The heterogenous subcellular distribution of a wide array of channels, pumps and exchangers allows extracellular stimuli to induce increases in cytoplasmic Ca2+ concentration ([Ca2+]c) with highly defined spatial and temporal patterns, that in turn induce specific cellular responses (e.g. contraction, secretion, proliferation or cell death). In this extreme complexity, the role of mitochondria was considered marginal, till the direct measurement with targeted indicators allowed to appreciate that rapid and large increases of the [Ca2+] in the mitochondrial matrix ([Ca2+]m) invariably follow the cytosolic rises. Given the low affinity of the mitochondrial Ca2+ transporters, the close proximity to the endoplasmic reticulum (ER) Ca2+-releasing channels was shown to be responsible for the prompt responsiveness of mitochondria. In this review, we will summarize the current knowledge of: i) the mitochondrial and ER Ca2+ channels mediating the ion transfer, ii) the structural and molecular foundations of the signaling contacts between the two organelles, iii) the functional consequences of the [Ca2+]m increases, and iv) the effects of oncogene-mediated signals on mitochondrial Ca2+ homeostasis. Despite the rapid progress carried out in the latest years, a deeper molecular understanding is still needed to unlock the secrets of Ca2+ signaling machinery.

Regeneration of mesenchymal tissues depends on a resident stem cell population, that in most cases remains elusive in terms of cellular identity and differentiation signals. We here show that primary cell cultures derived from adipose tissue or skeletal muscle differentiate into adipocytes when cultured in high glucose. High glucose induces ROS production and PKC␤ activation. These two events appear crucial steps in this differentiation process that can be directly induced by oxidizing agents and inhibited by PKC␤ siRNA silencing. The differentiated adipocytes, when implanted in vivo, form viable and vascularized adipose tissue. Overall, the data highlight a previously uncharacterized differentiation route triggered by high glucose that drives not only resident stem cells of the adipose tissue but also uncommitted precursors present in muscle cells to form adipose depots. This process may represent a feed-forward cycle between the regional increase in adiposity and insulin resistance that plays a key role in the pathogenesis of diabetes mellitus.adipocyte ͉ hyperglycemia ͉ PKC ͉ ROS

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Human T cell leukemia virus type 1 (HTLV-1) encodes p13, an 87-amino-acid protein that accumulates in the inner mitochondrial membrane. Recent studies performed using synthetic p13 and isolated mitochondria demonstrated that the protein triggers an inward potassium (K+) current and inner membrane depolarization. The present study investigated the effects of p13 on mitochondrial inner membrane potential (Deltapsi) in living cells. Using the potential-dependent probe tetramethyl rhodamine methyl ester (TMRM), we observed that p13 induced dose-dependent mitochondrial depolarization in HeLa cells. This effect was abolished upon mutation of 4 arginines in p13's alpha-helical domain that were previously shown to be essential for its activity in in vitro assays. As Deltapsi is known to control mitochondrial calcium (Ca2+) uptake, we next analyzed the effect of p13 on Ca2+ homeostasis. Experiments carried out in HeLa cells expressing p13 and organelle-targeted aequorins revealed that the protein specifically reduced mitochondrial Ca2+ uptake. These observations suggest that p13 might control key processes regulated through Ca2+ signaling such as activation and death of T cells, the major targets of HTLV-1 infection.

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