Concentration and proteolysis of CX3CL1 may regulate the microglial response to CX3CL1.

Fractalkine (FKN) is a membrane-bound chemokine that can be cleaved by proteases such as ADAM 10, ADAM 17, and cathepsin S to generate soluble fragments. Studies using different forms of the soluble FKN yield conflicting results in vivo. These observations prompted us to investigate the function and pharmacology of two commonly used isoforms of FKN, a human full-length soluble FKN (sFKN), and a human chemokine domain only FKN (cdFKN). Both are prevalent in the literature and are often assumed to be functionally equivalent. We observed that recombinant sFKN and cdFKN exhibit similar potencies in a cell-based cAMP assay, but binding affinity for CX3CR1 was modestly different. There was a 10-fold difference in potency between sFKN and cdFKN when assessing their ability to stimulate ß-arrestin recruitment. Interestingly, high concentrations of FKN, regardless of cleavage variant, were ineffective at reducing pro-inflammatory microglial activation and may induce a pro-inflammatory response. This effect was observed in mouse and rat primary microglial cells as well as microglial cell lines. The inflammatory response was exacerbated in aged microglia, which is known to exhibit age-related inflammatory phenotypes. We observed the same effects in Cx3cr1-/- primary microglia and therefore speculate that an alternative FKN receptor may exist. Collectively, these data provide greater insights into the function and pharmacology of these common FKN reagents, which may clarify conflicting reports and urge greater caution in the selection of FKN peptides for use in in vitro and in vivo studies and the interpretation of results obtained using these differing peptides.

CX3CL1/CX3CR1 signaling targets for the treatment of neurodegenerative diseases.

Neuroinflammation was initially thought of as a consequence of neurodegenerative disease pathology, but more recently it is becoming clear that it plays a significant role in the development and progression of disease. Thus, neuroinflammation is seen as a realistic and valuable therapeutic target for neurodegeneration. Neuroinflammation can be modulated by neuron-glial signaling through various soluble factors, and one such critical modulator is Fractalkine or C-X3-C Motif Chemokine Ligand 1 (CX3CL1). CX3CL1 is produced in neurons and is a unique chemokine that is initially translated as a transmembrane protein but can be proteolytically processed to generate a soluble chemokine. CX3CL1 has been shown to signal through its sole receptor CX3CR1, which is located on microglial cells within the central nervous system (CNS). Although both the membrane bound and soluble forms of CX3CL1 appear to interact with CX3CR1, they do seem to have different signaling capabilities. It is believed that the predominant function of CX3CL1 within the CNS is to reduce the proinflammatory response and many studies have shown neuroprotective effects. However, in some cases CX3CL1 appears to be promoting neurodegeneration. This review focusses on presenting a comprehensive overview of the complex nature of CX3CL1/CX3CR1 signaling in neurodegeneration and how it may present as a therapeutic in some neurodegenerative diseases but not others. The role of CX3CL1/CXCR1 is reviewed in the context of Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), ischemia, retinopathies, spinal cord and neuropathic pain, traumatic brain injury, amyotrophic lateral sclerosis, multiple sclerosis, and epilepsy.

T cell infiltration and upregulation of MHCII in microglia leads to accelerated neuronal loss in an α-synuclein rat model of Parkinson's disease.

BACKGROUND: Parkinson's disease (PD) is the second most prevalent movement disorder characterized by up to 80% loss of dopamine (DA) neurons and accumulation of Lewy body deposits composed of α-synuclein (α-syn). Accumulation of α-syn is associated with microglial activation, leading to a pro-inflammatory environment linked with the pathogenesis of PD. Along with microglia, CD4 and CD8 T cells are observed in SNpc. The contribution of T-cells to PD development remains unclear with studies demonstrating that they may mediate neurodegeneration or act in a neuroprotective manner. METHODS: Here, we assessed the contribution of T cells to PD neurodegeneration using an adeno-associated virus (AAV) coding human wild-type α-syn or GFP injected into the substantia nigra pars compacta (SNpc) in T cell deficient (athymic nude) and T cell competent (heterozygous) rats. The rats were behaviorally assessed with cylinder test to test paw bias. Following behavior testing, brains were collected and analyzed for markers of dopamine neuron, microglial activation, T cells, and α-syn expression. RESULTS: Injection of AAV9-α-syn unilaterally into the SN of T cell competent rats resulted in a significant paw bias in comparison to the controls at 60 days post-injection. Conversely, T cell-deficient rats injected with AAV9-α-syn showed no deficit in paw bias. As expected, injected T cell competent rats demonstrated a significant increase in microglial activation (MHCII staining) as well as significant dopaminergic neuron loss. In contrast, the T cell-deficient counterparts did not show a significant increase in microglial activation or significant neuron loss compared to the control animals. We also observed CD4 and CD8 T cells in SNpc following microglial MHCII expression and dopaminergic neuron loss. The time course of T cell entry correlates with upregulation of MHCII and the peak loss of TH+ cells in the SNpc. CONCLUSION: These data demonstrate that T cell infiltration and microglial upregulation of MHCII are involved in α-synuclein-mediated DA neuron loss in this rat model of PD.

