Different immortalized keratinocyte cell lines display distinct capabilities to differentiate and reconstitute an epidermis in vitro.

Dermatological research relies on the availability of suitable models that most accurately reflect the in vivo situation. Primary keratinocytes obtained from skin reduction surgeries are not only limited by availability but have a short lifespan and show donor-specific variations, which hamper the understanding of general mechanisms. The spontaneously immortalized keratinocyte cell line HaCaT displays chromosomal aberrations and is known to differentiate in an abnormal manner. To overcome these issues, we validated different engineered immortalized cell lines created from primary human keratinocytes (NHK) as model systems to study epidermal function. Cell lines either immortalized by the expression of SV40 large T antigen and hTERT (NHK-SV/TERT) or by transduction with HPV E6/E7 (NHK-E6/E7) were analysed for their growth and differentiation behaviour using 2D and 3D culture systems and compared to primary keratinocytes. Both cell lines displayed a robust proliferative behaviour but were still sensitive to contact inhibition. NHK-E6/E7 could be driven into differentiation by Ca2+ switch, while NHK-SV/TERT needed withdrawal from any proliferative signal to initiate a delayed onset of differentiation. In 3D epidermal models both cell lines were able to reconstitute a stratified epidermis and functional epidermal barrier. However, only NHK-E6/E7 showed a degree of epidermal maturation and stratification that was comparable to primary keratinocytes.

The Ca2+ channel TRPV4 is dispensable for Ca2+ influx and cell volume regulation during hypotonic stress response in human keratinocyte cell lines.

Cell swelling as a result of hypotonic stress is counteracted in mammalian cells by a process called regulatory volume decrease (RVD). We have recently discovered that RVD of human keratinocytes requires the LRRC8 volume-regulated anion channel (VRAC) and that Ca2+ exerts a modulatory function on RVD. However, the ion channel that is responsible for Ca2+ influx remains unknown. We investigated in this study whether the Ca2+-permeable TRPV4 ion channel, which functions as cell volume sensor in many cell types, may be involved in cell volume regulation during hypotonic stress response of human keratinocytes. We interfered with TRPV4 function in two human keratinocyte cell lines (HaCaT and NHEK-E6/E7) by using two TRPV4-specific inhibitors (RN1734 and GSK2193874), and by creating a CRISPR/Cas9-mediated genetic TRPV4-/- knockout in HaCaT cells. We employed electrophysiological patch clamp analysis, fluorescence-based Ca2+ imaging and cell volume measurements to determine the functional importance of TRPV4. We could show that both hypotonic stress and direct activation of TRPV4 by the specific agonist GSK1016790A triggered intracellular Ca2+ response. Strikingly, the Ca2+ increase upon hypotonic stress was neither affected by genetic knockout of TRPV4 in HaCaT cells nor by pharmacological inhibition of TRPV4 in both keratinocyte cell lines. Accordingly, hypotonicity-induced cell swelling, downstream activation of VRAC currents as well as subsequent RVD were unaffected both in TRPV4 inhibitor-treated keratinocytes and in HaCaT-TRPV4-/- cells. In summary, our study shows that keratinocytes do not require TRPV4 for coping with hypotonic stress, which implies the involvement of other, yet unidentified Ca2+ channels.

Forming megakaryocytes from murine-induced pluripotent stem cells by the inducible overexpression of supporting factors.

