Prdm15 acts upstream of Wnt4 signaling in anterior neural development of Xenopus laevis

Mutations in PRDM15 lead to a syndromic form of holoprosencephaly (HPE) known as the Galloway-Mowat syndrome (GAMOS). While a connection between PRDM15, a zinc finger transcription factor, and WNT/PCP signaling has been established, there is a critical need to delve deeper into their contributions to early development and GAMOS pathogenesis. We used the South African clawed frog Xenopus laevis as the vertebrate model organism and observed that prdm15 was enriched in the tissues and organs affected in GAMOS. Furthermore, we generated a morpholino oligonucleotide-mediated prdm15 knockdown model showing that the depletion of Prdm15 leads to abnormal eye, head, and brain development, effectively recapitulating the anterior neural features in GAMOS. An analysis of the underlying molecular basis revealed a reduced expression of key genes associated with eye, head, and brain development. Notably, this reduction could be rescued by the introduction of wnt4 RNA, particularly during the induction of the respective tissues. Mechanistically, our data demonstrate that Prdm15 acts upstream of both canonical and non-canonical Wnt4 signaling during anterior neural development. Our findings describe severe ocular and anterior neural abnormalities upon Prdm15 depletion and elucidate the role of Prdm15 in canonical and non-canonical Wnt4 signaling.

Effects of aircraft noise cessation on blood pressure, cardio- and cerebrovascular endothelial function, oxidative stress, and inflammation in an experimental animal model

Large epidemiological studies have shown that traffic noise promotes the development of cardiometabolic diseases. It remains to be established how long these adverse effects of noise may persist in response to a noise-off period. We investigated the effects of acute aircraft noise exposure (mean sound level of 72 dB(A) applied for 4d) on oxidative stress and inflammation mediating vascular dysfunction and increased blood pressure in male C57BL/6 J mice. 1, 2 or 4d of noise cessation after a 4d continuous noise exposure period completely normalized noise-induced endothelial dysfunction of the aorta (measured by acetylcholine-dependent relaxation) already after a 1d noise pause. Vascular oxidative stress and the increased blood pressure were partially corrected, while markers of inflammation (VCAM-1, IL-6 and leukocyte oxidative burst) showed a normalization within 4d of noise cessation. In contrast, endothelial dysfunction, oxidative stress, and inflammation of the cerebral microvessels of noise-exposed mice did not improve at all. These data demonstrate that the recovery from noise-induced damage is more complex than expected demonstrating a complete restoration of large conductance vessel function but persistent endothelial dysfunction of the microcirculation. These findings also imply that longer noise pauses are required to completely reverse noise-induced vascular dysfunction including the resistance vessels.

NECAB2 is an endosomal protein important for striatal function

Synaptic signaling depends on ATP generated by mitochondria. Dysfunctional mitochondria shift the redox balance towards a more oxidative environment. Due to extensive connectivity, the striatum is especially vulnerable to mitochondrial dysfunction. We found that neuronal calcium-binding protein 2 (NECAB2) plays a role in striatal function and mitochondrial homeostasis. NECAB2 is a predominantly endosomal striatal protein which partially colocalizes with mitochondria. This colocalization is enhanced by mild oxidative stress. Global knockout of Necab2 in the mouse results in increased superoxide levels, increased DNA oxidation and reduced levels of the antioxidant glutathione which correlates with an altered mitochondrial shape and function. Striatal mitochondria from Necab2 knockout mice are more abundant and smaller and characterized by a reduced spare capacity suggestive of intrinsic uncoupling respectively mitochondrial dysfunction. In line with this, we also found an altered stress-induced interaction of endosomes with mitochondria in Necab2 knockout striatal cultures. The predominance of dysfunctional mitochondria and the pro-oxidative redox milieu correlates with a loss of striatal synapses and behavioral changes characteristic of striatal dysfunction like reduced motivation and altered sensory gating. Together this suggests an involvement of NECAB2 in an endosomal pathway of mitochondrial stress response important for striatal function.

Neuroanatomical changes of ionotropic glutamatergic and GABAergic receptor densities in male mice modeling idiopathic and syndromic autism spectrum disorder

Autism spectrum disorder (ASD) comprises a wide range of neurodevelopment conditions primarily characterized by impaired social interaction and repetitive behavior, accompanied by a variable degree of neuropsychiatric characteristics. Synaptic dysfunction is undertaken as one of the key underlying mechanisms in understanding the pathophysiology of ASD. The excitatory/inhibitory (E/I) hypothesis is one of the most widely held theories for its pathogenesis. Shifts in E/I balance have been proven in several ASD models. In this study, we investigated three mouse lines recapitulating both idiopathic (the BTBR strain) and genetic (Fmr1 and Shank3 mutants) forms of ASD at late infancy and early adulthood. Using receptor autoradiography for ionotropic excitatory (AMPA and NMDA) and inhibitory (GABAA) receptors, we mapped the receptor binding densities in brain regions known to be associated with ASD such as prefrontal cortex, dorsal and ventral striatum, dorsal hippocampus, and cerebellum. The individual mouse lines investigated show specific alterations in excitatory ionotropic receptor density, which might be accounted as specific hallmark of each individual line. Across all the models investigated, we found an increased binding density to GABAA receptors at adulthood in the dorsal hippocampus. Interestingly, reduction in the GABAA receptor binding density was observed in the cerebellum. Altogether, our findings suggest that E/I disbalance individually affects several brain regions in ASD mouse models and that alterations in GABAergic transmission might be accounted as unifying factor.

Fixation and staining methods for macroscopical investigation of the brain

The proper preservation of human brain tissue is an indispensable requirement for post-mortem investigations. Neuroanatomical teaching, neuropathological examination, neurosurgical training, basic and clinical neuroscientific research are some of the possible downstream applications of brain specimens and, although much apart from one another, proper tissue fixation and preservation is a common denominator to all of them. In this review, the most relevant procedures to fixate brain tissue are described. In situ and immersion fixation approaches have been so far the most widespread ways to deliver the fixatives inside the skull. Although most of them rely on the use of formalin, alternative fixative solutions containing lower amounts of this compound mixed with other preservative agents, have been attempted. The combination of fixation and freezing paved the way for fiber dissection, particularly relevant for the neurosurgical practice and clinical neuroscience. Moreover, special techniques have been developed in neuropathology to tackle extraordinary problems, such as the examination of highly infective specimens, as in the case of the Creutzfeldt-Jakob encephalopathy, or fetal brains. Fixation is a fundamental prerequisite for further staining of brain specimens. Although several staining techniques have been developed for the microscopical investigation of the central nervous system, numerous approaches are also available for staining macroscopic brain specimens. They are mostly relevant for neuroanatomical and neuropathological teaching and can be divided in white and gray matter staining techniques. Altogether, brain fixation and staining techniques are rooted in the origins of neuroscience and continue to arouse interest in both preclinical and clinical neuroscientists also nowadays.

