TMP21 degradation is mediated by the ubiquitin-proteasome pathway

RHBDL4-triggered downregulation of COPII adaptor protein TMED7 suppresses TLR4-mediated inflammatory signaling

The toll-like receptor 4 (TLR4) is a central regulator of innate immunity that primarily recognizes bacterial lipopolysaccharide cell wall constituents to trigger cytokine secretion. We identify the intramembrane protease RHBDL4 as a negative regulator of TLR4 signaling. We show that RHBDL4 triggers degradation of TLR4's trafficking factor TMED7. This counteracts TLR4 transport to the cell surface. Notably, TLR4 activation mediates transcriptional upregulation of RHBDL4 thereby inducing a negative feedback loop to reduce TLR4 trafficking to the plasma membrane. This secretory cargo tuning mechanism prevents the over-activation of TLR4-dependent signaling in an in vitro Mycobacterium tuberculosis macrophage infection model and consequently alleviates septic shock in a mouse model. A hypomorphic RHBDL4 mutation linked to Kawasaki syndrome, an ill-defined inflammatory disorder in children, further supports the pathophysiological relevance of our findings. In this work, we identify an RHBDL4-mediated axis that acts as a rheostat to prevent over-activation of the TLR4 pathway.

Tmed10 deficiency results in impaired exocrine pancreatic differentiation in zebrafish larvae

Transmembrane p24 trafficking protein 10 (TMED10) is a conserved vesicle trafficking protein. It is dysregulated in Alzheimer disease and plays a pivotal role in the pathogenesis of Alzheimer disease. In addition to the brain, TMED10 is highly expressed in the exocrine pancreas; however, its biological functions and underlying mechanisms remain largely unknown. We studied reduced Tmed10 in zebrafish embryos by morpholino oligonucleotide knockdown and CRISPR-Cas9 mutagenesis. Tmed10-deficient embryos showed extensive loss of acinar mass and impaired acinar differentiation. TMED10 has been reported to have an inhibitory effect on γ-secretase. As one of the substrates of γ-secretase, membrane-bound β-catenin was significantly reduced in Tmed10-deficient embryos. Increased γ-secretase activity in wild-type embryos resulted in a phenotype similar to that of tmed10 mutants. And the mutant phenotype could be rescued by treatment with the γ-secretase inhibitor, N-[N-(3, 5-difluorophenacetyl)-l-alanyl]-s-phenylglycinet-butyl ester (DAPT). In addition, the reduced membrane-bound β-catenin was accompanied with up-regulated β-catenin target genes in Tmed10-deficient embryos. Overexpression of β-catenin signaling inhibitor Dickkopf-1 (DKK-1) could rescue the exocrine pancreas defects. Taken together, our study reveals that Tmed10 regulates exocrine pancreatic differentiation through γ-secretase. Reduced membrane-bound β-catenin, accompanied with hyperactivation of β-catenin signaling, is an important cause of exocrine pancreas defects in Tmed10-deficient embryos. Our study reaffirms the importance of appropriate β-catenin signaling in exocrine pancreas development. These findings may provide a theoretical basis for the development of treatment strategies for TMED10-related diseases.

CNTNAP2 Protein Is Degraded by the Ubiquitin-Proteasome System and the Macroautophagy-Lysosome Pathway

Contactin-associated protein-like 2 (CNTNAP2) gene, located on chromosome 7q35, is one of the largest genes in the human genome. CNTNAP2 protein is a type-I transmembrane protein specifically expressed in the nervous system, with versatile roles in the axonal organization, synaptic functions, neuronal migration, and functional connectivity. CNTNAP2 has been widely investigated as a risk gene for autism spectrum disorder (ASD), and recent studies also implicated CNTNAP2 in Alzheimer's disease (AD). Knowledge of the regulations on CNTNAP2's life cycle is necessary for understanding the related physiological functions and pathological conditions. However, the mechanisms underlying CNTNAP2 protein degradation remain elusive. Therefore, we systematically investigated the half-life and degradation pathway of the human CNTNAP2 protein. We discovered that CNTNAP2 has C-terminal fragments (CTF), which may have essential physiological functions. Our results demonstrated that CNTNAP2 full-length protein and CTF have a short half-life of about 3-4 h. CNTNAP2 proteins are degraded by the ubiquitin-proteasome system and the macroautophagy-lysosome pathway, while the lysosome pathway is more common for CNTNAP2 degradation. This study will provide novel insights and valuable tools for CNTNAP2 functional research in physiological and pathological scenarios.

