| Mesenchymal stem cells (MSCs), originally identified from bone marrow, have gained popularity due to their multilineage differentiation capability and promising results from preclinical and clinical applications. In addition to their well established differentiation routes to mesodermal lineage cells, such as osteoblasts, adipocytes and chondrocytes, MSCs are also capable of differentiating into neural lineage cells. The ubiquitous messenger cyclic adenosine monophosphate (cAMP) has been used frequently to induce neural lineage differentiation in MSCs. However, a clear understanding of how cAMP induces MSCs into functional neurons is lacking. Ongoing research employing in vitro pre-differentiated MSCs to treat neuronal diseases, including the combinatorial therapy of cAMP and MSCs for spinal cord injury repair, necessitate a better understanding of cAMP induced differentiation of MSCs to neural lineages. Accordingly, the goal of the current study is to determine the role of cAMP in MSC differentiation to neural lineages. We assessed if cAMP can enable MSCs to gain neuronal function and investigated a potential mechanism by which cAMP regulates MSC differentiation to neural cells. The results suggested that cAMP initiated neuron-like morphological changes early and induced neural marker expression much later. These two processes are regulated differentially downstream of protein kinase A (PKA). The early-phase neuron- like morphology is the result of cell shrinkage, which gradually decreased with cAMP treatment, whereas the expression of neural markers increased with exposure time. In addition to neural marker expression, cAMP also enabled MSCs to gain some neuronal function, namely inducing a calcium rise upon stimulation by neuronal activators. Further studies suggested that cAMP response element binding protein (CREB) plays a critical role in mediating the calcium rise upon stimulation by the neuronal activators. While CREB exerts a positive affect on calcium signaling, it appears to negatively impact the adoption of a neuron-like morphology, since a dominant negative CREB promoted the appearance of a neuron-like morphology.;In addition to differentiation, cAMP also participates in various other important cellular processes, such as cell death and survival. In a separate study we found that saturated free fatty acids (FFAs), i.e., palmitate, initiated cell death in hepatocellular carcinoma cells (HepG2) cells. cAMP has been shown to be protective of liver cells from cell death due to various insults. Therefore, we also investigated the role of cAMP in palmitate-induced cell death of HepG2 cells. However, we found that cAMP enhanced palmitate-induced cell death of HepG2 cells. cAMP enhanced palmitate-induced mitochondrial fragmentation and mitochondrial reactive oxygen species (ROS) generation. Mitochondrial fragmentation precedes mitochondrial ROS generation and may contribute to the mitochondrial ROS overproduction. Fragmentation of mitochondria also facilitated the release of cytotoxic proteins from the mitochondria and subsequent activation of caspases. However, the cell death induced by palmitate and cAMP was caspase-independent and predominantly necrotic. |
