Biophysical Studies of LLPS and Aggregation of TDP-43 LCD.

Growing evidence indicates that liquid-liquid phase separation (LLPS), a phenomenon whereby transient, weak interactions can facilitate self-assembly of proteins into liquid-like droplets and can contribute to the formation of amyloid fibrils. Such an observation has posited that LLPS and the associated formation of membrane-less organelles in the cell can contribute to protein aggregation in neurodegenerative disease. In this chapter, we describe methods for performing biophysical studies on the transactive response DNA-binding protein of 43 kDa (TDP-43), a protein that forms aggregates in amyotrophic lateral sclerosis and frontotemporal lobar degeneration. We describe purification of the disordered low-complexity domain (LCD) of TDP-43 and provide a methodology for studying the protein's behavior using site-directed spin labeling coupled with electron paramagnetic resonance. We additionally discuss visualization of TDP-43 LCD liquid droplets and methods for quantifying LLPS and aggregation into amyloid fibrils.

Cryo-EM structure of amyloid fibrils formed by the entire low complexity domain of TDP-43.

Amyotrophic lateral sclerosis and several other neurodegenerative diseases are associated with brain deposits of amyloid-like aggregates formed by the C-terminal fragments of TDP-43 that contain the low complexity domain of the protein. Here, we report the cryo-EM structure of amyloid formed from the entire TDP-43 low complexity domain in vitro at pH 4. This structure reveals single protofilament fibrils containing a large (139-residue), tightly packed core. While the C-terminal part of this core region is largely planar and characterized by a small proportion of hydrophobic amino acids, the N-terminal region contains numerous hydrophobic residues and has a non-planar backbone conformation, resulting in rugged surfaces of fibril ends. The structural features found in these fibrils differ from those previously found for fibrils generated from short protein fragments. The present atomic model for TDP-43 LCD fibrils provides insight into potential structural perturbations caused by phosphorylation and disease-related mutations.

Small molecules as potent biphasic modulators of protein liquid-liquid phase separation.

Liquid-liquid phase separation (LLPS) of proteins that leads to formation of membrane-less organelles is critical to many biochemical processes in the cell. However, dysregulated LLPS can also facilitate aberrant phase transitions and lead to protein aggregation and disease. Accordingly, there is great interest in identifying small molecules that modulate LLPS. Here, we demonstrate that 4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonic acid (bis-ANS) and similar compounds are potent biphasic modulators of protein LLPS. Depending on context, bis-ANS can both induce LLPS de novo as well as prevent formation of homotypic liquid droplets. Our study also reveals the mechanisms by which bis-ANS and related compounds modulate LLPS and identify key chemical features of small molecules required for this activity. These findings may provide a foundation for the rational design of small molecule modulators of LLPS with therapeutic value.

Studying Protein Aggregation in the Context of Liquid-liquid Phase Separation Using Fluorescence and Atomic Force Microscopy, Fluorescence and Turbidity Assays, and FRAP.

Liquid-liquid phase separation (LLPS) underlies the physiological assembly of many membrane-less organelles throughout the cell. However, dysregulation of LLPS may mediate the formation of pathological aggregates associated with neurodegenerative diseases. Here, we present complementary experimental approaches to study protein aggregation within and outside the context of LLPS in order to ascertain the impact of LLPS on aggregation kinetics. Techniques described include imaging-based approaches [fluorescence microscopy, atomic force microscopy (AFM), fluorescence recovery after photobleaching (FRAP)] as well as plate reader assays [Thioflavin-T (ThT) fluorescence intensity and turbidity]. Data and conclusions utilizing these approaches were recently reported for the low complexity domain (LCD) of the transactive response DNA binding protein of 43 kDa (TDP-43).

Liquid-Liquid Phase Separation and Its Mechanistic Role in Pathological Protein Aggregation.

Liquid-liquid phase separation (LLPS) of proteins underlies the formation of membrane-less organelles. While it has been recognized for some time that these organelles are of key importance for normal cellular functions, a growing number of recent observations indicate that LLPS may also play a role in disease. In particular, numerous proteins that form toxic aggregates in neurodegenerative diseases, such as amyotrophic lateral sclerosis, frontotemporal lobar degeneration, and Alzheimer's disease, were found to be highly prone to phase separation, suggesting that there might be a strong link between LLPS and the pathogenic process in these disorders. This review aims to assess the molecular basis of this link through exploration of the intermolecular interactions that underlie LLPS and aggregation and the underlying mechanisms facilitating maturation of liquid droplets into more stable assemblies, including so-called labile fibrils, hydrogels, and pathological amyloids. Recent insights into the structural basis of labile fibrils and potential mechanisms by which these relatively unstable structures could transition into more stable pathogenic amyloids are also discussed. Finally, this review explores how the environment of liquid droplets could modulate protein aggregation by altering kinetics of protein self-association, affecting folding of protein monomers, or changing aggregation pathways.

