The Mitochondria is the Powerhouse of the Cell
How Mitochondria Contribute to Neuron Development
Background
Every high school or college-level biology student can tell you that mitochondria are the powerhouse of the cell. This is true; mitochondria are very important in cells because they produce energy in the form of ATP through cellular respiration. However, mitochondria can also impact the cell in ways you don’t learn about in high school.
Have you ever wondered about the differences between the brains of humans and other animals? They vary in size, cortex mass, neuron number, and developmental patterns. Human neurons develop more slowly than animal neurons, and this slower development is linked with slower maturation rates. Human and nonhuman cortical neurons (neurons within the cerebral cortex of the brain) both develop from stem cells. This development follows a species-specific timeline (each species has a different rate of cortical neuron maturation). It is unclear exactly how this is accomplished. However, the rate of mitochondrial activity, which is lower in humans than in other animals, is thought to drive the difference in maturation rates.
Summary
To measure mitochondrial development, the researchers developed a method to identify neurons born at different times. This method was called NeuroD1-dependent Newborn Neuron labeling (NNN). In their experiments, NNN was used to observe mitochondrial size and number throughout the process of neuronal maturation. The neuronal cells were also treated with a gamma secretase inhibitor, which is a chemical that increases the rate of cortical neurogenesis (development of new neurons in the cerebral cortex).
After new neurons were born, the researchers used another technique to track mitochondrial development. Iwata et al. tagged the mitochondria with a fluorescent protein that binds to Complex IV of the Electron Transport Chain (a set of structures in the inner mitochondrial membrane that are used for the final steps of cellular respiration). The fluorescent protein tag on Complex IV allowed the rate of mitochondrial output (how much oxygen is being used by the mitochondria) to be measured. The researchers found that the mouse neurons had high rates of mitochondrial oxygen consumption, while the human neurons had lower rates. From this data, they concluded that mouse neurons develop quickly, within weeks, while human neurons take several months to mature.
The researchers also measured levels of intermediate molecules involved in the Krebs cycle (a cellular respiration process that occurs in the mitochondrial matrix) in both human and mouse neurons. The number of Krebs cycle intermediates in human neurons was found to be lower than the number in mouse neurons. This indicated that less metabolic activity was taking place in the human mitochondria.
The researchers also discovered that NAD+ (nicotinamide adenine dinucleotide) and NADH (the reduced form of NAD+) levels varied across species. Like Krebs cycle intermediates, the levels of NADH and NAD+ can tell us how efficiently cellular respiration (the cell’s process to produce energy in the form of ATP) is occurring in a cell. In human neurons, the lower levels of Krebs cycle intermediates, NADH, and NAD+ tell us that human neurons do not mature or use oxygen as quickly as mouse neurons.
Next, glucose tracer experiments were performed on the human and mouse mitochondria. This type of experiment tracks how quickly the cell can convert between two different chemicals called pyruvate and lactate. An enzyme called lactate dehydrogenase (LDH) catalyzes this reaction. LDH has two subunits that do different jobs. LDH subunit A (LDHA) converts pyruvate to lactate, and LDH subunit B (LDHB) converts lactate to pyruvate. In their experiment, the researchers targeted LDHA with an inhibitor molecule, which turned off LDHA. Turning off LDHA allowed LDHB to convert more lactate to pyruvate. This, in turn, increased the mitochondrial oxygen consumption rate (OCR) in human neurons.
After turning off LDHA, the researchers assessed neural excitability (the extent to which neurons responded to stimuli) by measuring their response strength to membrane depolarization (changes in electrical charge across the neurons’ cell membranes). They found that the excitability rates had changed: the neurons that had been treated with the LDHA inhibitor had a stronger response to membrane depolarization than neurons that were not treated with an inhibitor. In addition to this stronger neural response, neurons with LDHA inhibition had faster neuronal differentiation (the last stage of neural development, growing dendrites and increasing in size). This data showed that the treated neurons matured faster than the control group, and supported the idea that neural maturation is interconnected with development.
The study found that while mitochondria are not the sole determinants of maturation rates, they play a significant role. This discovery suggests that further research should be done to determine how mitochondria affect other cells’ maturation rates. The powerhouse of the cell may control much more than we previously imagined! Further discoveries like this one may have a profound impact on biotechnology, and open doors for new treatments of many diseases.
