Depletion of GGA1 and GGA3 mediates postinjury elevation of BACE1

BACE1-dependent cleavage of GABAA receptor contributes to neural hyperexcitability and disease progression in Alzheimer's disease

Neural hyperexcitability has been clinically associated with amyloid-β (Aβ) pathology and cognitive impairment in Alzheimer's disease (AD). Here, we show that decreased GABAA receptor (GABAAR) currents are linked to hippocampal granule cell hyperexcitability in the AD mouse model APP23. Elevated levels of β-secretase (BACE1), the β-secretase responsible for generating Aβ peptides, lead to aberrant cleavage of GABAAR β1/2/3 subunits in the brains of APP23 mice and AD patients. Moreover, BACE1-dependent cleavage of the β subunits leads to a decrease in GABAAR-mediated inhibitory currents in BACE1 transgenic mice. Finally, we show that the neural hyperexcitability, Aβ load, and spatial memory deficit phenotypes of APP23 mice are significantly reduced upon the granule cell expression of a non-cleavable β3 subunit mutant. Collectively, our study establishes that BACE1-dependent cleavage of GABAAR β subunits promotes the pathological hyperexcitability known to drive neurodegeneration and cognitive impairment in the AD brain, suggesting that prevention of the cleavage could slow disease progression.

GGA1 interacts with the endosomal Na+/H+ exchanger NHE6 governing localization to the endosome compartment

Mutations in the endosomal Na+/H+ exchanger 6 (NHE6) cause Christianson syndrome, an X-linked neurological disorder. NHE6 functions in regulation of endosome acidification and maturation in neurons. Using yeast two-hybrid screening with the NHE6 carboxyl terminus as bait, we identify Golgi-associated, gamma adaptin ear-containing, ADP-ribosylation factor (ARF) binding protein 1 (GGA1) as an interacting partner for NHE6. We corroborated the NHE6-GGA1 interaction using: coimmunoprecipitation; overexpressed constructs in mammalian cells; and coimmunoprecipitation of endogenously expressed GGA1 and NHE6 from neuroblastoma cells, as well as from the mouse brain. We demonstrate that GGA1 interacts with organellar NHEs (NHE6, NHE7, and NHE9) and that there is significantly less interaction with cell-surface localized NHEs (NHE1 and NHE5). By constructing hybrid NHE1/NHE6 exchangers, we demonstrate the cytoplasmic tail of NHE6 interacts most strongly with GGA1. We demonstrate the colocalization of NHE6 and GGA1 in cultured, primary hippocampal neurons, using super-resolution microscopy. We test the hypothesis that the interaction of NHE6 and GGA1 functions in the localization of NHE6 to the endosome compartment. Using subcellular fractionation experiments, we show that NHE6 is mislocalized in GGA1 KO cells, wherein we find less NHE6 in endosomes, but more NHE6 transport to lysosomes, and more Golgi retention of NHE6, with increased exocytosis to the surface plasma membrane. Consistent with NHE6 mislocalization, and Golgi retention, we find the intraluminal pH in Golgi to be alkalinized in GGA1-null cells. Our study demonstrates a new interaction between NHE6 and GGA1 which functions in the localization of this intracellular NHE to the endosome compartment.

E2F1 Mediates Traumatic Brain Injury and Regulates BDNF-AS to Promote the Progression of Alzheimer's Disease

Traumatic brain injury (TBI) is one of the important risk factors for the development of Alzheimer's disease (AD). However, the molecular mechanism by which TBI promotes the progression of AD is not elucidated. In this study, we showed that the abnormal production of E2F1 is a major factor in promoting the neuropathological and cognitive deterioration of AD post-TBI. We found that repeated mild TBI can aggravate the neuropathology of AD in APP/PS1 mice. At the same time, the co-expression of E2F1 and beta-site APP cleaving enzyme 1 (BACE1) was upregulated when the mouse hippocampus was dissected. BACE1 is recognized as a rate-limiting enzyme for the production of Aβ. Here, we speculate that E2F1 may play a role in promoting BACE1 expression in AD. Therefore, we collected peripheral blood from patients with AD. Interestingly, there is a positive correlation between E2F1 and brain-derived neurotrophic factor-antisense (BDNF-AS), whereas BDNF-AS in AD can promote the expression of BACE1 and exhibit a neurotoxic effect. We established a cell model and found a regulatory relationship between E2F1 and BDNF-AS. Therefore, based on our results, we concluded that E2F1 regulates BDNF-AS, promotes the expression of BACE1, and affects the progression of AD. Furthermore, E2F1 mediates the TBI-induced neurotoxicity of AD.

GGA1 interacts with the endosomal Na+/H+ Exchanger NHE6 governing localization to the endosome compartment

Mutations in the endosomal Na+/H+ exchanger (NHE6) cause Christianson syndrome (CS), an X-linked neurological disorder. Previous studies have shown that NHE6 functions in regulation of endosome acidification and maturation in neurons. Using yeast two-hybrid screening with the NHE6 carboxyl-terminus as bait, we identify Golgi-associated, Gamma adaptin ear containing, ARF binding protein 1 (GGA1) as an interacting partner for NHE6. We corroborated the NHE6-GGA1 interaction using co-immunoprecipitation (co-IP): using over-expressed constructs in mammalian cells; and co-IP of endogenously-expressed GGA1 and NHE6 from neuroblastoma cells, as well as from mouse brain. We demonstrate that GGA1 interacts with organellar NHEs (NHE6, NHE7 and NHE9) but not with cell-surface localized NHEs (NHE1 and NHE5). By constructing hybrid NHE1/NHE6 exchangers, we demonstrate that the cytoplasmic tail of NHE6 is necessary and sufficient for interactions with GGA1. We demonstrate the co-localization of NHE6 and GGA1 in cultured, primary hippocampal neurons, using super-resolution microscopy. We test the hypothesis that the interaction of NHE6 and GGA1 functions in the localization of NHE6 to the endosome compartment. Using subcellular fractionation experiments, we show that NHE6 is mis-localized in GGA1 knockout cells wherein we find less NHE6 in endosomes but more NHE6 transport to lysosomes, and more Golgi retention of NHE6 with increased exocytosis to the surface plasma membrane. Consistent with NHE6 mis-localization, and Golgi retention, we find the intra-luminal pH in Golgi to be alkalinized. Our study demonstrates a new interaction between NHE6 and GGA1 which functions in the localization of this intra-cellular NHE to the endosome compartment.

Discovery of potent and selective dual cholinesterases and β-secretase inhibitors in pomegranate as a treatment for Alzheimer's disease

Pomegranate (Punica granatum L.) extract has been reported to inhibit cholinesterase and the β-site amyloid precursor protein cleaving enzyme 1 (BACE1); however, most of its constituents' potential inhibition of these enzymes remains unknown. Thus, we investigated the anti-Alzheimer's disease (anti-AD) potential of 16 ellagitannin and gallotannin, and nine anthocyanin derivatives' inhibition of BACE1, AChE, and BChE, and gallagic acid inhibited both the best. Further, a kinetic study identified different modes of inhibition, and a molecular docking simulation revealed that active compounds inhibited these three enzymes with low binding energy through hydrophilic and hydrophobic interactions in the active site cavities. Gallagic acid and castalagin decreased Aβ peptides secretion from neuroblastoma cells that overexpressed human β-amyloid precursor protein significantly by 10 μM. Further, treatment with gallagic acid and castalagin reduced BACE1 and APPsβ expression levels significantly without affecting amyloid precursor protein (APP) levels in the amyloidogenic pathway. Co-incubation of Aβ42 with gallagic acid reduced Aβ42-induced intracellular reactive oxygen species (ROS) production significantly. Our results suggest that pomegranate constituents, specifically gallagic acid, may be useful in developing therapeutic treatment modalities for AD.

