A diverse array of environmental factors contribute to the overall control of stem cell activity. In particular, new data continues to mount on the influence of the extracellular matrix (ECM) on stem cell fate through physical interactions with cells, such as the control of cell geometry, ECM geometry/topography at the nanoscale, ECM mechanical properties, and the transmission of mechanical or other biophysical factors to the cell. Here we review some of the physical processes by which cues from the ECM can influence stem cell fate, with particular relevance to the use of stem cells in tissue engineering and regenerative medicine.

The detection of osmotic stimuli is essential for all organisms, yet few osmoreceptive proteins are known, none of them in vertebrates. By employing a candidate-gene approach based on genes encoding members of the TRP superfamily of ion channels, we cloned cDNAs encoding the vanilloid receptor-related osmotically activated channel (VR-OAC) from the rat, mouse, human, and chicken. This novel cation-selective channel is gated by exposure to hypotonicity within the physiological range. In the central nervous system, the channel is expressed in neurons of the circumventricular organs, neurosensory cells responsive to systemic osmotic pressure. The channel also occurs in other neurosensory cells, including inner-ear hair cells, sensory neurons, and Merkel cells.

Major features of the transcellular signaling mechanism responsible for endothelium-dependent regulation of vascular smooth muscle tone are unresolved. We identified local calcium (Ca2+) signals (“sparklets”) in the vascular endothelium of resistance arteries that represent Ca2+ influx through single TRPV4 cation channels. Gating of individual TRPV4 channels within a four-channel cluster was cooperative, with activation of as few as three channels per cell causing maximal dilation through activation of endothelial cell intermediate (IK)- and small (SK)-conductance, Ca2+-sensitive potassium (K+) channels. Endothelial-dependent muscarinic receptor signaling also acted largely through TRPV4 sparklet-mediated stimulation of IK and SK channels to promote vasodilation. These results support the concept that Ca2+ influx through single TRPV4 channels is leveraged by the amplifier effect of cooperative channel gating and the high Ca2+ sensitivity of IK and SK channels to cause vasodilation.

Osmotic homeostasis is one of the most aggressively defended physiological parameters in vertebrates. However, the molecular mechanisms underlying osmotic regulation are poorly understood. The transient receptor potential channel, vanilloid subfamily (TRPV4), is an osmotically activated ion channel that is expressed in circumventricular organs in the mammalian CNS, which is an important site of osmotic sensing. We have generated trpv4-null mice and observed abnormalities of their osmotic regulation. trpv4 ؊/؊ mice drank less water and became more hyperosmolar than did wild-type littermates, a finding that was seen with and without administration of hypertonic saline. In addition, plasma levels of antidiuretic hormone were significantly lower in trpv4 ؊/؊ mice than in wild-type littermates after a hyperosmotic challenge. Continuous s.c. infusion of the antidiuretic hormone analogue, dDAVP, resulted in systemic hypotonicity in trpv4 ؊/؊ mice, despite the fact that their renal water reabsorption capacity was normal. Thus, the response to both hyper-and hypoosmolar stimuli is impaired in trpv4 ؊/؊ mice. After a hyperosmolar challenge, there was markedly reduced expression of c-FOS in the circumventricular organ, the organum vasculosum of the lamina terminalis, of trpv4 ؊/؊ mice compared with wild-type mice. This finding suggests that there is an impairment of osmotic sensing in the CNS of trpv4 ؊/؊ mice. These data indicate that TRPV4 is necessary for the normal response to changes in osmotic pressure and functions as an osmotic sensor in the CNS. In vertebrate organisms, maintenance of an equilibrium of the internal environment is essential for viability, and osmotic equilibrium is aggressively defended against changes with a set-point value ranging between 280-300 milliosmol͞kg in most mammals (295 in humans and rodents) (1, 2). Systemic osmotic pressure is maintained by feedback regulation in which neurons in the circumventricular organs, the organum vasculosum of the lamina terminalis (OVLT), and, perhaps, also the subfornical organ (SFO), sense osmotic pressure. These neurons in turn project to magnocellular neurons in supraoptic and paraventricular nucleus of the hypothalamus (3). Magnocellular neurons themselves possess intrinsic osmosensitivity and they synthesize antidiuretic hormone (ADH) and secrete it into the blood (1, 2, 4-9). The ADH directs free-water reabsorption in the collecting ducts of the kidney (10). With respect to the regulation of water intake behavior, CNS lesioning experiments implicate a role for the lamina terminalis that comprises the circumventricular organs, the OVLT and the SFO, and the median preoptic area (2,(5)(6)(7)(11)(12)(13).Recently, we and others (14-17) have isolated, to our knowledge, a new vertebrate member of the transient receptor potential family, vanilloid subfamily (TRPV) of ion channels, the vanilloid receptor-related osmotically activated ion channel (VR-OAC), now named TRPV4. In heterologous expression systems, TRPV4, a nonselective cation channel, is activated b...

