Memantine Attenuates Alzheimer's Disease-Like Pathology and Cognitive Impairment.

Deficiency of protein phosphatase-2A is a key event in Alzheimer's disease. An endogenous inhibitor of protein phosphatase-2A, inhibitor-1, I1PP2A, which inhibits the phosphatase activity by interacting with its catalytic subunit protein phosphatase-2Ac, is known to be upregulated in Alzheimer's disease brain. In the present study, we overexpressed I1PP2A by intracerebroventricular injection with adeno-associated virus vector-1-I1PP2A in Wistar rats. The I1PP2A rats showed a decrease in brain protein phosphatase-2A activity, abnormal hyperphosphorylation of tau, neurodegeneration, an increase in the level of activated glycogen synthase kinase-3beta, enhanced expression of intraneuronal amyloid-beta and spatial reference memory deficit; littermates treated identically but with vector only, i.e., adeno-associated virus vector-1-enhanced GFP, served as a control. Treatment with memantine, a noncompetitive NMDA receptor antagonist which is an approved drug for treatment of Alzheimer's disease, rescued protein phosphatase-2A activity by decreasing its demethylation at Leu309 selectively and attenuated Alzheimer's disease-like pathology and cognitive impairment in adeno-associated virus vector-1-I1PP2A rats. These findings provide new clues into the possible mechanism of the beneficial therapeutic effect of memantine in Alzheimer's disease patients.

Sera from children with autism induce autistic features which can be rescued with a CNTF small peptide mimetic in rats.

Autism is a neurodevelopmental disorder characterized clinically by impairments in social interaction and verbal and non-verbal communication skills as well as restricted interests and repetitive behavior. It has been hypothesized that altered brain environment including an imbalance in neurotrophic support during early development contributes to the pathophysiology of autism. Here we report that sera from children with autism which exhibited abnormal levels of various neurotrophic factors induced cell death and oxidative stress in mouse primary cultured cortical neurons. The effects of sera from autistic children were rescued by pre-treatment with a ciliary neurotrophic factor (CNTF) small peptide mimetic, Peptide 6 (P6), which was previously shown to exert its neuroprotective effect by modulating CNTF/JAK/STAT pathway and LIF signaling and by enhancing brain derived neurotrophic factor (BDNF) expression. Similar neurotoxic effects and neuroinflammation were observed in young Wistar rats injected intracerebroventricularly with autism sera within hours after birth. The autism sera injected rats demonstrated developmental delay and deficits in social communication, interaction, and novelty. Both the neurobiological changes and the behavioral autistic phenotype were ameliorated by P6 treatment. These findings implicate the involvement of neurotrophic imbalance during early brain development in the pathophysiology of autism and a proof of principle of P6 as a potential therapeutic strategy for autism.

Inhibition of Protein Phosphatase-2A (PP2A) by I1PP2A Leads to Hyperphosphorylation of Tau, Neurodegeneration, and Cognitive Impairment in Rats.

Protein phosphatase-2A (PP2A) deficiency is a cause of the abnormal hyperphosphorylation of tau, which composes neurofibrillary tangles (NFTs) in Alzheimer's disease (AD) brain. We previously reported that both mRNA and protein expression of inhibitor I of PP2A (I(1)(PP2A)) are elevated in AD brain and that this inhibitor induces a dose-dependent inhibition of PP2A activity and tau hyperphosphorylation in NIH3T3 cells. However, whether I(1)(PP2A) can induce AD neurofibrillary degeneration and cognitive impairment was not known. In the present study, we infected the brains of rat pups within 24 hours of birth with adeno-associated virus serotype 1 (AAV1) carrying I(1)(PP2A). In the adult AAV1-I(1)(PP2A) rats, we found a decrease in PP2A activity and abnormal hyperphosphorylation of tau in the brain. Immunohistochemistry showed a significant reduction of MAP2 and synapsin 1 in AAV1- I(1)(PP2A) animals, suggesting that I(1)(PP2A) can induce a loss of dendritic and synaptic plasticity markers. Behavioral tests revealed that infection with AAV1- I(1)(PP2A) induced deficits in exploratory activity, spatial reference memory, and memory consolidation in adult rats. These studies suggest that I(1)(PP2A) can inhibit PP2A activity, and in turn induce AD neurofibrillary degeneration and cognitive deficits in rats.

