Gamma-secretase: substrates and inhibitors

Notch signaling in reproduction

The Notch signaling pathway is conserved among mammalian species and controls proliferation, differentiation, and cell death in many organs throughout the body including the reproductive tract. Notch signaling plays critical roles in the development and function of both the male and female reproductive systems. Specifically, within the female reproductive tract, Notch signaling is hormone regulated and mediates key reproductive events important for ovarian and uterine function. In this review, we highlight the tissues that express Notch receptors, ligands, and downstream effectors and distinguish how these molecules regulate reproductive function in male and female mice, non-human primates, and humans. Finally, we describe some of the aberrations in Notch signaling in female reproductive pathologies and identify opportunities for future investigation.

Essential Oils as a Potential Neuroprotective Remedy for Age-Related Neurodegenerative Diseases: A Review

Despite the improvements in life expectancy, neurodegenerative conditions have arguably become the most dreaded maladies of older people. The neuroprotective and anti-ageing potentials of essential oils (EOs) are widely evaluated around the globe. The objective of this review is to analyse the effectiveness of EOs as neuroprotective remedies among the four common age-related neurodegenerative diseases. The literature was extracted from three databases (PubMed, Web of Science and Google Scholar) between the years of 2010 to 2020 using the medical subject heading (MeSH) terms "essential oil", crossed with "Alzheimer's disease (AD)", "Huntington's disease (HD)", "Parkinson's disease (PD)" or "amyotrophic lateral sclerosis (ALS)". Eighty three percent (83%) of the studies were focused on AD, while another 12% focused on PD. No classifiable study was recorded on HD or ALS. EO from Salvia officinalis has been recorded as one of the most effective acetylcholinesterase and butyrylcholinesterase inhibitors. However, only Cinnamomum sp. has been assessed for its effectiveness in both AD and PD. Our review provided useful evidence on EOs as potential neuroprotective remedies for age-related neurodegenerative diseases.

Multi-Target β-Protease Inhibitors from Andrographis paniculata: In Silico and In Vitro Studies

Natural products derived from plants play a vital role in the discovery of new drug candidates, and these are used for novel therapeutic drug development. Andrographis paniculata and Spilanthes paniculata are used extensively as medicinal herbs for the treatment of various ailments, and are reported to have neuroprotective properties. β-amyloid is a microscopic brain protein whose significant aggregation is detected in mild cognitive impairment and Alzheimer’s disease (AD) brains. The accumulation of β-amyloid disrupts cell communication and triggers inflammation by activating immune cells, leading to neuronal cell death and cognitive disabilities. The proteases acetylcholinesterase (AChE), butyrylcholinesterase (BChE), and beta secretase-1 (BACE-1) have been reported to be correlated with the synthesis and growth of β-amyloid plaques in the brains of AD patients. In the present study, the phenolic compounds from A. paniculata and S. paniculata that have been reported in the literature were selected for the current investigation. Furthermore, we employed molecular docking and molecular dynamics studies of the phenolic compounds with the proteins AChE, BChE, and BACE-1 in order to evaluate the binding characteristics and identify potent anti-amyloid agents against the neurodegenerative diseases such as AD. In this investigation, we predicted three compounds from A. paniculata with maximum binding affinities with cholinesterases and BACE-1. The computational investigations predicted that these compounds follow the rule of five. We further evaluated these molecules for in vitro inhibition activity against all the enzymes. In the in vitro investigations, 3,4-di-o-caffeoylquinic acid (5281780), apigenin (5280443), and 7-o-methylwogonin (188316) were found to be strong inhibitors of AChE, BChE, and BACE-1. These findings suggest that these compounds can be potent multi-target inhibitors of the proteases that might cumulatively work and inhibit the initiation and formation of β-amyloid plaques, which is a prime cause of neurotoxicity and dementia. According to our knowledge, these findings are the first report on natural compounds isolated from A. paniculata as multi-target potent inhibitors and anti-amyloid agents.

New type of BACE1 siRNA delivery to cells

BACKGROUND

Small interfering RNA (siRNA) gene therapy is a new molecular approach in the search for an efficient therapy for Alzheimer disease (AD), based on the principle of RNA interference. Reducing BACE activity can have great therapeutic potential for the treatment of AD. In this study, receptor-mediated delivery was used to deliver opioid peptide-conjugated BACE 1 to INR-32 human neuroblastoma cells.

