Anatomy and Cell Biology of Autism Spectrum Disorder: Lessons from Human Genetics.

Until recently autism spectrum disorder (ASD) was regarded as a neurodevelopmental condition with unknown causes and pathogenesis. In the footsteps of the revolution of genome technologies and genetics, and with its high degree of heritability, ASD became the first neuropsychiatric disorder for which clues towards molecular and cellular pathogenesis were uncovered by genetic identification of susceptibility genes. Currently several hundreds of risk genes have been assigned, with a recurrence below 1% in the ASD population. The multitude and diversity of known ASD genes has extended the clinical notion that ASD comprises very heterogeneous conditions ranging from severe intellectual disabilities to mild high-functioning forms. The results of genetics have allowed to pinpoint a limited number of cellular and molecular processes likely involved in ASD including protein synthesis, signal transduction, transcription/chromatin remodelling and synaptic function all playing an essential role in the regulation of synaptic homeostasis during brain development. In this context, we highlight the role of protein synthesis as a key process in ASD pathogenesis as it might be central in synaptic deregulation and a potential target for intervention. These current insights should lead to a rational design of interventions in molecular and cellular pathways of ASD pathogenesis that may be applied to affected individuals in the future.

Association of Cell Adhesion Molecules Contactin-6 and Latrophilin-1 Regulates Neuronal Apoptosis.

In view of important neurobiological functions of the cell adhesion molecule contactin-6 (Cntn6) that have emerged from studies on null-mutant mice and autism spectrum disorders patients, we set out to examine pathways underlying functions of Cntn6 using a proteomics approach. We identified the cell adhesion GPCR latrophilin-1 (Lphn1, a.k.a. CIRL1/CL, ADGRL1) as a binding partner for Cntn6 forming together a heteromeric cis-complex. Lphn1 expression in cultured neurons caused reduction in neurite outgrowth and increase in apoptosis, which was rescued by coexpression of Cntn6. In cultured neurons derived from Cntn6-/- mice, Lphn1 knockdown reduced apoptosis, suggesting that the observed apoptosis was Lphn1-dependent. In line with these data, the number of apoptotic cells was increased in the cortex of Cntn6-/- mice compared to wild-type littermate controls. These results show that Cntn6 can modulate the activity of Lphn1 by direct binding and suggests that Cntn6 may prevent apoptosis thereby impinging on neurodevelopment.

Contactin-5 expression during development and wiring of the thalamocortical system.

The gene encoding the neural cell adhesion molecule Cntn5 (a.k.a. NB-2) has been put forward as a candidate in neurodevelopmental disorders, like autism spectrum disorder (ASD), by recent genetic findings. Little is known about the expression pattern and function of the gene, and its functional involvement in brain development has remained elusive. So far, most research has focused on its early postnatal expression in the auditory system, where the absence of Cntn5 causes abnormal responses to acoustic stimuli and a decrease in fiber density. The current study shows that the Cntn5 gene is expressed in forebrain structures during embryonic development, starting at E15.5, and that it continues to be expressed into adulthood. Sites of strong expression included the thalamus, the caudate putamen (CPu) and to a lesser extent layer Va of the cerebral cortex. Cntn5-positive thalamic nuclei include the laterodorsal (LD), ventrolateral (VL) and posterior group (Po), which contain glutamatergic neurons. Visualization of the expression pattern through the Tau-LacZ fusion protein coded by an insert in the Cntn5 gene, demonstrated that Cntn5-positive nuclei of the thalamus project to the cortex, based on co-localization with thalamocortical markers L1 and Calretinin. These results indicate that the cell adhesion functions of Cntn5 are exploited for circuit formation and connectivity in early development and for synaptic maintenance during adulthood. Subtle alterations in the formation of the thalamocortical circuit may contribute to neurodevelopmental disorders, such as ASD.

A double hit implicates DIAPH3 as an autism risk gene.

