The deficiency of PIP2 5-phosphatase in Lowe syndrome affects actin polymerization

Abstract

Lowe syndrome is a rare X-linked disorder characterized by bilateral congenital cataracts, renal Fanconi syndrome, and mental retardation. Lowe syndrome results from mutations in the OCRL1 gene, which encodes a phosphatidylinositol 4,5 bisphosphate 5-phosphatase located in the trans-Golgi network. As a first step in identifying the link between ocrl1 deficiency and the clinical disorder, we have identified a reproducible cellular abnormality of the actin cytoskeleton in fibroblasts from patients with Lowe syndrome. The cellular abnormality is characterized by a decrease in long actin stress fibers, enhanced sensitivity to actin depolymerizing agents, and an increase in punctate F-actin staining in a distinctly anomalous distribution in the center of the cell. We also demonstrate an abnormal distribution of two actin-binding proteins, gelsolin and alpha-actinin, proteins regulated by both PIP(2) and Ca(+2) that would be expected to be altered in Lowe cells. Actin polymerization plays a key role in the formation, maintenance, and proper function of tight junctions and adherens junctions, which have been demonstrated to be critical in renal proximal tubule function, and in the differentiation of the lens. These findings point to a general mechanism to explain how this PIP(2) 5-phosphatase deficiency might produce the Lowe syndrome phenotype.

Figure 1

Actin stress-fiber staining is reduced in Lowe fibroblasts. F-actin staining was assessed in Lowe and normal fibroblasts, using Alexa 488 conjugated phalloidin to detect F-actin. Stained cells were classified into one of four types of actin staining: examples of each type (a–d), are shown in normal cells. Scale bar=0.016 μm. The histograms (e and f) show the results of two experiments with different Lowe and normal fibroblast lines, in which 100 (e) and 220 cells (f) of each type were scored by an observer masked to the genotype. Lowe syndrome fibroblasts had fewer cells with type a and b staining patterns and more cells with type c and d staining patterns, as compared with control fibroblasts (χ²=38.00, df 7, P<10^–11). These results demonstrate that Lowe fibroblasts have a reduced number of long actin stress fibers.

Figure 2

The F-actin staining pattern of Lowe cells shows a punctate pattern in the center of the cell, essentially absent from control cells. Lowe (a) and normal (b) fibroblasts were stained for F-actin using a fluorescent conjugated phalloidin, as described in figure 1. Scale bar=0.02 μm.

Figure 3

Lowe fibroblasts had a more rapid response than controls to the actin depolymerizing agents, cytochalasin D and latrunculin A. Normal (a–c) and two Lowe fibroblast lines (d–f and g–i) were stained for F-actin using fluorescent conjugated phalloidin, as described in figure 1. Fibroblasts are shown untreated (a, d, and g) or incubated 5 min with 1 μmol cytochalasin D (b, e, and h) or with 0.24 μmol latrunculin A (c, f, and i). Lowe cells (e, f, h, and i) showed a greater response to depolymerizing agents than did control cells (b and c). Scale bar=0.02 μm.

Figure 4

Gelsolin staining is altered in Lowe fibroblasts. Control and two Lowe fibroblast cultures were fixed, permeabilized, and immunostained with a monoclonal antibody to gelsolin, followed by an Alexa 594 conjugated secondary antibody. In control cells (a), gelsolin staining was distributed along stress fibers. In Lowe fibroblasts (b and c), there was an accumulation of punctate staining in the perinuclear area that was virtually absent in normal fibroblasts. Scale bar=0.02 μm.

Figure 5

Punctate gelsolin staining in Lowe fibroblasts coincides with the punctate F-actin filament staining. Lowe fibroblasts were fixed and incubated with a monoclonal antibody to gelsolin followed by an Alexa 594 conjugated anti-rabbit secondary antibody, along with Alexa 488 conjugated phalloidin. Fluorescence was visualized by confocal microscopy. The punctate gelsolin staining (a) and punctate F-actin staining (b) coincided, as shown in the merged image (c). Scale bar=0.02 μm.

Figure 6

Distribution of the punctate gelsolin staining does not colocalize with the TGN but does overlap with that of an ER marker. Lowe fibroblasts (a–c) were fixed and incubated with a monoclonal antibody to gelsolin and a polyclonal antibody to the mannose 6-phosphate receptor (M6-PR), followed by an Alexa 594 conjugated anti-mouse secondary antibody and an Alexa 488 conjugated anti-rabbit secondary antibody. The gelsolin staining (a) did not overlap with that of the TGN marker, M6-PR (b), as seen in the merged image (c). Scale bar=0.06 μm. The localization of the punctate gelsolin staining was further studied (d–f) by double-labeling Lowe fibroblasts, as described above, with anti-gelsolin antibody and with Alexa 488 conjugated concanavalin A, a probe for the endoplasmic reticulum. Fluorescence was visualized by confocal microscopy (d–f), scale bar=0.063 μm. The gelsolin staining (d) and the concanavalin A staining (e) partially overlapped, as shown in the merged image (f).

Figure 7

Lowe cells transfected with ocrl1 cDNA are lacking the punctate gelsolin staining seen in most untransfected fibroblasts. Images show gelsolin staining of transfected (arrows) and untransfected cells. Following transfection, cells were plated, fixed, and double-labeled with an antibody to ocrl1, to identify transfected cells, and with gelsolin antibody. Scale bar=0.05 μm.

Figure 8

Immunofluorescence staining of α-actinin is altered in Lowe fibroblasts. Control and two Lowe fibroblast cultures were fixed, permeabilized, and immunostained with a monoclonal antibody to α-actinin followed by an Alexa 488 conjugated secondary antibody. In control cells (a), α-actinin staining was present at focal adhesions and along stress fibers. In Lowe fibroblasts (b and c), there was an accumulation of punctate staining in the perinuclear area that was virtually absent in normal fibroblasts. Scale bar=0.05 μm.