Leptin elicits a metabolic response that cannot be explained by its anorectic effects alone. To examine the mechanism underlying leptin's metabolic actions, we used transcription profiling to identify leptin-regulated genes in ob/ob liver. Leptin was found to specifically repress RNA levels and enzymatic activity of hepatic stearoyl-CoA desaturase-1 (SCD-1), which catalyzes the biosynthesis of monounsaturated fatty acids. Mice lacking SCD-1 were lean and hypermetabolic. ob/ob mice with mutations in SCD-1 were significantly less obese than ob/ob controls and had markedly increased energy expenditure. ob/ob mice with mutations in SCD-1 had histologically normal livers with significantly reduced triglyceride storage and VLDL (very low density lipoprotein) production. These findings suggest that down-regulation of SCD-1 is an important component of leptin's metabolic actions.

Mechanical loading of joints plays a critical role in maintaining the health and function of articular cartilage. The mechanism(s) of chondrocyte mechanotransduction are not fully understood, but could provide important insights into new physical or pharmacologic therapies for joint diseases. Transient receptor potential vanilloid 4 (TRPV4), a Ca 2+ -permeable osmomechano-TRP channel, is highly expressed in articular chondrocytes, and loss of TRPV4 function is associated with joint arthropathy and osteoarthritis. The goal of this study was to examine the hypothesis that TRPV4 transduces dynamic compressive loading in articular chondrocytes. We first confirmed the presence of physically induced, TRPV4-dependent intracellular Ca 2+ signaling in agarose-embedded chondrocytes, and then used this model system to study the role of TRPV4 in regulating the response of chondrocytes to dynamic compression. Inhibition of TRPV4 during dynamic loading prevented acute, mechanically mediated regulation of proanabolic and anticatabolic genes, and furthermore, blocked the loadinginduced enhancement of matrix accumulation and mechanical properties. Furthermore, chemical activation of TRPV4 by the agonist GSK1016790A in the absence of mechanical loading similarly enhanced anabolic and suppressed catabolic gene expression, and potently increased matrix biosynthesis and construct mechanical properties. These findings support the hypothesis that TRPV4-mediated Ca 2+ signaling plays a central role in the transduction of mechanical signals to support cartilage extracellular matrix maintenance and joint health. Moreover, these insights raise the possibility of therapeutically targeting TRPV4-mediated mechanotransduction for the treatment of diseases such as osteoarthritis, as well as to enhance matrix formation and functional properties of tissue-engineered cartilage as an alternative to bioreactor-based mechanical stimulation. mechanobiology | ion channel | calcium signaling | TGF-beta | tissue engineering

Here we provide evidence for a critical role of the transient receptor potential cation channel, subfamily V, member 4 (TRPV4) in normal bladder function. Immunofluorescence demonstrated TRPV4 expression in mouse and rat urothelium and vascular endothelium, but not in other cell types of the bladder. Intracellular Ca 2+ measurements on urothelial cells isolated from mice revealed a TRPV4-dependent response to the selective TRPV4 agonist 4α-phorbol 12,13-didecanoate and to hypotonic cell swelling. Behavioral studies demonstrated that TRPV4 -/-mice manifest an incontinent phenotype but show normal exploratory activity and anxietyrelated behavior. Cystometric experiments revealed that TRPV4 -/-mice exhibit a lower frequency of voiding contractions as well as a higher frequency of nonvoiding contractions. Additionally, the amplitude of the spontaneous contractions in explanted bladder strips from TRPV4 -/-mice was significantly reduced. Finally, a decreased intravesical stretch-evoked ATP release was found in isolated whole bladders from TRPV4 -/-mice. These data demonstrate a previously unrecognized role for TRPV4 in voiding behavior, raising the possibility that TRPV4 plays a critical role in urothelium-mediated transduction of intravesical mechanical pressure.

Exacerbated sensitivity to mechanical stimuli that are normally innocuous or mildly painful (mechanical allodynia and hyperalgesia) occurs during inflammation and underlies painful diseases. Proteases that are generated during inflammation and disease cleave protease-activated receptor 2 (PAR 2 ) on afferent nerves to cause mechanical hyperalgesia in the skin and intestine by unknown mechanisms. We hypothesized that PAR 2 -mediated mechanical hyperalgesia requires sensitization of the ion channel transient receptor potential vanilloid 4 (TRPV4). The ability to detect mechanical stimuli allows organisms to respond to their environment. High-intensity mechanical stimuli can damage tissue and provoke pain, leading to avoidance behaviours. Inflammatory mediators enhance sensitivity to mechanical stimuli that are normally innocuous or mildly painful (mechanical allodynia or hyperalgesia, respectively), resulting in pain associated with disorders such as arthritis, inflammatory bowel disease and irritable bowel syndrome. However, the ion channels that transduce mechanical stimuli are