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CADM1 isoforms differentially regulate human mast cell survival and homotypic adhesion.

Moiseeva EP, Leyland ML, Bradding P - Cell. Mol. Life Sci. (2012)

Bottom Line: CADM1 downregulation resulted in reduced viability and decreased expression of the pro-survival protein Mcl-1(L), but not Blc-2 or Bcl-X(L), and increased caspase-3/7 activity in both HMC-1 cells and HLMCs.This coincided with decreased basal Kit levels in HLMCs.The most highly expressed SP4 isoform is likely to contribute to MC aggregation and longevity in mastocytosis, and augment the pathophysiology of allergic diseases.

View Article: PubMed Central - PubMed

Affiliation: Department of Infection, Immunity and Inflammation, Institute for Lung Health, Clinical Sciences Wing, Glenfield Hospital, University of Leicester, Leicester LE3 9QP, UK. em9@le.ac.uk

ABSTRACT
Cell adhesion molecule 1 (CADM1), expressed by human lung mast cells (HLMCs), mediates their adhesion to airway smooth muscle (ASM), and contributes to ASM-dependent HLMC proliferation and survival. CADM1 is expressed in alternatively spliced isoforms, but those present in HLMCs and their function are not known. We cloned three functional and one cryptic non-functional isoform with alternative splicing between exons 7/11 and 1/2, respectively, from HLMCs and human MC lines (HMC-1 and LAD2). Differentiated HLMCs and LAD2 cells expressed the functional isoform SP4 containing exons 7/8/11 (~80% of clones), as well as SP1 (exons 7/8/9/11) and a novel SP6 (exons 7/8/9/10/11). In contrast, immature HMC-1 cells expressed only functional SP4. SP4 overexpression in HMC-1 cells and HLMCs augmented homotypic adhesion to a greater extent than SP1 in various conditions. In contrast, CADM1 downregulation abolished homotypic adhesion, indicating that CADM1 is the sole receptor mediating mast cell aggregation. CADM1-mediated adhesion was enhanced by the presence of cell survival factors. SP1 overexpression in HMC-1 cells compromised survival compared to SP4 overexpression or control. CADM1 downregulation resulted in reduced viability and decreased expression of the pro-survival protein Mcl-1(L), but not Blc-2 or Bcl-X(L), and increased caspase-3/7 activity in both HMC-1 cells and HLMCs. This coincided with decreased basal Kit levels in HLMCs. In summary, human MCs express multiple CADM1 isoforms which exhibit differential regulation of survival and homotypic adhesion. The most highly expressed SP4 isoform is likely to contribute to MC aggregation and longevity in mastocytosis, and augment the pathophysiology of allergic diseases.

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A model of adhesion mediated by SP4 and SP1. CADM1 monomers, shown as grey molecules, are inactive in adhesion. When cells adhere, CADM1 forms dimers, shown as red molecules, which are actively involved in adhesion. a Two cells express only SP4, which forms active dimers involved in cell adhesion. b Two cells express both SP4 and SP1 (molecule with additional protein sequence encoded by exon 9, shown as a blue box in SP1 stalk near cell membrane) with the same density along cell membrane (orange line). Because SP4 and SP1 extracellular domains have different lengths, they cannot align properly and form dimers. The total number of CADM1 dimers, involved in cell adhesion, is reduced on the cell surface. However, over time, most of the CADM1 molecules would be able to find a matching partner to dimerise with and cells would be able to adhere as strongly as cells expressing only one CADM1 isoform
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Fig7: A model of adhesion mediated by SP4 and SP1. CADM1 monomers, shown as grey molecules, are inactive in adhesion. When cells adhere, CADM1 forms dimers, shown as red molecules, which are actively involved in adhesion. a Two cells express only SP4, which forms active dimers involved in cell adhesion. b Two cells express both SP4 and SP1 (molecule with additional protein sequence encoded by exon 9, shown as a blue box in SP1 stalk near cell membrane) with the same density along cell membrane (orange line). Because SP4 and SP1 extracellular domains have different lengths, they cannot align properly and form dimers. The total number of CADM1 dimers, involved in cell adhesion, is reduced on the cell surface. However, over time, most of the CADM1 molecules would be able to find a matching partner to dimerise with and cells would be able to adhere as strongly as cells expressing only one CADM1 isoform

Mentions: SP4 and SP1 displayed differences in function in terms of adhesion and survival. Both isoforms promoted aggregation in some conditions, but SP4 overexpression promoted fast (within 3 h) cell aggregation of both HLMCs and HMC-1 cells, whereas SP1 increased aggregation only after prolonged incubation (48 h). To explain these facts, we propose the model illustrated in Fig. 7. Since SP1 has a longer extracellular domain, it is possible that it cannot align the immunoglobulin domains with those in SP4 and, as a result, does not form dimers with SP4. Hence, cells expressing only the SP4 isoform would form CADM1 dimers on the cell surface faster than cells expressing both SP4 and SP1 with the same CADM1 density on the cell surface. Since CADM1 dimers are involved in cell adhesion, cells expressing only SP4 would have more functional CADM1 dimers on the surface and would be able to adhere faster. However, cells expressing both SP4 and SP1 would form more SP4/SP4 and SP1/SP1 dimers over a longer period of time, which would provide strength of adhesion similar to that in cells expressing SP4 only.Fig. 7


CADM1 isoforms differentially regulate human mast cell survival and homotypic adhesion.

