DNA methylation in the mouse cochlea promotes maturation of supporting cells and contributes to the failure of hair cell regeneration

Mammalian hair cells do not functionally regenerate in adulthood but can regenerate at embryonic and neonatal stages in mice by direct transdifferentiation of neighboring supporting cells into new hair cells. Previous work showed loss of transdifferentiation potential of supporting cells is in part due to H3K4me1 enhancer decommissioning of the hair cell gene regulatory network during the first postnatal week. However, inhibiting this decommissioning only partially preserves transdifferentiation potential. Therefore, we explored other repressive epigenetic modifications that may be responsible for this loss of plasticity. We find supporting cells progressively accumulate DNA methylation at promoters of developmentally regulated hair cell genes. Specifically, DNA methylation overlaps with binding sites of Atoh1, a key transcription factor for hair cell fate. We further show that DNA hypermethylation replaces H3K27me3-mediated repression of hair cell genes in mature supporting cells, and is accompanied by progressive loss of chromatin accessibility, suggestive of facultative heterochromatin formation. Another subset of hair cell loci is hypermethylated in supporting cells, but not in hair cells. Ten-eleven translocation (TET) enzyme-mediated demethylation of these hypermethylated sites is necessary for neonatal supporting cells to transdifferentiate into hair cells. We also observe changes in chromatin accessibility of supporting cell subtypes at the single-cell level with increasing age: Gene programs promoting sensory epithelium development loses chromatin accessibility, in favor of gene programs that promote physiological maturation and function of the cochlea. We also find chromatin accessibility is partially recovered in a chronically deafened mouse model, which holds promise for future translational efforts in hearing restoration.

Cellular reprogramming with ATOH1, GFI1, and POU4F3 implicate epigenetic changes and cell-cell signaling as obstacles to hair cell regeneration in mature mammals

Reprogramming of the cochlea with hair-cell-specific transcription factors such as ATOH1 has been proposed as a potential therapeutic strategy for hearing loss. ATOH1 expression in the developing cochlea can efficiently induce hair cell regeneration but the efficiency of hair cell reprogramming declines rapidly as the cochlea matures. We developed Cre-inducible mice to compare hair cell reprogramming with ATOH1 alone or in combination with two other hair cell transcription factors, GFI1 and POU4F3. In newborn mice, all transcription factor combinations tested produced large numbers of cells with the morphology of hair cells and rudimentary mechanotransduction properties. However, 1 week later, only a combination of ATOH1, GFI1 and POU4F3 could reprogram non-sensory cells of the cochlea to a hair cell fate, and these new cells were less mature than cells generated by reprogramming 1 week earlier. We used scRNA-seq and combined scRNA-seq and ATAC-seq to suggest at least two impediments to hair cell reprogramming in older animals. First, hair cell gene loci become less epigenetically accessible in non-sensory cells of the cochlea with increasing age. Second, signaling from hair cells to supporting cells, including Notch signaling, can prevent reprogramming of many supporting cells to hair cells, even with three hair cell transcription factors. Our results shed light on the molecular barriers that must be overcome to promote hair cell regeneration in the adult cochlea.

N-Acetyl-L-cysteine Promotes Ex Vivo Growth and Expansion of Single Circulating Tumor Cells by Mitigating Cellular Stress Responses

Circulating tumor cells (CTC) can be isolated via a minimally invasive blood draw and are considered a "liquid biopsy" of their originating solid tumors. CTCs contain a small subset of metastatic precursors that can form metastases in secondary organs and provide a resource to identify mechanisms underlying metastasis-initiating properties. Despite technological advancements that allow for highly sensitive approaches of detection and isolation, CTCs are very rare and often present as single cells, posing an extreme challenge for ex vivo expansion after isolation. Here, using previously established patient-derived CTC lines, we performed a small-molecule drug screen to identify compounds that can improve ex vivo culture efficiency for single CTCs. We found that N-acetyl-L-cysteine (NAC) and other antioxidants can promote ex vivo expansion of single CTCs, by reducing oxidative and other stress particularly at the initial stage of single-cell expansion. RNA-seq analysis of growing clones and nongrowing clones confirmed the effect by NAC, but also indicates that NAC-induced decrease in oxidative stress is insufficient for promoting proliferation of a subset of cells with predominant senescent features. Despite the challenge in expanding all CTCs, NAC treatment led to establishment of single CTC clones that have similar tumorigenic features. IMPLICATIONS: Through a small molecule screen and validation study, we found that NAC could improve the success of ex vivo expansion of single CTCs by mitigating the initial stress, with the potential to facilitate the investigation of functional heterogeneity in CTCs.

Lithium bis-catechol borate as an effective reductive quencher in photoredox catalysis

The use of lithium bis-catechol borate (LiB(cat)₂) as a reductive quencher for the photoredox mediated intermolecular C-H functionalization of various heteroaromatics with bromopyrroloindolines is described. LiB(cat)₂ offers a financial benefit over state-of-the-art quenchers currently in use while eliminating the side reactions that typically plague these couplings. The advantage of this methodology is highlighted by the synthesis of C3-C2' (-) gliocladin C. Furthermore, additional examples of reactivity with various bromopyrroloindolines sets the stage for expedient routes towards other pharmaceutically active hexahydropyrroloindoline alkaloids and their analogues.