The Emergence of Model Systems to Investigate the Link Between Traumatic Brain Injury and Alzheimer's Disease

Numerous epidemiological studies have demonstrated that individuals who have sustained a traumatic brain injury (TBI) have an elevated risk for developing Alzheimer's disease and Alzheimer's-related dementias (AD/ADRD). Despite these connections, the underlying mechanisms by which TBI induces AD-related pathology, neuronal dysfunction, and cognitive decline have yet to be elucidated. In this review, we will discuss the various in vivo and in vitro models that are being employed to provide more definite mechanistic relationships between TBI-induced mechanical injury and AD-related phenotypes. In particular, we will highlight the strengths and weaknesses of each of these model systems as it relates to advancing the understanding of the mechanisms that lead to TBI-induced AD onset and progression as well as providing platforms to evaluate potential therapies. Finally, we will discuss how emerging methods including the use of human induced pluripotent stem cell (hiPSC)-derived cultures and genome engineering technologies can be employed to generate better models of TBI-induced AD.

Role of zinc and copper ions in the pathogenetic mechanisms of traumatic brain injury and Alzheimer's disease

The disruption of homeostasis of zinc (Zn2+) and copper (Cu2+) ions in the central nervous system is involved in the pathogenesis of many neurodegenerative diseases, such as amyotrophic lateral sclerosis, Wilson's, Creutzfeldt-Jakob, Parkinson's, and Alzheimer's diseases (AD), and traumatic brain injury (TBI). The last two pathological conditions of the brain are the most common; moreover, it is possible that TBI is a risk factor for the development of AD. Disruptions of Zn2+ and Cu2+ homeostasis play an important role in the mechanisms of pathogenesis of both TBI and AD. This review attempts to summarize and systematize the currently available research data on this issue. The neurocytotoxicity of Cu2+ and Zn2+, the synergism of the toxic effect of calcium and Zn2+ ions on the mitochondria of neurons, and the interaction of Zn2+ and Cu2+ with β-amyloid (Abeta) and tau protein are considered.

Gga3 deletion and a GGA3 rare variant associated with late onset Alzheimer's disease trigger BACE1 accumulation in axonal swellings

Axonal dystrophy, indicative of perturbed axonal transport, occurs early during Alzheimer's disease (AD) pathogenesis. Little is known about the mechanisms underlying this initial sign of the pathology. This study proves that Golgi-localized γ-ear-containing ARF binding protein 3 (GGA3) loss of function, due to Gga3 genetic deletion or a GGA3 rare variant that cosegregates with late-onset AD, disrupts the axonal trafficking of the β-site APP-cleaving enzyme 1 (BACE1) resulting in its accumulation in axonal swellings in cultured neurons and in vivo. We show that BACE pharmacological inhibition ameliorates BACE1 axonal trafficking and diminishes axonal dystrophies in Gga3 null neurons in vitro and in vivo. These data indicate that axonal accumulation of BACE1 engendered by GGA3 loss of function results in local toxicity leading to axonopathy. Gga3 deletion exacerbates axonal dystrophies in a mouse model of AD before β-amyloid (Aβ) deposition. Our study strongly supports a role for GGA3 in AD pathogenesis, where GGA3 loss of function triggers BACE1 axonal accumulation independently of extracellular Aβ, and initiates a cascade of events leading to the axonal damage distinctive of the early stage of AD.

An Overview of β-Amyloid Cleaving Enzyme 1 (Bace1) in Alzheimer's Disease Therapy Elucidating its Exosite-Binding Antibody and Allosteric Inhibitor

Over decades of its identification, numerous past and ongoing researches have focused on the therapeutic roles of β-amyloid cleaving enzyme 1 (BACE1) as a target in treating Alzheimer's disease (AD). Although the initial BACE1 inhibitors at phase-3 clinical trials tremendously reduced β-amyloid-associated plaques in patients with AD, the researchers eventually discontinued the tests due to the lack of potency. This discontinuation has resulted in limited drug development and discovery targeted at BACE1, despite the high demand for dementia and AD therapies. It is, therefore, imperative to describe the detailed underlying biological basis of the BACE1 therapeutic option in neurological diseases. Herein, we highlight BACE1 bioactivity, genetic properties, and role in neurodegenerative therapy. We review research contributions to BACE1 exosite-binding antibody and allosteric inhibitor development as AD therapies. The review also covers BACE1 biological function, the disease-associated mechanisms, and the enzyme conditions for amyloid precursor protein sites splitting. Based on the present review, we suggest further studies on anti-BACE1 exosite antibodies and BACE1 allosteric inhibitors. Non-active site inhibition might be the way forward to BACE1 therapy in Alzheimer's neurological disorder.

In vivo KPT-350 treatment decreases cortical hyperexcitability following traumatic brain injury

PRIMARY OBJECTIVE

We tested whether KPT-350, a novel selective inhibitor of nuclear export, could attenuate cortical network hyperexcitability, a major risk factor for developing post-traumatic epilepsy (PTE) following traumatic brain injury (TBI).

RESEARCH DESIGN

All mice in this study underwent TBI and were subsequently treated with either KPT-350 or vehicle during the post-injury latent period. Half of the animal cohort was used for electrophysiology while the other half was used for immunohistochemical analysis.

METHODS AND PROCEDURES

TBI was induced using the controlled cortical impact (CCI) model. Cortical network activity was recorded by evoking field potentials from deep layers of the cortex and analyzed using Matlab software. Immunohistochemistry coupled with fluorescence microscopy and Image J analysis detected changes in neuronal and glial markers.

MAIN OUTCOMES AND RESULTS

KPT-350 attenuated TBI-associated epileptiform activity and restored disrupted input-output responses in cortical brain slices. In vivo KPT-350 treatment reduced the loss of parvalbumin-(+) GABAergic interneurons after CCI but did not significantly change CCI-induced loss of somatostatin-(+) GABAergic interneurons, nor did it reduce reactivity of GFAP and Iba1 glial markers.

CONCLUSION

KPT-350 can prevent cortical hyperexcitability and reduce the loss of parvalbumin-(+) GABAergic inhibitory neurons, making it a potential therapeutic option for preventing PTE.

Thymoquinone as a Potential Neuroprotector in Acute and Chronic Forms of Cerebral Pathology

Thymoquinone is one of the main active components of the essential oil from black cumin (Nigella sativa) seeds. Thymoquinone exhibits a wide range of pharmacological activities, including neuroprotective action demonstrated in the models of brain ischemia/reperfusion, Alzheimer's and Parkinson's diseases, and traumatic brain injury. The neuroprotective effect of thymoquinone is mediated via inhibition of lipid peroxidation, downregulation of proinflammatory cytokines, maintenance of mitochondrial membrane potential, and prevention of apoptosis through inhibition of caspases-3, -8, and -9. Thymoquinone-based mitochondria-targeted antioxidants are accumulated in the mitochondria and exhibit neuroprotective properties in nanomolar concentrations. Thymoquinone reduces the negative effects of acute and chronic forms of brain pathologies. The mechanisms of the pharmacological action of thymoquinone and its chemical derivatives require more comprehensive studying. In this paper, we formulated the prospects of application of thymoquinone and thymoquinone-based drugs in the therapy of neurodegenerative diseases.

Regulation of BACE1 expression after injury is linked to the p75 neurotrophin receptor

BACE1 is a transmembrane aspartic protease that cleaves various substrates and it is required for normal brain function. BACE1 expression is high during early development, but it is reduced in adulthood. Under conditions of stress and injury, BACE1 levels are increased; however, the underlying mechanisms that drive BACE1 elevation are not well understood. One mechanism associated with brain injury is the activation of injurious p75 neurotrophin receptor (p75), which can trigger pathological signals. Here we report that within 72 h after controlled cortical impact (CCI) or laser injury, BACE1 and p75 are increased and tightly co-expressed in cortical neurons of mouse brain. Additionally, BACE1 is not up-regulated in p75 null mice in response to focal cortical injury, while p75 over-expression results in BACE1 augmentation in HEK-293 and SY5Y cell lines. A luciferase assay conducted in SY5Y cell line revealed that BACE1 expression is regulated at the transcriptional level in response to p75 transfection. Interestingly, this effect does not appear to be dependent upon p75 ligands including mature and pro-neurotrophins. In addition, BACE1 activity on amyloid precursor protein (APP) is enhanced in SY5Y-APP cells transfected with a p75 construct. Lastly, we found that the activation of c-jun n-terminal kinase (JNK) by p75 contributes to BACE1 up-regulation. This study explores how two injury-induced molecules are intimately connected and suggests a potential link between p75 signaling and the expression of BACE1 after brain injury.

