Inhibition of Urea Transporter (UT)-B Modulates LPS-Induced Inflammatory Responses in BV2 Microglia and N2a Neuroblastoma Cells.

Urea is the major nitrogen-containing product of protein metabolism, and the urea cycle is intrinsically linked to nitric oxide (NO) production via the common substrate L-arginine. Urea accumulates in the brain in neurodegenerative states, including Alzheimer's and Huntington's disease. Urea transporter B (UT-B, SLC14A1) is the primary transport protein for urea in the CNS, identified most abundantly in astrocytes. Moreover, enhanced expression of the Slc14a1 gene has been reported under neurodegenerative conditions. While the role of UT-B in disease pathology remains unclear, UT-B-deficient mice display behavioural impairment coupled with urea accumulation, NO disruption and neuronal loss. Recognising the role of inflammation in neurodegenerative disease pathology, the current short study evaluates the role of UT-B in regulating inflammatory responses. Using the specific inhibitor UTBinh-14, we investigated the impact of UT-B inhibition on LPS-induced changes in BV2 microglia and N2a neuroblastoma cells. We found that UTBinh-14 significantly attenuated LPS-induced production of TNFα and IL-6 from BV2 cells, accompanied by reduced release of NO. While we observed a similar reduction in supernatant concentration of IL-6 from N2a cells, the LPS-stimulated NO release was further augmented by UTBinh-14. These changes were accompanied by a small, but significant downregulation in UT-B expression in both cell types following incubation with LPS, which was not restored by UTBinh-14. Taken together, the current evidence implicates UT-B in regulation of inflammatory responses in microglia and neuronal-like cells. Moreover, our findings offer support for the further investigation of UT-B as a novel therapeutic target for neuroinflammatory conditions.

Bone marrow-derived macrophages from aged rats are more responsive to inflammatory stimuli.

BACKGROUND: Lipopolysaccharide (LPS) and interferon-γ (IFNγ) increase expression of tumour necrosis factor-α (TNFα) that characterizes the M1 activation state of macrophages. Whereas it is accepted that the immune system undergoes changes with age, there is inconsistency in the literature with respect to the impact of age on the response of macrophages to inflammatory stimuli. Here, we investigate the effect of age on the responsiveness of bone marrow-derived macrophages (BMDMs) to LPS and IFNγ. The context for addressing this question is that macrophages, which infiltrate the brain of aged animals, will encounter the neuroinflammatory environment that has been described with age. METHODS: Brain tissue, prepared from young and aged rats, was assessed for expression of inflammatory markers by PCR and for evidence of infiltration of macrophages by flow cytometry. BMDMs were prepared from the long bones of young and aged rats, maintained in culture for 8 days and incubated in the presence or absence of LPS (100 ng/ml) or IFNγ (50 ng/ml). Cells were harvested and assessed for mRNA expression of markers of M1 activation including TNFα and NOS2, or for expression of IFNγR1 and TLR4 by western immunoblotting. To assess whether BMDMs induced glial activation, mixed glial cultures were incubated in the presence of conditioned media obtained from unstimulated BMDMs of young and aged rats and evaluated for expression of inflammatory markers. RESULTS: Markers associated with M1 activation were expressed to a greater extent in BMDMs from aged rats in response to LPS and IFNγ, compared with cells from young rats. The increased responsiveness was associated with increases in IFNγ receptor (IFNγR) and Toll-like receptor 4 (TLR4). The data show that conditioned media from BMDMs of aged rats increased the expression of pro-inflammatory mediators in glial cells. Significantly, there was an age-related increase in macrophage infiltration into the brain, and this was combined with increased expression of IFNγ and the Toll-like receptor 4 agonist, high-mobility group protein B1 (HMGB1). CONCLUSION: Exposure of infiltrating macrophages to the inflammatory microenvironment that develops in the brain with age is likely to contribute to a damaging cascade that negatively impacts neuronal function.

α-TLR2 antibody attenuates the Aß-mediated inflammatory response in microglia through enhanced expression of SIGIRR.

