Phosphate is essential to life. Now, researchers have discovered a tiny structure inside animal cells that acts like a reservoir of phosphate, helping to regulate levels of the nutrient inside cells and triggering processes that maintain tissues when it is in short supply ¹ . The researchers classify the structure as a new type of organelle — fundamental structures in cells, such as the nucleus, mitochondria and the membrane, that function as miniature organs in its body.

“This is one of the first studies to actually find phosphate storage in an animal cell,” says Rebekka Wild, a structural biologist at the French national research agency CNRS in Grenoble, who was not involved in the research. “It’s really exciting.”

In plants, bacteria and yeast, phosphate is important for cell growth and helps cells to communicate and generate energy. Although it is known to be essential in animal tissues and cells, few studies had explored its specific functions. Charles Xu, a geneticist at the Rockefeller University in New York City, was curious about what part phosphate played in regulating tissue renewal in the fruit-fly gut, a useful model for studying how diseases affect cells in the human intestine. “That’s not really well known, especially in animal cells,” says Xu.

Fruit-fly findings

Xu and his colleagues fed fruit flies ( Drosophila melanogaster ) phosphonoformic acid (PFA), which inhibits absorption of phosphorous in cells. When the researchers stained and imaged cells from the flies’ intestinal lining, they noticed that the lack of phosphate led to a spike in cell numbers. This rapid cell multiplication also occurred when Xu and his colleagues fed the flies food that contained 10% less phosphate than standard levels, indicating that the phosphate did indeed have an impact on cell numbers.

To find out how phosphate was having this effect, Xu and his team investigated whether low phosphate levels affected gene expression. A gene that the authors call PXo is similar to a mammalian gene that encodes a phosphate-sensing protein. Xu and colleagues found that PXo ’s expression was weaker when cells were deprived of phosphate. This reduced gene expression also kicked cell division into overdrive. However, cell division slowed down when the researchers tweaked the gene to overexpress the PXo protein.

The researchers labelled the PXo protein with a fluorescent tag and noticed that it was associated with an array of oval-shaped structures in the cells that did not seem to be any of the known organelles.

Phospholipid reservoirs

“These were quite visible, and we wondered what they were,” says Xu. When the scientists took a closer look at the mysterious structures, they saw they had several membrane layers, and the PXo protein was transporting phosphate across them. Once inside the unfamiliar organelles, the phosphate was converted to phospholipids, the main building blocks of cellular membranes.

When the fly cells were deprived of phosphate, the organelles broke apart and released the stored phospholipids into each cell, indicating that they function like reservoirs, says Xu. This breakdown activated cellular machinery known as Cka, triggering a stress signal that increased the production of new cells. This could be a way for the intestinal lining to keep phosphate levels stable, because the increased number of cells can absorb more of the nutrient, says Xu. “It’s beneficial for the organism to regenerate more of these healthy [cells],” he says.

Wild says that the findings lay the groundwork for exploring whether there are similar phosphate-storing organelles in other animals, including humans. She adds that it could be useful to take a deeper look at the structure of the PXo protein, to unravel how it transports phosphate into the organelles. “This would be very interesting, especially for people who come from the structural-biology side,” she says.

Xu says a next step could be to investigate how these phosphate-storing organelles interact with other organelles, and how their dynamics change over time. “It’s opened the door to many other questions,” he says.

