Therapeutic Ultrasound as a Treatment Modality for Physiological and Pathological Ageing Including Alzheimer's Disease

Improving Cognition Without Clearing Amyloid: Effects of Tau and Ultrasound Neuromodulation

Alzheimer's disease is characterized by progressive impairment of neuronal functions culminating in neuronal loss and dementia. A universal feature of dementia is protein aggregation, a process by which a monomer forms intermediate oligomeric assembly states and filaments that develop into end-stage hallmark lesions. In Alzheimer's disease, this is exemplified by extracellular amyloid-β (Aβ) plaques which have been placed upstream of tau, found in intracellular neurofibrillary tangles and dystrophic neurites. This implies causality that can be modeled as a linear activation cascade. When Aβ load is reduced, for example, in response to an anti-Aβ immunotherapy, cognitive functions improve in plaque-forming mice. They also deteriorate less in clinical trial cohorts although real-world clinical benefits remain to be demonstrated. Given the existence of aged humans with unimpaired cognition despite a high plaque load, the central role of Aβ has been challenged. A counter argument has been that clinical symptoms would eventually develop if these aged individuals were to live long enough. Alternatively, intrinsic mechanisms that protect the brain in the presence of pathology may exist. In fact, Aβ toxicity can be abolished by either reducing or manipulating tau (through which Aβ signals), at least in preclinical models. In addition to manipulating steps in this linear pathocascade model, mechanisms of restoring brain reserve can also counteract Aβ toxicity. Low-intensity ultrasound is a neuromodulatory modality that can improve cognitive functions in Aβ-depositing mice without the need for removing Aβ. Together, this highlights a dissociation of Aβ and cognition, with important implications for therapeutic interventions.

Disease-Modifying Effects of Non-Invasive Electroceuticals on β-Amyloid Plaques and Tau Tangles for Alzheimer's Disease

Electroceuticals refer to various forms of electronic neurostimulators used for therapy. Interdisciplinary advances in medical engineering and science have led to the development of the electroceutical approach, which involves therapeutic agents that specifically target neural circuits, to realize precision therapy for Alzheimer's disease (AD). To date, extensive studies have attempted to elucidate the disease-modifying effects of electroceuticals on areas in the brain of a patient with AD by the use of various physical stimuli, including electric, magnetic, and electromagnetic waves as well as ultrasound. Herein, we review non-invasive stimulatory systems and their effects on β-amyloid plaques and tau tangles, which are pathological molecular markers of AD. Therefore, this review will aid in better understanding the recent technological developments, applicable methods, and therapeutic effects of electronic stimulatory systems, including transcranial direct current stimulation, 40-Hz gamma oscillations, transcranial magnetic stimulation, electromagnetic field stimulation, infrared light stimulation and ionizing radiation therapy, and focused ultrasound for AD.

Ultrasound-Mediated Bioeffects in Senescent Mice and Alzheimer's Mouse Models

Ultrasound is routinely used for a wide range of diagnostic imaging applications. However, given that ultrasound can operate over a wide range of parameters that can all be modulated, its applicability extends far beyond the bioimaging field. In fact, the modality has emerged as a hybrid technology that effectively assists drug delivery by transiently opening the blood-brain barrier (BBB) when combined with intravenously injected microbubbles, and facilitates neuromodulation. Studies in aged mice contributed to an insight into how low-intensity ultrasound brings about its neuromodulatory effects, including increased synaptic plasticity and improved cognitive functions, with a potential role for neurogenesis and the modulation of NMDA receptor-mediated neuronal signalling. This work is complemented by studies in mouse models of Alzheimer's disease (AD), a form of pathological ageing. Here, ultrasound was mainly employed as a BBB-opening tool that clears protein aggregates via microglial activation and neuronal autophagy, thereby restoring cognition. We discuss the currently available ultrasound approaches and how studies in senescent mice are relevant for AD and can accelerate the application of low-intensity ultrasound in the clinic.