MATERIAL AND METHODS

An INR-32 human neuroblastoma cell line was stably transfected to express the APP cDNA coding fragment containing the predicted sites for cleavage by α, β, or γ-secretase. This was then treated with BACE 1 siRNA to silence BACE gene expression. BACE gene transcription and translation was determined using BACE-1 siRNA cross-linked with opioid peptide, together with RT-PCR, Western blot analysis, and ELISA.

RESULTS

Receptor-mediated delivery was used to introduce BACE1 siRNA to the APP - INR 32 human neuroblastoma cells. Decreased BACE mRNA and protein expression were observed after the cells were transfected with BACE1 siRNA.

CONCLUSIONS

Delivery of BACE1 siRNA appears to specifically reduce the cleavage of APP by inhibiting BACE1 activity.

Presenilin-1 influences processing of the acetylcholinesterase membrane anchor PRiMA

Presenilin-1 (PS1) is the catalytic component of the γ-secretase complex. In this study, we explore if PS1 participates in the processing of the cholinergic acetylcholinesterase (AChE). The major AChE variant expressed in the brain is a tetramer (G(4)) bound to a proline-rich membrane anchor (PRiMA). Overexpression of the transmembrane PRiMA protein in Chinese hamster ovary cells expressing AChE and treated with the γ-secretase inhibitor N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester have enabled us to study whether, through its γ-secretase activity, PS1 participates in the processing of PRiMA-linked AChE. γ-Secretase inhibition led to a notable increase in the level of PRiMA-linked AChE, suggesting that γ-secretase is involved in the cleavage of PRiMA. We demonstrate that cleavage of PRiMA by γ-secretase results in a C-terminal PRiMA fragment. Immunofluorescence labeling allowed us to identify this PRiMA fragment in the nucleus. Moreover, we have determined changes in the proportion of the raft-residing AChE-PRiMA in a PS1 conditional knockout mouse. Our results are of interest as both enzymes have therapeutic relevance for Alzheimer's disease.

ApoER2 processing by presenilin-1 modulates reelin expression

The reelin signaling protein and its downstream components have been associated with synaptic plasticity and neurotransmission. The reelin signaling pathway begins with the binding of reelin to the transmembrane lipoprotein receptor apolipoprotein E receptor 2 (ApoER2), which in turns induces the sequential cleavage of ApoER2 by the sequential action of α- and γ-secretases. Using conditional-knockout mice of the catalytic component of the γ-secretase complex, presenilin 1 (PS1), we demonstrated increased brain ApoER2 and reelin protein and transcript levels, with no changes in the number of reelin-positive cells. Using the human SH-SY5Y neuroblastoma cell line, we showed that ApoER2 processing occurs in the presence of PS1, producing an intracellular ApoER2 C-terminal fragment. In addition, the pharmacologic inhibition of γ-secretase in SH-SY5Y cells led to increased reelin levels. Overexpression of ApoER2 decreased reelin mRNA levels in these cells. A luciferase reporter gene assay and nuclear fractionation confirmed that increased amounts of intracellular fragment of ApoER2 suppressed reelin expression at a transcriptional level. Chromatin immunoprecipitation experiments corroborated that the intracellular fragment of ApoER2 bound to the RELN promoter region. Our study suggests that PS1/γ-secretase-dependent processing of the reelin receptor ApoER2 inhibits reelin expression and may regulate its signaling.

Γ-secretase modulators do not induce Aβ-rebound and accumulation of β-C-terminal fragment

γ-secretase inhibitors (GSIs) have been developed to reduce amyloid-β (Aβ) production for the treatment of Alzheimer's disease by inhibiting the cleavage of amyloid precursor protein (APP). However, cross-inhibitory activity on the processing of Notch can cause adverse reactions. To avoid these undesirable effects, γ-secretase modulators (GSMs) are being developed to selectively reduce toxic Aβ production without perturbing Notch signaling. As it is also known that GSIs can cause a paradoxical increase of plasma Aβ over the baseline after a transient reduction (known as Aβ-rebound), we asked if GSMs would cause a similar rebound and what the potential mechanism might be. Our studies were performed with one GSI (LY-450139) and two chemically distinct GSMs. Although LY-450139 caused Aβ-rebound as expected in rat plasma, the two GSMs did not. Inhibition of APP processing by LY-450139 induced an accumulation of γ-secretase substrates, α- and β-C-terminal fragments of APP, but neither GSM caused such an accumulation. In conclusion, we discover that GSMs, unlike GSIs, do not cause Aβ-rebound, possibly because of the lack of accumulation of β-C-terminal fragments. GSMs may be superior to GSIs in the treatment of Alzheimer's disease not only by sparing Notch signaling but also by avoiding Aβ-rebound.

