The Role of Bid as a positive regulator of mutant SOD1 - induced TLR4 - NF-ҡB signalling in microglia

Amyotrophic Lateral Sclerosis is a debilitating, progressive and fatal neurodegenerative disorder resulting from the degeneration and death of motoneurons in the spinal cord and motor cortex which leads to muscle paralysis and death. Neuroinflammation is prominent in ALS pathogenesis, and other neurodegenerative disorders, and is believed to contribute to disease progression. Mutations in the superoxide dismutase 1 gene (SOD1) account for approximately 12 – 20 % of familial ALS cases. Microglia, the resident immune cells of the CNS, are activated concurrent with neuronal injury and mediate mutant SOD1 – induced neurotoxicity thus exacerbating disease progression. Additionally, mutant SOD1 has been shown to bind to the innate immune receptors Toll – like receptors -2 and -4 (TLR2, TLR4), which are highly expressed on microglia.

The transcription factor nuclear factor ҡB (NF-ҡB) elicits the transcription of multiple pro-inflammatory genes, and its activation is upregulated in ALS pathology. The Bcl-2 family member Bid is demonstrated to promote NF-ҡB activation in astrocytes in response to TLR4 activation, and here it is also identified that Bid promotes TLR4 - NF-ҡB signalling in microglia, and that bid – deficiency attenuates the TLR4 – and mutant SOD1 - induced pro-inflammatory response.

Detailed TLR4 - NF-ҡB pathway analysis identified an interaction between Bid and tumour necrosis factor α receptor adaptor protein 6 (TRAF6) in microglia, with Bid demonstrated to promote TRAF6 polyubiquitination. Further experiments demonstrated that Bid does not affect the activation of the Interleukin 1 receptor - associated kinase (IRAK) complex upon TLR4 activation; however reduced Peli1 protein levels are evident in the absence of Bid in TLR4 – activated microglia.

Collectively the results demonstrate that Bid regulates TLR4 signalling by promoting TRAF6 ubiquitin ligase activity and therefore promotes downstream signalling to NF-ҡB. Depletion of microglial Bid offers a potential therapeutic target for the attenuation of the SOD1 – induced microglial mediated toxicity evident in ALS pathogenesis.