Inhibition of β-site amyloid precursor protein cleaving enzyme 1 and cholinesterases by pterosins via a specific structure-activity relationship with a strong BBB permeability

We extracted 15 pterosin derivatives from Pteridium aquilinum that inhibited β-site amyloid precursor protein cleaving enzyme 1 (BACE1) and cholinesterases involved in the pathogenesis of Alzheimer's disease (AD). (2R)-Pterosin B inhibited BACE1, acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) with an IC₅₀ of 29.6, 16.2 and 48.1 µM, respectively. The K_i values and binding energies (kcal/mol) between pterosins and BACE1, AChE, and BChE corresponded to the respective IC₅₀ values. (2R)-Pterosin B was a noncompetitive inhibitor against human BACE1 and BChE as well as a mixed-type inhibitor against AChE, binding to the active sites of the corresponding enzymes. Molecular docking simulation of mixed-type and noncompetitive inhibitors for BACE1, AChE, and BChE indicated novel binding site-directed inhibition of the enzymes by pterosins and the structure-activity relationship. (2R)-Pterosin B exhibited a strong BBB permeability with an effective permeability (P_e) of 60.3×10^-6 cm/s on PAMPA-BBB. (2R)-Pterosin B and (2R,3 R)-pteroside C significantly decreased the secretion of Aβ peptides from neuroblastoma cells that overexpressed human β-amyloid precursor protein at 500 μM. Conclusively, our study suggested that several pterosins are potential scaffolds for multitarget-directed ligands (MTDLs) for AD therapeutics.

The Golgi-localized, gamma ear-containing, ARF-binding (GGA) protein family alters alpha synuclein (α-syn) oligomerization and secretion

Several age-related neurodegenerative disorders are associated with protein misfolding and aggregation of toxic peptides. α-synuclein (α-syn) aggregation and the resulting cytotoxicity is a hallmark of Parkinson's disease (PD) as well as dementia with Lewy bodies. Rising evidence points to oligomeric and pre-fibrillar forms as the pathogenic species, and oligomer secretion seems to be crucial for the spreading and progression of PD pathology. Recent studies implicate that dysfunctions in endolysosomal/autophagosomal pathways increase α-syn secretion. Mutation in the retromer-complex protein VPS35, which is involved in endosome to Golgi transport, was suggested to cause familial PD. GGA proteins regulate vesicular traffic between Golgi and endosomes and might work as antagonists for retromer complex mediated transport. To investigate the role of the GGAs in the α-syn oligomerization and/or secretion process we utilized protein-fragment complementation assays (PCA). We here demonstrate that GGAs alter α-syn oligomer secretion and α-syn oligomer-mediated toxicity. Specifically, we determined that GGA3 modifies extracellular α-syn species in an exosome-independent manner. Our data suggest that GGA3 drives α-syn oligomerization in endosomal compartments and thus facilitates α-syn oligomer secretion. Preventing the early events in α-syn oligomer release may be a novel approach to halt disease spreading in PD and other synucleinopathies.

BACE1 SUMOylation increases its stability and escalates the protease activity in Alzheimer's disease

BACE1 is a rate-limiting enzyme for amyloid beta polypeptide production, which plays a crucial role in Alzheimer’s disease (AD) pathogenesis. However, how this essential protease is posttranslationally regulated remains incompletely understood. In the current study, we show that K501 residue on BACE1, a ubiquitin modification site, is also competitively SUMOylated. We discovered that SUMOylation of BACE1 augments its stability and enzymatic activity, resulting in senile plaque formation and cognitive defect. Identification of the posttranslational modification on BACE1 provides insight into the molecular mechanism in AD.

Amyloid beta (Aβ) is a major pathological marker in Alzheimer’s disease (AD), which is principally regulated by the rate-limiting β-secretase (i.e., BACE1) cleavage of amyloid precursor protein (APP). However, how BACE1 activity is posttranslationally regulated remains incompletely understood. Here, we show that BACE1 is predominantly SUMOylated at K501 residue, which escalates its protease activity and stability and subsequently increases Aβ production, leading to cognitive defect seen in the AD mouse model. Compared with a non-SUMOylated K501R mutant, injection of wild-type BACE1 significantly increases Aβ production and triggers cognitive dysfunction. Furthermore, overexpression of wild-type BACE1, but not non-SUMOylated K501R mutant, facilitates senile plaque formation and aggravates the cognitive deficit seen in the APP/PS1 AD mouse model. Together, our data strongly suggest that K501 SUMOylation on BACE1 plays a critical role in mediating its stability and enzymatic activity.

BACE1 elevation engendered by GGA3 deletion increases β-amyloid pathology in association with APP elevation and decreased CHL1 processing in 5XFAD mice

BACKGROUND

β-site amyloid precursor protein cleaving enzyme 1 (BACE1) is the rate-limiting enzyme in the production of amyloid beta (Aβ), the toxic peptide that accumulates in the brains of Alzheimer's disease (AD) patients. Our previous studies have shown that the clathrin adaptor Golgi-localized γ-ear-containing ARF binding protein 3 (GGA3) plays a key role in the trafficking of BACE1 to lysosomes, where it is normally degraded. GGA3 depletion results in BACE1 stabilization both in vitro and in vivo. Moreover, levels of GGA3 are reduced and inversely related to BACE1 levels in post-mortem brains of AD patients.

METHOD

In order to assess the effect of GGA3 deletion on AD-like phenotypes, we crossed GGA3 -/- mice with 5XFAD mice. BACE1-mediated processing of APP and the cell adhesion molecule L1 like protein (CHL1) was measured as well as levels of Aβ42 and amyloid burden.

RESULTS

In 5XFAD mice, we found that hippocampal and cortical levels of GGA3 decreased while BACE1 levels increased with age, similar to what is observed in human AD brains. GGA3 deletion prevented age-dependent elevation of BACE1 in GGA3KO;5XFAD mice. We also found that GGA3 deletion resulted in increased hippocampal levels of Aβ42 and amyloid burden in 5XFAD mice at 12 months of age. While levels of BACE1 did not change with age and gender in GGAKO;5XFAD mice, amyloid precursor protein (APP) levels increased with age and were higher in female mice. Moreover, elevation of APP was associated with a decreased BACE1-mediated processing of CHL1 not only in 12 months old 5XFAD mice but also in human brains from subjects affected by Down syndrome, most likely due to substrate competition.

CONCLUSION

This study demonstrates that GGA3 depletion is a leading candidate mechanism underlying elevation of BACE1 in AD. Furthermore, our findings suggest that BACE1 inhibition could exacerbate mechanism-based side effects in conditions associated with APP elevation (e.g. Down syndrome) owing to impairment of BACE1-mediated processing of CHL1. Therefore, therapeutic approaches aimed to restore GGA3 function and to prevent the down stream effects of its depletion (e.g. BACE1 elevation) represent an attractive alternative to BACE inhibition for the prevention/treatment of AD.

Effects of altered RTN3 expression on BACE1 activity and Alzheimer's neuritic plaques

Reticulon 3 (RTN3), which is a member of the reticulon family of proteins, has a biochemical function of shaping tubular endoplasmic reticulum. RTN3 has also been found to interact with β-site amyloid precursor protein cleaving enzyme 1 (BACE1), which initiates the generation of β-amyloid peptides (Aβ) from amyloid precursor protein. Aβ is the major proteinaceous component in neuritic plaques, which constitute one of the major pathological features in brains of Alzheimer's disease (AD) patients. Mice deficient in or overexpressing RTN3 have altered amyloid deposition through effects on BACE1 expression and activity. In this review, we will summarize the current findings concerning the role of RTN3 in AD pathogenesis and demonstrate that RTN3 protein levels act as age-dependent modulators of BACE1 activity and Aβ deposition during the pathogenic progression of AD.

