Rex E. Martin

Docosahexaenoic acid decreases phospholipase A2 activity in the neurites/nerve growth cones of PC12 cells.

Docosahexaenoic acid (DHA) accumulates in nerve growth cones (NGC) during perinatal development and it is neuroprotective in ischemia. Because the phospholipases A2 (PLA2) are present in NGC and these enzymes function in both ischemia and long‐term potentiation, the relationship between DHA and PLA2 was investigated in the NGC of nerve growth factor‐differentiated PC12 cells. When PC12 cells were incubated with [3H]DHA, it primarily esterified in ethanolamine glycerolipids and concentrated initially in cell bodies with similar levels present in the neurite/nerve growth cone (N/NGC) fraction after 4 days. PLA2 activity in the N/NGC fraction was investigated using [14C]arachidonic acid‐labeled phosphatidylinositol ([14C‐AA]PI) as substrate. Heat denaturation and pharmacological inhibition showed that much of the PLA2 activity was calcium‐independent and secretory rather than cytosolic. Supplementing the media with as little as 33 nM DHA significantly reduced PLA2 activity in the N/NGC fraction. J. Neurosci. Res. 54:805–813, 1998.u2003

Diabetes Alters Sphingolipid Metabolism in the Retina: A Potential Mechanism of Cell Death in Diabetic Retinopathy

Dysregulated sphingolipid metabolism causes neuronal cell death and is associated with insulin resistance and diseases. Thus, we hypothesized that diabetes-induced changes in retinal sphingolipid metabolism may contribute to neuronal pathologies in diabetic retinopathy. ESI-MS/MS was used to measure ceramide content and ceramide metabolites in whole retinas after 2, 4, and 8 weeks of streptozotocin-induced diabetes. After 4 and 8 weeks of diabetes, a ∼30% decrease in total ceramide content was observed, concomitant with a significant ∼30% increase in glucosylceramide levels in fed diabetic rats compared with their age-matched controls. Acute insulin therapy as well as a short-term lowering of glucose via fasting did not affect the increase in glucosylceramide composition. To assess the putative biological consequences of the increase in glucosylceramide composition, R28 retinal neurons were treated with glucosylceramide synthase inhibitors. Inhibiting glycosphingolipid metabolism increased insulin sensitivity in retinal neurons. Glycosphingolipid inhibitors augmented insulin-stimulated p70 S6kinase activity in the presence of inhibitory concentrations of high glucose or glucosamine. Inhibition of glycosphingolipid synthesis also suppressed glucosamine- and interleukin-1β–induced death. Consistent with these inhibitor studies, pharmacological accumulation of glycosphingolipids increased activation of the endoplasmic reticulum stress response, a putative modulator of insulin resistance and neuronal apoptosis. It is speculated that an increase in glucosylceramide, and possibly higher-order glycosphingolipids, could contribute to the pathogenesis of diabetic retinopathy by contributing to local insulin resistance, resulting in neuronal cell death. Thus, dysfunctional glycosphingolipid metabolism may contribute to metabolic stress in diabetes, and therapeutic strategies to restore normal sphingolipid metabolism may be a viable approach for treatment of diabetic retinopathy.

Altered expression and changing distribution of the nerve growth associated protein GAP-43 during ocular HSV-1 infection in the rabbit

This research examines changes that occur in neurons during corneal herpes simplex virus (HSV-1) infection and focuses on the nerve growth associated protein GAP-43. Cornea and trigeminal ganglion (TG) of New Zealand white rabbits were examined after inoculation of the McKrae and 17 Syn+ strains of HSV-1 to the cornea. Rabbit tissues were taken during acute, latent and induced reactivation stages of infection. Systemic immunosuppression (intravenous injections of cyclophosphamide and dexamethasone) was used to induce reactivation. Western blotting, immunoblotting and autoradiography with the same antibody were used respectively to verify antibody specificity, measure changes in GAP-43 concentration and localize GAP-43 to neurons in the TG. During acute infection, corneal GAP-43 increased significantly while no change was seen in the TG. GAP-43 content was elevated in TG and cornea during viral latency (post-inoculation days 84-154) for both HSV-1 strains. When latent virus was reactivated, the corneal concentration of GAP-43 was more than double that of normal rabbits and the concentration of GAP-43 in TG was reduced compared to the non-reactivated, latently-infected TG. In summary, HSV-1 infected TG neurons expressed more GAP-43 than control neurons and immunosuppressive therapy led not only to viral reactivation and increased GAP-43 concentrations in cornea but also to decreased GAP-43 concentrations in TG. These results suggest that factors which maintain HSV-1 latency and induce reactivation could be linked to elements regulating GAP-43 expression.