Design of a Potent, Selective, and Brain-Penetrant Inhibitor of Wnt-Deactivating Enzyme Notum by Optimization of a Crystallographic Fragment Hit

Notum is a carboxylesterase that suppresses Wnt signaling through deacylation of an essential palmitoleate group on Wnt proteins. There is a growing understanding of the role Notum plays in human diseases such as colorectal cancer and Alzheimer's disease, supporting the need to discover improved inhibitors, especially for use in models of neurodegeneration. Here, we have described the discovery and profile of 8l (ARUK3001185) as a potent, selective, and brain-penetrant inhibitor of Notum activity suitable for oral dosing in rodent models of disease. Crystallographic fragment screening of the Diamond-SGC Poised Library for binding to Notum, supported by a biochemical enzyme assay to rank inhibition activity, identified 6a and 6b as a pair of outstanding hits. Fragment development of 6 delivered 8l that restored Wnt signaling in the presence of Notum in a cell-based reporter assay. Assessment in pharmacology screens showed 8l to be selective against serine hydrolases, kinases, and drug targets.

E3 ligase mahogunin (MGRN1) influences amyloid precursor protein maturation and secretion

Altered processing of the Amyloid Precursor Protein (APP) is a well-recognized central pathogenic mechanism in Alzheimer's Disease (AD), and regulation of APP processing is a major focus of research in the AD field. However, how age-associated cellular and molecular changes contribute to changes in the amyloidogenic processing of APP have not been extensively clarified so far. We here provide evidence that the processing of APP is influenced by the e3 ubiquitin ligase Mahogunin (MGRN1), a neuroprotective molecule whose levels decrease with aging. Specifically, the expression of MGRN1 inhibits the maturation of APP by sequestering it in the secretory pathway. This sequestration significantly delayed the proteolytic processing of APP, resulting in a reduced β-amyloid (Aβ) peptide release into the extracellular environment. Accordingly, a reduction of MGRN1 levels in hippocampal neurons, as it occurs during physiological aging, leads to an increased Aβ40 and Aβ42 release. We therefore propose that age contributes to the amyloidogenic processing of APP by altering its intracellular trafficking along the secretory pathway due in part to the down-regulation of MGRN1.

Sphingomyelin-induced inhibition of the plasma membrane calcium ATPase causes neurodegeneration in type A Niemann-Pick disease

Niemann-Pick disease type A (NPA) is a rare lysosomal storage disorder characterized by severe neurological alterations that leads to death in childhood. Loss-of-function mutations in the acid sphingomyelinase (ASM) gene cause NPA, and result in the accumulation of sphingomyelin (SM) in lysosomes and plasma membrane of neurons. Using ASM knockout (ASMko) mice as a NPA disease model, we investigated how high SM levels contribute to neural pathology in NPA. We found high levels of oxidative stress both in neurons from these mice and a NPA patient. Impaired activity of the plasma membrane calcium ATPase (PMCA) increases intracellular calcium. SM induces PMCA decreased activity, which causes oxidative stress. Incubating ASMko-cultured neurons in the histone deacetylase inhibitor, SAHA, restores PMCA activity and calcium homeostasis and, consequently, reduces the increased levels of oxidative stress. No recovery occurs when PMCA activity is pharmacologically impaired or genetically inhibited in vitro. Oral administration of SAHA prevents oxidative stress and neurodegeneration, and improves behavioral performance in ASMko mice. These results demonstrate a critical role for plasma membrane SM in neuronal calcium regulation. Thus, we identify changes in PMCA-triggered calcium homeostasis as an upstream mediator for NPA pathology. These findings can stimulate new approaches for pharmacological remediation in a disease with no current clinical treatments.

Neuronal activity controls Bdnf expression via Polycomb de-repression and CREB/CBP/JMJD3 activation in mature neurons

It has been recently described that in embryonic stem cells, the expression of some important developmentally regulated genes is repressed, but poised for fast activation under the appropriate stimuli. In this work we show that Bdnf promoters are repressed by Polycomb Complex 2 in mature hippocampal neurons, and basal expression is guaranteed by the coexistence with activating histone marks. Neuronal stimulation triggered by N-methyl-D-aspartate application induces the transcription of these promoters by H3K27Me3 demethylation and H3K27Me3 phosphorylation at Serine 28 leading to displacement of EZH2, the catalytic subunit of Polycomb Repressor Complex 2. Our data show that the fast transient expression of Bdnf promoters II and VI after neuronal stimulation is dependent on acetylation of histone H3K27 by CREB-p/CBP. Thus, regulatory mechanisms established during development seem to remain after differentiation controlling genes induced by different stimuli, as would be the case of early memory genes in mature neurons.

