👤 Tomonobu Hida

Excessive accumulation of toxic amyloid-β (Aβ) species in the brain is a major pathological process triggering neurodegeneration in Alzheimer's disease (AD). Recent studies indicate that both neurons and glial cells, including oligodendrocyte lineages (OLs), contribute to brain homeostasis and affect AD pathology; however, the roles of oligodendrocyte precursor cells (OPCs) and oligodendrocytes (OLGs) in AD remain to be fully elucidated. This study examined Aβ production and related protein expression in primary cultured OLs. Primary cultured OLs produced Aβ40 and Aβ42 and expressed amyloid precursor protein (APP), β-secretase (BACE1) and γ-secretase (PS1) as well as α-secretase (ADAM10). OLGs express APP770 in addition to APP695. Treatment with a γ-secretase inhibitor reduced Aβ40 and Aβ42 production levels derived from OPCs/OLGs and suppressed OPC differentiation. Additionally, conditioned media from OLGs improved neuronal cell viability under oxidative stress and contained higher levels of sAPPα compared to OPCs. The neuroprotective effect of OLG was diminished by a sAPPα inhibitor, suggesting that OLG-derived sAPPα protects neurons under oxidative stress. These findings revealed that OLs produce pathogenic Aβ40/42 via the amyloidogenic pathway and secrete neuroprotective sAPPα via the non-amyloidogenic pathway. Elucidating the pathological shift from beneficial non-amyloidogenic to harmful amyloidogenic processes in OLs during AD onset and progression would provide crucial insights into novel therapeutic approaches. Show less

The establishment of neuronal polarity is necessary for proper neuronal wiring. Semaphorin3A (Sema3A), originally identified as a repulsive axon guidance molecule, exerts a wide variety of biological functions through signaling pathways including sequential phosphorylation of collapsin response mediator protein by cyclin-dependent kinase-5 (Cdk5) and glycogen synthase kinase-3β (GSK3β). Sema3A acts on its receptor neuropilin-1 to regulate axonal transport. To delineate mechanism by which Sema3A induces axonal transport, we investigate whether GSK3β is involved in mediating Sema3A-induced axonal transport. 4-Benzyl-2-methyl-1,2,4-thiadiazolidine-3,5-dione, an inhibitor of GSK3β, suppressed Sema3A-induced antero- and retrograde axonal transport. Introduction of either GSK3β mutants, GSK3β-L128A or K85M, suppressed Sema3A-induced axonal transport. On the other hand, introduction of GSK3β-R96A did not affect the Sema3A effect, suggesting that unprimed substrates are primarily involved in Sema3A-induced axonal transport. Overexpression of a partial fragment of frequently rearranged in advanced T-cell lymphomas 1 (FRATtide), which interferes the interaction between GSK3β and Axis inhibitor-1 (Axin-1), also suppressed Sema3A-induced transport. siRNA knockdown of Axin-1, an unprimed substrate of GSK3β, suppressed Sema3A-induced antero- and retrograde axonal transport. These results indicate that GSK3β and Axin-1 are involved in Sema3A-induced bidirectional axonal transport. This finding should provide a clue for understanding of mechanisms of a wide variety of biological activities of Sema3A. Show less

Semaphorin3A (Sema3A) exerts a wide variety of biological functions by regulating reorganization of actin and tubulin cytoskeletal proteins through signaling pathways including sequential phosphorylation of collapsin response mediator protein 1 (CRMP1) and CRMP2 by cyclin-dependent kinase-5 and glycogen synthase kinase-3β (GSK3β). To delineate how GSK3β mediates Sema3A signaling, we here determined the substrates of GSK3β involved. Introduction of either GSK3β mutants, GSK3β-R96A, L128A, or K85M into chick dorsal root ganglion (DRG) neurons suppressed Sema3A-induced growth cone collapse, thereby suggesting that unprimed as well as primed substrates are involved in Sema3A signaling. Axin-1, a key player in Wnt signaling, is an unprimed substrate of GSK3β. The phosphorylation of Axin-1 by GSK3β accelerates the association of Axin-1 with β-catenin. Immunocytochemical studies revealed that Sema3A induced an increase in the intensity levels of β-catenin in the DRG growth cones. Axin-1 siRNA knockdown suppressed Sema3A-induced growth cone collapse. The reintroduction of RNAi-resistant Axin-1 (rAxin-1)-wt rescued the responsiveness to Sema3A, while that of nonphosphorylated mutants, rAxin S322A/S326A/S330A and T485A/S490A/S497A, did not. Sema3A also enhanced the colocalization of GSK3β, Axin-1, and β-catenin in the growth cones. The increase of β-catenin in the growth cones was suppressed by the siRNA knockdown of Axin-1. Furthermore, either Axin-1 or β-catenin RNAi knockdown suppressed the internalization of Sema3A. These results suggest that Sema3A induces the formation of GSK3β/Axin-1/β-catenin complex, which regulates signaling cascade of Sema3A via an endocytotic mechanism. This finding should provide clue for understanding of mechanisms of a wide variety of biological functions of Sema3A. Show less

We have previously shown that expression of SIAH1 is frequently down-regulated in HCCs and associated with their advanced stages. It has been shown that SIAH1 functions in the phosphorylation-independent degradation of beta-catenin and induces apoptosis and growth arrest. To examine if the effects of SIAH1 overexpression depend on the altered beta-catenin signaling pathway, we transferred the SIAH1 gene into three hepatoma cell lines with different genetic backgrounds: HepG2 (mutant beta-catenin), SNU475 (mutant AXIN1), and Huh7 cells (wild type beta-catenin and AXIN1). SIAH1 significantly decreased aberrant beta-catenin signal in HepG2 and SNU475 cells and induced growth arrest and apoptosis. However, SIAH1 also induced apoptosis in Huh7 cells, which retained a normal membranous distribution pattern of beta-catenin. Immunoblotting study demonstrated that SIAH1 also reduces the amount of PEG10 protein, which is known to be frequently overexpressed in HCC and to promote cell proliferation. These data suggest that PEG10 is another target protein of SIAH1 to induce apoptosis in hepatoma cells. Our results should lead to a better understanding of the relationship between deregulation of beta-catenin signals and hepatocarcinogenesis. Further investigations into the mechanisms by which SIAH1 promotes apoptosis and suppresses cell growth should also allow for the discovery of new therapeutic strategies. Show less