Huntington's disease. Glutamate and aspartate metabolism in blood platelets

Platelets in Neurodegenerative Conditions-Friend or Foe?

It is now apparent that platelet function is more diverse than originally thought, shifting the view of platelets from blood cells involved in hemostasis and wound healing to major contributors to numerous regulatory processes across different tissues. Given their intriguing ability to store, produce and release distinct subsets of bioactive molecules, including intercellular signaling molecules and neurotransmitters, platelets may play an important role in orchestrating healthy brain function. Conversely, a number of neurodegenerative conditions have recently been associated with platelet dysfunction, further highlighting the tissue-independent role of these cells. In this review we summarize the requirements for platelet-neural cell communication with a focus on neurodegenerative diseases, and discuss the therapeutic potential of healthy platelets and the proteins which they release to counteract these conditions.

Role of platelets in neurodegenerative diseases: a universal pathophysiology

Platelets play an important role in a variety of disorders, namely, cardiovascular, psychosomatic, psychiatric, thrombosis, HIV/AIDS in addition to various neurodegenerative diseases (NDDs). Recent evidence indicates that platelet react to diverse stressors, thereby offering an interesting vantage point for understanding their potential role in contemporary medical research. This review addresses the possible role of platelets as a systemic probe in various NDDs, such as amyotrophic lateral sclerosis, Parkinson's disease, Huntington's disease, Alzheimer's disease, multiple sclerosis, etc. The current review based on published literature, describes a probable link between platelets and pathophysiology of various NDDs. It also discusses how platelets epitomize ultrastructural, morphological, biochemical and molecular changes, highlighting their emerging role as systemic tools in different NDDs.

Of mice, rats and men: Revisiting the quinolinic acid hypothesis of Huntington's disease

The neurodegenerative disease Huntington's disease (HD) is caused by an expanded polyglutamine (polyQ) tract in the protein huntingtin (htt). Although the gene encoding htt was identified and cloned more than 15 years ago, and in spite of impressive efforts to unravel the mechanism(s) by which mutant htt induces nerve cell death, these studies have so far not led to a good understanding of pathophysiology or an effective therapy. Set against a historical background, we review data supporting the idea that metabolites of the kynurenine pathway (KP) of tryptophan degradation provide a critical link between mutant htt and the pathophysiology of HD. New studies in HD brain and genetic model organisms suggest that the disease may in fact be causally related to early abnormalities in KP metabolism, favoring the formation of two neurotoxic metabolites, 3-hydroxykynurenine and quinolinic acid, over the related neuroprotective agent kynurenic acid. These findings not only link the excitotoxic hypothesis of HD pathology to an impairment of the KP but also define new drug targets and therefore have direct therapeutic implications. Thus, pharmacological normalization of the imbalance in brain KP metabolism may provide clinical benefits, which could be especially effective in early stages of the disease.

Platelet and plasma levels of glutamate and glutamine in migraine with and without aura

We evaluated plasma and platelet glutamate and glutamine levels in migraine with and without aura during headache-free periods and compared the results with those of normal controls. The plasma and platelet levels of glutamine in migraine with and without aura were normal. Migraine without aura patients had higher glutamate levels in plasma, and normal platelet levels. In migraine with aura patients, glutamate levels were high in platelets, but not in plasma. This suggests different profiles of excitatory amino acid metabolism in migraine with and without aura.

Huntington's disease: the neuroexcitotoxin aspartate is increased in platelets and decreased in plasma

The neural degeneration observed in the striata of patients with Huntington's disease (HD) can be reproduced by excitatory NMDA receptor agonists such as aspartate and glutamate in striatal cell cultures and in striata of vertebrates injected with these substances. Therefore, we decided to investigate the role of aspartate and glutamate in HD. Aspartate, glutamate, glutamine, and phenylalanine were measured in platelets and plasma of HD patients and age- and sex-matched healthy controls (C), using HPLC methods. In HD platelets the mean aspartate concentration was significantly (p < 0.01) increased (8.9 +/- 3.8 (SD) nmol/mg protein, n = 28) compared to C (4.6 +/- 1.4 (SD) nmol/mg protein, n = 24), whereas plasma aspartate was significantly (p < 0.01) decreased in HD (0.092 +/- 0.023 (SD) mg/dl, n = 16) versus C (0.179 +/- 0.109 (SD) mg/dl, n = 21). The increase in platelet aspartate should be a direct or indirect consequence of the dominant gene defect in HD. It might therefore be present in neurons as well, especially since platelets share many characteristics with neurons. Hence, chronically increased release of aspartate with consecutive overstimulation of postsynaptic neurons via NMDA receptors might be responsible for the damage observed in striatal target cells of corticostriatal glutamatergic and aspartatergic projection fibers in HD.

Platelet glycine, glutamate and aspartate in primary headache

Platelet levels of glutamic and aspartic acid and glycine were measured in patients with migraine with aura, migraine without aura, tension headache and cluster headache. High levels of these amino acids were found in patients with migraine with aura compared to normal subjects and other headache groups. During headache, glutamate levels further increased in migraine with aura patients. These findings may have relevance to the neurological symptoms of migraine with aura.

Evidence for normal fibroblast cell membranes from individuals with Huntington's disease. A fluorescence probe study

We used fluorescence spectroscopy in an attempt to identify differences in the fibroblast cell membranes from individuals with Huntington's disease (HD), compared to those from normal individuals. Eight pairs of age- and sex-matched fibroblast cultures were examined, each using four different fluorescent probes. To maximize the possibility of finding a difference between the normal and HD cells fluorescence probes were selected which localize in different regions of the membrane. Also, these probes differed in their sensitivity to the surrounding environment. Procedures were developed for labeling of the fibroblasts while they were still attached to glass coverslips. All fluorescence measurements were done on attached cells, which were all found to be viable by staining with trypan blue. Both low (5-7) and high (11-13) passage cultures were examined since earlier reports indicated that normal and HD fibroblasts could be distinguished at low passage using a fluorescence method. Using these 16 cultures, and multiple determinations of the fluorescence emission spectra, anisotropies, and lifetimes, we found no significant difference between normal and HD fibroblasts. If such a difference exists, it appears to be too small for use as a diagnostic indicator for Huntington's disease.