Recent studies have shown that more than 10% of autism cases are caused by de novo structural genomic rearrangements. Given that some heritable copy number variants (CNVs) have been observed in patients as well as in healthy controls, to date little attention has been paid to the potential function of these non-de novo CNVs in causing autism. A normally intelligent patient with autism, with non-affected parents, was identified with a maternally inherited 10 Mb deletion at 13q21.2. Sequencing of the genes within the deletion identified a paternally inherited nonsynonymous amino-acid substitution at position 614 of diaphanous homolog 3 (DIAPH3) (proline to threonine; Pro614Thr). This variant, present in a highly conserved domain, was not found in 328 healthy subjects. Experiments showed a transient expression of Diaph3 in the developing murine cerebral cortex, indicating it has a function in brain development. Transfection of Pro614Thr in murine fibroblasts showed a significant reduction in the number of induced filopodia in comparison to the wild-type gene. DIAPH3 is involved in cell migration, axon guidance and neuritogenesis, and is suggested to function downstream of SHANK3. Our findings strongly suggest DIAPH3 as a novel autism susceptibility gene. Moreover, this report of a 'double-hit' compound heterozygote for a large, maternally inherited, genomic deletion and a paternally inherited rare missense mutation shows that not only de novo genomic variants in patients should be taken seriously in further study but that inherited CNVs may also provide valuable information.

Movies of dopamine transporter occupancy with ultra-high resolution focusing pinhole SPECT.

A pivotal question in neuropharmacology is how the function of neurotransmitter systems relates to psychiatric diseases. In experimental neuropharmacology, we have dreamt about a looking glass that would allow us to see neurotransmitter systems in action, and about animals that would faithfully serve us as models for human psychiatric disease. Analysis of animal models has been limited by the availability of methods to study in vivo neurotransmitter dynamics. Now, a single photon emission computed tomography system called U-SPECT can localize dopamine transporters in sub-compartments of the mouse brain during a range of points in time. Applied to the midbrain dopamine system of different models of disease, this will aid the understanding of dynamic processes of this neurotransmitter that underlie brain functions and human brain pathology.

Strategies to unravel molecular codes essential for the development of meso-diencephalic dopaminergic neurons.

Understanding the development of neuronal systems has become an important asset in the attempt to solve complex questions about neuropathology as found in Parkinson's disease, schizophrenia and other complex neuronal diseases. The development of anatomical and functional divergent structures in the brain is achieved by a combination of early anatomical patterning and highly coordinated neuronal migration and differentiation events. Fundamental to the existence of divergent structures in the brain is the early region-specific molecular programming. Neuronal progenitors located along the neural tube can still adapt many different identities. Their exact position in the developing brain, however, determines early molecular specification by region-specific signalling molecules. These signals determine time and region-specific expression of early regulatory genes, leading to neuronal differentiation. Here, we focus on a well-described neuronal group, the meso-diencephalic dopaminergic neurons, of which heterogeneity based on anatomical position could account for the difference in vulnerability of specific subgroups as observed in Parkinson's disease. The knowledge of their molecular coding helps us to understand how the meso-diencephalic dopaminergic system is built and could provide clues that unravel mechanisms associated with the neuropathology in complex diseases such as Parkinson's disease.

Neurohypophysial dysmorphogenesis in mice lacking the homeobox gene Uncx4.1.

A number of transcription factors have been implicated in the development of the hypothalamo-neurohypophysial system (HNS). Null mutations for these factors caused severe defects in proliferation, migration and survival during early embryogenesis. While they have informed about early events of HNS developments no insights in mechanisms of late development and maturation of this major peptidergic system have been obtained as yet. In a screen for adult-expressed homeobox genes we identified Uncx4.1 as a gene expressed in adult and embryonic magnocellular neurons of the (HNS). Null mutation of Uncx4.1 left these neurons viable and able to express neuropeptides. However, the connectivity of magnocellular neurons with posterior pituitary elements was compromised. As a consequence neuronal fibres traversed to the adenohypophysis. The penetrance of this phenotype was about 50%. The data show a selective role of Uncx4.1 in controlling the development of connections of hypothalamic neurons to pituitary elements, allowing central neurons to reach the peripheral blood circulation and to deliver hormones for control of peripheral functions.