RCAN1 Inhibits BACE2 Turnover by Attenuating Proteasome-Mediated BACE2 Degradation

Amyloid-β protein (Aβ) is the main component of neuritic plaques, the pathological hallmark of Alzheimer's disease (AD). β-site APP cleaving enzyme 1 (BACE1) is a major β-secretase contributing to Aβ generation. β-site APP cleaving enzyme 2 (BACE2), the homolog of BACE1, is not only a θ-secretase but also a conditional β-secretase. Previous studies showed that regulator of calcineurin 1 (RCAN1) is markedly increased by AD and promotes BACE1 expression. However, the role of RCAN1 in BACE2 regulation remains elusive. Here, we showed that RCAN1 increases BACE2 protein levels. Moreover, RCAN1 inhibits the turnover of BACE2 protein. Furthermore, RCAN1 attenuates proteasome-mediated BACE2 degradation, but not lysosome-mediated BACE2 degradation. Taken together, our work indicates that RCAN1 inhibits BACE2 turnover by attenuating proteasome-mediated BACE2 degradation. It advances our understanding of BACE2 regulation and provides a potential mechanism of BACE2 dysregulation in AD.

BACE2 degradation is mediated by both the proteasome and lysosome pathways

Background

Alzheimer’s disease is the most common neurodegenerative disease in the elderly. Amyloid-β protein (Aβ) is the major component of neuritic plaques which are the hallmark of AD pathology. β-site APP cleaving enzyme 1 (BACE1) is the major β-secretase contributing to Aβ generation. β-site APP-cleaving enzyme 2 (BACE2), the homolog of BACE1, might play a complex role in the pathogenesis of Alzheimer’s disease as it is not only a θ-secretase but also a conditional β-secretase. Dysregulation of BACE2 is observed in Alzheimer’s disease. However, the regulation of BACE2 is less studied compared with BACE1, including its degradation pathways. In this study, we investigated the turnover rates and degradation pathways of BACE2 in both neuronal cells and non-neuronal cells.

Results

Both lysosomal inhibition and proteasomal inhibition cause a time- and dose-dependent increase of transiently overexpressed BACE2 in HEK293 cells. The half-life of transiently overexpressed BACE2 protein is approximately 6 h. Moreover, the half-life of endogenous BACE2 protein is approximately 4 h in both HEK293 cells and mouse primary cortical neurons. Furthermore, both lysosomal inhibition and proteasomal inhibition markedly increases endogenous BACE2 in HEK293 cells and mouse primary cortical neurons.

Conclusions

This study demonstrates that BACE2 is degraded by both the proteasome and lysosome pathways in both neuronal and non-neuronal cells at endogenous level and in transient overexpression system. It indicates that BACE2 dysregulation might be mediated by the proteasomal and lysosomal impairment in Alzheimer’s disease. This study advances our understanding of the regulation of BACE2 and provides a potential mechanism of its dysregulation in Alzheimer’s disease.

RAB21 interacts with TMED10 and modulates its localization and abundance

Membrane trafficking controls vesicular transport of cargo between cellular compartments. Vesicular trafficking is essential for cellular homeostasis and dysfunctional trafficking is linked to several pathologies such as neurodegenerative diseases. Following endocytosis, early endosomes act as sorting stations of internalized materials, routing cargo toward various fates. One important class of membrane trafficking regulators are RAB GTPases. RAB21 has been associated with multiple functions and regulates integrin internalization, endosomal sorting of specific clathrin-independent cargo and autophagy. Although RAB21 is mostly associated with early endosomes, it has been shown to mediate a specific sorting event at the Golgi. From mass spectrometry data, we identified a GTP-favored interaction between RAB21 and TMED10 and 9, essential regulators of COPI and COPII vesicles. Using RAB21 knockout cells, we describe the role of RAB21 in modulating TMED10 Golgi localization. Taken together, our study suggests a new potential function of RAB21 in modulating TMED10 trafficking, with relevance to neurodegenerative disorders.

