Generation of muscle aquaporin 4 overexpressing transgenic mouse: Its characterization at RNA and protein levels including freeze-fracture study

CPT-11-Induced Delayed Diarrhea Develops via Reduced Aquaporin-3 Expression in the Colon

While irinotecan (CPT-11) has a potent anti-cancer effect, it also causes serious diarrhea as an adverse reaction. In this study, we analyzed the pathogenic mechanism of CPT-11-induced delayed diarrhea by focusing on water channel aquaporin-3 (AQP3) in the colon. When rats received CPT-11, the expression level of AQP3 was reduced during severe diarrhea. It was found that the expression levels of inflammatory cytokines and the loss of crypt cells were increased in the colon when CPT-11 was administered. When celecoxib, an anti-inflammatory drug, was concomitantly administered, both the diarrhea and the reduced expression of AQP3 induced by CPT-11 were suppressed. The inflammation in the rat colon during diarrhea was caused via activated macrophage by CPT-11. These results showed that when CPT-11 is administered, the expression level of AQP3 in the colon is reduced, resulting in delayed diarrhea by preventing water transport from the intestinal tract. It was also suggested that the reduced expression of AQP3 might be due to the inflammation that occurs following the loss of colonic crypt cells and to the damage caused by the direct activation of macrophages by CPT-11. Therefore, it was considered that anti-inflammatory drugs that suppress the reduction of AQP3 expression could prevent CPT-11-induced delayed diarrhea.

Marked decrease of aquaporin-4 protein is independent of the changes in α1-syntrophin and TRPV4 levels in response to denervation-induced muscle atrophy in vivo

Aquaporin-4 (AQP4) is a selective water channel mediating water transport across cell membranes in skeletal muscles. Recently, it was noted that AQP4 is one of the key molecules regulating muscle morphology. Indeed, the AQP4 accumulation level was stably maintained in hypertrophied skeletal muscles. On the other hand, whether the AQP4 accumulation level is stably maintained in atrophied muscles remains poorly understood. The present study investigated the changes in the AQP4 accumulation level in the atrophied muscles at 2 weeks after denervation. As a result, the accumulation level of AQP4 in the atrophied muscle was significantly decreased compared with that in the control muscle (p < 0.05). Interestingly, the accumulation level of α1-syntrophin, which is an essential factor in regulating the stable accumulation level of AQP4, was stably maintained in the atrophied muscles. On the other hand, the accumulation level of the transient receptor potential vanilloid 4 (TRPV4), which contributes to cell volume control via interaction with AQP4, was significantly increased in the atrophied muscles compared with that in the control muscle (p < 0.05). Therefore, the present study suggested that the imbalance between the AQP4 accumulation level and skeletal muscle volume may be induced in the atrophied muscles by denervation, and the decrease in the accumulation level of AQP4 may be accompanied by defects in the functional and structural relationships with α1-syntrophin and TRPV4.

Aquaporin-4 Protein Is Stably Maintained in the Hypertrophied Muscles by Functional Overload

Aquaporin-4 (AQP4) is a selective water channel that is located on the plasma membrane of myofibers in skeletal muscle and is bound to α1-syntrophin. It is considered that AQP4 is involved in the modulation of homeostasis in myofibers through the regulation of water transport and osmotic pressure. However, it remains unclear whether AQP4 expression is altered by skeletal muscle hypertrophy to modulate water homeostasis in myofibers. The present study investigated the effect of muscle hypertrophy on the changes in AQP4 and α1-syntrophin expression patterns in myofibers. Novel findings indicated in the present study were as follows: 1) Expression levels of AQP4 and α1-syntrophin were stably maintained in hypertrophied muscles, and 2) AQP4 was not expressed in the myofibers containing the slow-type myosin heavy chain isoform (MHC) with or without the presence of fast-type MHC. The present study suggests that AQP4 may regulate the efficiency of water transport in hypertrophied myofibers through its interaction with α1-syntrophin. In addition, this study suggests that AQP4 expression may be inhibited by a regulatory mechanism activated under physiological conditions that induces the expression of slow-type MHC in skeletal muscles.

