Exposure to man-made electromagnetic fields (EMFs), which increasingly pollute our environment, have consequences for human health about which there is continuing ignorance and debate. Whereas there is considerable ongoing concern about their harmful effects, magnetic fields are at the same time being applied as therapeutic tools in regenerative medicine, oncology, orthopedics, and neurology. This paradox cannot be resolved until the cellular mechanisms underlying such effects are identified. Here, we show by biochemical and imaging experiments that exposure of mammalian cells to weak pulsed electromagnetic fields (PEMFs) stimulates rapid accumulation of reactive oxygen species (ROS), a potentially toxic metabolite with multiple roles in stress response and cellular ageing. Following exposure to PEMF, cell growth is slowed, and ROS-responsive genes are induced. These effects require the presence of cryptochrome, a putative magnetosensor that synthesizes ROS. We conclude that modulation of intracellular ROS via cryptochromes represents a general response to weak EMFs, which can account for either therapeutic or pathological effects depending on exposure. Clinically, our findings provide a rationale to optimize low field magnetic stimulation for novel therapeutic applications while warning against the possibility of harmful synergistic effects with environmental agents that further increase intracellular ROS. Repetitive low-intensity magnetic stimulation has been used in the treatment of disease for over 50 years. Associated benefits have included alleviation of depression, memory loss, and symptoms of Parkinson disease, as well as accelerated bone and wound healing and the treatment of certain cancers, independently of surgery or drugs. However, the cellular mechanisms underlying these effects remain unclear. Here, we demonstrate that repetitive magnetic field exposure in human cells stimulates production of biological stress response chemicals known as reactive oxygen species (ROS). At moderate doses, we find that reactive oxygen actively stimulates cellular repair and stress response pathways, which might account for the observed therapeutic effects to repetitive magnetic stimulation. We further show that this response requires the function of a well-characterized, evolutionarily conserved flavoprotein receptor known as cryptochrome, which has been implicated in magnetic sensing in organisms ranging from plants to flies, including migratory birds. We conclude that exposure to weak magnetic fields induces the production of ROS in human cells and that this process requires the presence of the cryptochrome receptor.

Neuronal maturation during development is a multistep process regulated by transcription factors. The transcription factor RORα (retinoic acid-related orphan receptor α) is necessary for early Purkinje cell (PC) maturation but is also expressed throughout adulthood. To identify the role of RORα in mature PCs, we used Cre-lox mouse genetic toolsin vivothat delete it specifically from PCs between postnatal days 10–21. Up to 14 d of age, differences between mutant and control PCs were not detectable: both were mono-innervated by climbing fibers (CFs) extending along their well-developed dendrites with spiny branchlets. By week 4, mutant mice were ataxic, some PCs had died, and remaining PC soma and dendrites were atrophic, with almost complete disappearance of spiny branchlets. The innervation pattern of surviving RORα-deleted PCs was abnormal with several immature characteristics. Notably, multiple functional CF innervation was reestablished on these mature PCs, simultaneously with the relocation of CF contacts to the PC soma and their stem dendrite. This morphological modification of CF contacts could be induced even later, using lentivirus-mediated depletion ofrorafrom adult PCs. These data show that the late postnatal expression of RORα cell-autonomously regulates the maintenance of PC dendritic complexity, and the CF innervation status of the PC (dendritic vs somatic contacts, and mono-innervation vs multi-innervation). Thus, the differentiation state of adult neurons is under the control of transcription factors; and in their absence, adult neurons lose their mature characteristics and acquire some characteristics of an earlier developmental stage.

Biomimetic patterns of weak magnetic pulses modify gene expression and need cryptochrome to induce cerebellar reinnervation.

The primary cilium regulates neuronal migration via cyclic AMP production activating protein kinase A at the centrosome.

Neurons in the CNS of higher vertebrates lose their ability to regenerate their axons at a stage of development that coincides with peak circulating thyroid hormone (T₃) levels. Here, we examined whether this peak in T₃is involved in the loss of axonal regenerative capacity in Purkinje cells (PCs). This event occurs at the end of the first postnatal week in mice. Using organotypic culture, we found that the loss of axon regenerative capacity was triggered prematurely by early exposure of mouse PCs to T₃, whereas it was delayed in the absence of T₃. Analysis of mutant mice showed that this effect was mainly mediated by the T₃receptor α1. Using gain- and loss-of-function approaches, we also showed that Krüppel-like factor 9 was a key mediator of this effect of T₃. These results indicate that the sudden physiological increase in T₃during development is involved in the onset of the loss of axon regenerative capacity in PCs. This loss of regenerative capacity might be part of the general program triggered by T₃throughout the body, which adapts the animal to its postnatal environment.

Netrin-1 is a secreted protein that was first identified 20 years ago as an axon guidance molecule that regulates midline crossing in the CNS. It plays critical roles in various tissues throughout development and is implicated in tumorigenesis and inflammation in adulthood. Despite extensive studies, no inherited human disease has been directly associated with mutations in NTN1, the gene coding for netrin-1. Here, we have identified 3 mutations in exon 7 of NTN1 in 2 unrelated families and 1 sporadic case with isolated congenital mirror movements (CMM), a disorder characterized by involuntary movements of one hand that mirror intentional movements of the opposite hand. Given the diverse roles of netrin-1, the absence of manifestations other than CMM in NTN1 mutation carriers was unexpected. Using multimodal approaches, we discovered that the anatomy of the corticospinal tract (CST) is abnormal in patients with NTN1-mutant CMM. When expressed in HEK293 or stable HeLa cells, the 3 mutated netrin-1 proteins were almost exclusively detected in the intracellular compartment, contrary to WT netrin-1, which is detected in both intracellular and extracellular compartments. Since netrin-1 is a diffusible extracellular cue, the pathophysiology likely involves its loss of function and subsequent disruption of axon guidance, resulting in abnormal decussation of the CST.