Summary: A small early endosomal RAB GTPase is found to interact with p24 family members, with potential impacts on p24 functions.

Proteasome Inhibition Activates Autophagy-Lysosome Pathway Associated With TFEB Dephosphorylation and Nuclear Translocation

Ubiquitin-proteasome pathway (UPS) and autophagy-lysosome pathway (ALP) are the two major protein degradation pathways, which are critical for proteostasis. Growing evidence indicates that proteasome inhibition-induced ALP activation is an adaptive response. Transcription Factor EB (TFEB) is a master regulator of ALP. However, the characteristics of TFEB and its role in proteasome inhibition-induced ALP activation are not fully investigated. Here we reported that the half-life of TFEB is around 13.5 h in neuronal-like cells, and TFEB is degraded through proteasome pathway in both neuronal-like and non-neuronal cells. Moreover, proteasome impairment not only promotes TFEB accumulation but also facilitates its dephosphorylation and nuclear translocation. In addition, proteasome inhibition-induced TFEB accumulation, dephosphorylation and nuclear translocation significantly increases the expression of a number of TFEB downstream genes involved in ALP activation, including microtubule-associated protein 1B light chain-3 (LC3), particularly LC3-II, cathepsin D and lysosomal-associated membrane protein 1 (LAMP1). Furthermore, we demonstrated that proteasome inhibition increases autophagosome biogenesis but not impairs autophagic flux. Our study advances the understanding of features of TFEB and indicates that TFEB might be a key mediator of proteasome impairment-induced ALP activation.

TMP21 in Alzheimer's Disease: Molecular Mechanisms and a Potential Target

Alzheimer’s disease (AD) is the most common form of dementia in the elderly, which is characterized by progressive cognitive impairment. Neuritic plaques, neurofibrillary tangles and neuronal loss are the major neuropathological hallmarks in AD brains. TMP21 is a key molecule for protein trafficking in cells. Growing evidence indicates that TMP21 is dysregulated in AD, which plays a pivotal role in neuritic plaque formation. Therefore, we aim to review the dysregulation of TMP21 in AD, the role of TMP21 in neuritic plaque formation and underlying mechanisms. Moreover, the potential role of TMP21 in neurofibrillary tangle formation, synaptic impairment and neuronal loss is discussed. It will provide an outlook into the potential of regulating TMP21 as a therapeutic approach for AD treatment.

Regulation of global gene expression in brain by TMP21

TMP21, a type I transmembrane protein of thep24 protein family, mediates protein trafficking and maturation. Dysregulation of TMP21 is implicated in the pathogenesis of Alzheimer’s disease (AD). However, underlying mechanisms remain elusive. To reveal the function of TMP21 in the brain and the pathogenic role of TMP21 in the brain of AD, the global gene expression was profiled in the brain of TMP21 knockdown mice. We found that 8196 and 8195 genes are significantly altered in the hippocampus and cortex, respectively. The genes are involved in a number of brain function-related pathways, including glutamatergic synapse pathway, serotonergic synapse pathway, synaptic vesicle pathway, and long-term depression pathway. Moreover, the network analysis suggests that the TMP21 may contribute to the pathogenesis of AD by regulatingPI3K/Akt/GSK3β signalling pathway. Our study provides an insight into the physiological function of TMP21 in the brain and pathological role of TMP21 in AD.