Passive immunization targeting the N-terminal projection domain of tau decreases tau pathology and improves cognition in a transgenic mouse model of Alzheimer disease and tauopathies.

Intraneuronal accumulation of abnormally hyperphosphorylated tau in the brain is a histopathological hallmark of Alzheimer's disease and a family of related neurodegenerative disorders collectively called tauopathies. At present there is no effective treatment available for these progressive neurodegenerative diseases which are clinically characterized by dementia in mid to old-age. Here we report the treatment of 14-17-months-old 3xTg-AD mice with tau antibodies 43D (tau 6-18) and 77E9 (tau 184-195) to the N-terminal projection domain of tau or mouse IgG as a control by intraperitoneal injection once a week for 4 weeks, and the effects of the passive immunization on reduction of hyperphosphorylated tau, Aß accumulation and cognitive performance in these animals. We found that treatment with tau antibodies 43D and 77E9 reduced total tau level, decreased tau hyperphosphorylated at Ser199, Ser202/Thr205 (AT8), Thr205, Ser262/356 (12E8), and Ser396/404 (PHF-1) sites, and a trend to reduce Aß pathology. Most importantly, targeting N-terminal tau especially by 43D (tau 6-18) improved reference memory in the Morris water maze task in 3xTg-AD mice. We did not observe any abnormality in general physical characteristics of the treated animals with either of the two antibodies during the course of this study. Taken together, our studies demonstrate for the first time (1) that passive immunization targeting normal tau can effectively clear the hyperphosphorylated protein and possibly reduce Aß pathology from the brain and (2) that targeting N-terminal projection domain of tau containing amino acid 6-18 is especially beneficial. Thus, targeting selective epitopes of N-terminal domain of tau may present a novel effective therapeutic opportunity for Alzheimer disease and other tauopathies.

Cytoplasmic retention of protein phosphatase 2A inhibitor 2 (I2PP2A) induces Alzheimer-like abnormal hyperphosphorylation of Tau.

Abnormal hyperphosphorylation of Tau leads to the formation of neurofibrillary tangles, a hallmark of Alzheimer disease (AD), and related tauopathies. The phosphorylation of Tau is regulated by protein phosphatase 2A (PP2A), which in turn is modulated by endogenous inhibitor 2 (I2 (PP2A)). In AD brain, I2 (PP2A) is translocated from neuronal nucleus to cytoplasm, where it inhibits PP2A activity and promotes abnormal phosphorylation of Tau. Here we describe the identification of a potential nuclear localization signal (NLS) in the C-terminal region of I2 (PP2A) containing a conserved basic motif, (179)RKR(181), which is sufficient for directing its nuclear localization. The current study further presents an inducible cell model (Tet-Off system) of AD-type abnormal hyperphosphorylation of Tau by expressing I2 (PP2A) in which the NLS was inactivated by (179)RKR(181) → AAA along with (168)KR(169) → AA mutations. In this model, the mutant NLS (mNLS)-I2 (PP2A) (I2 (PP2A)AA-AAA) was retained in the cell cytoplasm, where it physically interacted with PP2A and inhibited its activity. Inhibition of PP2A was associated with the abnormal hyperphosphorylation of Tau, which resulted in microtubule network instability and neurite outgrowth impairment. Expression of mNLS-I2 (PP2A) activated CAMKII and GSK-3ß, which are Tau kinases regulated by PP2A. The immunoprecipitation experiments showed the direct interaction of I2 (PP2A) with PP2A and GSK-3ß but not with CAMKII. Thus, the cell model provides insights into the nature of the potential NLS and the mechanistic relationship between I2 (PP2A)-induced inhibition of PP2A and hyperphosphorylation of Tau that can be utilized to develop drugs preventing Tau pathology.

