Led Astray: Pathology Tied to “Grid Cell” Malfunction in Tauopathy Model
Patients in the early stages of Alzheimer’s disease (AD) often wander and get lost in familiar settings. In a paper to be published in the February 9 issue of Neuron, researchers unravel a bit of the biology that might underlie this disorientation, at least in transgenic mice. Karen Duff and Abid Hussaini of Columbia University Medical Center, New York, and their colleagues correlate tau pathology in the medial entorhinal cortex (EC) with three things: loss of excitatory neurons, disruption of “grid cells” that control spatial navigation, and impairment of spatial memory. “This is the first time such a close relationship between neurofibrillary tangles and dysfunction of the medial EC has been demonstrated,” said Davide Moretti at the S. John of God Hospital in Brescia, Italy. “This is very important because the grid cells in the EC are strictly related to spatial navigation in mice and in humans.”
Previous studies indicated that the entorhinal cortex (EC) is one of the first brain areas to accumulate tau in AD. But if and how that tau pathology might encumber navigation was unclear. Duff and colleagues set out to find a connection at the cellular level between this protein pathology and patients’ impaired navigation. “We wanted to tie these two observations together and see what types of cells are impacted,” Duff told Alzforum.
Duff’s team further analyzed the EC-tau mice they created to examine how tau pathology spreads in the brain (Liu et al., 2012). EC-tau mice express a mutant version of the human tau gene (P301L) that becomes hyperphosphorylated and is more prone to aggregate than normal tau. The gene is regulated by the neuropsin gene promoter, which is active predominantly in layer 2 of the EC. In this model, neurofibrillary tangles first appear in the EC and progressively show up in anatomically connected brain regions, following a similar pattern of pathology as described for AD (see Braak and Braak, 1991).
To probe how spatial memory and navigation become impaired, the team tracked tau pathology, the activity of grid cells, and the performance on spatial cognitive tests in EC-tau and normal mice. As expected, mice older than 30 months, both wild-type and transgenic, navigated the T and Morris water mazes less well than 14-month-old mice. However, aging took a greater toll on the EC-tau mice. At 30-plus months, they performed much worse than normal mice that also reached this ripe old age.
By 30 months, EC-tau mice had also accumulated human tau in the soma and dendrites of neurons in the EC, the hippocampus, and even in regions of the neocortex. “Finally, we have 3-year-old mice in which pathology has spread beyond the EC to the hippocampal formation and to the neocortex, a critical step in human AD,” said Duff. “That’s when you start seeing cognitive impairment.”
Antibodies that recognize different conformations and phosphorylation states of tau (MC1, CP27, AT8, AT180) also revealed the presence of hyperphosphorylated tau and mature tau tangles in these brain areas. Intriguingly, immunofluorescence indicated that tangles (MC1 staining) co-localized with TBR1, an excitatory neuron receptor, but not with parvalbumin or somatostatin, markers of inhibitory neurons. The number of excitatory cells plummeted by 70 percent in mEC layers II and by 40 percent in layers III/IV in aged EC-tau mice compared to controls, while the number of inhibitory neurons remained essentially unchanged, noted co-first author Harry Fu.
“We decided to place electrodes in this exact spot to see if we could measure changes in the function of specific cell types,” said Hussaini. Co-first author Gustavo Rodriguez implanted 16-channel electrodes to listen in on the neurons. He targeted the dorsal medial EC, an area recently reported by Jonathan Brown at the University of Exeter in the U.K. to be most vulnerable to tau pathology (Booth et al., 2016). Rodriguez recorded single-unit and local field potentials in freely moving mice as they explored open-field arenas. “We found that the balance between excitation and inhibition was tipped toward inhibition,” said Hussaini. This was not just because excitatory neurons were lost. Inhibitory neurons fired, on average, approximately 50 percent more often than in age-matched normal mice.
The scientists also found an unusual pattern of grid cell activity in aged EC-tau mice. Grid cells map the spatial layout of an organism’s environment. “They function like the brain’s GPS,” explained Rodriguez. In EC-tau mice, maps of the firing rates of grid cells were weaker compared to controls (see image above).
“In several respects, these results parallel, and may even provide a mechanistic explanation for, recent neuroimaging findings in humans at risk of developing AD,” wrote Nikolai Axmacher at Ruhr University in Bochum, Germany. Axmacher recently reported disrupted grid-cell activity and impaired navigation in healthy, young adults carrying the AD risk gene ApoE4 (Oct 2015 news). It remains to be seen whether those adults have any tau or other pathology in the entorhinal cortex.
Along with the alterations in grid cell activity, Fu and colleagues found a strengthening of theta rhythms in the EC of older EC-tau mice. These 4-12 Hz oscillations reflect activities of whole networks rather than firing of individual cells, and are thought to be paced by inhibitory neurons. “This is in complete agreement with results in humans,” noted Moretti. “Theta increases when performing navigation and working memory tasks.” Theta rhythms may also be enhanced in early AD and in people with mild cognitive impairment (see, for example, Moretti et al., 2009; Montez et al., 2009).
The causes and consequences of this altered theta rhythm are still unclear. Moretti and Brown hypothesized it could be a compensatory mechanism to preserve navigational skills as grid cell function breaks down. Duff and colleagues point out that the strengthening of theta may simply reflect the shift toward stronger inhibitory firing. Axmacher was surprised by the enhancement of inhibitory activity given that people with early AD typically exhibit hyperactivity and even an increased incidence of epileptic seizures. However, Moretti noted that rearrangement of network activities in one region (in this case, the medial EC) can result in different, or even opposing, effects in other cortical areas.
The findings raise many questions: Why do only certain neurons develop tangles? Is excitatory vulnerability characteristic of all tauopathies? Can targets for early treatment emerge from understanding this susceptibility? For her part, Duff is most interested in finding ways to tie this research to processes underlying pathogenesis in people by, for example, using fMRI to monitor EC activity while testing navigation.
The information gleaned from current navigation tests is limited, and scientists know little about different navigation strategies people use to compensate (e.g., remembering landmarks on a specified route versus creating a mental map of an environment). Hoping to make navigational testing more informative, Michael Hornberger at the University of East Anglia, Norwich, U.K., led a team of researchers, in association with the British game company Glitchers. They designed a mobile navigation game to collect baseline data on spatial navigation (Sea Hero Quest, 2016). The idea is to use artificial intelligence techniques to better understand how normal people navigate and to search for patterns associated with navigational decline. A more refined understanding of navigation could lead to better diagnostics and be a powerful tool to explore how tau pathology and grid cell function correlate with early AD symptoms, said Duff.—Marina Chicurel
Marina Chicurel is a writer based in Santa Cruz, California.
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No Available Further Reading
- Fu H, Rodriguez GA, Herman M, Emrani S, Nahmani E, Barrett G, Figueroa HY, Goldberg E, Hussaini SA, Duff KE. Tau Pathology Induces Excitatory Neuron Loss, Grid Cell Dysfunction, and Spatial Memory Deficits Reminiscent of Early Alzheimer's Disease. Neuron. 2017 Feb 8;93(3):533-541.e5. Epub 2017 Jan 19 PubMed.
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