Down-regulated TMED10 in Alzheimer disease induces autophagy via ATG4B activation

Several studies have shown that dysfunction of macroautophagy/autophagy is associated with many human diseases, including neurodegenerative disease and cancer. To explore the molecular mechanisms of autophagy, we performed a cell-based functional screening with SH-SY5Y cells stably expressing GFP-LC3, using an siRNA library and identified TMED10 (transmembrane p24 trafficking protein 10), previously known as the γ-secretase-modulating protein, as a novel regulator of autophagy. Further investigations revealed that depletion of TMED10 induced the activation of autophagy. Interestingly, protein-protein interaction assays showed that TMED10 directly binds to ATG4B (autophagy related gene 4B cysteine peptidase), and the interaction is diminished under autophagy activation conditions such as rapamycin treatment and serum deprivation. In addition, inhibition of TMED10 significantly enhanced the proteolytic activity of ATG4B for LC3 cleavage. Importantly, the expression of TMED10 in AD (Alzheimer disease) patients was considerably decreased, and downregulation of TMED10 increased amyloid-β (Aβ) production. Treatment with Aβ increased ATG4B proteolytic activity as well as dissociation of TMED10 and ATG4B. Taken together, our results suggest that the AD-associated protein TMED10 negatively regulates autophagy by inhibiting ATG4B activity.Abbreviations: Aβ: amyloid-β; AD: Alzheimer disease; ATG: autophagy related; BECN1: beclin 1; BiFC: bimolecular fluorescence complementation; CD: cytosolic domain; GFP: green fluorescent protein; GLUC: Gaussia luciferase; IP: immunoprecipitation; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; LD: luminal domain; PD: Parkinson disease; ROS: reactive oxygen species; siRNA: small interfering RNA; SNP: single-nucleotide polymorphisms; TD: transmembrane domain; TMED10: transmembrane p24 trafficking protein 10; VC: C terminus of Venus fluorescent protein; VN: N terminus of Venus fluorescent protein.

Mild traumatic brain injury induces memory deficits with alteration of gene expression profile

Repeated mild traumatic brain injury (rmTBI), the most common type of traumatic brain injuries, can result in neurological dysfunction and cognitive deficits. However, the molecular mechanisms and the long-term consequence of rmTBI remain elusive. In this study, we developed a modified rmTBI mouse model and found that rmTBI-induced transient neurological deficits and persistent impairments of spatial memory function. Furthermore, rmTBI mice had long-lasting detrimental effect on cognitive function, exhibiting memory deficits even 12 weeks after rmTBI. Microarray analysis of whole genome gene expression showed that rmTBI significantly altered the expression level of 87 genes which are involved in apoptosis, stress response, metabolism, and synaptic plasticity. The results indicate the potential mechanism underlying rmTBI-induced acute neurological deficits and its chronic effect on memory impairments. This study suggests that long-term monitoring and interventions for rmTBI individuals are essential for memory function recovery and reducing the risk of developing neurodegenerative diseases.

A Novel Alzheimer-Associated SNP in Tmp21 Increases Amyloidogenesis

Recent studies suggest that TMP21 is a selective modulator of γ-secretase and its dysregulation affects APP processing, leading to increased Aβ generation. However, the genetic association between Tmp21 and Alzheimer's disease (AD) remains elusive. In this study, we identified that a novel single-nucleotide polymorphism (SNP) rs12435391 (IVS4-28T>C) in intron 4 of Tmp21 was genetically associated with AD. We found that allele C of the SNP rs12435391 did not affect splicing site recognition, but it significantly increased TMP21 gene expression. The stability of Tmp21 pre-mRNA and the transcription of Tmp21 were not affected by allele C of the SNP rs12435391. However, allele C of the SNP rs12435391 significantly increased the splicing efficiency of Tmp21 pre-mRNA, leading to the elevation of mature mRNA. Furthermore, allele C of the SNP rs12435391 significantly reduced C83 level and increased Aβ generation. Taken together, our study suggests that TMP21 is genetically associated with Alzheimer's disease, with the novel Tmp21 SNP as a risk factor for Alzheimer's pathogenesis.

p24 family proteins: key players in the regulation of trafficking along the secretory pathway

p24 family proteins have been known for a long time, but their functions have remained elusive. However, they are emerging as essential regulators of protein trafficking along the secretory pathway, influencing the composition, structure, and function of different organelles in the pathway, especially the ER and the Golgi apparatus. In addition, they appear to modulate the transport of specific cargos, including GPI-anchored proteins, G-protein-coupled receptors, or K/HDEL ligands. As a consequence, they have been shown to play specific roles in signaling, development, insulin secretion, and the pathogenesis of Alzheimer's disease. The search of new putative ligands may open the way to discover new functions for this fascinating family of proteins.

