Developmental retardation, microcephaly, and peptiduria in mice without aminopeptidase P1

Suppression of exaggerated NMDAR activity by memantine treatment ameliorates neurological and behavioral deficits in aminopeptidase P1-deficient mice

Inborn errors of metabolism (IEMs) are common causes of neurodevelopmental disorders, including microcephaly, hyperactivity, and intellectual disability. However, the synaptic mechanisms of and pharmacological interventions for the neurological complications of most IEMs are unclear. Here, we report that metabolic dysfunction perturbs neuronal NMDA receptor (NMDAR) homeostasis and that the restoration of NMDAR signaling ameliorates neurodevelopmental and cognitive deficits in IEM model mice that lack aminopeptidase P1. Aminopeptidase P1-deficient (Xpnpep1^–/–) mice, with a disruption of the proline-specific metalloprotease gene Xpnpep1, exhibit hippocampal neurodegeneration, behavioral hyperactivity, and impaired hippocampus-dependent learning. In this study, we found that GluN1 and GluN2A expression, NMDAR activity, and the NMDAR-dependent long-term potentiation (LTP) of excitatory synaptic transmission were markedly enhanced in the hippocampi of Xpnpep1^–/– mice. The exaggerated NMDAR activity and NMDAR-dependent LTP were reversed by the NMDAR antagonist memantine. A single administration of memantine reversed hyperactivity in adult Xpnpep1^–/– mice without improving learning and memory. Furthermore, chronic administration of memantine ameliorated hippocampal neurodegeneration, hyperactivity, and impaired learning and memory in Xpnpep1^–/– mice. In addition, abnormally enhanced NMDAR-dependent LTP and NMDAR downstream signaling in the hippocampi of Xpnpep1^–/– mice were reversed by chronic memantine treatment. These results suggest that the metabolic dysfunction caused by aminopeptidase P1 deficiency leads to synaptic dysfunction with excessive NMDAR activity, and the restoration of synaptic function may be a potential therapeutic strategy for the treatment of neurological complications related to IEMs.

Addressing neurological symptoms may offer new treatments for inborn errors of metabolism (IEMs) affecting neurodevelopment. In such IEMs, mutation of an enzyme disrupts a metabolic pathway, causing buildup or lack of key molecules, with symptoms including hyperactivity, developmental delay, and intellectual disability. Because the detailed pathological mechanisms of most IEMs are unknown, there are no treatments for resulting neurological issues. Myoung-Hwan Kim at Seoul National University and co-workers investigated whether they could treat the neurological symptoms of the IEM, aminopeptidase P1 (APP1) deficiency. They found that APP1 deficiency in mice caused an increase in the neural receptor NMDAR. Suppressing NMDAR reduced both neurological and behavioral symptoms. These findings suggest potential treatments for APP1 deficiency, and indicate that neurodevelopmental disorders in IEMs may be treated by repairing the neural circuitry instead of the root metabolic cause.

Modulation of autoimmune diabetes by N-ethyl-N-nitrosourea- induced mutations in non-obese diabetic mice

Genetic association studies of type 1 diabetes (T1D) in humans, and in congenic non-obese diabetic (NOD) mice harboring DNA segments from T1D-resistant mice, face the challenge of assigning causation to specific gene variants among many within loci that affect disease risk. Here, we created random germline mutations in NOD/NckH mice and used automated meiotic mapping to identify mutations modifying T1D incidence and age of onset. In contrast with association studies in humans or congenic NOD mice, we analyzed a relatively small number of genetic changes in each pedigree, permitting implication of specific mutations as causative. Among 844 mice from 14 pedigrees bearing 594 coding/splicing changes, we identified seven mutations that accelerated T1D development, and five that delayed or suppressed T1D. Eleven mutations affected genes not previously known to influence T1D (Xpnpep1, Herc1, Srrm2, Rapgef1, Ppl, Zfp583, Aldh1l1, Col6a1, Ccdc13, Cd200r1, Atrnl1). A suppressor mutation in Coro1a validated the screen. Mutagenesis coupled with automated meiotic mapping can detect genes in which allelic variation influences T1D susceptibility in NOD mice. Variation of some of the orthologous/paralogous genes may influence T1D susceptibility in humans.