The immunoregulatory function of single-Ig-interleukin-1 related receptor (SIGIRR) is derived from its ability to constrain the inflammatory consequences of interleukin (IL)-1R and toll-like receptor (TLR)4 activation. This role extends to the brain, where SIGIRR deficiency increases the synaptic and cognitive dysfunction associated with IL-1R- and TLR4-mediated signalling. The current study set out to investigate the interaction between SIGIRR and TLR2 in brain tissue and the data demonstrate that the response to the TLR2 agonist, Pam3CysSK4 (Pam3Cys4), is enhanced in glial cells from SIGIRR(-/-) animals. Consistent with the view that ß-amyloid peptide (Aß) signals through activation of TLR2, the data also show that Aß-induced changes are exaggerated in glia from SIGIRR(-/-) animals. We report that microglia, rather than astrocytes, are the primary glial cell expressing both TLR2 and SIGIRR. While Aß increased TLR2 expression, it decreased SIGIRR expression in microglia. This was mimicked by direct activation of TLR2 with Pam3Cys4. We investigated the effect of an anti-TLR2 antibody (αTLR2) on the Aß-induced inflammatory responses and demonstrate that it prevented the expression and release of the pro-inflammatory cytokines TNFα and IL-6 from microglia. In addition, application of αTLR2 alleviated the Aß-mediated impairment in long-term potentiation (LTP) of hippocampal synaptic activity. The protective effects of αTLR2 were accompanied by an up-regulation in SIGIRR expression. We propose that a mechanism involving activation of PI3 kinase/Akt and the transcription factor peroxisome proliferator-activated receptor (PPAR)γ may facilitate this increase in SIGIRR. These findings highlight a novel role of SIGIRR as a negative regulator of TLR2-mediated inflammation in the brain.

Toll-like receptor 3 activation modulates hippocampal network excitability, via glial production of interferon-ß.

The family of toll-like receptors (TLR) plays a major role in innate immunity due to their pathogen-recognition abilities. TLR3 is a sensor for double-stranded RNA, and regulates host-defense responses to several viruses, via the production of type I interferons. Interferon-ß (IFNß) is a primary product of TLR3 activation, and its transcription is elevated in the CNS response to the synthetic TLR3 ligand, polyinosinic-polycytidylic acid (poly(I:C)). Peripheral infections, along with TLR-induced inflammatory mediators, are known to have detrimental effects on brain function, exerting a negative impact on cognition and enhancing seizure susceptibility. In this study, we assessed hippocampal function in vitro, in response to systemic delivery of a TLR3 agonist. Unlike agonists of other TLRs, intraperitoneal injection of poly(I:C) did not adversely affect evoked short- and long-term synaptic plasticity in mouse hippocampal slices. However, sustained and interictal-like spontaneous activity was observed in CA1 pyramidal cells in response to poly(I:C) and this was associated with alterations in the expression of phosphorylated NR2B subunit-containing NMDA receptors and an astrocyte-specific glutamate/aspartate transporter (GLAST) which impact on extracellular glutamate concentration and contribute to the genesis of epileptiform activity. We provide evidence for the production of IFNß from microglia and astrocytes, and using mice deficient in the type I IFN receptor α 1 (IFNAR1), demonstrate that its subsequent activation is likely to underlie the TLR3-mediated modulation of hippocampal excitability.

Regional investigation of UT-B urea transporters in the rat brain.

