The role of inflammation in neurodegeneration: novel insights into the role of the immune system in C9orf72 HRE-mediated ALS/FTD

Abstract

Neuroinflammation is an important hallmark of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). An inflammatory reaction to neuronal injury is deemed vital for neuronal health and homeostasis. However, a continued activation of the inflammatory response can be detrimental to remaining neurons and aggravate the disease process. Apart from a disease modifying role, some evidence suggests that neuroinflammation may also contribute to the upstream cause of the disease. In this review, we will first focus on the role of neuroinflammation in the pathogenesis of chromosome 9 open reading frame 72 gene (C9orf72) hexanucleotide repeat expansions (HRE)-mediated ALS/FTD (C9-ALS/FTD). Additionally, we will discuss evidence from ex vivo and in vivo studies and finally, we briefly summarize the trials and progress of anti-inflammatory therapies.

Keywords: Anti-inflammatory therapies; Astrocytes; C9orf72 HRE-mediated ALS/FTD; In-vivo and ex-vivo models; Microglia; Neuroinflammation; Peripheral immune cells.

Fig. 1

Overview of the different disease mechanisms at play in C9orf72-ALS/FTD. First, the (GGGGCC)n hexanucleotide repeat expansion could reduce transcription of the C9orf72 gene leading to reduced C9orf72 protein levels (haploinsufficiency) that may ultimately cause dysfunction of the autophagy-lysosome pathway. Second, bidirectional transcription of the repeat expansion forms sense (GGGGCC) and antisense (CCCCGG) RNA transcripts that form secondary structures and may cause RNA toxicity by sequestering essential RNA-binding proteins (RBPs). Finally, unconventional repeat-associated non-ATG (RAN) translation produces toxic dipeptide repeat proteins (DPRs): polyGP, polyGR and polyGA from the sense strand and polyGP, polyPR and polyPA from the antisense strand. These toxic DPRs are able to form aggregates and may affect several essential cellular pathways such as mitochondrial function, axonal transport, proteasome function and protein translation. Moreover, these three mechanisms may work separately or in synergy to cause alterations in nuclear-cytoplasmic transport, TDP-43 subcellular localization and stress granule dynamics that are toxic to motor neurons. DPR: dipeptide repeat protein; RAN: repeat associated non-ATG translation; RBP: RNA-binding protein; TDP-43: TAR DNA-binding protein 43

Fig. 2

Major inflammatory pathways. A The JAK-STAT pathway regulates the cellular response to cytokines and growth factors and signals through dimerization of receptors upon binding of these molecules. Upon dimerization, a series of autophosphorylation and transphosphorylation events of the receptors and their associated JAK proteins results in an active STAT dimer that functions as transcription activator. B The MAPK and NF-κB pathway is characterized by a multi-step signaling cascade upon receptor activation eventually resulting in MAPK activation. In this cascade, generally a MAPK kinase kinase (MAP3K) phosphorylates and activates a MAPK kinase (MAP2K) which in turn activates the MAPK. Meanwhile, the MAP3K can also induce NF-κB signaling. Together with the transcription factor activator protein 1 (AP-1) activated by phosphorylation of the MAPK, NF-κB translocates to the nucleus and functions as a transcription factor. C The NLRP3 inflammasome pathway can be initiated by a plethora of different mechanisms that result in activation of the NF-κB pathway. The presence of additional cellular insults will result in the formation of the NLRP3 inflammasome which is able to activate caspase 1. The activated caspase 1 dimer is subsequently able to process gasdermin D, pro-IL-1β and pro-IL-18 into their mature forms causing the formation of a gasdermin D pore resulting in pyroptosis and the release of inflammatory cytokines. AP-1: activator protein 1; ATP: adenosine triphosphate; ASC: apoptosis associated speck-like protein containing a CARD; DAMP: damage-associated molecular pattern; IκBα: nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor α; IKKα/β: Inhibitor of nuclear factor kappa-B kinase subunit alpha α/β; IL: interleukin; IL1-R: interleukin-1 receptor; IRAK1/4: interleukin-1 receptor-associated kinase 1/4; JAK: janus kinase; JNK: c-Jun N-terminal kinase; MAP2Ks: mitogen-activated protein kinase kinases; mtDNA: mitochondrial DNA; mtROS: mitochondrial reactive oxygen species; MyD88: myeloid differentiation primary response 88; NEK7: NIMA-related kinase 7; NEMO: NF-κB essential modulator; NF-κB: nuclear factor kappa-light-chain-enhancer of activated B cells; NLRP3: NLR family pyrin domain containing 3; P2X4/7: ATP-gated P2X receptor cation channel 4/7; p38: p38 mitogen-activated protein kinase; PAMP: pathogen-associated molecular pattern; STAT: signal transducer and activator of transcription; TAB1,2,3: TGF-β activated kinase 1 binding protein 1, 2 or 3; TAK1: TGF-β activated kinase 1; TLR: toll-like receptor; TRAF6: TNF receptor associated factor 6