Par3 and aPKC regulate BACE1 endosome-to-TGN trafficking through PACS1

The cleavage of amyloid precursor protein (APP) by β-site APP cleaving enzyme 1 (BACE1) is the rate-limiting step in beta amyloid generation during Alzheimer's disease (AD) pathogenesis. In AD brains, BACE1 is abnormally accumulated in endocytic compartments, where the acidic pH is optimal for its activity. However, mechanisms regulating the endosome-to-trans-Golgi network (TGN) retrieval of BACE1 remain unclear. Here, we show that partitioning defective 3 (Par3) facilitates BACE1 retrograde trafficking from endosomes to the TGN. Par3 functions through aPKC-mediated phosphorylation of BACE1 on Ser498, which in turn promotes the interaction between BACE1 and phosphofurin acidic cluster sorting protein 1 and facilitates the retrograde trafficking of BACE1 to the TGN. In human AD brains, there is a significant decrease in Ser498 phosphorylation of BACE1 suggesting that defective phosphorylation-dependent retrograde transport of BACE1 is important in AD pathogenesis. Together, our studies provide mechanistic insight into a novel role for Par3 and aPKC in regulating the retrograde endosome-to-TGN trafficking of BACE1 and shed light on the mechanisms of AD pathogenesis.

Lack of BACE1 S-palmitoylation reduces amyloid burden and mitigates memory deficits in transgenic mouse models of Alzheimer's disease

Alzheimer's disease (AD) is a devastating neurodegenerative disorder characterized by pathological brain lesions and a decline in cognitive function. β-Amyloid peptides (Aβ), derived from proteolytic processing of amyloid precursor protein (APP), play a central role in AD pathogenesis. β-Site APP cleaving enzyme 1 (BACE1), the transmembrane aspartyl protease which initiates Aβ production, is axonally transported in neurons and accumulates in dystrophic neurites near cerebral amyloid deposits in AD. BACE1 is modified by S-palmitoylation at four juxtamembrane cysteine residues. S-palmitoylation is a dynamic posttranslational modification that is important for trafficking and function of several synaptic proteins. Here, we investigated the in vivo significance of BACE1 S-palmitoylation through the analysis of knock-in mice with cysteine-to-alanine substitution at the palmitoylated residues (4CA mice). BACE1 expression, as well as processing of APP and other neuronal substrates, was unaltered in 4CA mice despite the lack of BACE1 S-palmitoylation and reduced lipid raft association. Whereas steady-state Aβ levels were similar, synaptic activity-induced endogenous Aβ production was not observed in 4CA mice. Furthermore, we report a significant reduction of cerebral amyloid burden and BACE1 accumulation in dystrophic neurites in the absence of BACE1 S-palmitoylation in mouse models of AD amyloidosis. Studies in cultured neurons suggest that S-palmitoylation is required for dendritic spine localization and axonal targeting of BACE1. Finally, the lack of BACE1 S-palmitoylation mitigates cognitive deficits in 5XFAD mice. Using transgenic mouse models, these results demonstrate that intrinsic posttranslational S-palmitoylation of BACE1 has a significant impact on amyloid pathogenesis and the consequent cognitive decline.

Physiological Functions of the β-Site Amyloid Precursor Protein Cleaving Enzyme 1 and 2

BACE1 was discovered as the β-secretase for initiating the cleavage of amyloid precursor protein (APP) at the β-secretase site, while its close homology BACE2 cleaves APP within the β-amyloid (Aβ) domain region and shows distinct cleavage preferences in vivo. Inhibition of BACE1 proteolytic activity has been confirmed to decrease Aβ generation and amyloid deposition, and thus specific inhibition of BACE1 by small molecules is a current focus for Alzheimer’s disease therapy. While BACE1 inhibitors are being tested in advanced clinical trials, knowledge regarding the properties and physiological functions of BACE is highly important and this review summarizes advancements in BACE1 research over the past several years. We and others have shown that BACE1 is not only a critical enzyme for testing the “Amyloid Hypothesis” associated with Alzheimer’s pathogenesis, but also important for various functions such as axon growth and pathfinding, astrogenesis, neurogenesis, hyperexcitation, and synaptic plasticity. BACE2 appears to play different roles such as glucose homeostasis and pigmentation. This knowledge regarding BACE1 functions is critical for monitoring the safe use of BACE1 inhibitors in humans.

Neuroprotective properties of mitochondria-targeted antioxidants of the SkQ-type

In 2008, using a model of compression brain ischemia, we presented the first evidence that mitochondria-targeted antioxidants of the SkQ family, i.e. SkQR1 [10-(6′-plastoquinonyl)decylrhodamine], have a neuroprotective action. It was shown that intraperitoneal injections of SkQR1 (0.5–1 μmol/kg) 1 day before ischemia significantly decreased the damaged brain area. Later, we studied in more detail the anti-ischemic action of this antioxidant in a model of experimental focal ischemia provoked by unilateral intravascular occlusion of the middle cerebral artery. The neuroprotective action of SkQ family compounds (SkQR1, SkQ1, SkQTR1, SkQT1) was manifested through the decrease in trauma-induced neurological deficit in animals and prevention of amyloid-β-induced impairment of long-term potentiation in rat hippocampal slices. At present, most neurophysiologists suppose that long-term potentiation underlies cellular mechanisms of memory and learning. They consider inhibition of this process by amyloid-β1-42as anin vitromodel of memory disturbance in Alzheimer’s disease. Further development of the above studies revealed that mitochondria-targeted antioxidants could retard accumulation of hyperphosphorylated τ-protein, as well as amyloid-β1-42, and its precursor APP in the brain, which are involved in developing neurodegenerative processes in Alzheimer’s disease.

Alzheimer's Disease: An Exacerbation of Senile Phenoptosis

Alzheimer's disease is characterized by progressive memory loss and cognitive decline accompanied by degeneration of neuronal synapses, massive loss of neurons in the brain, eventually resulting in complete degradation of personality and death. Currently, the cause of the disease is not fully understood, but it is believed that the person's age is the major risk factor for development of Alzheimer's disease. People who have survived after cerebral stroke or traumatic brain injury have substantially increased risk of developing Alzheimer's disease. Social exclusion, low social activity, physical inactivity, poor mental performance, and low level of education are among risk factors for development of this neurodegenerative disease, which is consistent with the concept of phenoptosis (Skulachev, V. P., et al. (1999) Biochemistry (Moscow), 64, 1418-1426; Skulachev, M. V., and Skulachev, V. P. (2014) Biochemistry (Moscow), 79, 977-993) stating that rate of aging is related to psychological and social aspects in human behavior. Here we assumed that Alzheimer's disease might be considered as an exacerbation of senile phenoptosis. If so, then development of this disease could be slowed using mitochondria-targeted antioxidants due to the accumulated data demonstrating a link between mitochondrial dysfunction and oxidative stress both with normal aging and Alzheimer's disease.

Inhibitory activities of major anthraquinones and other constituents from Cassia obtusifolia against β-secretase and cholinesterases

ETHNOPHARMACOLOGICAL RELEVANCE

Semen Cassiae has been traditionally used as an herbal remedy for liver, eye, and acute inflammatory diseases. Recent pharmacological reports have indicated that Cassiae semen has neuroprotective effects, attributable to its anti-inflammatory actions, in ischemic stroke and Alzheimer's disease (AD) models.

AIM OF THE STUDY

The basic goal of this study was to evaluate the anti-AD activities of C. obtusifolia and its major constituents. Previously, the extract of C. obtusifolia seeds, was reported to have memory enhancing properties and anti-AD activity to ameliorate amyloid β-induced synaptic dysfunction. However, the responsible components of C. obtusifolia seeds in an AD are currently still unknown. In this study, we investigated the inhibitory effects of C. obtusifolia and its constituents against acetylcholinesterase (AChE), butyrylcholinesterase (BChE), and β-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) enzyme activity.

