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Biomedical subjects

Riqiang Yan

Publications and source records attributed to Riqiang Yan.

10 recordsLinked to original sources
PubMed2006

Bace1 modulates myelination in the central and peripheral nervous system.

Bace1 is an endopeptidase that cleaves the amyloid precursor protein at the beta-secretase site. Apart from this cleavage, the functional importance of Bace1 in other physiological events is unknown. We show here that Bace1 regulates the process of myelination and myelin sheath thickness in the central and peripheral nerves. In Bace1-null mice, the process of myelination was delayed and myelin thickness was markedly reduced, indicating that genetic deletion of Bace1 causes hypomyelination. Bace1-null mice also showed altered neurological behaviors such as elevated pain sensitivity and reduced grip strength. Further mechanistic studies showed an altered neuregulin-Akt signaling pathway in Bace1-null mice. Full-length neuregulin-1 was increased and its cleavage product was decreased in the CNS of Bace1-null mice. Furthermore, phosphorylated Akt was also reduced. Based upon these and previous studies, we postulate that neuronally enriched Bace1 cleaves neuregulin-1 and that processed neuregulin-1 regulates myelination by means of phosphorylation of Akt in myelin-forming cells.

Amyloid Precursor Protein Secretases↗
PubMed2006

Mapping of interaction domains mediating binding between BACE1 and RTN/Nogo proteins.

BACE1 is a membrane-bound aspartyl protease that specifically cleaves amyloid precursor protein (APP) at the beta-secretase site. Membrane bound reticulon (RTN) family proteins interact with BACE1 and negatively modulate BACE1 activity through preventing access of BACE1 to its cellular APP substrate. Here, we focused our study on RTN3 and further show that a C-terminal QID triplet conserved among mammalian RTN members is required for the binding of RTN to BACE1. Although RTN3 can form homo- or heterodimers in cells, BACE1 mainly binds to the RTN monomer and disruption of the QID triplet does not interfere with the dimerization. Correspondingly, the C-terminal region of BACE1 is required for the binding of BACE1 to RTNs. Furthermore, we show that the negative modulation of BACE1 by RTN3 relies on the binding of RTN3 to BACE1. The knowledge from this study may potentially guide discovery of small molecules that can mimic the effect of RTN3 on the inhibition of BACE1 activity.

Alzheimer Disease↗
PubMed2004

Reticulon family members modulate BACE1 activity and amyloid-beta peptide generation.

Inhibiting the activity of the beta-amyloid converting enzyme 1 (BACE1) or reducing levels of BACE1 in vivo decreases the production of amyloid-beta. The reticulon family of proteins has four members, RTN1, RTN2, RTN3 and RTN4 (also known as Nogo), the last of which is well known for its role in inhibiting neuritic outgrowth after injury. Here we show that reticulon family members are binding partners of BACE1. In brain, BACE1 mainly colocalizes with RTN3 in neurons, whereas RTN4 is more enriched in oligodendrocytes. An increase in the expression of any reticulon protein substantially reduces the production of Abeta. Conversely, lowering the expression of RTN3 by RNA interference increases the secretion of Abeta, suggesting that reticulon proteins are negative modulators of BACE1 in cells. Our data support a mechanism by which reticulon proteins block access of BACE1 to amyloid precursor protein and reduce the cleavage of this protein. Thus, changes in the expression of reticulon proteins in the human brain are likely to affect cellular amyloid-beta and the formation of amyloid plaques.

Amyloid Precursor Protein Secretases↗
PubMed2004

Processing amyloid precursor protein at the beta-site requires proper orientation to be accessed by BACE1.

