A cartoon shows a zoomed in section of brain tissue in which cerebrospinal fluid is washing over cells and big yellow amyloid plaques.

Systems-level analyses

In addition to investigating the molecular and cellular processes underlying Alzheimer’s disease and other neurodegenerative conditions, my lab also seeks to understand disease pathogenesis on the systems level, i.e., on the level of neuronal circuits or the entire brain. Our goals are to understand how neuronal activity is impacted by brain pathology, to reveal how it contributes to progression of pathologies through the brain, and to identify strategies for detecting and preventing this progression.

To this end, we use a wide range of methods in mouse models of disease, including classical electrophysiological techniques such as intra-cranial EEG and patch-clamping as well as real-time monitoring of biosensors through fiber optics or two-photon microscopy; targeted stimulation or silencing of neurons using opto- and chemogenetics; and rabies virus-mediated retrograde tracing of neurons and whole-brain clearing and light-sheet microscopy to visualize neuronal circuits. Using these methods on a mouse model of Alzheimer’s disease, we found that dense early amyloid-beta deposits in the mammillary body (MB), a subcortical node of the medial limbic circuit, correlate with neuronal hyper-excitability in lateral MB neurons. Attenuating this aberrant hyperactivity in lateral MB neurons ameliorated memory deficits in the Alzheimer’s mice, while inducing hyperactivity in lateral MB neurons in wild-type mice impaired their performance on memory tasks [1,2].

A recent focus in our lab has been the effects of non-invasive sensory stimulation on Alzheimer’s disease symptoms. Synchronized rhythmic activity of groups of neurons, producing the electrical oscillations detectable by electroencephalography (EEG), is believed to mediate long-range communication within the brain. Oscillations in the gamma frequency range of 30 Hertz (Hz) and above, which are decreased in both Alzheimer’s patients and Alzheimer’s mouse models, are thought to be particularly important for learning and memory. Some years ago, we found that increasing gamma oscillations through external stimuli pulsating at 40 Hz not only reduced pathologies such as amyloid plaques, tau tangles, and neuronal loss in the brain of Alzheimer’s model mice, but also led to their improved performance in tasks related to learning and memory [3,4,5]. While we initially applied this gamma stimulation directly to parvalbumin-positive interneurons through use of optogenetics, similar effects could be achieved by non-invasive sensory stimulation with a flickering light and/or a clicking sound. Recording from intra-cranial electrodes showed that gamma oscillations were not only induced in the sensory cortices but could also be detected in the hippocampus and prefrontal cortex. We termed this non-invasive sensory stimulation “GENUS,” for Gamma ENtrainment Using Sensory stimuli.

In recent studies, we found that the benefits of GENUS may not be limited to Alzheimer’s disease but extend to other neurodegenerative disease. We showed that daily GENUS treatment over several weeks reduced neuroinflammation and demyelination observed in mouse models of chemotherapy-related cognitive impairment, known as “chemobrain,” and of cuprizone-induced demyelination, a model of multiple sclerosis [6,7].

Given the promising results in mouse models and the completely non-invasive nature of GENUS, clinical trials were quickly initiated. Several small clinical studies conducted by us and others showed GENUS to be safe and tolerable in humans and gave encouraging early treatment results such as less brain loss, improved sleep quality, and better performance on a memory test in patients with mild Alzheimer’s disease [8]. Ongoing clinical trials in our lab and by others are exploring the effects of long-term GENUS treatment on cognitive performance as well as brain-imaging- and blood-based biomarkers.

