Lentivirus-mediated gene transfer to human epidermis

Cellular heterogeneity and microenvironmental control of skin cancer

Healthy tissues harbour a surprisingly high number of cells that carry well-known cancer-causing mutations without impacting their physiological function. In recent years, strong evidence accumulated that the immediate environment of mutant cells profoundly impact their prospect of malignant progression. In this review, focusing on the skin, we investigate potential key mechanisms that ensure tissue homeostasis despite the presence of mutant cells, as well as critical factors that may nudge the balance from homeostasis to tumour formation. Functional in vivo studies and single-cell transcriptome analyses have revealed a tremendous cellular heterogeneity and plasticity within epidermal (stem) cells and their respective niches, revealing for example wild-type epithelial cells, fibroblasts or immune-cell subsets as critical in preventing cancer formation and malignant progression. It's the same cells, however, that can drive carcinogenesis. Therefore, understanding the abundance and molecular variation of cell types in health and disease, and how they interact and modulate the local signalling environment will thus be key for new therapeutic avenues in our battle against cancer.

Single cell transcriptomics of human epidermis identifies basal stem cell transition states

How stem cells give rise to epidermis is unclear despite the crucial role the epidermis plays in barrier and appendage formation. Here we use single cell-RNA sequencing to interrogate basal stem cell heterogeneity of human interfollicular epidermis and find four spatially distinct stem cell populations at the top and bottom of rete ridges and transitional positions between the basal and suprabasal epidermal layers. Cell-cell communication modeling suggests that basal cell populations serve as crucial signaling hubs to maintain epidermal communication. Combining pseudotime, RNA velocity, and cellular entropy analyses point to a hierarchical differentiation lineage supporting multi-stem cell interfollicular epidermal homeostasis models and suggest that transitional basal stem cells are stable states essential for proper stratification. Finally, alterations in differentially expressed transitional basal stem cell genes result in severe thinning of human skin equivalents, validating their essential role in epidermal homeostasis and reinforcing the critical nature of basal stem cell heterogeneity.

Opposing growth regulatory roles of protein kinase D isoforms in human keratinocytes

PKD is a family of three serine/threonine kinases (PKD-1, -2, and -3) involved in the regulation of diverse biological processes including proliferation, migration, secretion, and cell survival. We have previously shown that despite expression of all three isoforms in mouse epidermis, PKD1 plays a unique and critical role in wound healing, phorbol ester-induced hyperplasia, and tumor development. In translating our findings to the human, we discovered that PKD1 is not expressed in human keratinocytes (KCs) and there is a divergence in the expression and function of other PKD isoforms. Contrary to mouse KCs, treatment of cultured human KCs with pharmacological inhibitors of PKDs resulted in growth arrest. We found that PKD2 and PKD3 are expressed differentially in proliferating and differentiating human KCs, with the former uniformly present in both compartments whereas the latter is predominantly expressed in the proliferating compartment. Knockdown of individual PKD isoforms in human KCs revealed contrasting growth regulatory roles for PKD2 and PKD3. Loss of PKD2 enhanced KC proliferative potential while loss of PKD3 resulted in a progressive proliferation defect, loss of clonogenicity and diminished tissue regenerative ability. This proliferation defect was correlated with up-regulation of CDK4/6 inhibitor p15(INK4B) and induction of a p53-independent G1 cell cycle arrest. Simultaneous silencing of PKD isoforms resulted in a more pronounced proliferation defect consistent with a predominant role for PKD3 in proliferating KCs. These data underline the importance and complexity of PKD signaling in human epidermis and suggest a central role for PKD3 signaling in maintaining human epidermal homeostasis.

Regulatory role for the profilaggrin N-terminal domain in epidermal homeostasis

It is well known that profilaggrin, after its release from keratohyalin granules through dephosphorylation, becomes enzymatically processed into individual filaggrin monomers. The roles for filaggrin monomers in aggregating keratin filaments, as a component of the cornified cell envelope, and as a source of natural moisturizing factor are well established. A specific N-terminal fragment, called the PF-AB domain, becomes proteolytically released as well, but much less is known about its functional role in epidermal development. Here, the functional role of profilaggrin N-terminal (PF-N) domain was addressed by overexpressing three overlapping fragments from a lentiviral expression vector in the epidermis of living skin equivalents. The PF-N domain expression impaired the epidermal development through reducing keratinocyte proliferation and impairing differentiation. The expression of well-known differentiation markers profilaggrin, loricrin, and keratin 10 was considerably downregulated in PF-N domain overexpressing-skin equivalents. The activation of caspase 14 was also substantially affected. In contrast, total silencing of profilaggrin expression, obtained with a lentiviral miR vector, resulted in a hyperproliferative epidermis. We propose a hypothesis that profilaggrin AB domain provides a key feedback mechanism that controls epidermal homeostasis.

