Long-Term Intranasal Insulin Aspart: A Profile of Gene Expression, Memory, and Insulin Receptors in Aged F344 Rats.

Intranasal insulin is a safe and effective method for ameliorating memory deficits associated with pathological brain aging. However, the impact of different formulations and the duration of treatment on insulin's efficacy and the cellular processes targeted by the treatment remain unclear. Here, we tested whether intranasal insulin aspart, a short-acting insulin formulation, could alleviate memory decline associated with aging and whether long-term treatment affected regulation of insulin receptors and other potential targets. Outcome variables included measures of spatial learning and memory, autoradiography and immunohistochemistry of the insulin receptor, and hippocampal microarray analyses. Aged Fischer 344 rats receiving long-term (3 months) intranasal insulin did not show significant memory enhancement on the Morris water maze task. Autoradiography results showed that long-term treatment reduced insulin binding in the thalamus but not the hippocampus. Results from hippocampal immunofluorescence revealed age-related decreases in insulin immunoreactivity that were partially offset by intranasal administration. Microarray analyses highlighted numerous insulin-sensitive genes, suggesting insulin aspart was able to enter the brain and alter hippocampal RNA expression patterns including those associated with tumor suppression. Our work provides insights into potential mechanisms of intranasal insulin and insulin resistance, and highlights the importance of treatment duration and the brain regions targeted.

A T3R3 hexamer of the human insulin variant B28Asp.

Insulin shows a complex equilibrium between monomers and hexamers, involving varying conformers and association states. We sought to perform a structural characterization of the fast-acting human insulin variant B28Asp ("aspart"). Small-angle X-ray scattering measurements reveal similar globular behavior in both the aspart and regular human insulin, with a Rg of 19Å and a Dmax of approximately 50Å, indicating similar mean quaternary assembly distribution. Crystallographic assays revealed a T3R3 assembly of the aspart insulin formed by the TR dimer in the asymmetric unit, with all the first 8 residues of the B chain in the R-state monomer in helical conformation and the participation of its B3Asn in the stabilization of the hexamer. Our data provide access to novel structural information on aspart insulin such as an aspart insulin dimer in solution, the aspart insulin in T conformation and a pure R-state conformer establishing a T3R3 assembly, providing further insight on the stepwise conformational transition and assembly of this fast-insulin.