How do endoplasmic reticulum and nucleus communicate?

How cells achieve inner communication between their organelles is one of the fundamental questions in biology. The endoplasmic reticulum (ER) and the nucleus are the two largest compartments within our cells. The ER is the site where many proteins and fat molecules are built. The nucleus contains most of our genetic material and is separated from the rest of the cell by a boundary called the nuclear membrane, which is continuous with the ER membrane. When a cell grows, the nucleus has to grow and new proteins and fat molecules need to be supplied from the ER to the nucleus. The connection between the ER and the nucleus is hereby essential. However, it has been unclear how the ER is connected to the nucleus, how shape and number of the connections change to support the key cellular functions, and which molecules regulate the connection and its function in inner cell communication. This gap in our knowledge is due to the technical challenge of visualising these connections, which requires high-resolution microscopy that can spatially distinguish the connections from the rest of the ER/nuclear membranes. I have previously established a microscopy method that allows for visualisation of structures inside cells at nano-meter resolution and at different stages of nuclear growth. This is achieved by observing a living human cell in which the nucleus is growing and then studying it using advanced techniques such as high-resolution three-dimensional electron tomography. Here, I propose a research program to reveal the structure and function of the connection between the ER and the nucleus and their molecular regulation, by combining the novel microscopic technique that I have established with quantitative live cell imaging and molecular perturbations. We will first elucidate systematically how the structure of the ER- nucleus connection changes during nuclear growth and how it correlates with the transport efficiency of membrane proteins between the ER and the nucleus, and form initial hypotheses for how the ER-nucleus connectivity controls the ER-to-nucleus transport and nuclear growth. In parallel, we will identify molecular players regulating the ER-nucleus connection by deleting candidate molecules, which will enable us to manipulate the structural features of the connection by perturbing identified molecules. By manipulating the ER- nucleus connectivity, we will test our hypotheses for how the ER-nucleus connection mechanistically controls the ER-to-nucleus transport and nuclear growth, and finally reveal the molecular mechanism that governs the structure and function of the ER-nucleus connection. As a result, this work will provide fundamental knowledge of how the two largest membrane- bounded compartments in the cell communicate with each other, and will substantially advance our understanding of communication inside cells.

How our cells achieve inner communication between their organelles is one of the fundamental questions in biology. The endoplasmic reticulum (ER) and the nucleus are the two largest compartments within our cells. The ER is the site where many proteins and fat molecules are built. The nucleus contains most of our genetic material and is separated from the rest of the cell by a boundary called the nuclear membrane, which is continuous with the ER membrane. When a cell grows, the nucleus has to grow and new proteins and fat molecules need to be supplied from the ER to the nucleus. The connection between the ER and the nucleus is hereby essential. However, it has been unclear how the ER is connected to the nucleus and how its size and number is regulated to support inner cell communication. This gap in our knowledge is due to the technical challenge of visualising these connections, which requires high-resolution microscopy that can spatially distinguish the connections from the rest of the ER/nuclear membranes. We established a microscopy method that allows for visualisation of structures inside cells at nanometer resolution and at different stages of cell growth. Using this technique, we investigated the structure of the connection between the ER and the nucleus in three different mammalian cells. Strikingly, we discovered that ER-nucleus junctions form narrow hourglass-shaped structures (~15 nm in diameter), which are distinct from the junctions within the ER. Before our research, ER-nucleus junctions were assumed to resemble the junctions within the ER network. Our observation strongly suggests that a novel mechanism, distinct from that known to remodel the ER, likely regulates ER-nucleus junctions. Furthermore, the narrow and constricted nature of junctions implies their role in ensuring correct ER-nucleus communication and homeostasis, possibly by acting as barriers that selectively regulate protein and fat molecule transport. This project lays the groundwork for many exciting future mechanistic and functional studies, which will shed light on gene expression, nuclear organisation, and disease mechanisms.