Abstract
Pulsed electromagnetic field (PEMF) is emerging as innovative treatment for regulation of inflammation, which could have significant effects on tissue regeneration. PEMF modulates inflammatory processes through the regulation of pro- and anti-inflammatory cytokine secretion during different stages of inflammatory response. Consistent outcomes in studies involving animal and human tissue have shown promise for the use of PEMF as an alternative or complementary treatment to pharmaceutical therapies. Thus, PEMF treatment could provide a novel nonpharmaceutical means of modulating inflammation in injured tissues resulting in enhanced functional recovery. This review examines the effect of PEMF on immunomodulatory cells (e.g., mesenchymal stem/stromal cells [MSCs] and macrophages [MΦ]) to better understand the potential for PEMF therapy to modulate inflammatory signaling pathways and improve tissue regeneration. This review cites published data that support the use of PEMF to improve tissue regeneration. Our studies included herein confirm anti-inflammatory effects of PEMF on MSCs and MΦ.
Introduction
The immune system plays an essential role in tissue regeneration following tissue damage as well as during cell signaling homeostasis. The immune response to tissue injury is crucial in determining the efficacy and rate of the healing process, including the extent of scarring and the restoration of organ function.1 To integrate the immune system into regenerative strategies, one of the first challenges is to modulate the precise functions of the different immune components during the tissue healing process. The regulatory interactions of the immune system with tissue regeneration are not unidirectional, and stem cells, as key players in regeneration, can modulate the immune system in several ways to facilitate regeneration.1,2
However, the immune system does not always perform a complementary role in regeneration, and several reports have suggested that increased inflammation can inhibit the regeneration process. An argument can be made that there are immune-mediated mechanisms of regeneration and repair that can be modulated by pulsed electromagnetic field (PEMF) therapy to improve the ability of tissue to regenerate. Until recently, allopathic medicine rejected the possibility that an electromagnetic field (EMF) could affect biochemical mechanisms with weak electrical fields. Biochemistry, however, is based on an understanding of the flow of energy that drives chemical reactions.3
Physical properties of molecules can be combined to express internal energy and thermodynamic potentials, which are necessary for equilibrium and homeostasis in spontaneous processes.4 New models of biophysics emphasize cooperative electrical activity of highly ordered elements at all levels of physiology: cells, tissues, organs, organ systems, as well as the entire human organism. Research has shown that effects caused by low-frequency or weak EMF therapies can induce changes in cell proliferation, alterations in membrane structure and function, changes in nucleic acids, protein phosphorylation and adenosine triphosphate (ATP) synthesis, as well as entrainment of brain rhythms and conditioned brain response in vitro and in vivo.5–7 Parameters of these EMFs include frequency, intensity (field strength), waveform, and time of exposure. Recognition of physiological sensitivities to exogenous EMF came from the observation of endogenous internal electrical processes.4 For example, the piezoelectric properties of bone use electromechanical control to determine either osteoblastic or osteoclastic phenotype of cells.8 However, biophysical properties of cell function have mostly been ignored when choosing treatments for inflammation/immune modulation and regenerative medicine therapies.
Using PEMF to regulate cell signaling mechanisms involved in the inflammatory/immune response pathways of different cell types has become an innovative alternative treatment in the pursuit of regenerative therapies.9,10 Several studies have reported that PEMF can modulate both cell surface receptor expression/activation and downstream signal transduction pathways, thereby restoring homeostatic cell functions such as viability, proliferation, differentiation, communication with neighboring cells, and interaction with components of the extracellular matrix (ECM).11–18 PEMF can activate multiple intracellular pathways, including numerous processes and biochemical mechanisms within both the immune system and tissue regenerative processes, such as the musculoskeletal system7,19,20 and the nervous system.21–23 PEMFs are physical stimuli that affect biological systems through the production of coherent or interfering fields that modify fundamental electromagnetic frequencies generated by living organisms.7,24 These endogenous frequencies are ubiquitous in tissue, for instance, frequencies from 5 to 30 Hz have been found during postural muscle activity (quiet standing) and 10 Hz during walking.25 Successful regeneration requires a balanced immune cell response, with the recruitment of accurately polarized immune cells in an appropriate quantity.2 Here is where PEMF could have an influential role in the inflammatory process and thereby support tissue regeneration.
Mechanisms of Action
The immune response is a tightly regulated process where any imbalance in its strict regulation could lead to pathological conditions. The important role of ion channel stability in immune function is becoming more apparent. After immune activation, changes in the cells’ microenvironment are integrated into a survival response by complex signal transduction mechanisms.79 Lipid nanopores forming stable ion channel conduction pathways in the plasma membrane of cells80 explain the conduction of ions into the cell from the extracellular space.80 It has been postulated that a direct effect of PEMF on phospholipids within the plasma membrane stimulates the production of second messengers, initiating multiple intracellular signal transduction pathways.81–83 PEMF can alter cell function by triggering the forced vibration of free ions on the surface of the plasma membrane, causing external oscillating field disruptions in the electrochemical balance of transmembrane proteins (ion channels).24,84
The formation of a complex multicellular organism from a single cell is one of the most amazing processes of biology. Embryonic development is characterized by the careful regulation of cell behaviors such as cell proliferation, migration, differentiation, and tissue formation at the perfect time and place. These processes are dependent on the activities of genetics, signaling pathways, and information processing that coordinate cellular interactions leading to organogenesis.85 During human development, lineage-committed cells of the three embryonic germ layers migrate and proliferate in the form of endogenous ionic currents, giving rise to EFs.86 While endogenous EFs are present in all developing and regenerating animal tissues, their existence in inflammatory/immune modulation and tissue regeneration has been largely ignored. Ion flux is closely involved in differentiation control as stem cells migrate and proliferate in specific directions to form tissues and organs, each having their own signature characteristics to form specific cell and tissue types. Applying the PEMF would modulate mechanisms of action that play significant roles in action potential/voltage-gated ion regulation. The density of the musculoskeletal system versus the delicacy of the immune system shows two very different characteristics in human physiology; therefore, the targeted tissue would require different dosimetry.