Tau pathology involves protein phosphatase 2A in parkinsonism-dementia of Guam.

Parkinsonism-dementia (PD) of Guam is a neurodegenerative disease with parkinsonism and early-onset Alzheimer-like dementia associated with neurofibrillary tangles composed of hyperphosphorylated microtubule-associated protein, tau. ß-N-methylamino-l-alanine (BMAA) has been suspected of being involved in the etiology of PD, but the mechanism by which BMAA leads to tau hyperphosphorylation is not known. We found a decrease in protein phosphatase 2A (PP2A) activity associated with an increase in inhibitory phosphorylation of its catalytic subunit PP2Ac at Tyr(307) and abnormal hyperphosphorylation of tau in brains of patients who had Guam PD. To test the possible involvement of BMAA in the etiopathogenesis of PD, we studied the effect of this environmental neurotoxin on PP2A activity and tau hyperphosphorylation in mouse primary neuronal cultures and metabolically active rat brain slices. BMAA treatment significantly decreased PP2A activity, with a concomitant increase in tau kinase activity resulting in elevated tau hyperphosphorylation at PP2A favorable sites. Moreover, we found an increase in the phosphorylation of PP2Ac at Tyr(307) in BMAA-treated rat brains. Pretreatment with metabotropic glutamate receptor 5 (mGluR5) and Src antagonists blocked the BMAA-induced inhibition of PP2A and the abnormal hyperphosphorylation of tau, indicating the involvement of an Src-dependent PP2A pathway. Coimmunoprecipitation experiments showed that BMAA treatment dissociated PP2Ac from mGluR5, making it available for phosphorylation at Tyr(307). These findings suggest a scenario in which BMAA can lead to tau pathology by inhibiting PP2A through the activation of mGluR5, the consequent release of PP2Ac from the mGluR5-PP2A complex, and its phosphorylation at Tyr(307) by Src.

Alzheimer disease and amyotrophic lateral sclerosis: an etiopathogenic connection.

The etiopathogenesis of neither the sporadic form of Alzheimer disease (AD) nor of amyotrophic lateral sclerosis (ALS) is well understood. The activity of protein phosphatase-2A (PP2A), which regulates the phosphorylation of tau and neurofilaments, is negatively regulated by the myeloid leukemia-associated protein SET, also known as inhibitor-2 of PP2A, I2(PP2A). In AD brain, PP2A activity is compromised, probably because I2(PP2A) is overexpressed and is selectively cleaved at asparagine 175 into an N-terminal fragment, I2NTF, and a C-terminal fragment, I2CTF, and both fragments inhibit PP2A. Here, we analyzed the spinal cords from ALS and control cases for I2(PP2A) cleavage and PP2A activity. As observed in AD brain, we found a selective increase in the cleavage of I2(PP2A) into I2NTF and I2CTF and inhibition of the activity and not the expression of PP2A in the spinal cords of ALS cases. To test the hypothesis that both AD and ALS could be triggered by I2CTF, a cleavage product of I2(PP2A), we transduced by intracerebroventricular injections newborn rats with adeno-associated virus serotype 1 (AAV1) containing human I2CTF. AAV1-I2CTF produced reference memory impairment and tau pathology, and intraneuronal accumulation of Aß by 5-8 months, and motor deficit and hyperphosphorylation and proliferation of neurofilaments, tau and TDP-43 pathologies, degeneration and loss of motor neurons and axons in the spinal cord by 10-14 months in rats. These findings suggest a previously undiscovered etiopathogenic relationship between sporadic forms of AD and ALS that is linked to I2(PP2A) and the potential of I2(PP2A)-based therapeutics for these diseases.