Degradation of gamma secretase activating protein by the ubiquitin-proteasome pathway

A major hallmark feature of Alzheimer's disease is the accumulation of amyloid β (Aβ), whose formation is regulated by the γ-secretase complex and its activating protein (also known as γ-secretase activating protein, or GSAP). Because GSAP interacts with the γ-secretase without affecting the cleavage of Notch, it is an ideal target for a viable anti-Aβ therapy. GSAP derives from a C-terminal fragment of a larger precursor protein of 98 kDa via a caspase 3-mediated cleavage. However, the mechanism(s) involved in its degradation remain unknown. In this study, we show that GSAP has a short half-life of approximately 5 h. Neuronal cells treated with proteasome inhibitors markedly prevented GSAP protein degradation, which was associated with a significant increment in Aβ levels and γ-secretase cleavage products. In contrast, treatment with calpain blocker and lysosome inhibitors had no effect. In addition, we provide experimental evidence that GSAP is ubiquitinated. Taken together, our findings reveal that GSAP is degraded through the ubiquitin-proteasome system. Modulation of the GSAP degradation pathway may be implemented as a viable target for a safer anti-Aβ therapeutic approach in Alzheimer's disease. The GSAP derives from a precursor via a caspase 3-mediated cleavage, is up-regulated in Alzheimer's disease brains and facilitates Aβ production by interacting directly with the γ-secretase complex. Here, we demonstrate that GSAP is ubiquitinated and then selectively degraded via the proteasome system but not the calpains or lysosome pathways. These findings provide further evidence for the involvement of the proteasome system in the regulation of amyloid beta (Aβ) precursor protein metabolism and Aβ formation. AICD, APP intracellular domain; APP, amyloid precursor protein; ATP, adenosine triphosphate; CTF-α, alpha-C-terminal fragment; CTF-β, beta-C-terminal fragment; GSAP, γ-secretase activating protein; Ub, ubiquitin.

Overexpression of ubiquitin carboxyl-terminal hydrolase L1 (UCHL1) delays Alzheimer's progression in vivo

Deposition of amyloid β protein (Aβ) to form neuritic plaques in the brain is the pathological hallmark of Alzheimer's disease (AD). Aβ is produced by β- and γ-cleavages of amyloid β precursor protein (APP). Ubiquitin carboxyl-terminal hydrolase L1 (UCHL1) is a de-ubiquitinating enzyme that cleaves ubiquitin at its carboxyl terminal. Dysfunction of UCHL1 has been reported in neurodegenerative diseases. However, whether UCHL1 affects Aβ production and AD progression remains unknown. Here we report that UCHL1 interacts with APP and regulates Aβ production. UCHL1 increases free ubiquitin level and accelerates the lysosomal degradation of APP by promoting its ubiquitination. Furthermore, we demonstrate that overexpression of UCHL1 by intracranial injection of UCHL1-expressing rAAV reduces Aβ production, inhibits neuritic plaque formation and improves memory deficits in AD transgenic model mice. Our study suggests that UCHL1 may delay Alzheimer's progression by regulating APP degradation in a long-term fashion, and that overexpression of UCHL1 may be a safe and effective disease-modifying strategy to treat AD.