Proteolytic and non-proteolytic roles of membrane type-1 matrix metalloproteinase in malignancy

This manuscript provides an overview of the dynamic interactions which play an important role in regulating cancer cell functions. We describe and discuss, primarily, those interactions which involve membrane type-1 matrix metalloproteinase (MT1-MMP), its physiological inhibitor tissue inhibitor of metalloproteinases-2 (TIMP-2), furin-like proprotein convertases and the low density lipoprotein-related protein 1 (LRP1) signaling scavenger receptor. The interaction among these cellular proteins controls the efficiency of the activation of MT1-MMP and the unorthodox intracellular signaling which is generated by the catalytically inert complex of MT1-MMP with TIMP-2 and which plays a potentially important role in the migration of cancer cells. Our in-depth understanding of these cellular mechanisms may provide the key to solving the puzzling TIMP-2 paradox. This unsolved paradox arises from the fact that TIMP-2 is a powerful inhibitor of MMPs including MT1-MMP, but at the same time high levels of TIMP-2 positively correlate with an unfavorable prognosis in cancer patients. Solving the TIMP-2 paradox may lead to solving a similar PAI-1 paradox and produce a clearer understanding of the biochemical mechanisms which control the functionality of the urokinase-type plasminogen activator*urokinase receptor*plasminogen activator inhibitor type-1 (uPAR*uPA*PAI-1) system in cancer.

Alzheimer's disease and the amyloid-beta peptide

Alzheimer's disease (AD) pathogenesis is widely believed to be driven by the production and deposition of the amyloid-beta peptide (Abeta). For many years, investigators have been puzzled by the weak to nonexistent correlation between the amount of neuritic plaque pathology in the human brain and the degree of clinical dementia. Recent advances in our understanding of the development of amyloid pathology have helped solve this mystery. Substantial evidence now indicates that the solubility of Abeta, and the quantity of Abeta in different pools, may be more closely related to disease state. The composition of these pools of Abeta reflects different populations of amyloid deposits and has definite correlates with the clinical status of the patient. Imaging technologies, including new amyloid imaging agents based on the chemical structure of histologic dyes, are now making it possible to track amyloid pathology along with disease progression in the living patient. Interestingly, these approaches indicate that the Abeta deposited in AD is different from that found in animal models. In general, deposited Abeta is more easily cleared from the brain in animal models and does not show the same physical and biochemical characteristics as the amyloid found in AD. This raises important issues regarding the development and testing of future therapeutic agents.

BACE1 and BACE2 enzymatic activities in Alzheimer's disease

beta-Secretase is the rate limiting enzymatic activity in the production of the amyloid-beta peptide (Abeta) and is thought to be involved in Alzheimer's disease (AD) pathogenesis. Although BACE1 (beta-site APP Cleaving Enzyme 1, EC 3.4.23.46) has received significant attention, the related BACE2 (EC 3.4.23.45) has not. Though BACE2 is also expressed in the brain, its potential role in AD has not been resolved. In this study, we compared the activities of both BACE1 and BACE2, which were isolated from the same samples of frontal cortex from both AD-affected individuals and age-matched controls. BACE1 activity showed a significant positive correlation with the amount of extractable Abeta, and BACE1 protein and activity were significantly increased in AD cases. Unexpectedly, there were substantial total amounts of BACE2 protein and enzymatic activity in the human brain. BACE2 activity did not change significantly in the AD brain, and was not related to Abeta concentration. These data indicate that BACE1 likely accounts for most of the Abeta produced in the human brain, and that BACE2 activity is not a likely contributor. However, as both forms of BACE compete for the same substrate pool, even small changes in BACE2 activity could have consequences for human disease.