Activation of asparaginyl endopeptidase leads to Tau hyperphosphorylation in Alzheimer disease.

Neurofibrillary pathology of abnormally hyperphosphorylated Tau is a key lesion of Alzheimer disease and other tauopathies, and its density in the brain directly correlates with dementia. The phosphorylation of Tau is regulated by protein phosphatase 2A, which in turn is regulated by inhibitor 2, I2(PP2A). In acidic conditions such as generated by brain ischemia and hypoxia, especially in association with hyperglycemia as in diabetes, I2(PP2A) is cleaved by asparaginyl endopeptidase at Asn-175 into the N-terminal fragment (I2NTF) and the C-terminal fragment (I2CTF). Both I2NTF and I2CTF are known to bind to the catalytic subunit of protein phosphatase 2A and inhibit its activity. Here we show that the level of activated asparaginyl endopeptidase is significantly increased, and this enzyme and I2(PP2A) translocate, respectively, from neuronal lysosomes and nucleus to the cytoplasm where they interact and are associated with hyperphosphorylated Tau in Alzheimer disease brain. Asparaginyl endopeptidase from Alzheimer disease brain could cleave GST-I2(PP2A), except when I2(PP2A) was mutated at the cleavage site Asn-175 to Gln. Finally, an induction of acidosis by treatment with kainic acid or pH 6.0 medium activated asparaginyl endopeptidase and consequently produced the cleavage of I2(PP2A), inhibition of protein phosphatase 2A, and hyperphosphorylation of Tau, and the knockdown of asparaginyl endopeptidase with siRNA abolished this pathway in SH-SY5Y cells. These findings suggest the involvement of brain acidosis in the etiopathogenesis of Alzheimer disease, and asparaginyl endopeptidase-I2(PP2A)-protein phosphatase 2A-Tau hyperphosphorylation pathway as a therapeutic target.

Neurogenic and neurotrophic effects of BDNF peptides in mouse hippocampal primary neuronal cell cultures.

The level of brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, is down regulated in Alzheimer's disease (AD), Parkinson's disease (PD), depression, stress, and anxiety; conversely the level of this neurotrophin is increased in autism spectrum disorders. Thus, modulating the level of BDNF can be a potential therapeutic approach for nervous system pathologies. In the present study, we designed five different tetra peptides (peptides B-1 to B-5) corresponding to different active regions of BDNF. These tetra peptides were found to be non-toxic, and they induced the expression of neuronal markers in mouse embryonic day 18 (E18) primary hippocampal neuronal cultures. Additionally, peptide B-5 induced the expression of BDNF and its receptor, TrkB, suggesting a positive feedback mechanism. The BDNF peptides induced only a moderate activation (phosphorylation at Tyr 706) of the TrkB receptor, which could be blocked by the Trk's inhibitor, K252a. Peptide B-3, when combined with BDNF, potentiated the survival effect of this neurotrophin on H(2)O(2)-treated E18 hippocampal cells. Peptides B-3 and B-5 were found to work as partial agonists and as partial antagonists competing with BDNF to activate the TrkB receptor in a dose-dependent manner. Taken together, these results suggest that the described BDNF tetra peptides are neurotrophic, can modulate BDNF signaling in a partial agonist/antagonist way, and offer a novel therapeutic approach to neural pathologies where BDNF levels are dysregulated.

CREB regulates the expression of neuronal glucose transporter 3: a possible mechanism related to impaired brain glucose uptake in Alzheimer's disease.

