A study published in the EMBO Journal by the Amoyel Lab has uncovered how developing sperm cells receive the nutrients they need to survive despite being physically isolated from the bloodstream.
In the testes, male germ cells develop behind a protective barrier formed by surrounding somatic cells. In mammals, this structure is known as the blood-testis barrier. While the barrier shields germ cells from potentially harmful substances, it also prevents them from directly accessing nutrients circulating in the body. How germ cells obtain sufficient energy for survival has remained an important unanswered question.
The new study reveals that somatic support cells solve this problem by acting as metabolic intermediaries. By studying metabolism in the testis of the fruit fly Drosophila, the researchers showed in vivo that these cells take up circulating sugars and break them down through glycolysis, producing lactate that is then transferred to neighbouring germ cells. The germ cells depend on this lactate supply for their survival.
The team also identified a previously uncharacterised transporter responsible for moving lactate between cells. Because of its role in delivering this essential metabolic fuel, the researchers named the transporter “milkman”.
“Developing germ cells are effectively cut off from direct access to nutrients in the bloodstream,” said Dr Amoyel, co-corresponding author in the study. “We found that surrounding support cells act as dedicated metabolic providers, converting sugars into lactate and delivering it to the germline. The discovery of the transporter we named ‘milkman’ provides a key missing piece of this nutrient-sharing system.”
Importantly, the study found that somatic support cells actively prevent themselves from consuming these sugar-derived resources. When the researchers increased carbohydrate consumption within the somatic cells, less lactate was available to support the germline, leading to increased germ cell death.
“Our results show that successful reproduction depends on a carefully controlled division of metabolic labour,” said Dr Sainz de la Maza, co-corresponding author in the study. “The support cells must prioritise feeding the germline rather than themselves. When that balance is disrupted, germ cells begin to die.”
While previous experiments performed in laboratory cultures had suggested that lactate may support germ cells in mammalian testes, this study using fruit flies provides the first direct demonstration of the process in a living organism. The findings suggest that a similar nutrient-sharing mechanism may operate in mammals, including humans.
“This is the first time this metabolic support system has been demonstrated in vivo,” said Dr Amoyel. “Earlier studies hinted that the same process occurs in mammalian testes, so our findings may have important implications for understanding male fertility in humans.”