Mouse models, antibodies, and neuroimaging: Current knowledge and future perspectives in neuropsychiatric systemic lupus erythematosus (NPSLE)

As a chronic autoimmune disease systemic lupus erythematosus (SLE) can also affect the central and the peripheral nervous system causing symptoms which are summed up as neuropsychiatric systemic lupus erythematosus (NPSLE). These symptoms are heterogenous including cognitive impairment, seizures, and fatigue, leading to morbidity or even mortality. At present, little is known about the pathophysiological processes involved in NPSLE. This review focuses on the current knowledge of the pathogenesis of NPSLE gained from the investigation of animal models, autoantibodies, and neuroimaging techniques. The antibodies investigated the most are anti-ribosomal P protein antibodies (Anti-rib P) and anti-N-Methyl-D-Aspartic Acid Receptor 2 antibodies (Anti-NR2), which represent a subpopulation of anti-dsDNA autoantibodies. Experimental data demonstrates that Anti-rib P and Anti-NR2 cause different neurological pathologies when applied intravenously (i.v.), intrathecally or intracerebrally in mice. Moreover, the investigation of lupus-prone mice, such as the MRL/MpJ-Faslpr/lpr strain (MRL/lpr) and the New Zealand black/New Zealand white mice (NZB × NZW F1) showed that circulating systemic antibodies cause different neuropsychiatric symptoms compared to intrathecally produced antibodies. Furthermore, neuroimaging techniques including magnetic resonance imaging (MRI) and positron emission tomography (PET) are commonly used tools to investigate structural and functional abnormalities in NPSLE patients. Current research suggests that the pathogenesis of NPSLE is heterogenous, complex and not yet fully understood. However, it demonstrates that further investigation is needed to develop individual therapy in NPSLE.

Striatal increase of dopamine receptor 2 density in idiopathic and syndromic mouse models of autism spectrum disorder

Autism spectrum disorder (ASD) comprises a wide range of neurodevelopmental phenotypes united by impaired social interaction and repetitive behavior. Environmental and genetic factors are associated with the pathogenesis of ASD, while other cases are classified as idiopathic. The dopaminergic system has a profound impact in the modulation of motor and reward-motivated behaviors, and defects in dopaminergic circuits are implicated in ASD. In our study, we compare three well-established mouse models of ASD, one idiopathic, the BTBR strain, and two syndromic, Fmr1 and Shank3 mutants. In these models, and in humans with ASD, alterations in dopaminergic metabolism and neurotransmission were highlighted. Still, accurate knowledge about the distribution of dopamine receptor densities in the basal ganglia is lacking. Using receptor autoradiography, we describe the neuroanatomical distribution of D1 and D2 receptors in dorsal and ventral striatum at late infancy and adulthood in the above-mentioned models. We show that D1 receptor binding density is different among the models irrespective of the region. A significant convergence in increased D2 receptor binding density in the ventral striatum at adulthood becomes apparent in BTBR and Shank3 lines, and a similar trend was observed in the Fmr1 line. Altogether, our results confirm the involvement of the dopaminergic system, showing defined alterations in dopamine receptor binding density in three well-established ASD lines, which may provide a plausible explanation to some of the prevalent traits of ASD. Moreover, our study provides a neuroanatomical framework to explain the utilization of D2-acting drugs such as Risperidone and Aripiprazole in ASD.

Functional characterization of a novel TP53RK mutation identified in a family with Galloway-Mowat syndrome

Galloway-Mowat syndrome (GAMOS) is a very rare condition characterized by early-onset nephrotic syndrome and microcephaly with variable neurologic features. While considerable genetic heterogeneity of GAMOS has been identified, the majority of cases are caused by pathogenic variants in genes encoding the four components of the Kinase, endopeptidase, and other proteins of small size (KEOPS) complex, one of which is TP53RK. Here we describe a 3-year-old male with progressive microcephaly, neurodevelopmental deficits, and glomerular proteinuria. He was found to carry a novel homozygous TP53RK missense variant, c.163C>G (p.Arg55Gly), which was considered as potentially disease-causing. We generated a morpholino tp53rk knockdown model in Xenopus laevis showing that the depletion of endogenous Tp53rk caused abnormal eye and head development. This phenotype could be rescued by the expression of human wildtype TP53RK but not by the c.163C>G mutant nor by another previously described GAMOS-associated mutant c.125G>A (p.Gly42Asp). These findings support the pathogenic role of the novel TP53RK variant.

A morphometric analysis of the circumolivary fiber bundle of the human brainstem

The circumolivary fiber bundle (CFB) is considered to be an anatomical variation, which can be found on the surface of the human medulla oblongata. The macroscopical fiber bundle runs downwards from either the anterior median fissure, the pyramid, or both, around the inferior pole of the olive and turns upwards to reach the restiform body of the inferior cerebellar peduncle. Multiple fiber systems feed the constitution of the CFB (collateral corticospinal fibers, fibers connecting to the reticular formation, anterior external arcuate fibers). With this examination we provide a systematic analysis of the frequency of occurrence (6.14%), size, and laterality of the CFB. Including all three fiber bundle parts (descending part, genu, and ascending part), the left-sided sizes were increased. Likewise, the appearance of an unilateral left-sided CFB could be detected in more than 60% of our cases. Our morphometrical analysis currently covers the largest sample of investigated brainstem sides (n = 489) so far. This investigation should widen the perspective on how anatomists, neuroradiologists, and neurosurgeons expect the anterolateral surface of the human medulla oblongata.

TMBIM5 loss of function alters mitochondrial matrix ion homeostasis and causes a skeletal myopathy

TMBIM5 deficiency reduces mitochondrial K⁺/H⁺ exchange. Mutation of the channel pore in mice destabilizes the protein and results in increased embryonic lethality and a skeletal myopathy.

Ion fluxes across the inner mitochondrial membrane control mitochondrial volume, energy production, and apoptosis. TMBIM5, a highly conserved protein with homology to putative pH-dependent ion channels, is involved in the maintenance of mitochondrial cristae architecture, ATP production, and apoptosis. Here, we demonstrate that overexpressed TMBIM5 can mediate mitochondrial calcium uptake. Under steady-state conditions, loss of TMBIM5 results in increased potassium and reduced proton levels in the mitochondrial matrix caused by attenuated exchange of these ions. To identify the in vivo consequences of TMBIM5 dysfunction, we generated mice carrying a mutation in the channel pore. These mutant mice display increased embryonic or perinatal lethality and a skeletal myopathy which strongly correlates with tissue-specific disruption of cristae architecture, early opening of the mitochondrial permeability transition pore, reduced calcium uptake capability, and mitochondrial swelling. Our results demonstrate that TMBIM5 is an essential and important part of the mitochondrial ion transport system machinery with particular importance for embryonic development and muscle function.

Chronic social stress disrupts the intracellular redistribution of brain hexokinase 3 induced by shifts in peripheral glucose levels

Abstract

Chronic stress has the potential to impair health and may increase the vulnerability for psychiatric disorders. Emerging evidence suggests that specific neurometabolic dysfunctions play a role herein. In mice, chronic social defeat (CSD) stress reduces cerebral glucose uptake despite hyperglycemia. We hypothesized that this metabolic decoupling would be reflected by changes in contact sites between mitochondria and the endoplasmic reticulum, important intracellular nutrient sensors, and signaling hubs. We thus analyzed the proteome of their biochemical counterparts, mitochondria-associated membranes (MAMs) from whole brain tissue obtained from CSD and control mice. This revealed a lack of the glucose-metabolizing enzyme hexokinase 3 (HK3) in MAMs from CSD mice. In controls, HK3 protein abundance in MAMs and also in striatal synaptosomes correlated positively with peripheral blood glucose levels, but this connection was lost in CSD. We conclude that the ability of HK3 to traffic to sites of need, such as MAMs or synapses, is abolished upon CSD and surmise that this contributes to a cellular dysfunction instigated by chronic stress.

