Physiological, Pathological, and Targetable Membraneless Organelles in Neurons

Abstract

Neurons require unique subcellular compartmentalization to function efficiently. Formed from proteins and RNAs through liquid-liquid phase separation, membraneless organelles (MLOs) have emerged as one way in which cells form distinct, specialized compartments in the absence of lipid membranes. We first discuss MLOs that are common to many cell types as well as those that are specific to neurons. Interestingly, many proteins associated with neurodegenerative disease are found in MLOs, particularly in stress and transport granules. We next review possible links between neurodegeneration and MLOs, and the hypothesis that the protein and RNA inclusions formed in disease are related to the functional complexes occurring inside these MLOs. Finally, we discuss the hypothesis that protein post-translational modifications (PTMs), which can alter phase separation, can modulate MLO formation and provide potential new therapeutic strategies for currently untreatable neurodegenerative diseases.

Keywords: LLPS; RNA/RNP granule; membraneless organelle; neurodegeneration; post-translational modification.

Figure 1:. Interactions driving MLO formation.

Many different types of interactions have been shown to drive or contribute to LLPS and MLO formation. These include RNA-RNA, protein-RNA (through RNA-RNA binding domain (RBD) interactions or RNA-IDR/IDP interactions), and different kinds of protein-protein interactions, like self-interactions between IDRs or interactions between an IDP and structured protein. Granule components exchange with the surrounding environment, from the dilute phase to the condensed phase. Within the granule, RNA and protein molecules both form multivalent interactions, meaning that one molecule can interact with multiple others simultaneously.

Figure 2:. Membraneless organelles in the nervous system.

The nervous system relies heavily on MLOs and has a few specialized membraneless compartments, including the postsynaptic density (PSD), the synapsin-synaptic vesicle phase, and the active zone. Neurons use transport granules to move RNAs to sites of local translation. The myelin sheath is dependent on MLOs to transport myelin basic protein mRNA and hold lipid membranes in close proximity with myelin basic protein.

Figure 3:. Hypotheses relating MLOs and aggregate formation in neurodegenerative disease.

A number of hypotheses exist to explain the connection between MLOs and protein inclusions/aggregates found in disease. Increased toxic oligomer/aggregate formation: The high protein concentration in a granule may increase the likelihood that mutant proteins will aggregate and not be dissociated with the rest of the granule. Solidification: What was once a liquid phase may persist and solidify. Accumulation of misfolded proteins: Misfolded or aggregated proteins may accumulate in MLOs. The high concentration inside MLOs may increase the formation of toxic oligomers stochastically, so they are not dissolved when the fluid parts of the granule dissociate.

Figure 4:. Granules and neurodegenerative diseases/associated proteins.

(A) Neurodegenerative disease-associated proteins grouped by the type of granule where they have been identified in cells (stress or transport granules) or whether they are capable of undergoing LLPS in vitro. (B) Neurodegenerative diseases grouped by the type of granule(s) that has been implicated in that disease. Fragile X has been added to the schematic given the roles of FMRP, its associated protein, as an mRNA transport granule component and the recently raised possibility that it undergoes LLPS (see main text).