Membrane-bound BACE1 naturally cleaves its transmembrane substrate amyloid precursor protein (APP) at the two adjacent beta- and beta'-sites. Cleavage at these two sites generates the heterogeneous N-terminal end of APP C-terminal fragments that are further processed by gamma-secretase to release Abeta-(1-40/42) or Abeta-(11-40/42). The significance underlying Abeta-(11-40/42) in Alzheimer's disease pathogenesis has remained to be experimentally elucidated, but increased production of Abeta-(1-40/42) has been broadly demonstrated to contribute to amyloid depositions in senile plaques. In this study, we show that the cleavage of APP at the beta-site by BACE1 is readily disrupted through limited structural twists, whereas the beta'-site is relatively better positioned to gain access to the BACE1 catalytic cavity. Radical insertion or deletion of residues between beta- and beta'-site also favors cleavage of APP at the beta'-site. On the other hand, either lengthening or shortening the loop region of BACE1 has a minor impact on the selective cleavage of APP at these two adjacent sites, but significantly shortening the loop region impairs the ability of BACE1 to process APP at both sites. Thus, processing of APP by BACE1 is clearly dependent on a mutual structural compatibility in addition to the sequence feature. The knowledge gained from this study will potentially offer an opportunity for rational design of small molecule drugs to block the cleavage of APP specifically at the beta-site while not disturbing the functions of other cellular aspartyl proteases.

Amino Acid Sequence↗
PubMed2004

Amyloid beta peptide load is correlated with increased beta-secretase activity in sporadic Alzheimer's disease patients.

Whether elevated beta-secretase (BACE) activity is related to plaque formation or amyloid beta peptide (Abeta) production in Alzheimer's disease (AD) brains remains inconclusive. Here, we report that we used sandwich enzyme-linked immunoabsorbent assay to quantitate various Abeta species in the frontal cortex of AD brains homogenized in 70% formic acid. We found that most of the Abeta species detected in rapidly autopsied brains (<3 h) with sporadic AD were Abeta(1-x) and Abeta(1-42), as well as Abeta(x-42). To establish a linkage between Abeta levels and BACE, we examined BACE protein, mRNA expression and enzymatic activity in the same brain region of AD brains. We found that both BACE mRNA and protein expression is elevated in vivo in the frontal cortex. The elevation of BACE enzymatic activity in AD is correlated with brain Abeta(1-x) and Abeta(1-42) production. To examine whether BACE elevation was due to mutations in the BACE-coding region, we sequenced the entire ORF region of the BACE gene in these same AD and nondemented patients and performed allelic association analysis. We found no mutations in the ORF of the BACE gene. Moreover, we found few changes of BACE protein and mRNA levels in Swedish mutated amyloid precursor protein-transfected cells. These findings demonstrate correlation between Abeta loads and BACE elevation and also suggest that as a consequence, BACE elevation may lead to increased Abeta production and enhanced deposition of amyloid plaques in sporadic AD patients.

Alzheimer Disease↗
PubMed2003

Identification of a mutant amyloid peptide that predominantly forms neurotoxic protofibrillar aggregates.

The amyloid peptide (Abeta), derived from the proteolytic cleavage of the amyloid precursor protein (APP) by beta- and gamma-secretases, undergoes multistage assemblies to fibrillar depositions in the Alzheimer's brains. Abeta protofibrils were previously identified as an intermediate preceding insoluble fibrils. While characterizing a synthetic Abeta variant named EV40 that has mutations in the first two amino acids (D1E/A2V), we discerned unusual aggregation profiles of this variant. In comparison of the fibrillogenesis and cellular toxicity of EV40 to the wild-type Abeta peptide (Abeta40), we found that Abeta40 formed long fibrillar aggregates while EV40 formed only protofibrillar aggregates under the same in vitro incubation conditions. Cellular toxicity assays indicated that EV40 was slightly more toxic than Abeta40 to human neuroblastoma SHEP cells, rat primary cortical, and hippocampal neurons. Like Abeta40, the neurotoxicity of the protofibrillar EV40 could be partially attributed to apoptosis since multiple caspases such as caspase-9 were activated after SHEP cells were challenged with toxic concentrations of EV40. This suggested that apoptosis-induced neuronal loss might occur before extensive depositions of long amyloid fibrils in AD brains. This study has been the first to show that a mutated Abeta peptide formed only protofibrillar species and mutations of the amyloid peptide at the N-terminal side affect the dynamic amyloid fibrillogenesis. Thus, the identification of EV40 may lead to further understanding of the structural perturbation of Abeta to its fibrillation.

