The KitL/c-Kit pathway constitutes a cellular communication system where the KitL signal acts on its receptor cells mostly in a paracrine fashion. When bound to the membrane, KitL can also signal in a “juxtacrine” manner; this is in the contacting cell. What are the mechanisms that transform the KitL signal into a specific biological response? How can the same signal lead to different biological responses in different cells or even within the same cell? This section describes various aspects of KitL.
The c-Kit ligand is the Stem Cell Factor (SCF) (Williams et al., 1990), also called "mast cell growth factor," "steel factor," or "kit ligand" hereafter KitL. Various cell types produce KitL including fibroblasts, endothelial cells, keratinocytes, hematopoietic stromal cells, Sertoli cells and some tumor cells (Ashman, 1999). It is encoded by the Sl locus in the mouse chromosome 10 (human chromosome 12) (reviewed in (Broudy, 1997). It has been included in the family of helical cytokine structural superfamily (Jiang et al., 2000). KitL primary structure consists of an extracellular domain, followed by a transmembrane region and an intracellular c-tail (Zsebo et al., 1990).
KitL is a 30 kDa glycoprotein that exists in soluble or membrane-bound form. Both isoforms result from alternative splicing and proteolytic cleavage (Anderson et al., 1991).
The soluble KitL (s-KitL) derives from the proteolytic cleavage in exon 6 of a 248-amino acid membrane protein (KitL248) (Figure 1). This proteolytic cleavage occurs on the cell surface (Huang et al., 1992) and may depend on the action of metalloproteinases including MMP-9 (Heissig et al., 2002). Soluble-KitL circulates as glycosylated noncovalent homodimers at a plasma concentration of about 3.3 ng/ml (Langley et al., 1993; Langley et al., 1992).
The membrane-bound KitL (m-KitL) originates from the alternative splicing of exon six which leads to a glycoprotein lacking the proteolytic site and thus remains anchored to the membrane (KitL220) (Figure 1). Nevertheless, this form can be cleaved at exon 7 to generate a soluble form (Majumdar et al., 1994).
Figure 1. Alternative splicing of KitL. Two transmembrane precursors arise from alternative splicing of exon 6 of the mRNA encoding KitL: KitL248 and KitL220. KitL248 is cleaved at a proteolytic site (*) contained in exon 6 and releases the soluble-KitL (s-KitL). The KitL220 lack the cleavage site and remains mostly anchored to the membrane (m-KitL). A minor cleavage can occur in exon 7 to release a soluble form which is insignificant.
Both KitL isoforms are biologically active, but their level of expression is tissue-dependent (Huang et al., 1992). In adults, s-KitL is the predominant form in many organs. Under physiological conditions, s-KitL is produced by pulmonary fibroblasts (Kiener et al., 2000) and bronchial smooth muscle cells (Kassel et al., 1999). In contrast, mast cells and mouse testis and Sertoli cells preferentially produce m-KitL (Mauduit, Chatelain, et al., 1999; Welker et al., 1999). Within the same tissue, the level of expression of the two KitL isoforms may vary according to the stage of development. These differences are exemplified in Sl/Sld mice which revealed that germinal cells do not proliferate at puberty. During the development of male gonads, Sertoli cells express both KitL isoforms at different stages. While the soluble form is expressed just before and after birth, the membrane form persists. Interestingly, the division of PGCs correlates well with m-KitL expression (Mauduit, Hamamah, et al., 1999). In Sertoli cells, the shift between the two isoforms depends on the pH, with the m-KitL being produced at acidic pH (Mauduit, Chatelain, et al., 1999). Therefore, in adulthood, the developmental cycle of spermatogonia could be directed by the expression of different forms of KitL at various stages.
Although both KitL isoforms bind to and activate the receptor c-Kit, their functions are not equivalent and cannot be mutually compensated in vivo. Steel dickie (Sld) mice carry an intragenic deletion that restricts the expression of KitL to the soluble form (Brannan et al., 1991; Flanagan et al., 1991). The Sld/Sld mice are viable but have a severe phenotype, with pigmentation defects, macrocytic anemia, reduced the number of tissue mast cells and sterility (Lev et al., 1994; Nakayama et al., 1988). Conversely, the expression of only a membrane-restricted from of KitL in mice reduces the number of mast cells and increased sensitivity to irradiation (Tajima et al., 1998). The complete absence of KitL expression is embryonic lethal because of anemia.
The differential effects of KitL isoforms were also studied in vitro by using fetal liver stromal cells from Sl mice expressing s-KitL or m-KitL. The study demonstrated that growth of HSCs and progenitor cells is maintained longer with the membrane form than with the soluble one (Toksoz et al., 1992). Moreover, expression of KitL by endothelial and perivascular cells promotes the maintenance of HSCs in the bone marrow (Ding et al., 2012). In contrast, the release of stem and progenitor cells from the bone marrow requires the soluble form (Heissig et al., 2002). Biochemically, c-Kit stimulation by m-KitL drives to a long-lasting activation and half-life of the receptor. In contrast, s-KitL leads to faster receptor degradation (Miyazawa et al., 1995).
Therefore, the activation levels of c-Kit may have different consequences regarding endocytosis and intracellular signaling that in turns may affect the biological response.
Indeed, it is possible that if s-KitL stimulation of c-Kit can occur on the whole cell surface, m-KitL stimulation must be focal. Indeed, m-m-KitL stimulates adhesion of c-Kit expressing mast cells (Flanagan et al., 1991) a phenomenon that is accompanied by the accumulation of KitL at the point of contact between two cell types comparable to an immunological synapse (S.
Tabone-Eglinger et al., 2014).