Key messages

Chronic social defeat (CSD) alters brain glucose metabolism

CSD depletes hexokinase 3 (HK3) from mitochondria-associated membranes (MAMs)

CSD results in loss of positive correlation between blood glucose and HK3 in MAMs and synaptosomes

The Sexual Dimorphic Synapse: From Spine Density to Molecular Composition

A synaptic sexual dimorphism is relevant in the context of multiple neurodevelopmental, neurodegenerative, and neuropsychiatric disorders. Many of these disorders show a different prevalence and progression in woman and man. A similar variance is also present in corresponding animal models. To understand and characterize this dimorphism in pathologies it is important to first understand sex differences in unaffected individuals. Therefore, sexual differences have been studied since 1788, first focusing on brain weight, size, and volume. But as these measures are not directly related to brain function, the investigation of sexual dimorphism also expanded to other organizational levels of the brain. This review is focused on sexual dimorphism at the synaptic level, as these specialized structures are the smallest functional units of the brain, determining cell communication, connectivity, and plasticity. Multiple differences between males and females can be found on the levels of spine density, synaptic morphology, and molecular synapse composition. These differences support the importance of sex-disaggregated data. The specificity of changes to a particular brain region or circuit might support the idea of a mosaic brain, in which each tile individually lies on a continuum from masculinization to feminization. Moreover, synapses can be seen as the smallest tiles of the mosaic determining the classification of larger areas.

Long-Term Effects of Aircraft Noise Exposure on Vascular Oxidative Stress, Endothelial Function and Blood Pressure: No Evidence for Adaptation or Tolerance Development

Transportation noise is recognized as an important cardiovascular risk factor. Key mechanisms are noise-triggered vascular inflammation and oxidative stress with subsequent endothelial dysfunction. Here, we test for adaptation or tolerance mechanisms in mice in response to chronic noise exposure. C57BL/6J mice were exposed to aircraft noise for 0, 4, 7, 14 and 28d at a mean sound pressure level of 72 dB(A) and peak levels of 85 dB(A). Chronic aircraft noise exposure up to 28d caused persistent endothelial dysfunction and elevation of blood pressure. Likewise, reactive oxygen species (ROS) formation as determined by dihydroethidium (DHE) staining and HPLC-based measurement of superoxide formation in the aorta/heart/brain was time-dependently increased by noise. Oxidative burst in the whole blood showed a maximum at 4d or 7d of noise exposure. Increased superoxide formation in the brain was mirrored by a downregulation of neuronal nitric oxide synthase (Nos3) and transcription factor Foxo3 genes, whereas Vcam1 mRNA, a marker for inflammation was upregulated in all noise exposure groups. Induction of a pronounced hearing loss in the mice was excluded by auditory brainstem response audiometry. Endothelial dysfunction and inflammation were present during the entire 28d of aircraft noise exposure. ROS formation gradually increases with ongoing exposure without significant adaptation or tolerance in mice in response to chronic noise stress at moderate levels. These data further illustrate health side effects of long-term noise exposure and further strengthen a consequent implementation of the WHO noise guidelines in order to prevent the development of noise-related future cardiovascular disease.

Translating the Role of mTOR- and RAS-Associated Signalopathies in Autism Spectrum Disorder: Models, Mechanisms and Treatment

Mutations affecting mTOR or RAS signaling underlie defined syndromes (the so-called mTORopathies and RASopathies) with high risk for Autism Spectrum Disorder (ASD). These syndromes show a broad variety of somatic phenotypes including cancers, skin abnormalities, heart disease and facial dysmorphisms. Less well studied are the neuropsychiatric symptoms such as ASD. Here, we assess the relevance of these signalopathies in ASD reviewing genetic, human cell model, rodent studies and clinical trials. We conclude that signalopathies have an increased liability for ASD and that, in particular, ASD individuals with dysmorphic features and intellectual disability (ID) have a higher chance for disruptive mutations in RAS- and mTOR-related genes. Studies on rodent and human cell models confirm aberrant neuronal development as the underlying pathology. Human studies further suggest that multiple hits are necessary to induce the respective phenotypes. Recent clinical trials do only report improvements for comorbid conditions such as epilepsy or cancer but not for behavioral aspects. Animal models show that treatment during early development can rescue behavioral phenotypes. Taken together, we suggest investigating the differential roles of mTOR and RAS signaling in both human and rodent models, and to test drug treatment both during and after neuronal development in the available model systems.

Interclavicularis anticus digastricus muscle in a female body donor: a case report

Background and objectives

Muscular variations of the ventral thoracic wall are generally common and of great clinical interest.

Materials and methods

An unusual muscular variation of the ventral thoracic wall was observed and dissected in a West-European female body donor.

Results

An interclavicularis anticus digastricus muscle was observed and studied. It originated from the manubrium sterni and inserted bilaterally to the clavicles. Both muscle bellies were interconnected by a tendon on the ventral surface of the manubrium sterni. The muscle was innervated by branches of the lateral pectoral nerve.

Conclusions

The interclavicularis anticus digastricus muscle is a muscular variation of the ventral thoracic wall of unknown prevalence. This variation might be of clinical interest in orthopaedics and thoracic surgery. It is also a vulnerable structure during infraclavicular insertion of a subclavian vein catheter or fractures of the clavicle.

Galloway-Mowat syndrome: New insights from bioinformatics and expression during Xenopus embryogenesis

Galloway-Mowat syndrome (GAMOS) is a rare developmental disease. Patients suffer from congenital brain anomalies combined with renal abnormalities often resulting in an early-onset steroid-resistant nephrotic syndrome. The etiology of GAMOS has a heterogeneous genetic contribution. Mutations in more than 10 different genes have been reported in GAMOS patients. Among these are mutations in four genes encoding members of the human KEOPS (kinase, endopeptidase and other proteins of small size) complex, including OSGEP, TP53RK, TPRKB and LAGE3. Until now, these components have been functionally mainly investigated in bacteria, eukarya and archaea and in humans in the context of the discovery of its role in GAMOS, but the KEOPS complex members' expression and function during embryogenesis in vertebrates is still unknown. In this study, in silico analysis showed that both gene localization and the protein sequences of the three core KEOPS complex members Osgep, Tp53rk and Tprkb are highly conserved across different species including Xenopus laevis. In addition, we examined the spatio-temporal expression pattern of osgep, tp53rk and tprkb using RT-PCR and whole mount in situ hybridization approaches during early Xenopus development. We observed that all three genes were expressed during early embryogenesis and enriched in tissues and organs affected in GAMOS. More precisely, KEOPS complex genes are expressed in the pronephros, but also in neural tissue such as the developing brain, eye and cranial cartilage. These findings suggest that the KEOPS complex plays an important role during vertebrate embryonic development.

Criss-crossing autism spectrum disorder and adult neurogenesis

Autism spectrum disorder (ASD) comprises a group of multifactorial neurodevelopmental disorders primarily characterized by deficits in social interaction and repetitive behavior. Although the onset is typically in early childhood, ASD poses a lifelong challenge for both patients and caretakers. Adult neurogenesis (AN) is the process by which new functional neurons are created from neural stem cells existing in the post-natal brain. The entire event is based on a sequence of cellular processes, such as proliferation, specification of cell fate, maturation, and ultimately, synaptic integration into the existing neural circuits. Hence, AN is implicated in structural and functional brain plasticity throughout life. Accumulating evidence shows that impaired AN may underlie some of the abnormal behavioral phenotypes seen in ASD. In this review, we approach the interconnections between the molecular pathways related to AN and ASD. We also discuss existing therapeutic approaches targeting such pathways both in preclinical and clinical studies. A deeper understanding of how ASD and AN reciprocally affect one another could reveal important converging pathways leading to the emergence of psychiatric disorders.

