A Missense Mutation in Kynurenine Aminotransferase-1 in Spontaneously Hypertensive Rats

Abstract

Spontaneously hypertensive rats (SHR) are the most extensively used animal model for genetic hypertension, increased stroke damage, and insulin resistance syndromes; however, the identification of target genes has proved difficult. SHR show elevated sympathetic nerve activity, and stimulation of the central blood pressure control centers with glutamate or nicotine results in exaggerated blood pressure responses, effects that appear to be genetically determined. Kynurenic acid, a competitive glutamate antagonist and a non-competitive nicotinic antagonist, can be synthesized in the brain by the enzyme kynurenine aminotransferase-1 (KAT-1). We have previously shown that KAT-1 activity is significantly reduced in SHR compared with normotensive Wistar Kyoto rats (WKY). Here we show that KAT-1 contains a missense mutation, E61G, in all the strains of SHR examined but not in any of the WKY or outbred strains. Previous studies on F2 rats from a cross of stroke-prone SHR and WKY have shown a suggestive level of linkage between elevated blood pressure and the KAT-1 locus on chromosome 3. In addition, the mutant enzyme expressed in Escherichia coli displays altered kinetics. This mutation may explain the enhanced sensitivity to glutamate and nicotine seen in SHR that may be related to an underlying mechanism of hypertension and increased sensitivity to stroke. The spontaneously hypertensive rat (SHR)1 displays essential hypertension as well as insulin resistance and as such has become the most widely used model for the study of genetic hypertension and related disorders commonly coexisting in human patients. The complexity of these syndromes and the known heterogeneity of the SHR genotype have, however, made the identification of target genes difficult (1-3).Increased sympathetic outflow is a key factor contributing to some types of hypertension in humans and in many animal models of hypertension (4). The major sympathetic output pathway for the tonic and reflex control of blood pressure, which uses glutamate as the transmitter, arises from the rostral ventrolateral medulla (RVLM) (5). In addition the RVLM receives numerous cholinergic and glutamatergic inputs, making this pathway especially sensitive to altered receptor activity (4, 5). Enhanced excitation of the RVLM has been implicated in human and in animal models of hypertension (4).Recent evidence suggests that alterations of the levels of the endogenous antagonist kynurenic acid, which shows highest affinity for the "glycine site" of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors (6), can affect glutamate receptor function (7). In SHR, microinjection of glutamate or NMDA into the caudal ventrolateral medulla results in exaggerated depressor responses; however, the responses to the "NMDA glycine site" agonist D-serine are diminished (8). Kynurenic acid injection into the RVLM reduces basal blood pressure in SHR but not in normotensive WKY (9). This suggests that there is enhanced sensitivity to glutamate in SHR, possibly due to reduced levels of the e...

Biochemical and Structural Properties of Mouse Kynurenine Aminotransferase III

“…By genetic analysis, a missense mutation (E61G) in the KAT I gene was identified in spontaneously hypertensive rats and shown to lead to reduced KAT I on May 12, 2018 by guest http://mcb.asm.org/ enzymatic activity and higher blood pressure (29,34). Our previous study dealing with structural characterization of hKAT I revealed that the evolutionarily conserved Glu27 residue (i.e., Glu61 in the full-length sequence of KAT I), positioned at the C terminus of the H1 helix, is a pivotal residue in fixing the helix in the observed conformations.…”

“…In addition to its role as an excitatory amino acid antagonist, KYNA is also involved in the control of cardiovascular function by acting at the rostral ventrolateral medulla of the central nervous system (8). Spontaneously hypertensive rats, the most widely used animal model for studying genetic hypertension, have abnormally low KYNA levels in the area of the central nervous system that tunes physiological blood pressure (29,34).…”

Crystal Structure of Human Kynurenine Aminotransferase I

“…This defect has been associated with abnormally low KA levels in the central nervous system-specific districts tuning physiological blood pressure (49,50). Sequencing analysis revealed the presence of a single point mutation in the KAT-I gene in all of the examined spontaneously hypertensive rats, leading to the substitution of a glutamate by a glycine at position 61 in the enzyme (51). Remarkably, the resulting mutated protein showed a severely reduced enzymatic activity, hinting at the key role of position 61 in catalysis (51).…”

Podocyte Hypertrophy, “Adaptation,” and “Decompensation” Associated with Glomerular Enlargement and Glomerulosclerosis in the Aging Rat

“…Within the renal cortex and medullary DNA arrays, we identified marker genes that were highly expressed in either renal cortex or medulla but were expressed by at least a 10-fold lower amount in glomeruli and were unaffected by diet and age. We used kynurenine aminotransferase as a marker of proximal tubules (34). We used claudin 16, an epithelial tight junction protein present in the thick ascending limb of Henle but not in glomerulus (35), as a medullary marker.…”