Astrocytic calcium transients associated with slow oscillations are disrupted in young APP mice. To determine whether astrocytic calcium signaling was altered in 4-6-month-old APP mice, we monitored calcium transients within individual cortical astrocytes using imaging with multiphoton microscopy. We targeted the genetically encoded calcium indicator Yellow Cameleon 3.6 (YC3.6) to astrocytes in the somatosensory cortex via the GFAP promoter, since cortical slow oscillations traverse somatosensory cortex 57 and slow oscillations are disrupted when imaged in somatosensory cortex 26. YC3.6 expression was visualized in the somas, processes, and microdomains of astrocytes in non-transgenic control (NTG) and APP mice (Fig. 1A). YC3.6 is a ratiometric (R) probe. Representative ΔR/R0 traces are shown in Fig. 1B. Here we focused the analyses on the low amplitude, high frequency astrocytic calcium transients that were implicated in slow oscillations 53 (Fig. 1B, blue box), not large amplitude, low frequency astrocytic calcium transients 55 (Fig. 1B, red box). Next, we used Fourier transform analysis to characterize the astrocytic calcium transients. The percentage of astrocytes cycling at the slow-wave frequency (0.2-1 Hz) was high and did not differ significantly between APP and NTG mice (Supplemental Fig. 1, 72.11 ± 3.01% of astrocytes in NTG vs 70.15 ± 2.93% of astrocytes in APP mice; n = 300 cells in 7 NTG mice, n = 282 cells in 7 APP mice). The average power spectral density plots of somatic calcium transients for NTG (n = 300 cells in 7 mice) and APP mice (n = 282 cells in 7 mice) are shown in Fig. 1C. The calcium transient power, which is proportional to the square of the amplitude [A]2, at the slow-wave frequency was significantly lower in astrocytic somas, processes and microdomains of APP mice compared to that in NTG controls (Fig. 1D, 0.0080 ± 0.00012 for NTG vs 0.0069 ± 0.00007 for APP; Fig. 1E, 0.020 ± 0.00008 for NTG vs 0.016 ± 0.00015 for APP; Fig. 1F, 0.032 ± 0.00022 for NTG vs 0.020 ± 0.00015 for APP; ****p < 0.0001). The frequency at peak power remained comparable in astrocytic somas, processes, and microdomains across genotypes (Fig. 1D, 0.62 ± 0.006 for NTG vs 0.63 ± 0.016 for APP; Fig. 1E, 0.62 ± 0.02 for NTG vs 0.63 ± 0.022 for APP; Fig. 1F, 0.61 ± 0.016 for NTG vs 0.064 ± 0.019 for APP). Thus, astrocytic calcium transients associated with SWA are disrupted in APP mice.
Optogenetic activation of cortical astrocytes restores slow waves in young APP mice.
We had previously shown that APP mice exhibit disruptions in SWA compared to NTG controls measured with voltage sensitive dyes 26. The power of slow waves was significantly lower in APP mice compared to NTG mice starting at 3 months of age, while the frequency remained comparable at 0.6 Hz 26. Since astrocytes actively contribute to slow oscillations and astrocytic calcium transients associated with slow waves are disrupted, we tested whether optogenetic activation of astrocytes would restore neuronal slow oscillations in APP mice. 3-5-month-old APP and NTG mice received viral delivery of ChannelRhodopsin2-mCherry (ChR2) or mCherry lacking ChR2. Viral vectors targeted astrocytes via the GFAP promoter in the left hemisphere (Fig. 2A).
In addition to ChR2 or mCherry injections in the left hemisphere, animals received viral injections of a voltage sensor Voltron and AAV9-hsyn-Cre into the contralateral somatosensory cortex (Fig. 2A). Subsequently, a cannula was implanted over the ChR2/mCherry injection site and a 5mm cranial window was installed over the right somatosensory cortex expressing Voltron (Fig. 2B, C). Cortical slow waves were monitored using wide-field microscopy, before and during light activation of ChR2/mCherry-expressing astrocytes (Fig. 2D-F). Since slow oscillations propagate as traveling waves, optogenetic stimulation in anterior left cortex was expected to activate the local circuit that would then propagate the wave into the contralateral hemisphere, where slow waves would be monitored with voltage sensors.
