Calcineurin inhibitor, FK506, prevents reduction in the binding capacity of cyclic AMP-dependent protein kinase in ischemic gerbil brain

MEK/ERK- and calcineurin/NFAT-mediated mechanism of cerebral hyperemia and brain injury following NMDA receptor activation

N-methyl-d-aspartate (NMDA) receptor activation increases regional cerebral blood flow (rCBF) and induces neuronal injury, but similarities between these processes are poorly understood. In this study, by measuring rCBF in vivo, we identified a clear correlation between cerebral hyperemia and brain injury. NMDA receptor activation induced brain injury as a result of rCBF increase, which was attenuated by an inhibitor of mitogen-activated protein kinase or calcineurin. Moreover, NMDA induced phosphorylation of extracellular signal-regulated kinase (ERK) and nuclear translocation of nuclear factor of activated T-cell (NFAT) in neurons. Therefore, a MEK/ERK- and calcineurin/NFAT-mediated mechanism of neurovascular coupling underlies the pathophysiology of neurovascular disorders.

Effects of FK506 on Hippocampal CA1 Cells Following Transient Global Ischemia/Reperfusion in Wistar Rat

Transient global cerebral ischemia causes loss of pyramidal cells in CA1 region of hippocampus. In this study, we investigated the neurotrophic effect of the immunosuppressant agent FK506 in rat after global cerebral ischemia. Both common carotid arteries were occluded for 20 minutes followed by reperfusion. In experimental group 1, FK506 (6 mg/kg) was given as a single dose exactly at the time of reperfusion. In the second group, FK506 was administered at the beginning of reperfusion, followed by its administration intraperitoneally (IP) 6, 24, 48, and 72 hours after reperfusion. FK506 failed to show neurotrophic effects on CA1 region when applied as a single dose of 6 mg/kg. The cell number and size of the CA1 pyramidal cells were increased, also the number of cell death decreased in this region when FK506 was administrated 48 h after reperfusion. This work supports the possible use of FK506 in treatment of ischemic brain damage.

Neuroprotectant FK506 inhibits glutamate-induced apoptosis of astrocytes in vitro and in vivo

Neuron-astrocyte interactions are critical for signalling, energy metabolism, extracellular ion and glutamate homeostasis, volume regulation and neuroprotection in the CNS. Glutamate uptake by astrocytes may prevent excitotoxic glutamate elevation and determine neuronal survival. However, an excess of glutamate can cause the death of astrocytes. FK506, an inhibitor of calcineurin, and an immunosuppressive drug, is neuroprotective in animal models of neurologic diseases, including focal and global ischaemia. In the present work, we demonstrate that a single injection of FK506 60 min after a transient middle cerebral artery occlusion (MCAo) significantly decreases the number of terminal deoxynucleotidyl transferase nick-end labelling (TUNEL)-positive cells in the ischaemic cortex and striatum. Using 3-D confocal microscopy we found that, 24 h after MCAo, many TUNEL-positive cells in the ischaemic striatum and cortex are astrocytes. Furthermore, we demonstrate that exposure of cultured cortical astrocytes to 50-100 mM Glu for 24 h induces apoptotic alterations in nuclear morphology, DNA fragmentation, dissipation of mitochondrial transmembrane potential (DeltaPsi) and caspase activation. FK506 (1 muM) efficiently inhibits Glu-induced apoptosis of cultured astrocytes, DNA fragmentation and changes in mitochondrial DeltaPsi. Our findings suggest that modulation of glutamate-induced astrocyte death early after reperfusion may be a novel mechanism of FK506-mediated neuroprotection in ischaemia.

A novel mechanism of FK506-mediated neuroprotection: Downregulation of cytokine expression in glial cells