MATERIALS AND METHODS

In vitro cholinesterase enzyme assays by using AChE, BChE, and BACE1 were performed. We also scrutinized the potentials of Cassiae semen active component as BACE1 inhibitors via enzyme kinetics and molecular docking simulation.

RESULTS

In vitro enzyme assays demonstrated that C. obtusifolia and its major constituents have promising inhibitory potential against AChE, BChE, and BACE1. All Cassiae semen constituents exhibited potent inhibitory activities against AChE and BACE1 with IC50 values of 6.29-109µg/mL and 0.94-190µg/mL, whereas alaternin, questin, and toralactone gentiobioside exhibited significant inhibitory activities against BChE with IC50 values of 113.10-137.74µg/mL. Kinetic study revealed that alaternin noncompetitively inhibited, whereas cassiaside and emodin showed mixed-type inhibition against BACE1. Furthermore, molecular docking simulation results demonstrated that hydroxyl group of alaternin and emodin tightly interacted with the active site residues of BACE1 and their relevant binding energies (-6.62 and -6.89kcal/mol), indicating a higher affinity and tighter binding capacity of these compounds for the active site of BACE1.

CONCLUSION

The findings of the present study suggest the potential of C. obtusifolia and its major constituents for use in the development of therapeutic or preventive agents for AD, especially through inhibition of AChE, BChE and BACE1 activities.

A Compact Blast-Induced Traumatic Brain Injury Model in Mice

Blast-induced traumatic brain injury (bTBI) is a common injury on the battlefield and often results in permanent cognitive and neurological abnormalities. We report a novel compact device that creates graded bTBI in mice. The injury severity can be controlled by precise pressures that mimic Friedlander shockwave curves. The mouse head was stabilized with a head fixator, and the body was protected with a metal shield; shockwave durations were 3 to 4 milliseconds. Reflective shockwave peak readings at the position of the mouse head were 12 6 2.6 psi, 50 6 20.3 psi, and 100 6 33.1 psi at 100, 200, and 250 psi predetermined driver chamber pressures, respectively. The bTBIs of 250 psi caused 80% mortality, which decreased to 27% with the metal shield. Brain and lung damage depended on the shockwave duration and amplitude. Cognitive deficits were assessed using the Morris water maze, Y-maze, and open-field tests. Pathological changes in the brain included disruption of the blood-brain barrier, multifocal neuronal and axonal degeneration, and reactive gliosis assessed by Evans Blue dye extravasation, silver and Fluoro-Jade B staining, and glial fibrillary acidic protein immunohistochemistry, respectively. Behavioral and pathological changes were injury severity-dependent. This mouse bTBI model may be useful for investigating injury mechanisms and therapeutic strategies associated with bTBI.

Neuronal human BACE1 knockin induces systemic diabetes in mice

AIMS

β-Secretase 1 (BACE1) is a key enzyme in Alzheimer's disease pathogenesis that catalyses the amyloidogenic cleavage of amyloid precursor protein (APP). Recently, global Bace1 deletion was shown to protect against diet-induced obesity and diabetes, suggesting that BACE1 is a potential regulator of glucose homeostasis. Here, we investigated whether increased neuronal BACE1 is sufficient to alter systemic glucose metabolism, using a neuron-specific human BACE1 knockin mouse model (PLB4).

METHODS

Glucose homeostasis and adiposity were determined by glucose tolerance tests and EchoMRI, lipid species were measured by quantitative lipidomics, and biochemical and molecular alterations were assessed by western blotting, quantitative PCR and ELISAs. Glucose uptake in the brain and upper body was measured via (18)FDG-PET imaging.

RESULTS

Physiological and molecular analyses demonstrated that centrally expressed human BACE1 induced systemic glucose intolerance in mice from 4 months of age onward, alongside a fatty liver phenotype and impaired hepatic glycogen storage. This diabetic phenotype was associated with hypothalamic pathology, i.e. deregulation of the melanocortin system, and advanced endoplasmic reticulum (ER) stress indicated by elevated central C/EBP homologous protein (CHOP) signalling and hyperphosphorylation of its regulator eukaryotic translation initiation factor 2α (eIF2α). In vivo (18)FDG-PET imaging further confirmed brain glucose hypometabolism in these mice; this corresponded with altered neuronal insulin-related signalling, enhanced protein tyrosine phosphatase 1B (PTP1B) and retinol-binding protein 4 (RBP4) levels, along with upregulation of the ribosomal protein and lipid translation machinery. Increased forebrain and plasma lipid accumulation (i.e. ceramides, triacylglycerols, phospholipids) was identified via lipidomics analysis.

CONCLUSIONS/INTERPRETATION

Our data reveal that neuronal BACE1 is a key regulator of metabolic homeostasis and provide a potential mechanism for the high prevalence of metabolic disturbance in Alzheimer's disease.

The Endosome-associated Deubiquitinating Enzyme USP8 Regulates BACE1 Enzyme Ubiquitination and Degradation

The β-site amyloid precursor protein-cleaving enzyme (BACE1) is the rate-limiting enzyme in the production of amyloid-β, the toxic peptide that accumulates in the brain of subjects affected by Alzheimer disease. Our previous studies have shown that BACE1 is degraded via the lysosomal pathway and that that depletion of the trafficking molecule Golgi-localized γ-ear-containing ARF-binding protein 3 (GGA3) results in increased BACE1 levels and activity because of impaired lysosomal degradation. We also determined that GGA3 regulation of BACE1 levels requires its ability to bind ubiquitin. Accordingly, we reported that BACE1 is ubiquitinated at lysine 501 and that lack of ubiquitination at lysine 501 produces BACE1 stabilization. Ubiquitin conjugation is a reversible process mediated by deubiquitinating enzymes. The ubiquitin-specific peptidase 8 (USP8), an endosome-associated deubiquitinating enzyme, regulates the ubiquitination, trafficking, and lysosomal degradation of several plasma membrane proteins. Here, we report that RNAi-mediated depletion of USP8 reduced levels of both ectopically expressed and endogenous BACE1 in H4 human neuroglioma cells. Moreover, USP8 depletion increased BACE1 ubiquitination, promoted BACE1 accumulation in the early endosomes and late endosomes/lysosomes, and decreased levels of BACE1 in the recycling endosomes. We also found that decreased BACE1 protein levels were accompanied by a decrease in BACE1-mediated amyloid precursor protein cleavage and amyloid-β levels. Our findings demonstrate that USP8 plays a key role in the trafficking and degradation of BACE1 by deubiquitinating lysine 501. These studies suggest that therapies able to accelerate BACE1 degradation (e.g. by increasing BACE1 ubiquitination) may represent a potential treatment for Alzheimer disease.

Inhibiting BACE1 to reverse synaptic dysfunctions in Alzheimer's disease

Over the past two decades, many studies have identified significant contributions of toxic β-amyloid peptides (Aβ) to the etiology of Alzheimer's disease (AD), which is the most common age-dependent neurodegenerative disease. AD is also recognized as a disease of synaptic failure. Aβ, generated by sequential proteolytic cleavages of amyloid precursor protein (APP) by BACE1 and γ-secretase, is one of major culprits that cause this failure. In this review, we summarize current findings on how BACE1-cleaved APP products impact learning and memory through proteins localized on glutamatergic, GABAergic, and dopaminergic synapses. Considering the broad effects of Aβ on all three types of synapses, BACE1 inhibition emerges as a practical approach for ameliorating Aβ-mediated synaptic dysfunctions. Since BACE1 inhibitory drugs are currently in clinical trials, this review also discusses potential complications arising from BACE1 inhibition. We emphasize that the benefits of BACE1 inhibitory drugs will outweigh the concerns.

