Bone marrow structure and its possible significance for hematopoietic cell renewal

Pathophysiological Significance of Store-Operated Calcium Entry in Megakaryocyte Function: Opening New Paths for Understanding the Role of Calcium in Thrombopoiesis

Store-Operated Calcium Entry (SOCE) is a universal calcium (Ca²⁺) influx mechanism expressed by several different cell types. It is now known that Stromal Interaction Molecule (STIM), the Ca²⁺ sensor of the intracellular compartments, together with Orai and Transient Receptor Potential Canonical (TRPC), the subunits of Ca²⁺ permeable channels on the plasma membrane, cooperate in regulating multiple cellular functions as diverse as proliferation, differentiation, migration, gene expression, and many others, depending on the cell type. In particular, a growing body of evidences suggests that a tight control of SOCE expression and function is achieved by megakaryocytes along their route from hematopoietic stem cells to platelet production. This review attempts to provide an overview about the SOCE dynamics in megakaryocyte development, with a focus on most recent findings related to its involvement in physiological and pathological thrombopoiesis.

The secret life of a megakaryocyte: emerging roles in bone marrow homeostasis control

Megakaryocytes are rare cells found in the bone marrow, responsible for the everyday production and release of millions of platelets into the bloodstream. Since the discovery and cloning, in 1994, of their principal humoral factor, thrombopoietin, and its receptor c-Mpl, many efforts have been directed to define the mechanisms underlying an efficient platelet production. However, more recently different studies have pointed out new roles for megakaryocytes as regulators of bone marrow homeostasis and physiology. In this review we discuss the interaction and the reciprocal regulation of megakaryocytes with the different cellular and extracellular components of the bone marrow environment. Finally, we provide evidence that these processes may concur to the reconstitution of the bone marrow environment after injury and their deregulation may lead to the development of a series of inherited or acquired pathologies.

Developmental profiling of postnatal dentate gyrus progenitors provides evidence for dynamic cell-autonomous regulation

The dentate gyrus of the hippocampus is one of the most prominent regions in the postnatal mammalian brain where neurogenesis continues throughout life. There is tremendous speculation regarding the potential implications of adult hippocampal neurogenesis, though it remains unclear to what extent this ability becomes attenuated during normal aging, and what genetic changes in the progenitor population ensue over time. Using defined elements of the nestin promoter, we developed a transgenic mouse that reliably labels neural stem and early progenitors with green fluorescent protein (GFP). Using a combination of immunohistochemical and flow cytometry techniques, we characterized the progenitor cells within the dentate gyrus and created a developmental profile from postnatal day 7 (P7) until 6 months of age. In addition, we demonstrate that the proliferative potential of these progenitors is controlled at least in part by cell-autonomous cues. Finally, to identify what may underlie these differences, we performed stem cell-specific microarrays on GFP-expressing sorted cells from isolated P7 and postnatal day 28 (P28) dentate gyrus. We identified several differentially expressed genes that may underlie the functional differences that we observe in neurosphere assays from sorted cells and differentiation assays at these different ages. These data suggest that neural progenitors from the dentate gyrus are differentially regulated by cell-autonomous factors that change over time.

Efficient generation of human hepatocytes by the intrahepatic delivery of clonal human mesenchymal stem cells in fetal sheep

Alternative methods to whole liver transplantation require a suitable cell that can be expanded to obtain sufficient numbers required for successful transplantation while maintaining the ability to differentiate into hepatocytes. Mesenchymal stem cells (MSCs) possess several advantageous characteristics for cell-based therapy and have been shown to be able to differentiate into hepatocytes. Thus, we investigated whether the intrahepatic delivery of human MSCs is a safe and effective method for generating human hepatocytes and whether the route of administration influences the levels of donor-derived hepatocytes and their pattern of distribution throughout the parenchyma of the recipient's liver. Human clonally derived MSCs were transplanted by an intraperitoneal (n = 6) or intrahepatic (n = 6) route into preimmune fetal sheep. The animals were analyzed 56-70 days after transplantation by immunohistochemistry, enzyme-linked immunosorbent assay, and flow cytometry. The intrahepatic injection of human MSCs was safe and resulted in more efficient generation of hepatocytes (12.5% +/- 3.5% versus 2.6% +/- 0.4%). The animals that received an intrahepatic injection exhibited a widespread distribution of hepatocytes throughout the liver parenchyma, whereas an intraperitoneal injection resulted in a preferential periportal distribution of human hepatocytes that produced higher amounts of albumin. Furthermore, hepatocytes were generated from MSCs without the need to first migrate/lodge to the bone marrow and give rise to hematopoietic cells.