Ligand functionalization as a deactivation pathway in a fac-Ir(ppy)3-mediated radical addition

Knowledge of the kinetic behavior of catalysts under synthetically relevant conditions is vital for the efficient use of compounds that mediate important transformations regardless of their composition or driving force. In particular, these data are of great importance to add perspective to the growing number of applications of photoactive transition metal complexes. Here we present kinetic, synthetic, and spectroscopic evidence of the mechanistic behavior of fac-Ir(ppy)3 in a visible light-mediated radical addition to 3-methylindole, demonstrating the instability of fac-Ir(ppy)3 under these conditions. During the reaction, rapid in situ functionalization of the photocatalyst occurs, eventually leading to deactivation. These findings demonstrate a conceivable deactivation process for catalytic single electron reactions in the presence of radicophilic ligands. Attempts to inhibit photocatalyst deactivation through structural modification provide further insight into catalyst selection for a given system of interest.

The Development of Visible-Light Photoredox Catalysis in Flow

Visible-light photoredox catalysis has recently emerged as a viable alternative for radical reactions otherwise carried out with tin and boron reagents. It has been recognized that by merging photoredox catalysis with flow chemistry, slow reaction times, lower yields, and safety concerns may be obviated. While flow reactors have been successfully applied to reactions carried out with UV light, only recent developments have demonstrated the same potential of flow reactors for the improvement of visible-light-mediated reactions. This review examines the initial and continuing development of visible-light-mediated photoredox flow chemistry by exemplifying the benefits of flow chemistry compared with conventional batch techniques.

Batch to flow deoxygenation using visible light photoredox catalysis

Herein we report a one-pot deoxygenation protocol for primary and secondary alcohols developed via the combination of the Garegg-Samuelsson reaction, visible light-photoredox catalysis, and flow chemistry. This procedure is characterized by mild reaction conditions, easy-to-handle reactants and reagents, excellent functional group tolerance, and good yields.

Radical carbon-carbon bond formations enabled by visible light active photocatalysts

This mini-review highlights the Stephenson group's contribution to the field of photoredox catalysis with emphasis on carbon-carbon bond formation. The realization of photoredox mediated reductive dehalogenation initiated investigations toward both intra- and intermolecular coupling reactions. These reactions commenced via visible light-mediated reduction of activated halogens to give carbon-centered radicals that were subsequently involved in carbon-carbon bond forming transformations. The developed protocols using Ru and Ir based polypyridyl complexes as photoredox catalysts were further tuned to efficiently catalyze overall redox neutral atom transfer radical addition reactions. Most recently, a simplistic flow reactor technique has been utilized to affect a broad scope of photocatalytic transformations with significant enhancement in reaction efficiency.

Visible light-mediated atom transfer radical addition via oxidative and reductive quenching of photocatalysts

Herein, the development of visible light-mediated atom transfer radical addition (ATRA) of haloalkanes onto alkenes and alkynes using the reductive and oxidative quenching of [Ir{dF(CF(3))ppy}(2)(dtbbpy)]PF(6) and [Ru(bpy)(3)]Cl(2) is presented. Initial investigations indicated that the oxidative quenching of photocatalysts could effectively be utilized for ATRA, and since that report, the protocol has been expanded by broadening the scope of the reaction in terms of the photocatalysts, substrates, and solvents. In addition, further modifications of the reaction conditions allowed for the efficient ATRA of perfluoroalkyl iodides onto alkenes and alkynes utilizing the reductive quenching cycle of [Ru(bpy)(3)]Cl(2) with sodium ascorbate as the sacrificial electron donor. These results signify the complementary nature of the oxidative and reductive quenching pathways of photocatalysts and the ability to predictably direct reaction outcome through modification of the reaction conditions.

Intermolecular atom transfer radical addition to olefins mediated by oxidative quenching of photoredox catalysts

Atom transfer radical addition of haloalkanes and α-halocarbonyls to olefins is efficiently performed with the photocatalyst Ir[(dF(CF(3))ppy)(2)(dtbbpy)]PF(6). This protocol is characterized by excellent yields, mild conditions, low catalyst loading, and broad scope. In addition, the atom transfer protocol can be used to quickly and efficiently introduce vinyl trifluoromethyl groups to olefins and access 1,1-cyclopropane diesters.

Synthesis and characterization of all four diastereomers of 3,4-dichloro-2-pentanol, motifs relevant to the chlorosulfolipids

All four diastereomers of 3,4-dichloro-2-pentanol were synthesized by anti-dichlorination of the precursor allylic alcohols; their stereochemistry was elucidated by X-ray crystallographic analysis of tosylate derivatives. Complete NMR data is provided in the hope that this information will facilitate structural elucidation and synthesis studies on the chlorosulfolipid family of natural products, such as malhamensilipin A.

Atypical lipomatous tumor/well-differentiated liposarcoma of the parotid gland: case report and literature review

Although liposarcoma is the second most common soft-tissue sarcoma in adults, it is exceedingly rare in the head and neck. According to our MEDLINE search, only 11 cases of liposarcoma of the parotid gland have been reported since 1968. We report a new case of primary atypical lipomatous tumor/well-differentiated liposarcoma of the parotid gland in a 77-year-old man. Because only a very limited number of case reports and small series have been published on liposarcoma in the head and neck, we also provide a review of the literature on this uncommon disease entity.