An association between chronic high blood pressure and obstructive sleep apnea has been described. We hypothesized that repetitive episodic hypoxia patterned after the hypoxia seen in sleep apnea could contribute to diurnal elevation of blood pressure. Using 12-second infusions of nitrogen into daytime sleeping chambers, four groups of male rats (250-375 g) were subjected to intermittent hypoxia (3-5% nadir ambient oxygen) every 30 seconds, 7 hours per day for up to 35 days. In one group, blood pressure was measured weekly by the tail-cuff method in conscious animals during 5 weeks of episodic hypoxia. In the other three groups, blood pressure was measured in conscious animals via femoral artery catheters at baseline and after 20, 30, or 35 days of exposure. Additional groups served as controls: two sham groups housed in identical "hypoxia" chambers received compressed air instead of nitrogen (35 days) while two other groups remained unhandled in their usual cages (35 days). Both groups challenged with 35 days episodic hypoxia showed significant increases in blood pressure compared with controls: the tail-cuff rats showed a 21 mm Hg increase in systolic pressure (/?<0.05) and the intra-arterially measured rats a 13.7 mm Hg increase in mean arterial pressure (p<0.05). The 30-day exposed rats also showed a 5.7 mm Hg increase in mean pressure over baseline (p<0.05). Blood pressure did not change significantly from baseline in the control groups. Left ventricle-to-body weight ratio was higher in both 35-day exposed groups than in unhandled or sham controls. This duration-of-exposure-related blood pressure response to hypoxia along with increased left ventricular size after 35 days indicates that chronic intermittent hypoxia could be a mechanism directly contributing to diurnal arterial blood pressure elevation. 1 Chronic hypertension seen in OSA patients may be reversed by treatment of the apnea.2 -4 Other reports show an increased prevalence of sleep apnea in populations of middle-aged men with primary hypertension, 5 -8 but this association has been challenged by more recent epidemiological studies.910 Possible mechanisms for the development of long-term diurnal blood pressure (BP) elevation in this setting are stress related to episodic repetitive hypoxia, disruption of sleep architecture, and modification of the cardiovascular system (including fluid balance) in response to marked fluctuations in intrathoracic pressure. There have been no studies to date examining the effect of any of these individual mechanisms on long-term diurnal BP. It would be quite difficult to prospectively examine factors effecting diurnal BP in humans since such changes may take many years to manifest in the face of progressively more severe, recurrent OSA. One approach is to develop an animal model with a sufficiently short life span in which some conditions of OSA can be duplicated. We chose the rat as a suitable animal in which to examine the effects of hypoxia on BP because of extensive knowledge about mechanisms of systemic hyperte...

Abnormal hyperphosphorylation of tau appears to be crucial in neurofibrillary degeneration in Alzheimer's disease (AD). Previous studies suggest that a down-regulation of protein phosphatase 2A (PP2A), the major tau phosphatase in human brain, contributes to tau hyperphosphorylation in AD. However, the effects of PP2A down-regulation on site-specific tau hyperphosphorylation is not well understood. In the present study, we showed that PP2A dephosphorylated tau at several phosphorylation sites with different efficiencies. Among the sites studied, Thr205, Thr212, Ser214, and Ser262 were the most favorable sites, and Ser199 and Ser404 were the least favorable sites for PP2A in vitro. Inhibition of PP2A with okadaic acid in metabolically active rat brain slices caused inhibition of glycogen synthase kinase-3beta (GSK-3beta) via an increase in its phosphorylation at Ser9. GSK-3beta phosphorylated tau at many sites, with Ser199, Thr205, and Ser396 being the most favorable sites in cells. The overall alterations in tau phosphorylation induced by PP2A inhibition were the result of the combined effects of both reduced tau dephosphorylation due to PP2A inhibition directly and reduced phosphorylation by GSK-3beta due to its inhibition. Because the impacts of tau phosphorylation on its biological activity and on neurofibrillary degeneration are site-specific, this study provides a new insight into the role of PP2A down-regulation in neurofibrillary degeneration in AD.