Neuronal population imaging using Voltron revealed disruptions of slow oscillations in APP mice, consistent with earlier reports (Fig. 2D) 26. To determine whether optogenetic targeting of astrocytes was able to drive slow oscillations, we stimulated astrocytes at twice the endogenous frequency of slow waves, 1.2Hz, in NTG mice. This allowed us to distinguish between endogenous slow oscillations and optogenetically elicited slow waves. Optogenetic targeting of astrocytes elicited slow waves at twice the endogenous frequency (Fig. 2E Spon v.s. ChR2 2XRx). However, light stimulation of mCherry in absence of ChR2 failed to do so (Fig. 2E Spon v.s. mCherry 2XRx). Furthermore, light activation of ChR2-expressing astrocytes at the endogenous frequency of slow waves, 0.6Hz, restored the SWA in APP mice (Fig. 2F Spon v.s. ChR2 1XRx). Yet, light activation of mCherry in absence of ChR2 failed to do so (Fig. 2F Spon v.s. mCherry 1XRx).
Fourier transform analysis was performed to elucidate the power-frequency relationships. The power spectral densities of slow oscillations were plotted using Matlab (Fig. 2G-I). Endogenous slow waves oscillated spontaneously at 0.4–0.6 Hz in NTG and APP mice (Fig. 2G) 26. Fourier transform analysis revealed decreases in the power of spontaneous slow waves in APP mice compared to NTG group (Fig. 2G, J; n = 37 traces in 6 NTG mice, n = 30 traces in 6 APP mice). The slow-wave power was significantly lower in young APP mice (Fig. 2J; 1.27x10− 6±3.54x10− 7 for NTG vs 1.93x10− 7±6.48x10− 8 for APP; ***p < 0.001). A spectral power density plot revealed a pronounced peak around 1.2 Hz in NTG mice whose astrocytes were optogenetically driven at 1.2 Hz (ChR2 2XRx), but not in mCherry-expressing (mCherry 2XRx) controls (Fig. 2H, red box). The power at 1.2 Hz increased significantly during optogenetic stimulation, but not light stimulation of mCherry (Fig. 2K; 5.19x10− 8±7.45x10− 9 for Spon vs 1.29x10− 7±2.56x10− 8 for mCherry 2XRx vs 1.17x10− 6±1.28x10− 7 for ChR2 2XRx; ****p < 0.0001) (Fig. 2H, K; NTG Spontaneous: n = 37 traces in 6 mice, NTG mCherry 2XRx: n = 61 traces in 5 mice, NTG ChR2 2XRx: n = 40 traces in 5 mice). This suggested that optogenetic stimulation of cortical astrocytes was able to drive SWA at twice the endogenous frequency in NTG mice.
We then determined whether optogenetic targeting of astrocytes had an effect on slow-oscillation power in anesthetized APP mice. Optogenetic stimulation of ChR2-expressing astrocytes at the endogenous frequency of slow waves, 0.6Hz, restored the power of slow waves in APP mice (Fig. 2I, red box, L; 1.93x10− 7±6.48x10− 8 for Spon vs 2.56x10− 7±4.07x10− 8 for mCherry 1XRx vs 5.82x10− 7±9.63x10− 8 for ChR2 1XRx; **p < 0.01; ****p < 0.0001) (Fig. 2I, L; APP Spontaneous: n = 30 traces in 6 mice, APP mCherry 1XRx: n = 45 traces in 5 mice, APP ChR2 1XRx: n = 53 traces in 5 mice). Thus, the SWA was restored following light activation of astrocytes at the endogenous frequency of slow waves.
To determine whether optogenetic stimulation of astrocytes during slow wave sleep (SWS) impacted the cortical local field potential (LFP), mice underwent injection of ChR2 in the left hemisphere and implantation of a stereotrode array in the contralateral hemisphere, targeting the region imaged with Voltron. The cortical LFP was recorded as animals slept and underwent optogenetic stimulation, and SWS was identified using the theta delta ratio in the setting of prolonged immobility in a sleep posture. Compared to baseline LFP, optogenetic stimulation of ChR2-expressing astrocytes at 0.6 Hz was associated with a small but significant increase in LFP power at 0.6 Hz (n = 3 mice, p = 0.030, 1-tailed paired t-test; Supplemental Fig. 2A-C). These results indicated that optogenetic targeting of astrocytes could be used to successfully manipulate neuronal slow oscillations in APP mice.