Carl Toldt Centennial, Surgeon and Anatomist

Carl Florian Toldt was an Austrian anatomist who made meaningful contributions worldwide and defined what is one of the most important surgical landmarks in abdominal surgery. Through his research studies, the embryologic dissection plane known as the "White Line of Toldt" represents an important anatomical landmark that helps to mobilize either the ascending or descending colon. His career spanned over 45 years, beginning in Verona and continuing to Prague and Vienna. He was an author of several innovative books and scientific articles regarding micro- and macroscopic anatomy. In addition, he received numerous recognitions and prizes for his work, making him an essential figure in the medical scientific community. Even a street in Vienna, Karl-Toldt-Weg, is named in his honor. The purpose of this historical article is to celebrate and honor Toldt 100 years following his death, remembering his scientific contributions to the medical and surgical fields and giving thanks for his numerous accomplishments. This article brings light to the man behind the eponym.

Mutations in PRDM15 Are a Novel Cause of Galloway-Mowat Syndrome

BACKGROUND

Galloway-Mowat syndrome (GAMOS) is characterized by neurodevelopmental defects and a progressive nephropathy, which typically manifests as steroid-resistant nephrotic syndrome. The prognosis of GAMOS is poor, and the majority of children progress to renal failure. The discovery of monogenic causes of GAMOS has uncovered molecular pathways involved in the pathogenesis of disease.

METHODS

Homozygosity mapping, whole-exome sequencing, and linkage analysis were used to identify mutations in four families with a GAMOS-like phenotype, and high-throughput PCR technology was applied to 91 individuals with GAMOS and 816 individuals with isolated nephrotic syndrome. In vitro and in vivo studies determined the functional significance of the mutations identified.

RESULTS

Three biallelic variants of the transcriptional regulator PRDM15 were detected in six families with proteinuric kidney disease. Four families with a variant in the protein's zinc-finger (ZNF) domain have additional GAMOS-like features, including brain anomalies, cardiac defects, and skeletal defects. All variants destabilize the PRDM15 protein, and the ZNF variant additionally interferes with transcriptional activation. Morpholino oligonucleotide-mediated knockdown of Prdm15 in Xenopus embryos disrupted pronephric development. Human wild-type PRDM15 RNA rescued the disruption, but the three PRDM15 variants did not. Finally, CRISPR-mediated knockout of PRDM15 in human podocytes led to dysregulation of several renal developmental genes.

CONCLUSIONS

Variants in PRDM15 can cause either isolated nephrotic syndrome or a GAMOS-type syndrome on an allelic basis. PRDM15 regulates multiple developmental kidney genes, and is likely to play an essential role in renal development in humans.

Genetic influences of autism candidate genes on circuit wiring and olfactory decoding

Olfaction supports a multitude of behaviors vital for social communication and interactions between conspecifics. Intact sensory processing is contingent upon proper circuit wiring. Disturbances in genetic factors controlling circuit assembly and synaptic wiring can lead to neurodevelopmental disorders, such as autism spectrum disorder (ASD), where impaired social interactions and communication are core symptoms. The variability in behavioral phenotype expression is also contingent upon the role environmental factors play in defining genetic expression. Considering the prevailing clinical diagnosis of ASD, research on therapeutic targets for autism is essential. Behavioral impairments may be identified along a range of increasingly complex social tasks. Hence, the assessment of social behavior and communication is progressing towards more ethologically relevant tasks. Garnering a more accurate understanding of social processing deficits in the sensory domain may greatly contribute to the development of therapeutic targets. With that framework, studies have found a viable link between social behaviors, circuit wiring, and altered neuronal coding related to the processing of salient social stimuli. Here, the relationship between social odor processing in rodents and humans is examined in the context of health and ASD, with special consideration for how genetic expression and neuronal connectivity may regulate behavioral phenotypes.

Physiological relevance of the neuronal isoform of inositol-1,4,5-trisphosphate 3-kinases in mice

Inositol-1,4,5-trisphosphate 3-kinase-A (ITPKA) is the neuronal isoform of ITPKs and exhibits both actin bundling and InsP₃kinase activity. In addition to neurons, ITPKA is ectopically expressed in tumor cells, where its oncogenic activity increases tumor cell malignancy. In order to analyze the physiological relevance of ITPKA, here we performed a broad phenotypic screening of itpka deficient mice. Our data show that among the neurobehavioral tests analyzed, itpka deficient mice reacted faster to a hotplate, prepulse inhibition was impaired and the accelerating rotarod test showed decreased latency of itpka deficient mice to fall. These data indicate that ITPKA is involved in the regulation of nociceptive pathways, sensorimotor gating and motor learning. Analysis of extracerebral functions in control and itpka deficient mice revealed significantly reduced glucose, lactate, and triglyceride plasma concentrations in itpka deficient mice. Based on this finding, expression of ITPKA was analyzed in extracerebral tissues and the highest level was found in the small intestine. However, functional studies on CaCo-2 control and ITPKA depleted cells showed that glucose, as well as triglyceride uptake, were not significantly different between the cell lines. Altogether, these data show that ITPKA exhibits distinct functions in the central nervous system and reveal an involvement of ITPKA in energy metabolism.

Hyperactivity and Hypermotivation Associated With Increased Striatal mGluR1 Signaling in a Shank2 Rat Model of Autism

Mutations in the SHANK family of genes have been consistently identified in genetic and genomic screens of autism spectrum disorder (ASD). The functional overlap of SHANK with several other ASD-associated genes suggests synaptic dysfunction as a convergent mechanism of pathophysiology in ASD. Although many ASD-related mutations result in alterations to synaptic function, the nature of those dysfunctions and the consequential behavioral manifestations are highly variable when expressed in genetic mouse models. To investigate the phylogenetic conservation of phenotypes resultant of Shank2 loss-of-function in a translationally relevant animal model, we generated and characterized a novel transgenic rat with a targeted mutation of the Shank2 gene, enabling an evaluation of gene-associated phenotypes, the elucidation of complex behavioral phenotypes, and the characterization of potential translational biomarkers. The Shank2 loss-of-function mutation resulted in a notable phenotype of hyperactivity encompassing hypermotivation, increased locomotion, and repetitive behaviors. Mutant rats also expressed deficits in social behavior throughout development and in the acquisition of operant tasks. The hyperactive phenotype was associated with an upregulation of mGluR1 expression, increased dendritic branching, and enhanced long-term depression (LTD) in the striatum but opposing morphological and cellular alterations in the hippocampus (HP). Administration of the mGluR1 antagonist JNJ16259685 selectively normalized the expression of striatally mediated repetitive behaviors and physiology but had no effect on social deficits. Finally, Shank2 mutant animals also exhibited alterations in electroencephalography (EEG) spectral power and event-related potentials, which may serve as translatable EEG biomarkers of synaptopathic alterations. Our results show a novel hypermotivation phenotype that is unique to the rat model of Shank2 dysfunction, in addition to the traditional hyperactive and repetitive behaviors observed in mouse models. The hypermotivated and hyperactive phenotype is associated with striatal dysfunction, which should be explored further as a targetable mechanism for impairment in ASD.