Recent studies have reported increased levels of urea in the aging brain and various neurological disorders. Additionally, these diseased tissues also have increased expression of the UT-B transporter that regulates urea transport in the brain. However, little is known regarding the actual UT-B protein distribution across the brain in either normal or diseased states. This current study investigated UT-B protein abundance across three regions of the rat brain - anterior, posterior and cerebellum. Endpoint RT-PCR experiments showed that there were no regional differences in UT-B RNA expression (NS, N = 3, ANOVA), whilst Western blotting confirmed no difference in the abundance of a 35 kDa UT-B protein (NS, N = 3-4, ANOVA). In contrast, there was a significant variation in a non-UT-B 100 kDa protein (P < 0.001, N = 3-4, ANOVA), which was also detected by anti-UT-B antibodies. Using the C6 rat astrocyte cell line, Western blot analysis showed that 48-h incubation in either 5 mM or 10 mM significantly increased a 30-45 kDa UT-B protein signal (P < 0.05, N = 3, ANOVA). Furthermore, investigation of compartmentalized C6 protein samples showed the 30-45 kDa signal in the membrane fraction, whilst the 100 kDa non-UT-B signal was predominantly in the cytosolic fraction. Finally, immunolocalization studies gave surprisingly weak detection of rat UT-B, except for strong staining of red blood cells in the cerebellum. In conclusion, this study confirmed that RNA expression and protein abundance of UT-B were equal across all regions of the rat brain, suggesting that urea levels were also similar. However, it also highlighted some of the technical challenges of studying urea transporters at the protein level.

Interleukin-1alpha and HMGB1 mediate hippocampal dysfunction in SIGIRR-deficient mice.

Single-Ig-interleukin-1 related receptor (SIGIRR) is a member of the interleukin (IL)-1/Toll-like receptor (TLR) family. It negatively regulates inflammation, rendering SIGIRR(-/-) mice more susceptible to inflammatory challenge. This susceptibility extends to the brain, where increased responsiveness to lipopolysaccharide has been observed in SIGIRR-deficient mice. While this is likely due to enhanced TLR4-mediated signaling, the functional consequences of these changes have not yet been described. In the current study, we have investigated the impact of SIGIRR deficiency on hippocampal function, and show that novel object recognition, spatial reference memory, and long-term potentiation (LTP) were impaired in SIGIRR(-/-) mice. These changes were accompanied by increased expression of IL-1RI and TLR4, and upregulation of their downstream signaling events, namely IRAK1 (IL-1R-associated kinase 1), c-Jun N-terminal protein kinase (JNK), and nuclear factor κB (NF-κB). The deficit in LTP was attenuated by the endogenous IL-1 receptor antagonist (IL-1ra) and an anti-TLR4 antibody, and also by inhibition of JNK and NF-κB. We propose that IL-1RI is activated by IL-1α and TLR4 is activated by the endogenous agonist, high mobility group box 1 (HMGB1), as we identified enhanced expression of both cytokines in the hippocampus of SIGIRR(-/-) mice. Additionally, application of HMGB1 increased the activation of JNK and NF-κB and was found to be detrimental to LTP in a TLR4-dependent manner. These findings highlight the functional role of SIGIRR in regulating inflammatory-mediated synaptic and cognitive decline, and describe evidence of the key role of HMGB1 in this process.

Long term potentiation is impaired in membrane glycoprotein CD200-deficient mice: a role for Toll-like receptor activation.

The membrane glycoprotein CD200 is expressed on several cell types, including neurons, whereas expression of its receptor, CD200R, is restricted principally to cells of the myeloid lineage, including microglia. The interaction between CD200 and CD200R maintains microglia and macrophages in a quiescent state; therefore, CD200-deficient mice express an inflammatory phenotype exhibiting increased macrophage or microglial activation in models of arthritis, encephalitis, and uveoretinitis. Here, we report that lipopolysaccharide (LPS) and Pam(3)CysSerLys(4) exerted more profound effects on release of the proinflammatory cytokines, interleukin (IL)-1ß, IL-6, and tumor necrosis factor-α (TNFα), in glia prepared from CD200(-/-) mice compared with wild type mice. This effect is explained by the loss of CD200 on astrocytes, which modulates microglial activation. Expression of Toll-like receptors 4 and 2 (TLR4 and -2) was increased in glia prepared from CD200(-/-) mice, and the evidence indicates that microglial activation, assessed by the increased numbers of CD11b(+) cells that stained positively for both MHCII and CD40, was enhanced in CD200(-/-) mice compared with wild type mice. These neuroinflammatory changes were associated with impaired long term potentiation (LTP) in CA1 of hippocampal slices prepared from CD200(-/-) mice. One possible explanation for this is the increase in TNFα in hippocampal tissue prepared from CD200(-/-) mice because TNFα application inhibited LTP in CA1. Significantly, LPS and Pam(3)CysSerLys(4), at concentrations that did not affect LTP in wild type mice, inhibited LTP in slices prepared from CD200(-/-) mice, probably due to the accompanying increase in TLR2 and TLR4. Thus, the neuroinflammatory changes that result from CD200 deficiency have a negative impact on synaptic plasticity.

