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Tumour stromal cells support tumourigenesis. We report that Syndecan‐2 (SDC2) is expressed on a nonepithelial, nonhaematopoietic, nonendothelial stromal cell population within breast cancer tissue. In vitro, syndecan‐2 modulated TGFβ signalling (SMAD7, PAI‐1), migration and immunosuppression of patient‐derived tumour‐associated stromal cells (TASCs). In an orthotopic immunocompromised breast cancer model, overexpression of syndecan‐2 in TASCs significantly enhanced TGFβ signalling (SMAD7, PAI‐1), tumour growth and metastasis, whereas reducing levels of SDC2 in TASCs attenuated TGFβ signalling (SMAD7, PAI‐1, CXCR4), tumour growth and metastasis. To explore the potential for therapeutic application, a syndecan‐2‐peptide was generated that inhibited the migratory and immunosuppressive properties of TASCs in association with reduced expression of TGFβ‐regulated immunosuppressive genes, such as CXCR4 and PD‐L1. Moreover, using an orthotopic syngeneic breast cancer model, overexpression of syndecan‐2‐peptide in TASCs reduced tumour growth and immunosuppression within the TME. These data provide evidence that targeting stromal syndecan‐2 within the TME inhibits tumour growth and metastasis due to decreased TGFβ signalling and increased immune control., What's new? Tumour‐associated stromal cells within the tumour microenvironment play a significant role in promoting tumorigenesis. Current strategies to inhibit tumour‐associated stromal cell function must however reduce the potential side‐effects associated with affecting normal stromal cells. In this study, the authors report that Syndecan‐2 is expressed on a non‐epithelial, non‐haematopoietic, non‐endothelial stromal cell population within breast cancer tissue. Syndecan‐2 contributes to the oncogenic properties of tumour‐associated stromal cells by promoting TGF‐β signalling, tumour growth, metastasis, and immunosuppression. Altogether, the results highlight pro‐tumorigenic, immunosuppressive Syndecan‐2+ stromal cells within the breast tumour microenvironment as a potential therapeutic target.

The marrow microenvironment is a complex and heterogeneous mixture of hematopoietic and stromal progenitors necessary for haematopoiesis. Whilst the hematopoietic progenitors are well described, the stromal cellular composition is not fully elucidated due to the low cells numbers, localisation-distribution-accessibility, and the lack of specific biomarkers. Cellular taxonomy studies have recently identified new populations of stromal subsets with distinct gene signature and regulatory properties of hematopoietic regeneration. Fibroblast activation protein-α (FAP), a stromal cell type first identified in cancer is also rarely found in normal tissues but might play an essential role in tissue homeostasis. Using FAPDM2 transgenic mouse in which FAP-expressing cells can be ablated with Diphtheria Toxin (DTX) FAP+ cells were depleted in healthy mice. Whilst FAP+ cells constituted 5% of all marrow cells; its ablation caused a rapid loss of PDGFR-α, Leptin-R, gp38 and SDC2 stromal cells populations, endothelial cells and vascular disruption. These resulted in anaemia, thrombocytopenia and neutropenia in peripheral blood (PB) and extreme hypo-cellularity in marrow with abnormalities within the hematopoietic progenitors. In an effort to reverse the phenotypes caused by FAP+ cell loss, a single intravenous injection of syngeneic bone marrow-derived stromal cells was administered. In a short-term evaluation, anaemia, thrombocytopenia and neutropenia ameliorated in PB and the numbers of marrow hematopoietic progenitors increased. Our data suggest FAP-expressing cells are a non-redundant component of the marrow microenvironment, necessary for marrow homeostasis and haematopoiesis. These data also provided evidence that stromal cell ablation can be rescued by stromal cell therapy.Significance StatementFAP-expressing cells depletion led to collateral damage in PB and marrow, including haematological defects that can be ameliorated by adoptive transfer of low-dose, ex-vivo expanded FAP-expressing marrow stromal cells. We suggest that stromal cell loss is a feature of severe immune-mediated inflammatory diseases – such as Graft versus Host Disease and sepsis - and that FAPDM2 model represents a novel tool to explore the native function of the recently identified stromal cell sub-populations.

Heterozygous disruptions of the Forkhead transcription factor FoxP2 impair acquisition of speech and language. Experimental downregulation in brain region Area X of the avian ortholog FoxP2 disrupts song learning in juvenile male zebra finches. In vitro, transcriptional activity of FoxP2 requires dimerization with itself or with paralogs FoxP1 and FoxP4. Whether this is the case in vivo is unknown. To provide the means for future functional studies we cloned FoxP4 from zebra finches and compared regional and cellular coexpression of FoxP1, FoxP2, and FoxP4 mRNA and protein in brains of juvenile and adult male zebra finches. In the telencephalic song nuclei HVC, RA, and Area X, the three investigated FoxPs were either expressed alone or occurred in specific combinations with each other, as shown by double in situ hybridization and triple immunohistochemistry. FoxP1 and FoxP4 but not FoxP2 were expressed in RA and in the HVCRA and HVCX projection neurons. In Area X and the surrounding striatum the density of neurons expressing all three FoxPs together or FoxP1 and FoxP4 together was significantly higher than the density of neurons expressing other combinations. Interestingly, the proportions of Area X neurons expressing particular combinations of FoxPs remained constant at all ages. In addition, FoxP-expressing neurons in adult Area X express dopamine receptors 1A, 1B, and 2. Together, these data provide the first evidence that Area X neurons can coexpress all avian FoxP subfamily members, thus allowing for a variety of regulatory possibilities via heterodimerization that could impact song behavior in zebra finches.

Heterozygous disruptions of the Forkhead transcription factor FoxP2 impair acquisition of speech and language. Experimental downregulation in brain region Area X of the avian ortholog FoxP2 disrupts song learning in juvenile male zebra finches. In vitro, transcriptional activity of FoxP2 requires dimerization with itself or with paralogs FoxP1 and FoxP4. Whether this is the case in vivo is unknown. To provide the means for future functional studies we cloned FoxP4 from zebra finches and compared regional and cellular coexpression of FoxP1, FoxP2, and FoxP4 mRNA and protein in brains of juvenile and adult male zebra finches. In the telencephalic song nuclei HVC, RA, and Area X, the three investigated FoxPs were either expressed alone or occurred in specific combinations with each other, as shown by double in situ hybridization and triple immunohistochemistry. FoxP1 and FoxP4 but not FoxP2 were expressed in RA and in the HVCRA and HVCX projection neurons. In Area X and the surrounding striatum the density of neurons expressing all three FoxPs together or FoxP1 and FoxP4 together was significantly higher than the density of neurons expressing other combinations. Interestingly, the proportions of Area X neurons expressing particular combinations of FoxPs remained constant at all ages. In addition, FoxP-expressing neurons in adult Area X express dopamine receptors 1A, 1B, and 2. Together, these data provide the first evidence that Area X neurons can coexpress all avian FoxP subfamily members, thus allowing for a variety of regulatory possibilities via heterodimerization that could impact song behavior in zebra finches.