Ying-Hua Liu

Hyperhomocysteinemia Increases β-Amyloid by Enhancing Expression of γ-Secretase and Phosphorylation of Amyloid Precursor Protein in Rat Brain

Hyperhomocysteinemia and beta-amyloid (Abeta) overproduction are critical etiological and pathological factors in Alzheimer disease, respectively; however, the intrinsic link between them is still missing. Here, we found that Abeta levels increased and amyloid precursor protein (APP) levels simultaneously decreased in hyperhomocysteinemic rats after a 2-week induction by vena caudalis injection of homocysteine. Concurrently, both the mRNA and protein levels of presenilin-1, a component of gamma-secretase, were elevated, whereas the expression levels of beta-secretase and presenilin-2 were not altered. We also observed that levels of phosphorylated APP at threonine-668, a crucial site facilitating the amyloidogenic cleavage of APP, increased in rats with hyperhomocysteinemia, although the phosphorylation per se did not increase the binding capacity of pT668-APP to the secretases. The enhanced phosphorylation of APP in these rats was not relevant to either c-Jun N-terminal kinase or cyclin-dependent kinase-5. A prominent spatial memory deficit was detected in rats with hyperhomocysteinemia. Simultaneous supplementation of folate and vitamin-B12 attenuated the hyperhomocysteinemia-induced abnormal processing of APP and improved memory. Our data revealed that hyperhomocysteinemia could increase Abeta production through the enhanced expression of gamma-secretase and APP phosphorylation, causing memory deficits that could be rescued by folate and vitamin-B12 treatment in these rats. It is suggested that hyperhomocysteinemia may serve as an upstream factor for increased Abeta production as seen in patients with Alzheimer disease.

Peroxynitrite induces Alzheimer-like tau modifications and accumulation in rat brain and its underlying mechanisms

To investigate the upstream effector that led to tau hyperphosphorylation, nitration, and accumulation as seen in Alzheimers disease brain, and the underlying mechanisms, we bilaterally injected SIN‐1, a recognized peroxynitrite donor, into the hippocampus of rat brain. We observed that the level of nitrated and hyperphosphorylated tau was markedly increased in rat hippocampus 24 h after drug administration, and these alterations were prevented by preinjection of uric acid, a natural scavenger of peroxynitrite. Concomitantly, we detected a significant activation in glycogen synthase kinase‐3β (GSK‐3β) and p38 MAPKs, including p38α, p38β, and p38δ, but no obvious change was measured in the activity of p38γ, ERK, and c‐Jun amino‐terminal kinase (JNK). Both nitrated tau and hyperphosphorylated tau were aggregated in the hippocampus, in which the activity of 20S proteasome was significantly arrested in SIN‐1‐injected rats. Further studies demonstrated that the hyperphosphorylated tau was degraded as efficiently as normal tau by 20S proteasome, but the nitrated tau with an unorderly secondary structure became more resistant to the proteolysis. These results provide the first in vivo evidence showing that peroxynitrite simultaneously induces tau hyperphosphorylation, nitration, and accumulation, and that activation of GSK‐3β, p38α, p38β, p38δ isoforms and the inhibition of proteasome activity are respectively responsible for the peroxynitrite‐induced tau hyperphosphorylation and accumulation. Our findings reveal a common upstream stimulator and a potential therapeutic target for Alzheimer‐like neurodegeneration.—Zhang, Y‐J., Xu, Y‐F., Liu, Y‐H., Yin, J., Li, H‐L., Wang, Q., Wang, J‐Z. Peroxynitrite induces Alzheimer‐like tau modifications and accumulation in rat brain and its underlying mechanisms. FASEB J. 20, 1431–1442 (2006)

Proteasome inhibition increases tau accumulation independent of phosphorylation

An intrinsic link between proteasome and tau degradation in Alzheimers disease (AD) has been suggested, however, the role of proteasome in the proteolysis of tau is still uncertain. Here, we investigated the influence of proteasome inhibition on the accumulation, phosphorylation, ubiquitination, solubility of tau and the memory retention in rats. We observed that lactacystin inhibited the proteasome activities and increased the level and insolubility of different tau species, including phosphorylated tau. The elevation of the phosphorylated tau was no longer present and the level of pS214 and pT231 tau was even lower than normal level after normalized to total tau. Inhibition of proteasome resulted in activation of cAMP-dependent protein kinase, glycogen synthase kinases-3beta and cyclin-dependent kinase-5, and inhibition of protein phosphatase-2A and c-Jun N-terminal kinase (JNK). Proteasome inhibition did not affect the memory retention of the rats. We conclude that proteasome inhibition increases accumulation and insolubility of tau proteins independent of tau phosphorylation, and JNK inhibition may be partially responsible for the relatively decreased phosphorylation of tau in the rat brains.

