Loss-of-function mutations in Rab escort protein 1 (REP-1) affect intracellular transport in fibroblasts and monocytes of choroideremia patients

Abstract

Background: Choroideremia (CHM) is a progressive X-linked retinopathy caused by mutations in the CHM gene, which encodes Rab escort protein-1 (REP-1), an escort protein involved in the prenylation of Rabs. Under-prenylation of certain Rabs, as a result of loss of function mutations in REP-1, could affect vesicular trafficking, exocytosis and secretion in peripheral cells of CHM patients.

Methodology/principal findings: To evaluate this hypothesis, intracellular vesicle transport, lysosomal acidification and rates of proteolytic degradation were studied in monocytes (CD14+ fraction) and primary skin fibroblasts from the nine age-matched controls and thirteen CHM patients carrying 10 different loss-of-function mutations. With the use of pHrodo BioParticles conjugated with E. coli, collagen I coated FluoSpheres beads and fluorescent DQ ovalbumin with BODYPY FL dye, we demonstrated for the first time that lysosomal pH was increased in monocytes of CHM patients and, as a consequence, the rates of proteolytic degradation were slowed. Microarray analysis of gene expression revealed that some genes involved in the immune response, small GTPase regulation, transcription, cell adhesion and the regulation of exocytosis were significantly up and down regulated in cells from CHM patients compared to controls. Finally, CHM fibroblasts secreted significantly lower levels of cytokine/growth factors such as macrophage chemoattractant protein-1 (MCP-1), pigment epithelial derived factor (PEDF), tumor necrosis factor (TNF) alpha, fibroblast growth factor (FGF) beta and interleukin (lL)-8.

Conclusions/significance: We demonstrated for the first time that peripheral cells of CHM patients had increased pH levels in lysosomes, reduced rates of proteolytic degradation and altered secretion of cytokines. Peripheral cells from CHM patients expose characteristics that were not previously recognized and could used as an alternative models to study the effects of different mutations in the REP-1 gene on mechanism of CHM development in human population.

Figure 1. Experimental design.

Collection of monocyte fractions and culture of primary dermal fibroblasts for the evaluation of gene expression and functional differences between CHM patients and age-matched controls.

Figure 2. Fundus photographs of the control and CHM patients.

a. CHM patient 22 y.o. characterized by RPE depigmentation and widespread RPE disruption b. CHM patient 74 y.o. characterized by loss of RPE and choroid, scattered pigment in macula, faint deep choroidal vessels and severely narrowed retinal vessels and optic nerve pallor ( Table 1 , CHM 9 and 10 respectively). c. Female CHM carrier, age 50 showing patchy RPE hypopigmentation without pigment dispersion and control subject. d. Fundus photograph of the normal eye.

Figure 3. Effect of different mutations on the structure and levels of REP-1 mRNA and protein.

a, Effect of different nonsense mutations on the structure of REP-1 protein (Q273X, I460X, M1I and K234X). b, Distribution and position of the mutations in the REP-1 protein, note that 4 of 9 mutations localized in the beta sheet of the REP-1 (blue) and 7 of 9 mutations (P179X, K234X, I244X, I460X, Y504 X, L550P and I553X, Table 1 ) localized to domain 2 of the REP-1 protein. c. Levels of mRNA determined by the microarray analysis of the expression profiles from monocytes and fibroblasts from CHM and control patients. Control group, n = 5; group CHM1 includes patients with low levels of REP1 mRNA, n = 7; group CHM2 includes patients with REP-1 mRNA similar to the controls, n = 6. d. Expression levels of REP-1 and REP-2 in different cell types derived from CHM and control patients. Lane: 1, 10 ng of rat recombinant REP-1 or 10 ng of rat recombinant REP-2 with HisTag; cell lysates (40 µg of protein for each) 2, ARPE19; 3, human fetal RPE; 4, MO- monocytes from control; 5, MO-monocytes from patient CHM4; 6, cultured human umbilical vein endothelial cells (HUVECs); 7, primary fibroblasts from control; 8, primary fibroblasts from CHM2 patient. β-actin was used as a loading control.