Visualization of plasmid delivery to keratinocytes in mouse and human epidermis

The accessibility of skin makes it an ideal target organ for nucleic acid-based therapeutics; however, effective patient-friendly delivery remains a major obstacle to clinical utility. A variety of limited and inefficient methods of delivering nucleic acids to keratinocytes have been demonstrated; further advances will require well-characterized reagents, rapid noninvasive assays of delivery, and well-developed skin model systems. Using intravital fluorescence and bioluminescence imaging and a standard set of reporter plasmids we demonstrate transfection of cells in mouse and human xenograft skin using intradermal injection and two microneedle array delivery systems. Reporter gene expression could be detected in individual keratinocytes, in real-time, in both mouse skin as well as human skin xenografts. These studies revealed that non-invasive intravital imaging can be used as a guide for developing gene delivery tools, establishing a benchmark for comparative testing of nucleic acid skin delivery technologies.

Preclinical corrective gene transfer in xeroderma pigmentosum human skin stem cells

Xeroderma pigmentosum (XP) is a devastating disease associated with dramatic skin cancer proneness. XP cells are deficient in nucleotide excision repair (NER) of bulky DNA adducts including ultraviolet (UV)-induced mutagenic lesions. Approaches of corrective gene transfer in NER-deficient keratinocyte stem cells hold great hope for the long-term treatment of XP patients. To face this challenge, we developed a retrovirus-based strategy to safely transduce the wild-type XPC gene into clonogenic human primary XP-C keratinocytes. De novo expression of XPC was maintained in both mass population and derived independent candidate stem cells (holoclones) after more than 130 population doublings (PD) in culture upon serial propagation (>10(40) cells). Analyses of retrovirus integration sequences in isolated keratinocyte stem cells suggested the absence of adverse effects such as oncogenic activation or clonal expansion. Furthermore, corrected XP-C keratinocytes exhibited full NER capacity as well as normal features of epidermal differentiation in both organotypic skin cultures and in a preclinical murine model of human skin regeneration in vivo. The achievement of a long-term genetic correction of XP-C epidermal stem cells constitutes the first preclinical model of ex vivo gene therapy for XP-C patients.

Increased interstitial pressure improves nucleic acid delivery to skin enabling a comparative analysis of constitutive promoters

Nucleic acid-based therapies hold great promise for treatment of skin disorders if delivery challenges can be overcome. To investigate one mechanism of nucleic acid delivery to keratinocytes, a fixed mass of expression plasmid was intradermally injected into mouse footpads in different volumes, and reporter expression was monitored by intravital imaging or skin sectioning. Reporter gene expression increased with higher delivery volumes, suggesting that pressure drives nucleic acid uptake into cells after intradermal injections similar to previously published studies for muscle and liver. For spatiotemporal analysis of reporter gene expression, a dual-axis confocal (DAC) fluorescence microscope was used for intravital imaging following intradermal injections. Individual keratinocytes expressing hMGFP were readily visualized in vivo and initially appeared to preferentially express in the stratum granulosum and subsequently migrate to the stratum corneum over time. Fluorescence microscopy of frozen skin sections confirmed the patterns observed by intravital imaging. Intravital imaging with the DAC microscope is a noninvasive method for probing spatiotemporal control of gene expression and should facilitate development and testing of new nucleic acid delivery technologies.

Loss of transgene following ex vivo gene transfer is associated with a dominant Th2 response: implications for cutaneous gene therapy

Host responses to therapeutic gene products are potentially serious complications in cutaneous gene therapy. Controlling immune responses to the therapeutic antigen may therefore be critical for an effective therapy. Both ex vivo and in vivo gene transfer to epidermal stem cells has been shown to induce transgene-specific immune responses; however, whether the mechanism of immune activation is the same is not clear. In this study, we have characterized transgene-specific immune responses in an ex vivo model of epidermal gene transfer using green fluorescent protein as a model antigen and retrovirus-mediated gene delivery. Contrary to T helper (Th)1-type responses induced following in vivo gene transfer to epidermis, rejection of ex vivo-transduced keratinocytes was associated with Th2/eosinophilc responses. These responses were characterized by interleukin (IL)-4 and IL-5 production by T cells, a predominance of anti-green fluorescent protein IgG1 in serum, the presence of numerous eosinophils within rejected skin, and a lack of class I-restricted cytotoxic T lymphocyte response. Pretreatment of mice receiving ex vivo transduced keratinocytes with neutralizing anti-IL-5 antibody prevented eosinophil infiltration and prolonged survival of transduced epidermis. These data indicate a role for the Th2/eosinophilic pathway in rejection of ex vivo-transduced keratinocytes, suggesting different requirements for achieving tolerance for ex vivo and in vivo approaches to gene therapy.

