Blockade of Astrocytic Calcineurin/NFAT Signaling Helps to Normalize Hippocampal Synaptic Function and Plasticity in a Rat Model of Traumatic Brain Injury

Abstract

Increasing evidence suggests that the calcineurin (CN)-dependent transcription factor NFAT (Nuclear Factor of Activated T cells) mediates deleterious effects of astrocytes in progressive neurodegenerative conditions. However, the impact of astrocytic CN/NFAT signaling on neural function/recovery after acute injury has not been investigated extensively. Using a controlled cortical impact (CCI) procedure in rats, we show that traumatic brain injury is associated with an increase in the activities of NFATs 1 and 4 in the hippocampus at 7 d after injury. NFAT4, but not NFAT1, exhibited extensive labeling in astrocytes and was found throughout the axon/dendrite layers of CA1 and the dentate gyrus. Blockade of the astrocytic CN/NFAT pathway in rats using adeno-associated virus (AAV) vectors expressing the astrocyte-specific promoter Gfa2 and the NFAT-inhibitory peptide VIVIT prevented the injury-related loss of basal CA1 synaptic strength and key synaptic proteins and reduced the susceptibility to induction of long-term depression. In conjunction with these seemingly beneficial effects, VIVIT treatment elicited a marked increase in the expression of the prosynaptogenic factor SPARCL1 (hevin), especially in hippocampal tissue ipsilateral to the CCI injury. However, in contrast to previous work on Alzheimer's mouse models, AAVGfa2-VIVIT had no effects on the levels of GFAP and Iba1, suggesting that synaptic benefits of VIVIT were not attributable to a reduction in glial activation per se. Together, the results implicate the astrocytic CN/NFAT4 pathway as a key mechanism for disrupting synaptic remodeling and homeostasis in the hippocampus after acute injury.

Calcineurin proteolysis in astrocytes: Implications for impaired synaptic function

“…Hyperactivation of CN signaling in astrocytes has been linked to the elevated expression of inflammatory mediators, impaired glutamate regulation, Aβ production, and altered astrocytic Ca 2+ dynamics [3, 12–14, 18, 52, 53]. Using adeno-associated virus (AAV) vectors, equipped with astrocyte specific promoters, we have shown that the selective inhibition of astrocytic CN/NFAT signaling provides numerous beneficial effects in a mouse model of AD [2], and a rat model of traumatic brain injury [17]. These results suggest that hyperactivation of CN in astrocytes can mediate and/or exacerbate pathophysiological processes.…”

“…Both ΔCN and DsRed2 were transcriptionally regulated by a truncated human GFAP promoter (Gfa104), which provides astrocyte-specific expression [31, 32]. Using confocal microscopy and/or immunodepletion assays, we previously showed that AAV-mediated expression of EGFP was driven exclusively in astrocytes (in mice and rats) when a similar Gfa2 promoter was used [2, 17]. Consistent with these observations, immunofluorescent labeling of the DsRed2 tag in the present study was limited to astrocytes and showed no co-localization with the neuron-specific protein MAP2b (Figure 6B).…”

“…The Gfa104 promoter has been characterized previously, and has been shown to drive transgene expression selectively in astrocytes of intact animals [31, 32], similar to that seen with the Gfa2 promoter (e.g. see [17]).…”

“…In astrocytes and microglia, CN controls immune/inflammatory phenotypes through activation of key transcription factors including nuclear factor of activated T cells (NFAT), nuclear factor κ B (NFκB), and FOXO, among others [9, 12–16]. Previously, we showed that the selective blockade of astrocytic CN/NFAT signaling with the NFAT-inhibitory peptide, VIVIT, suppresses markers of glial activation, alleviates amyloid pathology, and protects against cognitive deficits in experimental models of AD [1, 2, 14] and prevents synapse dysfunction in models of AD and acute brain injury [2, 17]. These results suggest that astrocytes are a key locus of hyperactive CN signaling during the progression of AD.…”

Enhanced astrocytic d-serine underlies synaptic damage after traumatic brain injury

“…Specialized membranes, such as at synapses, are susceptible to unique environmental changes within the injured brain, where astrocytes in the tripartite synapse contribute to synaptic transmission and function (1)(2)(3). TBI is known to result in astrocyte reactivity, which has both beneficial and deleterious effects on injury progression and recovery; however, few studies have examined the effects of astrogliosis on synaptic stability in the CNS (4)(5)(6).…”