Fig 2: Single-cell RNA-Seq identifies adult SC gene expression. (A) Schematic of adult LfngEGFP organ of Corti. GFP-expressing cells include inner phalangeal cells, Deiters’ cells, and, to a lesser extent, pillar cells. IHC = inner hair cell, OHC = outer hair cell. Mice that are homozygous and heterozygous for the transgene display the same phenotype. (B) Expression of genes that are known to be expressed by adult cochlear SCs (Myh9, Cdkn1b). Feature plots show the expression level of each gene in each cell. An expression is shown in log2 [nTPM] with the maximum expression value (Max) shown in the lower-left corner of each plot. The expression histogram is shown with blue indicating higher expression. (C) smFISH localization of RNA expression for Myh9, and Cdkn1b in cross-sections of the adult organ of Corti demonstrates localization in all SCs (border, inner phalangeal, the pillar, and Deiters cells). Upper left image demonstrates the Myh9 probe in red along with Lfng-EGFP in green, antibody labeling forMYO7A (HCs), and DAPI-labeling of nuclei. Lower left image shows the Cdkn1b probe in red, Lfng-EGFP in green, MYO7A (HCs), and DAPI-labeling of nuclei. Upper and lower panels to the right illustrates the smFISH channel with DAPI-labeling of nuclei. (D) Expression of previously uncharacterized adult cochlear SC genes (Lcp1, Notch2, Nlrp3, Slc2a3) presented as in panel (B). (E) smFISH localization of RNA expression for Lcp1, Notch2, Nlrp3, and Slc2a3 in adult cross-sections of the organ of Corti. Images to the right demonstrate the RNA probe of interest in red along with anti-MYO7A (HCs) in blue, Lfng-EGFP in green and DAPI in white where applicable. Images to the right are the smFISH probe alone in grayscale. The IHC (arrowhead) and OHC regions (bracket) are indicated. Myh9 = Myosin heavy chain 9 gene; Cdkn1b = Cyclin-dependent kinase inhibitor 1b gene; Lcp1 = Lymphocyte cytosolic protein 1 gene; Notch2 = Notch receptor 2 gene; Nlrp3 = NLR family pyrin domain containing 3 gene; Slc2a3 = Solute carrier family 2 member 3 gene; MYO7A = myosin 7A protein; DAPI = 4’, 6-diaminodino-2-phenylindole; Scale bars in all panels, 20 μm.

Fig 3: Phosphorylation of L‐plastin at serine (Ser) residues 5 was absent in Lrrk1 deficient cells. OC precursors derived from Lrrk1 knockout and control wild‐type mice were differentiated in the presence of RANKL and M‐CSF into mature OCs and lysed for protein extraction. Cellular proteins were digested with trypsin and the phospho‐peptides were enriched with spherical porous titanium dioxide (TiO2) resin spin tips for LS/MS analyses. Phosphorylation of L‐plastin, an actin bundling protein, at Ser5 was only present in WT mature OCs but not in Lrrk1 deficient cells as detected by LC/MS. LC/MS, liquid chromatography/mass spectrometry; M‐CSF, macrophage colony‐stimulating factor; OC, osteoclasts; RANKL, receptor activator of nuclear factor‐ κB ligan; WT, wild type

Fig 4: L‐Plastin KO mice exhibit mild osteopetrosis in long bones. Femurs isolated from 16 to 21‐week‐old WT and L‐plastin KO female mice were scanned by micro‐CT images. A & B, Representative micro‐CT images show an increased bone volume in the distal metaphysis of the femur in L‐plastin KO female as compared to WT control mouse. C‐H, Quantitative data of the trabecular bone parameters of the distal femoral metaphysis in WT control (N = 6) and L‐Plastin KO (N = 8) mice show increases in the ratio of BV to tissue volume (BV/TV) and connectivity density. P < 0.05 indicates statistical significance. BV, bone volume; KO, knockout; OC, osteoclasts; TV, tissue volume; WT, wild type