Transcriptional response to mitochondrial protease IMMP2L knockdown in human primary astrocytes

Inner Mitochondrial Membrane Peptidase 2-Like Deletion Aggravates Mitochondrial Apoptosis and Inhibits Autophagy After Hyperglycemia Stroke

This study investigated the effects of inner mitochondrial membrane peptidase 2-like (Immp2l) deletion on mitochondrial apoptosis and mitochondrial autophagy under hyperglycemic conditions. The middle cerebral artery occlusion (MCAO) model was established in wild-type (WT) mice and Immp2l+/- mice; animals were then exposed to hyperglycemic (induced using 1% streptozotocin) and normoglycemic conditions. Tissues were collected at various time points post-reperfusion. The production of reactive oxygen species (ROS) was assessed by fluorescent measurements, and mitochondrial membrane potential was evaluated using a JC-1 assay kit. Autophagy was analyzed by measuring LC3II/LC3I protein expression and Beclin 1 expression. Mitochondrial ultrastructure was examined through transmission electron microscopy (TEM); neuronal autophagosomes were also assessed. Immp2l mutation in a hyperglycemic environment exacerbated brain injury by increasing ROS production, compromising mitochondrial membrane potential, inducing apoptotic cascades, and impairing mitochondrial autophagy. These findings highlight the critical role of Immp2l in modulating the response to hyperglycemic cerebral ischemia-reperfusion (I/R) injury. Furthermore, the deficiency of Immp2l appears to contribute to increased oxidative stress, mitochondrial dysfunction, and cell death, thereby exacerbating brain injury. These data may provide new insights into therapeutic strategies for reducing the impact of diabetes on stroke outcomes.

Immp2l knockdown in male mice increases stimulus-driven instrumental behaviour but does not alter goal-directed learning or neuron density in cortico-striatal circuits in a model of Tourette syndrome and autism spectrum disorder

Cortico-striatal neurocircuits mediate goal-directed and habitual actions which are necessary for adaptive behaviour. It has recently been proposed that some of the core symptoms of autism spectrum disorder (ASD) and Gilles de la Tourette syndrome (GTS), such as tics and other repetitive behaviours, may emerge because of imbalances in these neurocircuits. We have recently developed a model of ASD and GTS by knocking down Immp2l, a mitochondrial gene frequently associated with these disorders. The current study sought to determine whether Immp2l knockdown (KD) in male mice alters flexible, goal- or cue- driven behaviour using procedures specifically designed to examine response-outcome and stimulus-response associations, which underlie goal-directed and habitual behaviour, respectively. Whether Immp2l KD alters neuron density in cortico-striatal neurocircuits known to regulate these behaviours was also examined. Immp2l KD mice and wild type-like mice (WT) were trained on Pavlovian and instrumental learning procedures where auditory cues predicted food delivery and lever-press responses earned a food outcome. It was demonstrated that goal-directed learning was not changed for Immp2l KD mice compared to WT mice, as lever-press responses were sensitive to changes in the value of the food outcome, and to contingency reversal and degradation. There was also no difference in the capacity of KD mice to form habitual behaviours compared to WT mice following extending training of the instrumental action. However, Immp2l KD mice were more responsive to auditory stimuli paired with food as indicated by a non-specific increase in lever response rates during Pavlovian-to-instrumental transfer. Finally, there were no alterations to neuron density in striatum or any prefrontal cortex or limbic brain structures examined. Thus, the current study suggests that Immp2l is not necessary for learned maladaptive goal or stimulus driven behaviours in ASD or GTS, but that it may contribute to increased capacity for external stimuli to drive behaviour. Alterations to stimulus-driven behaviour could potentially influence the expression of tics and repetitive behaviours, suggesting that genetic alterations to Immp2l may contribute to these core symptoms in ASD and GTS. Given that this is the first application of this battery of instrumental learning procedures to a mouse model of ASD or GTS, it is an important initial step in determining the contribution of known risk-genes to goal-directed versus habitual behaviours, which should be more broadly applied to other rodent models of ASD and GTS in the future.

Exonic deletions in IMMP2L in schizophrenia with enhanced glycation stress subtype

We previously identified a subtype of schizophrenia (SCZ) characterized by increased plasma pentosidine, a marker of glycation and oxidative stress (PEN-SCZ). However, the genetic factors associated with PEN-SCZ have not been fully clarified. We performed a genome-wide copy number variation (CNV) analysis to identify CNVs associated with PEN-SCZ to provide an insight into the novel therapeutic targets for PEN-SCZ. Plasma pentosidine was measured by high-performance liquid chromatography in 185 patients with SCZ harboring rare CNVs detected by array comparative genomic hybridization. In three patients with PEN-SCZ showing additional autistic features, we detected a novel deletion at 7q31.1 within exons 2 and 3 of IMMP2L, which encodes the inner mitochondrial membrane peptidase subunit 2. The deletion was neither observed in non-PEN-SCZ nor in public database of control subjects. IMMP2L is one of the SCZ risk loci genes identified in a previous SCZ genome-wide association study, and its trans-populational association was recently described. Interestingly, deletions in IMMP2L have been previously linked with autism spectrum disorder. Disrupted IMMP2L function has been shown to cause glycation/oxidative stress in neuronal cells in an age-dependent manner. To our knowledge, this is the first genome-wide CNV study to suggest the involvement of IMMP2L exons 2 and 3 in the etiology of PEN-SCZ. The combination of genomic information with plasma pentosidine levels may contribute to the classification of biological SCZ subtypes that show additional autistic features. Modifying IMMP2L functions may be useful for treating PEN-SCZ if the underlying biological mechanism can be clarified in further studies.

