Transient IKK2 activation in astrocytes initiates selective non-cell-autonomous neurodegeneration

Abstract

BackgroundNeuroinflammation is associated with a wide range of neurodegenerative disorders, however the specific contribution to individual disease pathogenesis and selective neuronal cell death is not well understood. Inflammatory cerebellar ataxias are neurodegenerative diseases occurring in various autoimmune/inflammatory conditions, e.g. paraneoplastic syndromes. However, how inflammatory insults can cause selective cerebellar neurodegeneration in the context of these diseases remains open, and appropriate animal models are lacking. A key regulator of neuroinflammatory processes is the NF-κB signalling pathway, which is activated by the IκB kinase 2 (IKK2) in response to various pathological conditions. Importantly, its activation is sufficient to initiate neuroinflammation on its own.MethodsTo investigate the contribution of IKK/NF-κB-mediated neuroinflammation to neurodegeneration, we established conditional mouse models of cerebellar neuroinflammation, which depend either on the tetracycline-regulated expression of IKK2 in astrocytes or Cre-recombination based IKK2 activation in Bergmann glia.ResultsWe demonstrate that IKK2 activation for a limited time interval in astrocytes is sufficient to induce neuroinflammation, astrogliosis and loss of Purkinje neurons, resembling the pathogenesis of inflammatory cerebellar ataxias. We identified IKK2-driven irreversible dysfunction of Bergmann glia as critical pathogenic event resulting in Purkinje cell loss. This was independent of Lipocalin 2, an acute phase protein secreted by reactive astrocytes and well known to mediate neurotoxicity. Instead, downregulation of the glutamate transporters EAAT1 and EAAT2 and ultrastructural alterations suggest an excitotoxic mechanism of Purkinje cell degeneration.ConclusionsOur results suggest a novel pathogenic mechanism how diverse inflammatory insults can cause inflammation/autoimmune-associated cerebellar ataxias. Disease-mediated elevation of danger signals like TLR ligands and inflammatory cytokines in the cerebellum activates IKK2/NF-κB signalling in astrocytes, which as a consequence triggers astrogliosis-like activation of Bergmann glia and subsequent non-cell-autonomous Purkinje cell degeneration. Notably, the identified hit and run mechanism indicates only an early window for therapeutic interventions.Electronic supplementary materialThe online version of this article (doi:10.1186/s13024-017-0157-0) contains supplementary material, which is available to authorized users.

Questions and (some) answers on reactive astrocytes

“…For example, JAK-STAT3 pathway activation in astrocytes induces many reactive markers and is sufficient to alter synaptic transmission and plasticity in the hippocampus (Ceyzériat et al, 2018). Activation of the NF-κB pathway by expression of a constitutive form of the upstream IκB kinase in adult astrocytes also induces a reactive profile and Purkinje cell loss in the cerebellum (Lattke et al, 2017;Oeckl, Lattke, Wirth, Baumann, & Ferger, 2012). Such approaches are useful to isolate functional changes in reactive astrocytes, independently of a specific disease condition.…”

Signaling Pathways Regulating the Pathophysiological Responses of Astrocytes: A Focus on the IKK/NF-κB System

“…Remarkably, both in the developing, early postnatal brain and in the adult brain, the selective activation of astroglial NF-κB in this model was sufficient to initiate and maintain a prominent global neuroinflammatory response [57,58,64]. This inflammatory response was characterized by strong astrogliosis and microgliosis, as well as a prominent infiltration of innate immune cells, and, in adult animals, also T cells [57,58,64]. On a molecular level, in particular the chemokines CCL2(MCP-1), CCL5(Rantes) and CXCL10(IP-10), the cell adhesion molecule Madcam1, the MHC class II protein CD74 and some acute phase effector genes (Lcn2, C3) were highly upregulated by astroglial IKK/NF-κB activation [57,58].…”

“…Vice versa, to genetically activate IKK/NF-κB signaling, we generated a mouse model allowing the inducible overexpression of a constitutively active mutant of IKK2 specifically in astrocytes (GFAP.tTA/tetO.IKK2-CA) [57]. Using a so-called Tet-off conditional expression system, the expression of this transgene is driven by the GFAP promoter and can be reversibly repressed by the administration of doxycycline, allowing both constitutive and timecontrolled transient activation of astroglial IKK/NF-κB signaling [57,58].…”

“…To address these questions, we generated the GFAP.tTA/tetO.IKK2-CA model, which allows astroglial NF-κB activation in normal physiological conditions, that is, in the absence of an external pathogenic trigger that could induce neuroinflammation [57]. Remarkably, both in the developing, early postnatal brain and in the adult brain, the selective activation of astroglial NF-κB in this model was sufficient to initiate and maintain a prominent global neuroinflammatory response [57,58,64]. This inflammatory response was characterized by strong astrogliosis and microgliosis, as well as a prominent infiltration of innate immune cells, and, in adult animals, also T cells [57,58,64].…”

Short and long TNF‐alpha exposure recapitulates canonical astrogliosis events in human‐induced pluripotent stem cells‐derived astrocytes

“…Astrocytes are prompt to react after inflammatory insults, experiencing a continuum of temporally ordered physiological changes that culminates in a process called astrogliosis. Initially, mechanical or pathological injuries in the CNS trigger NF‐kB signaling in astrocytes leading to an increased production of NF‐kB‐dependent cytokines, which potentially activate these cells (Lattke et al, ; Saggu et al, ). Once activated, astrocytes show progressive upregulation of intermediate filaments GFAP and Vimentin (Liu et al, ) and heterogeneous degrees of cell hypertrophy (Kang, Lee, Han, Choi, & Song, ).…”