BACKGROUND: Platelets are small anucleate cells that circulate in the blood in a resting state but can be activated by external cues. In case of need, platelets from blood donors can be transfused. As an alternative source, platelets can be produced from induced pluripotent stem cells (iPSCs); however, recovered numbers are low. OBJECTIVES: To optimize megakaryocyte (MK) and platelet output from murine iPSCs, we investigated overexpression of the transcription factors GATA-binding factor 1 (GATA1); nuclear factor, erythroid 2; and pre-B-cell leukemia transcription factor 1 (Pbx1) and a hyperactive variant of the small guanosine triphosphatase RhoA (RhoAhc). METHODS: To avoid off-target effects, we generated iPSCs carrying the reverse tetracycline-responsive transactivator M2 (rtTA-M2) in the Rosa26 locus and expressed the factors from Tet-inducible gammaretroviral vectors. Differentiation of iPSCs was initiated by embryoid body (EB) formation. After EB dissociation, early hematopoietic progenitors were enriched and cocultivated on OP9 feeder cells with thrombopoietin and stem cell factor to induce megakaryocyte (MK) differentiation. RESULTS: Overexpression of GATA1 and Pbx1 increased MK output 2- to 2.5-fold and allowed prolonged collection of MK. Cytologic and ultrastructural analyses identified typical MK with enlarged cells, multilobulated nuclei, granule structures, and an internal membrane system. However, GATA1 and Pbx1 expression did not improve MK maturation or platelet release, although in vitro-generated platelets were functional in spreading on fibrinogen or collagen-related peptide. CONCLUSION: We demonstrate that the use of rtTA-M2 transgenic iPSCs transduced with Tet-inducible retroviral vectors allowed for gene expression at later time points during differentiation. With this strategy we could identify factors that increased in vitro MK production.

Cell Volume Regulation in the Epidermis.

In order to cope with external stressors such as changes in humidity and temperature or irritating substances, the epidermis as the outermost skin layer forms a continuously renewing and ideally intact protective barrier. Under certain circumstances, this barrier can be impaired and epidermal cells have to counteract cell swelling or shrinkage induced by osmotic stress via regulatory volume decrease (RVD) or increase (RVI). Here, we will review the current knowledge regarding the molecular machinery underlying RVD and RVI in the epidermis. Furthermore, we will discuss the current understanding how cell volume changes and its regulators are associated with epidermal renewal and barrier formation.

Spinocerebellar Ataxia Type 28-Phenotypic and Molecular Characterization of a Family with Heterozygous and Compound-Heterozygous Mutations in AFG3L2.

While heterozygous mutations in the AFG3L2 gene have been linked to spinocerebellar ataxia 28 (SCA28), homozygous mutations in the same gene can cause spastic ataxia 5 (SPAX5). AFG3L2 encodes a mitochondrial ATP-dependent metalloprotease. We here report a SCA28 patient with biallelic AFG3L2 variants and his heterozygous mother. The patient and his mother underwent a detailed neurological examination and fibroblast lines were established. The effect of the two missense variants on mitochondria was assessed by form factor analysis and quantification of mitochondrial proteins (TOMM70, complex V). The 39-year-old index patient presented with a slowly progressive cerebellar gait disorder for 19 years, bilateral ptosis, and dysarthria. A cranial MRI showed mild cerebellar atrophy. He carried two compound-heterozygous, rare, missense variants (c.1847A>G [p.Y616C], c.2167G>A [p.V723M]) in AFG3L2, while his mother was heterozygous for the first change that had previously been described in SPAX5. Altered mitochondrial morphology and interconnectivity, together with reduced protein levels of TOMM70 and complex V (ATPase), suggest mitochondrial structural defects in the patient's fibroblasts. No significant abnormalities were found in his mother's fibroblast cultures albeit all measurements were slightly below the control level. We here present a SCA28 patient with compound-heterozygous AFG3L2 variants and demonstrate mitochondrial abnormalities in skin fibroblast cultures from this patient. Thus, AFG3L2 variants should be considered in both slowly progressive ataxias and phenotypes with clinical features reminiscent of mitochondrial disease. Of note, ptosis was present in both mutation carriers and may serve as a red flag in the diagnosis of SCA28.

Generation and characterization of eight human-derived iPSC lines from affected and unaffected THAP1 mutation carriers.

Mutations in THAP1 (THAP domain-containing apoptosis-associated protein 1) cause a form of early-onset, isolated dystonia (DYT-THAP1, aka DYT6). Here, we describe the generation of eight human induced pluripotent stem cell (iPSC) lines of manifesting and non-manifesting carriers of the THAP1 mutations p.Lys158Asnfs*23 or p.Arg13His (each 4 lines). Dermal fibroblasts were reprogrammed using non-integrating Sendai virus. The iPSC lines were comprehensively characterized including expression analyses of pluripotency markers, the potential to differentiate into cells of all three germ layers, and stable karyotypes. These lines provide a valuable resource for studying the impact of THAP1 mutations on the pathology of dystonia.