The collection and analysis of formalin-fixed paraffin-embedded (FFPE) human skin sections was approved by the local ethics-committee at the Department of Pathology, University of Cologne, Germany. Written informed consentwas obtained from all patients in accordance with the Declaration of Helsinki. For biopsy materials from archival paraffin blocks of human skin, an informed consent was obtained from all the subjects and ethical approval obtained from the Ethics Committee at the University of Cologne. Surgically removed human FFPE skin biopsies were sectioned into 4 µm sections. Sections were dewaxed, and indirectly immunofluorescently labeled against nuclear phosphatidylinositol 4,5-bisphosphate (nPI(4,5)P2) using 5 µg/mL rabbit primary polyclonal antibody (Echelon Biosciences Inc. Z-A045, clone 2C11). The primary antibody against nPI(4,5)P2 was recognized by the goat secondary antibody conjugated with Abberrior Star 635P (Abberior 2-0002-007-5). Sections were indirectly immunofluorescently labeled against nuclear speckle marker SON using 1 µg/mL rabbit primary polyclonal antibody (Abcam ab121759). The primary antibody against SON was recognized by the goat secondary antibody conjugated with Abberrior Star 580 (Abberrior ST580-1002). Sections were co-stained by DAPI 1:1000 in PBS for 5 min. Imaging of nPI(4,5)P2-635P channel was performed on Leica TCS SP8 STED 3x inverted DMi8 microscope with pulsed white light laser 470-640 nm 1.5 mW and 775 nm pulse STED laser >1.5 W controlled by Leica Application Suite X software and equipped with HC PL APO CS2 100x/1.40 OIL objective used with Leica Type F immersion oil n=1.518. Unidirectional xyz scanning speed was 400 Hz, line accumulation 8. Pixel size was 20 nm in X and Y. Channel settings: 7% 633 nm laser; 775 Notch filter; 50% 775 nm STED laser; 30% 3D STED; HyD 639-698 nm, photon-counting mode, gain 100, gating 0.3-10 ns. Imaging of SON-580 channel was performed on Leica TCS SP8 STED 3x inverted DMi8 microscope with pulsed white light laser 470-640 nm 1.5 mW and 775 nm pulse STED laser >1.5 W controlled by Leica Application Suite X software and equipped with HC PL APO CS2 100x/1.40 OIL objective used with Leica Type F immersion oil n=1.518. Unidirectional xyz scanning speed was 400 Hz, line accumulation 8. Pixel size was 20 nm in X and Y. Channel settings: 10% 585 nm laser; 775 Notch filter; 80% 775 nm STED laser, 30% 3D STED; Hybrid detector (HyD) 589-616 nm, photon-counting mode, gain 100, gating 0.4-10 ns. Z-stacks of STED images were deconvolved using Huygens Professional 22.10 software (Scientific Imaging B.V.). Data sets were processed using Workflow Processor. The workflow consisted of selecting images, setting up the microscopy and deconvolution parameters and saving deconvolved images as 8-bit TIFF single files for individual channels (which were later used for the quantitative analyses; see below). Microscopy parameters were optimized and set as follows. Sampling intervals were ≤20 nm in X and Y and ≤20 nm in Z. Numerical aperture was 1.4; refractive indexes of the lens immersion oil was 1.518 and of the embedding media 1.458; objective quality was good, coverslip position was 0 µm and imaging direction was downward. For nPI(4,5)P2-635P STED channel the backprojected pinhole was 216 nm; excitation (ex.) and emission (em.) wavelengths (λ) were 633 and 651 nm, resp., ex. fill factor 2. STED depletion mode was pulsed, saturation factor 25, STED λ = 775, STED immunity factor 10 and STED 3X was 30%. Classic MLE algorithm with stabilization of Z-slices was used and signal-to-noise ratio was 5.1. For SON-580 STED channel the backprojected pinhole was 195 nm; excitation (ex.) and emission (em.) wavelengths (λ) were 585 and 602 nm, resp., ex. fill factor 2. STED depletion mode was pulsed, saturation factor 20, STED λ = 775, STED immunity factor 10 and STED 3X was 30%. Classic MLE algorithm with stabilization of Z-slices was used and signal-to-noise ratio was 4.

Individual channels of the images were merged into 3-chennel images by Ondrej Sebesta, Vinicna Microscopy Core Facility (VMCF) Faculty of Science Charles University Prague Czechia.