CONCLUSION

Our studies provide evidence that MSCs are a valuable source of cells for liver repair and regeneration and that, by the alteration of the site of injection, the generation of hepatocytes occurs in different hepatic zones, suggesting that a combined transplantation approach may be necessary to successfully repopulate the liver with these cells.

Induction of tolerance to hind limb allografts in rats receiving cyclosporine A and antilymphocyte serum: effect of duration of the treatment

BACKGROUND

This study assessed the ability of antilymphocyte serum (ALS) and cyclosporine A (CsA) to induce tolerance for hind limb composite tissue allograft in rats without chronic immunosuppression.

METHODS

Hind limb transplantations were performed in Lewis-Brown-Norway (LBN, RT1(1+n)) and Lewis (LEW, RT1(1)) rats. Treatment consisted of ALS only (0.4 mL/kg), CsA only (16 mg/kg), and a combination of CsA and ALS, and it was administered 12 hr before surgery at three different intervals (7, 14, and 21 days). Long-term survivors were tested for tolerance by standard skin grafting from the recipient (LEW), the donor (LBN), and the third party (ACI, RT1 ) 60 days after cessation of the treatment and by mixed lymphocyte reaction at 100 days. T-cell lines were analyzed with flow cytometry.

RESULTS

Single use of ALS in all treatment intervals did not prolong allograft survival. Single use of CsA extended survival up to 23 days in the 21-day protocol group. CsA and ALS caused indefinite survival in two of six rats in the 14-day protocol and in all six rats in the 21-day protocol (>420 days). The six long-term survivors in the 21-day protocol accepted the skin grafts from the donor (LBN) and the recipient (LEW) and rejected third-party grafts (ACI). Tolerant animals showed a donor-specific hematopoietic chimerism of 35% to 42% in the peripheral blood. Mixed lymphocyte reaction assay demonstrated tolerance to the host and donor alloantigens and increased response to the third party.

CONCLUSIONS

Administration of CsA and ALS for 21 days induced donor-specific tolerance in the recipients of the rat hind limb composite tissue allografts. The mechanism of tolerance should be investigated further.

Bone marrow tissue engineering

The creation of mixed hematopoietic chimerism has become an important clinical strategy for tolerance induction for cellular and organ transplantation, and for the treatment of numerous hematopoietic diseases. Clinical success has been limited however, by host immune response and by competition from host hematopoiesis. Recent data suggests that limited donor stem cell engraftment after minimally myeloablative hematopoietic stem cell (HSC) transplantation may in part be due to MHC associated microenvironmental mismatch resulting in a competitive disadvantage for donor HSC. A strategy to overcome this barrier to stable mixed hematopoietic chimerism would involve concurrent transplantation of a donor bone marrow microenvironment. To test this possibility, we set out to develop a method to tissue engineer a bone marrow microenvironment. One to two murine femurs were mechanically crushed to a fine suspension and were combined in vitro with various delivery vehicles. These constructs were transplanted into syngeneic animals in locations that are known to support transplantation of other tissues. Although bone formation was observed with several conditions, bone marrow formation was noted only within the small bowel mesentery when type I collagen was used as the delivery vehicle. No bone marrow formed when the vehicle was changed to polyglycolic acid or type IV collagen. We have demonstrated that the small bowel mesentery can support bone marrow formation under specific in vivo conditions. Future work will focus on strategies for transplantation of an engineered donor bone marrow environment to facilitate creation of allogeneic mixed hematopoietic chimerism.