Minor disturbances in central nervous system function in familial neurohypophysial diabetes insipidus.

Familial neurohypophysial diabetes insipidus (FNDI) is caused by a defect in vasopressin synthesis and release as a result of a heterozygous mutation in the gene for the vasopressin prohormone. The predominant characteristic of FNDI is excessive thirst and urine production. However, vasopressin not only has peripheral endocrine effects, but also regulates numerous brain functions. We investigated whether central functions are affected in FNDI, by studying neuropsychological functioning of 23 affected members (15 males, 8 females) of a large family carrying a T/G transition mutation at nucleotide 2110 (codon 116) of the vasopressin prohormone gene (Cys116Gly). The relatively large number of family members with FNDI made it possible to compare cognitive and other CNS effects in these subjects with those of family members without FNDI. Thirty-seven adult volunteers (20 males, 17 females) from the same family and 11 non-family members (2 males, 9 females) from northern part of The Netherlands were tested. The mean age of the subjects was 35+/-12 years. Of the 63 quantified neuropsychological parameters few were statistically different between the subjects with FDNI and control subjects. Memory retrieval processes and sustained attention were worse in the subjects with FDNI. Moreover, these individuals reported significantly fewer symptoms of agoraphobia and miscellaneous symptoms, and had significantly lower scores on a scale measuring anger. The performance of FNDI subjects on an auditory verbal learning test (the 15-word test learning trial) was worse, but not significantly so, than that of the subjects without FDNI. There were subjective complaints of forgetfulness and slow recalls and those were observed in daily life by non-affected family members. These moderate differences in neuropsychological performance indicate that in human FNDI parvocellular vasopressin systems that supply the brain may be less affected or give no such serious disabilities, than the magnocellular hypothalamo-neurohypophysial system that provides vasopressin for endocrine regulation of water homeostasis.

Signalling through phospholipase C beta 4 is not essential for midbrain dopaminergic neuron survival.

The most prominent progressive neurodegenerative movement disorder, Parkinson's disease, is attributed to selective loss of dopamine neurons in the substantia nigra pars compacta, resulting in severe deficiency of dopamine. The homeo-domain gene, Pit x 3, is essential for proper development of midbrain dopaminergic neurons in the substantia nigra pars compacta and might be involved in midbrain dopaminergic survival pathways. The mGluR1-signaling downstream-effector phospholipase C beta 4 was identified in a suppression subtractive hybridization screen comparing wild-type and Pit x 3-deficient Aphakia midbrain dopaminergic neurons. Expression pattern analysis revealed that phospholipase C beta 4 was expressed in midbrain dopaminergic neurons of the substantia nigra pars compacta and part of the ventral tegmental area, whereas expression of mGluR1alpha was predominantly observed in the more vulnerable midbrain dopaminergic neurons in the lateral substantia nigra pars compacta. However, clear expression of phospholipase C beta 4 in spared midbrain dopaminergic neurons of Aphakia mice located in the ventral tegmental area, indicated that induction and maintenance of phospholipase C beta 4 expression is Pit x 3-independent in these neurons. Furthermore, we report here a normal distribution of midbrain dopaminergic cell bodies and axonal projection to the striatum in phospholipase C beta 4-/- mice, indicating that signaling of phospholipase C beta 4 is not essential for the survival of midbrain dopaminergic neurons.

Increased gabaergic input to ventral tegmental area dopaminergic neurons associated with decreased cocaine reinforcement in mu-opioid receptor knockout mice.