The discovery of a new organelle in animal cells also highlights how much there is still to learn about cell physiology, adds Xu. “The beauty is there, it's just waiting for us to discover it,” he says.

article_text: Phosphate is essential to life. Now, researchers have discovered a tiny structure inside animal cells that acts like a reservoir of phosphate, helping to regulate levels of the nutrient inside cells and triggering processes that maintain tissues when it is in short supply1. The researchers classify the structure as a new type of organelle — fundamental structures in cells, such as the nucleus, mitochondria and the membrane, that function as miniature organs in its body. “This is one of the first studies to actually find phosphate storage in an animal cell,” says Rebekka Wild, a structural biologist at the French national research agency CNRS in Grenoble, who was not involved in the research. “It’s really exciting.” In plants, bacteria and yeast, phosphate is important for cell growth and helps cells to communicate and generate energy. Although it is known to be essential in animal tissues and cells, few studies had explored its specific functions. Charles Xu, a geneticist at the Rockefeller University in New York City, was curious about what part phosphate played in regulating tissue renewal in the fruit-fly gut, a useful model for studying how diseases affect cells in the human intestine. “That’s not really well known, especially in animal cells,” says Xu. Xu and his colleagues fed fruit flies (Drosophila melanogaster) phosphonoformic acid (PFA), which inhibits absorption of phosphorous in cells. When the researchers stained and imaged cells from the flies’ intestinal lining, they noticed that the lack of phosphate led to a spike in cell numbers. This rapid cell multiplication also occurred when Xu and his colleagues fed the flies food that contained 10% less phosphate than standard levels, indicating that the phosphate did indeed have an impact on cell numbers. To find out how phosphate was having this effect, Xu and his team investigated whether low phosphate levels affected gene expression. A gene that the authors call PXo is similar to a mammalian gene that encodes a phosphate-sensing protein. Xu and colleagues found that PXo’s expression was weaker when cells were deprived of phosphate. This reduced gene expression also kicked cell division into overdrive. However, cell division slowed down when the researchers tweaked the gene to overexpress the PXo protein. The researchers labelled the PXo protein with a fluorescent tag and noticed that it was associated with an array of oval-shaped structures in the cells that did not seem to be any of the known organelles. “These were quite visible, and we wondered what they were,” says Xu. When the scientists took a closer look at the mysterious structures, they saw they had several membrane layers, and the PXo protein was transporting phosphate across them. Once inside the unfamiliar organelles, the phosphate was converted to phospholipids, the main building blocks of cellular membranes. When the fly cells were deprived of phosphate, the organelles broke apart and released the stored phospholipids into each cell, indicating that they function like reservoirs, says Xu. This breakdown activated cellular machinery known as Cka, triggering a stress signal that increased the production of new cells. This could be a way for the intestinal lining to keep phosphate levels stable, because the increased number of cells can absorb more of the nutrient, says Xu. “It’s beneficial for the organism to regenerate more of these healthy [cells],” he says. Wild says that the findings lay the groundwork for exploring whether there are similar phosphate-storing organelles in other animals, including humans. She adds that it could be useful to take a deeper look at the structure of the PXo protein, to unravel how it transports phosphate into the organelles. “This would be very interesting, especially for people who come from the structural-biology side,” she says. Xu says a next step could be to investigate how these phosphate-storing organelles interact with other organelles, and how their dynamics change over time. “It’s opened the door to many other questions,” he says. The discovery of a new organelle in animal cells also highlights how much there is still to learn about cell physiology, adds Xu. “The beauty is there, it's just waiting for us to discover it,” he says. vocabulary:

{'Phosphate': '磷酸盐，是生命必需的元素，可以帮助细胞产生能量、传递信息和生长发育','Organelle': '细胞器，是细胞内的基本结构，具有像器官一样的功能','Phosphonoformic acid': '磷酸甲酯酸，是一种抑制磷酸盐吸收的物质','Drosophila melanogaster': '果蝇，是一种常用的模式生物，用于研究疾病如何影响人类肠道细胞','Phospholipids': '磷脂，是细胞膜的主要组成部分','Cka': '细胞活性蛋白，可以触发细胞分裂','Phosphate-storing': '磷酸盐储存，指细胞内的磷酸盐储存结构','Phosphate-sensing': '磷酸盐感应，指细胞内的磷酸盐感应蛋白','Fluorescent': '荧光，指物质受到光照射后发出的荧光','Overexpress': '过表达，指基因表达水平超过正常水平','Staining': '染色，指用染料染色的一种技术','Transporting': '运输，指物质从一个地方运输到另一个地方','Membrane': '膜，指细胞膜','Nucleus': '核，指细胞核','Mitochondria': '线粒体，指细胞内的能量工厂','Regenerate': '再生，指细胞的再生过程','Physiology': '生理学，指研究生物体内的生理过程','Interact': '相互作用，指两个或多个物质之间的相互作用','Dynamics': '动力学，指研究物质运动的规律','Deprived': '剥夺，指物质被剥夺的状态','Triggering': '触发，指物质被触发的状态'} readguide:

{'reading_guide': '研究人員發現動物細胞內的一個小結構可以像磷酸鹽儲存器一樣，幫助調節細胞內營養素的水平，並在缺乏時觸發維持組織的過程。研究人員將這種結構分類為一種新型的細胞器，它在細胞內充當微型器官的功能。研究人員發現，當細胞缺乏磷酸鹽時，PXo基因的表達會減弱，這也會導致細胞分裂加快。研究人員發現，PXo蛋白質與細胞內的一系列橢圓形結構有關，這些結構似乎不是已知的細胞器。研究人員發現，當細胞缺乏磷酸鹽時，這些結構會分裂，釋放出儲存的磷脂，激活細胞機制Cka，觸發壓力信號，增加新細胞的產生。這可能是腸粘膜維持磷酸鹽水平穩定的一種方式，因為增加的細胞數量可以吸收更多的營養素。研究發現，動物細胞內存在一種新型的細胞器，它們可以像儲存器一樣儲存磷酸鹽，這也凸顯出我們對細胞生理學還有多少東西還不了解。'} long_sentences:

{'sentence 1': '研究人員將這個結構歸類為一種新型的細胞器，它們在細胞中充當微型器官的功能，像是細胞核、細胞質和膜等，', 'sentence 2': 'Xu和他的同事們給果蠅（Drosophila melanogaster）喂食磷酸甲酯（PFA），這種物質可以抑制細胞吸收磷，當研究人員對果蠅腸表面的細胞進行染色和成像時，他們發現缺乏磷導致細胞數量激增。'}

sentence 1: 研究人員將這個結構歸類為一種新型的細胞器，它們在細胞中充當微型器官的功能，像是細胞核、細胞質和膜等， 句子結構：主詞為「研究人員」，動詞為「將歸類」，修飾詞為「這個結構」，受詞為「一種新型的細胞器」，其中「它們在細胞中充當微型器官的功能，像是細胞核、細胞質和膜等」為修飾語。 语义分析：本句描述研究人員將某種結構歸類為一種新型的細胞器，它們在細胞中充當微型器官的功能，像是細胞核、細胞質和膜等。可以看出，本句描述的是研究人員對某種結構的歸類，以及它們在細胞中的功能。

sentence 2: Xu和他的同事們給果蠅（Drosophila melanogaster）喂食磷酸甲酯（PFA），這種物質可以抑制細胞吸收磷，當研究人員對果蠅腸表面的細胞進行染色和成像時，他們發現缺乏磷導致細胞數量激增。 句子結構：主詞為「Xu和他的同事們」，動詞為「給喂食」，受詞為「果蠅（Drosophila melanogaster）」，修飾詞為「磷酸甲酯（PFA）」，其中「這種物質可以抑制細胞吸收磷，當研究人員對果蠅腸表面的細胞進行染色和成像時，他們發現缺乏磷導致細胞數量激增」為修飾語。 语义分析：本句描述Xu和他的同事們給果蠅喂食磷酸甲酯，這種物質可以抑制細胞吸收磷，當研究人員對果蠅腸表面的細胞進行染色和成像時，他們發現缺乏磷導致細胞數量激增。可以看出，本句描述的是Xu和他的同事們給果蠅喂食磷酸甲酯，以及缺乏磷導致細胞數量激增的情況。