Using biochemical characterization of fusion proteins associated with endometrial stromal sarcoma, we identified JAZF1 as a new subunit of the NuA4 acetyltransferase complex and CXORF67 as a subunit of the Polycomb Repressive Complex 2 (PRC2). Since CXORF67’s interaction with PRC2 leads to decreased PRC2-dependent H3K27me2/3 deposition, we propose a new name for this gene: CATACOMB (catalytic antagonist of Polycomb; official gene name: EZHIP). We map CATACOMB’s inhibitory function to a short highly conserved region and identify a single methionine residue essential for diminution of H3K27me2/3 levels. Remarkably, the amino acid sequence surrounding this critical methionine resembles the oncogenic histone H3 Lys27-to-methionine (H3K27M) mutation found in high-grade pediatric gliomas. As CATACOMB expression is regulated through DNA methylation/demethylation, we propose CATACOMB as the potential interlocutor between DNA methylation and PRC2 activity. We raise the possibility that similar regulatory mechanisms could exist for other methyltransferase complexes such as Trithorax/COMPASS.

https://advances.sciencemag.org/content/advances/5/7/eaax2887.full.pdf

CATACOMB: An endogenous inducible gene that antagonizes H3K27 methylation activity of Polycomb repressive complex 2 via an H3K27M-like mechanism.

Histone H3 Lys4 (H3K4) methylation is a chromatin feature enriched at gene cis-regulatory sequences such as promoters and enhancers. Here we identify an evolutionarily conserved factor, BRWD2/PHIP, which colocalizes with histone H3K4 methylation genome-wide in human cells, mouse embryonic stem cells, and Drosophila Biochemical analysis of BRWD2 demonstrated an association with the Cullin-4-RING ubiquitin E3 ligase-4 (CRL4) complex, nucleosomes, and chromatin remodelers. BRWD2/PHIP binds directly to H3K4 methylation through a previously unidentified chromatin-binding module related to Royal Family Tudor domains, which we named the CryptoTudor domain. Using CRISPR-Cas9 genetic knockouts, we demonstrate that COMPASS H3K4 methyltransferase family members differentially regulate BRWD2/PHIP chromatin occupancy. Finally, we demonstrate that depletion of the single Drosophila homolog dBRWD3 results in altered gene expression and aberrant patterns of histone H3 Lys27 acetylation at enhancers and promoters, suggesting a cross-talk between these chromatin modifications and transcription through the BRWD protein family.

/pdf/a-cryptic-tudor-domain-links-brwd2-phip-to-compass-mediated-5b1lk4w0s1.pdf

A cryptic Tudor domain links BRWD2/PHIP to COMPASS-mediated histone H3K4 methylation

Control of the dimensions of actin-rich processes like filopodia, lamellipodia, microvilli, and stereocilia requires the coordinated activity of many proteins. Each of these actin structures relies on heterodimeric capping protein (CAPZ), which blocks actin polymerization at barbed ends. Because dimension control of the inner ear’s stereocilia is particularly precise, we studied the CAPZB subunit in hair cells. CAPZB, present at ∼100 copies per stereocilium, concentrated at stereocilia tips as hair cell development progressed, similar to the CAPZB-interacting protein TWF2. We deleted Capzb specifically in hair cells using Atoh1-Cre, which eliminated auditory and vestibular function. Capzb-null stereocilia initially developed normally but later shortened and disappeared; surprisingly, stereocilia width decreased concomitantly with length. CAPZB2 expressed by in utero electroporation prevented normal elongation of vestibular stereocilia and irregularly widened them. Together, these results suggest that capping protein participates in stereocilia widening by preventing newly elongating actin filaments from depolymerizing.

/pdf/heterodimeric-capping-protein-is-required-for-stereocilia-3megu5vz8j.pdf

Heterodimeric capping protein is required for stereocilia length and width regulation.

The mammalian cochlea loses its ability to regenerate new hair cells prior to the onset of hearing. In contrast, the adult vestibular system can produce new hair cells in response to damage, or by reprogramming of supporting cells with the hair cell transcription factor Atoh1. We used RNA-seq and ATAC-seq to probe the transcriptional and epigenetic responses of utricle supporting cells to damage and Atoh1 transduction. We show that the regenerative response of the utricle correlates with a more accessible chromatin structure in utricle supporting cells compared to their cochlear counterparts. We also provide evidence that Atoh1 transduction of supporting cells is able to promote increased transcriptional accessibility of some hair cell genes. Our study offers a possible explanation for regenerative differences between sensory organs of the inner ear, but shows that additional factors to Atoh1 may be required for optimal reprogramming of hair cell fate.

/pdf/transcriptomic-and-epigenetic-regulation-of-hair-cell-2z3p1yj96u.pdf

Transcriptomic and epigenetic regulation of hair cell regeneration in the mouse utricle and its potentiation by Atoh1.