The role of liquid-liquid phase separation in aggregation of the TDP-43 low-complexity domain.

Pathological aggregation of the transactive response DNA-binding protein of 43 kDa (TDP-43) is associated with several neurodegenerative disorders, including ALS, frontotemporal dementia, chronic traumatic encephalopathy, and Alzheimer's disease. TDP-43 aggregation appears to be largely driven by its low-complexity domain (LCD), which also has a high propensity to undergo liquid-liquid phase separation (LLPS). However, the mechanism of TDP-43 LCD pathological aggregation and, most importantly, the relationship between the aggregation process and LLPS remains largely unknown. Here, we show that amyloid formation by the LCD is controlled by electrostatic repulsion. We also demonstrate that the liquid droplet environment strongly accelerates LCD fibrillation and that its aggregation under LLPS conditions involves several distinct events, culminating in rapid assembly of fibrillar aggregates that emanate from within mature liquid droplets. These combined results strongly suggest that LLPS may play a major role in pathological TDP-43 aggregation, contributing to pathogenesis in neurodegenerative diseases.

Emission tuning of fluorescent kinase inhibitors: conjugation length and substituent effects.

Fluorescent N-phenyl-4-aminoquinazoline probes targeting the ATP-binding pocket of the ERBB family of receptor tyrosine kinases are reported. Extension of the aromatic quinazoline core with fluorophore "arms" through substitution at the 6- position of the quinazoline core with phenyl, styryl, and phenylbutadienyl moieties was predicted by means of TD-DFT calculations to produce probes with tunable photoexcitation energies and excited states possessing charge-transfer character. Optical spectroscopy identified several synthesized probes that are nonemissive in aqueous solutions and exhibit emission enhancements in solvents of low polarity, suggesting good performance as turn-on fluorophores. Ligand-induced ERBB2 phosphorylation assays demonstrate that despite chemical modification to the quinazoline core these probes still function as ERBB2 inhibitors in MCF7 cells. Two probes were found to exhibit ERBB2-induced fluorescence, demonstrating the utility of these probes as turn-on, fluoroescent kinase inhibitors.

Binding-induced fluorescence of serotonin transporter ligands: A spectroscopic and structural study of 4-(4-(dimethylamino)phenyl)-1-methylpyridinium (APP(+)) and APP(+) analogues.

The binding-induced fluorescence of 4-(4-(dimethylamino)-phenyl)-1-methylpyridinium (APP(+)) and two new serotonin transporter (SERT)-binding fluorescent analogues, 1-butyl-4-[4-(1-dimethylamino)phenyl]-pyridinium bromide (BPP(+)) and 1-methyl-4-[4-(1-piperidinyl)phenyl]-pyridinium (PPP(+)), has been investigated. Optical spectroscopy reveals that these probes are highly sensitive to their chemical microenvironment, responding to variations in polarity with changes in transition energies and responding to changes in viscosity or rotational freedom with emission enhancements. Molecular docking calculations reveal that the probes are able to access the nonpolar and conformationally restrictive binding pocket of SERT. As a result, the probes exhibit previously not identified binding-induced turn-on emission that is spectroscopically distinct from dyes that have accumulated intracellularly. Thus, binding and transport dynamics of SERT ligands can be resolved both spatially and spectroscopically.

Fluorescent stilbazolium dyes as probes of the norepinephrine transporter: structural insights into substrate binding.

We report the synthesis, binding kinetics, optical spectroscopy and predicted binding modes of a series of sterically demanding, fluorescent norepinephrine transporter (NET) ligands. A series of bulky stilbazolium dyes, including six newly synthesized compounds, were evaluated to determine the effect of extending the molecular probes' 'heads' or 'tails'. Taking advantage of the dyes' characteristic 'turn-on' emission, the kinetic binding parameters, k(on) and k(off) were determined revealing that extension of the molecules' tails is well tolerated while expansion of the head is not. Additionally, a 'headfirst' orientation appears to be preferred over a 'tail-first' binding pose. Further details of the possible binding modes were obtained from the emission spectra of the bound probes. A small range of interplanar twist angles, approximately 35° to 60°, is predicted to produce the observed emission. Docking experiments and molecular modelling support the kinetic and spectroscopic data providing structural insights into substrate binding.