26 Nov 2016

The multidimensional relationship between Aβ and insulin just gained another twist. According to a live monitoring study published in the Journal of Neuroscience on November 16, the peptide hormone may influence Aβ more from without than within—the brain, that is. Researchers led by David Holtzman at Washington University in St. Louis reported that a spike in blood insulin levels (akin to what occurs after a typical meal) elevated Aβ both in the brain and blood of mice that model AD. However, insulin delivered directly into the animals' brains had no effect on Aβ, despite activating insulin receptors there. These results held true in both young and older, plaque-ridden animals. The findings suggest that systemic insulin somehow influences brain Aβ production or clearance mechanisms and that intranasal insulin, which is being tested as a means to improve cognition, may not affect amyloidosis in the brain.

“This paper does a great job at isolating the acute effects of insulin elevation on Aβ concentration, which is hard to do” commented Suzanne Craft of Wake Forest University in Winston-Salem, North Carolina. “Now we need to understand how this relates to multifactorial, complex human disease.”

Type II diabetes (T2D), with its insulin resistance and elevated blood insulin and glucose, increases the risk for AD (see AlzRisk). So does hyperinsulinemia (see Luchsinger et al., 2004). But how? Postmortem studies have indicated the AD brain responds poorly to insulin, and linked its insulin resistance to a greater burden of amyloid plaques and memory problems (see Talbot et al., 2012; Willette et al., 2014). Some scientists proposed that Aβ oligomers exacerbate insulin resistance by reducing the expression of insulin receptors in the brain (see Zhao et al., 2008; Bomfim et al., 2012), while others suggested that insulin receptor signaling boosts Aβ production (see Stöhr et al., 2011). Amidst this complex backdrop, early stage clinical trials hint that intranasal insulin, which reportedly slips into the brain bypassing the blood, might boost cognitive function in people with mild cognitive impairment or AD (see Claxton et al., 2015; Reger and Craft, 2006). The approach has advanced to Phase 2/3, with a federally funded trial called SNIFF.

Given these complex relationships, first author Molly Stanley and colleagues sought to parse the effects of blood versus brain insulin on levels of Aβ. First, they focused on the effect of elevated blood insulin. In three-month-old APP/PS1 animals, which do not yet have plaques, the researchers set up hyperinsulinemia/euglycemic clamps. In other words, they held plasma insulin and glucose levels artificially high through an intravenous line into the jugular vein. At the same time, they used a microdialysis method perfected in Holtzman’s lab to probe hippocampal interstitial fluid (ISF) for Aβ and other molecules. Once these devices were placed, the animals roamed freely in their cages.

When the researchers pinned blood insulin to 4 mU/kg/min—quadrupling fasting serum insulin concentrations to a postprandial level—they detected a 10-fold increase in Aβ in the ISF. A similar ISF Aβ boost occurred when they brought blood insulin to 20 mU/kg/min, suggesting a limit to how high insulin can elevate brain Aβ. Plasma Aβ shot up even higher, rising by about 20- and 35-fold in response to the 4 and 20 mU/g/min insulin clamps, respectively. Because the researchers monitored and continually adjusted blood glucose to stay at a fixed level throughout the experiment, they were able to attribute any alteration in Aβ concentration to insulin, rather than to the fluctuations in glucose known to occur in response to elevated insulin.

Did increased insulin signaling in the brain facilitate this rise in ISF Aβ? To find out, the researchers measured insulin levels in the ISF, CSF, and cortex to see if the insulin they administered peripherally crossed into the brain. They found no rise in insulin concentration in any of these fluids or tissues, indicating that the infused insulin did not reach the brain. In support of this idea, the researchers also found no increase in insulin signaling, as measured by the phosphorylation of the kinase AKT in the hippocampus or hypothalamus. This suggested that insulin in the blood somehow raised Aβ in the brain and plasma in ways other than switching on insulin signaling in the brain. The results were essentially the same in 12-month-old APP/PS1 mice, which bear a substantial plaque burden.

