Our work has revealed that intracellular organelles, known as ribonucleoprotein (RNP) bodies or granules, behave as liquid phase droplets of RNA and protein, which assemble by a type of intracellular phase transition. Phase transitions within living cells appear to represent a fundamental mechanism for organizing the cytoplasm and nucleoplasm (see figure below, from our 2012 Cell review; see also the article on Princeton's Bioengineering website). We study the assembly, properties, and function of these liquid states of biological matter, and the way in which they are coupled to cell size and growth control. Our work combines the tools and techniques of engineering and soft condensed matter physics with cutting edge cell and molecular biology methods.
Nucleolar Assembly and Function
Nucleoli are membrane-less organelles found within the nucleus of growing cells. Our prior work has shown that nucleoli behave as non-equilibrium liquid-like droplets. We are working to test the hypothesis that these structures assemble by a type of intracellular phase transition. We are also seeking to understand how nucleolar assembly is linked to ribosomal RNA (rRNA) transcription, and the functional consequences for cell growth.
The movie below shows nucleoli undergoing cycles of assembly and disassembly within the C.elegans embryo. Typically two bright foci are visible in each nucleus, corresponding to the two rDNA gene clusters in C.elegans. (Steph Weber)
Biophysical Properties of RNA/Protein Droplets
We are interested in understanding the biophysical properties of liquid phase RNA/protein droplets. These organelle droplets function as intracellular micro-reactors, and we aim to understand their biophysical properties and how these properties are coupled to molecular transport and reactivity within the droplets. Their properties will be important for understanding not only their biological function, but also its pathological dysregulation in diseases from cancer to neurodegeneration.
The movie below shows liquid phase droplets composed of protein and RNA. The droplets have formed via a phase transition in solution, and proceed to "rain down" onto a coverslip (Shani Elbaum).
Mechanics and Organization of the Cytoskeleton
The cytoskeleton is a dynamic network of biopolymer filaments that plays a central role in many fundamental biological processes, including cell migration, cell division, and intracellular transport. We are interested in collective properties of the cytoskeleton, and the way in which these collective properties play a role in cytoplasmic organization and cell growth.
We have a particular interest in the role of the cytoskeleton in the nucleus of cells. The potential role of actin as a mechanical scaffold within the nucleus is poorly understood. We utilize Xenopous laevis frogs to study nuclear actin and its potential role in cell growth control.
Our work has revealed that this nuclear actin is essential for stabilizing the emulsion of RNP droplets (nucleoli, HLBs, etc.) within this nucleus. As can be seen in the movie below, when we disrupt the actin network, RNP droplets (nucleoli in red and histone locus bodies in green) rapidly sediment and coalesce at the bottom of the nucleus (Marina Feric).
Cell Growth and Size Control
Many cells grow orders of magnitude in size. But how these cells "know" how big to grow remains a mystery. We use the frog X.laevis, and the worm C.elegans, as model systems to elucidate the biophysical mechanisms governing cell growth and size control. Our work increasingly relies on the development of custom microfabricated devices ("lab on a chip").
The movie below is a 3D rendering from fluorescence image stacks taken using a laser scanning confocal microscope. The move shows nucleoli within a growing C.elegans worm. We labelled these nucleoli using the CRISPR/Cas9 genome editing technique.
The image below shows the stages of growth of a frog oocyte. Although it is still a single cell, it ultimately reaches a size of > 1mm. (Marina Feric)
Funding and Support
We thank these sources of funding and support for helping make our work possible.