NCLX pumps up the heat

Mitochondria as intracellular signalling organelles. An update

Traditionally, mitochondria are known as "the powerhouse of the cell," responsible for energy (ATP) generation (by the electron transport chain, oxidative phosphorylation, the tricarboxylic acid cycle, and fatty acid ß-oxidation), and for the regulation of several metabolic processes, including redox homeostasis, calcium signalling, and cellular apoptosis. The extensive studies conducted in the last decades portray mitochondria as multifaceted signalling organelles that ultimately command cells' survival or death. Based on current knowledge, we'll outline the mitochondrial signalling to other intracellular compartments in homeostasis and pathology-related mitochondrial stress conditions here. The following topics are discussed: (i) oxidative stress and mtROS signalling in mitohormesis, (ii) mitochondrial Ca²⁺ signalling; (iii) the anterograde (nucleus-to-mitochondria) and retrograde (mitochondria-to-nucleus) signal transduction, (iv) the mtDNA role in immunity and inflammation, (v) the induction of mitophagy- and apoptosis - signalling cascades, (vi) the mitochondrial dysfunctions (mitochondriopathies) in cardiovascular, neurodegenerative, and malignant diseases. The novel insights into molecular mechanisms of mitochondria-mediated signalling can explain mitochondria adaptation to metabolic and environmental stresses to achieve cell survival.

Transferrin Receptor Functionally Marks Thermogenic Adipocytes

Background

Thermogenic adipocytes, including beige and brown adipocytes, are critical for thermogenesis and energy homeostasis. Identification of functional cell surface markers of thermogenic adipocytes is of significance for potential application in biological and clinical practices.

Methods

With a combination of RNA-sequencing of in vivo and in vitro models, we identified transferrin receptor (Tfr1), a receptor specialized for cellular iron uptake, as a previously unappreciated cell surface molecule for thermogenic adipocytes compared to white adipocytes. The alternation of Tfr1 levels under physiological and pathological stimuli was assessed, and the mitochondria functionality, browning capacity, and iron metabolism of mature adipocytes were examined with Tfr1 knockdown.

Results

Tfr1 was expressed predominantly in thermogenic adipocytes versus white adipocyte, and its expression levels were tightly correlated with the activation or inhibition status of thermogenic adipocytes under external stimuli. Besides, Tfr1 gene deficiency in thermogenic adipocytes led to reduced thermogenic gene programs and mitochondrial integrity.

Conclusion

Tfr1 functionally marks thermogenic adipocytes and could serve as a potential thermogenic adipocyte surface marker.