Remote and Selective Control of Astrocytes by Magnetomechanical Stimulation

Abstract Astrocytes play crucial and diverse roles in brain health and disease. The ability to selectively control astrocytes provides a valuable tool for understanding their function and has the therapeutic potential to correct dysfunction. Existing technologies such as optogenetics and chemogenetics require the introduction of foreign proteins, which adds a layer of complication and hinders their clinical translation. A novel technique, magnetomechanical stimulation (MMS), that enables remote and selective control of astrocytes without genetic modification is described here. MMS exploits the mechanosensitivity of astrocytes and triggers mechanogated Ca2+ and adenosine triphosphate (ATP) signaling by applying a magnetic field to antibody‐functionalized magnetic particles that are targeted to astrocytes. Using purpose‐built magnetic devices, the mechanosensory threshold of astrocytes is determined, a sub‐micrometer particle for effective MMS is identified, the in vivo fate of the particles is established, and cardiovascular responses are induced in rats after particles are delivered to specific brainstem astrocytes. By eliminating the need for device implantation and genetic modification, MMS is a method for controlling astroglial activity with an improved prospect for clinical application than existing technologies.

. Characterization of astrocyte cultures. a) Labeling of the glial fibrillary acidic protein (GFAP) by an anti-GFAP antibody and staining of the cell nuclei by 4',6-diamidino-2-phenylindole (DAPI) in an astrocyte culture.
Scale bar = 20 µm. b) Purity of astrocyte cultures was determined using images such as the one in Panel a. A total of 1,338 cells were counted from 11 cultures derived from 4 rats.  B field (input current = 0.9 A) a magnitudes of the vectors; the white dashed ellipse demarcates the region of high uniformity at 2 mm above magnet base. f) The minimum forces required to trigger Ca 2+ signals in 290 astrocytes. The data exhibit a lognormal distribution. g) Scanning electron microscopy (SEM) images of astrocyte cultures were analysed to determine the base area and volume of the  SiMAG (500 nm) BioMag Maxi (3-12 μm) different depending on the presence or absence of ligand on the surface of the particles. In comparison to the no particle condition: collagen-coated SiMAG particles had no effect on [ATP] e (p = 0.869); the use of anti-GLAST-coupled SiMAG particles led to appreciable elevations in [ATP] e (p < 0.001); and unmodified SiMAG particles caused [ATP] e to rise conspicuously (p < 10 -16 ). See Table S5. This phenomenon was likely due to the negative charge of the carboxyl groups on the surface of the SiMAG particles. When making collagencoated particles, the ratio between ligand mass in µg and particle mass in mg was 50.0, resulting in complete masking of the carboxyl groups, whereas for anti-GLAST-coupled ones, the ratio was only 5.0, resulting in partial coverage of the particle surface. From left to right, n = 48, 16,32,16,24,16,16,24,32 and 16 measurements. In Panel (d,f): data shown as mean ± standard deviation (S.D.).    (Table S6). Data shown as mean ± S.D.; n = 8 measurements for each condition; **, p < 0.01, two-tailed t-test of mean [ATP] e change equaling zero; ***, p < 0.001, same test; #, p < 0.05, two-sample two-tailed t-test. e,f) Maps and summary statistics of SiMAG magnetization values estimated for the Magnetic Mangle and the MRI scanner fringe field.  Each rat received a unilateral 1 µL injection of an anti-GLAST-coupled SiMAG particle suspension. MRI was performed and the acquired images were affinely registered to each other. The cross hairs in MR images mark the same anatomical location in each brain.
Animals were sacrificed at different time points and brainstem sections were stained for the microglial marker cluster of differentiation 68 (CD68), the astroglial markers GFAP, GLAST and excitatory amino acid transporter 2 (EAAT2), and the neuronal markers neuronal nuclei (NeuN) and tyrosine hydroxylase (TH). Three consecutive sections are shown for each rat.
a) values from product data sheets. b) values assumed to be equal to those of BioMag Plus particles.
Abbreviations: [Particle], the concentration of the particle suspension applied to the astrocyte culture; df, degree of freedom; S.E., standard error; CI, confidence interval.