Immunosuppressant FK506 is neuroprotective in experimental models of cerebral ischemia, but the molecular mechanisms underlying this neuroprotection remain unknown. We have demonstrated that FK506 inhibits the signaling pathways that regulate hypertrophic/proliferative responses in cultured astrocytes. Ischemia/reperfusion injury is associated with the proliferation and hypertrophy of astrocytes and with inflammatory responses. In the present work, we sought to determine whether FK506 neuroprotection after middle cerebral artery occlusion (MCAo) in rat is mediated via suppression of glia activation and changes in cytokine expression. Neurological deficits, infarct size, and astrocyte/microglial response were quantified in rats subjected to 90 min of MCAo. Changes in the mRNA expression of interleukin-1beta (IL-1beta), IL-6, and tumor necrosis factor-alpha (TNF-alpha) in ipsilateral and contralateral cortices were determined by reverse transcription-polymerase chain reaction (RT-PCR). FK506 administered at 1 mg/kg, 60 min after MCAo, produced a significant improvement in neurological function and reduction of infarct volume. In FK506-treated rats, a significant reduction of IL-1beta, IL-6, and TNF-alpha expression was observed 12 h after reperfusion. FK506 neuroprotection was associated with a significant downregulation of IL-1beta expression in astrocytes and microglia in the injured side. FK506 selectively decreased the levels of TNF-alpha, and IL-1beta mRNAs in astrocytes in vitro, with no effect on transforming growth factor-beta 1 (TGF-beta1) and IL-6 expression. Moreover, FK506 inhibits lipopolysaccharide (LPS)-induced activation and cytokine expression in microglia in vitro. Our findings suggest that astrocytes and microglia are targets for FK506, and that modulation of glial response and inflammation may be a mechanism of FK506-mediated neuroprotection in ischemia.

Molecular mechanisms of neuroprotective action of immunosuppressants--facts and hypotheses

Cyclosporin A (CsA) and FK506 (Tacrolimus) are short polypeptides which block the activation of lymphocytes and other immune system cells. Immunosuppressants exert neuroprotective and neurotrophic action in traumatic brain injury, sciatic nerve injury, focal and global ischemia in animals. Their neuroprotective actions are not understood and many hypotheses have been formed to explain such effects. We discuss a role of drug target--calcineurin in neuroprotective action of immunosuppressants. Protein dephosphorylation by calcineurin plays an important role in neuronal signal transduction due to its ability to regulate the activity of ion channels, glutamate release, and synaptic plasticity. In vitro FK506 protects cortex neurons from NMDA-induced death, augments NOS phosphorylation inhibiting its activity and NO synthesis. However, in vivo experiments demonstrated that FK506 in neuroprotective doses did not block excitotoxic cell death nor did it alter NO production during ischemia/reperfusion. Tissue damage in ischemia is the result of a complex pathophysiological cascade, which comprises a variety of distinct pathological events. Resident non-neuronal brain cells respond rapidly to neuronal cell death and may have both deleterious and useful role in neuronal damage. There is increasing evidence that reactive gliosis and post-ischemic inflammation involving microglia contribute to ischemic damage. We have demonstrated that FK506 modulates hypertrophic/proliferative responses and proinflammatory cytokine expression in astrocytes and microglia in vitro and in focal transient brain ischemia. Our findings suggest that astrocytes and microglia are direct targets of FK506 and modulation of glial response and inflammation is a possible mechanism of FK506-mediated neuroprotection in ischemia.

Alteration of second messengers during acute cerebral ischemia - adenylate cyclase, cyclic AMP-dependent protein kinase, and cyclic AMP response element binding protein

A variety of neurotransmitters and other chemical substances are released into the extracellular space in the brain in response to acute ischemic stress, and the biological actions of these substances are exclusively mediated by receptor-linked second messenger systems. One of the well-known second messenger systems is adenylate cyclase, which catalyzes the generation of cyclic AMP, triggering the activation of cyclic AMP-dependent protein kinase (PKA). PKA controls a number of cellular functions by phosphorylating many substrates, including an important DNA-binding transcription factor, cyclic AMP response element binding protein (CREB). CREB has recently been shown to play an important role in many physiological and pathological conditions, including synaptic plasticity and neuroprotection against various insults, and to constitute a convergence point for many signaling cascades. The autoradiographic method developed in our laboratory enables us to simultaneously quantify alterations of the second messenger system and local cerebral blood flow (lCBF). Adenylate cyclase is diffusely activated in the initial phase of acute ischemia (< or = 30 min), and its activity gradually decreases in the late phase of ischemia (2-6 h). The areas of reduced adenylate cyclase activity strictly coincide with infarct areas, which later become visible. The binding activity of PKA to cyclic AMP, which reflects the functional integrity of the enzyme, is rapidly suppressed during the initial phase of ischemia in the ischemic core, especially in vulnerable regions, such as the CA1 of the hippocampus, and it continues to decline. By contrast, PKA binding activity remains enhanced in the peri-ischemia area. These changes occur in a clearly lCBF-dependent manner. CREB phosphorylation at a serine residue, Ser(133), which suggests the activation of CREB-mediated transcription of genes containing a CRE motif in the nuclei, remains enhanced in the peri-ischemia area, which is spared of infarct damage. On the other hand, CREB phosphorylation at Ser133 rapidly diminishes in the ischemic core before the histological damage becomes manifest. The Ca2+ influx during membrane depolarization contributes to CREB phosphorylation in the initial phase of post-ischemic recirculation, while PKA activation and other signaling elements seem to be responsible in the later phase. These findings suggest that derangement of cyclic AMP-related intracellular signal transduction closely parallels ischemic neuronal damage and that persistent enhancement of this signaling pathway is important for neuronal survival in acute cerebral ischemia.