There is general agreement that dopaminergic neurons projecting from the ventral tegmental area (VTA) to the nucleus accumbens and prefrontal cortex play a key role in drug reinforcement. The activity of these neurons is strongly modulated by the inhibitory and excitatory input they receive. Activation of mu-opioid receptors, located on GABAergic neurons in the VTA, causes hyperpolarization of these GABAergic neurons, thereby causing a disinhibition of VTA dopaminergic neurons. This effect of mu-opioid receptors upon GABA neurotransmission is a likely mechanism for mu-opioid receptor modulation of drug reinforcement. We studied mu-opioid receptor signaling in relation to cocaine reinforcement in wild-type and mu-opioid receptor knockout mice using a cocaine self-administration paradigm and in vitro electrophysiology. Cocaine self-administration was reduced in mu-opioid receptor knockout mice, suggesting a critical role of mu-opioid receptors in cocaine reinforcement. The frequency of spontaneous inhibitory post-synaptic currents onto dopaminergic neurons in the ventral tegmental area was increased in mu-opioid receptor knockout mice compared with wild-type controls, while the frequency of spontaneous excitatory post-synaptic currents was unaltered. The reduced cocaine self-administration and increased GABAergic input to VTA dopaminergic neurons in mu-opioid receptor knockout mice supports the notion that suppression of GABAergic input onto dopaminergic neurons in the VTA contributes to mu-opioid receptor modulation of cocaine reinforcement.

Developmental regulation of homeobox gene expression in dorsal root ganglion neurons is not recapitulated during regeneration of the crushed sciatic nerve.

The adult peripheral nervous system is able to regenerate after injury. Regeneration is associated with the expression of new genes and proteins. Proteins abundant in developing axons increase in expression after injury, whereas proteins involved in neurotransmission are downregulated. It has been hypothesized that molecular mechanisms underlying regeneration-associated alterations in gene expression may be a recapitulation of developmental processes. These gene expression changes are likely to be regulated by changes in the gene expression of transcription factors. As homeobox genes play important roles in embryonic development of the nervous system, it makes them candidates for a regulatory role in the process of regeneration. Here we show that the relative mRNA expression levels of Isl1 decreased shortly after crush, but those of DRG11, Lmx1b, and Pax3 did not change after crush. These data indicate that the developmental expression patterns of the homeobox genes studied here are not recapitulated during regeneration of the dorsal root ganglia neurons. We conclude that developmental gene expression programs controlled by these homeobox genes are not directly involved in sciatic nerve regeneration.

[The 2004 Nobel Prize for Physiology or Medicine for research into smell receptors and the organization of the olfactory system]. / Nobelprijs Fysiologie of Geneeskunde 2004 voor de ontdekking van geurreceptoren en de organisatie van het reuksysteem.

The 2004 Nobel Prize for Physiology or Medicine has been awarded to Richard Axel and Linda B. Buck, for their discovery of smell receptors and the organisation of the olfactory system. Their original discovery concerned the identification of some 1000 genes that code for smell receptors in the olfactory epithelium of the rat. They also demonstrated that each receptor can only be activated by a limited number of odourants and that there is some overlap in specificity with other smell receptors. Odourants in inhaled air are specifically recognized and bound by the smell receptors on the olfactory neurones in the nasal epithelium. The activated neurones send an electrical signal to the mitral cells, the dendrites of which lie in the glomeruli of the olfactory bulb. In each olfactory neuron only one smell receptor gene is expressed. Neurones with the same type of receptor are spread throughout the epithelium but converge in the same glomerulus. An olfactory map is formed by means of mitral-cell projections which run to the cerebral cortex as well as to other parts of the brain. Possibly the information gained about odourants will be applied in the areas of physiology and pathophysiology; in the field of pharmacology for example where odourants may be used in the treatment of disorders of fertility, behaviour or mood.

The Hormone Domain of the Vasopressin Prohormone is Required for the Correct Prohormone Trafficking Through the Secretory Pathway.

