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Blood-tissue exchange and homeostasis within the organs depend on various interactions between endothelial and perivascular cells (Buniatian, 2001). Podocytes possess anatomical and cellular features intermediate between those of astrocytes and hepatic stellate cells (HSCs). Podocytes, like HSCs, are associated with fenestrated capillaries and, similar to astrocytes, interact with the capillaries via the basement membrane and participate in permeability-limiting ultrafiltration. The fact that podocytes come in direct contact with xenobiotics prompted us to investigate whether they express metallothionein (MT), an anticytotoxic system characteristic of astrocytes. In comparative studies, cryosections of 1-and 3-month-old rat kidney and adult rat brain, as well as podocytes and astrocytes from early and prolonged primary cultures of glomerular explants and newborn rat brain, respectively, were investigated. The cells were double-labeled with antiserum against glial fibrillary acidic protein (GFAP) and monoclonal antibody (MAb) against the lysine-containing epitope of Cd/Zn-MT-I (MAb MT) or MAb against alpha-actin. In kidney sections, MT immunoreactivity was detected in GFAP-positive glomerular cells and in interstitial fibroblasts. The pattern of staining for MT and GFAP in glomerular cells was similar to that of astrocytes in vivo. In glomerular cell cultures, MT was expressed in cobblestone-like podocytes which contained Wilms' tumor protein and lacked desmin. MT was upregulated at later culture periods, during which podocytes acquired features typical of undifferentiated astrocytes. This study hints at the existence of common regulatory mechanisms of blood-tissue interactions by neural and non-neural perivascular cells. These mechanisms appear to be used in an organ-specific manner.
Blood-tissue exchange and homeostasis within the organs depend on various interactions between endothelial and perivascular cells (Buniatian, 2001). Podocytes possess anatomical and cellular features intermediate between those of astrocytes and hepatic stellate cells (HSCs). Podocytes, like HSCs, are associated with fenestrated capillaries and, similar to astrocytes, interact with the capillaries via the basement membrane and participate in permeability-limiting ultrafiltration. The fact that podocytes come in direct contact with xenobiotics prompted us to investigate whether they express metallothionein (MT), an anticytotoxic system characteristic of astrocytes. In comparative studies, cryosections of 1-and 3-month-old rat kidney and adult rat brain, as well as podocytes and astrocytes from early and prolonged primary cultures of glomerular explants and newborn rat brain, respectively, were investigated. The cells were double-labeled with antiserum against glial fibrillary acidic protein (GFAP) and monoclonal antibody (MAb) against the lysine-containing epitope of Cd/Zn-MT-I (MAb MT) or MAb against alpha-actin. In kidney sections, MT immunoreactivity was detected in GFAP-positive glomerular cells and in interstitial fibroblasts. The pattern of staining for MT and GFAP in glomerular cells was similar to that of astrocytes in vivo. In glomerular cell cultures, MT was expressed in cobblestone-like podocytes which contained Wilms' tumor protein and lacked desmin. MT was upregulated at later culture periods, during which podocytes acquired features typical of undifferentiated astrocytes. This study hints at the existence of common regulatory mechanisms of blood-tissue interactions by neural and non-neural perivascular cells. These mechanisms appear to be used in an organ-specific manner.
Glial fibrillary acidic protein (GFAP) has recently been shown to be expressed in the glomerular podocytes and mesangial cells (MC) of kidney (Buniatian et al (1998) Biol Cell 90, 53-61). The different localization of GFAP in podocytes and MC has raised the question whether this might reflect specific cellular functions. To address this question, in the present study podocytes and MC in early (2, 3 day-old), prolonged (5, 7 day-old) and late (14, 21 day-old) primary cultures from out-growths of glomerular explants were used. Double-immunolabeling studies demonstrated that podocytes transiently acquire myofibroblastic features, characterized by the expression of smooth muscle alpha-actin (SMAA) and increased perinuclear reaction of GFAP in prolonged cultures. The morphological differentiation of cobblestone-like podocytes into process-bearing cells was followed by loss of the myofibroblastic marker, SMAA, de novo expression of desmin, and distribution of GFAP, vimentin and desmin into the processes. In late culture, GFAP and SMAA were nearly absent from the podocytes which maintained the cobblestone-like morphology. By contrast, the myofibroblastic features gained by MC during prolonged culturing increased with time. A myofibroblast-like cytoskeleton of podocytes and MC similar to that of healthy astrocytes suggest an increased spectrum of functional activities of these cells during the acquisition of myofibroblastic features. In addition, the present study provides a new combination of biochemical and biological features by which podocytes and MC can be distinguished in culture.
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