Melatonin arrests peroxynitrite‐induced tau hyperphosphorylation and the overactivation of protein kinases in rat brain

Abstract:u2002 The purpose of this study was to examine the in vivo effect of melatonin (MEL) on peroxynitrite‐induced tau hyperphosphorylation and the involvement of glycogen synthase kinase‐3β (GSK‐3β) and mitogen‐activated protein kinase (MAPK) families. Melatonin was injected into the right cerebroventricle of the rats 1u2003hr before the bilateral hippocampal injection of 3‐morpholino‐sydnonimine chloride (SIN‐1), the recognized donor of peroxynitrite. Thereafter, the phosphorylation level of tau and the activity of the kinases were analyzed. The injection of SIN‐1 induced hyperphosphorylation of tau at pS396 epitope with a concomitant activation of GSK‐3β and selective MAPK isoforms including p38α, p38β, and p38δ but not p38γ. The effect of peroxynitrite was confirmed using uric acid, a recognized scavenger of peroxynitrite. Preinjection of MEL significantly arrested the peroxynitrite‐induced hyperphosphorylation of tau and the activation of GSK‐3β and MAPKs. Melatonin also ameliorated peroxynitrite‐induced oxidative stress. We conclude that MEL can efficiently arrest peroxynitrite‐induced tau hyperphosphorylation, and the underlying mechanism may involve scavenging the reactive species and suppressing the activated GSK‐3β and p38 MAPK family.

Folate/vitamin-B12 prevents chronic hyperhomocysteinemia-induced tau hyperphosphorylation and memory deficits in aged rats.

Hyperhomocysteinemia is associated with an increased risk of Alzheimers disease (AD). Our previous work has demonstrated that combined folate and vitamin B12 (vit-B12) supplementation prevents tau hyperphosphorylation and memory deficits induced by acute administration of homocysteine in young rats. Here, we further investigated whether folate/vit-B12 supplementation is also effective in aged rats with a chronically high level of homocysteine. 18-month-old rats were injected with homocysteine via the vena caudalis with or without a concurrent folate/vit-B12 supplementation for 28 weeks. We found that hyperhomocysteinemia induced tau hyperphosphorylation and accumulation in hippocampus and cortex. Concurrent signaling changes included the activation of glycogen synthase kinases-3β, cyclin-dependent kinase-5, c-Jun N-terminal kinase, extracellular signal-regulated kinase, and p38MAPK, and inhibition of protein phosphatase 2A. Although the ability to learn was not affected, the aged rats exhibited significant memory deficits. Folate/vit-B12 supplementation attenuated these biochemical and behavioral correlates. These data demonstrate that folate/vit-B12 supplementation is also effective in a chronic hyperhomocysteinemia model in reversing the AD-like tau pathologies and memory deficits.

Relationship of Adult Neurogenesis with Tau Phosphorylation and GSK-3β Activity in Subventricular Zone

Altered neurogenesis has been reported in Alzheimer disease (AD), the most common neurodegenerative disorder characterized with hyperphosphorylated tau and accumulation of β-amyloid (Aβ). Recent studies suggest that tau phosphorylation is essential for hippocampal neurogenesis, however, it is not known whether tau phosphorylation also play a role in neurogenesis of subventricular zone (SVZ), another main progenitor niche in the brain. Here, we examined the expression of phosphorylated tau (p-tau) in SVZ and analyzed the role of p-tau in adult SVZ neurogenesis. We found that the expression of p-tau increased during postnatal development and remains at a high level until adulthood, and the p-tau was colocalized with some SVZ neural precursors. However, up-regulating glycogen synthase kinase-3 (GSK-3), a crucial tau kinase, had no effect on SVZ neurogenesis in adult rat brain. The SVZ neurogenesis was also unaffected in tau knockout and human tau transgenic mice. These results suggest that tau phosphorylation and GSK-3 activation may not be essential for adult SVZ neurogenesis.