Behavioural Phenotypes and Neural Circuit Dysfunctions in Mouse Models of Autism Spectrum Disorder

Autism spectrum disorder (ASD) is a neurodevelopmental condition primarily characterised by alterations in social interaction and communication combined with the presence of restricted interests and stereotyped behaviours. Mutations in several genes have been associated with ASD resulting in the generation of corresponding mouse models. Here, we focus on the behavioural (social and stereotyped behaviours), functional and structural traits of mice with mutations in genes encoding defined synaptic proteins including adhesion proteins, scaffolding proteins and subunits of channels and receptors. A meta-analysis on ASD mouse models shows that they can be divided into two subgroups. Cluster I gathered models highly impaired in social interest, stereotyped behaviours, synaptic physiology and protein composition, while Cluster II regrouped much less impaired models, with typical social interactions. This distribution was not related to gene families. Even within the large panel of mouse models carrying mutations in Shank3, the number of mutated isoforms was not related to the severity of the phenotype. Our study points that the majority of structural or functional analyses were performed in the hippocampus. However, to robustly link the structural and functional impairments with the behavioural deficits observed, brain structures forming relevant nodes in networks involved in social and stereotyped behaviours should be targeted in the future. In addition, the characterisation of core ASD-like behaviours needs to be more detailed using new approaches quantifying the variations in social motivation, recognition and stereotyped behaviours.

Genetic Animal Models for Autism Spectrum Disorder

Autism spectrum disorder (ASD) affects approximately 1 % of the human population and has a strong genetic component. Hence, the recent discovery of major "ASD genes" has subsequently resulted in the generation of several genetic animal models of ASD. Careful analysis of behavioral phenotypes and characterization of the underlying neurobiological mechanisms in these models should further help us to identify novel therapeutic targets and develop more effective strategies in the future to ameliorate or even reverse core symptoms and comorbidities of ASD. In this review, we will focus on the mutant mouse as animal model and outline how to characterize both behavioral and neurobiological phenotypes in this organism. We will further discuss a selection of major ASD mutant mouse lines. Our conclusions will finally address the current goals and perspectives in the field to obtain a more comprehensive and possibly also converging picture of ASD pathogenesis, which could be most useful for the desired bench-to-bedside strategy of translational medicine for this complex disorder.

Proteomic Analysis of Post-synaptic Density Fractions from Shank3 Mutant Mice Reveals Brain Region Specific Changes Relevant to Autism Spectrum Disorder

Disruption of the human SHANK3 gene can cause several neuropsychiatric disease entities including Phelan-McDermid syndrome, autism spectrum disorder (ASD), and intellectual disability. Although, a wide array of neurobiological studies strongly supports a major role for SHANK3 in organizing the post-synaptic protein scaffold, the molecular processes at synapses of individuals harboring SHANK3 mutations are still far from being understood. In this study, we biochemically isolated the post-synaptic density (PSD) fraction from striatum and hippocampus of adult Shank3Δ11^-/- mutant mice and performed ion-mobility enhanced data-independent label-free LC–MS/MS to obtain the corresponding PSD proteomes (Data are available via ProteomeXchange with identifier PXD005192). This unbiased approach to identify molecular disturbances at Shank3 mutant PSDs revealed hitherto unknown brain region specific alterations including a striatal decrease of several molecules encoded by ASD susceptibility genes such as the serine/threonine kinase Cdkl5 and the potassium channel K_Ca1.1. Being the first comprehensive analysis of brain region specific PSD proteomes from a Shank3 mutant line, our study provides crucial information on molecular alterations that could foster translational treatment studies for SHANK3 mutation-associated synaptopathies and possibly also ASD in general.

Cerebellar and Striatal Pathologies in Mouse Models of Autism Spectrum Disorder

Autism spectrum disorder (ASD) is a complex neurodevelopmental condition with a strong genetic component. To date, several hundred different genetic mutations have been identified to play a role in its aetiology. The heterogeneity of genetic abnormalities combined with the different brain regions where aberrations are found makes the search for causative mechanisms a daunting task. Even within a limited number of brain regions, a myriad of different neural circuit dysfunctions may lead to ASD. Here, we review mouse models that incorporate mutations of ASD risk genes causing pathologies in the cerebellum and striatum and highlight the vulnerability of related circuit dysfunctions within these brain regions in ASD pathophysiology.

Neuroanatomy and Neuropathology of Autism Spectrum Disorder in Humans

Autism spectrum disorder (ASD) is a lifelong heterogeneous neurodevelopmental condition that is associated with differences in brain anatomy and connectivity. Yet, the molecular and cellular mechanisms that underpin the atypical developmental of the brain in ASD remain poorly understood. Here, we review the findings of in vivo neuroimaging studies examining the time course of atypical brain development in ASD and relate the different neurodevelopmental stages that are atypical in ASD to the known neurobiological mechanisms that drive the maturation of the typically developing brain. In particular, we focus on the notion of 'early brain overgrowth' in ASD, which may lead to differences in the formation of the brain's micro- and macro-circuitry. Moreover, we attempt to link the in vivo reports describing differences in brain anatomy and connectivity on the macroscopic level to the increasing number of post-mortem studies examining the neural architecture of the brain in ASD on the microscopic level. In addition, we discuss future directions and outstanding questions in this particular field of research and highlight the need for establishing the link between micro- and macro-pathology in the same set of individuals with ASD based on advances in genetic, molecular and imaging techniques. In combination, these may proof to be invaluable for patient stratification and the development of novel pharmacotherapies in the future.

Dysfunctional cerebellar Purkinje cells contribute to autism-like behaviour in Shank2-deficient mice

Loss-of-function mutations in the gene encoding the postsynaptic scaffolding protein SHANK2 are a highly penetrant cause of autism spectrum disorders (ASD) involving cerebellum-related motor problems. Recent studies have implicated cerebellar pathology in the aetiology of ASD. Here we evaluate the possibility that cerebellar Purkinje cells (PCs) represent a critical locus of ASD-like pathophysiology in mice lacking Shank2. Absence of Shank2 impairs both PC intrinsic plasticity and induction of long-term potentiation at the parallel fibre to PC synapse. Moreover, inhibitory input onto PCs is significantly enhanced, most prominently in the posterior lobe where simple spike (SS) regularity is most affected. Using PC-specific Shank2 knockouts, we replicate alterations of SS regularity in vivo and establish cerebellar dependence of ASD-like behavioural phenotypes in motor learning and social interaction. These data highlight the importance of Shank2 for PC function, and support a model by which cerebellar pathology is prominent in certain forms of ASD.

The Shank3 Interaction Partner ProSAPiP1 Regulates Postsynaptic SPAR Levels and the Maturation of Dendritic Spines in Hippocampal Neurons

The postsynaptic density or PSD is a submembranous compartment containing a wide array of proteins that contribute to both morphology and function of excitatory glutamatergic synapses. In this study, we have analyzed functional aspects of the Fezzin ProSAP-interacting protein 1 (ProSAPiP1), an interaction partner of the well-known PSD proteins Shank3 and SPAR. Using lentiviral-mediated overexpression and knockdown of ProSAPiP1, we found that this protein is dispensable for the formation of both pre- and postsynaptic specializations per se. We further show that ProSAPiP1 regulates SPAR levels at the PSD and the maturation of dendritic spines. In line with previous findings on the ProSAPiP1 homolog PSD-Zip70, we conclude that Fezzins essentially contribute to the maturation of excitatory spine synapses.

