. Protection of synapses against Alzheimer's-linked toxins: insulin signaling prevents the pathogenic binding of Abeta oligomers. Proc Natl Acad Sci U S A. 2009 Feb 10;106(6):1971-6. PubMed.

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  1. Insulin as a Potential Therapeutic Agent in Alzheimer Disease
    The belief that insulin has little if any role in brain function is outdated. There is now abundant evidence that insulin plays important roles in the brain, including synaptic diverse roles affecting cognition (van der Heide, 2006). Definitive proof for insulin generation by adult neurons is still lacking, but it is well established that pancreatic insulin is actively taken up across the blood-brain barrier and that relatively high levels of insulin receptors occur in various forebrain areas, including the hippocampal formation.

    The new paper by De Felice et al. in PNAS suggests that insulin may serve a protective function in Alzheimer disease. The authors demonstrate that insulin can protect cultured hippocampal neurons from deleterious effects of amyloid-β-derived diffusible ligands or ADDLs. They specifically show that insulin and, where tested, the insulin sensitizing drug rosiglitazone can impair or even prevent neuronal binding of ADDLs, dendritic spine loss, dendritic insulin receptor (IR) internalization, and oxidative stress. This raises the prospect of a new therapeutic strategy for retarding the progression of Alzheimer disease.

    Several questions are nevertheless raised by this report. The most basic is whether the effects are exclusively the result of effects on the IR. The effects reported were achieved with 100 nM and 1 uM insulin, which is far higher than the 1-5 nM concentrations at which insulin selectively activates the IR (Li et al., 2005). Even at 10 nM, insulin can activate insulin-like growth factor 1 (IGF-1) receptors (Li et al., 2005; Denley et al., 2007). Since IGF-1 receptors are abundant in the hippocampal formation (Doré et al., 1997) and in cultured hippocampal neurons (Doré et al., 1997), it remains possible that both IRs and/or IGF-1Rs mediate the neuroprotective effects reported by De Felice et al.

    Another question is whether the ADDL binding sites are IRs. This difficult question has been addressed previously by the same investigators (Zhao et al., 2008). Arguing against the idea are colocalization studies showing that all neurons in hippocampal cultures have abundant IRs, but only a lesser number of neurons binding ADDLs. This is evident in Fig. 1F of the De Felice paper, which shows at least one neuron with IRs but no ADDL binding in contrast to adjacent neurons. More importantly, co-immunoprecipitation studies of Zhao et al. (2008) established only that IRs are closely associated with ADDL binding sites. While their work also showed that ADDLs can bind isolated IRs, this occurred only under insulin stimulation. Similarly, De Felice et al. found that ADDL binding can be abolished simply by inhibiting the protein kinase activity of the IR’s intracellular segment with AG1024, which should not affect the extracellular alpha chain's ability to bind ADDL. So there is good reason to think that ADDLs do not directly bind IRs even though they may bind membrane receptors very close to them.

    What, then, are the ADDL binding sites? Could they be NMDA receptors? Despite some seemingly supportive evidence, this also proves unlikely. ADDLs promote rapid decreases in cell surface expression of not only IRs, but also NMDARs (Lacor et al., 2007). Moreover, the Zhao et al. (2008) showed that the effect of ADDLs on IRs was blocked by NMDAR antagonists. This raised the possibility that NMDARs may be binding sites of ADDLs, consistent with the finding that pretreatment of hippocampal cultures with antibodies to the extracellular domain of NMDARs reduces both ADDL binding and generation of ADDL-induced reactive oxygen species (Klein et al., chapter in Memories: Molecules and Circuits [B. Bontempi et al., eds.], 2007). Yet direct evidence for NMDARs as ADDL binding sites has not been obtained, and Zhao et al. (2008) note that after ADDL treatment dendritic loss of IRs occurs more quickly than that of NMDARs. Consequently, the results presented by De Felice et al. cannot be attributed entirely to an effect of ADDLs on NMDARs.

    How does ADDL binding promote IR internalization? Insulin binding of IRs is well known to promote internalization of those receptors, a process heavily dependent on phosphorylation of tyrosines 953 and 960 in the intracellular portion of the IR (Backer et al., 1992). ADDLs are known to inhibit phosphorylation of tyrosines 1162 and 1163 of the (Zhao et al., 2008), the sites important for transmitting insulin signals from the receptor to targets within cells. Though speculative, it is possible that decreased phosphorylation of tyrosines 1162 and 1163 promotes phosphorylation of tyrosines 953 and 960 and thereby triggers receptor internalization.

    While the findings of De Felice et al. do suggest that using insulin and/or an insulin sensitizer such as rosiglitazone may be a viable therapeutic approach for treating Alzheimer disease, it must be recognized that much work remains to be done before its actual potential can be assessed. There are two reasons for this. The findings are based on in vitro work using rat embryonic hippocampal neurons and, as noted above, 100 nM-1 uM insulin (with or without rosiglitazone), which is far beyond the normal levels of insulin in CSF (i.e., about 360 pM = 0.360 nM; see Craft et al., 1998). (CSF provides our best estimate of insulin in the extracellular spaces in vivo.) It thus remains to be shown if the effects reported by De Felice occur in adults in vivo without unwanted side effects. This can be tested in animal models of AD, in which it would be worth exploring whether the necessary amount of insulin and perhaps rosiglitazone can be administered intranasally given reports that intranasal insulin administration improves memory in Alzheimer cases (Reger et al., 2008).

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