Why is homocysteine toxic for the nervous and immune systems?

Hyperhomocysteinemia is a risk factor for a number of neurodegenerative and cardiovascular diseases. We have shown that homocysteine induces excitotoxic effects in cells expressing glutamate receptors of the NMDA class. These receptors were found not only in neurons but also in immune-competent cells, neutrophils, red blood cells, cardiomyocytes, and osteoblasts. Activation of these cells by homocysteine results in an increase in cytoplasmic calcium ions, accumulation of reactive oxygen species, and activation of MAP kinase. An overload of immune-competent cells activates both necrotic and apoptotic cell death, whereas the neuropeptide carnosine (an antioxidant and immune modulator) protects cells against both processes. In a model of prenatal hyperhomocysteinemia in rats, we have found that carnosine protects animals against homocysteine toxicity with no change of the blood homocysteine levels. The efficiency of carnosine has also been demonstrated in clinical trials of chronic brain ischemia and Parkinson's disease.

Pinealon protects the rat offspring from prenatal hyperhomocysteinemia.

The offspring of rats with experimental hyperhomocysteinemia caused by alimentary loading with dietary methionine within pregnancy has been studied. Using pinealon (Glu-Asp-Arg) under these conditions was found to result in the offspring cognitive function being improved significantly and their cerebellum neurons becoming more resistant to oxidative stress. This may be proved by the fact that the administration of pinealon to pregnant rats loaded with methionine improved their offspring spatial orientation and learning ability and decreased both reactive oxygen species accumulation and the number of necrotic cells in the population of the neurons isolated from the cerebellum of the offspring developed under the prenatal hyperhomocysteinemia. Our experiments allowed confirming the neuroprotective properties of pinealon, which is in agreement with the previous data obtained by us in vitro.

NMDA receptors are expressed in lymphocytes activated both in vitro and in vivo.

There is increasing evidence showing that the interplay between neuronal and immune systems may be regulated by neuromediators. However, little is known about the involvement of glutamatergic system in such neuro-immune relations. In the present study, we have shown that some intact lymphocytes express N-methyl-D: -aspartate activated receptors (NMDA receptors), an important constituent of glutamatergic system. The activation of lymphocytes with phytohemagglutinin (PHA) induces a time-dependent increase in the amount of NMDA receptor presenting cells, and NMDA stimulates this process. Immune response of such lymphocytes is suppressed and the amount of cells producing interferon gamma (IFN-gamma) in vitro is decreased to the level corresponding to intact (non-activated) cells. Furthermore, lymphocytes in the region of inflammation, induced by spinal cord injury (SCI), are also NMDA-positive. We suggest that expression of NMDA receptors in lymphocytes is regulated by central nervous system, which controls the inflammation process.

The excitotoxic effect of NMDA on human lymphocyte immune function.

N-Methyl-d-aspartate (NMDA)-activated glutamate receptors are expressed in lymphocytes, but their roles have not yet been defined. We show that incubation of human peripheral blood lymphocytes with NMDA resulted in increased intracellular calcium and reactive oxygen species (ROS) levels through effects on NMDA-activated glutamate receptors. In terms of ROS production, T cells were most affected, followed by NK cells, whereas B cell ROS levels were not increased. In unstimulated T and NK cells, interferon-gamma (IFN-gamma) production was unaffected by NMDA, whereas interleukin-2 stimulation of IFN-gamma production was significantly suppressed by NMDA. Simultaneous incubation of the cells with NMDA and IL-2 resulted in a dramatic increase in the amount of cells expressing the NR1 subunit of the NMDA-activated receptors. We conclude that NMDA-activated glutamate receptor activation, accompanied by the changes in intracellular calcium and ROS levels, may be involved in the modification of immune functions of human T and NK cells.

Rodent lymphocytes express functionally active glutamate receptors.

RT-PCR demonstrated that ionotropic (iGluR NR1) and metabotropic (mGluR Group III) glutamate receptors are expressed in rodent lymphocytes. Flow cytometry showed that activation of iGluR NR1 by N-methyl-D-aspartate (NMDA) increased intracellular free calcium and reactive oxygen species (ROS) levels and activated caspase-3. The latter effect was attenuated by the NMDA antagonist, 5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801), by the antioxidant N-acetylcysteine and by cyclosporin A. Treatment with L-2-amino-4-phosphonobutyric acid (L-AP4), an mGluR Group III agonist, increased lymphocyte ROS levels but to a lower extent than did NMDA. Activation of lymphocytes with both NMDA and L-AP4 caused a synergistic increase in ROS levels and induced necrotic cellular death without elevating the caspase-3 activation observed in the presence of NMDA alone. These results show that lymphocyte iGluR NR1 and mGluR Group III receptors may be involved in controlling rodent lymphocyte functions and longevity as they regulate events in cell proliferation, maturation, and death.