatty acid β-oxidation, which is also nonuniform and increased in the rim of the wing disc, as evidenced by a high mitochondrial membrane potential in this tissue region. Rim histone acetylation epigenetically impacts expression of genes in the wing disc implicated in developmentally important signaling pathways, including Hippo, Notch, and Hedgehog. Jointly, my findings suggest a novel role for nuclear positioning and a concordantly aligned metabolism in determining cell fate and, consequently, wing disc development

Cellular uptake of steroid carrier proteins - mechanisms and implications

Steroid hormones are believed to enter cells solely by free diffusion through the plasma membrane. However, recent studies suggest the existence of cellular uptake pathways for carrier-bound steroids. Similar to the clearance of cholesterol via lipoproteins, these pathways involve the recognition of carrier proteins by endocytic receptors on the surface of target cells, followed by internalization and cellular delivery of the bound sterols. Here, we discuss the emerging concept that steroid hormones can selectively enter steroidogenic tissues by receptor-mediated endocytosis; and we discuss the implications of these uptake pathways for steroid hormone metabolism and action in vivo

Protein sorting gone wrong - VPS10P domain receptors in cardiovascular and metabolic diseases

VPS10P domain receptors are a unique class of sorting receptors that direct intracellular transport of target proteins in neurons and that play central roles in neurodegenerative processes. Surprisingly, genome-wide association studies now implicate the very same receptors in cardiovascular and metabolic disturbances. In this review, we discuss current findings that uncovered some of the molecular mechanisms whereby sorting receptors, such as SORLA, sortilin, and SORCS1 control homeostasis in cardiovascular and metabolic tissues, and how they promote hypercholesterolemia, atherosclerosis, obesity, and diabetes, when being altered

Excitatory amino acid transporters in physiology and disorders of the central nervous system

Excitatory amino acid transporters (EAATs) encompass a class of five transporters with distinct expression in neurons and glia of the central nervous system (CNS). EAATs are mainly recognized for their role in uptake of the amino acid glutamate, the major excitatory neurotransmitter. EAATs-mediated clearance of glutamate released by neurons is vital to maintain proper glutamatergic signalling and to prevent toxic accumulation of this amino acid in the extracellular space. In addition, some EAATs also act as chloride channels or mediate the uptake of cysteine, required to produce the reactive oxygen speciesscavenger glutathione. Given their central role in glutamate homeostasis in the brain, as well as their additional activities, it comes as no surprise that EAAT dysfunctions have been implicated in numerous acute or chronic diseases of the CNS, including ischemic stroke and epilepsy, cerebellar ataxias, amyotrophic lateral sclerosis, Alzheimer´s disease and Huntington´s disease. Here we review the studies in cellular and animal models, as well as in humans that highlight the roles of EAATs in the pathogenesis of these devastating disorders. We also discuss the mechanisms regulating EAATs expression and intracellular trafficking and new exciting possibilities to modulate EAATs and to provide neuroprotection in course of pathologies affecting the CNS

A dynamical model of lipoprotein metabolism

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Risk factor SORL1: from genetic association to functional validation in Alzheimer's disease

Alzheimer's disease (AD) represents one of the most dramatic threats to healthy aging and devising effective treatments for this devastating condition remains a major challenge in biomedical research. Much has been learned about the molecular concepts that govern proteolytic processing of the amyloid precursor protein to amyloid-{beta} peptides (A{beta}), and how accelerated accumulation of neurotoxic A{beta} peptides underlies neuronal cell death in rare familial but also common sporadic forms of this disease. Out of a plethora of proposed modulators of amyloidogenic processing, one protein emerged as a key factor in AD pathology, a neuronal sorting receptor termed SORLA. Independent approaches using human genetics, clinical pathology, or exploratory studies in animal models all converge on this receptor that is now considered a central player in AD-related processes by many. This review will provide a comprehensive overview of the evidence implicating SORLA-mediated protein sorting in neurodegenerative processes, and how receptor gene variants in the human population impair functional receptor expression in sporadic but possibly also in autosomal-dominant forms of AD