Proline-specific aminopeptidase P prevents replication-associated genome instability

Genotoxic stress during DNA replication constitutes a serious threat to genome integrity and causes human diseases. Defects at different steps of DNA metabolism are known to induce replication stress, but the contribution of other aspects of cellular metabolism is less understood. We show that aminopeptidase P (APP1), a metalloprotease involved in the catabolism of peptides containing proline residues near their N-terminus, prevents replication-associated genome instability. Functional analysis of C. elegans mutants lacking APP-1 demonstrates that germ cells display replication defects including reduced proliferation, cell cycle arrest, and accumulation of mitotic DSBs. Despite these defects, app-1 mutants are competent in repairing DSBs induced by gamma irradiation, as well as SPO-11-dependent DSBs that initiate meiotic recombination. Moreover, in the absence of SPO-11, spontaneous DSBs arising in app-1 mutants are repaired as inter-homologue crossover events during meiosis, confirming that APP-1 is not required for homologous recombination. Thus, APP-1 prevents replication stress without having an apparent role in DSB repair. Depletion of APP1 (XPNPEP1) also causes DSB accumulation in mitotically-proliferating human cells, suggesting that APP1’s role in genome stability is evolutionarily conserved. Our findings uncover an unexpected role for APP1 in genome stability, suggesting functional connections between aminopeptidase-mediated protein catabolism and DNA replication.

The accurate duplication of DNA that occurs before cells divide is an essential aspect of the cell cycle that is also crucial for the correct development of multicellular organisms. Mutations that compromise the normal function of the DNA replication machinery can lead to the accumulation of replication-related DNA damage, a known cause of human disease and a common feature of cancer and precancerous cells. Therefore, identifying factors that prevent replication-related DNA damage is highly relevant for human health. In this manuscript, we identify aminopeptidase P, an enzyme involved in the breakdown of proteins containing the amino acid Proline at their N-terminus, as a novel factor that prevents replication-related DNA damage. Analysis of C. elegans nematodes lacking aminopeptidase P reveals that this protein is required for normal fertility and development, and that in its absence proliferating germ cells display DNA replication defects, including cell cycle arrest and accumulation of extensive DNA damage. We also show that removal of aminopeptidase P induces DNA damage in proliferating human cells, suggesting that its role in preventing replication defects is evolutionarily conserved. These findings uncover functional connections between aminopeptidase-mediated protein degradation and DNA replication.

Altered hippocampal gene expression, glial cell population, and neuronal excitability in aminopeptidase P1 deficiency

Inborn errors of metabolism are often associated with neurodevelopmental disorders and brain injury. A deficiency of aminopeptidase P1, a proline-specific endopeptidase encoded by the Xpnpep1 gene, causes neurological complications in both humans and mice. In addition, aminopeptidase P1-deficient mice exhibit hippocampal neurodegeneration and impaired hippocampus-dependent learning and memory. However, the molecular and cellular changes associated with hippocampal pathology in aminopeptidase P1 deficiency are unclear. We show here that a deficiency of aminopeptidase P1 modifies the glial population and neuronal excitability in the hippocampus. Microarray and real-time quantitative reverse transcription-polymerase chain reaction analyses identified 14 differentially expressed genes (Casp1, Ccnd1, Myoc, Opalin, Aldh1a2, Aspa, Spp1, Gstm6, Serpinb1a, Pdlim1, Dsp, Tnfaip6, Slc6a20a, Slc22a2) in the Xpnpep1^−/− hippocampus. In the hippocampus, aminopeptidase P1-expression signals were mainly detected in neurons. However, deficiency of aminopeptidase P1 resulted in fewer hippocampal astrocytes and increased density of microglia in the hippocampal CA3 area. In addition, Xpnpep1^−/− CA3b pyramidal neurons were more excitable than wild-type neurons. These results indicate that insufficient astrocytic neuroprotection and enhanced neuronal excitability may underlie neurodegeneration and hippocampal dysfunction in aminopeptidase P1 deficiency.