Moiseeva EP, Leyland ML, Bradding P - Cell. Mol. Life Sci. (2012)

A model of adhesion mediated by SP4 and SP1. CADM1 monomers, shown as grey molecules, are inactive in adhesion. When cells adhere, CADM1 forms dimers, shown as red molecules, which are actively involved in adhesion. a Two cells express only SP4, which forms active dimers involved in cell adhesion. b Two cells express both SP4 and SP1 (molecule with additional protein sequence encoded by exon 9, shown as a blue box in SP1 stalk near cell membrane) with the same density along cell membrane (orange line). Because SP4 and SP1 extracellular domains have different lengths, they cannot align properly and form dimers. The total number of CADM1 dimers, involved in cell adhesion, is reduced on the cell surface. However, over time, most of the CADM1 molecules would be able to find a matching partner to dimerise with and cells would be able to adhere as strongly as cells expressing only one CADM1 isoform
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Related In: Results  -  Collection

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Fig7: A model of adhesion mediated by SP4 and SP1. CADM1 monomers, shown as grey molecules, are inactive in adhesion. When cells adhere, CADM1 forms dimers, shown as red molecules, which are actively involved in adhesion. a Two cells express only SP4, which forms active dimers involved in cell adhesion. b Two cells express both SP4 and SP1 (molecule with additional protein sequence encoded by exon 9, shown as a blue box in SP1 stalk near cell membrane) with the same density along cell membrane (orange line). Because SP4 and SP1 extracellular domains have different lengths, they cannot align properly and form dimers. The total number of CADM1 dimers, involved in cell adhesion, is reduced on the cell surface. However, over time, most of the CADM1 molecules would be able to find a matching partner to dimerise with and cells would be able to adhere as strongly as cells expressing only one CADM1 isoform
Mentions: SP4 and SP1 displayed differences in function in terms of adhesion and survival. Both isoforms promoted aggregation in some conditions, but SP4 overexpression promoted fast (within 3 h) cell aggregation of both HLMCs and HMC-1 cells, whereas SP1 increased aggregation only after prolonged incubation (48 h). To explain these facts, we propose the model illustrated in Fig. 7. Since SP1 has a longer extracellular domain, it is possible that it cannot align the immunoglobulin domains with those in SP4 and, as a result, does not form dimers with SP4. Hence, cells expressing only the SP4 isoform would form CADM1 dimers on the cell surface faster than cells expressing both SP4 and SP1 with the same CADM1 density on the cell surface. Since CADM1 dimers are involved in cell adhesion, cells expressing only SP4 would have more functional CADM1 dimers on the surface and would be able to adhere faster. However, cells expressing both SP4 and SP1 would form more SP4/SP4 and SP1/SP1 dimers over a longer period of time, which would provide strength of adhesion similar to that in cells expressing SP4 only.Fig. 7

Bottom Line: CADM1 downregulation resulted in reduced viability and decreased expression of the pro-survival protein Mcl-1(L), but not Blc-2 or Bcl-X(L), and increased caspase-3/7 activity in both HMC-1 cells and HLMCs.This coincided with decreased basal Kit levels in HLMCs.The most highly expressed SP4 isoform is likely to contribute to MC aggregation and longevity in mastocytosis, and augment the pathophysiology of allergic diseases.

View Article: PubMed Central - PubMed

Affiliation: Department of Infection, Immunity and Inflammation, Institute for Lung Health, Clinical Sciences Wing, Glenfield Hospital, University of Leicester, Leicester LE3 9QP, UK. em9@le.ac.uk

ABSTRACT
Cell adhesion molecule 1 (CADM1), expressed by human lung mast cells (HLMCs), mediates their adhesion to airway smooth muscle (ASM), and contributes to ASM-dependent HLMC proliferation and survival. CADM1 is expressed in alternatively spliced isoforms, but those present in HLMCs and their function are not known. We cloned three functional and one cryptic non-functional isoform with alternative splicing between exons 7/11 and 1/2, respectively, from HLMCs and human MC lines (HMC-1 and LAD2). Differentiated HLMCs and LAD2 cells expressed the functional isoform SP4 containing exons 7/8/11 (~80% of clones), as well as SP1 (exons 7/8/9/11) and a novel SP6 (exons 7/8/9/10/11). In contrast, immature HMC-1 cells expressed only functional SP4. SP4 overexpression in HMC-1 cells and HLMCs augmented homotypic adhesion to a greater extent than SP1 in various conditions. In contrast, CADM1 downregulation abolished homotypic adhesion, indicating that CADM1 is the sole receptor mediating mast cell aggregation. CADM1-mediated adhesion was enhanced by the presence of cell survival factors. SP1 overexpression in HMC-1 cells compromised survival compared to SP4 overexpression or control. CADM1 downregulation resulted in reduced viability and decreased expression of the pro-survival protein Mcl-1(L), but not Blc-2 or Bcl-X(L), and increased caspase-3/7 activity in both HMC-1 cells and HLMCs. This coincided with decreased basal Kit levels in HLMCs. In summary, human MCs express multiple CADM1 isoforms which exhibit differential regulation of survival and homotypic adhesion. The most highly expressed SP4 isoform is likely to contribute to MC aggregation and longevity in mastocytosis, and augment the pathophysiology of allergic diseases.

Show MeSH
Related in: MedlinePlus