Genetic Deletion of the Clathrin Adaptor GGA3 Reduces Anxiety and Alters GABAergic Transmission

Golgi-localized γ-ear-containing ARF binding protein 3 (GGA3) is a monomeric clathrin adaptor that has been shown to regulate the trafficking of the Beta-site APP-cleaving enzyme (BACE1), which is required for production of the Alzheimer’s disease (AD)-associated amyloid βpeptide. Our previous studies have shown that BACE1 is degraded via the lysosomal pathway and that depletion of GGA3 results in increased BACE1 levels and activity owing to impaired lysosomal trafficking and degradation. We further demonstrated the role of GGA3 in the regulation of BACE1 in vivo by showing that BACE1 levels are increased in the brain of GGA3 null mice. We report here that GGA3 deletion results in novelty-induced hyperactivity and decreased anxiety-like behaviors. Given the pivotal role of GABAergic transmission in the regulation of anxiety-like behaviors, we performed electrophysiological recordings in hippocampal slices and found increased phasic and decreased tonic inhibition in the dentate gyrus granule cells (DGGC). Moreover, we found that the number of inhibitory synapses is increased in the dentate gyrus of GGA3 null mice in further support of the electrophysiological data. Thus, the increased GABAergic transmission is a leading candidate mechanism underlying the reduced anxiety-like behaviors observed in GGA3 null mice. All together these findings suggest that GGA3 plays a key role in GABAergic transmission. Since BACE1 levels are elevated in the brain of GGA3 null mice, it is possible that at least some of these phenotypes are a consequence of increased processing of BACE1 substrates.

BIN1 regulates BACE1 intracellular trafficking and amyloid-β production

BIN1 is a genetic risk factor of late-onset Alzheimer disease (AD), which was identified in multiple genome-wide association studies. BIN1 is a member of the amphiphysin family of proteins, and contains N-terminal Bin-Amphiphysin-Rvs and C-terminal Src homology 3 domains. BIN1 is widely expressed in the mouse and human brains, and has been reported to function in the endocytosis and the endosomal sorting of membrane proteins. BACE1 is a type 1 transmembrane aspartyl protease expressed predominantly in neurons of the brain and responsible for the production of amyloid-β peptide (Aβ). Here we report that the depletion of BIN1 increases cellular BACE1 levels through impaired endosomal trafficking and reduces BACE1 lysosomal degradation, resulting in increased Aβ production. Our findings provide a mechanistic role of BIN1 in the pathogenesis of AD as a novel genetic regulator of BACE1 levels and Aβ production.

SEPT8 modulates β-amyloidogenic processing of APP by affecting the sorting and accumulation of BACE1

Dysfunction and loss of synapses are early pathogenic events in Alzheimer's disease. A central step in the generation of toxic amyloid-β (Aβ) peptides is the cleavage of amyloid precursor protein (APP) by β-site APP-cleaving enzyme (BACE1). Here, we have elucidated whether downregulation of septin (SEPT) protein family members, which are implicated in synaptic plasticity and vesicular trafficking, affects APP processing and Aβ generation. SEPT8 was found to reduce soluble APPβ and Aβ levels in neuronal cells through a post-translational mechanism leading to decreased levels of BACE1 protein. In the human temporal cortex, we identified alterations in the expression of specific SEPT8 transcript variants in a manner that correlated with Alzheimer's-disease-related neurofibrillary pathology. These changes were associated with altered β-secretase activity. We also discovered that the overexpression of a specific Alzheimer's-disease-associated SEPT8 transcript variant increased the levels of BACE1 and Aβ peptides in neuronal cells. These changes were related to an increased half-life of BACE1 and the localization of BACE1 in recycling endosomes. These data suggest that SEPT8 modulates β-amyloidogenic processing of APP through a mechanism affecting the intracellular sorting and accumulation of BACE1.

Post-translational regulation of the β-secretase BACE1

β-Secretase, widely known as β-site APP cleaving enzyme 1 (BACE1), is a membrane-associated protease that cleaves amyloid precursor protein (APP) to generate amyloid β-protein (Aβ). As this cleavage is a pathologically relevant event in Alzheimer's disease, BACE1 is considered a viable therapeutic target. BACE1 can be regulated at the transcriptional, post-transcriptional, translational, and post-translational levels. Elucidation of the regulatory pathways of BACE1 is critical, not only for understanding the pathological mechanisms of AD but also developing effective therapeutic strategies to inhibit activity of the protease. This mini-review focuses on the post-translational regulation of BACE1, as modulation at this level is closely associated with both physiological and pathological conditions. Current knowledge on the mechanisms underlying such BACE1 regulation and their implications for therapy are discussed.

Pulsed arterial spin labeling effectively and dynamically observes changes in cerebral blood flow after mild traumatic brain injury

Cerebral blood flow is strongly associated with brain function, and is the main symptom and diagnostic basis for a variety of encephalopathies. However, changes in cerebral blood flow after mild traumatic brain injury remain poorly understood. This study sought to observe changes in cerebral blood flow in different regions after mild traumatic brain injury using pulsed arterial spin labeling. Our results demonstrate maximal cerebral blood flow in gray matter and minimal in the white matter of patients with mild traumatic brain injury. At the acute and subacute stages, cerebral blood flow was reduced in the occipital lobe, parietal lobe, central region, subcutaneous region, and frontal lobe. Cerebral blood flow was restored at the chronic stage. At the acute, subacute, and chronic stages, changes in cerebral blood flow were not apparent in the insula. Cerebral blood flow in the temporal lobe and limbic lobe diminished at the acute and subacute stages, but was restored at the chronic stage. These findings suggest that pulsed arterial spin labeling can precisely measure cerebral blood flow in various brain regions, and may play a reference role in evaluating a patient's condition and judging prognosis after traumatic brain injury.

Cerebral amyloid-β accumulation and deposition following traumatic brain injury--A narrative review and meta-analysis of animal studies

Traumatic brain injury (TBI) increases the risk of neurodegenerative disorders many years post-injury. However, molecular mechanisms underlying the relationship between TBI and neurodegenerative diseases, such as Alzheimer's disease (AD), remain to be elucidated. Nevertheless, previous studies have demonstrated a link between TBI and increased amyloid-β (Aβ), a protein involved in AD pathogenesis. Here, we review animal studies that measured Aβ levels following TBI. In addition, from a pool of initially identified 1209 published papers, we examined data from 19 eligible animal model studies using a meta-analytic approach. We found an acute increase in cerebral Aβ levels ranging from 24h to one month following TBI (overall log OR=2.97 ± 0.40, p<0.001). These findings may contribute to further understanding the relationship between TBI and future dementia risk. The methodological inconsistencies of the studies discussed in this review suggest the need for improved and more standardised data collection and study design, in order to properly elucidate the role of TBI in the expression and accumulation of Aβ.

Bisecting GlcNAc modification stabilizes BACE1 protein under oxidative stress conditions

BACE1 is a protease essential for amyloid-β production in Alzheimer's disease. We report that bisecting GlcNAc modification on BACE1 stabilizes BACE1 protein under oxidative stress conditions. This suggests that bisecting GlcNAc is a therapeutic target for Alzheimer's disease.

Amyloid β-protein oligomers upregulate the β-secretase, BACE1, through a post-translational mechanism involving its altered subcellular distribution in neurons

Background

β-Site amyloid precursor protein cleaving enzyme 1 (BACE1) is a membrane-bound aspartyl protease that initiates amyloid β-protein (Aβ) generation. Aberrant elevation of BACE1 levels in brains of Alzheimer’s disease (AD) patients may involve Aβ. In the present study, we used a neuron culture model system to investigate the effects of Aβ on BACE1 expression as well as the underlying mechanisms.

Results

Rat primary cortical neurons were treated with relatively low concentrations (2.5 μM) of Aβ42 oligomers (Aβ-O) or fibrils (Aβ-F) for 2–3 days. Aβ-O induced a significant increase in protein levels of BACE1, while Aβ-F only had a marginal effect. Levels of amyloid precursor protein (APP) and the major α-secretase, ADAM10, remained unaltered upon treatment with both types of Aβ. Aβ-O treatment resulted in activation of eIF2α and caspase 3 in a time-dependent manner, with no changes in the endoplasmic reticulum (ER) stress marker, GRP78, indicating that a typical ER stress response is not induced under our experimental conditions. Furthermore, Aβ-O did not affect BACE1 mRNA expression but augmented the levels of exogenous BACE1 expressed via recombinant adenoviruses, indicating regulation of BACE1 protein expression, not at the transcriptional or translational but the post-translational level. Immunocytochemical analysis revealed that Aβ-O causes a significant increase in BACE1 immunoreactivity in neurites (both axons and dendrites), but not soma of neurons; this change appears relevant to the mechanism of Aβ-O-induced BACE1 elevation, which may involve impairment of BACE1 trafficking and degradation. In contrast, Aβ-O had no effect on APP immunoreactivity.