Amyloid-β precursor protein facilitates the regulator of calcineurin 1-mediated apoptosis by downregulating proteasome subunit α type-5 and proteasome subunit β type-7

Individuals with Down syndrome (DS), caused by trisomy of chromosome 21, inevitably develop characteristic Alzheimer's disease (AD) neuropathology, including neuritic plaques, neurofibrillary tangles, and neuronal loss. Amyloid-β protein, the major component of neuritic plaques, is the proteolytic product of amyloid-β precursor protein (APP). APP and the regulator of calcineurin 1 (RCAN1) genes on chromosome 21 play a pivotal role in promoting plaque formation and neuronal apoptosis. However, the mechanism underlying AD pathogenesis in DS is not well defined. In this study, we demonstrated that APP significantly increased RCAN1 level in both cells and transgenic mice. Overexpression of APP significantly reduced the expression of 2 proteasome subunits, proteasome subunit α type-5 and proteasome subunit β type-7, leading to the inhibition of proteasomal degradation of RCAN1. Furthermore, knockdown of RCAN1 expression attenuated APP-induced neuronal apoptosis. Taken together, the results clearly showed that APP has a previously unknown function in regulating RCAN1-mediated neuronal apoptosis through the proteasome pathway. Our study demonstrates a novel mechanism by which overexpression of APP and RCAN1 causes neurodegeneration and AD pathogenesis in DS, and it provides new insights into the potential of targeting APP-induced proteasomal impairment and RCAN1 accumulation for AD and DS treatment.

BACE2 degradation mediated by the macroautophagy-lysosome pathway

Neuritic plaque is the pathological hallmark in Alzheimer's disease (AD). Amyloid-β protein (Aβ), the central component of neuritic plaques, is generated from amyloid-β precursor protein (APP) by β-site APP cleaving enzyme 1 (BACE1) and γ-secretase. β-site APP cleaving enzyme 2 (BACE2), a homolog of BACE1, functions differently from BACE1 in APP processing. BACE1 is the β-secretase essential for Aβ production, and BACE2, a θ-secretase, cleaves APP within the Aβ domain, preventing Aβ production. Elucidation of the mechanism underlying BACE2 degradation is important for defining its biological features and its potential role in Alzheimer's disease drug development. In this report we first showed that the half-life of BACE2 is approximately 20 h. Lysosomal inhibition increased BACE2 protein levels whereas proteasomal inhibition had no effect on BACE2 protein expression. Furthermore, we identified that macroautophagy mediated BACE2 degradation. Finally, we showed that lysosomal inhibition increased BACE2 cleavage of APP. Taken together, our in vitro study showed that BACE2 is degraded through the macrophagy-lysosome pathway and that lysosomal inhibition affects BACE2 processing of APP. Modulation of BACE2 degradation via the lysosomal pathway could be a new target for AD drug development.

Putative p24 complexes in Arabidopsis contain members of the delta and beta subfamilies and cycle in the early secretory pathway

p24 proteins are a family of type I membrane proteins localized to compartments of the early secretory pathway and to coat protein I (COPI)- and COPII-coated vesicles. They can be classified, by sequence homology, into four subfamilies, named p24α, p24β, p24γ, and p24δ. In contrast to animals and fungi, plants contain only members of the p24β and p24δ subfamilies, the latter probably including two different subclasses. It has previously been shown that transiently expressed red fluorescent protein (RFP)-p24δ5 (p24δ1 subclass) localizes to the endoplasmic reticulum (ER) at steady state as a consequence of highly efficient COPI-based recycling from the Golgi apparatus. It is now shown that transiently expressed RFP-p24δ9 (p24δ2 subclass) also localizes to the ER. In contrast, transiently expressed green fluorescent protein (GFP)-p24β3 mainly localizes to the Golgi apparatus (as p24β2) and exits the ER in a COPII-dependent manner. Immunogold electron microscopy in Arabidopsis root tip cells using specific antibodies shows that endogenous p24δ9 localizes mainly to the ER but also partially to the cis-Golgi. In contrast, endogenous p24β3 mainly localizes to the Golgi apparatus. By a combination of experiments using transient expression, knock-out mutants, and co-immunoprecipitation, it is proposed that Arabidopsis p24 proteins form different heteromeric complexes (including members of the β and δ subfamilies) which are important for their stability and their coupled trafficking at the ER-Golgi interface. Evidence is also provided for a role for p24δ5 in retrograde Golgi-ER transport of the KDEL-receptor ERD2.

