Growth factor delivery from hydrogel particle aggregates to promote tubular regeneration after acute kidney injury.

Local delivery of growth factors (GFs) can accelerate regeneration of injured tissue, but for many medical applications, injectable GF delivery systems are required for clinical success. Viscoelastic, injectable aggregates of micrometer-sized hydrogel particles made of multiarmed polyethylene glycol (starPEG) and heparin were prepared and tested for site-specific paracrine stimulation of tissue regeneration. Heparin was used as it binds, protects and releases numerous GFs. Hydrogel based delivery of basic fibroblast growth factor (bFGF) and murine epidermal growth factor (EGF) was monitored utilizing enzyme-linked immunosorbent assay (ELISA). bFGF was released slowly because of its high affinity to the heparin while the significantly higher release of the non-specific binding EGF was controlled by diffusion only. To investigate GF delivery in vivo, a hydrogel loaded with murine EGF or bFGF was injected subcapsularly into the left kidney of mice with experimental acute kidney injury caused by glycerol induced rhabdomyolysis. Visual examination confirmed sustained stability of the injected gel aggregates during the timescale of the experiment. The number of proliferating kidney tubular epithelial cells was quantified both in the injected kidney and the non-injected contralateral kidney. bFGF delivery from hydrogels induced a significant increase in cell proliferation in the injected kidney, although small effects were also seen in the non-injected kidney due to a systemic effect. EGF delivery strongly increased cell proliferation for both kidneys, but also showed a local effect on the injected kidney. The hydrogel without loaded GFs was used as a control and showed no increase in cell proliferation. Our results suggest that this novel starPEG-heparin hydrogel system can be an effective approach to deliver GFs locally.

Cystogenic potential of CD133+ progenitor cells of human polycystic kidneys.

In autosomal dominant polycystic kidney disease, cysts arise focally and disrupt normal renal tissue leading to renal failure. In the present study, we show that cyst-lining cells express the stem cell marker CD133. CD133+ progenitor cells isolated from polycystic kidney, carrying mutations of PKD genes, showed a dedifferentiated phenotype similar to CD133+ progenitor cells from normal kidney. However, these cells were more proliferative and presented a defective epithelial differentiation phenotype with respect to normal renal CD133+ cells as they were not able to express all tubular epithelial cell markers when cultured in epithelial differentiation medium. Polycystic CD133+ cells, in contrast to normal renal CD133+ cells, formed cysts in vitro in a three-dimensional culture system and in vivo when injected subcutaneously within Matrigel in SCID mice. Rapamycin treatment reduced in vitro proliferation of polycystic CD133+ cells and decreased cystogenesis both in vitro and in vivo. The in vitro epithelial differentiation was only partially improved by rapamycin. These results indicate that polycystic CD133+ cells retain a dedifferentiated phenotype and the ability to generate cysts.

Contribution of stem cells to kidney repair.

A current explanation for development of chronic renal injury is the imbalance between injurious mechanism and regenerative repair. The possibility that stem cells contribute to the repair of glomerular and tubular damage is of great interest for basic and translational research. Endogenous bone marrow-derived stem cells have been implicated in the repair of renal tissue, although the lineage of stem cells recruited has not been determined. If endogenous bone marrow-derived stem cells repopulate injured nephrons directly or act indirectly over a paracrine/endocrine mechanism remains also controversial. Therapeutic administration of exogenous bone marrow derived stem cells in animal models of acute renal injury suggests that a stem cell-based therapy may improve the recovery of both glomerular and tubular compartments. Whereas the therapeutic benefit of sorted hematopoietic stem cells remains uncertain, several studies showed a beneficial effect of mesenchymal stem cell administration in models of acute tubular injury and of endothelial progenitors in acute glomerular injury. Recent studies demonstrate the presence of resident stem cells within the adult kidney. These cells are capable, when injected in animals with acute tubular injury, to localize to renal compartments and contribute to regeneration. This review summarizes the current literature on the physiological role of endogenous stem cells in renal regeneration and on the therapeutic potential of exogenous stem cell administration. Moreover, critical points that still need clarification, such as the homing mechanisms of stem cells to injured tissue, the secreted factors underlying the paracrine/endocrine mechanisms and the long-term behaviour of in vivo administered stem cells, are discussed.