You are hereJanuary 5, 2015 | Haematopoetic Stem Cells
Nanotubes from Stem Cell Progeny Help to Correct Storage Disorder
Review “Lysosomal cross-correction by hematopoietic stem cell-derived macrophages via tunneling nanotubes” from Stem Cells by Stuart P. Atkinson
A recent report from the group of Stephanie Cherqui (University of California, San Diego) reported the use of hematopoietic stem cells (HSCs) to correct cystinosis, a lysosomal storage disorder leading to kidney dysfunction . In this disorder, a defect in the transmembrane transport protein Cystinosin (Ctns) leads to cystine accumulation leading to impairment of the renal tubules . Engraftment of bone-marrow derived-cells reduces cystine levels [3-5], with evidence suggesting that cross-connection, the transfer of a functional protein from one cell into adjacent deficient cells , mediated this reduction. Now in a new report in stem cells Naphade et al demonstrate that HSCs from the bone-marrow differentiate in macrophages, and these cells mediate cystine cross-correction through the formation of “tunneling nanotubes” (TNTs) between cells to mediate bidirectional lysosomal exchange .
The group first transplanted green fluorescent protein (GFP) -expressing HSCs into Ctns‐/‐ mice which expressed the DsRed fluorescent reporter gene ubiquitously, so creating a bifluorescent mouse model of cystinosis to further study HSC-mediated repair. Tracking of cells to the liver and kidney demonstrated no double fluorescent cells, removing cell-fusion as a possible mediator of repair, but GFP+ cells in the kidney did express a macrophage marker, indicating that HSCS differentiated into macrophages, and these mediated the correction of cystine levels.
In vitro co-culture experiments demonstrated that high levels of cystine removal from Ctns‐/‐ fibroblasts required cell-cell contact with HSC-derived macrophages, discounting a major role for cell-excreted microvesicles in cross-correction. Examination of the contacts between cells found that the macrophages contacted fibroblasts via long TNTs, and macrophages carrying a cystinosin‐GFP fusion protein allowed the authors to demonstrate the passage of Cystinosin-laden vesicles/lysosomes along the TNTs towards deficient fibroblasts. Further assessment found that the Ctns‐/‐ fibroblasts triggered nanotube formation suggesting that “cellular stress” significantly enhances TNT formation and the transfer of cellular components [7, 8]. The appearance of DsRed-labeled vesicles in TNTs and macrophages indicated a bidirectional transfer process, with deficient lysosomes moving from deficient cells to the macrophages where they can fuse with lysosomes to enhance cystine clearance (See Figure).
Naphade et al then assessed if these in vitro findings held true in an in vivo setting – HSC-grafting in the Ctns‐/‐ mouse kidney. High resolution microscopy found bone marrow-derived cells close to the proximal tubular cells (PTCs) of the kidney with tubular extensions passing from the GFP‐expressing HSC‐derived macrophages through the tubular basement membrane (TBM) which surrounds PTCs. The appearance of GFP structures demonstrated the transfer of macrophage content to the PTCs, including the cystinosin‐GFP fusion protein.
The authors propose that cross‐correction is the likely mechanism behind long‐term kidney preservation in Ctns‐/‐ mice , and provide the first evidence of direct transfer of proteins from macrophages via TNTs. The authors also recommend the examination of this new mechanism towards creating strategies to restore functional protein levels in multiple tissue compartments, and furthermore, the wider application of HSCs in tissue repair, a strategy which is not generally considered.
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