Impaired brain glucose uptake and metabolism precede the appearance of clinical symptoms in Alzheimer disease (AD). Neuronal glucose transporter 3 (GLUT3) is decreased in AD brain and correlates with tau pathology. However, what leads to the decreased GLUT3 is yet unknown. In this study, we found that the promoter of human GLUT3 contains three potential cAMP response element (CRE)-like elements, CRE1, CRE2 and CRE3. Overexpression of CRE-binding protein (CREB) or activation of cAMP-dependent protein kinase significantly increased GLUT3 expression. CREB bound to the CREs and promoted luciferase expression driven by human GLUT3-promoter. Among the CREs, CRE2 and CRE3 were required for the promotion of GLUT3 expression. Full-length CREB was decreased and truncation of CREB was increased in AD brain. This truncation was correlated with calpain I activation in human brain. Further study demonstrated that calpain I proteolysed CREB at Gln28-Ala29 and generated a 41-kDa truncated CREB, which had less activity to promote GLUT3 expression. Importantly, human brain GLUT3 was correlated with full-length CREB positively and with activation of calpain I negatively. These findings suggest that overactivation of calpain I caused by calcium overload proteolyses CREB, resulting in a reduction of GLUT3 expression and consequently impairing glucose uptake and metabolism in AD brain.

Abnormal hyperphosphorylation of tau: sites, regulation, and molecular mechanism of neurofibrillary degeneration.

Microtubule associated protein tau is a phosphoprotein which potentially has 80 serine/threonine and 5 tyrosine phosphorylation sites. Normal brain tau contains 2-3 moles of phosphate per mole of the protein. In Alzheimer's disease brain, tau is abnormally hyperphosphorylated to a stoichiometry of at least three-fold greater than normal tau, and in this altered state it is aggregated into paired helical filaments forming neurofibrillary tangles, a histopathological hallmark of the disease. The abnormal hyperphosphorylation of tau is also a hallmark of several other related neurodegenerative disorders, called tauopathies. The density of neurofibrillary tangles in the neocortex correlates with dementia and, hence, is a rational therapeutic target and an area of increasing research interest. Development of rational tau-based therapeutic drugs requires understanding of the role of various phosphorylation sites, protein kinases and phosphatases, and post-translational modifications that regulate the phosphorylation of this protein at various sites, as well as the molecular mechanism by which the abnormally hyperphosphorylated tau leads to neurodegeneration and dementia. In this article we briefly review the progress made in these areas of research.

A non-transgenic mouse model (icv-STZ mouse) of Alzheimer's disease: similarities to and differences from the transgenic model (3xTg-AD mouse).

Alzheimer's disease (AD) can be divided into sporadic AD (SAD) and familial AD (FAD). Most AD cases are sporadic and result from multiple etiologic factors, including environmental, genetic, and metabolic factors, whereas FAD is caused by mutations in the presenilins or amyloid-ß (Aß) precursor protein (APP) genes. A commonly used animal model for AD is the 3xTg-AD transgenic mouse model, which harbors mutated presenilin 1, APP, and tau genes and thus represents a model of FAD. There is an unmet need in the field to characterize animal models representing different AD mechanisms, so that potential drugs for SAD can be evaluated preclinically in these animal models. A mouse model generated by intracerebroventricular (icv) administration of streptozocin (STZ), the icv-STZ mouse, shows many aspects of SAD. In this study, we compared the non-cognitive and cognitive behaviors as well as biochemical and immunohistochemical alterations between the icv-STZ mouse and the 3xTg-AD mouse. We found that both mouse models showed increased exploratory activity as well as impaired learning and spatial memory. Both models also demonstrated neuroinflammation, altered synaptic proteins and insulin/IGF-1 (insulin-like growth factor-1) signaling, and increased hyperphosphorylated tau in the brain. The most prominent brain abnormality in the icv-STZ mouse was neuroinflammation, and in the 3xTg-AD mouse it was elevation of hyperphosphorylated tau. These observations demonstrate the behavioral and neuropathological similarities and differences between the icv-STZ mouse and the 3xTg-AD mouse models and will help guide future studies using these two mouse models for the development of AD drugs.

Brain gene expression of a sporadic (icv-STZ Mouse) and a familial mouse model (3xTg-AD mouse) of Alzheimer's disease.

