Astrocytic localization of kynurenine aminotransferase II in the rat brain visualized by immunocytochemistry

Abstract

Kynurenic acid (KYNA), a metabolite of the kynurenine pathway of tryptophan degradation, is a neuroinhibitory agent present in the mammalian brain. Endogenous KYNA preferentially affects the alpha7 nicotinic acetylcholine (alpha7nACh) receptor and, possibly, the glycine co-agonist (glycineB) site of the NMDA receptor. Functionally relevant fluctuations in brain KYNA occur under both physiological and pathological conditions, affecting cholinergic and glutamatergic neurotransmission. Kynurenine aminotransferase II (KAT II), the major biosynthetic enzyme of KYNA in the rat brain, catalyzes the irreversible formation of KYNA from its immediate bioprecursor, kynurenine. We now purified rat kidney KAT II to homogeneity, generated a polyclonal rabbit anti-rat KAT II antibody, and purified the antibody using routine biochemical methods. The antibody selectively recognized KAT II by Western blot analysis and in immunotitration experiments. Used for immunocytochemistry, the antibody revealed discrete, specific staining of KAT II-positive astrocyte-like cells throughout the adult rat brain. The presence of KAT II in astrocytes was confirmed by double fluorescence immunostaining with an antibody against the astrocyte-specific marker glial fibrillary acidic protein (GFAP). No specific labeling was detected in neurons or microglia. However, KAT II-positive astrocytes were intimately associated with select neuron populations, supporting a neuromodulatory role of KYNA. Intense staining was frequently seen around brain capillaries, with astrocytic end feet contacting the capillary wall. This may explain the rapid access of blood-derived kynurenine to KAT II-containing astrocytes. The new anti-KAT II antibody should be useful in the further elucidation of the presumed role of KYNA in brain physiology and pathology.

Fluctuations in Endogenous Kynurenic Acid Control Hippocampal Glutamate and Memory

“…Although several of these enzymes may participate in cerebral KYNA biosynthesis under physiological and pathological conditions, it appears that the pool of KYNA that can be most readily mobilized in the brain is largely provided by KAT-II (Amori et al, 2009). This enzyme is almost exclusively localized in astrocytes (Guidetti et al, 2007b;Guillemin et al, 2001), which rapidly liberate newly synthesized KYNA into the extracellular milieu (Curatolo et al, 1996;Guillemin et al, 2000). Although the precise mechanism controlling the release of KYNA has not been elucidated, this insight led to the development of specific KAT-II inhibitors, which are designed to target astrocytes and reduce extracellular KYNA concentrations.…”

“…This process can be catalyzed by several distinct kynurenine aminotransferases (KATs), of which KAT-II is functionally best characterized in the brain (Guidetti et al, 2007a;Han et al, 2010). Thus, KAT-II is preferentially localized in astrocytes (Guidetti et al, 2007b), which readily release newly synthesized KYNA into the extracellular milieu for possible inhibition of a7nAChRs and NMDARs (Schwarcz and Pellicciari, 2002).…”

The Tryptophan Metabolite 3-Hydroxyanthranilic Acid Plays Anti-Inflammatory and Neuroprotective Roles During Inflammation

“…Data were expressed as percentage of inhibition by 3-HAA calculated as follows: 100 ϫ (1 Ϫ cytokine produced in the presence of 3-HAA/cytokine produced by stimulus alone). The compiled results from multiple [5][6][7] independent experiments are shown in Figure 2, B and C. A single-sample t-test was performed to determine whether the inhibition by 3-HAA was significant.…”

“…For example, kynurenine monooxygenase (KMO) is expressed in macrophages and microglia, [2][3][4] whereas kynurenine aminotransferase II (KAT II) is present in astrocytes. 5 A well-appreciated biological activity of IDO is T-cell suppression. IDO expressed in antigen-presenting cells (dendritic cells, macrophages, and microglia) can suppress T-cell immunity against viruses, tumors, or transplanted tissues by suppressing T-cell proliferation.…”

Altered Microbiota Composition Mediates Depressive Behavior During Chronic Stress

“…The two glial cell types reportedly process kynurenine differently. Astrocytes express KAT2 and metabolize kynurenine to kynurenic acid, whereas microglia express KMO and have quinolinic acid as its product 167,184,191,192 . Both molecules have neuromodulatory effects, as they can both alter glutamatergic transmission: kynurenic acid is considered an NMDA receptor antagonist, whereas quinolinic acid is considered an agonist 167,184,193 .…”