Neuroactive metabolites of the kynurenine pathway (KP) of tryptophan degradation have been closely linked to the pathogenesis of several neurodegenerative diseases. Tryptophan is an essential amino acid required for protein synthesis, and in higher eukaryotes is also converted into the key neurotransmitters serotonin and tryptamine. However, in mammals >95% of tryptophan is metabolized through the KP, ultimately leading to the production of nicotinamide adenosine dinucleotide (NAD + ). A number of the pathway metabolites are neuroactive; e.g. can modulate activity of several glutamate receptors and generate/scavenge free radicals. Imbalances in absolute and relative levels of KP metabolites have been strongly associated with neurodegenerative disorders including Huntington's, Alzheimer's, and Parkinson's diseases. The KP has also been implicated in the pathogenesis of other brain disorders (e.g. schizophrenia, bipolar disorder), as well as several cancers and autoimmune disorders such as HIV. Pharmacological and genetic manipulation of the KP has been shown to ameliorate neurodegenerative phenotypes in a number of model organisms, suggesting that it could prove to be a viable target for the treatment of such diseases. Here, we provide an overview of the KP, its role in neurodegeneration and the current strategies for therapeutic targeting of the pathway.

Due to their post-mitotic state, metabolic demands and often large polarised morphology, the function and survival of neurons is dependent on an efficient cellular waste clearance system both for generation of materials for metabolic processes and removal of toxic components. It is not surprising therefore that deficits in protein clearance can tip the balance between neuronal health and death. Here we discuss how autophagy and lysosome-mediated degradation pathways are disrupted in several neurological disorders. Both genetic and cell biological evidence show the diversity and complexity of vesicular clearance dysregulation in cells, and together may ultimately suggest a unified mechanism for neuronal demise in degenerative conditions. Causative and risk-associated mutations in Alzheimer’s disease, Frontotemporal Dementia, Amyotrophic Lateral Sclerosis, Parkinson’s disease, Huntington’s disease and others have given the field a unique mechanistic insight into protein clearance processes in neurons. Through their broad implication in neurodegenerative diseases, molecules involved in these genetic pathways, in particular those involved in autophagy, are emerging as appealing therapeutic targets for intervention in neurodegeneration.

BACKGROUND Kynurenine 3-monooxygenase (KMO) converts kynurenine to 3-hydroxykynurenine, and its inhibition shunts the kynurenine pathway - which is implicated as dysfunctional in various psychiatric disorders - towards enhanced synthesis of kynurenic acid (KYNA), an antagonist of both α7 nicotinic acetylcholine and NMDA receptors. Possibly as a result of reduced KMO activity, elevated central nervous system levels of KYNA have been found in patients with psychotic disorders, including schizophrenia (SZ). METHODS In the present study, we investigated adaptive – and possibly regulatory – changes in mice with a targeted deletion of Kmo (Kmo−/−) and characterized the KMO-deficient mice using six behavioral assays relevant for the study of SZ. RESULTS Genome-wide differential gene expression analyses in the cerebral cortex and cerebellum of these mice identified a network of SZ- and psychosis-related genes, with more pronounced alterations in cerebellar tissue. KYNA levels were also increased in these brain regions in Kmo−/− mice, with significantly higher levels in the cerebellum than in the cerebrum. Kmo−/− mice exhibited impairments in contextual memory and spent less time than controls interacting with an unfamiliar mouse in a social interaction paradigm. The mutant animals displayed increased anxiety-like behavior in the elevated plus maze and in a light-dark box. After a D-amphetamine challenge (5 mg/kg, i.p.), Kmo−/− mice showed potentiated horizontal activity in the open field paradigm. CONCLUSIONS Taken together, these results demonstrate that the elimination of Kmo in mice is associated with multiple gene and functional alterations that appear to duplicate aspects of the psychopathology of several neuropsychiatric disorders.

Huntington’s disease is a progressive neurodegenerative disorder caused by polyglutamine-expanded mutant huntingtin (mHTT). Here, we show that the deubiquitinase Usp12 rescues mHTT-mediated neurodegeneration in Huntington’s disease rodent and patient-derived human neurons, and in Drosophila. The neuroprotective role of Usp12 may be specific amongst related deubiquitinases, as the closely related homolog Usp46 does not suppress mHTT-mediated toxicity. Mechanistically, we identify Usp12 as a potent inducer of neuronal autophagy. Usp12 overexpression accelerates autophagic flux and induces an approximately sixfold increase in autophagic structures as determined by ultrastructural analyses, while suppression of endogenous Usp12 slows autophagy. Surprisingly, the catalytic activity of Usp12 is not required to protect against neurodegeneration or induce autophagy. These findings identify the deubiquitinase Usp12 as a regulator of neuronal proteostasis and mHTT-mediated neurodegeneration.