Abnormal alternative splicing of tau exon 10 results in imbalance of 3R-tau and 4R-tau expression, which is sufficient to cause neurofibrillary degeneration. Splicing factor SC35, a member of the superfamily of the serine/arginine-rich (SR) proteins, promotes tau exon 10 inclusion. The molecular mechanism by which SC35 participates in tau exon 10 splicing remains elusive. In the present study, we found that tau pre-mRNA was coprecipitated by SC35 tagged with HA. Mutation of the SC35-like exonic splicing enhancer located at exon 10 of tau affected both the binding of SC35 to tau pre-mRNA and promotion of tau exon 10 inclusion, suggesting that SC35 acts on the SC35-like exonic splicing enhancer to promote tau exon 10 inclusion. Dyrk1A (dual-specificity tyrosine-phosphorylated and regulated kinase 1A) phosphorylated SC35 in vitro and interacted with it in cultured cells. Overexpression of Dyrk1A suppressed SC35′s ability to promote tau exon 10 inclusion. Downregulation of Dyrk1A promoted 4R-tau expression. Therefore, upregulation of Dyrk1A in Down syndrome brain or Alzheimer’s brain may cause dysregulation of tau exon 10 splicing through SC35, and probably together with other splicing factors, leading to the imbalance in 3R-tau and 4R-tau expression, which may initiate or accelerate tau pathology and cause neurofibrillary degeneration in the diseases.

The neuronal microtubule-associated protein tau is abnormally hyperphosphorylated and aggregated into neurofibrillary tangles in the brains of individuals with Alzheimer's disease and related neurodegenerative disorders. The adult human brain expresses six isoforms of tau generated by alternative splicing of exons 2, 3, and 10 of its pre-mRNA. Exon 10 encodes the second microtubule-binding repeat of tau. Its alternative splicing produces tau isoforms with either three or four microtubule-binding repeats, termed 3R-tau and 4R-tau. In the normal adult human brain, the level of 3R-tau is approximately equal to that of 4R-tau. Several silent and intronic mutations of the tau gene associated with FTDP-17T (frontotemporal dementia with Parkinsonism linked to chromosome 17 and specifi cally characterized by tau pathology) only disrupt exon 10 splicing, but do not infl uence the primary sequence of the tau protein. Thus, abnormal exon 10 splicing is suffi cient to cause neurodegeneration and dementia. Here, we review the regulation of tau exon 10 splicing by cis-elements and trans-factors and summarize all the mutations associated with FTDP-17T and related tauopathies. The fi ndings suggest that correction of exon 10 splicing may be a potential target for tau exon 10 splicing-related tauopathies.Keywords: alternative splicing; tau; tau exon 10; tauopathies IntroductionTau is a major neuronal microtubule-associated protein, which promotes the assembly of microtubules and stabilizes the microtubule network [1] . As a phosphoprotein, the degree of phosphorylation of tau regulates its biological functions. However, abnormally hyperphosphorylated tau easily aggregates into neurofibrillary tangles (NFTs) and deposits in the affected neurons in the brains of individuals with Alzheimer's disease (AD) [2,3] . It is believed that hyperphosphorylation of tau is responsible for its loss of biological function, gain of toxicity, and aggregation into NFTs [4][5][6] .In addition to AD, aggregation of hyperphosphorylated tau in the brain has been found in other neurodegenerative diseases, such as corticobasal degeneration (CBD), Down syndrome (DS), frontotemporal dementia with Parkinsonism linked to chromosome 17 and specifi cally characterized by tau pathology (FTDP-17T), Niemann-Pick disease, Pick's disease, postencephalitic Parkinsonism, and progressive supranuclear palsy (PSP). These sporadic and familial neurodegenerative disorders are defi ned as "tauopathies" [7, 8] .The discovery of tau gene (MAPT) mutations causing FTDP-17Tprovides clear evidence that aberrant tau can cause neurodegeneration by itself without amyloid plaques [9] .Human tau is encoded by a single gene located on chromosome 17q21, which is composed of 16 exons. By alternative splicing of exons 2, 3, and 10, six isoforms of tau are expressed in the normal adult human brain [10] . Exon 10 encodes the second microtubule-binding repeat. Thus, [6,12] . The normal adult human brain expresses approximately equal levels of 3R-tau and 4R-tau [10, 13] . However, almo...

