TRPV4 (Transient Receptor Potentials, vanilloid family, type 4) is widely expressed in vertebrate tissues and is activated by several stimuli, including by mechanical forces. Certain TRPV4 mutations cause complex hereditary bone or neuronal pathologies in human. Wild-type or mutant TRPV4 transgenes are commonly expressed in cultured mammalian cells and examined by
Fura-2 fluorometry and by
electrodes. In terms of the mechanism of mechanosensitivity and the molecular bases of the diseases, the current literature is confusing and controversial. To
complement existing methods, we describe two additional methods to examine the molecular properties of TRPV4. (1) Rat TRPV4 and an
aequorin transgene are transformed into budding yeast. A hypo-osmtic
shock of the transformant population yields a luminometric signal due to the combination of
aequorin with Ca(2+), released through the TRPV4 channel. Here TRPV4 is isolated from its usual mammalian partner
proteins and reveals its own mechanosensitivity. (2)
cRNA of TRPV4 is injected into Xenopus oocytes. After a suitable period of incubation, the macroscopic TRPV4 current is examined with a two-
electrode voltage clamp. The current rise upon removal of inert osmoticum from the oocyte bath is indicative of mechanosensitivity. The microAmpere (10(-6) to 10(-4) A) currents from oocytes are much larger than the subnano- to nanoAmpere (10(-10) to 10(-9) A) currents from cultured cells, yielding clearer quantifications and more confident assessments. Microscopic currents reflecting the activities of individual channel
proteins can also be directly registered under a patch clamp, in on-cell or excised mode. The same oocyte provides multiple patch samples, allowing better data replication.
Suctions applied to the patches can activate TRPV4 to directly assess mechanosensitivity. These methods should also be useful in the study of other types of TRP channels.