Pressure effects on neurons: investigations into the pathogenesis of glaucoma.

Abstract

Cellular responses to changes in pressure are implicated in numerous disease processes. In glaucoma apoptosis of retinal ganglion cells (RGCs) is associated with elevated intra-ocular pressure (IOP), however the exact cellular basis of this link remains unclear. This research aimed to examine the direct response of neuronal cells to elevated hydrostatic pressure in terms of apoptosis. We developed an in vitro model consisting of a pressure chamber to adjust ambient hydrostatic pressure, a source of neuronal cells and methods to measure apoptosis in these cells. The neural cells examined were primary retinal cultures, four neuronal cell lines (B35, PC12, C17, NT2), and the RGC-5 cell line. Pressure conditions selected were within physiological limits; 100 mmHg above atmospheric pressure (as seen clinically in severe acute glaucoma) and extended in RGC-5 neurons to 30 mmHg (chronic glaucoma) and 15 mmHg (normal IOP). Apoptosis was detected by cell morphology and specific immunochemical markers: TUNEL and Annexin V. Caspase-3 activation, a known pathway of apoptosis, was also investigated in RGC-5 neurons. These fluorescent markers were detected and quantified by automated Laser Scanning Cytometry. Negative controls were treated identically except for the application of pressure, while positive controls were generated by treatment with a known apoptotic stimulus. The results showed that neurons responded to elevated hydrostatic pressure directly and that an apoptotic process was induced. There was a greater level of apoptosis in pressurised cells compared to the negative controls. This apoptotic effect at high pressures was seen in primary rat retinal cultures and in both undifferentiated (B35, C17, NT2, RGC-5) and differentiated (PC12, RGC-5) neuronal cell lines. RGC-5 neurons showed a graded response, proportionate to the level of pressure elevation, representative of the severity of analogous clinical settings (acute, chronic glaucoma & normal). RGC-5 neurons also showed increased activation of Capsase-3. Thus this pathway may play a role in pressure induced apoptosis. Our findings indicate that pressure alone may act as a stimulus for apoptosis in neuronal cells. We suggest the possibility of novel mechanisms of pressure related mechanotransduction and cell death, relevant to the pathogenesis of glaucoma.