In our previous studies it has been demonstrated that phosphorylation of the neuron specific intermediate filament proteins, such as neurofilaments (NFs) were tightly regulated developmentally and topographically, and generally confined to the axonal compartment and selectively occurs on proline directed serine (Ser) and threonine (Thr) residues. It was recognized that in neurodegenerative disorders such as Alzheimers disease (AD) and Amyotrophic lateral sclerosis (ALS), the pathology was characterized by an accumulation of aberrantly phosphorylated cytoskeletal proteins in perikarya on these residues, suggesting that topographic regulation had been compromised. Neurodegenerative diseases including AD are complex and chronic disorders that involve the disruption of the neuronal network in the human brain. One of the major factors is the compartment specific tightly regulate kinases and phosphatases. One of the principle pathologies diagnostic of AD a is hyperphosphorylated tau and phospho-NF-M/H assembled into intraneuronal paired helical filaments (PHF) and aggregated phospho-NFPs (pNFPs). It is proposed that this pathology inhibits axonal transport and may lead to neuronal apoptosis. An extensive repertoire of site-specific protein kinases have been implicated in tau and neurofilament phosphorylation which are balanced, in turn, by the activity of Ser/Thr protein phosphatases, principally protein phosphatase 2A (PP2A). PP2A is colocalized with tau and NFs in the brain and is probably a key another phosphatase that maintains tau and pNFs in a normal dephosphorylated state in the neuronal cell bodies. It has been reported that the mRNA of PP2A level is significantly down regulated in AD brains. This suggests that accumulation of PHF tau and aggregated pNFPs in AD and ALS brains (and other neurodegenerative disorders), may also result from the failure to dephosphorylate hyperphosphorylated tau and -NFM/H. This has stimulated the study of model systems in which dephosphorylating activity of PP2A, inhibited by okadaic acid, induces aberrant cytoskeletal phosphorylation in perikarya resembling AD pathology. Phosphatase inhibitors okadaic acid, microcystine and fostriecine (specific to PP2A) induce robust perikaryal phosphorylation of NF-M/H in rat cortical neurons, whereas, the PP2B inhibitor, cyclosporine, has no effect, suggesting that PP2A is principally involved in modulating NF phosphorylation. It has become evident that another rational approach to AD therapy is a study of the efficacy of phosphatase activators, particularly those that specifically activate PP2A. We have demonstrated that in addition of kinases the activity of phosphatase, specifically , protein phosphatase 2A activity is down regulated in AD and ALS. The other approach, we have proposed to reduce the pathological burden is to use PP2A activators as therapeutic reagents. To evaluate the effects of the PP2A activator FTY720, fingolimod, we employed an okadaic acid protocol that induces an aberrant hyperphosphorylation in neuronal perikarya of NF-M/H in primary rat cortical neurons, Indeed FTY720 reduced the level of NF-M/H phosphorylated neurofilaments in a dose and time dependent manner. In a similar experiment the aberrant hyperphosphorylation of Tau was also reduced in FTY720 treated cortical neurons. FTY720, is an immunosuppressant, extensively used to prevent organ transplant rejection. It is important to note that its low concentration is highly effective in activating PP2A. Furthermore, it succeeds in rescuing cells from okadaic-induced apoptosis. We propose animal models to explore the efficacy of phosphatase activators on the progress of AD and ALS. In one such protocol, adult rats infused with okadaic acid into the dorsal hippocampus region induced an AD-like syndrome. We propose the ALS and AD transgenic mice will be an ideal model to study therapeutic effects of PP2A activators.
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