The researchers next asked whether deliberately switching on insulin signaling in the brains of three-month-old APP/PS1 mice would alter Aβ levels. They delivered either 40 nM or 400 nM insulin through a probe placed in the hippocampus, and looked for signs of insulin signaling there one hour later. As expected, insulin levels, as well as phosphorylated AKT, ramped up. However, Aβ concentrations in hippocampal ISF stayed flat over the course of eight hours following the insulin treatment, indicating that insulin signaling in the brain did not alter Aβ levels.

Similarly, 400 nM insulin injected into the brains of 12-month-old APP/PS1 mice did not budge Aβ levels, though insulin signaling ramped up. Interestingly, the researchers reported similar levels of AKT activation in the old APP/PS1 animals as in wild-type mice of the same age, or in three-month-old APP/PS1 mice. This suggested that, contrary to the prevailing view that AD brains are less sensitive to insulin, these plaque-ridden animals responded to insulin just fine.

All told, the findings indicate that elevations in blood insulin, but not brain insulin, affect Aβ concentration. Stanley told Alzforum that it is unclear how elevated blood insulin can modulate Aβ levels from outside of the brain. It could perhaps block Aβ clearance via some other molecule that alters the permeability of the blood-brain barrier or that crosses the barrier to act directly on brain tissue, Stanley suggested. Glucose seems to play no role since the researchers kept its plasma concentration steady throughout the experiments, and also because they found no changes in ISF lactate—a sign of increased neuronal activity in response to glucose. Holtzman’s group previously reported that hyperglycemia, a major symptom of diabetes, elevated brain Aβ (see McCauley et al., 2015).

Stanley acknowledged these studies were limited in only elevating insulin for a short time, but said the findings could hold true in the context of hyperinsulinemia or diabetes, where baseline insulin is chronically elevated. “It supports the idea that high blood insulin plays a causal role in elevating Aβ levels,” she said. Furthermore, that the brains of older APP/PS1 mice respond to insulin without elevating soluble Aβ levels supports the use of intranasal insulin to improve cognition in people with MCI or AD, she said.

Christian Holscher of Lancaster University in England was not convinced the findings applied to chronic hyperinsulinemia or diabetes in people. He previously reported that the brains of AD patients were insulin-resistant, likely due to the downregulation of insulin receptors by Aβ oligomers. He added that the response of older APP/PS1 brains to insulin may be an artifact of delivering large amounts of the hormone directly to the brain, a situation that never occurs physiologically.

Fernanda De Felice of the Federal University of Rio De Janeiro in Brazil, who led insulin-resistance studies along with Holscher, commented that she would have liked to see a deeper examination of signaling pathways in mice that received insulin directly into the hippocampus, including analysis of phosphorylated insulin receptor substrate 1 (IRS-1), a common biomarker for insulin resistance. “In an insulin-resistance context, multiple IRS-1 serines are phosphorylated and a more complete analysis could have provided a broader view on insulin pathway [disturbances] in this study,” she said.

Craft commented that while the study was simplistic relative to the complexity of human metabolic disorders, its straightforward approach was necessary to isolate the acute effects of insulin on Aβ. In the human condition—in which hyperinsulinemia arises a decade or more prior to the onset of T2D—small increases in Aβ as reported by the authors could translate into substantial Aβ accumulation, she said. Alternatively, compensatory modulation could either correct or magnify the effects on Aβ.

Craft led clinical trials of intranasal insulin. She said these data support the idea that insulin delivered to the AD brain might not elevate Aβ levels. This concurs with her own findings on intranasal insulin reducing amyloid accumulation and tau phosphorylation (see Schiöth et al., 2012). Craft emphasized that the goal of intranasal insulin delivery is to correct defects in insulin signaling. Therefore, the drug may work in people with insulin resistance, but produce unwanted effects in people with normal insulin signaling. Indeed, in early trials, AD patients with insulin resistance responded best to the drug.—Jessica Shugart