Conclusion

Our results collectively suggest that Aβ oligomers induce BACE1 elevation via a post-translational mechanism involving its altered subcellular distribution in neurons, which possibly triggers a vicious cycle of Aβ generation, thus contributing to the pathogenetic mechanism of AD.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-015-0163-5) contains supplementary material, which is available to authorized users.

An aberrant sugar modification of BACE1 blocks its lysosomal targeting in Alzheimer's disease

The β-site amyloid precursor protein cleaving enzyme-1 (BACE1), an essential protease for the generation of amyloid-β (Aβ) peptide, is a major drug target for Alzheimer's disease (AD). However, there is a concern that inhibiting BACE1 could also affect several physiological functions. Here, we show that BACE1 is modified with bisecting N-acetylglucosamine (GlcNAc), a sugar modification highly expressed in brain, and demonstrate that AD patients have higher levels of bisecting GlcNAc on BACE1. Analysis of knockout mice lacking the biosynthetic enzyme for bisecting GlcNAc, GnT-III (Mgat3), revealed that cleavage of Aβ-precursor protein (APP) by BACE1 is reduced in these mice, resulting in a decrease in Aβ plaques and improved cognitive function. The lack of this modification directs BACE1 to late endosomes/lysosomes where it is less colocalized with APP, leading to accelerated lysosomal degradation. Notably, other BACE1 substrates, CHL1 and contactin-2, are normally cleaved in GnT-III-deficient mice, suggesting that the effect of bisecting GlcNAc on BACE1 is selective to APP. Considering that GnT-III-deficient mice remain healthy, GnT-III may be a novel and promising drug target for AD therapeutics.

The Golgi-Localized γ-Ear-Containing ARF-Binding (GGA) Proteins Alter Amyloid-β Precursor Protein (APP) Processing through Interaction of Their GAE Domain with the Beta-Site APP Cleaving Enzyme 1 (BACE1)

Proteolytic processing of amyloid-β precursor protein (APP) by beta-site APP cleaving enzyme 1 (BACE1) is the initial step in the production of amyloid beta (Aβ), which accumulates in senile plaques in Alzheimer's disease (AD). Essential for this cleavage is the transport and sorting of both proteins through endosomal/Golgi compartments. Golgi-localized γ-ear-containing ARF-binding (GGA) proteins have striking cargo-sorting functions in these pathways. Recently, GGA1 and GGA3 were shown to interact with BACE1, to be expressed in neurons, and to be decreased in AD brain, whereas little is known about GGA2. Since GGA1 impacts Aβ generation by confining APP to the Golgi and perinuclear compartments, we tested whether all GGAs modulate BACE1 and APP transport and processing. We observed decreased levels of secreted APP alpha (sAPPα), sAPPβ, and Aβ upon GGA overexpression, which could be reverted by knockdown. GGA-BACE1 co-immunoprecipitation was impaired upon GGA-GAE but not VHS domain deletion. Autoinhibition of the GGA1-VHS domain was irrelevant for BACE1 interaction. Our data suggest that all three GGAs affect APP processing via the GGA-GAE domain.

Clec4g (LSECtin) interacts with BACE1 and suppresses Aβ generation

β-Site amyloid precursor protein cleaving enzyme-1 (BACE1) is a central molecule in Alzheimer's disease (AD). It cleaves amyloid precursor protein (APP) to produce the toxic amyloid-β (Aβ) peptides. Thus, a novel BACE1 modulator could offer a new therapeutic strategy for AD. We report that C-type lectin-like domain family 4, member g (Clec4g, also designated as LSECtin) interacts with BACE1 in mouse brain and cultured cells. Overexpression of Clec4g suppressed BACE1-mediated Aβ generation, and affected the intracellular distribution of BACE1 but not its catalytic activity. These results highlight a novel role of Clec4g in negatively regulating BACE1 function.

Evidence of a novel mechanism for partial γ-secretase inhibition induced paradoxical increase in secreted amyloid β protein

BACE1 (β-secretase) and α-secretase cleave the Alzheimer's amyloid β protein (Aβ) precursor (APP) to C-terminal fragments of 99 aa (CTFβ) and 83 aa (CTFα), respectively, which are further cleaved by γ-secretase to eventually secrete Aβ and Aα (a.k.a. P3) that terminate predominantly at residues 40 and 42. A number of γ-secretase inhibitors (GSIs), such as N-[N-(3,5-Difluorophenacetyl-L-alanyl)]-S-phenylglycine t-butyl ester (DAPT), have been developed with the goal of reducing Aβ to treat Alzheimer's disease (AD). Although most studies show that DAPT inhibits Aβ in a dose-dependent manner several studies have also detected a biphasic effect with an unexpected increase at low doses of DAPT in cell cultures, animal models and clinical trials. In this article, we confirm the increase in Aβ40 and Aβ42 in SH-SY5Y human neuroblastoma cells treated with low doses of DAPT and identify one of the mechanisms for this paradox. We studied the pathway by first demonstrating that stimulation of Aβ, a product of γ-secretase, was accompanied by a parallel increase of its substrate CTFβ, thereby demonstrating that the inhibitor was not anomalously stimulating enzyme activity at low levels. Secondly, we have demonstrated that inhibition of an Aβ degrading activity, endothelin converting enzyme (ECE), yielded more Aβ, but abolished the DAPT-induced stimulation. Finally, we have demonstrated that Aα, which is generated in the secretory pathway before endocytosis, is not subject to the DAPT-mediated stimulation. We therefore conclude that impairment of γ-secretase can paradoxically increase Aβ by transiently skirting Aβ degradation in the endosome. This study adds to the growing body of literature suggesting that preserving γ-secretase activity, rather than inhibiting it, is important for prevention of neurodegeneration.

Impact of genetic background on neonatal lethality of Gga2 gene-trap mice

The functional redundancy of the three mammalian Golgi-localized, γ-ear-containing, ADP-ribosylation factor-binding proteins (GGAs) was addressed in a previous study. Using insertional mutagenesis, we found that Gga1 or Gga3 homozygous knockout mice were for the most part normal, whereas mice homozygous for two different Gga2 gene-trap alleles exhibited either embryonic or neonatal lethality in the C57BL/6 background, depending on the source of the vector utilized (Byg vs. Tigm, respectively). We now show that the Byg strain harbors a disrupted Gga2 allele that is hypomorphic, indicating that the Byg lethality is attributable to a mechanism independent of GGA2. This is in contrast to the Tigm Gga2 allele, which is a true knockout and establishes a role for GGA2 during the neonatal period. Placement of the Tigm Gga2 allele into the C57BL6/Ola129Sv mixed background results in a lower incidence of neonatal lethality, showing the importance of genetic background in determining the requirement for GGA2 during this period. The Gga2(-/-) mice that survive have reduced body weight at birth and this runted phenotype is maintained through adulthood.

Traumatic Brain Injury Increases Cortical Glutamate Network Activity by Compromising GABAergic Control

Traumatic brain injury (TBI) is a major risk factor for developing pharmaco-resistant epilepsy. Although disruptions in brain circuitry are associated with TBI, the precise mechanisms by which brain injury leads to epileptiform network activity is unknown. Using controlled cortical impact (CCI) as a model of TBI, we examined how cortical excitability and glutamatergic signaling was altered following injury. We optically mapped cortical glutamate signaling using FRET-based glutamate biosensors, while simultaneously recording cortical field potentials in acute brain slices 2-4 weeks following CCI. Cortical electrical stimulation evoked polyphasic, epileptiform field potentials and disrupted the input-output relationship in deep layers of CCI-injured cortex. High-speed glutamate biosensor imaging showed that glutamate signaling was significantly increased in the injured cortex. Elevated glutamate responses correlated with epileptiform activity, were highest directly adjacent to the injury, and spread via deep cortical layers. Immunoreactivity for markers of GABAergic interneurons were significantly decreased throughout CCI cortex. Lastly, spontaneous inhibitory postsynaptic current frequency decreased and spontaneous excitatory postsynaptic current increased after CCI injury. Our results suggest that specific cortical neuronal microcircuits may initiate and facilitate the spread of epileptiform activity following TBI. Increased glutamatergic signaling due to loss of GABAergic control may provide a mechanism by which TBI can give rise to post-traumatic epilepsy.