Deficiency of aminopeptidase P1 causes behavioral hyperactivity, cognitive deficits, and hippocampal neurodegeneration

Metabolic diseases affect various organs including the brain. Accumulation or depletion of substrates frequently leads to brain injury and dysfunction. Deficiency of aminopeptidase P1, a cytosolic proline-specific peptidase encoded by the Xpnpep1 gene, causes an inborn error of metabolism (IEM) characterized by peptiduria in humans. We previously reported that knockout of aminopeptidase P1 in mice causes neurodevelopmental disorders and peptiduria. However, little is known about the pathophysiological role of aminopeptidase P1 in the brain. Here, we show that loss of aminopeptidase P1 causes behavioral and neurological deficits in mice. Mice deficient in aminopeptidase P1 (Xpnpep1^-/- ) display abnormally enhanced locomotor activities in both the home cage and open-field box. The aminopeptidase P1 deficiency in mice also resulted in severe impairments in novel-object recognition, the Morris water maze task, and contextual, but not cued, fear memory. These behavioral dysfunctions were accompanied by epileptiform electroencephalogram activity and neurodegeneration in the hippocampus. However, mice with a heterozygous mutation for aminopeptidase P1 (Xpnpep1^+/- ) exhibited normal behaviors and brain structure. These results suggest that loss of aminopeptidase P1 leads to behavioral, cognitive and neurological deficits. This study may provide insight into new pathogenic mechanisms for brain dysfunction related to IEMs.

Evidence for genetic heterogeneity between clinical subtypes of bipolar disorder

We performed a genome-wide association study of 6447 bipolar disorder (BD) cases and 12 639 controls from the International Cohort Collection for Bipolar Disorder (ICCBD). Meta-analysis was performed with prior results from the Psychiatric Genomics Consortium Bipolar Disorder Working Group for a combined sample of 13 902 cases and 19 279 controls. We identified eight genome-wide significant, associated regions, including a novel associated region on chromosome 10 (rs10884920; P=3.28 × 10-8) that includes the brain-enriched cytoskeleton protein adducin 3 (ADD3), a non-coding RNA, and a neuropeptide-specific aminopeptidase P (XPNPEP1). Our large sample size allowed us to test the heritability and genetic correlation of BD subtypes and investigate their genetic overlap with schizophrenia and major depressive disorder. We found a significant difference in heritability of the two most common forms of BD (BD I SNP-h2=0.35; BD II SNP-h2=0.25; P=0.02). The genetic correlation between BD I and BD II was 0.78, whereas the genetic correlation was 0.97 when BD cohorts containing both types were compared. In addition, we demonstrated a significantly greater load of polygenic risk alleles for schizophrenia and BD in patients with BD I compared with patients with BD II, and a greater load of schizophrenia risk alleles in patients with the bipolar type of schizoaffective disorder compared with patients with either BD I or BD II. These results point to a partial difference in the genetic architecture of BD subtypes as currently defined.

Crystal structure of X-prolyl aminopeptidase from Caenorhabditis elegans: A cytosolic enzyme with a di-nuclear active site

Eukaryotic aminopeptidase P1 (APP1), also known as X‐prolyl aminopeptidase (XPNPEP1) in human tissues, is a cytosolic exopeptidase that preferentially removes amino acids from the N‐terminus of peptides possessing a penultimate N‐terminal proline residue. The enzyme has an important role in the catabolism of proline containing peptides since peptide bonds adjacent to the imino acid proline are resistant to cleavage by most peptidases. We show that recombinant and catalytically activeCaenorhabditis elegans APP‐1 is a dimer that uses dinuclear zinc at the active site and, for the first time, we provide structural information for a eukaryotic APP‐1 in complex with the inhibitor, apstatin. Our analysis reveals thatC. elegans APP‐1 shares similar mode of substrate binding and a common catalytic mechanism with other known X‐prolyl aminopeptidases.

We present the crystal structure ofC. elegans APP‐1 both in bound and unbound forms.

We showC. elegans APP‐1 uses dinuclear zinc at the active site.

We confirm thatC. elegans APP‐1 is biological dimer.

Our analysis reveals thatC. elegans APP‐1 shares a common catalytic mechanism with other X‐prolyl aminopeptidases.