To verify whether the ChR2 and mCherry targeted astrocytes and not other brain cells, we performed immunohistochemical analyses of the localization of mCherry with astrocytic (GFAP), neuronal (NeuN) and microglial (Iba1) markers in post-mortem brain tissue (Supplemental Fig. 3). mCherry-expressing cells colocalized with GFAP + positive cells (Supplemental Fig. 3E; 92.74 ± 1.21% of cells were GFAP+; 825/891 cells from 10 sections in 5 mice), not NeuN (Supplemental Fig. 3F; 98.43 ± 0.47% cells were NeuN-; 877/891 cells from 10 sections in 5 mice), nor Iba1 (Supplemental Fig. 3K; 98.24 ± 0.52% of mCherry-expressing cells lacked Iba-1; 421/431 cells from 10 sections in 6 mice). Thus, the GFAP promoter targeted ChR2-mCherry or mCherry specifically to astrocytes, not neurons or microglia.
The rhythmic optogenetic stimulation of astrocytes decreases amyloid beta deposition. Since optogenetic targeting of astrocytes restored slow waves, and SWA actively contributed to the clearance of Aβ 58, we next determined whether optogenetic restoration of SWA via astrocytes could slow the accumulation of Aβ in APP mice. To that end, ChR2- or mCherry-expressing cortical astrocytes were stimulated with light at 0.6 Hz, 24 hours per day continuously for 2 or 4 weeks in aged APP mice. We decided to stimulate during the day and night, since prior work showed APP mice exhibiting deficits in NREM sleep during the day and night 59,60. At the end of the light treatment, cranial windows were installed over the right posterior cortex, contralateral to the ChR2/mCherry site. High-resolution multiphoton microscopy allowed monitoring of methoxy-X04-positive amyloid plaques after treatment in vivo (Fig. 3A). Multiphoton images revealed the presence of amyloid plaques in 8-10-month-old APP mice (Fig. 3B). Restoring slow oscillations decreased amyloid deposition in the ChR2 group compared to mCherry controls (Fig. 3B). The frequency distribution of plaque size is shown in Fig. 3C. The size of the plaques was not significantly different across conditions (Fig. 3C). However, amyloid plaque burden, which takes into account plaque numbers and size, was significantly lower after optogenetic treatment compared to light activation of mCherry (Fig. 3D; 1.43 ± 0.11% burden for mCherry vs 1.01 ± 0.06% burden for ChR2; mCherry: n = 57 fields of view (FOV) in 8 APP mice; ChR2: n = 82 FOV in 10 APP mice; **P < 0.01). Also, the number of amyloid plaques was significantly lower in the ChR2 group compared to that in mCherry controls (Fig. 3E; 185.9 ± 14.44 plaques/mm3 for mCherry vs 139.0 ± 5.69 plaques/mm3 for ChR2; mCherry: n = 57 FOV in 8 APP mice; ChR2: n = 82 FOV in 10 APP mice; *P < 0.05). Therefore, astrocyte-dependent rescue of slow waves reduced the rate of amyloid plaque deposition in APP mice. Random optogenetic stimulation of astrocytes failed to significantly alter amyloid plaque number or amyloid plaque burden in APP mice compared to non-treated APP mice (Supplemental Fig. 4A, B).
The rhythmic optogenetic stimulation of astrocytes normalizes neuronal calcium levels in APP mice. Intracellular calcium levels are tightly regulated for healthy neuronal function. APP mice exhibit disruptions in neuronal calcium homeostasis that result in elevated resting calcium levels, or calcium overload 61,62. Calcium levels were monitored using multiphoton microscopy after 2–4 weeks of optogenetic stimulation. The genetically encoded ratiometric calcium reporter YC3.6 under the CBA promoter was used to target cortical neuronal processes, neurites. YC3.6 allows the determination of absolute calcium concentration in neurons, as well as dynamic changes. We determined whether light activation of ChR2-expressing astrocytes would alter neuronal calcium levels in APP mice. Thus, neuronal calcium was used as a functional readout of treatment efficacy. While the majority of neuronal processes, or neurites, exhibited normal calcium levels in APP mice (Fig. 4A, blue neurons), some neurites exhibited abnormally high calcium (Fig. 4A, red neuron, white arrows). Restoration of slow waves led to a decrease in the percentage of neurons with calcium overload (Fig. 4B). A YC3.6 ratio greater than two standard deviations above the mean value (> 1.73) in NTG neurons was defined as calcium overload (Fig. 4C red box) 26,27,61,62. The percentage of neuronal processes exhibiting calcium overload was lower in the ChR2 group compared to the mCherry group (Fig. 4C, D; 12.65 ± 1.8% for mCherry vs 5.79 ± 1.2% for ChR2; mCherry: n = 2182 neurites in 23 FOV of 7 mice; ChR2: n = 2014 neurites in 24 FOV of 6 mice; *p < 0.05). Thus, astrocyte-dependent rescue of slow waves decreased neuronal calcium overload and restored neuronal calcium homeostasis in APP mice.