Selective Localization of Shanks to VGLUT1-Positive Excitatory Synapses in the Mouse Hippocampus

Members of the Shank family of multidomain proteins (Shank1, Shank2, and Shank3) are core components of the postsynaptic density (PSD) of excitatory synapses. At synaptic sites Shanks serve as scaffolding molecules that cluster neurotransmitter receptors as well as cell adhesion molecules attaching them to the actin cytoskeleton. In this study we investigated the synapse specific localization of Shank1-3 and focused on well-defined synaptic contacts within the hippocampal formation. We found that all three family members are present only at VGLUT1-positive synapses, which is particularly visible at mossy fiber contacts. No costaining was found at VGLUT2-positive contacts indicating that the molecular organization of VGLUT2-associated PSDs diverges from classical VGLUT1-positive excitatory contacts in the hippocampus. In light of SHANK mutations in neuropsychiatric disorders, this study indicates which glutamatergic networks within the hippocampus will be primarily affected by shankopathies.

Shank3 is localized in axons and presynaptic specializations of developing hippocampal neurons and involved in the modulation of NMDA receptor levels at axon terminals

Autism-related Shank1, Shank2, and Shank3 are major postsynaptic scaffold proteins of excitatory glutamatergic synapses. A few studies, however, have already indicated that within a neuron, the presence of Shank family members is not limited to the postsynaptic density. By separating axons from dendrites of developing hippocampal neurons in microfluidic chambers, we show that RNA of all three Shank family members is present within axons. Immunostaining confirms these findings as all three Shanks are indeed found within separated axons and further co-localize with well-known proteins of the presynaptic specialization in axon terminals. Therefore, Shank proteins might not only serve as postsynaptic scaffold proteins, but also play a crucial role during axonal outgrowth and presynaptic development and function. This is supported by our findings that shRNA-mediated knockdown of Shank3 results in up-regulation of the NMDA receptor subunit GluN1 in axon terminals. Taken together, our findings will have major implications for the future analysis of neuronal Shank biology in both health and disease. Shank1, Shank2, and Shank3 are major postsynaptic scaffold proteins of excitatory glutamatergic synapses strongly related to several neuropsychiatric disorders. However, a few studies have already implicated a functional role of the Shanks beyond the postsynaptic density (PSD). We here show that all three Shanks are localized in both axons and pre-synaptic specializiations of developing hippocampal neurons in culture. We further provide evidence that Shank3 is involved in the modulation of NMDA receptor levels at axon terminals. Taken together, our study will open up novel avenues for the future analysis of neuronal Shank biology in both health and disease.

Super-Resolution Microscopy Reveals Presynaptic Localization of the ALS/FTD Related Protein FUS in Hippocampal Neurons

Fused in Sarcoma (FUS) is a multifunctional RNA-/DNA-binding protein, which is involved in the pathogenesis of the neurodegenerative disorders amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). A common hallmark of these disorders is the abnormal accumulation of mutated FUS protein in the cytoplasm. Under normal conditions FUS is confined to the nuclear compartment, in neurons, however, additional somatodendritic localization can be observed. In this study, we carefully analyzed the subcellular localization of endogenous FUS at synaptic sites of hippocampal neurons which are among the most affected cell types in FTD with FUS pathology. We could confirm a strong nuclear localization of FUS as well as its prominent and widespread neuronal expression throughout the adult and developing rat brain, particularly in the hippocampus, the cerebellum and the outer layers of the cortex. Intriguingly, FUS was also consistently observed at synaptic sites as detected by neuronal subcellular fractionation as well as by immunolabeling. To define a pre- and/or postsynaptic localization of FUS, we employed super-resolution fluorescence localization microscopy. FUS was found to be localized within the axon terminal in close proximity to the presynaptic vesicle protein Synaptophysin1 and adjacent to the active zone protein Bassoon, but well separated from the postsynaptic protein PSD-95. Having shown the presynaptic localization of FUS in the nervous system, a novel extranuclear role of FUS at neuronal contact sites has to be considered. Since there is growing evidence that local presynaptic translation might also be an important mechanism for plasticity, FUS - like the fragile X mental retardation protein FMRP - might act as one of the presynaptic RNA-binding proteins regulating this machinery. Our observation of presynaptic FUS should foster further investigations to determine its role in neurodegenerative diseases such as ALS and FTD.

An Epha4/Sipa1l3/Wnt pathway regulates eye development and lens maturation

The signal-induced proliferation associated family of proteins comprises four members, SIPA1 and SIPA1L1-1L3. Mutations of the human SIPA1L3 gene result in congenital cataracts. In Xenopus, loss of Sipa1l3 function led to a severe eye phenotype that was distinguished by smaller eyes and lenses including lens fiber cell maturation defects. We found a direct interaction between Sipa1l3 and Epha4, building a functional platform for proper ocular development. Epha4 deficiency phenocopied loss of Sipa1l3 and rescue experiments demonstrated that Epha4 acts up-stream of Sipa1l3 during eye development. Both, Sipa1l3 and Epha4 are required for early eye specification. The ocular phenotype, upon loss of either Epha4 or Sipa1l3, was partially mediated by rax. We demonstrated that canonical Wnt signaling is inhibited downstream of Epha4/Sipa1l3 during normal eye development. Depletion of either Sipa1l3 or Epha4 resulted in an up-regulation of axin2 expression, a direct Wnt/β-catenin target gene. In line with this, Sipa1l3 or Epha4 depletion could be rescued by blocking Wnt/β-catenin or activating non-canonical Wnt signaling. We therefore conclude that this pathomechanism prevents proper eye development and maturation of lens fiber cells resulting in congenital cataracts.

Sipa1l3/SPAR3 is targeted to postsynaptic specializations and interacts with the Fezzin ProSAPiP1/Lzts3

Rap GTPase-activating proteins (RapGAPs) are essential for synaptic function as they tightly regulate synaptic Rap signaling. Among the most abundant synaptic RapGAPs in brain are the Spine-associated RapGAPs (SPARs) Sipa1l1/SPAR and Sipa1l2/SPAR2, whereas nothing has been reported on Sipa1l3/SPAR3. In this study, we show that Sipa1l3/SPAR3 is conserved across species, has a distinct expression pattern in the developing rat brain and is localized at excitatory postsynapses. We further demonstrate that the Sipa1l3/SPAR3 C-terminus is required for postsynaptic targeting and represents an interaction module for Fezzins such as ProSAPiP1/Lzts3, a binding partner of the postsynaptic scaffold protein Shank3. Taken together, our data imply that Sipa1l3/SPAR3 is a hitherto unknown synaptic RapGAP, which is targeted to postsynaptic specializations and interacts with Fezzins. Spine-associated RapGAPs (SPARs) are essential modulators of synaptic signaling. Our study is the first to characterize the SPAR family member Sipa1l3/SPAR3 in neuronal tissue. We show that Sipa1l3/SPAR3 is conserved across species, has a distinct expression pattern in brain and is localized to excitatory postsynapses via its C-terminus, which represents an interaction module for other postsynaptic proteins including the Fezzin ProSAPiP1/Lzts3.

Phenotypic and functional analysis of SHANK3 stop mutations identified in individuals with ASD and/or ID

BACKGROUND

SHANK proteins are crucial for the formation and plasticity of excitatory synapses. Although mutations in all three SHANK genes are associated with autism spectrum disorder (ASD), SHANK3 appears to be the major ASD gene with a prevalence of approximately 0.5% for SHANK3 mutations in ASD, with higher rates in individuals with ASD and intellectual disability (ID). Interestingly, the most relevant mutations are typically de novo and often are frameshift or nonsense mutations resulting in a premature stop and a truncation of SHANK3 protein.