While the mechanisms underlying the beneficial effects of GENUS have not yet been fully elucidated, we recently showed that amyloid reduction observed in Alzheimer’s mice after GENUS is at least partially mediated through enhancement of glymphatic clearance [9]. Glymphatic clearance describes the “flushing out” of waste products from brain tissue by the directed flow of cerebrospinal fluid (CSF) from the perivascular spaces surrounding arterial brain vessels through the brain parenchyma into perivenous spaces and finally lymphatic vessels. This CSF flow is believed to be driven by arterial pulsation and supported by the water channel aquaporin-4 (AQP4) located on astrocytic endfeet surrounding the brain blood vessels. We showed that GENUS promoted arterial pulsatility, AQP4 polarization along astrocytic endfeet, influx of a CSF tracer into and its efflux from the cortex, and amyloid accumulation in cervical lymph nodes in a mouse model of Alzheimer’s disease. Pharmacological or genetic inhibition of AQP4 reduced CSF tracer influx into the brain and attenuated stimulation-mediated amyloid clearance.

References

  1. Gail Canter, Rebecca, Wen-Chin Huang, Heejin Choi, Jun Wang, Lauren Ashley Watson, Christine G. Yao, Fatema Abdurrob, et al. 2019. “3D Mapping Reveals Network-Specific Amyloid Progression and Subcortical Susceptibility in Mice.” Communications Biology 2 (October): 360.
  2. Huang, Wen-Chin, Zhuyu Peng, Mitchell H. Murdock, Liwang Liu, Hansruedi Mathys, Jose Davila-Velderrain, Xueqiao Jiang, et al. 2023. “Lateral Mammillary Body Neurons in Mouse Brain Are Disproportionately Vulnerable in Alzheimer’s Disease.” Science Translational Medicine 15 (692): eabq1019.
  3. Iaccarino, Hannah F., Annabelle C. Singer, Anthony J. Martorell, Andrii Rudenko, Fan Gao, Tyler Z. Gillingham, Hansruedi Mathys, et al. 2016. “Gamma Frequency Entrainment Attenuates Amyloid Load and Modifies Microglia.” Nature 540 (7632): 230–35.
  4. Adaikkan, Chinnakkaruppan, Steven J. Middleton, Asaf Marco, Ping-Chieh Pao, Hansruedi Mathys, David Nam-Woo Kim, Fan Gao, et al. 2019. “Gamma Entrainment Binds Higher-Order Brain Regions and Offers Neuroprotection.” Neuron 102 (5): 929-943.e8.
  5. Martorell, Anthony J., Abigail L. Paulson, Ho-Jun Suk, Fatema Abdurrob, Gabrielle T. Drummond, Webster Guan, Jennie Z. Young, et al. 2019. “Multi-Sensory Gamma Stimulation Ameliorates Alzheimer’s-Associated Pathology and Improves Cognition.” Cell 177 (2): 256-271.e22.
  6. Kim, Taehyun, Benjamin T. James, Martin C. Kahn, Cristina Blanco-Duque, Fatema Abdurrob, Md Rezaul Islam, Nicolas S. Lavoie, Manolis Kellis, and Li-Huei Tsai. 2024. “Gamma Entrainment Using Audiovisual Stimuli Alleviates Chemobrain Pathology and Cognitive Impairment Induced by Chemotherapy in Mice.” Science Translational Medicine 16 (737): eadf4601.
  7. Rodrigues-Amorim, Daniela, P. Lorenzo Bozzelli, Taehyun Kim, Liwang Liu, Oliver Gibson, Cheng-Yi Yang, Mitchell H. Murdock, et al. 2024. “Multisensory Gamma Stimulation Mitigates the Effects of Demyelination Induced by Cuprizone in Male Mice.” Nature Communications 15 (1): 6744.
  8. Chan, Diane, Ho-Jun Suk, Brennan L. Jackson, Noah P. Milman, Danielle Stark, Elizabeth B. Klerman, Erin Kitchener, et al. 2022. “Gamma Frequency Sensory Stimulation in Mild Probable Alzheimer’s Dementia Patients: Results of Feasibility and Pilot Studies.” PloS One 17 (12): e0278412.
  9. Murdock, Mitchell H., Cheng-Yi Yang, Na Sun, Ping-Chieh Pao, Cristina Blanco-Duque, Martin C. Kahn, Taehyun Kim, et al. 2024. “Multisensory Gamma Stimulation Promotes Glymphatic Clearance of Amyloid.” Nature 627 (8002): 149–56.