The rhythmic optogenetic stimulation in astrocytes restores sleep-dependent memory performance in APP mice. To determine whether optogenetic targeting of astrocytes enhanced memory consolidation, we subjected NTG controls and APP mice to contextual fear conditioning test (Fig. 5A). Fear acquisition was performed on day 13 of light treatment. The freezing levels at baseline were similar in all four groups (Fig. 5B; 2.44 ± 1.13% for NTG mCherry vs 2.27 ± 0.76% for NTG ChR2, 2.77 ± 0.79% for APP mCherry vs 2.90 ± 0.77% for APP ChR2). Delivery of 3 consecutive electric shocks resulted in progressive increases in freezing. Additionally, the freezing levels after the delivery of first, second and third shocks were comparable between respective mCherry and ChR2 groups (Fig. 5B; 1st shock: 18.17 ± 10.7% for NTG mCherry vs 8.86 ± 3.67% for NTG ChR2, 12.04 ± 5.93% for APP mCherry vs 9.97 ± 4.13% for APP ChR2; 2nd shock: 38.04 ± 7.81% for NTG mCherry vs 28.97 ± 8.75% for NTG ChR2, 15.25 ± 6.48% for APP mCherry vs 31.52 ± 6.41% for APP ChR2; 3rd shock: 46.23 ± 9.54% for NTG mCherry vs 52.16 ± 13.03% for NTG ChR2, 27.82 ± 9.49% for APP mCherry vs 33.12 ± 7.32% for APP ChR2). Thus, fear acquisition was not significantly altered by optogenetic treatment of APP mice. Following fear acquisition, the mice were allowed to sleep and consolidate their memories overnight during light activation of ChR2 or mCherry. Fear recall was tested the following day. The contextual fear memory was impaired in APP mice (Fig. 5C; 87.29 ± 1.67% for NTG mCherry vs 65.73 ± 4.92% for APP mCherry; **p < 0.01). The freezing levels were significantly higher in APP ChR2 group compared to that in APP mCherry group (Fig. 5C; 65.73 ± 4.92% for APP mCherry vs 82.46 ± 6.73% for APP ChR2; *p < 0.05). In addition, the freezing levels were comparable between NTG mCherry group and APP ChR2 group (Fig. 5C; 87.29 ± 1.67% for NTG mCherry vs 82.46 ± 6.73% for APP ChR2), indicating that optogenetic treatment restored sleep-dependent memory consolidation in APP mice to that in NTG littermates. Optogenetic treatment of NTG mice failed to further improve memory (Fig. 5C; 87.29 ± 1.67% for NTG mCherry vs 82.09 ± 5.99% for NTG ChR2).
The same mice were subjected to a locomotor test to determine whether increases in freezing could be attributed to deficits in locomotor activity. The distance travelled in the open field was not significantly different for the four groups (Fig. 5D; 2754 ± 285.3cm for NTG mCherry vs 2785 ± 110.2cm for NTG ChR2, 3329 ± 285cm for APP mCherry vs 3125 ± 346.1cm for APP ChR2). The average velocity of the APP mice did not show significant group differences (Fig. 5E; 4.59 ± 0.48cm/s for NTG mCherry vs 4.52 ± 0.20cm/sfor NTG ChR2, 5.61 ± 0.47cm/s for APP mCherry vs 5.21 ± 0.58cm/s for APP ChR2). Our data indicated that the locomotion of the APP mice was not impaired by optogenetic treatment. Thus, slow wave rescue via optogenetic targeting of astrocytes improved sleep-dependent memory consolidation in APP mice.
We subsequently verified that optogenetic stimulation of astrocytes did not result in neurodegeneration in APP mice (Supplemental Fig. 5). Thus, blue light did not result in significant phototoxicity.