. A potent mechanism-inspired O-GlcNAcase inhibitor that blocks phosphorylation of tau in vivo. Nat Chem Biol. 2008 Aug;4(8):483-90. PubMed.

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  1. Inhibition of Alzheimer Neurofibrillary Degeneration by Inhibition of O-GlcNAcase: A Sweet Approach With Some Bitter Hurdles Ahead
    The development of a potent O-GlcNAcase inhibitor and its ability to inhibit abnormal hyperphosphorylation of tau by Yuzwa et al. (2008), while very promising, might at the same time produce contraindicated effects by inhibiting phosphorylation of PI-3 kinase cascade enzymes upstream of glycogen synthase kinase-3 (GSK3).

    Tau is a major microtubule-associated protein in the neuron. It is abnormally hyperphosphorylated and aggregated into neurofibrillary tangles in AD brains (Grundke-Iqbal et al., 1986a; Grundke-Iqbal et al., 1986b). Unlike normal tau, which promotes assembly of tubulin into microtubules and stabilizes them, the AD abnormally hyperphosphorylated tau sequesters normal microtubule associated proteins and inhibits microtubule assembly as well as self-assembling into paired helical filaments (Alonso et al., 1994; 2001a). Many studies have demonstrated that abnormal hyperphosphorylation of tau is crucial to neurodegeneration in AD and other tauopathies (Iqbal et al., 2005). Thus, inhibiting and/or reversing tau hyperphosphorylation has been one of the major objectives of research in the AD field.

    The exact causes leading to abnormal hyperphosphorylation of tau in AD are poorly understood. We and others have found that protein phosphatase 2A, the major tau phosphatase in the brain, is downregulated in AD brain (Gong et al., 1993; Gong et al., 1995; Vogelsberg-Ragaglia et al., 2001; Sontag et al., 2004; Liu, F et al., 2005; Liu, R et al., 2008; Zhou et al., 2008), suggesting that this downregulation may be a possible cause of tau hyperphosphorylation. In addition to being regulated by tau kinases and phosphatases, tau phosphorylation is also regulated by O-GlcNAcylation. In 2004, we found that human brain tau is modified by O-GlcNAc in addition to phosphates (Liu, F et al., 2004). O-GlcNAcylation regulates phosphorylation of tau inversely both in cultured cells and in metabolically active rat brain slices. More importantly, O-GlcNAcylation is decreased in AD brain. In a mouse model of decreased brain glucose metabolism induced by fasting, we observed decreased O-GlcNAcylation and concurrently increased tau phosphorylation at multiple phosphorylation sites (Liu, F et al., 2004; Li et al., 2006). On the basis of these observations, we proposed a mechanism by which impaired brain glucose metabolism leads to Alzheimer neurofibrillary degeneration via decrease in O-GlcNAcylation and consequently hyperphosphorylation of tau (Gong et al., 2006).

    According to the above proposed mechanism, upregulation of tau O-GlcNAcylation could be a novel approach to inhibit and reverse hyperphosphorylation of tau and thus to treat AD and other tauopathies. The new O-GlcNAcase inhibitor, thiamet-G, developed in Dr. Vocadlo’s laboratory (Yuzwa et al., 2008) appears to be an excellent agent for upregulation of protein O-GlcNAcylation. Compared to previously developed O-GlcNAcase inhibitors, thiamet-G is very potent (Ki = 21 nM) and highly specific to human O-GlcNAcase. It exhibits 37,000-fold selectivity for human O-GlcNAcase over human lysosomal β-hexosaminidase and does not inhibit other glycoside hydrolases at as high as 500 μM concentration. Furthermore, its competitive inhibition to O-GlcNAcase, extreme stability, and ability to cross the blood-brain barrier makes thiamet-G very attractive for drug development for treating AD. Furthermore, treatment of PC12 cells with thiamet-G induced increased protein O-GlcNAcylation and decreased tau phosphorylation at Thr231, Ser396, and Ser422. Similar results were observed in vivo when rats are treated with thiamet-G either intravenously or orally. This in vivo study with oral administration is especially attractive for drug development.

    To date, more than 40 phosphorylation sites have been identified in tau protein isolated from AD brain (Gong et al., 2005; Hanger et al., 2007). It is generally believed that hyperphosphorylation at multiple sites converts the normal tau into the pathological tau, and that different phosphorylation sites of tau play different roles in this conversion (Iqbal et al., 2005; Wang et al., 2007). The current study investigated only seven phosphorylation sites of tau, among which only three sites were found decreased upon treatment with thiamet-G. It will be important to examine the effects of thiamet-G on tau phosphorylation at other sites as well, especially at those sites known to be involved in its pathological activity, i.e., sequestration of normal tau, MAP1, and MAP2, and its self-assembly into paired helical filaments (Iqbal et al., 1986; Alonso et al., 1994; 1997; 2001b; Iqbal et al., 2005).