Alzheimer's disease (AD) can be divided into sporadic AD (SAD) and familial AD (FAD). Most AD cases are sporadic and may result from multiple etiologic factors, including environmental, genetic and metabolic factors, whereas FAD is caused by mutations of presenilins or amyloid-ß (Aß) precursor protein (APP). A commonly used mouse model for AD is 3xTg-AD mouse, which is generated by over-expression of mutated presenilin 1, APP and tau in the brain and thus represents a mouse model of FAD. A mouse model generated by intracerebroventricular (icv) administration of streptozocin (STZ), icv-STZ mouse, shows many aspects of SAD. Despite the wide use of these two models for AD research, differences in gene expression between them are not known. Here, we compared the expression of 84 AD-related genes in the hippocampus and the cerebral cortex between icv-STZ mice and 3xTg-AD mice using a custom-designed qPCR array. These genes are involved in APP processing, tau/cytoskeleton, synapse function, apoptosis and autophagy, AD-related protein kinases, glucose metabolism, insulin signaling, and mTOR pathway. We found altered expression of around 20 genes in both mouse models, which affected each of above categories. Many of these gene alterations were consistent with what was observed in AD brain previously. The expression of most of these altered genes was decreased or tended to be decreased in the hippocampus of both mouse models. Significant diversity in gene expression was found in the cerebral cortex between these two AD mouse models. More genes related to synaptic function were dysregulated in the 3xTg-AD mice, whereas more genes related to insulin signaling and glucose metabolism were down-regulated in the icv-STZ mice. The present study provides important fundamental knowledge of these two AD mouse models and will help guide future studies using these two mouse models for the development of AD drugs.

Developmental regulation of protein O-GlcNAcylation, O-GlcNAc transferase, and O-GlcNAcase in mammalian brain.

O-GlcNAcylation is a common posttranslational modification of nucleocytoplasmic proteins by ß-N-acetylglucosamine (GlcNAc). The dynamic addition and removal of O-GlcNAc groups to and from proteins are catalyzed by O-linked N-acetylglucosamine transferase (O-GlcNAc transferase, OGT) and ß-N-acetylglucosaminidase (O-GlcNAcase, OGA), respectively. O-GlcNAcylation often modulates protein phosphorylation and regulates several cellular signaling and functions, especially in the brain. However, its developmental regulation is not well known. Here, we studied protein O-GlcNAcylation, OGT, and OGA in the rat brain at various ages from embryonic day 15 to the age of 2 years. We found a gradual decline of global protein O-GlcNAcylation during developmental stages and adulthood. This decline correlated positively to the total protein phosphorylation at serine residues, but not at threonine residues. The expression of OGT and OGA isoforms was regulated differently at various ages. Immunohistochemical studies revealed ubiquitous distribution of O-GlcNAcylation at all ages. Strong immunostaining of O-GlcNAc, OGT, and OGA was observed mostly in neuronal cell bodies and processes, further suggesting the role of O-GlcNAc modification of neuronal proteins in the brain. These studies provide fundamental knowledge of age-dependent protein modification by O-GlcNAc and will help guide future studies on the role of O-GlcNAcylation in the mammalian brain.

Dual-specificity tyrosine phosphorylation-regulated kinase 1A (Dyrk1A) modulates serine/arginine-rich protein 55 (SRp55)-promoted Tau exon 10 inclusion.