Cell-type-specific analysis has become a major focus for many investigators in the field of neuroscience, particularly because of the large number of different cell populations found in brain tissue that play roles in a variety of developmental and behavioral disorders. However, isolation of these specific cell types can be challenging due to their nonuniformity and complex projections to different brain regions. Moreover, many analytical techniques used for protein detection and quantitation remain insensitive to the low amounts of protein extracted from specific cell populations. Despite these challenges, methods to improve proteomic yield and increase resolution continue to develop at a rapid rate. In this review, we highlight the importance of cell-type-specific proteomics in neuroscience and the technical difficulties associated. Furthermore, current progress and technological advancements in cell-type-specific proteomics research are discussed with an emphasis in neuroscience.

https://www.mdpi.com/2227-7382/6/4/51/pdf

Cell-Type-Specific Proteomics: A Neuroscience Perspective

The mammalian inner ear (IE) subserves auditory and vestibular sensations via highly specialized cells and proteins. Sensory receptor hair cells (HCs) are necessary for transducing mechanical inputs and stimulating sensory neurons by using a host of known and as yet unknown protein machinery. To understand the protein composition of these unique postmitotic cells, in which irreversible protein degradation or damage can lead to impaired hearing and balance, we analyzed IE samples by tandem mass spectrometry to generate an unbiased, shotgun-proteomics view of protein identities and abundances. By using Pou4f3/eGFP-transgenic mice in which HCs express GFP driven by Pou4f3, we FACS purified a population of HCs to analyze and compare the HC proteome with other IE subproteomes from sensory epithelia and whole IE. We show that the mammalian HC proteome comprises hundreds of uniquely or highly expressed proteins. Our global proteomic analysis of purified HCs extends the existing HC transcriptome, revealing previously undetected gene products and isoform-specific protein expression. Comparison of our proteomic data with mouse and human databases of genetic auditory/vestibular impairments confirms the critical role of the HC proteome for normal IE function, providing a cell-specific pool of candidates for novel, important HC genes. Several proteins identified exclusively in HCs by proteomics and verified by immunohistochemistry map to human genetic deafness loci, potentially representing new deafness genes. SIGNIFICANCE STATEMENT Hearing and balance rely on specialized sensory hair cells (HCs) in the inner ear (IE) to convey information about sound, acceleration, and orientation to the brain. Genetically and environmentally induced perturbations to HC proteins can result in deafness and severe imbalance. We used transgenic mice with GFP-expressing HCs, coupled with FACS sorting and tandem mass spectrometry, to define the most complete HC and IE proteome to date. We show that hundreds of proteins are uniquely identified or enriched in HCs, extending previous gene expression analyses to reveal novel HC proteins and isoforms. Importantly, deafness-linked proteins were significantly enriched in HCs, suggesting that this in-depth proteomic analysis of IE sensory cells may hold potential for deafness gene discovery.

https://www.jneurosci.org/content/jneuro/37/5/1320.full.pdf

Global Analysis of Protein Expression of Inner Ear Hair Cells.

Noise Exposures Causing Hearing Loss Generate Proteotoxic Stress and Activate the Proteostasis Network

Exposure to damaging levels of noise is the most common cause of hearing loss and impairs high frequency hearing in more than 15 % of adult Americans. Using mice exposed to increasing levels of noise in combination with quantitative proteomics, we tested how noise insults remodel the cochlear proteome both acutely and after a two-week recovery period. We used ABR & DPOAE recordings to define the intensity of noise exposure necessary to produce temporary or permanent threshold shifts (TTS, PTS) in young adult mice and found noise at 94 and 105 dB SPL levels for 30 minutes elicits TTS and PTS, respectively. We quantified thousands of proteins and found that noise insults cause a rapid increase rather than a decrease in the levels of many proteins involved with protein homeostasis, myelin, cytoskeletal structures, and cell junctions such as the synapse. The vast majority of proteins with increased levels immediately after noise exposure showed normal levels after two weeks of recovery. However, several proteins involved in oxidative stress and neuroprotection had significantly increased levels only after the recovery period suggesting they play in important role in regeneration. Interestingly, a small panel of mitochondrial proteins were significantly altered only in PTS conditions suggesting potential discrete protein mechanisms. Our discovery-based proteomic analysis extends the recent description of noise-induced cochlear synaptopathy and shows that noise insults drive a robust proteostasis response. These data provide a new understanding of noise sensitive proteins and may inform the development of effective preventiative strategies or therapies for NIHL.

https://www.biorxiv.org/content/biorxiv/early/2018/10/25/452698.full.pdf

Miguel A. Ramirez

Papers

Global Analysis of Protein Expression of Inner Ear Hair Cells.

Noise Exposures Causing Hearing Loss Generate Proteotoxic Stress and Activate the Proteostasis Network

Altered protein quality control contributes to noise-induced hearing loss