Amino Acid Substitution↗
PubMed2003

Mutations in tau gene exon 10 associated with FTDP-17 alter the activity of an exonic splicing enhancer to interact with Tra2 beta.

Mutations in the human tau gene leading to aberrant splicing have been identified in FTDP-17, an autosomal dominant hereditary neurodegenerative disorder. Molecular mechanisms by which such mutations cause tau aberrant splicing were not understood. We characterized two mutations in exon 10 of the tau gene, N279K and Del280K. Our results revealed an exonic splicing enhancer element located in exon 10. The activity of this AG-rich splicing enhancer was altered by N279K and Del280K mutations. This exonic enhancer element interacts with human Tra2 beta protein. The interaction between Tra2 beta and the exonic splicing enhancer correlates with the activity of this enhancer element in stimulating splicing. Biochemical studies including in vitro splicing and RNA interference experiments in transfected cells support a role for Tra2 beta protein in regulating alternative splicing of human tau gene. Our results implicate the human tau gene as a target gene for the alternative splicing regulator Tra2 beta, suggesting that Tra2 beta may play a role in aberrant tau exon 10 alternative splicing and in the pathogenesis of tauopathies.

Alternative Splicing↗
PubMed2003

Employing a superior BACE1 cleavage sequence to probe cellular APP processing.

The involvement of beta-secretase (BACE1; beta-site APP-cleaving enzyme) in producing the beta-amyloid component of plaques found in the brains of Alzheimer's patients, has fueled a major research effort to characterize this protease. Here, we describe work toward understanding the substrate specificity of BACE1 that began by considering the natural APP substrate and its Swedish mutant, APPSw, and proceeded on to include oxidized insulin B chain and ubiquitin substrates. From these findings, and the study of additional synthetic peptides, we determined that a decapeptide derived from APP in which the P3-P2' sequence, ...VKM--DA..., was replaced by ...ISY--EV... (-- = beta site of cleavage), yielded a substrate that was cleaved by BACE1 seven times faster than the corresponding APPSw peptide, SEVNL--DAEFR. The expanded peptide, GLTNIKTEEISEISY--EVEFRWKK, was cleaved an additional seven times faster than its decapeptide counterpart (boldface), and provides a substrate allowing assay of BACE1 at picomolar concentrations. Several APP mutants reflecting these beta-site amino acid changes were prepared as the basis for cellular assays. The APPISYEV mutant proved to be a cellular substrate that was superior to APPSw. The assay based on APPISYEV is highly specific for measuring BACE1 activity in cells; its homolog, BACE2, barely cleaved APPISYEV at the beta-site. Insertion of the optimized ISY--EV motif at either the beta-site (Asp1) or beta'-site (Glu11) directs the rate of cellular processing of APP at these two accessible sites. Thus, we have identified optimal BACE1 substrates that will be useful to elucidate the cellular enzymatic actions of BACE1, and for design of inhibitors that might be of therapeutic benefit in Alzheimer's disease.

Alzheimer Disease↗
PubMed2002

SEL-10 interacts with presenilin 1, facilitates its ubiquitination, and alters A-beta peptide production.

Mutations in the human presenilin genes (PS1 or PS2) have been linked to autosomal dominant, early onset Alzheimer's disease (AD). Presenilins, probably as an essential part of gamma-secretase, modulate gamma-cleavage of the amyloid protein precursor (APP) to the amyloid beta-peptide (Abeta). Mutations in sel-12, a Caenorhabditis elegans presenilin homologue, cause a defect in egg laying that can be suppressed by loss of function mutations in a second gene, SEL-10. SEL-10 protein is a homologue of yeast Cdc4, a member of the SCF (Skp1-Cdc53/CUL1-F-box protein) E2-E3 ubiquitin ligase family. In this study, we show that human SEL-10 interacts with PS1 and enhances PS1 ubiquitination, thus altering cellular levels of unprocessed PS1 and its N- and C-terminal fragments. Co-transfection of sel-10 and APP cDNAs in HEK293 cells leads to an alteration in the metabolism of APP and to an increase in the production of amyloid beta-peptide, the principal component of amyloid plaque in Alzheimer's disease.

Amino Acid Sequence↗