Figure 4. Levels of lysosomal acidification and proteolytic degradation in monocytes from CHM patients and age-matched controls.

a. Intralysosomal acidification measurements were performed using E. coli BioParticles conjugated with pH dependent dye (pHrodo). Representative confocal images of the monocytes from patient CHM8 (right panel) and age matched control (left panel) feed with BioParticles at 3 h following the feeding. Acidic environment of the lysosomes caused an increase in fluorescent intensity of the BioParticles engulfed by the cells (in red, blue is a DAPI nuclear staining). b. Levels of fluorescence of the monocytes from CHM (n = 7) and control (n = 6) patients feed with BioParticles were analysed by flow cytometry, data expressed as a mean fluorescence intensity+/−SED at 1, 3 and 5 h following the feeding. Fluorescence intensity of pH dependent BioParticles taken up by monocytes from CHM patients was lower compared to the control at each time point. c. Representative FACS histograms showing shift in fluorescence intensity between the CHM and control monocytes fed with E. coli pHrodo at 1, 3 and 5 h. d. The efficiency of lysosome-mediated proteolytic degradation by monocytes from CHM (n = 7) and control (n = 6) patients assessed with DQ-ovalbumin particles. DQ-ovalbumin is a self-quenched substrate for proteases, which becomes fluorescent after proteolytic cleavage. Increase in fluorescence intensity, corresponding to the rate of proteolytic degradation of DQ-ovalbumin, was measured by flow cytometry. Data was expressed as a mean fluorescence intensity of the cells+/−SED at 1, 3 and 5 h following the feeding. Rates of proteolytic degradation were significantly lower in monocytes from CHM patients compared to control (• p = 0.005).

Figure 5. Lysosomal acidification and rate of proteolytic degradation in monocytes from CHM and control patients treated with Bafilomycin-A1 (BafA1).

a. Lysosomal acidification and rate of proteolytic degradation in monocytes from CHM and control BafA1. Intralysosomal acidification measurements were performed using E. coli BioParticles conjugated with a pH dependent dye (pHrodo). Treatment caused an increase in lysosomal pH as evident by a decrease in the fluorescence of BioParticles (confocal images, left panel vehicle no effect, right panel cells pre- treated with BafA1 for 30 min, decreased fluorescence). b. Decrease in fluorescence levels of BioParticles following the treatment with BafA1 in monocytes from control and patient CHM4 measured by flow cytometry analysis at 1, 3 and 5 hours following the feeding. c. Representative FACS histograms showing a shift in fluorescence intensity of the CHM and control monocytes fed with BioParticles treated with BafA1 at 1, 3 and 5 h. d. Decreased rate of DQ-ovalbumin degradation in CHM (n = 3) and control (n = 3) patients before and after the treatment with BafA1 measured by flow cytometry analysis at 1, 3 and 5 hours following the feeding. Data expressed as a percent of fluorescence reduction in CHM and control cells treated with BafA1, compared to the non-treated (NT) cells.

Figure 6. Differential uptake of collagen coated beads (FluoSphere) by primary fibroblasts from CHM and control patients.

a. Confocal image of the fibroblasts 1 h after the uptake of collagen coated fluorescent beads. Note that beads were internalized into the cytoplasm. b. Rates of bead uptake 1 h following feeding was significantly higher in the control cells (n = 7) compared to the CHM patients (n = 6) as measured by flow cytometry. Data expressed as mean fluorescence intensity+/−SED at 1, 3, 5 and 16 h. After that CHM cells demonstrated a similar rate of the uptake. c. Percent of cells taking up beads was consistently higher in the control cells (16 h period). d. CHM fibroblasts (n = 5) cultures fed with beads less resistant to the oxidative stress initiated by the treatment with 1 µM and 10 µM of hydroquinone (HQ) when compared to controls (n = 3). Viability levels were measured using XTT and expressed as percent viability of non-treated non-fed cells.

Figure 7. Mutation in REP-1 affects gene expression and secretion in CHM patients.

a. Hierarchical cluster of 47 probe sets in control and CHM samples. Using consistency testing, twenty-six probe sets were found to be significantly over-expressed and 21 under-expressed in monocytes and primary fibroblasts cells from CHM patients 43 (CHM 6-15) compared to control (Cont 1-5)(p<0.0001, FDR30%) b-d. Level of secretion of the cytokines and growth factors by primary fibroblast cultures into conditioned media. MCP-1, TNF-alpha and FGF factors were detected at significantly higher levels in samples collected from the control cells (n = 9) compared to conditioned media samples from 8 CHM patients (p<0.005)