A moderate increase in P CO 2 (55 mmHg) closes Cx26 gap junctions. r This effect of CO 2 is independent of changes in intra-or extracellular pH. r The CO 2-dependent closing effect depends on the same residues (K125 and R104) that are required for the CO 2-dependent opening of Cx26 hemichannels. r Pathological mutations of Cx26 abolish the CO 2-dependent closing of the gap junction. r Elastic network modelling suggests that the effect of CO 2 on Cx26 hemichannels and gap junctions is mediated through changes in the lowest entropy state of the protein.

Mutations in the gene encoding DJ-1 are associated with autosomal recessive forms of Parkinson's disease (PD). DJ-1 plays a role in protection from oxidative stress, but how it functions as an Bupstream^oxidative stress sensor and whether this relates to PD is still unclear. Intriguingly, DJ-1 may act as an RNA binding protein associating with specific mRNA transcripts in the human brain. Moreover, we previously reported that the yeast DJ-1 homolog Hsp31 localizes to stress granules (SGs) after glucose starvation, suggesting a role for DJ-1 in RNA dynamics. Here, we report that DJ-1 interacts with several SG components in mammalian cells and localizes to SGs, as well as P-bodies, upon induction of either osmotic or oxidative stress. By purifying the mRNA associated with DJ-1 in mammalian cells, we detected several transcripts and found that subpopulations of these localize to SGs after stress, suggesting that DJ-1 may target specific mRNAs to mRNP granules. Notably, we find that DJ-1 associates with SGs arising from N-methyl-D-aspartate (NMDA) excitotoxicity in primary neurons and parkinsonism-inducing toxins in dopaminergic cell cultures. Thus, our results indicate that DJ-1 is associated with cytoplasmic RNA granules arising during stress and neurodegeneration, providing a possible link between DJ-1 and RNA dynamics which may be relevant for PD pathogenesis.

The enzyme kynurenine 3-monooxygenase (KMO) operates at a critical branch-point in the kynurenine pathway (KP), the major route of tryptophan metabolism. As the KP has been implicated in the pathogenesis of several human diseases, KMO and other enzymes that control metabolic flux through the pathway are potential therapeutic targets for these disorders. While KMO is localized to the outer mitochondrial membrane in eukaryotic organisms, no mitochondrial role for KMO has been described. In this study, KMO deficient Drosophila melanogaster were investigated for mitochondrial phenotypes in vitro and in vivo. We find that a loss of function allele or RNAi knockdown of the Drosophila KMO ortholog (cinnabar) causes a range of morphological and functional alterations to mitochondria, which are independent of changes to levels of KP metabolites. Notably, cinnabar genetically interacts with the Parkinson’s disease associated genes Pink1 and parkin, as well as the mitochondrial fission gene Drp1, implicating KMO in mitochondrial dynamics and mitophagy, mechanisms which govern the maintenance of a healthy mitochondrial network. Overexpression of human KMO in mammalian cells finds that KMO plays a role in the post-translational regulation of DRP1. These findings reveal a novel mitochondrial role for KMO, independent from its enzymatic role in the kynurenine pathway.

Cx26 hemichannels are opened by modest levels of CO2 (PCO2 55 mmHg) acting via a novel carbamylation mechanism to bias the hemichannel to the open state. In the hemichannel, CO2 nonenzymatically carbamylates Lys125, and the resulting carbamate forms a salt bridge to Arg104 of the neighbouring subunit. By contrast, similar modest levels of CO2 cause Cx26 gap junction closure.Gap junctions of Cx31, a beta connexin that lacks the carbamylation motif, are insensitive to these levels of CO2. Mutations of the carbamylation motif, Lys125Arg and Arg104Ala, which abrogate hemichannel opening in Cx26, also prevent gap junction closing. Elastic network modelling suggests that the lowest entropy state, when CO2 is bound, is in the open configuration for the hemichannel and the closed configuration for the gap junction Surprisingly, we conclude that the opposing actions of CO2 on Cx26 gap junctions and hemichannels results from carbamylation of the same residues.