FRAT1 peptide decreases Abeta production in swAPP(751) cells

Recently, LiCl has been shown to inhibit amyloid beta peptide secretion in association with diminished glycogen synthase kinase beta (GSK3beta) activity. However, it remains unclear if direct inhibition of GSK3beta activity will result in decreased Abeta production. Frequently rearranged in advanced T-cell lymphomas 1 (FRAT1) protein is a negative regulator of GSK3alpha/beta kinase activity. To examine whether direct inhibition of GSK3alpha/beta kinase activity can lower Abeta production, a FRAT1 peptide was expressed in swAPP(751) cells that produce high levels of Abeta. Our data demonstrate that cellular expression of FRAT1 peptide in swAPP(751) cells increases both GSK3alpha and beta phosphorylation on Ser21 and Ser9, respectively, while inhibiting kinase activity of both isoforms. Moreover, as a result of FRAT1 expression, the production of both total Abeta and Abeta(1-42) was significantly decreased. Thus, we provide evidence that direct regulation of GSK3alpha/beta by FRAT1 peptide significantly decreases Abeta production in swAPP(751) cells.

The low density lipoprotein receptor-related protein LRP is regulated by membrane type-1 matrix metalloproteinase (MT1-MMP) proteolysis in malignant cells

We demonstrate that the presentation of LRP and the subsequent uptake of its ligands by malignant cells are both strongly regulated by MT1-MMP. Because LRP is essential for the clearance of multiple ligands, these findings have important implications for many pathophysiological processes including the pericellular proteolysis in neoplastic cells as well as the fate of the soluble matrix-degrading proteases such as MMP-2. MT1-MMP is a key protease in cell invasion and a physiological activator of MMP-2. Cellular LRP consists of a non-covalently associated 515-kDa extracellular alpha-chain (LRP-515) and an 85-kDa membrane-spanning beta-chain, and plays a dual role as a multifunctional endocytic receptor and a signaling molecule. Through the capture and uptake of several soluble proteases, LRP is involved in the regulation of matrix proteolysis. LRP-515 associates with the MT1-MMP catalytic domain and is highly susceptible to MT1-MMP proteolysis in vitro. Similar to MT1-MMP, the metalloproteinases MT2-MMP, MT3-MMP and MT4-MMP also degrade LRP. The N-terminal and C-terminal parts of the LRP-515 subunit are resistant and susceptible, respectively, to MT1-MMP proteolysis. In cells co-expressing LRP and MT1-MMP, the proteolytically competent protease decreases the levels of cellular LRP and releases its N-terminal portion in the extracellular milieu while the catalytically inert protease co-precipitates with LRP. These events implicate MT1-MMP, not only in the activation of MMP-2, but also in the mechanisms that control the subsequent fate of MMP-2 in cells and tissues.

JLK isocoumarin inhibitors: Selective ?-secretase inhibitors that do not interfere with notch pathway in vitro or in vivo

gamma-Secretase activity is involved in the generation of Abeta and therefore likely contributes to the pathology of Alzheimer's disease. Blocking this activity was seen as a major therapeutic target to slow down or arrest Abeta-related AD progression. This strategy seemed more doubtful when it was established that gamma-secretase also targets other substrates including Notch, a particularly important transmembrane protein involved in vital functions, at both embryonic and adulthood stages. We have described previously new non-peptidic inhibitors able to selectively inhibit Abeta cellular production in vitro without altering Notch pathway. We show here that in vivo, these inhibitors do not alter the Notch pathway responsible for somitogenesis in the zebrafish embryo. In addition, we document further the selectivity of JLK inhibitors by showing that, unlike other described gamma-secretase inhibitors, these agents do not affect E-cadherin processing. Finally, we establish that JLKs do not inhibit beta-site APP cleaving enzymes (BACE) 1 and BACE2, alpha-secretase, the proteasome, and GSK3beta kinase. Altogether, JLK inhibitors are the sole agents to date that are able to prevent Abeta production without triggering unwanted cleavages of other proteins.

Peptide-based, irreversible inhibitors of gamma-secretase activity

The characterization of the enzymes responsible for amyloid beta-peptide (Abeta) production is considered to be a primary goal towards the development of future therapeutics for the treatment of Alzheimer's disease. Inhibitors of gamma-secretase activity were critical in demonstrating that the presenilins (PSs) likely comprised at least part of the active site of the gamma-secretase enzyme complex, with two highly conserved membrane aspartates presumably acting as catalytic residues. However, whether or not these aspartates are actually the catalytic residues of the enzyme complex or are merely essential for normal PS function and/or maturation is still unknown. In this paper, we report the development of reactive inhibitors of gamma-secretase activity that are functionally irreversible. Since such inhibitors have been shown to bind catalytic residues in other aspartyl proteases (e.g., HIV protease), they might be used to determine if the transmembrane aspartates of PSs are involved directly in substrate cleavage.