Tau exon 10, which encodes the second microtubule-binding repeat, is regulated by alternative splicing. Its alternative splicing generates Tau isoforms with three- or four-microtubule-binding repeats, named 3R-tau and 4R-tau. Adult human brain expresses equal levels of 3R-tau and 4R-tau. Imbalance of 3R-tau and 4R-tau causes Tau aggregation and neurofibrillary degeneration. In the present study, we found that splicing factor SRp55 (serine/arginine-rich protein 55) promoted Tau exon 10 inclusion. Knockdown of SRp55 significantly promoted Tau exon 10 exclusion. The promotion of Tau exon 10 inclusion by SRp55 required the arginine/serine-rich region, which was responsible for the subnucleic speckle localization. Dyrk1A (dual specificity tyrosine-phosphorylated and regulated kinase 1A) interacted with SRp55 and mainly phosphorylated its proline-rich domain. Phosphorylation of SRp55 by Dyrk1A suppressed its ability to promote Tau exon 10 inclusion. Up-regulation of Dyrk1A as in Down syndrome could lead to neurofibrillary degeneration by shifting the alternative splicing of Tau exon 10 to an increase in the ratio of 3R-tau/4R-tau.

Diverse regulation of AKT and GSK-3ß by O-GlcNAcylation in various types of cells.

Protein kinase B (AKT) and glycogen synthase kinase-3ß (GSK-3ß) are major components of insulin-AKT signaling that plays crucial roles in various types of tissue. Recent studies found that these two kinases are modified posttranslationally by O-GlcNAcylation. Here, we demonstrate that O-GlcNAcylation regulated phosphorylation/activation of AKT and GSK-3ß in different manners in kidney HEK-293FT cells, but did not affect these two kinases in hepatic HepG2 cells. In neuronal cells, O-GlcNAcylation regulated phosphorylation of AKT negatively, but had no effect on GSK-3ß. These results suggest protein-specific and cell type-specific regulation of AKT and GSK-3ß by O-GlcNAcylation. Therefore, studies on the roles of AKT and GSK-3ß O-GlcNAcylation should be done in a tissue- and cell type-specific manner.

Differential effects of an O-GlcNAcase inhibitor on tau phosphorylation.

Abnormal hyperphosphorylation of microtubule-associated protein tau plays a crucial role in neurodegeneration in Alzheimer's disease (AD). The aggregation of hyperphosphorylated tau into neurofibrillary tangles is also a hallmark brain lesion of AD. Tau phosphorylation is regulated by tau kinases, tau phosphatases, and O-GlcNAcylation, a posttranslational modification of proteins on the serine or threonine residues with ß-N-acetylglucosamine (GlcNAc). O-GlcNAcylation is dynamically regulated by O-GlcNAc transferase, the enzyme catalyzing the transfer of GlcNAc to proteins, and N-acetylglucosaminidase (OGA), the enzyme catalyzing the removal of GlcNAc from proteins. Thiamet-G is a recently synthesized potent OGA inhibitor, and initial studies suggest it can influence O-GlcNAc levels in the brain, allowing OGA inhibition to be a potential route to altering disease progression in AD. In this study, we injected thiamet-G into the lateral ventricle of mice to increase O-GlcNAcylation of proteins and investigated the resulting effects on site-specific tau phosphorylation. We found that acute thiamet-G treatment led to a decrease in tau phosphorylation at Thr181, Thr212, Ser214, Ser262/Ser356, Ser404 and Ser409, and an increase in tau phosphorylation at Ser199, Ser202, Ser396 and Ser422 in the mouse brain. Investigation of the major tau kinases showed that acute delivery of a high dose of thiamet-G into the brain also led to a marked activation of glycogen synthase kinase-3ß (GSK-3ß), possibly as a consequence of down-regulation of its upstream regulating kinase, AKT. However, the elevation of tau phosphorylation at the sites above was not observed and GSK-3ß was not activated in cultured adult hippocampal progenitor cells or in PC12 cells after thiamet-G treatment. These results suggest that acute high-dose thiamet-G injection can not only directly antagonize tau phosphorylation, but also stimulate GSK-3ß activity, with the downstream consequence being site-specific, bi-directional regulation of tau phosphorylation in the mammalian brain.

Regulation of alternative splicing of tau exon 10 by 9G8 and Dyrk1A.

