Decades before Alzheimer’s disease symptoms appear, neurons become hyperactive, and the reason could be that astrocytes fail to keep them in check. In a bioRXiv preprint uploaded on April 26, researchers led by Bart de Strooper, UK Dementia Research Institute, London, and first author Disha Shah, KU Leuven, Belgium, report that the cingulate cortex is hyperactive in cognitively normal older adults years before they develop amyloid plaques. They found the same in young APP knock-in mice. In the mouse cingulate, astrocytes did not maintain intracellular calcium signaling, but boosting calcium levels restored astrocyte activity, normalized neuron function, and calmed the hyperactive mice. This research places astrocytes into the pathological cascade much earlier than previously suspected.
- Healthy adults have overactive cingulate neurons years before developing plaques.
- In the same region in young APP knock-in mice, neurons are hyperactive, astrocytes sluggish.
- Jolting astrocytes normalized mouse connectivity and behavior.
“I’m really happy that the authors focused on the role of astrocytes in brain circuit dysfunctions, because we don’t know a lot about what causes these issues,” Brian Bacskai, Harvard Medical School, told Alzforum. “The researchers used an impressive array of techniques to show that altered calcium signaling in astrocytes leads to dysfunctional neuron circuits,” he added.
In the AD brain, a collection of regions called the default mode network (DMN), which includes the prefrontal and cingulate cortices, becomes overactive years before symptom onset (Sep 2005 news; Mar 2004 news).
Shah found much the same when she analyzed functional MRI scans from healthy adults ages 50 to 80 who had enrolled in the Flemish Prevent AD Cohort. F-PACK collected baseline cognitive assessment scores, structural and functional MRI scans, and amyloid PET scans from cognitively intact people between 2009 and 2015, and is following participants for 10 years. Shah focused on 34 people who had had two amyloid scans that were captured seven years apart on average. Ten of the 34 accumulated amyloid during that time. For comparison, she matched these “accumulators” with non-accumulators with roughly the same age, sex, years of education, and baseline mini-mental state exam scores.
F-PACK measured neural activity using blood oxygen level-dependent functional MRI. BOLD detects tiny oxygen changes generated by respiratory surges in nearby neurons (Aug 2005 news). Compared to controls at baseline, people who went on to develop amyloid plaques had heightened activity in the anterior-posterior cingulate cortex (see image below), in keeping with earlier data suggesting activity changes in this region early in disease. “This brain area may be particularly vulnerable because the default mode network spends so much time in an active state when a person isn’t focusing on a task,” Shah told Alzforum.
Calm Down, Cingulate. Baseline fMRI scans reveal a hyperactive anterior cingulate cortex in people who go on to accumulate amyloid (middle), but not in those who remain amyloid-free (left). The change in amyloid (right) correlated with baseline activity in this region (red diamonds are amyloid-positives; blue, amyloid negative). [Courtesy of Shah et al., bioRXiv, 2022.]
To find out what might cause the hyperactivity if there are no plaques around, the scientists turned to mice. Shah had seen hyperactivity in homologous brain regions in APP NL-F knock-in mice and other models of amyloidosis, also before amyloid had accumulated (Shah et al., 2018; Latif-Hernandez et al., 2017; Shah et al., 2016). Bacskai and others had shown that when plaques did emerge, they intensified calcium signaling within nearby neurons and astrocytes (Sep 2008 news; Feb 2009 news). Does calcium run amok even before plaques appear?
Shah assessed intracellular calcium signaling in mice by selectively expressing the calcium indicator, GCaMP6f, in neurons or in astrocytes in the cingulate cortices of 2-month-old APP NL-F knock-ins. She then imaged the reporter using transcranial two-photon microscopy. Compared to that in wild-type mice, calcium signaling in APP NL-F neurons within the cingulate was intense, but in APP NL-F astrocytes, it was weaker.
To test whether failing astrocytic calcium signaling might explain the neuronal hyperactivity, the scientists restored astrocyte calcium signaling. They expressed hM3Dq in the cells. This Designer Receptor Exclusively Activated by Designer Drugs, aka DREADD, releases calcium through the inositol 1,4,5-trisphosphate pathway only when the mice are given the designer drug clozapine-N-oxide (CNO). In APP NL-F/hM3Dq mice, CNO boosted astrocyte calcium responses. This calmed overzealous neuronal calcium and neuronal activity in the mouse DMN, lowering the BOLD signal.