Traumatic brain injury using mouse models

The use of mouse models in traumatic brain injury (TBI) has several advantages compared to other animal models including low cost of breeding, easy maintenance, and innovative technology to create genetically modified strains. Studies using knockout and transgenic mice demonstrating functional gain or loss of molecules provide insight into basic mechanisms of TBI. Mouse models provide powerful tools to screen for putative therapeutic targets in TBI. This article reviews currently available mouse models that replicate several clinical features of TBI such as closed head injuries (CHI), penetrating head injuries, and a combination of both. CHI may be caused by direct trauma creating cerebral concussion or contusion. Sudden acceleration-deceleration injuries of the head without direct trauma may also cause intracranial injury by the transmission of shock waves to the brain. Recapitulation of temporary cavities that are induced by high-velocity penetrating objects in the mouse brain are difficult to produce, but slow brain penetration injuries in mice are reviewed. Synergistic damaging effects on the brain following systemic complications are also described. Advantages and disadvantages of CHI mouse models induced by weight drop, fluid percussion, and controlled cortical impact injuries are compared. Differences in the anatomy, biomechanics, and behavioral evaluations between mice and humans are discussed. Although the use of mouse models for TBI research is promising, further development of these techniques is warranted.

Critical analysis of the use of β-site amyloid precursor protein-cleaving enzyme 1 inhibitors in the treatment of Alzheimer's disease

Alzheimer’s disease (AD) is the major cause of dementia in the elderly and an unmet clinical challenge. A variety of therapies that are currently under development are directed to the amyloid cascade. Indeed, the accumulation and toxicity of amyloid-β (Aβ) is believed to play a central role in the etiology of the disease, and thus rational interventions are aimed at reducing the levels of Aβ in the brain. Targeting β-site amyloid precursor protein-cleaving enzyme (BACE)-1 represents an attractive strategy, as this enzyme catalyzes the initial and rate-limiting step in Aβ production. Observation of increased levels of BACE1 and enzymatic activity in the brain, cerebrospinal fluid, and platelets of patients with AD and mild cognitive impairment supports the potential benefits of BACE1 inhibition. Numerous potent inhibitors have been generated, and many of these have been proved to lower Aβ levels in the brain of animal models. Over 10 years of intensive research on BACE1 inhibitors has now culminated in advancing half a dozen of these drugs into human trials, yet translating the in vitro and cellular efficacy of BACE1 inhibitors into preclinical and clinical trials represents a challenge. This review addresses the promises and also the potential problems associated with BACE1 inhibitors for AD therapy, as the complex biological function of BACE1 in the brain is becoming unraveled.

Molecular mechanisms of cognitive dysfunction following traumatic brain injury

Traumatic brain injury (TBI) results in significant disability due to cognitive deficits particularly in attention, learning and memory, and higher-order executive functions. The role of TBI in chronic neurodegeneration and the development of neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), Amyotrophic Lateral Sclerosis (ALS) and most recently chronic traumatic encephalopathy (CTE) is of particular importance. However, despite significant effort very few therapeutic options exist to prevent or reverse cognitive impairment following TBI. In this review, we present experimental evidence of the known secondary injury mechanisms which contribute to neuronal cell loss, axonal injury, and synaptic dysfunction and hence cognitive impairment both acutely and chronically following TBI. In particular we focus on the mechanisms linking TBI to the development of two forms of dementia: AD and CTE. We provide evidence of potential molecular mechanisms involved in modulating Aβ and Tau following TBI and provide evidence of the role of these mechanisms in AD pathology. Additionally we propose a mechanism by which Aβ generated as a direct result of TBI is capable of exacerbating secondary injury mechanisms thereby establishing a neurotoxic cascade that leads to chronic neurodegeneration.

Metabolic stress modulates Alzheimer's β-secretase gene transcription via SIRT1-PPARγ-PGC-1 in neurons

Classic cardio-metabolic risk factors such as hypertension, stroke, diabetes, and hypercholesterolemia all increase the risk of Alzheimer's disease. We found increased transcription of β-secretase/BACE1, the rate-limiting enzyme for Aβ generation, in eNOS-deficient mouse brains and after feeding mice a high-fat, high-cholesterol diet. Up- or downregulation of PGC-1α reciprocally regulated BACE1 in vitro and in vivo. Modest fasting in mice reduced BACE1 transcription in the brains, which was accompanied by elevated PGC-1 expression and activity. Moreover, the suppressive effect of PGC-1 was dependent on activated PPARγ, likely via SIRT1-mediated deacetylation in a ligand-independent manner. The BACE1 promoter contains multiple PPAR-RXR sites, and direct interactions among SIRT1-PPARγ-PGC-1 at these sites were enhanced with fasting. The interference on the BACE1 gene identified here represents a unique noncanonical mechanism of PPARγ-PGC-1 in transcriptional repression in neurons in response to metabolic signals that may involve recruitment of corepressor NCoR.

Mild oxidative stress induces redistribution of BACE1 in non-apoptotic conditions and promotes the amyloidogenic processing of Alzheimer's disease amyloid precursor protein

BACE1 is responsible for β-secretase cleavage of the amyloid precursor protein (APP), which represents the first step in the production of amyloid β (Aβ) peptides. Previous reports, by us and others, have indicated that the levels of BACE1 protein and activity are increased in the brain cortex of patients with Alzheimer's disease (AD). The association between oxidative stress (OS) and AD has prompted investigations that support the potentiation of BACE1 expression and enzymatic activity by OS. Here, we have established conditions to analyse the effects of mild, non-lethal OS on BACE1 in primary neuronal cultures, independently from apoptotic mechanisms that were shown to impair BACE1 turnover. Six-hour treatment of mouse primary cortical cells with 10-40 µM hydrogen peroxide did not significantly compromise cell viability but it did produce mild oxidative stress (mOS), as shown by the increased levels of reactive radical species and activation of p38 stress kinase. The endogenous levels of BACE1 mRNA and protein were not significantly altered in these conditions, whereas a toxic H2O2 concentration (100 µM) caused an increase in BACE1 protein levels. Notably, mOS conditions resulted in increased levels of the BACE1 C-terminal cleavage product of APP, β-CTF. Subcellular fractionation techniques showed that mOS caused a major rearrangement of BACE1 localization from light to denser fractions, resulting in an increased distribution of BACE1 in fractions containing APP and markers for trans-Golgi network and early endosomes. Collectively, these data demonstrate that mOS does not modify BACE1 expression but alters BACE1 subcellular compartmentalization to favour the amyloidogenic processing of APP, and thus offer new insight in the early molecular events of AD pathogenesis.

β-Secretase: its biology as a therapeutic target in diseases

β-Secretase (BACE1, β-site APP cleaving enzyme 1) is an aspartic proteinase that has multiple functions in various physiological processes, such as cell differentiation, immunoregulation, and cell death. There is increasing evidence that changes in BACE1 activity are involved in many diseases, such as Alzheimer's disease (AD), schizophrenia, epileptic behavior, and others. However, a deeper understanding of the molecular biology of BACE1 is necessary for further exploration of cell development, immunological regulation, and disease pathogenesis. Here, we review the molecular and cellular biology of BACE1, including its enzymatic properties, structure, biosynthesis, and physiological functions to provide a new perspective and rational assessment of drugability. Lastly, we discuss proposed strategies to control BACE1 activity for possible therapeutic application.