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Hyperphosphorylation and deposition of tau into neurofibrillary tangles is a hallmark of Alzheimer disease (AD).Alternative splicing of tau exon 10 generates tau isoforms containing three or four microtubule binding repeats (3R-tau and 4R-tau), which are equally expressed in adult human brain. Dysregulation of exon 10 causes neurofibrillary degeneration. Here, we report that cyclic AMP-dependent protein kinase, PKA, phosphorylates splicing factor SRSF1, modulates its binding to tau pre-mRNA, and promotes tau exon 10 inclusion in cultured cells and in vivo in rat brain. PKA-C␣, but not PKA-C␤, interacts with SRSF1 and elevates SRSF1-mediated tau exon 10 inclusion. In AD brain, the decreased level of PKA-C␣ correlates with the increased level of 3R-tau. These findings suggest that a downregulation of PKA dysregulates the alternative splicing of tau exon 10 and contributes to neurofibrillary degeneration in AD by causing an imbalance in 3R-tau and 4R-tau expression.Tau is a neuronal microtubule-associated protein, the function of which is to stimulate microtubule assembly and stabilize microtubules. Hyperphosphorylation of tau leads to its aggregation into neurofibrillary tangles, a hallmark of Alzheimer disease (AD) 2 and related neurodegenerative diseases called tauopathies (1-4). Adult human brain expresses six different tau isoforms from a single gene by alternative splicing of exons 2, 3, and 10 of its pre-mRNA (5). The exon 10 encodes the second microtubule binding repeat (6). Alternative splicing of exon 10 generates tau with three or four microtubule binding repeats (3R-tau or 4R-tau), which is under developmental and cell type-specific regulation. Only 3R-tau is expressed during embryogenesis, whereas 3R-tau and 4R-tau are expressed in approximately equal amounts in adult human brain (6, 7). Several mutations in tau gene result in either an increase or a decrease in 4R-tau expression and cause frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17), one of the tauopathies (8). Thus, alteration in the 3R-tau/ 4R-tau ratio is sufficient to trigger neurodegeneration in frontotemporal dementia and might also play a role in other neurodegenerative disorders such as Pick's disease, progressive nuclear palsy, or corticobasal degeneration in which the 3R-tau/4R-tau ratio is markedly altered (9 -12). Thus, the regulation of alternative splicing of human tau exon 10 has been of critical interest. However, results of studies of the alternative splicing of tau exon 10 in AD brain have been contradictory (13)(14)(15). Recent studies have shown that aggregation and deposition of 3R-tau may be associated with more advanced stages (16,17).Alternative splicing of tau exon 10 is regulated by several trans-acting factors, including serine-and arginine-rich (SR) proteins, and their phosphorylation (18 -24). Splicing factor 2/alternative splicing factor (ASF/SF2), now named SRSF1 (serine/arginine-rich splicing factor 1) (25), is a prototypical SR protein that participates in both constitutive and alternativ...

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In the brains of individuals with Alzheimer's disease (AD) and chronic traumatic encephalopathy, tau pathology is accompanied usually by intracellular aggregation of transactive response DNA-binding protein 43 (TDP-43). However, the role of TDP-43 in tau pathogenesis is not understood. Here, we investigated the role of TDP-43 in tau expression in vitro and in vivo. We found that TDP-43 suppressed tau expression by promoting its mRNA instability through the UG repeats of its 3΄-untranslated region (3΄-UTR). The C-terminal region of TDP-43 was required for this function. Neurodegenerative diseases-causing TDP-43 mutations affected tau mRNA instability differentially, in that some promoted and others did not significantly affect tau mRNA instability. The expression levels of tau and TDP-43 were inverse in the frontal cortex and the cerebellum. Accompanied with cytoplasmic accumulation of TDP-43, tau expression was elevated in TDP-43M337V transgenic mouse brains. The level of TDP-43, which is decreased in AD brains, was found to correlate negatively with the tau level in human brain. Our findings indicate that TDP-43 suppresses tau expression by promoting the instability of its mRNA. Down-regulation of TDP-43 may be involved in the tau pathology in AD and related neurodegenerative disorders.