Regulation of RCAN1 translation and its role in oxidative stress-induced apoptosis

Abnormal expression of regulator of calcineurin 1 (RCAN1) has been implicated in Alzheimer's disease (AD) and Down's syndrome (DS). There are two major isoforms of RCAN1, isoforms 1 and 4. RCAN1 isoform 1 is predominantly expressed in the brain, particularly in neurons. In this report, we showed that there are two translation start codons in RCAN1 exon 1 serving as a functional translation initiation site to generate a longer 41-kDa isoform 1 (RCAN1.1L) and a shorter 31-kDa isoform 1 (RCAN1.1S). The first translation initiation site has higher translation efficiency than the downstream second one, and the translation initiation of two AUG sites is by a Cap-dependent mechanism. Short-term expression of RCAN1.1L protected SH-SY5Y cells from oxidative stress-induced apoptosis by inhibiting caspase-3 activation. However, long-term accumulation of RCAN1.1L in SH-SY5Y cells promoted oxidative stress-induced apoptosis via caspase-3 activation, and terminal deoxynucleotidyl transferase dUTP nick end labeling assay showed that the apoptosis ratio was increased to 499.03 ± 47.56% in SH-1.1L cells compared with 283.93 ± 28.66% in control cells. Furthermore, we found that RCAN1.1L is significantly elevated in the AD brains and patients with DS. RCAN1.1S is expressed at a low level in both human cells and brain tissues. Our results defined the regulatory mechanism underlying RCAN1 expression and the roles of RCAN1.1 in oxidative stress-induced neurodegeneration in AD and DS pathogenesis.

Control of BACE1 degradation and APP processing by ubiquitin carboxyl-terminal hydrolase L1

Deposition of amyloid β protein (Aβ) in the brain is the hallmark of Alzheimer's disease (AD) pathogenesis. Beta-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1) is the β-secretase in vivo essential for generation of Aβ. Previously we demonstrated that BACE1 is ubiquitinated and the degradation of BACE1 is mediated by the ubiquitin-proteasome pathway (UPP). However the mechanism underlying regulation of BACE1 degradation by UPP remains elusive. Ubiquitin carboxyl-terminal hydrolase L1 (UCHL1) is a deubiquitinating enzyme highly specific to neuron, catalyzing the hydrolysis of ubiquitin conjugates from ubiquitinated substrates. UCHL1 regulates ubiquitin-dependent protein degradation. However, whether UCHL1 is particularly involved in the proteasomal degradation of BACE1 and what is the role of UCHL1 in AD pathogenesis remain elusive. To investigate the effect of UCHL1 on BACE1 degradation, HUCH cells, a UCHL1 stably over-expressed HEK293 cell line, was established. We found that inhibition of UCHL1 significantly increased BACE1 protein level in a time-dependent manner. Half life of BACE1 was reduced in HUCH cells compared with HEK. Over-expression of UCHL1 decreased APP C-terminal fragment C99 and Aβ levels in HUCH cells. Moreover, disruption of Uchl1 gene significantly elevated levels of endogenous BACE1, C99 and Aβ in the Uchl1-null gad mice. These results demonstrated that UCHL1 accelerates BACE1 degradation and affects APP processing and Aβ production. This study suggests that potentiation of UCHL1 might be able to reduce the level of BACE1 and Aβ in brain, which makes it a novel target for AD drug development.

Transcriptional Regulation of TMP21 by NFAT

BACKGROUND

TMP21 is a member of the p24 cargo protein family, which is involved in protein transport between the Golgi apparatus and ER. Alzheimer's Disease (AD) is the most common neurodegenerative disorder leading to dementia and deposition of amyloid β protein (Aβ) is the pathological feature of AD pathogenesis. Knockdown of TMP21 expression by siRNA causes a sharp increase in Aβ production; however the underlying mechanism by which TMP21 regulates Aβ generation is unknown, and human TMP21 gene expression regulation has not yet been studied.

