Pattern-dependent low-intensity repetitive magnetic stimulation enhances spinal cord repair through modulation of neuroinflammation
Spinal cord injuries (SCI) are traumatic lesions of the spinal cord that commonly result from physical trauma. They can lead to severe and permanent impairments in motor, sensory, and/or autonomic functions, often resulting in paraplegia or tetraplegia. Despite advances in this field, effective restorative treatments for SCI remain limited. Over the past few years, several therapeutic strategies have been investigated to promote functional recovery after SCI, including neuromodulation approaches. Repetitive magnetic stimulation has emerged as a promising non-invasive strategy, with encouraging results when the stimulation is applied directly over the spinal cord. However, most studies have focused on high-intensity stimulation, while the key parameters underlying its therapeutic efficacy remain poorly understood. In this study, we investigated the effects of low-intensity repetitive trans-spinal magnetic stimulation (LI-rTSMS) following SCI in mice, using BMS, histological and RNA sequencing approaches. We aimed to determine the optimal protocol by comparing three stimulation patterns (10 Hz, iTBS and BHFS) and two coil sizes. Our results show that LI-rTSMS modulates the injured spinal cord in a pattern- and coil size-dependent manner, with distinct effects on fibrotic, neural, and inflammatory responses. Notably, the BHFS pattern produced the most marked tissue-remodeling effects, reducing fibrosis, reactive astrogliosis, and phagocytosis of myelin debris. This reduction in phagocytosis was also observed with a smaller coil, supporting an effect of LI-rTSMS on this process. We also identified distinct inflammatory signatures depending on the stimulation pattern and time after injury. Transcriptomic analyses revealed a common early inflammatory response to LI-rTSMS, whereas each stimulation pattern subsequently led to distinct molecular signatures after 2 weeks stimualtion. In addition, ependymal cells are known to retain endogenous regenerative potential following SCI. We found that LI-rTSMS increased ependymal cell proliferation and migration toward the lesion site, without detectable changes in their differentiation. Finally, we assessed c-Fos expression as a marker of early transcriptional changes in neurons. LI-rTSMS did not result in significant changes, suggesting that the observed effects were unlikely to be mediated by early, widespread neuronal activation. Altogether, this study provides mechanistic insights into the pattern-dependent effects of LI-rTSMS and highlights its potential role for future clinical translation.