Ramakrishnaiah Siddappa

cAMP/PKA pathway activation in human mesenchymal stem cells in vitro results in robust bone formation in vivo

Tissue engineering of large bone defects is approached through implantation of autologous osteogenic cells, generally referred to as multipotent stromal cells or mesenchymal stem cells (MSCs). Animal-derived MSCs successfully bridge large bone defects, but models for ectopic bone formation as well as recent clinical trials demonstrate that bone formation by human MSCs (hMSCs) is inadequate. The expansion phase presents an attractive window to direct hMSCs by pharmacological manipulation, even though no profound effect on bone formation in vivo has been described so far using this approach. We report that activation of protein kinase A elicits an immediate response through induction of genes such as ID2 and FosB, followed by sustained secretion of bone-related cytokines such as BMP-2, IGF-1, and IL-11. As a consequence, PKA activation results in robust in vivo bone formation by hMSCs derived from orthopedic patients.

The response of human mesenchymal stem cells to osteogenic signals and its impact on bone tissue engineering.

Bone tissue engineering using human mesenchymal stem cells (hMSCs) is a multidisciplinary field that aims to treat patients with trauma, spinal fusion and large bone defects. Cell-based bone tissue engineering encompasses the isolation of multipotent hMSCs from the bone marrow of the patient, in vitro expansion and seeding onto porous scaffold materials. In vitro pre-differentiation of hMSCs into the osteogenic lineage augments their in vivo bone forming capacity. Differentiation of hMSCs into bone forming osteoblasts is a multi-step process regulated by various molecular signaling pathways, which warrants a thorough understanding of these signaling cues for the efficient use of hMSCs in bone tissue engineering. Recently, there has been a surge of knowledge on the molecular cues regulating osteogenic differentiation but extrapolation to hMSC differentiation is not guaranteed, because of species- and cell-type specificity. In this review, we describe a number of key osteogenic signaling pathways, which directly or indirectly regulate osteogenic differentiation of hMSCs. We will discuss how and to what extent the process is different from that in other cell types with special emphasis on applications in bone tissue engineering.

Timing, rather than the concentration of cyclic AMP, correlates to osteogenic differentiation of human mesenchymal stem cells

Previously, we demonstrated that protein kinase A (PKA) activation using dibutyryl‐cAMP in human mesenchymal stem cells (hMSCs) induces in vitro osteogenesis and bone formation in vivo. To further investigate the physiological role of PKA in hMSC osteogenesis, we tested a selection of G‐protein‐coupled receptor ligands which signal via intracellular cAMP production and PKA activation. Treatment of hMSCs with parathyroid hormone, parathyroid hormone‐related peptide, melatonin, epinephrine, calcitonin or calcitonin gene‐related peptide did not result in accumulation of cAMP or induction of alkaline phosphatase (ALP) expression. The only ligand that did induce cAMP, prostaglandin E2, even inhibited ALP expression and mineralization, suggesting that physiological levels of cAMP may inhibit osteogenesis. Furthermore, intermittent exposure of hMSCs to dibutyryl‐cAMP inhibited ALP expression, whereas we did not observe an inhibitive effect at low dibutyryl‐cAMP concentrations. Taken together, our results demonstrate that cAMP can either stimulate or inhibit osteogenesis in hMSCs, depending on the duration, rather than the strength, of the signal provided. Copyright

Tissue Engineering : An Introduction

This chapter gives an illustrative introduction into the field of tissue engineering. The introduction is supported by the discussion of three classical experiments presented in a clearly arranged box format. The chapter also provides a well-commented and -motivated outline of the books chapters. These chapters are substantially revised and updated compared to the first edition, covering subjects such as stem cells, extracellular matrix, biomaterials, scaffolds, drug-controlled release strategies, bioreactors, and actual “engineering” of different tissues and organ systems. Furthermore, the book is extended by a number of completely new chapters reporting on consolidated latest trends in the field. The latter includes topics such as “Materiomics,” here defined as the study of cell–material interactions employing high-throughput screening technology, or “organs on chips.” Finally, quality control, clinical translation, and ethical aspects are presented and discussed in view of the required steps to take care of when passing from the bench to the bed side.