A. Chiavegato et al., Differential expression of SM22 isoforms in myofibroblasts and smooth muscle cells from rabbit bladder, J MUSCLE R, 20(2), 1999, pp. 133-146
The E-11 and 1-B8 monoclonal antibodies raised to the smooth muscle (SM)-sp
ecific SM22 protein from pig stomach were used to study the in vivo and in
vitro phenotypic characteristics of myofibroblasts (MF) and SM cells (SMC)
from the bladder detrusor muscle and serosal thickening of male rabbit. The
22-kDa SM22 band found in the SM extract appeared to be composed of distin
ct isoforms when examined in non-equilibrium two-dimensional gel electropho
resis (2D-EF): alpha (the most basic), beta, gamma, and delta (the most aci
dic) in the ratio of 34(alpha):23(beta):36(gamma):8(delta). Western blots o
f 2D-electrophoresed bladder extracts treated with E-11 and 1-B8 showed tha
t alpha, beta, and delta, but not gamma isoforms were labeled with E-11, wh
ereas alpha, beta, and gamma isoforms were stained with 1-B8. This differen
tial immunoreactivity was not influenced by phosphorylation. The tissue dis
tribution of SM22 immunostaining was heterogeneous in the bladder SM and se
rosal thickening developed as a consequence of partial outflow obstruction
of the urinary bladder. At the cellular level, the 1-B8 and E-11 antibodies
stained the SMC in a "diffuse" (the whole cytoplasm) and "honeycomb" (the
peripheral cytoplasm) manner, whereas MF immunostaining was quite homogeneo
us. The two antibodies also reacted with cultured primary bladder SMC and M
F grown in low serum conditions showing a heterogeneous SM22 cell distribut
ion but an identical subcellular localization, i.e., the actin-containing f
ilamentous network, distinguishable in part from that found in vivo. The im
munocytochemical, Western blotting and 2D-EF patterns of MF from thickened
serosa indicated that the gamma isoform alone is expressed in this tissue.
This SM22 variant appeared before the completion of the cellular transition
from MF to fully differentiated SMC. This pattern is reminiscent of bladde
r ontogenesis where SM22 expression in the developing bladder wall precedes
that of SM myosin. Taken together these data suggest that: (i) SM22 isofor
ms are differently assorted in MF vs. SMC; (ii) SM22 is a SMC-lineage marke
r inasmuch as its expression occurs in an experimental condition characteri
zed by a time-related cell phenotypic transition from MF to SMC, and (iii)
cell conversion ability of serosal cells in the adult might take place via
the reactivation of a specific "foetal" gene programme.