Dissertation for Scientific Degree of Dr.habil.med.
Valerija Groma
Cytoskeleton of kidney
Medical Academy of Latvia, 1998
Cytoskeleton is a native integral part of every cell. This is localized in close spatial proximity of other
organelles, and constantly interacts with them. Cytoskeleton is assumed to be a modulator of
numerous cell functions. This plays a pivotal role in pathogenesis and progression of some human
diseases. Moreover, it is widely believed that some disorders are thought to be "diseases of
cytoskeleton". Peculiar morphological features of cytoskeleton elements in the cells of the kidney
nephron and their role in physiological and pathological processes appearing during urine formation
have not been elucidated yet.
Analysis of kidney biopsy appears to be a necessary step traditionally included in a number of
clinical and functional examinations made in case of renal disease. However, the traditional
morphological methods are lacking to indicate the very early changes in the course of kidney disease.
The present study aimed to elucidate the peculiar morphological features of nephron cell cytoskeleton
in case of renal pathology and kidney embryogenesis as well as to clarify the correlation between
cytoskeletal changes and progression of renal disease. Additionally, the role of detection of these
changes in prediction of renal disease is also discussed.
A wide range of renal biopsies that included both acute and chronic kidney disorders as well as
systematic and metabolic disorders was used in this study. Renal biopsy material was obtained from
patients admitted at the Centre of Nephrology at the Academy Hospital in Riga in 1991-1998 and
Aarhus University Hospital (Denmark) in 1989-1992. Human embryonic and fetal kidneys from 24
embryos and fetuses (ranging from 6 to 21 week of gestation) were obtained fresh from tissue
examined after therapeutic abortions. Experimental material was obtained from 11 white Wistar rats.
Routine
histochemical methods
as well as immunohistochemistry (direct
and indirect
immunofluorescence, indirect immunohistochemistry based on avidin-biotin and double staining) and
electron microscopy (standard and immunoelectron microscopy) were
applied. Subjective semiquantitative estimation of cytoskeleton proteins expression as well as
morphometry were used.
It has been shown that cytoskeleton changes along with cell differentiation, and, therefore is a good
indicator of cells' specialization. This could be used in order to predict the progression of renal
disease and chose an appropriate therapy.
The cytoskeleton in the nephron cells is composed of microfilaments, intermediate filaments which
are supplemented by straight, slender microtubules. These have been demonstrated to be connected
with organelles and are responsible for the special cellular functions.
It has been shown that along with the changes of podocyte shape and foot processes fusion, the
changes of actin cytoskeleton take part. The podocyte appears more dense microfilamental
organization and smoothly attaches to the glomerular basement membrane. Changes of microtubules
and intermediate filaments are not so prominent. Epithelial cells of the nephron bear a single cilium.
Striated rootlets appear to be associated with the base of it.
Immunohistochemical studies of glomerular a-smooth muscle actin expression revealed that this is a
reliable marker of mesangial cell activation and is a good predictor of glomerular injury. In case of
IgA nephropathy this appears to be expressed even in the practically normal at the light microscopical
level glomeruli. a-smooth muscle actin expression estimated quantitatively did not correlate with the
cell proliferation, proliferation index and cellularity. However, quantitative and semiquantitative asmooth muscle actin immunoreactivity data showed a good correlation. It is believed that a constant
glomerular a-smooth muscle actin expression is an indicator of disease progression toward
glomerulosclerosis.
It has been observed that in case of renal interstitial fibrosis excessive accumulation of collagen type
VI along with types I and III takes a place. This is associated with the phenotypic changes of renal
fibroblasts. Many of them transform into myofibroblasts. Some transitional forms like interstitial
cells with numerous lipid droplets have been observed ultrastructurally. Deposition of collagen type
VI in the renal interstitium
correlates with tubular atrophy. In case of prominent interstitial widening and tubular atrophy
collagen accumulation correlated with the serum creatinine level. Moreover, it was revealed that
TGF-p- and a-smooth muscle actin-positive cells were colocalized in the same fibrotic area.
It has been observed that nephrogenic zone of embryonic kidney contains small blood vessels. These
form arcades beneath the organ capsule. Different staged of glomerulogenesis could be observed in
one section. Mature glomeruli appear to have numerous a-smooth muscle actin-positive mesangial
areas. This phenomenon is coincident with the beginning of urine formation in the nephron.