CNO also quieted neurons artificially overstimulated with dihydrokainic acid, an inhibitor of glutamate transporter-1. Astrocytes use GLT-1 to recycle glutamate from synapses, and when it is blocked, neurotransmission ramps up. Boosting astrocytic calcium signaling in dihydrokainic acid-treat mice quieted neuron activity to baseline. To the authors, this indicated that calcium signaling within astrocytes plays a key role in regulating brain network activity.
Lastly, the authors found the CNO also calmed APP NL-F behavior. These mice tend to be hyperactive, scurrying around their cages more than do wild-types. Shah found that they are also more prone to drug-induced seizures. However, once they were given CNO and the astrocytic calcium flowed, they calmed down and had fewer seizures than did untreated mice.
Why would astrocytes struggle so early in AD, without astrocytosis or amyloid plaques to cause problems? Shah thinks soluble Aβ species are to blame. “Oligomers may bind astrocyte receptors and disturb signaling cascades within the cell,” she said. In fact, Aβ dimers may block GLT-1 to prevent glutamate uptake and overexcite neurons (Aug 2019 news). Bacskai agreed that Aβ, in some form, is the likeliest culprit, but its link to malfunctioning astrocytes and neuron activity needs to be proven. “The critical role of astrocytes in brain circuit dysfunction is an avenue that needs to be explored further,” he said.—Chelsea Weidman Burke
- Network News: Images of AD Brains Reveal Widespread Snafus
- Network Diagnostics: "Default-Mode" Brain Areas Identify Early AD
- MRI—The Good, the Bad and the…BOLD?
- Hyperactive Neurons and Amyloid, Side by Side
- Making Waves—Calcium Dysregulation in Astrocytes of AD Mice
- Aβ Dimers Block Glutamate Uptake, Fire Up Synapses
Research Models Citations
- Shah D, Latif-Hernandez A, De Strooper B, Saito T, Saido T, Verhoye M, D'Hooge R, Van der Linden A. Spatial reversal learning defect coincides with hypersynchronous telencephalic BOLD functional connectivity in APPNL-F/NL-F knock-in mice. Sci Rep. 2018 Apr 19;8(1):6264. PubMed.
- Latif-Hernandez A, Shah D, Craessaerts K, Saido T, Saito T, De Strooper B, Van der Linden A, D'Hooge R. Subtle behavioral changes and increased prefrontal-hippocampal network synchronicity in APPNL-G-F mice before prominent plaque deposition. Behav Brain Res. 2017 Nov 20; PubMed.
- Shah D, Praet J, Latif Hernandez A, Höfling C, Anckaerts C, Bard F, Morawski M, Detrez JR, Prinsen E, Villa A, De Vos WH, Maggi A, D'Hooge R, Balschun D, Rossner S, Verhoye M, Van der Linden A. Early pathologic amyloid induces hypersynchrony of BOLD resting-state networks in transgenic mice and provides an early therapeutic window before amyloid plaque deposition. Alzheimers Dement. 2016 Sep;12(9):964-76. Epub 2016 Apr 21 PubMed.
- Lee YF, Russ AN, Zhao Q, Maci M, Miller MR, Hou SS, Algamal M, Araque A, Galea E, Bacskai BJ, Kastanenka K. Optogenetic Targeting of Astrocytes Restores Sleep-Dependent Brain Rhythm Function and Slows Alzheimer's Disease. Cell Reports, 8 Apr 2022 Cell Reports
- Busche MA, Konnerth A. Neuronal hyperactivity--A key defect in Alzheimer's disease?. Bioessays. 2015 Jun;37(6):624-32. Epub 2015 Mar 14 PubMed.
- Shah D, Gsell W, Wahis J, Luckett ES, Jamoulle T, Vermaercke B, Preman P, Moechars D, Hendrickx V, Jaspers T, Craessaerts K, Horré K, Wolfs L, Fiers M, Holt M, Thal DR, Callaerts-Vegh Z, D’Hooge R, Vandenberghe R, Himmelreich U, Bonin V, De Strooper B. Astrocyte calcium dysfunction causes early network hyperactivity in Alzheimer’s Disease. bioRxiv, April 26, 2022 bioRxiv