Adult human brain expresses 6 isoforms of tau protein as a result of alternative splicing. Alternative splicing of exon 10 (E10) leads to tau isoforms containing either 3 (3R-tau) or 4 (4R-tau) microtubule-binding repeats. Imbalance in the 3R-tau/4R-tau ratio causes neurofibrillary degeneration and dementia. Here, we demonstrated that the dual-specificity tyrosine phosphorylation-regulated kinase 1A (Dyrk1A) interacted with the splicing factor 9G8 and phosphorylated it at several serine residues. Dyrk1A itself promoted tau E10 inclusion, whereas 9G8 inhibited E10 inclusion, and these actions were variable depending on the cell types. Coexpression of Dyrk1A and 9G8 led to their translocation from the nucleus to the cytoplasm and suppressed their ability to regulate tau exon 10 splicing. This action is probably due to their interaction-induced translocation from the nucleus, where the regulation of tau E10 splicing occurs, to the cytoplasm. These findings provide novel insights into the molecular mechanism of the regulation of tau E10 splicing and further our understanding of the neurodegeneration caused by dysregulation of tau E10 splicing.

An experimental rat model of sporadic Alzheimer's disease and rescue of cognitive impairment with a neurotrophic peptide.

Alzheimer's disease (AD) is multifactorial and, to date, no single cause of the sporadic form of this disease, which accounts for over 99% of the cases, has been established. In AD brain, protein phosphatase-2A (PP2A) activity is known to be compromised due to the cleavage and translocation of its potent endogenous inhibitor, I2PP2A, from the neuronal nucleus to the cytoplasm. Here, we show that adeno-associated virus vector-induced expression of the N-terminal I2NTF and C-terminal I2CTF halves of I2PP2A , also called SET, in brain reproduced key features of AD in Wistar rats. The I2NTF-CTF rats showed a decrease in brain PP2A activity, abnormal hyperphosphorylation and aggregation of tau, a loss of neuronal plasticity and impairment in spatial reference and working memories. To test whether early pharmacologic intervention with a neurotrophic molecule could rescue neurodegeneration and behavioral deficits, 2.5-month-old I2NTF-CTF rats and control littermates were treated for 40 days with Peptide 6, an 11-mer peptide corresponding to an active region of the ciliary neurotrophic factor. Peripheral administration of Peptide 6 rescued neurodegeneration and cognitive deficit in I2NTF-CTF animals by increasing dentate gyrus neurogenesis and mRNA level of brain derived neurotrophic factor. Moreover, Peptide 6-treated I2NTF-CTF rats showed a significant increase in dendritic and synaptic density as reflected by increased expression of synapsin I, synaptophysin and MAP2, especially in the pyramidal neurons of CA1 and CA3 of the hippocampus.

Rescue of synaptic failure and alleviation of learning and memory impairments in a trisomic mouse model of down syndrome.

Down syndrome (DS) is caused by the triplication of ∼240 protein-coding genes on chromosome 21 and is the most prevalent form of developmental disability. This condition results in abnormalities in many organ systems, as well as in intellectual retardation. Many previous efforts to understand brain dysfunction in DS have indicated that cognitive deficits are coincident with reduced synaptic plasticity and decreased neuronal proliferation. One therapeutic strategy for optimizing the microenvironment for neuronal proliferation and synaptic plasticity in the brain is the use of neurotrophins to restore the homeostasis of the brain biochemical milieu. Here, we show that peripheral administration of Peptide 6, an 11-mer corresponding to an active region of ciliary neurotrophic factor, amino acid residues 146 to 156, can inhibit learning and memory impairments in Ts65Dn mice, a trisomic mouse model of DS. Long-term treatment with Peptide 6 enhanced the pool of neural progenitor cells in the hippocampus and increased levels of synaptic proteins crucial for synaptic plasticity. These findings suggest a therapeutic potential of Peptide 6 in promoting functional neural integration into networks, thereby strengthening biologic substrates of memory processing.