Promotion of row 1-specific tip complex condensates by Gpsm2-Gαi provides insights into row identity of the tallest stereocilia

Biomolecular condensates and disease pathogenesis

Biomolecular condensates or membraneless organelles (MLOs) formed by liquid-liquid phase separation (LLPS) divide intracellular spaces into discrete compartments for specific functions. Dysregulation of LLPS or aberrant phase transition that disturbs the formation or material states of MLOs is closely correlated with neurodegeneration, tumorigenesis, and many other pathological processes. Herein, we summarize the recent progress in development of methods to monitor phase separation and we discuss the biogenesis and function of MLOs formed through phase separation. We then present emerging proof-of-concept examples regarding the disruption of phase separation homeostasis in a diverse array of clinical conditions including neurodegenerative disorders, hearing loss, cancers, and immunological diseases. Finally, we describe the emerging discovery of chemical modulators of phase separation.

Properties of biomolecular condensates defined by Activator of G-protein Signaling 3

Activator of G-protein signaling 3 (AGS3; also known as GPSM1), a receptor-independent activator of G-protein signaling, oscillates among defined subcellular compartments and biomolecular condensates (BMCs) in a regulated manner that is likely related to the functional diversity of the protein. We determined the influence of cell stress on the cellular distribution of AGS3 and core material properties of AGS3 BMCs. Cellular stress (oxidative, pHi and thermal) induced the formation of AGS3 BMCs in HeLa and COS-7 cells, as determined by fluorescent microscopy. Oxidative stress-induced AGS3 BMCs were distinct from G3BP1 stress granules and from RNA processing BMCs defined by the P-body protein Dcp1a. Immunoblots indicated that cellular stress shifted AGS3, but not the stress granule protein G3BP1 to a membrane pellet fraction following cell lysis. The stress-induced generation of AGS3 BMCs was reduced by co-expression of the signaling protein Gαi3, but not the AGS3-binding partner DVL2. Fluorescent recovery following photobleaching of individual AGS3 BMCs indicated that there are distinct diffusion kinetics and restricted fluidity for AGS3 BMCs. These data suggest that AGS3 BMCs represent a distinct class of stress granules that serve as a previously unrecognized signal processing node.

The actin cytoskeleton in hair bundle development and hearing loss

Inner ear hair cells assemble mechanosensitive hair bundles on their apical surface that transduce sounds and accelerations. Each hair bundle is comprised of ∼ 100 individual stereocilia that are arranged into rows of increasing height and width; their specific and precise architecture being necessary for mechanoelectrical transduction (MET). The actin cytoskeleton is fundamental to establishing this architecture, not only by forming the structural scaffold shaping each stereocilium, but also by composing rootlets and the cuticular plate that together provide a stable foundation supporting each stereocilium. In concert with the actin cytoskeleton, a large assortment of actin-binding proteins (ABPs) function to cross-link actin filaments into specific topologies, as well as control actin filament growth, severing, and capping. These processes are individually critical for sensory transduction and are all disrupted in hereditary forms of human hearing loss. In this review, we provide an overview of actin-based structures in the hair bundle and the molecules contributing to their assembly and functional properties. We also highlight recent advances in mechanisms driving stereocilia elongation and how these processes are tuned by MET.

