Hala Kazi

While a potential causal factor in Alzheimer's disease (AD), brain insulin resistance has not been demonstrated directly in that disorder. We provide such a demonstration here by showing that the hippocampal formation (HF) and, to a lesser degree, the cerebellar cortex in AD cases without diabetes exhibit markedly reduced responses to insulin signaling in the IR→IRS-1→PI3K signaling pathway with greatly reduced responses to IGF-1 in the IGF-1R→IRS-2→PI3K signaling pathway. Reduced insulin responses were maximal at the level of IRS-1 and were consistently associated with basal elevations in IRS-1 phosphorylated at serine 616 (IRS-1 pS⁶¹⁶) and IRS-1 pS⁶³⁶/⁶³⁹. In the HF, these candidate biomarkers of brain insulin resistance increased commonly and progressively from normal cases to mild cognitively impaired cases to AD cases regardless of diabetes or APOE ε4 status. Levels of IRS-1 pS⁶¹⁶ and IRS-1 pS⁶³⁶/⁶³⁹ and their activated kinases correlated positively with those of oligomeric Aβ plaques and were negatively associated with episodic and working memory, even after adjusting for Aβ plaques, neurofibrillary tangles, and APOE ε4. Brain insulin resistance thus appears to be an early and common feature of AD, a phenomenon accompanied by IGF-1 resistance and closely associated with IRS-1 dysfunction potentially triggered by Aβ oligomers and yet promoting cognitive decline independent of classic AD pathology.

Defective brain insulin signaling has been suggested to contribute to the cognitive deficits in patients with Alzheimer's disease (AD). Although a connection between AD and diabetes has been suggested, a major unknown is the mechanism(s) by which insulin resistance in the brain arises in individuals with AD. Here, we show that serine phosphorylation of IRS-1 (IRS-1pSer) is common to both diseases. Brain tissue from humans with AD had elevated levels of IRS-1pSer and activated JNK, analogous to what occurs in peripheral tissue in patients with diabetes. We found that amyloid-β peptide (Aβ) oligomers, synaptotoxins that accumulate in the brains of AD patients, activated the JNK/TNF-α pathway, induced IRS-1 phosphorylation at multiple serine residues, and inhibited physiological IRS-1pTyr in mature cultured hippocampal neurons. Impaired IRS-1 signaling was also present in the hippocampi of Tg mice with a brain condition that models AD. Importantly, intracerebroventricular injection of Aβ oligomers triggered hippocampal IRS-1pSer and JNK activation in cynomolgus monkeys. The oligomer-induced neuronal pathologies observed in vitro, including impaired axonal transport, were prevented by exposure to exendin-4 (exenatide), an anti-diabetes agent. In Tg mice, exendin-4 decreased levels of hippocampal IRS-1pSer and activated JNK and improved behavioral measures of cognition. By establishing molecular links between the dysregulated insulin signaling in AD and diabetes, our results open avenues for the investigation of new therapeutics in AD.

OBJECTIVE: Olfactory dysfunction is common in Alzheimer disease (AD) and other neurodegenerative diseases. Paired helical filament (PHF)-tau, alpha-synuclein, and amyloid-beta lesions occur early and severely in cerebral regions of the olfactory system, and they have also been observed in olfactory epithelium (OE). However, their frequency, abundance, and disease specificity, and the relationships of OE pathology to brain pathology have not been established. METHODS: We investigated the pathological expression of amyloid-beta, PHFtau, alpha-synuclein, and TDP-43 in postmortem OE of 79 cases with AD, 63 cases with various other neurodegenerative diseases, and 45 neuropathologically normal cases. RESULTS: Amyloid-beta was present as punctate and small patchy aggregates in 71% of AD cases, compared with 22% of normal cases and 14% of cases with other diseases, and in greater amounts in AD than in either of the other 2 diagnostic categories. PHFtau was evident in dystrophic neurites in 55% of cases with AD, 34% with normal brains, and 39% with other neurodegenerative diseases, also at higher densities in AD. alpha-Synuclein was present in dystrophic neurites in 7 cases, 6 of which also had cerebral Lewy bodies. Pathological TDP-43 inclusions were not observed in the OE in any cases. Amyloid-beta and to a lesser degree, PHFtau ratings in OE significantly correlated with cortical Abeta and PHFtau lesion ratings in the brain. INTERPRETATION: These data demonstrate that AD pathology in the OE is present in the majority of cases with pathologically verified AD and correlates with brain pathology. Future work may assess the utility of amyloid-beta and PHFtau measurement in OE as a biomarker for AD.