Dok2 mediates the CD200Fc attenuation of Aß-induced changes in glia.

BACKGROUND: The interaction between the membrane glycoprotein, CD200 and its cognate receptor CD200 receptor (CD200R), has been shown to play a role in maintaining microglia in a quiescent state. There is evidence of increased activation under resting and stimulated conditions in microglia prepared from CD200-deficient mice compared with wild-type mice, whereas activation of the receptor by CD200 fusion protein (CD200Fc) ameliorates inflammatory changes which are evident in the central nervous system (CNS) of the mouse model of multiple sclerosis (MS), experimental autoimmune encephalomyelitis (EAE) and also in the hippocampus of aged rats. Additionally, an inverse relationship between microglial activation and expression of CD200 has been observed in animals treated with lipopolysaccharide (LPS) or amyloid-ß (Aß). METHODS: We assessed the effect of CD200R activation by CD200Fc on Aß-induced production of the pro-inflammatory cytokines, interleukin-1ß (IL-1ß) and tumor necrosis factor-α (TNFα) and the expression of microglial activation markers, CD68 and CD40 in cultured glia. The role played by downstream of tyrosine kinase 2 (Dok2) phosphorylation in mediating the effects of CD200R activation was evaluated by siRNA knockdown of Dok2. To further examine the impact of inflammatory changes on synaptic plasticity, the effect of CD200Fc on Aß-induced impairment of long-term potentiation (LTP) in the CA1 region of hippocampal slices was also investigated. RESULTS: We demonstrate that Aß-induced increases in IL-1ß, TNFα, CD68 and CD40 were inhibited by CD200Fc. The evidence suggests that Dok2 phosphorylation is a key factor in mediating the effect of CD200Fc, since Dok2 knockdown by siRNA abrogated its effects on microglial activation and inflammatory cytokine production. Consistent with evidence that inflammatory changes negatively impact on LTP, we show that the Aß-induced impairment of LTP was attenuated by CD200Fc. CONCLUSIONS: The findings suggest that activation of CD200R and Dok2 is a valuable strategy for modulating microglial activation and may have therapeutic potential in neurodegenerative conditions.

Palmitic Acid and Oleic Acid Differently Modulate TLR2-Mediated Inflammatory Responses in Microglia and Macrophages.

The relationship between systemic immunity and neuroinflammation is widely recognised. Infiltration of peripheral immune cells to the CNS during certain chronic inflammatory states contributes significantly to neuropathology. Obesity and its co-morbidities are primary risk factors for neuroinflammatory and neurodegenerative conditions, including Alzheimer's disease (AD). Dietary fats are among the most proinflammatory components of the obesogenic diet and play a prominent role in the low-grade systemic inflammation associated with the obese state. Saturated fatty acid (SFA) is largely implicated in the negative consequences of obesity, while the health benefits of monounsaturated fatty acid (MUFA) are widely acknowledged. The current study sought to explore whether SFA and MUFA differently modulate inflammatory responses in the brain, compared with peripheral immune cells. Moreover, we assessed the neuroinflammatory impact of high-fat-induced obesity and hypothesised that a MUFA-rich diet might mitigate inflammation despite obesogenic conditions. Toll-like receptor (TLR)2 mediates the inflammation associated with both obesity and AD. Using the TLR2 agonist lipoteichoic acid (LTA), we report that pre-exposure to either palmitic acid (PA) or oleic acid (OA) attenuated cytokine secretion from microglia, but heightened sensitivity to nitric oxide (NO) production. The reduction in cytokine secretion was mirrored in LTA-stimulated macrophages following exposure to PA only, while effects on NO were restricted to OA, highlighting important cell-specific differences. An obesogenic diet over 12 weeks did not induce prominent inflammatory changes in either cortex or hippocampus, irrespective of fat composition. However, we reveal a clear disparity in the effects of MUFA under obesogenic and non-obesogenic conditions.