Control of stereocilia length during development of hair bundles

Assembly of the hair bundle, the sensory organelle of the inner ear, depends on differential growth of actin-based stereocilia. Separate rows of stereocilia, labeled 1 through 3 from tallest to shortest, lengthen or shorten during discrete time intervals during development. We used lattice structured illumination microscopy and surface rendering to measure dimensions of stereocilia from mouse apical inner hair cells during early postnatal development; these measurements revealed a sharp transition at postnatal day 8 between stage III (row 1 and 2 widening; row 2 shortening) and stage IV (final row 1 lengthening and widening). Tip proteins that determine row 1 lengthening did not accumulate simultaneously during stages III and IV; while the actin-bundling protein EPS8 peaked at the end of stage III, GNAI3 peaked several days later-in early stage IV-and GPSM2 peaked near the end of stage IV. To establish the contributions of key macromolecular assemblies to bundle structure, we examined mouse mutants that eliminated tip links (Cdh23v2J or Pcdh15av3J), transduction channels (TmieKO), or the row 1 tip complex (Myo15ash2). Cdh23v2J/v2J and Pcdh15av3J/av3J bundles had adjacent stereocilia in the same row that were not matched in length, revealing that a major role of these cadherins is to synchronize lengths of side-by-side stereocilia. Use of the tip-link mutants also allowed us to distinguish the role of transduction from effects of transduction proteins themselves. While levels of GNAI3 and GPSM2, which stimulate stereocilia elongation, were greatly attenuated at the tips of TmieKO/KO row 1 stereocilia, they accumulated normally in Cdh23v2J/v2J and Pcdh15av3J/av3J stereocilia. These results reinforced the suggestion that the transduction proteins themselves facilitate localization of proteins in the row 1 complex. By contrast, EPS8 concentrates at tips of all TmieKO/KO, Cdh23v2J/v2J, and Pcdh15av3J/av3J stereocilia, correlating with the less polarized distribution of stereocilia lengths in these bundles. These latter results indicated that in wild-type hair cells, the transduction complex prevents accumulation of EPS8 at the tips of shorter stereocilia, causing them to shrink (rows 2 and 3) or disappear (row 4 and microvilli). Reduced rhodamine-actin labeling at row 2 stereocilia tips of tip-link and transduction mutants suggests that transduction's role is to destabilize actin filaments there. These results suggest that regulation of stereocilia length occurs through EPS8 and that CDH23 and PCDH15 regulate stereocilia lengthening beyond their role in gating mechanotransduction channels.

Liquid-liquid phase separation in hair cell stereocilia development and maintenance

As an emerging concept, liquid-liquid phase separation (LLPS) in biological systems has shed light on the formation mechanisms of membrane-less compartments in cells. The process is driven by multivalent interactions of biomolecules such as proteins and/or nucleic acids, allowing them to form condensed structures. In the inner ear hair cells, LLPS-based biomolecular condensate assembly plays a vital role in the development and maintenance of stereocilia, the mechanosensing organelles located at the apical surface of hair cells. This review aims to summarize recent findings on the molecular basis governing the LLPS of Usher syndrome-related gene-encoding proteins and their binding partners, which may ultimately result in the formation of upper tip-link density and tip complex density in hair cell stereocilia, offering a better understanding of this severe inherited disease that causes deaf-blindness.

Myosin motors in sensory hair bundle assembly

Mechanosensory hair bundles are assembled from actin-based stereocilia that project from the apical surface of hair cells in the inner ear. Stereocilia architecture is critical for the transduction of sound and accelerations, and structural defects in these mechano-sensors are a clinical cause of hearing and balance disorders in humans. Unconventional myosin motors are central to the assembly and shaping of stereocilia architecture. A sub-group of myosin motors with MyTH4-FERM domains (MYO7A, MYO15A) are particularly important in these processes, and hypothesized to act as transporters delivering structural and actin-regulatory cargos, in addition to generating force and tension. In this review, we summarize existing evidence for how MYO7A and MYO15A operate and how their dysfunction leads to stereocilia pathology. We further highlight emerging properties of the MyTH4/FERM myosin family and speculate how these new functions might contribute towards the acquisition and maintenance of mechano-sensitivity.

Tackling cellular senescence by targeting miRNAs

Cellular senescence, which is characterized by permanent proliferation arrest, has become an important target for the amelioration of various human diseases. The activity of senescent cells is mainly related to the senescence-associated secretory phenotype (SASP). The SASP can cause chronic inflammation in local tissues and organs through autocrine and paracrine mechanisms, and a series of factors secreted by senescent cells can deteriorate the cellular microenvironment, promoting tumor formation and exacerbating aging-related diseases. Therefore, avoiding the promotion of cancer is an urgent problem. In recent years, increased attention has been given to the mechanistic study of microRNAs in senescence. As important posttranscriptional regulators, microRNAs possess unique tissue-specific expression in senescence. MicroRNAs can regulate the SASP by regulating proteins in the senescence signaling pathway, the reverse transcriptase activity of telomerase, the generation of reactive oxygen species and oxidative damage to mitochondria. Numerous studies have confirmed that removing senescent cells does not cause significant side effects, which also opens the door to the development of treatment modalities against senescent cells. Herein, this review discusses the double-edged sword of cellular senescence in tumors and aging-related diseases and emphasizes the roles of microRNAs in regulating the SASP, especially the potential of microRNAs to be used as therapeutic targets to inhibit senescence, giving rise to novel therapeutic approaches for the treatment of aging-associated diseases.