Neuronal Necroptosis Drives Neuroinflammation and Cognitive Decline Independent of Neuronal Cell Death

Markers of necroptosis increase in neurons with neurodegenerative diseases and aging. Using a novel knockin mouse model that overexpresses the terminal necroptosis effector gene MLKL specifically in neurons (nMlkl-KI), we studied the impact of inducing necroptosis in neurons of young/adult mice on cognition. At 6-months of age, nMlkl-KI mice exhibited a 7-fold and 3-fold increase in MLKL monomer expression in the cortex and hippocampus, respectively. Correspondingly, MLKL-oligomer levels increased 3- to 4-fold in these brain regions, indicating necroptosis activation. The increased necroptosis was associated with an induction of neuroinflammation as shown by an increase in transcript levels of inflammatory markers and increased Iba-1 expression in the cortex and hippocampus. At 12-months of age, nMlkl-KI mice exhibited significant cognitive impairment compared to control mice as measured by a continuous home-cage discrimination learning with the Noldus PhenoTyper. For example, cumulative learning index during the reversal phase and cognitive flexibility were dramatically reduced in nMlkl-KI mice as compared to the control mice. Unbiased stereological analysis revealed no loss in neuronal number in the cortex and hippocampus, suggesting neuronal dysfunction rather than neuronal death was responsible for the reduced cognition observed in the nMlkl-KI mice. Transcriptomic analysis of the cortex revealed an upregulation of pathways associated with age-related neurodegenerative diseases (e.g., Parkinson's, Alzheimer's, Huntington's) as well as chemokine and TNF signaling in the nMlkl-KI mice. In contrast, the neuroactive ligand-receptor interaction pathway was downregulated. Collectively, these data show for the first time that the overexpression of MLKL in neurons leads to a loss in cognition in the absence of neuronal cell death, demonstrating that increased MLKL can interfere with neuronal functions involved in cognition.