Microwaves (MW), electromagnetic waves with frequencies ranging from 300 MHz to 300 GHz, have been widely used in the telecommunications, agriculture, transportation, medical and military fields. The popularization of mobile phones, computers, household appliances and other electronic equipment has made learning, working and accessing entertainment much more convenient. With the intensive development of various advanced military weaponry equipment, such as early warning aircraft, electronic jammers and new radar, soldiers are always exposed to intricate environmental factors, including intensive and complex MW radiation. As the fourth largest source of pollution after air, water and noise, MW radiation induces many biological effects. The brain is the most sensitive target organ for MW radiation, where mitochondrial injury occurs earlier and more severely than in other organs. Studies on the effects of MW radiation on brain energy metabolism have aroused great concern.
Features of brain energy metabolism
The human body, the brain has the greatest demand for oxygen and is susceptible to disturbances in energy metabolism, which is determined by its high metabolic rate, high oxygen consumption and low energy reserves. Mitochondria are the key sites of oxidative phosphorylation (OXPHOS) and the synthesis of adenosine triphosphate (ATP). The redox enzymes and the coenzymes involved in the respiratory chain lie in the mitochondrial inner membrane in close proximity. Electrons passing through the respiratory chain drive protons from the matrix side to the cytoplasmic side across the mitochondrial inner membrane. When protons reflux along the concentration gradient, the energy released is used by ATP synthase to catalyze ATP synthesis.
In addition to energy conversion, mitochondria also play other important roles, such as in the regulation of apoptosis and Ca2+ storage. Mitochondria are not only the starting point of many signal transduction pathways but also the target.
MW radiation is detrimental to brain energy metabolism. Intrinsically, neurons are extremely sensitive to a reduced ATP availability. As the main source of energy, mitochondria are prone to MW radiation-induced injury.
Effects of MW radiation on mitochondrial structure
MW radiation leads to mitochondrial structural damage, primarily observed as mitochondrial swelling and cavitation and disorganized, broken and sparse cristae.
As the “cell power plant”, the most important function of mitochondria is to provide energy for the cell; therefore, intracellular ATP content is one of the most direct and objective indicators in the evaluation of mitochondrial function. In addition, ATPases hydrolyze ATP to ADP and release the energy stored in ATP.
Apoptotic death of neural cells
During the process of MW radiation-induced brain damage, apoptosis is one of the final outcomes of damaged cells. Blocking apoptosis to relieve the effect of MW radiation on the nervous system and to find new targets for prevention and treatment is of great value.
MW radiation activates the NADH oxidase-mediated increase in ROS, and in turn, excessive ROS damages the mitochondrial electron transport chain, which is the main source of ROS, ultimately forming a vicious cycle and aggravating the disturbance in brain energy metabolism
To date, the damaging effects of MW radiation on mitochondrial structure and function have been recognized, and studies at the cellular and molecular level on the related mechanisms have also made advances, enabling a number of potential molecular targets for the prevention and treatment of MW radiation to be proposed.