RIDC Redoxoma researchers have found that changes in mitochondrial calcium transport in astrocytes modify brain metabolism and improve cognitive performance in mice. They demonstrated that the NCLX transporter, responsible for mitochondrial calcium efflux, modulates glycolytic flux and lactate secretion, shaping cytosolic calcium signaling. In this way, NCLX may act in the control of brain metabolism, affecting the transport of lactate from astrocytes to neurons and, therefore, brain function.

The research, published in the Journal of Neurochemistry, was conducted by João Victor Cabral-Costa during his doctorate, under the supervision of Professor Alicia Kowaltowski, from the Instituto de Química at Universidade de São Paulo (USP) and the RIDC Redoxoma. The researcher developed part of the work in collaboration with professor Juan P. Bolaños’s group, at the University of Salamanca, Spain, with the support of a FAPESP Research Internship Abroad fellowship (BEPE).

"The actual major output of this work was the opening of a new line of research, a way to better understand NCLX function. We know that in Alzheimer’s and Parkinson’s it is key, besides of playing a role in other diseases and other organs. Ours was one of the first works that observed a modulation of metabolism by NCLX. It could be that NCLX acts as a sensor or is part of a hub that helps detecting astrocytes’ energy demand — which is directly related to neuronal energy demand brain activity. Thus, as a research tool, this finding is quite important because it reinforces the hypothesis of the lactate shuttle, of the metabolic coupling between astrocytes and neurons,” said Cabral-Costa.

Our cells depend on calcium to function. The intracellular calcium ion (Ca²⁺) concentration is strictly controlled by transporters in the plasma membrane, endoplasmic reticulum, and mitochondria. In mitochondria, calcium induces the activity of several enzymes associated with oxidative metabolism, coupling ATP supply to cellular energy demands. Thus, mitochondrial calcium transport — in particular, influx by the mitochondrial Ca²⁺ uniporter (MCU) and efflux by the mitochondrial Na⁺/Ca²⁺ exchanger (NCLX) — constitutes a critical point of the metabolic regulation.

Astrocytes

Astrocyte.

Figure: João Victor Cabral-Costa

From a metabolic point of view, the brain is considered our most active organ and, although it represents 2% of the body weight of an adult person, it consumes about 25% of the body’s energy (at rest).

Cabral-Costa says that the researchers were interested in studying the physiological role of mitochondrial calcium transport in brain function. In a search in two public mouse brain transcriptomic databases, i.e. the set of all RNA molecules expressed in the brain, they found that astrocytes express more NCLX messenger RNA than other brain cells, with an enrichment up to 10 times.

Calcium fluctuations within mitochondria and the cytosol impact metabolism and NCLX is a mitochondrial protein responsible for mitochondrial calcium flux, moving calcium ions from the mitochondrial matrix to the intermembrane space in exchange for extramitochondrial sodium ions. Mitochondrial calcium handling is essential for cerebral homeostasis, acting in several processes, such as the integration of astrocyte-neuron activity, control of energy metabolism, and neurodegeneration.

“In general, when we hear about the brain, it is natural to think of neurons, which are the cells that transmit electrical impulses. In fact, the literature is very focused on neurons. But we have several other additional cell types in the brain, mainly the so-called glial cells, of which astrocytes are a part, and which are equally important,” the researcher explains.

Astrocytes, so named because of their star-like shape, were once considered cells that only supported neurons, but today it is known that they perform several functions. They help control ionic balance in the brain, modulate communication at synapses, and play a role in neuron nutrition. They are also one of the components of the blood-brain barrier, helping to protect the brain against pathogens and toxins. In addition, they influence the migration of neurons and act as synaptic insulators, controlling neurotransmitters concentrations and preventing the disorderly propagation of nerve impulses. Basically, without astrocytes, neurons could not function properly.

To investigate the effects of NCLX on astrocyte function, the researchers pharmacologically inhibited the protein in a primary culture of mouse astrocytes. As a result, they observed an increase in glycolytic flux and lactate secretion, suggesting that NCLX has a key functional role in the metabolic homeostasis of these cells.

As the lactate secreted by these cells is used as a substrate by neurons, with known effects on memory and synaptic plasticity, the researchers decided to investigate the impact of these metabolic changes on brain function in vivo. For this, they deleted NCLX in astrocytes and neurons in mice’s hippocampus and subjected the animals to behavioral and cognitive evaluation tests. The hippocampus is a brain structure involved in forming new memories and is associated with learning and emotions.

The researchers observed that the deletion of astrocyte-specific NCLX in the hippocampus improved aspects of the mice’s cognitive performance. Deletion of NCLX in neurons promoted deleterious effects.

Cabral-Costa explains that astrocytes have very active mitochondrial respiration, i.e., they oxidize substrates through the mitochondria, but they are also very fermentative cells — they have very active glycolytic flux — and secrete lactate. "In 1994, Pellerin and Magistretti raised the hypothesis of a lactate shuttle produced by astrocytes to neurons. Although some points of it are still debated, nowadays this theory is well supported. The lactate shuttle between astrocytes and neurons is associated with various brain processes, including higher cognitive functions. Therefore, if you modulate this system, you affect electrophysiological parameters associated with memory, with synaptic plasticity in neurons, for instance”.

Basic research

With this work, Cabral-Costa received the award for best oral communication among young scientists and best poster of the day at the Young Scientists Forum (YSF 2022) held last year, in Portugal, during The Biochemistry Global Summit, which included the 25^th IUBMB Congress, the 46^th FEBS Congress, and the 15^th PABMB Congress.

For the researcher, this work emphasizes the importance of basic research. “We were investigating the physiological function of a mitochondrial calcium transporter in mice astrocytes. That is super interesting from the biochemical point of view — astrocyte culture, calcium tracing, lactate measurements. But what now? Do we know the whole mechanism involved? Did we find a cure? No. But the paths we have opened — or borrowing Marcelo Gleiser’s metaphor on the Island of Knowledge, the sand we have put onshore, increasing the edge of the island — create new possibilities for lines of research, and then we can even think about the impact ahead. Not that all basic research has to generate an application, but here we showed that basic research has the potential to contribute to the advancement of science, in one way or the other."

The article Mitochondrial sodium/calcium exchanger NCLX regulates glycolysis in astrocytes, impacting on cognitive performance, by João Victor Cabral-Costa, Carlos Vicente-Gutiérrez, Jesús Agulla, Rebeca Lapresa, John W. Elrod, Ángeles Almeida, Juan P. Bolaños and Alicia J. Kowaltowski, can be accessed here.