SIGIRR modulates the inflammatory response in the brain.

One of the more recently described members of the interleukin-1 (IL-1) receptor family, single-Ig-interleukin-1 related receptor (SIGIRR), has been identified as a negative regulator of inflammation in several tissues. It modulates the responses triggered by stimulation of Toll-like receptor (TLR) 4 and IL-1 in several peripheral cell types, possibly in an NFkappaB-dependent manner. Consistently, responses to lipopolysaccharide (LPS) are exaggerated in SIGIRR-deficient mice and the symptoms of experimental inflammatory conditions are more profound in these animals. Here, we set out to establish whether the absence of SIGIRR was associated with inflammatory changes in the brain and report that, LPS-induced a greater effect on CD40 and ICAM mRNA in mixed glia prepared from SIGIRR(-/-), compared with wildtype mice. This was associated with parallel changes in TNFalpha and IL-6 at mRNA and protein levels, an effect which was observed in purified microglia but not astrocytes. Similarly, LPS exerted a more profound effect on microglial activation and cytokine production in hippocampal tissue prepared from SIGIRR(-/-), compared with wildtype mice. The effect of LPS on exploratory behaviour was also accentuated in SIGIRR(-/-) mice. The evidence suggests that these changes are a likely consequence of increased hippocampal expression of CD14 and TLR4, and NFkappaB activation in SIGIRR(-/-) mice.

Characterising lipoteichoic acid as an in vitro model of acute neuroinflammation.

Toll-like receptor 2 (TLR2) is a primary sensor for pathogens, including those derived from gram-positive bacteria. It can also mediate the effects of endogenous inflammatory signals such as ß-amyloid peptide (Aß), thus promoting the microglial activation and subsequent neuronal dysfunction, characteristic of chronic neuroinflammatory conditions. More recently, a role for TLR2 has been proposed in the pathogenesis of disorders associated with acute inflammation, including anxiety and depression. The current study aims to characterise the acute effects of the TLR2 agonist lipoteichoic acid (LTA) on microglial activation and neuronal integrity, and to evaluate the influence of LTA exposure on sensitivity to the inflammation and neuronal dysfunction associated with Aß. Using BV2 and N2a cells as an in vitro model, we highlight that acute exposure to LTA robustly promotes inflammatory cytokine and nitric oxide (NO) production in microglia but also in neurons, similar to that reported under longer-term and chronic inflammatory conditions. Moreover, we find that exposure to LTA can enhance sensitivity to subthreshold Aß, promoting an 'M1'-like phenotype in microglia and provoking dysregulation of neuronal activity in acute hippocampal slices. Anti-inflammatory agents, including mimetics of brain-derived neurotrophic factor (BDNF), have proven effective at alleviating chronic neuroinflammatory complications. We further examined the effects of 7,8,3-trihydroxyflavone (7,8,3-THF), a small-molecule TrkB agonist, on LTA-induced microglial activation. We report that 7,8,3-THF can significantly ameliorate interleukin (IL)-6 and NO production in LTA-stimulated BV2 cells. Taken together, our findings offer support for exploration of TLR2 as a potential target for therapeutic intervention into acute neuroinflammatory conditions. Moreover we propose that exposure to gram-positive bacterial pathogens may promote sensitivity to the inflammatory changes characteristic of the aged brain.

Iron accumulation in microglia triggers a cascade of events that leads to altered metabolism and compromised function in APP/PS1 mice.

