Two- and three-dimensional culture of keratinocyte stem and precursor cells derived from primary murine epidermal cultures.

In the skin, multipotent keratinocyte stem cells (KSC) are localised in the hair follicle bulge region. Although, KSC can be cultivated and grown in two-dimensional (2D) culture they rapidly lose stem cell markers when isolated from their niche. Currently, there is no KSC culture method available which recapitulates an environment similar to the KSC niche in the hair follicle. Here we describe the successful establishment of an in vitro 3D stem cell culture model developed from clonally growing keratinocyte lines derived from neonatal mice using culture conditions previously established for human keratinocytes. After 20 passages, keratinocyte lines showed a stable ratio of holoclones (stem cells), meroclones (stem and precursor cells) and paraclones (differentiating cells), with approximately 29% holoclones, 54% meroclones and 17% paraclones, and were thus termed keratinocyte stem and precursor cell (KSPC) cultures. In high calcium medium, KSPC cultures grown at the air-liquid interphase differentiated and formed epidermal equivalents. Notably, and in contrast to primary keratinocytes, keratinocytes from KSPC cultures were able to aggregate and form spherical clusters in hanging drops, a characteristic hallmark shared with other stem cell types. Similar to the in vivo situation in the hair follicle bulge, KSPC aggregates also showed low proliferation, down-regulation of keratin 6, absence of keratin 1, and expression of the KSC markers keratin 15, Sox9, NFATc1 and Zfp145. KSPC aggregates therefore provide an optimal in vitro 3D environment for the further characterisation and study of normal and genetically modified KSPC.

Highly efficient zinc-finger nuclease-mediated disruption of an eGFP transgene in keratinocyte stem cells without impairment of stem cell properties.

Zinc-finger nucleases (ZFNs) are sequence-specific genome engineering tools with great potential for the development of gene therapies. The achievement of permanent cures through gene therapy requires targeting of stem cells but the effects and/or side effects of ZFN treatment on adult stem cell potency are largely unknown. Keratinocyte stem cells (KSCs) are attractive candidates for the development of gene therapies as their isolation, culture and grafting are well established. We derived KSCs from eGFP-transgenic mice and knocked out eGFP expression by disrupting the open reading frame with specific ZFNs in cell culture. EGFP-negative KSCs were then used as a model system to study the impact of ZFN treatment on stem cell potential. We achieved high gene disruption efficiencies with up to 18% eGFP-negative KSCs. As expected, ZFN cytotoxicity increased with rising ZFN concentrations. However, the ratio of correctly targeted KSCs among total treated cells was similar at different ZFN doses. Most importantly, our in vitro assays showed that ZFN-treated KSCs maintained their stem cell potential. They retained the capacity to both self-renew and form fully differentiated epidermal equivalents in culture. Moreover, they were able to form spherical aggregates in suspension culture, a characteristic hallmark shared with other stem cell types, and they expressed the in vivo KSC markers K15, NFATc1 and Sox9. Our data suggest that the stem cell potential of KSCs is not impaired by highly efficient ZFN treatment.

C-terminal tagging with eGFP yields new insights into expression of connexin45 but prevents rescue of embryonic lethal connexin45-deficient mice.

Connexin45 (Cx45) is a member of the connexin family which can form gap junction channels and is known to be expressed in several cell types in the embryonic as well as adult mouse including working cardiomyocytes and certain types of neurons. Until now its subcellular localization could not be unequivocally determined in certain tissues due to the lack of sensitive and specific antibodies. In order to investigate the localization of Cx45, we have generated a transgenic mouse expressing a fusion protein composed of Cx45 and eGFP under control of the endogenous Cx45 promoter using a bacterial artificial chromosome (BAC). In previous studies it had been shown that a C-terminal tag of connexin proteins only slightly altered the properties of gap junction channels in cultured cells and allowed direct visualization of the fusion protein. In the adult brain the expression of the Cx45eGFP protein was found in the subventricular zone in transient amplifying cells as well as in neuroblasts and ependymal cells. In addition Cx45eGFP is expressed in the atrial and ventricular working myocardium, i.e. regions of the heart where divergent results regarding Cx45 expression had previously been published. In the lung we identified Cx45eGFP in the smooth muscle cell layer of bronchioles. The Cx45eGFP transgene could not rescue embryonic lethality of Cx45-deficient mice, i.e. Cx45eGFP//Cx45(-/-) mice die around ED10.5 presumably due to altered properties of gap junction channels as a result of C-terminal tagging of Cx45.