Two forms of CX3CL1 display differential activity and rescue cognitive deficits in CX3CL1 knockout mice.

BACKGROUND: Fractalkine (CX3CL1; FKN) is a chemokine expressed by neurons that mediates communication between neurons and microglia. By regulating microglial activity, CX3CL1 can mitigate the damaging effects of chronic microglial inflammation within the brain, a state that plays a major role in aging and neurodegeneration. CX3CL1 is present in two forms, a full-length membrane-bound form and a soluble cleaved form (sFKN), generated by a disintegrin and metalloproteinase (ADAM) 10 or 17. Levels of sFKN decrease with aging, which could lead to enhanced inflammation, deficits in synaptic remodeling, and subsequent declines in cognition. Recently, the idea that these two forms of CX3CL1 may display differential activities within the CNS has garnered increased attention, but remains unresolved. METHODS: Here, we assessed the consequences of CX3CL1 knockout (CX3CL1-/-) on cognitive behavior as well as the functional rescue with the two different forms of CX3CL1 in mice. CX3CL1-/- mice were treated with adeno-associated virus (AAV) expressing either green fluorescent protein (GFP), sFKN, or an obligate membrane-bound form of CX3CL1 (mFKN) and then subjected to behavioral testing to assess cognition and motor function. Following behavioral analysis, brains were collected and analyzed for markers of neurogenesis, or prepared for electrophysiology to measure long-term potentiation (LTP) in hippocampal slices. RESULTS: CX3CL1-/- mice showed significant deficits in cognitive tasks for long-term memory and spatial learning and memory in addition to demonstrating enhanced basal motor performance. These alterations correlated with deficits in both hippocampal neurogenesis and LTP. Treatment of CX3CL1-/- mice with AAV-sFKN partially corrected changes in both cognitive and motor function and restored neurogenesis and LTP to levels similar to wild-type animals. Treatment with AAV-mFKN partially restored spatial learning and memory in CX3CL1-/- mice, but did not rescue long-term memory, or neurogenesis. CONCLUSIONS: These results are the first to demonstrate that CX3CL1 knockout causes significant cognitive deficits that can be rescued by treatment with sFKN and only partially rescued with mFKN. This suggests that treatments that restore signaling of soluble forms of CX3CL1 may be a viable therapeutic option for aging and disease.

Orthosteric- versus allosteric-dependent activation of the GABAA receptor requires numerically distinct subunit level rearrangements.

Anaesthetic molecules act on synaptic transmission via the allosteric modulation of ligand-gated chloride channels, such as hetero-oligomeric α1ß2γ2 GABAA receptors. To elucidate the overall activation paradigm via allosteric versus orthosteric sites, we used highly homologous, but homo-oligomeric, ρ1 receptors that are contrastingly insensitive to anaesthetics and respond partially to several full GABA α1ß2γ2 receptor agonists. Here, we coexpressed varying ratios of RNAs encoding the wild-type and the mutated ρ1 subunits, which are anaesthetic-sensitive and respond with full efficacy to partial GABA agonists, to generate distinct ensembles of receptors containing five, four, three, two, one, or zero mutated subunits. Using these experiments, we then demonstrate that, in the pentamer, three anaesthetic-sensitive ρ1 subunits are needed to impart full efficacy to the partial GABA agonists. By contrast, five anaesthetic-sensitive subunits are required for direct activation by anaesthetics alone, and only one anaesthetic-sensitive subunit is sufficient to confer the anaesthetic-dependent potentiation to the GABA current. In conclusion, our data indicate that GABA and anaesthetics holistically activate the GABAA ρ1 receptor through distinct subunit level rearrangements and suggest that in contrast to the global impact of GABA via orthosteric sites, the force of anaesthetics through allosteric sites may not propagate to the neighbouring subunits and, thus, may have only a local and limited effect on the ρ1 GABAA receptor model system.