Microglia in juvenile neuronal ceroid lipofuscinosis are primed toward a pro-inflammatory phenotype

Juvenile neuronal ceroid lipofuscinosis (JNCL) is a lysosomal storage disease caused by an autosomal recessive mutation in CLN3. Regions of microglial activation precede and predict areas of neuronal loss in JNCL; however, the functional role of activated microglia remains to be defined. The inflammasome is a key molecular pathway for activating pro-IL-1 in microglia, and IL-1 is elevated in the brains of JNCL patients and can induce neuronal cell death. Here, we utilized primary microglia isolated from CLN3(ex7/8) mutant and wild-type (WT) mice to examine the impact of CLN3 mutation on microglial activation and inflammasome function. Treatment with neuronal lysates and ceramide, a lipid intermediate elevated in the JNCL brain, led to inflammasome activation and IL-1 release in CLN3(ex7/8) microglia but not WT cells, as well as increased expression of additional pro-inflammatory mediators. Similar effects were observed following either TNF- or IL-1 treatment, suggesting that CLN3(ex7/8) microglia exist in primed state and hyper-respond to several inflammatory stimuli compared to WT cells. CLN3(ex7/8) microglia displayed constitutive caspase-1 activity that when blocked led to increased glutamate release that coincided with hemichannel opening. Conditioned medium from activated CLN3(ex7/8) or WT microglia induced significant cell death in CLN3(ex7/8) but not WT neurons, demonstrating that intrinsically diseased CLN3(ex7/8) neurons are less equipped to withstand cytotoxic insults generated by activated microglia. Collectively, aberrant microglial activation may contribute to the pathological chain of events leading to neurodegeneration during later stages of JNCL.