RESULTS

In this report we have cloned a 3.3-kb fragment upstream of the human TMP21 gene. The transcription start site (TSS) of the human TMP21 gene was identified. A series of nested deletions of the 5' flanking region of the human TMP21 gene were subcloned into the pGL3-basic luciferase reporter plasmid. We identified the -120 to +2 region as containing the minimal sequence necessary for TMP21 gene promoter activity. Gel shift assays revealed that the human TMP21 gene promoter contains NFAT response elements. Expression of NFAT increased TMP21 gene expression and inhibition of NFAT by siRNA reduced TMP21 gene expression.

CONCLUSION

NFAT plays a very important role in the regulation of human TMP21 gene expression. This study demonstrates that the human TMP21 gene expression is transcriptionally regulated by NFAT signaling.

The ubiquitin proteasome system in neuropathology

The ubiquitin proteasome system (UPS) orchestrates the turnover of innumerable cellular proteins. In the process of ubiquitination the small protein ubiquitin is attached to a target protein by a peptide bond. The ubiquitinated target protein is subsequently shuttled to a protease complex known as the 26S proteasome and subjected to degradative proteolysis. The UPS facilitates the turnover of proteins in several settings. It targets oxidized, mutant or misfolded proteins for general proteolytic destruction, and allows for the tightly controlled and specific destruction of proteins involved in development and differentiation, cell cycle progression, circadian rhythms, apoptosis, and other biological processes. In neuropathology, alteration of the UPS, or mutations in UPS target proteins may result in signaling abnormalities leading to the initiation or progression of tumors such as astrocytomas, hemangioblastomas, craniopharyngiomas, pituitary adenomas, and medulloblastomas. Dysregulation of the UPS may also contribute to tumor progression by perturbation of DNA replication and mitotic control mechanisms, leading to genomic instability. In neurodegenerative diseases caused by the expression of mutant proteins, the cellular accumulation of these proteins may overload the UPS, indirectly contributing to the disease process, e.g., sporadic Parkinsonism and prion diseases. In other cases, mutation of UPS components may directly cause pathological accumulation of proteins, e.g., autosomal recessive Parkinsonism and spinocerebellar ataxias. Defects or dysfunction of the UPS may also underlie cognitive disorders such as Angelman syndrome, Rett syndrome and autism, and muscle and nerve diseases, e.g., inclusion body myopathy and giant axon neuropathy. This paper describes the basic biochemical mechanisms comprising the UPS and reviews both its theoretical and proven involvement in neuropathological diseases. The potential for the UPS as a target of pharmacological therapy is also discussed.

Degradation of regulator of calcineurin 1 (RCAN1) is mediated by both chaperone-mediated autophagy and ubiquitin proteasome pathways

Regulator of calcineurin 1 (RCAN1), a gene identified from the critical region of Down syndrome, has been implied in pathogenesis of Alzheimer's disease (AD). RCAN1 expression was shown to be increased in AD brains; however, the mechanism of RCAN1 gene regulation is not well defined. The present study was designed to investigate the molecular mechanism of RCAN1 protein degradation. In addition to being degraded through the ubiquitin proteasome pathway, we found that lysosomal inhibition markedly increased RCAN1 protein expression in a time- and dosage-dependent manner. Inhibition of macroautophagy reduced RCAN1 expression, indicating that RCAN1 degradation is not through a macroautophagy pathway. However, disruption of chaperone-mediated autophagy (CMA) increased RCAN1 expression. Two CMA recognition motifs were identified in RCAN1 protein to mediate its degradation through a CMA-lysosome pathway. A promoter assay further demonstrated that inhibition of RCAN1 degradation in cells reduced calcineurin-NFAT activity. Dysfunctions of ubiquitin-proteasome and autophagy-lysosome pathways have been implicated in neurodegenerative diseases. Therefore, elucidation of RCAN1 degradation by a ubiquitin proteasome pathway and CMA-lysosome pathway in the present study may greatly advance our understanding of AD pathogenesis.