Fig. 3

Overview of the neuroinflammatory pathways at play in C9-ALS/FTD. Coinciding with the neurodegeneration, astrocytes and microglia undergo a switch towards a neurotoxic and (over)activated state in which they produce and secrete an array of pro-inflammatory factors including cytokines (TNFα, IL-6, IL-12, IL-1β, TGFβ, IFNγ), glutamate, NO and ROS. During disease progression, peripheral immune cells such as T cells, B cells, monocytes and NK cells migrate into the CNS. Upon CNS infiltration, monocytes may differentiate to macrophages that act similar to the CNS-residing microglia. Although an active involvement of T cells in ALS pathophysiology is debated, disease progression has been associated with a disbalance between neuroprotective and neurotoxic T cell populations. Next, NK cells are attracted to the CNS by chemokines and can stimulate neurotoxic glia differentiation or directly be toxic to the neurons. Finally, although their role has never been directly demonstrated, activated B cells may exert toxicity by the secretion of autoimmune antibodies. CCL2: C–C motif chemokine ligand 2; CNS, central nervous system; CX3CL1: CX3 chemokine ligand 1; CXCL10: C-X-C motif ligand 10; IFNγ, interferon γ; IL, interleukin; NK cell: natural killer cell; NO, nitric oxide; ROS, reactive oxygen species; TGFβ, transforming growth factor β; TNFα, tumor necrosis factor α

Fig. 4

Overview of the STING pathway, the involvement of C9orf72 and its relation to C9 ALS/FTD. Cytosolic (mislocalized) TDP-43 species and/or DPR proteins cause mitochondrial damage and lead to the increased production and release of toxic ROS and mitochondrial DNA (mtDNA). cGAS, which is a cytosolic DNA sensor, binds the mtDNA and catalyzes the production of cGAMP which in turn results in stimulator of interferon genes (STING) protein dimerization. STING dimers are able to translocate to the Golgi, where they bind and activate TANK binding kinase 1 (TBK1). This complex will then phosphorylate inhibitor of NF-κBα (IκBα) and interferon regulatory factor 3 (IRF3) and cause both transcription factors to localize to the nucleus and promote the transcription of proinflammatory cytokines and type I interferons. The active STING-TBK1 signalosome is involved in autophagosome formation, an initial step in the process of autophagy. Interestingly, STING is degraded itself by autophagy and thus has an elegant self-regulatory mechanism to control its levels and the activation of the STING-based immune response. Moreover, as C9orf72 is also involved in endolysosomal trafficking and autophagy, reduced levels of the C9orf72 protein may cause a delay or failure of STING degradation and a hyperactivation of the type I interferon response. ATP: adenosine triphosphate; cGAS: cyclic guanosine monophosphate–adenosine monophosphate (cGAMP) synthase; GTP: guanosine triphosphate; IL-6: interleukin 6; IRF3: interferon regulatory factor 3; mtDNA: mitochondrial DNA; NFκB: nuclear factor-κB; OPTN: optineurin; P: phosphorylation; P62: sequestome 1/ ubiquitin-binding protein p62; STING: stimulator of interferon genes; TBK1: TANK binding kinase 1; TDP-43: TAR DNA-binding protein 43; TNF: tumor necrosis factor