The TSG101 protein binds to connexins and is involved in connexin degradation.

Gap junctions mediate electrical and metabolic communication between cells in almost all tissues and are proposed to play important roles in cellular growth control, differentiation and embryonic development. Gap junctional communication and channel assembly were suggested to be regulated by interaction of connexins with different proteins including kinases and phosphatases. Here, we identified the tumor susceptibility gene 101 (TSG101) protein to bind to the carboxyterminal tail of connexin45 in a yeast two-hybrid protein interaction screen. Glutathione S-transferase pull down experiments and immunoprecipitation revealed that not only connexin45 but also connexin30.2, -36, and -43 carboxyterminal regions were associated with TSG101 protein in pull down analyses and that connexin31, -43 and -45 co-precipitate with endogenous TSG101 protein in lysates from HM1 embryonic stem cells. TSG101 has been shown to be involved in cell cycle control, transcriptional regulation and turnover of endocytosed proteins. Thus, we decided to study the functional role of this interaction. SiRNA mediated knock down of TSG101 in HM1 embryonic stem cells led to increased levels of connexin43 and -45, prolonged half life of these connexins and increased transfer of microinjected Lucifer yellow. Our results suggest that TSG101 is involved in the degradation of connexins via interaction with connexin proteins.

The neuronal connexin36 interacts with and is phosphorylated by CaMKII in a way similar to CaMKII interaction with glutamate receptors.

Electrical synapses can undergo activity-dependent plasticity. The calcium/calmodulin-dependent kinase II (CaMKII) appears to play a critical role in this phenomenon, but the underlying mechanisms of how CaMKII affects the neuronal gap junction protein connexin36 (Cx36) are unknown. Here we demonstrate effective binding of (35)S-labeled CaMKII to 2 juxtamembrane cytoplasmic domains of Cx36 and in vitro phosphorylation of this protein by the kinase. Both domains reveal striking similarities with segments of the regulatory subunit of CaMKII, which include the pseudosubstrate and pseudotarget sites of the kinase. Similar to the NR2B subunit of the NMDA receptor both Cx36 binding sites exhibit phosphorylation-dependent interaction and autonomous activation of CaMKII. CaMKII and Cx36 were shown to be significantly colocalized in the inferior olive, a brainstem nucleus highly enriched in electrical synapses, indicating physical proximity of these proteins. In analogy to the current notion of NR2B interaction with CaMKII, we propose a model that provides a mechanistic framework for CaMKII and Cx36 interaction at electrical synapses.

Protein kinase A-mediated phosphorylation of connexin36 in mouse retina results in decreased gap junctional communication between AII amacrine cells.

Gap junctions in AII amacrine cells of mammalian retina participate in the coordination of the rod and cone signaling pathway involved in visual adaptation. Upon stimulation by light, released dopamine binds to D(1) receptors on AII amacrine cells leading to increased intracellular cAMP (cyclic adenosine monophosphate) levels. AII amacrine cells express the gap junctional protein connexin36 (Cx36). Phosphorylation of Cx36 has been hypothesized to regulate gap junctional activity of AII amacrine cells. However, until now in vivo phosphorylation of Cx36 has not been reported. Indeed, it had been concluded that Cx36 in bovine retina is not phosphorylated, but in vitro phosphorylation for Cx35, the bass ortholog of Cx36, had been shown. To clarify this experimental discrepancy, we examined protein kinase A (PKA)-induced phosphorylation of Cx36 in mouse retina as a possible mechanism to modulate the extent of gap junctional coupling. The cytoplasmic domains of Cx36 and the total Cx36 protein were phosphorylated in vitro by PKA. Mass spectroscopy revealed that all four possible PKA consensus motifs were phosphorylated; however, domains point mutated at the sites in question showed a prevalent usage of Ser-110 and Ser-293. Additionally, we demonstrated that Cx36 was phosphorylated in cultured mouse retina. Furthermore, activation of PKA increased the level of phosphorylation of Cx36. cAMP-stimulated, PKA-mediated phosphorylation of Cx36 protein was accompanied by a decrease of tracer coupling between AII amacrine cells. Our results link increased phosphorylation of Cx36 to down-regulation of permeability through gap junction channels mediating light adaptation in the retina.