Abstract
Kynurenine 3-monooxygenase (KMO), a key player in the kynurenine pathway (KP) of tryptophan degradation, regulates the synthesis of the neuroactive metabolites 3-hydroxykynurenine (3-HK) and kynurenic acid (KYNA). KMO activity has been implicated in several major brain diseases including Huntington’s disease (HD) and schizophrenia. In the brain, KMO is widely believed to be predominantly localized in microglial cells, but verification in vivo has not been provided so far. Here, we examined KP metabolism in the brain after depleting microglial cells pharmacologically with the colony stimulating factor 1 receptor inhibitor PLX5622. Young adult mice were fed PLX5622 for 21 days and were euthanized either on the next day or after receiving normal chow for an additional 21 days. Expression of microglial marker genes was dramatically reduced on day 22 but had fully recovered by day 43. In both groups, PLX5622 treatment failed to affect Kmo expression, KMO activity or tissue levels of 3-HK and KYNA in the brain. In a parallel experiment, PLX5622 treatment also did not reduce KMO activity, 3-HK and KYNA in the brain of R6/2 mice (a model of HD with activated microglia). Finally, using freshly isolated mouse cells ex vivo, we found KMO only in microglia and neurons but not in astrocytes. Taken together, these data unexpectedly revealed that neurons contain a large proportion of functional KMO in the adult mouse brain under both physiological and pathological conditions.
Highlights
As a number of kynurenine pathway (KP) metabolites are increasingly perceived to have considerable significance in normal brain function, impaired Kynurenine 3-monooxygenase (KMO) activity may play a substantive role in the pathophysiology of several neurological and psychiatric diseases [9,10,11,12]
Because fluctuations in KMO activity play a critical role in the formation and function of the prominent neuromodulator kynurenic acid (KYNA), which is mainly synthesized in astrocytes (Figure 1) [30,31,32,33,34,35], we determined both enzyme activity and the brain tissue levels of 3-HK and KYNA in these studies. We examined both Kmo expression and KMO activity in microglial cells, neurons and astrocytes that were acutely purified from healthy mouse forebrain using magnetic beads [36] and a neuron isolation kit [37]
Real time PCR analysis of the microglial marker genes Aif1 (−89%), Csf1r (−97%), Cx3cr1 (−97%), Siglech (−97%) and Tmem119 (−90%) confirmed that daily administration of PLX5622 for 21 days caused the massive depletion of microglial cells in the forebrain of normal mice
Summary
As a number of KP metabolites are increasingly perceived to have considerable significance in normal brain function (see [8], for review), impaired KMO activity may play a substantive role in the pathophysiology of several neurological and psychiatric diseases [9,10,11,12]. For both conceptual and translationally relevant reasons, it is essential to have a clear understanding of the cellular localization of KMO in the brain in health and disease
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