Mitochondria are remarkably plastic organelles constantly changing their shape to fulfil their various functional activities. Although the osmotic movement of water into and out of the mitochondrion is central for its morphology and activity, the molecular mechanisms and the pathways for water transport across the inner mitochondrial membrane (IMM), the main barrier for molecules moving into and out of the organelle, are completely unknown. Here, we show the presence of a member of the aquaporin family of water channels, AQP8, and demonstrate the strikingly high water permeability (P f ) characterizing the rat liver IMM. Immunoblotting, electron microscopy, and biophysical studies show that the largest mitochondria feature the highest AQP8 expression and IMM P f . AQP8 was also found in the mitochondria of other organs, whereas no other known aquaporins were seen. The osmotic water transport of liver IMM was partially inhibited by the aquaporin blocker Hg 2؉ , while the related activation energy remained low, suggesting the presence of a Hg 2؉ -insensitive facilitated pathway in addition to AQP8. It is suggested that AQP8-mediated water transport may be particularly important for rapid expansions of mitochondrial volume such as those occurring during active oxidative phosphorylation and those following apoptotic signals.Mitochondrial volume is of pivotal importance for the activity of the electron transport chain (1) and a control point of apoptosis (2). Changes in mitochondrial volume occur in many other physiological and patho-physiological conditions, including intracellular signal transduction, liver regeneration, ischemia/reperfusion-induced damage, and anoxia (3-5). Mitochondria are well behaved osmometers, and swelling and contraction of the mitochondrial matrix and related changes to mitochondrial morphology are the consequence of the water movement that osmotically accompanies the net transport of solutes into and out of the mitochondrion (6), respectively. Mitochondrial volume changes are modulated by the net movement of solutes including K ϩ and Ca 2ϩ ions across the IMM 1 (7,8). The inner mitochondrial membrane acts as a major barrier for the solutes and water moving between the cytoplasm and the mitochondrial matrix, the outer membrane being freely permeable to molecules of up to 1.5 kDa due to the presence of the exceedingly large pores formed by VDAC, the voltage-dependent anion channel (9). However, although a number of IMM transport systems have been cloned and characterized for their ability to transport solutes across the IMM (10), the molecular pathway for the movement of water remains obscure. Important clues for understanding the molecular bases of the mitochondrial osmotic properties were recently provided by the identification of an aquaporin water channel (11), AQP8, in rat hepatocyte mitochondria (12). AQP8 was also found in intracellular vesicles that are shuttled to the hepatocyte apical membrane under choleretic stimuli such as those brought about by glucagon (13). This led us to hyp...

Skeletal growth relies on both biosynthetic and catabolic processes. While the role of the former is clearly established, how the latter contributes to growth-promoting pathways is less understood. Macroautophagy, hereafter referred to as autophagy, is a catabolic process that plays a fundamental part in tissue homeostasis. We investigated the role of autophagy during bone growth, which is mediated by chondrocyte rate of proliferation, hypertrophic differentiation and extracellular matrix (ECM) deposition in growth plates. Here we show that autophagy is induced in growth-plate chondrocytes during post-natal development and regulates the secretion of type II collagen (Col2), the major component of cartilage ECM. Mice lacking the autophagy related gene 7 (Atg7) in chondrocytes experience endoplasmic reticulum storage of type II procollagen (PC2) and defective formation of the Col2 fibrillary network in the ECM. Surprisingly, post-natal induction of chondrocyte autophagy is mediated by the growth factor FGF18 through FGFR4 and JNK-dependent activation of the autophagy initiation complex VPS34-beclin-1. Autophagy is completely suppressed in growth plates from Fgf18(-/-) embryos, while Fgf18(+/-) heterozygous and Fgfr4(-/-) mice fail to induce autophagy during post-natal development and show decreased Col2 levels in the growth plate. Strikingly, the Fgf18(+/-) and Fgfr4(-/-) phenotypes can be rescued in vivo by pharmacological activation of autophagy, pointing to autophagy as a novel effector of FGF signalling in bone. These data demonstrate that autophagy is a developmentally regulated process necessary for bone growth, and identify FGF signalling as a crucial regulator of autophagy in chondrocytes.

In this study we analyzed the expression of aquaporin-4 (AQP4) in mammalian skeletal muscle. Immunohistochemical experiments revealed that affinity-purified AQP4 antibodies stained selectively the sarcolemma of fast-twitch fibers. By immunogold electron microscopy, little or no intracellular labeling was detected. Western blot analysis showed the presence of two immunopositive bands with apparent molecular masses of 30 and 32 kD specifically present in membrane fraction of a fast-twitch rat skeletal muscle (extensor digitorum longus, EDL) and not revealed in a slow-twitch muscle (soleus). PCR Southern blot experiments resulted in a selective amplification in EDL of a 960-bp cDNA fragment encoding for the full-length rat form of AQP4. Functional experiments carried out on isolated skeletal muscle bundle fibers demonstrated that the osmotic response is faster in EDL than in soleus fibers isolated from the same rat. These results provide for the first time evidence for the expression of an aquaporin in skeletal muscle correlated to a specific fiber-type metabolism.