Liquid-Liquid Phase Separation in Physiology and Pathophysiology of the Nervous System

Abstract

Molecules within cells are segregated into functional domains to form various organelles. While some of those organelles are delimited by lipid membranes demarcating their constituents, others lack a membrane enclosure. Recently, liquid-liquid phase separation (LLPS) revolutionized our view of how segregation of macromolecules can produce membraneless organelles. While the concept of LLPS has been well studied in the areas of soft matter physics and polymer chemistry, its significance has only recently been recognized in the field of biology. It occurs typically between macromolecules that have multivalent interactions. Interestingly, these features are present in many molecules that exert key functions within neurons. In this review, we cover recent topics of LLPS in different contexts of neuronal physiology and pathology.

Keywords: liquid-liquid phase separation; local protein synthesis; membraneless organelle; neurodegenerative diseases; synapse.

Figure 1.

Phase separation illustrated by a simple two-component system. A, Free energy diagram showing phase separation of a two-component system (e.g., a protein indicated by blue dots; in water indicated by brown dots) under a certain condition. A uniformly mixed system can undergo phase separation by lowering the free energy to its minima, which results in a two-phase system: a dilute phase (Φ_d, expressed as fraction volume for the dilute phase) and a condensed phase (Φ_c, fraction volume for the condensed phase). B, Phase diagram of the two-component system constructed by plotting the free energy minima as a function of temperature. Blue curve indicates a sharp boundary (or the threshold concentration) of the system transitioning from a homogeneous single-phase state to a two-phase state. Within the phase separation region, two modes of phase separation, binodal nucleation and spinodal decomposition, can occur. C, In a phase-separated two-component system, a thermodynamic equilibrium is reached (i.e., ΔG_d/c = 0). A sharp gradient in the concentration of the blue molecule is established between the two phases. D, After phase separation, the components of the condensed phase and the diluted phase can freely exchange. However, there is no net flow of components between the two phases. E, An example of binodal nucleation-induced phase separation forming condensed spherical droplets (left) and an example of spinodal decomposition-induced phase separation forming worm-like condensed networks (right). F, In sharp contrast to membraneless condensates, spontaneous compartment fusion or materials exchange does not occur in membrane-separated organelles.

Figure 2.

RNA binding proteins are involved in RNA stability (P bodies), mRNA transport (mRNA transport granules), translation, and stress granules (SG) formation. Under transient stress, protein-protein and RNA interactions form a dense SG core. Several RNA binding proteins can be recruited to SG cores and undergo LLPS forming functional dynamic structures (physiological LLPS). Under conditions of transient stress, SGs are transiently formed but disassemble after the stress is gone. In case of prolonged stress, and after post-translational modifications, such as phosphorylation, proteins can become insoluble (pathologic LLPS). The same RNA binding proteins can participate in the formation of nontoxic hydrophobic aggregates and toxic cytoplasmic inclusions.

Figure 3.

Schematic diagram LLPS at synapses. Synapses contain various unique biological condensates, such as active zones and PSD. In a presynaptic bouton (light blue), the reserve pool of synaptic vesicles (SV) can form molecular condensates via coacervating with the synapsin condensates. The docked pool of synaptic vesicles instead coats the surface of active zone condensates formed by proteins, including RIM, RIM-BP, and ELKS. In the postsynaptic neuron (purple) and both in excitatory and inhibitory synapses, formation of PSD assemblies may also involve phase separation of synaptic scaffold proteins interacting with neurotransmitter receptors.