It has long been known that under intracellular conditions vasopressin associates tightly to neurophysin, which is present in the same prohormone. As the association has been suggested to play a role during hormone biosynthesis, its role was studied in a cellular context by expressing mutant vasopressin precursors in Neuro2A cells. Mutant vasopressin precursors, in which the association between the vasopressin and neurophysin domains was prevented either by deleting the vasopressin domain from the precursor or by substitution of the essential Tyr2 residue in vasopressin for Gly, were neither processed nor targeted into secretory granules. Rather, both provasopressin mutants were retained in the endoplasmic reticulum. Our results demonstrate that the vasopressin domain is crucial for correct trafficking of the prohormone through the secretory pathway, and suggest that vasopressin-neurophysin association provides correct prohormone folding in the endoplasmic reticulum.

ERK1/2 activation in rat ventral tegmental area by the mu-opioid agonist fentanyl: an in vitro study.

Opioid receptors in the ventral tegmental area, predominantly the mu-opioid receptors, have been suggested to modulate reinforcement sensitivity for both opioid and non-opioid drugs of abuse. The present study was conducted to study signal transduction proteins, which may mediate the functioning of mu-opioid receptors in the neurons of the ventral tegmental area. Therefore, brain slices of the ventral tegmental area were exposed in vitro to the specific mu-opioid agonist fentanyl and immunohistochemically stained for four different activated proteins using phospho-specific antibodies. Fentanyl dose-dependently activated extracellular signal-regulated protein in brain slices of the ventral tegmental area. This activation was reversible with naloxone. Furthermore, naloxone itself also activated extracellular signal-regulated protein kinase. Under the present conditions fentanyl did not affect extracellular signal-regulated protein kinase 1 and 2, Stat and cyclic AMP-response element-binding protein activity. The direct activation of extracellular signal-regulated protein kinase in ventral tegmental area slices by the mu-opioid agonist fentanyl may suggest a role of extracellular signal-regulated protein kinase in reward processes.

Survey for paired-like homeodomain gene expression in the hypothalamus: restricted expression patterns of Rx, Alx4 and goosecoid.

Homeobox genes are important regulators of cellular identity. Several homeobox genes are known to be specifically expressed in subsets of neurons in the forebrain, exclusively, or in distinct combinations. In this study, we explored the expression of homeobox genes in the forebrain of the adult rat by a degenerate polymerase chain reaction cloning strategy. We identified the expression of 12 homeobox genes, several of which display a remarkable restricted expression pattern in the adult brain. We demonstrated the expression of goosecoid in a very small set of neurons in the hypothalamus. By using Otp as a marker, these goosecoid-positive cells were found to constitute a small area just beside the paraventricular nucleus. Furthermore, we found expression of Rx in the pineal gland, along with Alx4. Rx was additionally found in the posterior pituitary and in cells aligning the bottom of the third ventricle. These findings form a starting point to reveal functions of the described homeobox genes in the forebrain.

The homeobox genes Lhx7 and Gbx1 are expressed in the basal forebrain cholinergic system.

The specific combination of homeobox genes is proposed to be decisive in the terminal differentiation of neuronal systems. In order to identify combined expression of homeobox genes in the ventral forebrain, a reverse transcriptase-polymerase chain reaction strategy using degenerated primers was employed. We identified, amongst others, Lhx7 and Gbx1, displaying a marked overlapping expression in septal and pallidal areas. Gbx1 and Lhx7 were both expressed in those adult brain nuclei that collectively form the basal forebrain cholinergic system, a prime target of neurodegeneration in Alzheimer's disease. Indeed, we detected Lhx7 within cholinergic neurons, whereas the related Lhx6 gene was found in adjacent neurons. From these data we suggest that combined expression of Lhx7 and Gbx1 plays a role in the development of the cholinergic system of the basal forebrain. It is speculated that both genes remain participating in molecular processes in the adult cholinergic neurons, and can be employed to study regulation and survival of these neurons under normal and pathological conditions.