FK506 ameliorates the discrimination learning impairment due to preventing the rarefaction of white matter induced by chronic cerebral hypoperfusion in rats

We examined the effects of the immunosuppressant tacrolimus (FK506) on the discrimination learning impairment induced by chronic cerebral hypoperfusion in rats. Chronic cerebral hypoperfusion was prepared by permanent ligation of bilateral common carotid arteries for male Wistar rats aged 9 weeks. FK506 (0.05 mg/kg, s.c.) recovered the learning impairment and also prevented the rarefaction of white matter and striatal neuronal cell damage. Our findings suggest that FK506 ameliorates the learning impairment mainly due to preventing neuropathological alterations.

Inhibition of cyclic AMP-dependent protein kinase in the acute phase of focal cerebral ischemia in the rat

Binding of cyclic AMP to the regulatory subunit of cyclic AMP-dependent protein kinase is an essential step in cyclic AMP-mediated intracellular signal transduction. In the present study, the binding capacity of cyclic AMP-dependent protein kinase for cyclic AMP was examined by autoradiography with local cerebral blood flow in focal cerebral ischemia in the rat, which was induced by occlusion of the middle cerebral artery using the intraluminal suture method. The binding capacity of cyclic AMP-dependent protein kinase and local cerebral blood flow were assessed by the in vitro [3H]cyclic AMP binding and the [14C]iodoantipyrine methods, respectively. At 3 h of occlusion, a significant reduction in the binding of cyclic AMP-dependent protein kinase to cyclic AMP was already noted in the lateral region of the caudate-putamen and the parietal cortex. Between three and five hours of occlusion, the area with reduced cyclic AMP binding was significantly expanded to the peri-ischemic regions including the frontal cortex and the medial region of the caudate-putamen. The threshold in local cerebral blood flow for reduced cyclic AMP binding was clearly noted at 5 h of ischemia, and was 45 ml/100 g per min in the cerebral cortices, and 38 ml/100 g per min in the caudate-putamen, respectively. No threshold was noted at 3 h of ischemia, since cyclic AMP binding showed a large variation ranging from reduced to normal values even when local cerebral blood flow was below 20 ml/100 g per min. Recirculation for 3.5 h following 1.5 h of ischemia restored the normal cyclic AMP binding in the cerebral cortices, but failed to normalize cyclic AMP binding in the caudate-putamen despite good recovery of local cerebral blood flow. Western blot analysis suggested that this reduction in cyclic AMP binding was not due to loss or degradation of the subunit protein of cyclic AMP-dependent protein kinase, and may therefore have resulted from conformational changes in the protein. A significant increase in cyclic AMP binding was noted after recirculation in the non-ischemic regions such as the frontal and the cingulate cortices on the occluded side and in the contralateral cortices. These data indicate that cyclic AMP-mediated signal transduction in the brain tissue may be very susceptible to ischemic stress, and the region of disrupted signal transduction may expand progressively from the ischemic core to peri-ischemic regions in the acute phase of ischemia. Such impairment of signal transduction may not be restored in the caudate-putamen even when cerebral circulation is fully recovered after short-term ischemia, suggesting that a regional vulnerability to ischemic stress may also exist in cyclic AMP-mediated signal transduction. A significant increase in cyclic AMP binding after recirculation in regions outside of ischemic area may be closely related with the protective mechanisms of brain tissue, since cyclic AMP has been reported to exert various neuroprotective actions.