Among the changes that typify Alzheimer's disease (AD) are neuroinflammation and microglial activation, amyloid deposition perhaps resulting from compromised microglial function and iron accumulation. Data from Genome Wide Association Studies (GWAS) identified a number of gene variants that endow a significant risk of developing AD and several of these encode proteins expressed in microglia and proteins that are implicated in the immune response. This suggests that neuroinflammation and the accompanying microglial activation are likely to contribute to the pathogenesis of the disease. The trigger(s) leading to these changes remain to be identified. In this study, we set out to examine the link between the inflammatory, metabolic and iron-retentive signature of microglia in vitro and in transgenic mice that overexpress the amyloid precursor protein (APP) and presenilin 1 (PS1; APP/PS1 mice), a commonly used animal model of AD. Stimulation of cultured microglia with interferon (IFN)γ and amyloid-ß (Aß) induced an inflammatory phenotype and switched the metabolic profile and iron handling of microglia so that the cells became glycolytic and iron retentive, and the phagocytic and chemotactic function of the cells was reduced. Analysis of APP/PS1 mice by magnetic resonance imaging (MRI) revealed genotype-related hypointense areas in the hippocampus consistent with iron deposition, and immunohistochemical analysis indicated that the iron accumulated in microglia, particularly in microglia that decorated Aß deposits. Isolated microglia prepared from APP/PS1 mice were characterized by a switch to a glycolytic and iron-retentive phenotype and phagocytosis of Aß was reduced in these cells. This evidence suggests that the switch to glycolysis in microglia may kick-start a cascade of events that ultimately leads to microglial dysfunction and Aß accumulation.

The age-related neuroinflammatory environment promotes macrophage activation, which negatively impacts synaptic function.

The impact of infiltration of macrophages into the brain is debatable with evidence of both beneficial and detrimental effects. Recent work suggests that inflammatory macrophages, with an inflammatory phenotype that resembles the M1 activation state, may be detrimental, whereas anti-inflammatory M2-like macrophages may be beneficial. We set up a model to examine the response of bone marrow-derived macrophages to the inflammatory milieu that occurs in the aged brain. Expression of MHCII and CD40 was increased in macrophages incubated with soluble brain extract prepared from aged, compared with young, mice and this was accompanied by increased production of tumor necrosis factor-α and interleukin-6. Analysis of soluble brain extract indicated that it contained increased concentrations of several inflammatory mediators and, importantly, when bone marrow-derived macrophages were incubated in the inflammatory cytokines that were increased and applied to hippocampal slices, long-term potentiation was inhibited. The data suggest that infiltrating macrophages respond to local conditions and, in the case of aging, adopt an inflammatory phenotype that ultimately has a neurodetrimental effect.

Agonists of peroxisome proliferator-activated receptor-gamma attenuate the Abeta-mediated impairment of LTP in the hippocampus in vitro.

The data we present here suggest that agonists of peroxisome proliferator-activated receptor-gamma (PPARgamma) can attenuate the effects of beta-amyloid peptide (Abeta). Alzheimer's disease is associated with elevated levels of Abeta, and enhanced expression of PPARgamma. In this study, we determined that application of Abeta([1-40]) could impair hippocampal post-tetanic potentiation (PTP) and long-term potentiation (LTP) in vitro. We investigated the effects of PPARgamma agonists; troglitazone, ciglitazone and 15-deoxy-delta(12,14) prostaglandin J2 (PGJ2) on synaptic transmission and plasticity in area CA1. Both ciglitazone and PGJ2 increased baseline synaptic transmission significantly, without altering paired-pulse facilitation. PGJ2 produced a significant reduction in LTP, whereas ciglitazone and troglitazone had no significant effect. In addition, prior application of each ligand attenuated the previously observed Abeta([1-40])-mediated impairment of LTP. The effect of troglitazone on the Abeta([1-40])-mediated impairment of LTP was not reversed by the PPARgamma antagonist, GW-9662. These findings demonstrate that PPARgamma agonists attenuate the effects of Abeta on LTP, and support the potential use of these agents to alleviate the symptoms of Alzheimer's disease. We also suggest that PPARgamma agonists may regulate expression of hippocampal LTP in vitro.