In neurons, brain-derived neurotrophic factor (BDNF) transcription is activated by synaptic activity, in part by epigenetic regulation of its promoter regions. Here the authors characterize histone modifications in response to NMDA treatment that result in different kinetics of Bdnf activation from its different promoter regions.

Developmental influence of the cellular prion protein on the gene expression profile in mouse hippocampus

The conversion of the cellular prion protein (PrP(C)) to an abnormal and protease-resistant isoform is the key event in prion diseases. Mice lacking PrP(C) are resistant to prion infection, and downregulation of PrP(C) during prion infection prevents neuronal loss and the progression to clinical disease. These results are suggestive of the potential beneficial effect of silencing PrP(C) during prion diseases. However, the silencing of a protein that is widely expressed throughout the central nervous system could be detrimental to brain homeostasis. The physiological role of PrP(C) remains still unclear, but several putative functions (e.g., neuronal development and maintenance) have been proposed. To assess the influence of PrP(C) on gene expression profile in the mouse brain, we undertook a microarray analysis by using RNA isolated from the hippocampus at two different developmental stages: newborn (4.5-day-old) and adult (3-mo-old) mice, both from wild-type and Prnp(0/0) animals. Comparing the different datasets allowed us to identify "commonly" co-regulated genes and "uniquely" deregulated genes during postnatal development. The absence of PrP(C) affected several biological pathways, the most representative being cell signaling, cell-cell communication and transduction processes, calcium homeostasis, nervous system development, synaptic transmission, and cell adhesion. However, there was only a moderate alteration of the gene expression profile in our animal models. PrP(C) deficiency did not lead to a dramatic alteration of gene expression profile and produced moderately altered gene expression levels from young to adult animals. Thus, our results may provide additional support to silencing endogenous PrP(C) levels as therapeutic approach to prion diseases.

Neurodevelopmental expression and localization of the cellular prion protein in the central nervous system of the mouse

Transmissible spongiform encephalopathies (TSEs) are neurodegenerative disorders caused by PrP(Sc), or prion, an abnormally folded form of the cellular prion protein (PrP(C)). The abundant expression of PrP(C) in the central nervous system (CNS) is a requirement for prion replication, yet despite years of intensive research the physiological function of PrP(C) still remains unclear. Several routes of investigation point out a potential role for PrP(C) in axon growth and neuronal development. Thus, we undertook a detailed analysis of the spatial and temporal expression of PrP(C) during mouse CNS development. Our findings show regional differences of the expression of PrP, with some specific white matter structures showing the earliest and highest expression of PrP(C). Indeed, all these regions are part of the thalamolimbic neurocircuitry, suggesting a potential role of PrP(C) in the development and functioning of this specific brain system.

Prion protein paralog doppel protein interacts with alpha-2-macroglobulin: a plausible mechanism for doppel-mediated neurodegeneration

Doppel protein (Dpl) is a paralog of the cellular form of the prion protein (PrP^C), together sharing common structural and biochemical properties. Unlike PrP^C, which is abundantly expressed throughout the central nervous system (CNS), Dpl protein expression is not detectable in the CNS. Interestingly, its ectopic expression in the brain elicits neurodegeneration in transgenic mice. Here, by combining native isoelectric focusing plus non-denaturing polyacrylamide gel electrophoresis and mass spectrometry analysis, we identified two Dpl binding partners: rat alpha-1-inhibitor-3 (α₁I₃) and, by sequence homology, alpha-2-macroglobulin (α₂M), two known plasma metalloproteinase inhibitors. Biochemical investigations excluded the direct interaction of PrP^C with either α₁I₃ or α₂M. Nevertheless, enzyme-linked immunosorbent assays and surface plasmon resonance experiments revealed a high affinity binding occurring between PrP^C and Dpl. In light of these findings, we suggest a mechanism for Dpl-induced neurodegeneration in mice expressing Dpl ectopically in the brain, linked to a withdrawal of natural inhibitors of metalloproteinase such as α₂M. Interestingly, α₂M has been proven to be a susceptibility factor in Alzheimer's disease, and as our findings imply, it may also play a relevant role in other neurodegenerative disorders, including prion diseases.