Cardiac aquaporins

Aquaporins are a group of proteins with high-selective permeability for water. A subgroup called aquaglyceroporins is also permeable to glycerol, urea and a few other solutes. Aquaporin function has mainly been studied in the brain, kidney, glands and skeletal muscle, while the information about aquaporins in the heart is still scarce. The current review explores the recent advances in this field, bringing aquaporins into focus in the context of myocardial ischemia, reperfusion, and blood osmolarity disturbances. Since the amount of data on aquaporins in the heart is still limited, examples and comparisons from better-studied areas of aquaporin biology have been used. The human heart expresses aquaporin-1, -3, -4 and -7 at the protein level. The potential roles of aquaporins in the heart are discussed, and some general phenomena that the myocardial aquaporins share with aquaporins in other organs are elaborated. Cardiac aquaporin-1 is mostly distributed in the microvasculature. Its main role is transcellular water flux across the endothelial membranes. Aquaporin-4 is expressed in myocytes, both in cardiac and in skeletal muscle. In addition to water flux, its function is connected to the calcium signaling machinery. It may play a role in ischemia-reperfusion injury. Aquaglyceroporins, especially aquaporin-7, may serve as a novel pathway for nutrient delivery into the heart. They also mediate toxicity of various poisons. Aquaporins cannot influence permeability by gating, therefore, their function is regulated by changes of expression-on the levels of transcription, translation (by microRNAs), post-translational modification, membrane trafficking, ubiquitination and subsequent degradation. Studies using mice genetically deficient for aquaporins have shown rather modest changes in the heart. However, they might still prove to be attractive targets for therapy directed to reduce myocardial edema and injury caused by ischemia and reperfusion.

Aquaporin expression in normal and pathological skeletal muscles: a brief review with focus on AQP4

Freeze-fracture electron microscopy enabled us to observe the molecular architecture of the biological membranes. We were studying the myofiber plasma membranes of health and disease by using this technique and were interested in the special assembly called orthogonal arrays (OAs). OAs were present in normal myofiber plasma membranes and were especially numerous in fast twitch type 2 myofibers; while OAs were lost from sarcolemmal plasma membranes of severely affected muscles with dystrophinopathy and dysferlinopathy but not with caveolinopathy. In the mid nineties of the last century, the OAs turned out to be a water channel named aquaporin 4 (AQP4). Since this discovery, several groups of investigators have been studying AQP4 expression in diseased muscles. This review summarizes the papers which describe the expression of OAs, AQP4, and other AQPs at the sarcolemma of healthy and diseased muscle and discusses the possible role of AQPs, especially that of AQP4, in normal and pathological skeletal muscles.

Aquaporin 4 mRNA levels in neuromuscular tissues of wild-type and dystrophin-deficient mice

Aquaporin (AQP) 4 is a water-specific channel protein and is abundant in central nervous tissues and skeletal muscles. Recently, the AQP4 molecule has been increasingly highlighted in its pathophysiological role of several neurological diseases, such as stroke, muscular dystrophy and neuromyelitis optica. We therefore measured the levels of AQP4 mRNA and glyceraldehyde-3 phosphate dehydrogenase mRNA (an internal control) in muscle and brain tissues of wild-type mice (C57BL10/ScSn) and age-matched dystrophin-deficient mdx mice (C57BL10/ScSn mdx) by real-time quantitative RT-PCR. The relative AQP4 mRNA level was highest in the spinal cord among the neuromuscular tissues examined in wild-type mice. Among the muscle tissues of wild-type mice, the relative AQP4 mRNA level was higher in extensor digitorum longus (EDL) muscles, and its descending order was EDL, quadriceps femoris, soleus and heart muscles. It is noteworthy that there was no difference in the relative AQP4 mRNA levels in the brain tissues between wild-type mice and age-matched mdx mice. In contrast, the AQP4 mRNA level in the quadriceps femoris muscle was significantly lower in mdx mice than in wild-type mice. The fact that the spinal cord contains the highest AQP4 mRNA may be related to the pathogenesis of neuromyelitis optica, in which AQP4 protein is the target antigen. In addition, the low expression level of AQP4 mRNA in the mdx mouse muscle suggests a functional link between AQP4 and dystrophin in the muscle tissue. We suggest that a similar pathomechanism may underlie the phenotypic consequences of the mdx mouse and Duchenne muscular dystrophy.