  H$_2$O MegaMaser emission may arise from thin gas discs surrounding the
massive nuclei of galaxies such as NGC\,4258, but the physical conditions
responsible for the amplified emission are unclear. A detailed view of these
regions is possible using the very high angular resolution afforded by space
very long baseline interferometry (SVLBI). Here we report SVLBI experiments
conducted using the orbiting RadioAstron Observatory that have resulted in
detections of the H$_2$O 22 GHz emission in NGC\,4258, with Earth-space
baselines of 1.3, 9.5 and 19.5 Earth diameters. Observations at the highest
angular resolution of 11 and 23 $\mu$as show distinct and regularly spaced
regions within the rotating disc, at an orbital radius of about 0.126 pc. These
observations at three subsequent epochs also indicate a time evolution of the
emission features, with a sudden rise in amplitude followed by a slow decay.
The formation of the emission regions, their regular spacing and their
time-dependent behaviour appear consistent with the occurrence of a periodic
magneto-rotational instability in the disc. This type of shear-driven
instability within the differentially rotating disc has been suggested to be
the mechanism governing the radial momentum transfer and viscosity within a
mass-accreting disc. The connection of the H$_2$O MegaMaser activity with the
magneto-rotational instability activity would make it an indicator of the
mass-accretion rate in the nuclear disc of the host galaxy.