Protection of normal human reconstructed epidermis from UV by catalase overexpression

Reactive oxygen species (ROS) generated by ultraviolet (UV) irradiation are counterbalanced by endogenous antioxidant systems. To test the hypothesis of a novel photoprotective approach, we irradiated epidermis reconstructed with normal human keratinocytes overexpressing sustainably lentivirus-mediated catalase (CAT), copper/zinc superoxide dismutase (CuZnSOD) or manganese superoxide dismutase (MnSOD) enzymes. We found that following UVB irradiation there was a marked decrease in sunburn cell formation, caspase-3 activation and p53 accumulation in human reconstructed epidermis overexpressing CAT. Moreover, UVA-induced hypertrophy and DNA oxidation (8-oxodeoxyguanosine) were decreased by CAT overexpression. These effects were not achieved by overexpression of CuZnSOD or MnSOD. In conclusion, vector-mediated CAT overexpression could be a promising photoprotective tool against deleterious effects of UV irradiation such skin cancer especially in monogenic/polygenic photosensitive disorders characterized by ROS accumulation.

Local gene transfer to calcified tissue cells using prolonged infusion of a lentiviral vector

Gene transfer using viral vectors offers the potential for the sustained expression of proteins in specific target tissues. However, in the case of calcified tissues, in vivo delivery remains problematic because of limited accessibility. The aim of this study was to test the efficiency of lentiviral vectors (LVs) on osteogenic cells in vitro, and determine the feasibility of directly transducing resident bone cells in vivo. LVs encoding for green fluorescent protein (GFP) and ameloblastin (AMBN), a protein associated with mineralization not reported in bone, were generated. The transduction efficiency of the LVs was evaluated using the MC3T3 cell line and primary calvaria-derived osteogenic cells. For in vivo delivery, the LVs were infused using osmotic minipumps through holes created in the bone of the rat hemimandible and tibia. The production of GFP and AMBN in vitro and in vivo was monitored using fluorescence microscopy. Both transgenes were expressed in MC3T3 and primary osteogenic cells. In vivo, GFP was detected at the infusion site and fibroblast-like cells, osteoblasts, osteocytes and osteoclasts expressed AMBN. Our data demonstrate, for the first time, that primary osteogenic cells are efficiently transduced with LVs and that their infusion is advantageous for locally delivering DNA to bone cells.

Evidence from the stop-EGFP mouse supports a niche-sharing model of epidermal proliferative units

The classical model of epidermal proliferative units (EPUs) postulates that each EPU is composed of a single column of corneocytes plus epidermal cells directly below the column and is maintained by a single stem cell within the unit. Using the stop-enhanced green fluorescent protein (stop-EGFP) transgenic mouse system, we previously showed epidermal stem cell clonal lineages could produce multiple adjacent corneocytes (i.e. epidermal cells belonging to multiple adjacent EPUs), contradicting the classical EPU model. One possible problem with our earlier study was that N-ethyl-N-nitrosourea (ENU) was used to generate mutations for clonal analysis. This could alter the normal environment of the epidermal tissue and might lead to an artificial expansion of stem cell clonal lineages. In this study, we replicate our earlier findings using untreated stop-EGFP mice and relying on spontaneous mutations to generate clonal cell lineages. We propose an alternative to the classical EPU model to explain the dynamic nature of epidermal proliferation. Our niche-sharing model of EPUs allows epidermal cells to horizontally migrate among EPUs, so that multiple stem cells cooperatively maintain a larger proliferative compartment.

Organization of stem cells and their progeny in human epidermis

Renewal of epidermis is achieved by an ordered replication of stem cells and transit amplifying cells followed by terminal differentiation. In mouse epidermis, renewal is organized around highly ordered structures termed epidermal proliferative units (EPU), each generated by a single stem cell. It has been difficult to apply these concepts to the human epidermis where the basal layer is undulating and the strata have variable thickness. For example, it is unclear whether stem cells in human epidermis are located at the base of rete ridges or overlying the tip of dermal papilla. Data are available to support both views. To gain a better understanding of EPU organization in human skin, we have genetically marked xenografts of human foreskin with a lentivirus encoding a fluorescent marker protein and have mapped labeled columns of cells over a 28-wk period. By following these columns to their origin in the epidermis we have been able to determine that stem cells are dispersed along the basal compartment. The widths of these columns do vary considerably, with the narrowest originating from cells located in the base of the rete ridge. These findings provide new insights into the dynamics of epidermal renewal in human glabrous skin.