The mechanisms through which PEMF exchanges information between cells, and how the conversion of this biochemical signaling is translated, have been researched for decades showing that the PEMF can permeate both the plasma and nuclear membranes of cells, thereby affecting a variety of cell functions and tissue types.87–89 For example, PEMF can induce depolarization in the cell membrane, followed by an increase or decrease of intracellular calcium (Ca2+).90 While Ca2+ release from voltage-gated Ca2+channels (VGCCs) regulates immune responses to pathogens,91 inhibiting VGCCs in infected macrophages can reduce calcium influx, upregulating the expression of proinflammatory genes.91 As biophysicists point out, a very important factor for regulating cell homeostasis is the level of the resting potentials, generated on the cell membrane.19,92 VGCCs are activated by membrane depolarization in action potentials,93 and when regulated by physical stimuli, VGCCs play a pivotal role in MSC differentiation. Levin and colleagues have shown that human MSC differentiation is accompanied by progressive hyperpolarization of voltage-gated ion channels.94 Artificial depolarization keeps these cells in an undifferentiated state, whereas artificial hyperpolarization accelerates differentiation.95 Poor regenerative capacity of musculoskeletal tissue has been the focus of regenerative medicine for many years. VGCCs are a group of membrane proteins that are predominantly found in excitable cells, such as cardiomyocytes, muscle, neurons and glial cells. VGCCs are known for their involvement in electrical current generation but are also expressed in nonexcitable cells including osteoblasts and chondrocytes.96 VGCCs increase intracellular Ca2+ concentration, which leads to the initiation of different physical stimuli, such as electrical, electromagnetic/magnetic, and mechanical function in regenerative processes.97 The bioelectric properties of a cell are mainly defined by the cellular membrane potential that controls different cell functions, which depend on the particular cell type.98 Electrically charged membranes tightly regulate the concentration of ions such as electrically charged Ca2+, sodium (Na+), and/or potassium (K+), which MSCs use as potent signal mediators.99 Here is where the effects of PEMF in cells occur, triggered at the membrane level. Evidence shows that PEMF can act on Ca2+ concentrations,100–102 Ca2+-dependent pathways,103 as well as Na+ and K+ pathways.104 PEMF can affect action potentials and hyperpolarization to modulate endogenous electrical potentials in plants, animals, and humans.7,95,105,106 Multiple factors cause discrepancies in the outcomes of PEMF-exposed cells during the inflammatory response. These variations include frequency, intensity, time of exposure and waveform, as well as the biological sample. The goal is to find the optimal PEMF dosimetry for creating homeostasis of cytokine signaling, transcription factors, and ion-flux-driven action potentials.
Conclusion
Poor regulation of inflammatory/immune function can allow acute-phase inflammatory response to become chronic, initiating disease and inhibiting tissue regeneration. Current theories of damage-associated molecules released by injured/infected cells or secreted by innate immune cells generate danger signals, activating the immune response. These signaling mechanisms are important to the subsequent activation of homeostatic mechanisms that control the immune response in pro- and anti-inflammatory reactions that allow for therapeutic treatments.9 In this review, we described the effects of PEMF on inflammatory/immune regulators and transcription factors relevant to the activation of danger signals and innate immune cells. Achieving homeostasis in the face of acute inflammatory/immune challenges in the human body involves maintaining a balance of highly complex biochemical and cellular interactions, such as cytokine expression and signal transduction. When this delicate balance is upset, acute inflammatory and immune responses designed to quickly eliminate a transient threat become chronic, and inflammatory and/or autoimmune disease sets in. The importance of maintaining healthy cytokine expression during this impactful time cannot be overstated. Our feasibility study shows that PEMF has the potential to regulate this very delicate balance. More investigative research is needed to discover therapies to regulate signaling molecules involved in inflammation and tissue regeneration. We propose PEMF as such a therapy and performed a proof-of-concept study using MSCs, MΦ, and C2C12 cells, exposing them to PEMF to investigate its effect on expression of inflammatory molecules after insult. Results show that the immunomodulatory effect of this therapy has the potential to decrease the production of proinflammatory secretion, while stabilizing or increasing anti-inflammatory cytokine production, and NF-κB expression during activated response. By modulating the expression of various signaling cascades and cellular information processing networks to restore them to homeostatic (healthy) production levels, PEMF is showing promise as a treatment for inflammatory regulation to be used to promote tissue regeneration.
