Role of Chitin in Alzheimer’s disease: a new cytotoxic pathway

The pathogenesis of Alzheimer’s disease (AD) is generally attributed to the abnormal production and accumulation of β-amyloid protein, in association with neurofibrillary tangle (NTF) formation. The production and subsequently accumulation of β-amyloid protein in AD brains finally results in direct neuronal toxicity and in microglial activation which, through the production of inflammatory mediators, contributes to neuronal damage. In recent years the scientific community has raised doubts regarding the exclusive pathological role of amyloid. Familiar AD, where amyloid deposition is supposed to play a prevalent pathogenetic role, represents a condition confirming this hypothesis, but the vast majority of AD cases are sporadic and in this condition the scenario is complicated by the possible role of additional components/pathways involved. In fact, a wide range of molecules are present in AD plaques, whose significance has not been clearly characterized. Among these, previous studies have identified chitin, an insoluble polymer of N-acetyl-glucosamine, in close association with β-amyloid in autoptic sporadic AD brains. Chitin was detected by Calcofluor staining both in amyloid plaques and within the cytoplasm of surrounding microglia. The aim of this study was to investigate whether chitin has a pathogenetic role in AD by assessing its biological effects on two important players: neurons and microglia. First of all, we have found chitin deposits only in sporadic AD but not in familiar AD and Down syndrome, emphasizing the complexity of amyloid-related pathology. Then we performed in vitro experiments, in which the exposure of microglial cultures to chitin showed that the cells were able to phagocyte small chitin particles, and the process was significantly inhibited by β-amyloid. Similarly to what described with β-amyloid, phagocytosis of chitin activated microglial cells. In addition, experiments with neuronal cultures clearly showed a significant cytotoxic effect induced by chitin on neurons to levels comparable to β-amyloid. A central point of this research concerned the production of chitin by mammalian cells, which lack chitin synthase. In sporadic AD glucose metabolism is frequently impaired with activation of the exosamine pathway with consequent production of N-acetyl-glucosamine. Previous studies suggested that, under such condition, the absence of a chitin synthesizing enzyme may be overcome by hyaluronan synthase-1 (HAS-1), that has been shown to convert UDP-N-acetyl-glucosamine to chito-oligosaccharides in vitro. We demonstrated that in the presence of UDP-N-acetyl-glucosamine, both microglia and neurons are able to produce chitin-like deposits that HPLC-MS analysis confirmed to be “new-formed” chitin-like compounds. Such treatment leads to activation of microglia as well as significant neuronal cytotoxicity, mimicking the effect of exogenous chitin. Our results indicate that in particular conditions of altered glucose metabolism both microglia and neurons produce chitin-like polymers, which may trigger a neurotoxic effect either by direct neuronal toxicity and by microglia activation. Moreover, preliminary experiments suggest that synaptic transmission is affected in murine hippocampal slice cultures treated with UDP-N-acetyl-glucosamine. Taken together, these results suggest a cytotoxic role of chitin-like molecules in AD and offer new insights in the understanding the complex pathogenesis of AD.