The keratinocyte stem cells differentiate into keratinocytes of basal layer epidermis, also known as transit amplifying cells (TA cells), in a process stimulated by the epidermal growth factor (EGF) (Jansen and Watt 2006). The keratinocyte stem cell must express the epidermal growth factor receptor (EGFR) on its surface in order to be responsive to the EGF ligand (Jansen and Watt 2006). Still, responsiveness of keratinocyte stem cells to EGF is reduced because of low levels of EGFR (Fortunel et al. 2003) and high levels of LRIG, a negative regulator of EGFR signaling (Jensen and Watt 2006). TGF-alpha (TGFA), another ligand of EGFR, may also stimulate proliferation of keratinocyte stem cells (reviewed in Fuchs et al. 1990).
In interfollicular epidermis, keratinocyte stem cells are found in the basal layer, where they account for about 10% of cells (Potten and Morris 1988). Protein markers of keratinocyte stem cells are summarized in the table below, titled "Table of markers of keratinocyte stem cells in interfollicular epidermis". CSPG4 is the only marker undetectable in other basal cells of the human epidermis, while others are detectable, but a gradient of marker expression exists that is thought to be consistent with the gradient of stemness (Niemann and Watt 2002).
Although keratinocyte stem cells of interfollicular epidermis can divide extensively to repair damaged epidermis, they predominantly do not actively divide in steady state epidermis. Lrig1 null mice exhibit epidermal hyperproliferation (Suzuki et al. 2002). LRIG1 expressing cells in the basal layer, considered to represent keratinocyte stem cells, are mainly negative for the proliferation marker MKI67 (Jensen and Watt 2006). LRIG1 knockdown stimulates renewal of keratinocyte stem cells without inhibiting terminal differentiation (Jensen and Watt 2006). The inhibitory effect of LRIG1 on proliferation of keratinocyte stem cells can partially be explained by reduced activation of MYC transcription factor when EGFR signaling is attenuated (Jensen and Watt 2006).
Table of markers of keratinocyte stem cells in interfollicular epidermis. Please note that the so-called keratinocyte progenitor cells in Cell Marker database or just keratinocytes in CellMarker database and PanlgaoDB correspond to keratinocyte stem cells in Reactome.
| Marker (protein/RNA) | Literature Reference | CellMarker database – RNA/Protein (Hu et al. 2022) | PanglaoDB – RNA (Franzén et al. 2019) |
|---|
| ABCG2 (protein) | Ma et al. 2015
Chang et al. 2016 | Yes | No |
| AQP3 (protein) | Sugiyama et al. 2001
Ma et al. 2002
Sougrat et al. 2002 | No | Yes |
| CSPG4 (protein, RNA) | Legg et al. 2003
Jensen and Watt 2006 | No | No |
| COL17A1 (protein, RNA) | Wang et al.,2020
Negri et al., 2023 | No | No |
| DLL1 (protein) | Lowell et al. 2000
Jensen and Watt 2006
Negri et al., 2023 | No | No |
| ITGA6 (protein) | Li, Simmons, and Kaur 1998
Negri et al., 2023 | Yes | No |
| ITGB1 (protein) | Zhou et al. 2004
Legg et al. 2003
Fu, Sun, and Sun 2001
Negri et al., 2023 | Yes | No |
| ITGB4 (protein) | Jensen and Watt 2006 Raymond et al. 2007 | No | No |
| KRT19 (protein) | Fradette et al. 1998
Fu, Sun, and Sun 2001
Wang et al.,2020 | Yes | No |
| LGALS7 (protein) | Magnaldo et al. 1995
Umayahara et al. 2020 | No | Yes |
| LRIG1 (protein, RNA) | Jensen and Watt 2006 | No | No |
| NUMB (protein) | Iannolo et al. 2016 | No | No |
| TP63 (protein) | Reis Filho et al. 2002
Zhou et al. 2004
Negri et al., 2023 | No | No |
| YAP1 (protein) | Zhang et al. 2011
Schlegelmilch et al. 2011
Beverdam et al. 2013 | Yes | No |
Basal cells of epidermis expressing ABCG2 have a higher proliferative capacity than TA cells, which places ABCG2 positive cells closer to keratinocyte stem cells (Ma et al. 2015). Ma et al. 2015 defines ABCG2 positive basal cells as holoclones, with the capacity to produce 120 to 140 cell generations, while meroclones are able to produce 20 to 40, and paraclones are able to produce 5 to10 cell generations. The proliferative capacity of paraclones matches the defined proliferative capacity of TA cells. Abcg2 null mice have delayed wound closure and reduced number of TP63 positive cells in the basal layer of the epidermis (Chang et al. 2016). AQP3 (Aquaporin 3) is detected in plasma membranes of all epidermal layers of human and mouse, including basal layer keratinocyte stem cells, except stratum corneum, and is thought to contribute to water permeability of human epidermis. AQP3 levels in human epidermis may be upregulated under conditions of osmotic stress (Sugiyama et al. 2001, Ma et al. 2002, Sougrat et al. 2002). LGALS7 gene encodes Galectin 7, also known as Gal 7, member of the S lectin family of soluble, unglycosylated lectins. LGALS7 mRNA is specifically expressed in keratinocytes, at all stages of differentiation, and is moderately repressed by retinoic acid (Magnaldo et al. 1995). By proteomic and immunohistochemical analyses, LGALS7 is expressed in all epidermal layers, including the area corresponding to the basal cell layer (Umayahara et al. 2020). Studies of human and mouse epidermis have shown that nuclear YAP1, a component of Hippo signaling, is expressed in basal keratinocyte stem cells of both interfollicular and follicular epidermis. YAP1 may regulate proliferation of keratinocyte stem cells in response to cell crowding. In higher layers of epidermis, YAP1 is expressed in the cytoplasm, excluded from the nucleus (Zhang et al. 2011, Schlegelmilch et al. 2011, Beverdam et al. 2013).
TFRC (CD71) (Li, Simmons, and Kaur 1998), DSG3 (Wan et al. 2003), DSP (Wan et al. 2003), and KRT14 (Jensen and Watt 2006) are expressed at low levels in keratinocyte stem cells of interfollicular epidermis and are not used as markers. GJA1 (Matic et al. 1997; Matic et al. 2002), and KRT10 (Jensen and Watt 2006) are not expressed in keratinocyte stem cells of interfollicular epidermis as they are suprabasal keratinocyte markers.
CSPG4 and LRIG1 were identified as RNA markers of keratinocyte stem cells of interfollicular epidermis by single cell microarray expression profiling as transcripts whose levels were consistently at least 7-fold higher than in the TA cells (Jensen and Watt 2006) and also in single RNA sequencing studies (Negri et al. 2023). Twelve other genes whose levels were consistently at least 7-fold higher in stem cells than in the TA cells by microarray expression profiling, but which have not been associated with interfollicular epidermal stem cells in other studies, are: ASXL1, FAM120B, FRMD4A, GNL1, KIF2A, MAP4, MUL1, OSER1, PHKA2, RNASE4, RNF169, and ZSCAN26 (ZNF187) (Jensen and Watt 2006).