The role of c-Jun N-terminal kinase in the A beta-mediated impairment of LTP and regulation of synaptic transmission in the hippocampus.

The effects of the beta-amyloid peptide (Abeta) fragment 25-35 were investigated on hippocampal synaptic transmission and long-term potentiation (LTP) in vitro. Abeta([25-35]) was found to impair both post-tetanic potentiation (PTP) and LTP in the hippocampal CA1. The anthra[1,9-cd]pyrazol-6(2H)-one, SP600125, was used to inhibit c-Jun N-terminal kinase (JNK) activity, which is believed to mediate cell death. Prior application of SP600125 attenuated the Abeta([25-35])-mediated impairment of PTP and LTP, when measured from the pre-drug baseline. In the presence of SP600125 alone, we observed an increase in baseline synaptic transmission and reduction in paired-pulse facilitation, consistent with an increase in synaptic transmission. There was no alteration in the level of PTP and LTP obtained, when measured from the pre-drug baseline. In the presence of both SP600125 and Abeta, however, PTP was greatly enhanced compared with controls. We therefore suggest that the activation of the JNK signalling pathway mediates the effects of Abeta on synaptic plasticity. Our data also indicate that endogenous JNK activity may regulate neurotransmitter release in the hippocampal CA1 in vitro.

Glial Activation in AßPP/PS1 Mice is Associated with Infiltration of IFNγ-Producing Cells.

Whereas the classical histological hallmarks of Alzheimer's disease (AD) are deposition of amyloid-containing plaques and development of neurofibrillary tangles, there is also clear evidence of inflammatory changes accompanied by the presence of activated microglia and astrocytosis. However, at this time, it remains uncertain whether inflammatory changes contribute to pathogenesis of the disease or if they are secondary to deposition of amyloid-ß or other pathological changes. A greater understanding of the sequence of events would clearly improve development of strategies to delay progression of the disease. There is a realistic expectation that advances in imaging technology may provide the key to uncovering this sequence. In this study, we employed non-invasive imaging techniques to examine changes in tissue state in hippocampus and cortex of transgenic mice which overexpress amyloid-ß protein precursor and presenilin 1 and show that the observed increase in T1 relaxation time was associated with astrogliosis while the decrease in T2 relaxation time was associated with microglial activation. We explored the possibility that interferon-γ might trigger glial activation and demonstrate a genotype-related infiltration of macrophages and natural killer cells, which release interferon-γ. The evidence suggests that IFNγ triggers glial activation and expression of proinflammatory cytokines, and these changes, in turn, contribute to the decrease in long-term potentiation.

Brain deletion of insulin receptor substrate 2 disrupts hippocampal synaptic plasticity and metaplasticity.

OBJECTIVE: Diabetes mellitus is associated with cognitive deficits and an increased risk of dementia, particularly in the elderly. These deficits and the corresponding neurophysiological structural and functional alterations are linked to both metabolic and vascular changes, related to chronic hyperglycaemia, but probably also defects in insulin action in the brain. To elucidate the specific role of brain insulin signalling in neuronal functions that are relevant for cognitive processes we have investigated the behaviour of neurons and synaptic plasticity in the hippocampus of mice lacking the insulin receptor substrate protein 2 (IRS-2). RESEARCH DESIGN AND METHODS: To study neuronal function and synaptic plasticity in the absence of confounding factors such as hyperglycaemia, we used a mouse model with a central nervous system- (CNS)-restricted deletion of IRS-2 (NesCreIrs2KO). RESULTS: We report a deficit in NMDA receptor-dependent synaptic plasticity in the hippocampus of NesCreIrs2KO mice, with a concomitant loss of metaplasticity, the modulation of synaptic plasticity by the previous activity of a synapse. These plasticity changes are associated with reduced basal phosphorylation of the NMDA receptor subunit NR1 and of downstream targets of the PI3K pathway, the protein kinases Akt and GSK-3ß. CONCLUSIONS: These findings reveal molecular and cellular mechanisms that might underlie cognitive deficits linked to specific defects of neuronal insulin signalling.