METHODS

We analyzed three different SHANK3 stop mutations that we identified in individuals with ASD and/or ID, one novel (c.5008A > T) and two that we recently described (c.1527G > A, c.2497delG). The mutations were inserted into the human SHANK3a sequence and analyzed for effects on subcellular localization and neuronal morphology when overexpressed in rat primary hippocampal neurons.

RESULTS

Clinically, all three individuals harboring these mutations had global developmental delays and ID. In our in vitro assay, c.1527G > A and c.2497delG both result in proteins that lack most of the SHANK3a C-terminus and accumulate in the nucleus of transfected cells. Cells expressing these mutants exhibit converging morphological phenotypes including reduced complexity of the dendritic tree, less spines, and less excitatory, but not inhibitory synapses. In contrast, the truncated protein based on c.5008A > T, which lacks only a short part of the sterile alpha motif (SAM) domain in the very SHANK3a C-terminus, does not accumulate in the nucleus and has minor effects on neuronal morphology.

CONCLUSIONS

In spite of the prevalence of SHANK3 disruptions in ASD and ID, only a few human mutations have been functionally characterized; here we characterize three additional mutations. Considering the transcriptional and functional complexity of SHANK3 in healthy neurons, we propose that any heterozygous stop mutation in SHANK3 will lead to a dysequilibrium of SHANK3 isoform expression and alterations in the stoichiometry of SHANK3 protein complexes, resulting in a distinct perturbation of neuronal morphology. This could explain why the clinical phenotype in all three individuals included in this study remains quite severe - regardless of whether there are disruptions in one or more SHANK3 interaction domains.

Translational neurobiology in Shank mutant mice--model systems for neuropsychiatric disorders

The Shank family comprises three core postsynaptic scaffold proteins of excitatory synapses in the mammalian brain: Shank1, Shank2 and Shank3. Since mutations in all three human SHANK genes are linked to neuropsychiatric disorders such as autism and schizophrenia, Shank mutant mice serve as corresponding in vivo model systems. Besides intriguing alterations in behavior, dysfunction of glutamatergic synapses has emerged as a pathological hallmark among several Shank mutant lines. However, there is very limited knowledge of the underlying pathomechanisms. Therefore, precise neurobiological evaluation of morphological, molecular and electrophysiological phenotypes in Shank mutants is crucially needed. In this brief review, I will focus on the Shank mutant mouse lines we have generated so far and discuss how they might help us to develop translational treatment studies in the future.

Meta-analysis of SHANK Mutations in Autism Spectrum Disorders: a gradient of severity in cognitive impairments

SHANK genes code for scaffold proteins located at the post-synaptic density of glutamatergic synapses. In neurons, SHANK2 and SHANK3 have a positive effect on the induction and maturation of dendritic spines, whereas SHANK1 induces the enlargement of spine heads. Mutations in SHANK genes have been associated with autism spectrum disorders (ASD), but their prevalence and clinical relevance remain to be determined. Here, we performed a new screen and a meta-analysis of SHANK copy-number and coding-sequence variants in ASD. Copy-number variants were analyzed in 5,657 patients and 19,163 controls, coding-sequence variants were ascertained in 760 to 2,147 patients and 492 to 1,090 controls (depending on the gene), and, individuals carrying de novo or truncating SHANK mutations underwent an extensive clinical investigation. Copy-number variants and truncating mutations in SHANK genes were present in ∼1% of patients with ASD: mutations in SHANK1 were rare (0.04%) and present in males with normal IQ and autism; mutations in SHANK2 were present in 0.17% of patients with ASD and mild intellectual disability; mutations in SHANK3 were present in 0.69% of patients with ASD and up to 2.12% of the cases with moderate to profound intellectual disability. In summary, mutations of the SHANK genes were detected in the whole spectrum of autism with a gradient of severity in cognitive impairment. Given the rare frequency of SHANK1 and SHANK2 deleterious mutations, the clinical relevance of these genes remains to be ascertained. In contrast, the frequency and the penetrance of SHANK3 mutations in individuals with ASD and intellectual disability—more than 1 in 50—warrant its consideration for mutation screening in clinical practice.

Autism spectrum disorders (ASD) are a heterogeneous group of neurodevelopmental disorders. Mutations altering genes involved in the junction between brain cells have been repeatedly associated in ASD. For example, SHANK1, SHANK2 and SHANK3 emerged as one family of genes that are associated with ASD. However, little was known about the number of patients carrying these mutations and the clinical outcome. Here, we performed a new genetic screen of SHANK mutations and these results were analyzed in combination with those of the literature. In summary, SHANK mutations account for ∼1% of patients with ASD and were detected in the whole spectrum of autism with a gradient of severity in cognitive impairment: mutations in SHANK1 were rare (0.04%) and present in males with normal IQ and autism; mutations in SHANK2 were present in 0.17% of patients with ASD and mild intellectual disability; mutations in SHANK3 were present in 0.69% of patients with ASD and up to 2.12% of the cases with moderate to profound intellectual disability. Given the high frequency and impact of SHANK3 mutations in individuals with ASD and intellectual disability—more than 1 in 50—this gene should be screened for mutations in clinical practice.

Neurobiology of autism gene products: towards pathogenesis and drug targets

RATIONALE

The genetic heterogeneity of autism spectrum disorders (ASDs) is enormous, and the neurobiology of proteins encoded by genes associated with ASD is very diverse. Revealing the mechanisms on which different neurobiological pathways in ASD pathogenesis converge may lead to the identification of drug targets.

OBJECTIVE

The main objective is firstly to outline the main molecular networks and neuronal mechanisms in which ASD gene products participate and secondly to answer the question how these converge. Finally, we aim to pinpoint drug targets within these mechanisms.

METHOD

Literature review of the neurobiological properties of ASD gene products with a special focus on the developmental consequences of genetic defects and the possibility to reverse these by genetic or pharmacological interventions.

RESULTS

The regulation of activity-dependent protein synthesis appears central in the pathogenesis of ASD. Through sequential consequences for axodendritic function, neuronal disabilities arise expressed as behavioral abnormalities and autistic symptoms in ASD patients. Several known ASD gene products have their effect on this central process by affecting protein synthesis intrinsically, e.g., through enhancing the mammalian target of rapamycin (mTOR) signal transduction pathway or through impairing synaptic function in general. These are interrelated processes and can be targeted by compounds from various directions: inhibition of protein synthesis through Lovastatin, mTOR inhibition using rapamycin, or mGluR-related modulation of synaptic activity.

CONCLUSIONS

ASD gene products may all feed into a central process of translational control that is important for adequate glutamatergic regulation of dendritic properties. This process can be modulated by available compounds but may also be targeted by yet unexplored routes.

The PSD protein ProSAP2/Shank3 displays synapto-nuclear shuttling which is deregulated in a schizophrenia-associated mutation

Recently, mutations in ProSAP2/Shank3 have been discovered as one of the genetic factors for schizophrenia (SCZ). Here, we show that the postsynaptic density protein ProSAP2/Shank3 undergoes activity dependent synapse-to-nucleus shuttling in hippocampal neurons. Our study shows that the de novo mutation (R1117X) in ProSAP2/Shank3 that was identified in a patient with SCZ leads to an accumulation of mutated ProSAP2/Shank3 within the nucleus independent of synaptic activity. Furthermore, we identified novel nuclear ProSAP2/Shank3 interaction partners. Nuclear localization of mutated ProSAP2/Shank3 alters transcription of several genes, among them already identified genetic risk factors for SCZ such as Synaptotagmin 1 and LRRTM1. Comparing the SCZ mutation of ProSAP2/Shank3 to the knockdown of ProSAP2/Shank3 we found some shared features such as reduced synaptic density in neuronal cultures. However, hippocampal neurons expressing the ProSAP2/Shank3 SCZ mutation furthermore show altered E/I ratio and reduced dendritic branching. Thus, we conclude that the uncoupling of ProSAP2/Shank3 nuclear shuttling from synaptic activity may represent a molecular mechanism that contributes to the pathology of SCZ in patients with mutations in ProSAP2/Shank3.