Mitochondrial Processing Peptidases-Structure, Function and the Role in Human Diseases

Mitochondrial proteins are encoded by both nuclear and mitochondrial DNA. While some of the essential subunits of the oxidative phosphorylation (OXPHOS) complexes responsible for cellular ATP production are synthesized directly in the mitochondria, most mitochondrial proteins are first translated in the cytosol and then imported into the organelle using a sophisticated transport system. These proteins are directed mainly by targeting presequences at their N-termini. These presequences need to be cleaved to allow the proper folding and assembly of the pre-proteins into functional protein complexes. In the mitochondria, the presequences are removed by several processing peptidases, including the mitochondrial processing peptidase (MPP), the inner membrane processing peptidase (IMP), the inter-membrane processing peptidase (MIP), and the mitochondrial rhomboid protease (Pcp1/PARL). Their proper functioning is essential for mitochondrial homeostasis as the disruption of any of them is lethal in yeast and severely impacts the lifespan and survival in humans. In this review, we focus on characterizing the structure, function, and substrate specificities of mitochondrial processing peptidases, as well as the connection of their malfunctions to severe human diseases.

Differential DNA Methylation of the IMMP2L Gene in Families with Maternally Inherited 7q31.1 Microdeletions is Associated with Intellectual Disability and Developmental Delay

Most copy number variations (CNVs) in the human genome display incomplete penetrance with unknown underlying mechanisms. One such mechanism may be epigenetic modification, particularly DNA methylation. The IMMP2L gene is located in a critical region for autism susceptibility on chromosome 7q (AUTS1). The level of DNA methylation was assessed by bisulfite sequencing of 87 CpG sites in the IMMP2L gene in 3 families with maternally inherited 7q31.1 microdeletions affecting the IMMP2L gene alone. Bisulfite sequencing revealed comparable levels of DNA methylation in the probands, healthy siblings without microdeletions, and their fathers. In contrast, a reduced DNA methylation index and increased IMMP2L expression were observed in lymphocytes from the healthy mothers compared with the probands. A number of genes were upregulated in the healthy mothers compared to controls and downregulated in probands compared to mothers. These genes were enriched in components of the ribosome and electron transport chain, as well as oxidative phosphorylation and various degenerative conditions. Differential expression in probands and mothers with IMMP2L deletions relative to controls may be due to compensatory processes in healthy mothers with IMMP2L deletions and disturbances of these processes in probands with intellectual disability. The results suggest a possible partial compensation for IMMP2L gene haploinsufficiency in healthy mothers with the 7q31.1 microdeletion by reducing the DNA methylation level. Differential DNA methylation of intragenic CpG sites may affect the phenotypic manifestation of CNVs and explain the incomplete penetrance of chromosomal microdeletions.

Mitochondrial retrograde signalling in neurological disease

Neuronal mitochondrial dysfunction causes primary mitochondrial diseases and likely contributes to neurodegenerative diseases including Parkinson's and Alzheimer's disease. Mitochondrial dysfunction has also been documented in neurodevelopmental disorders such as tuberous sclerosis complex and autism spectrum disorder. Only symptomatic treatments exist for neurodevelopmental disorders, while neurodegenerative diseases are largely untreatable. Altered mitochondrial function activates mitochondrial retrograde signalling pathways, which enable signalling to the nucleus to reprogramme nuclear gene expression. In this review, we discuss the role of mitochondrial retrograde signalling in neurological diseases. We summarize how mitochondrial dysfunction contributes to neurodegenerative disease and neurodevelopmental disorders. Mitochondrial signalling mechanisms that have relevance to neurological disease are discussed. We then describe studies documenting retrograde signalling pathways in neurons and glia, and in animal models of neuronal mitochondrial dysfunction and neurological disease. Finally, we suggest how specific retrograde signalling pathways can be targeted to develop novel treatments for neurological diseases. This article is part of the theme issue 'Retrograde signalling from endosymbiotic organelles'.

Genomic Deletion Involving the IMMP2L Gene in Two Cases of Autism Spectrum Disorder

Mutations/deletions of the IMMP2L gene have been associated with different cognitive/behavioral disturbances, including autism spectrum disorders (ASD). The penetrance of these defects is not complete since they often are inherited from a healthy parent. Using array-CGH in a cohort of 37 ASD patients, we found 2 subjects harboring a deletion inside the IMMP2L gene. In both cases, the IMMP2L gene deletion was inherited: from a healthy mother in one case and from a dyslectic father in the other. In the latter family, the IMMP2L deletion was also detected in the patient's brother, who showed delayed language development. In a cohort of 100 normal controls, no deletions including the IMMP2L gene were observed. However, a recent meta-analysis found no association between IMMP2L deletions and ASD. Our data would indicate that deletions involving the IMMP2L gene may contribute to the development of a subgroup of cognitive/behavioral disorders.

Association of IMMP2L deletions with autism spectrum disorder: A trio family study and meta-analysis

IMMP2L, the gene encoding the inner mitochondrial membrane peptidase subunit 2-like protein, has been reported as a candidate gene for Tourette syndrome, autism spectrum disorder (ASD) and additional neurodevelopmental disorders. Here we genotyped 100 trio families with an index proband with autism spectrum disorder in Han Chinese population and found three cases with rare exonic IMMP2L deletions. We have conducted a comprehensive meta-analysis to quantify the association of IMMP2L deletions with ASD using 5,568 cases and 10,279 controls. While the IMMP2L deletions carried non-recurrent breakpoints, in contrast to previous reports, our meta-analysis found no evidence of association (P > 0.05) between IMMP2L deletions and ASD. We also observed common exonic deletions impacting IMMP2L in a separate control (5,971 samples) cohort where subjects were screened for psychiatric conditions. This is the first systematic review and meta-analysis regarding the effect of IMMP2L deletions on ASD, but further investigations in different populations, especially Chinese population may be still needed to confirm our results.