Neuroprotective FK506 does not alter in vivo nitric oxide production during ischemia and early reperfusion in rats

BACKGROUND AND PURPOSE

Previous studies have demonstrated that the immunosuppressant FK506 provides neuroprotection in experimental brain injury and suggest that this action may be mediated by suppression of neuronal nitric oxide synthase activation that occurs after ischemic depolarization. We sought to determine whether FK506 reduces histological injury after middle cerebral artery occlusion (MCAO) in the rat and whether the neuroprotective effect is mediated via suppression of in vivo nitric oxide (NO) production during ischemia or early reperfusion.

METHODS

Under controlled conditions of normoxia, normocarbia, and normothermia, halothane-anesthetized male Wistar rats were subjected to 2 hours of MCAO by the intraluminal occlusion technique in a blinded, randomized experimental trial. Ipsilateral parietal cortical laser-Doppler flowmetry was monitored throughout ischemia. Animals were randomly assigned to 4 pretreatment groups: intravenous FK506 0.3 mg/kg or 1. 0 mg/kg, vehicle (cremaphor), or an equivalent volume of saline administered 30 minutes before MCAO. Infarction volume was assessed by a triphenyltetrazolium chloride staining at 22 hours of reperfusion. In separate experiments, microdialysis probes were placed bilaterally into the striatum. Rats were perfused with artificial cerebrospinal fluid containing 3 micromol/L [14C]- L-arginine for 3 hours and then subjected to 2 hours of right MCAO. Intravenous 0.3 mg/kg FK506 or cremaphor was given 30 minutes before right MCAO. Right-left differences between [14C]-L-citrulline in the effluent were assumed to reflect differences in NO production.

RESULTS

All values are mean+/-SE. FK506 at 0.3 mg/kg reduced infarction volume in cortex: 40+/-12 mm3 compared with saline (109+/-15 mm3) and cremaphor vehicle (148+/-23) (P<0.05). Striatal infarction was also reduced by low-dose FK506: 16+/-4 mm3 versus 36+/-4 mm3 and 34+/-4 mm3 in saline and vehicle groups, respectively (P<0.05). High-dose treatment reduced infarction volume in cortex (61+/-14 mm3, P<0.05 from saline and vehicle groups) and in striatum (22+/-5 mm3, P<0.05 from saline and vehicle groups). [14C]-L-citrulline recovery via microdialysis was markedly enhanced in ischemic compared with nonischemic striatum. However, ischemia-evoked [14C]-L-citrulline recovery was not different in FK506-treated rats compared with vehicle-treated animals.

CONCLUSIONS

These data demonstrate that FK506 provides robust neuroprotection against transient focal cerebral ischemia in the rat. The mechanism of protection in vivo is not through attenuation of ischemia-evoked NO production during MCAO and early reperfusion.

Binding capacity of FK506 binding protein after 2-hour hemispheric ischemia in gerbil brain

The binding capacity of FK506 binding protein (FKBP) was examined after 2-h hemispheric ischemia in the gerbil brain in order to clarify the precise mechanism of the neuroprotective effects of FK506. Firstly, the FK506 binding was evaluated in vitro in the normal gerbil brain using 1 nM [3H]dihydro-FK506 as a specific ligand. FK506 binding sites were distributed in a rather homogeneous manner, although the greatest binding was noted in the hippocampus CA1. Secondly, Scatchard analysis demonstrated that the binding sites of FK506 could be composed of two components in each brain region. Thirdly, 18 Mongolian gerbils were divided into two groups: an ischemia group (n = 12) and a sham group (n = 6). The right common carotid artery was ligated to induce hemispheric ischemia for 2 h in the ischemia group. The local cerebral blood flow was measured at the end of the experiment by the [14C]iodoantipyrine method. The ligated animals with levels of local cerebral blood flow in the lateral nuclei of the thalamus of less than 50 ml/100 g/min were utilized as the ischemia group (n=6) for further data analysis. No significant differences in FK506 binding between the ischemia and sham groups were observed in any regions. The above data indicate that the binding capacity of FKBP tends to remain normal during 2-h ischemia, suggesting that FK506 may exert its neuroprotective effects through its binding to FKBP in the brain during the early phase of cerebral ischemia.