The Nedd4-binding protein 3 (N4BP3) is crucial for axonal and dendritic branching in developing neurons

Background

Circuit formation in the nervous system essentially relies on the proper development of neurons and their processes. In this context, the ubiquitin ligase Nedd4 is a crucial modulator of axonal and dendritic branching.

Results

Herein we characterize the Nedd4-binding protein 3 (N4BP3), a Fezzin family member, during nerve cell development. In developing rat primary hippocampal neurons, endogenous N4BP3 localizes to neuronal processes, including axons and dendrites. Transient in vitro knockdown of N4BP3 in hippocampal cultures during neuritogenesis results in impaired branching of axons and dendrites. In line with these findings, in vivo knockdown of n4bp3 in Xenopus laevis embryos results in severe alteration of cranial nerve branching.

Conclusions

We introduce N4BP3 as a novel molecular element for the correct branching of neurites in developing neurons and propose a central role for an N4BP3-Nedd4 complex in neurite branching and circuit formation.

Autistic-like behaviours and hyperactivity in mice lacking ProSAP1/Shank2

Autism spectrum disorders comprise a range of neurodevelopmental disorders characterized by deficits in social interaction and communication, and by repetitive behaviour. Mutations in synaptic proteins such as neuroligins, neurexins, GKAPs/SAPAPs and ProSAPs/Shanks were identified in patients with autism spectrum disorder, but the causative mechanisms remain largely unknown. ProSAPs/Shanks build large homo- and heteromeric protein complexes at excitatory synapses and organize the complex protein machinery of the postsynaptic density in a laminar fashion. Here we demonstrate that genetic deletion of ProSAP1/Shank2 results in an early, brain-region-specific upregulation of ionotropic glutamate receptors at the synapse and increased levels of ProSAP2/Shank3. Moreover, ProSAP1/Shank2(-/-) mutants exhibit fewer dendritic spines and show reduced basal synaptic transmission, a reduced frequency of miniature excitatory postsynaptic currents and enhanced N-methyl-d-aspartate receptor-mediated excitatory currents at the physiological level. Mutants are extremely hyperactive and display profound autistic-like behavioural alterations including repetitive grooming as well as abnormalities in vocal and social behaviours. By comparing the data on ProSAP1/Shank2(-/-) mutants with ProSAP2/Shank3αβ(-/-) mice, we show that different abnormalities in synaptic glutamate receptor expression can cause alterations in social interactions and communication. Accordingly, we propose that appropriate therapies for autism spectrum disorders are to be carefully matched to the underlying synaptopathic phenotype.

Heterogeneous nuclear ribonucleoprotein k interacts with Abi-1 at postsynaptic sites and modulates dendritic spine morphology

Background

Abelson-interacting protein 1 (Abi-1) plays an important role for dendritic branching and synapse formation in the central nervous system. It is localized at the postsynaptic density (PSD) and rapidly translocates to the nucleus upon synaptic stimulation. At PSDs Abi-1 is in a complex with several other proteins including WASP/WAVE or cortactin thereby regulating the actin cytoskeleton via the Arp 2/3 complex.

Principal Findings

We identified heterogeneous nuclear ribonucleoprotein K (hnRNPK), a 65 kDa ssDNA/RNA-binding-protein that is involved in multiple intracellular signaling cascades, as a binding partner of Abi-1 at postsynaptic sites. The interaction with the Abi-1 SH3 domain is mediated by the hnRNPK-interaction (KI) domain. We further show that during brain development, hnRNPK expression becomes more and more restricted to granule cells of the cerebellum and hippocampal neurons where it localizes in the cell nucleus as well as in the spine/dendritic compartment. The downregulation of hnRNPK in cultured hippocampal neurons by RNAi results in an enlarged dendritic tree and a significant increase in filopodia formation. This is accompanied by a decrease in the number of mature synapses. Both effects therefore mimic the neuronal morphology after downregulation of Abi-1 mRNA in neurons.

Conclusions

Our findings demonstrate a novel interplay between hnRNPK and Abi-1 in the nucleus and at synaptic sites and show obvious similarities regarding both protein knockdown phenotypes. This indicates that hnRNPK and Abi-1 act synergistic in a multiprotein complex that regulates the crucial balance between filopodia formation and synaptic maturation in neurons.

Amyloid beta protein-induced zinc sequestration leads to synaptic loss via dysregulation of the ProSAP2/Shank3 scaffold

Background

Memory deficits in Alzheimer's disease (AD) manifest together with the loss of synapses caused by the disruption of the postsynaptic density (PSD), a network of scaffold proteins located in dendritic spines. However, the underlying molecular mechanisms remain elusive. Since it was shown that ProSAP2/Shank3 scaffold assembly within the PSD is Zn²⁺-dependent and that the amyloid beta protein (Aβ) is able to bind Zn²⁺, we hypothesize that sequestration of Zn²⁺ ions by Aβ contributes to ProSAP/Shank platform malformation.

Results

To test this hypothesis, we designed multiple in vitro and in vivo assays demonstrating ProSAP/Shank dysregulation in rat hippocampal cultures following Aβ oligomer accumulation. These changes were independent from alterations on ProSAP/Shank transcriptional level. However, application of soluble Aβ prevented association of Zn²⁺ ions with ProSAP2/Shank3 in a cell-based assay and decreased the concentration of Zn²⁺ clusters within dendrites. Zn²⁺ supplementation or saturation of Aβ with Zn²⁺ ions prior to cell treatment was able to counter the effects induced by Aβ on synapse density and ProSAP2/Shank3 levels at the PSD. Interestingly, intracellular Zn²⁺ levels in APP-PS1 mice and human AD hippocampus are reduced along with a reduction in synapse density and synaptic ProSAP2/Shank3 and Shank1 protein levels.

Conclusions

We conclude that sequestration of Zn²⁺ ions by Aβ significantly contributes to changes in ProSAP2/Shank3 platforms. These changes in turn lead to less consolidated (mature) synapses reflected by a decrease in Shank1 protein levels at the PSD and decreased synapse density in hippocampal neurons.

Postsynaptic ProSAP/Shank scaffolds in the cross-hair of synaptopathies

Intact synaptic homeostasis is a fundamental prerequisite for a healthy brain. Thus, it is not surprising that altered synaptic morphology and function are involved in the molecular pathogenesis of so-called synaptopathies including autism, schizophrenia (SCZ) and Alzheimer's disease (AD). Intriguingly, various recent studies revealed a crucial role of postsynaptic ProSAP/Shank scaffold proteins in all of the aforementioned disorders. Considering these findings, we follow the hypothesis that ProSAP/Shank proteins are key regulators of synaptic development and plasticity with clear-cut isoform-specific roles. We thus propose a model where ProSAP/Shank proteins are in the center of a postsynaptic signaling pathway that is disrupted in several neuropsychiatric disorders.