There are at least five human NFAT isoforms, with NFATs 1 to 4 being sensitive to calcineurin. Normally, NFATs are found in the cytosol; rising calcium levels activate calcineurin, which in turn dephosphorylates the transcription factors. This loss of phosphate exposes a nuclear translocation signal and the NFATs move to the nucleus where they regulate transcription in cooperation with NF-κB, AT-1, and other transcription factors. NFAT activation has been extensively studied in immune cells and in muscle, but the brain is another matter. “Relative to what happens outside of the brain, not much is known about the role of NFATs in the nervous system,” Norris told ARF.

Norris and colleagues previously found that calcineurin (CaN) activity is upregulated in astrocytes in normal aging and in cells surrounding amyloid plaques in mouse models of AD (see Norris et al., 2005). To see if NFAT activation might also be related to AD, first author Hafiz Abdul and colleagues measured nuclear and cytosolic levels of NFATs 1, 2, and 3 in hippocampal tissue from 18 AD patients, 10 patients with mild cognitive impairment, and 12 normal age-matched controls. They found that NFAT1, which is all but absent from the nucleus in control tissue samples, was significantly elevated in the nucleus in MCI tissue samples. Distribution of NFAT3, on the other hand, was normal in MCI tissue, but skewed in AD samples, where it was significantly elevated in the nucleus. More specifically, the researchers correlated nuclear NFAT1 and 3 with the severity of cognitive decline, as judged by Mini-Mental State Examination (MMSE) scores. In mild cases (MMSE 20-27), NFAT1 was significantly elevated in hippocampal nuclei, returned closer to normal levels in intermediate cases (MMSE 12-19), and then trended below normal levels in severe cases (MMSE 0-11). NFAT3 was only activated in intermediate and severe cases.

It is not yet clear which cells in the hippocampus are subject to the vagaries of NFAT activation, but immunofluorescent labeling of the tissue samples suggests that astrocytes may be involved. “We have not quantified that yet,” Norris said, “but at early cognitive decline, we see quite a lot of astrocytes label intensely for NFAT1, localized to somatic/nuclear regions.” The same appears true for NFAT3 in AD tissue samples. “Right now we can’t say definitely that one [NFAT] is getting activated in one cell type and the other in another cell type.”

It is also not clear if NFAT changes are a cause or consequence of disease, but there is evidence pointing to both. Abdul and colleagues found that in AD tissue samples, nuclear NFAT3 localization appears tied to levels of soluble Aβ42. “We got a pretty nice positive correlation,” said Norris. To investigate this further, the scientists grew astrocytes in culture in the presence of monomeric, oligomeric, and fibrillar Aβ. “We found that NFAT3 is robustly, rapidly, and selectively activated by Aβ oligomers,” said Norris.

NFAT activation may also exacerbate pathology. The researchers found that activating NFATs using interleukin-1β led to a downregulation in astrocyte expression of the glutamate transporter EAAT2. The same thing happened if the cultures were treated with oligomers of Aβ. Because this transporter sucks up glutamate released into the extracellular space by neurons, its downregulation could leave neurons exposed to toxic levels of the neurotransmitter. That’s just what the researchers found. Mixed cultures exposed to Aβ had a sevenfold increase in glutamate in the media and an almost fourfold increase in neuronal death. Both could be attenuated, though not fully, by blocking NFAT activation. “That was an interesting effect that we weren’t expecting,” said Norris. The researchers also found that EAAT2 expression was progressively lost in MCI and AD tissue samples, in agreement with previous observations that the transporter was downregulated in AD (see Simpson et al., 2008). Research from Dennis Selkoe’s lab at Brigham and Women’s Hospital, Boston, also points the finger at glutamate transporter defects in AD. Those researchers found that long-term depression induced by Aβ oligomers is due to an excess of glutamate that desensitizes neuronal receptors to the neurotransmitter. Inhibiting EAATs mimicked the effect of Aβ on LTD (see ARF related news story on Li et al., 2009).

Overall, Norris sees a situation where elevated calcium levels early in the disease process lead to activation of calcineurin and hence activation of NFAT1. The early NFAT1 activation could be linked to an inflammatory response, suggested Norris, but it is not clear whether that may mediate the toxic effects of Aβ. As for the later activation of NFAT3, Norris believes that may be more closely associated with cell death. “When NFAT3 has been investigated in nervous tissue, it has been linked to cell death processes, and that makes me think that it is being activated later on in disease process and is related to the degeneration that’s occurring” said Norris. Work from Brian Bacskai’s lab at Massachusetts General Hospital, Charlestown, also linked calcineurin activation to amyloid plaques (see ARF related news story on Kuchibhotla et al., 2008).

How would blocking NFAT activation affect the brain? There are drugs that block calcineurin, such as cyclosporin and FK506, but these are potent immunosuppressants and have side effects, including kidney damage (for a review, see Naesens et al., 2009) that render them unacceptable for long-term use in AD patients, said Norris. Nevertheless, research suggests FK506 improves learning and memory in the Tg2576 mouse model of AD (see Dineley et al., 2007) and also prevents tau pathology and increases lifespan in a mouse model of tauopathy (see Yoshiyama et al., 2007). Norris suggested that blocking NFAT directly would be one way to overcome the detrimental effects of calcineurin suppression. To investigate this further, he plans to use an adenovirus approach to express a peptide inhibitor of NFAT activation in mouse models of AD.—Tom Fagan.

Reference:
Abdul HM, Sama MA, Furman JL, Mathis DM, Beckett TL, Weidner AM, Patel ES, Baig I, Murphy MP, LeVine 3rd H, Kraner SD, Norris CM. Cognitive decline in Alzheimer’s disease is associated with selective changes in calcineurin/NFAT signaling. J. Neurosci. 2009 October 14; 29: 12957-12969. Abstract