    Thiamet-G can cause decreased tau phosphorylation at certain phosphorylation sites directly via elevation of tau O-GlcNAcylation. Thiamet-G might also elevate O-GlcNAcylation level of other neuronal proteins in the brain. Whether elevation of O-GlcNAcylation of these proteins has significant diverse effects remains to be elucidated. These affected proteins could include several protein kinases, especially those of PI-3 kinase pathway, that regulate tau phosphorylation. Glycogen synthase kinase-3β (GSK3β) is one of the most important tau kinases in the brain (Takashima, 2006; Avila and Hernandez, 2007). The activity of GSK3β is mainly regulated by its upstream kinase AKT via phosphorylation at Ser9. It has been reported that AKT and several components of the AKT signaling pathway are also modified by O-GlcNAc and that O-GlcNAcylation of AKT inhibits its kinase activity (Vosseller et al., 2002; Luo et al., 2008; Yang et al., 2008). Thus, it is possible that thiamet-G could also elevate AKT O-GlcNAcylation, which, in turn, leads to inhibition of AKT activity and, consequently, results in activation of GSK3β. It will not be surprising if increased phosphorylation at some of tau phosphorylation sites is induced in the brain by thiamet-G treatment. The ultimate changes of tau phosphorylation with thiamet-G treatment will be the combined consequence of direct effect through increase in tau O-GlcNAcylation and indirect effects through modifying activities of various tau kinases and phosphatases.

    In the end, if the net effect results in converting the abnormally hyperphosphorylated tau to a protein with normal-like biological activity or non-inhibitory molecule, which remains to be determined, it will be a major step forward.

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  2. I have carefully read this very good paper from David Vocadlo’s group. The data presented are clear and very interesting since the study opens a new strategy for a therapy against Alzheimer disease. To my knowledge it is one of the first times, together with the work of John Chatham’s group in the field of cardioprotection, that some scientists propose to target the O-GlcNAc dynamism with the intention to treat a disease.

    Contrary to complex glycosylations, O-GlcNAc is confined within the cytosol and the nucleus of eukaryotes. It is highly dynamic and it can counteract the effect of phosphorylation by modifying the same sites on the peptide backbone. Two enzymes are responsible for the versatility of O-GlcNAc: the O-GlcNAc transferase, simply named OGT, and the O-GlcNAcase, named OGA. To be aware of the impact of the work presented in the paper by Yuzwa and collaborators, one should know that O-GlcNAc level is tightly dependent upon glucose metabolism since UDP-GlcNAc, the OGT substrate that gives the GlcNAc moiety, comes from the extracellular glucose.

    Alzheimer disease is characterized by two hallmarks—the formation of senile plaques and the aggregation of hyperphosphorylated tau into paired helical filaments. It has been recently proposed by Suzanne de la Monte that Alzheimer disease represents a brain-specific form of diabetes, i.e., type 3 diabetes. Starting from this hypothesis, a link between the nucleocytoplasmic-specific glycosylation from O-GlcNAc and the impairment in glucose metabolism found in Alzheimer patients could be drawn. Tau proteins are extensively modified with O-GlcNAc residues (Arnold et al., 1996) and our group has observed a reciprocal relationship between this glycosylation and phosphorylation for this neuronal protein (Lefebvre et al., 2003). So, it is strongly suspected that the hyperphosphorylation of tau is a consequence of its own hypoglycosylation, which itself ensues from glucose impairment. The idea of Yuzwa’s paper is based on the specific inhibition of OGA, the enzyme that hydrolyses the O-GlcNAc moiety from the target proteins. Numerous OGA inhibitors have been described in the literature, PUGNAc being the most widely used. The major problem of PUGNAc is that it is not sufficiently selective to inhibit the cytosolic and nuclear hexosaminidase OGA (indeed numerous other glycosidases are found in other organelles such as the lysosome, for example) and moreover it cannot reach the blood-brain barrier. But the authors have designed and synthesized a new molecule they named thiamet-G. Thiamet-G is highly stable under aqueous conditions and is selective for nucleocytoplasmic OGA with a very interesting Ki of 21 nM. Experiments were conducted either on PC-12 cells, in which thiamet-G rapidly increases the O-GlcNAc levels, and in rats. The administration of thiamet-G in vivo clearly reduces tau phosphorylation in the CA-1 region of the hippocampus and in the cortex of these animals. Accordingly, thiamet-G reduces the phosphorylation at Thr231, Ser396, and Ser422, two of these sites being key priming sites that control the pathological hyperphosphorylation of tau in Alzheimer disease.

    In conclusion, the paper creates an opening in the treatment of Alzheimer disease by targeting one of the enzymes of the cycling O-GlcNAc. Nevertheless, and as the authors clearly recognize, the experiments conducted with thiamet-G were realized only on healthy, non-pathological animals, and at the moment it is not known if the drug could reverse or counteract the pathological phosphorylation process acting on tau. It would be also interesting to test if the opposite strategy—inhibition of OGT with specific inhibitors—would lead to an increase in tau phosphorylation in vivo. What we also do not know is what effect thiamet-G has on the O-GlcNAc levels in tissues other than brain, since O-GlcNAc is widely expressed in the entire organism and controls many crucial cellular processes such as transcription, trafficking, and protein stability.

    References:

    . The microtubule-associated protein tau is extensively modified with O-linked N-acetylglucosamine. J Biol Chem. 1996 Nov 15;271(46):28741-4. PubMed.

    . Evidence of a balance between phosphorylation and O-GlcNAc glycosylation of Tau proteins--a role in nuclear localization. Biochim Biophys Acta. 2003 Jan 20;1619(2):167-76. PubMed.