Selective dopamine D2 receptor deletion from Nkx6.2 expressing cells causes impaired cognitive, motivation and anxiety phenotypes in mice

Abnormal dopamine neurotransmission is a common trait of some psychiatric diseases, like schizophrenia or bipolar disorder. Excessive dopaminergic tone in subcortical brain regions is associated with psychotic episodes, while reduced prefrontal dopaminergic activity is associated with impaired cognitive performance and reduced motivation, among other symptoms. Inhibitory interneurons expressing the calcium binding protein parvalbumin are particularly affected in both schizophrenia and bipolar disorder, as they set a fine-tuned physiological inhibitory/excitatory balance. Parvalbumin and somatostatin interneuron subtypes, are born from the medial ganglionic eminence and require the sequential expression of specific transcription factors for their specification, such as Nkx6.2. Here, we aimed at characterizing in detail interneuron subtypes derived from Nkx6.2 expressing progenitors by the generation of an Nkx6.2 Cre transgenic mouse line. We show that Nkx6.2 specifies over a third part of the total population of cortical somatostatin interneurons, preferentially at early developmental time points, whereas at late developmental stages, Nkx6.2 expressing progenitors shift to parvalbumin interneuron specification. Dopamine D2 receptor deletion from Nkx6.2 expressing progenitors causes abnormal phenotypes restricted to cognitive, motivation and anxiety domains. Our results show that Nkx6.2 have the potential to specify both somatostatin and parvalbumin interneurons in an opposite timed program and that DRD2 expression is required in Nkx6.2 expressing progenitors to avoid impaired phenotypes commonly associated to the pathophysiology of psychiatric diseases.

Kinesin-1-mediated axonal transport of CB1 receptors is required for cannabinoid-dependent axonal growth and guidance

Endocannabinoids (eCB) modulate growth cone dynamics and axonal pathfinding through the stimulation of cannabinoid type-1 receptors (CB1R), the function of which depends on their delivery and precise presentation at the growth cone surface. However, the mechanism involved in the axonal transport of CB1R and its transport role in eCB signaling remains elusive. As mutations in the kinesin-1 molecular motor have been identified in patients with abnormal cortical development and impaired white matter integrity, we studied the defects in axonal pathfinding and fasciculation in mice lacking the kinesin light chain 1 (Klc1-/-) subunit of kinesin-1. Reduced levels of CB1R were found in corticofugal projections and axonal growth cones in Klc1-/- mice. By live-cell imaging of CB1R-eGFP we characterized the axonal transport of CB1R vesicles and described the defects in transport that arise after KLC1 deletion. Cofilin activation, which is necessary for actin dynamics during growth cone remodeling, is impaired in the Klc1-/- cerebral cortex. In addition, Klc1-/- neurons showed expanded growth cones that were unresponsive to CB1R-induced axonal elongation. Together, our data reveal the relevance of kinesin-1 in CB1R axonal transport and in eCB signaling during brain wiring.

Impaired brain glucose metabolism and presynaptic dopaminergic functioning in a mouse model of schizophrenia

Background

Schizophrenia is a disease diagnosed by visible signs and symptoms from late adolescence to early adulthood. The etiology of this disease remains unknown. An objective diagnostic approach is required. Here, we used a mouse model that shows schizophrenia-like phenotypes to study brain glucose metabolism and presynaptic dopaminergic functioning by positron emission tomography (PET) and immunohistochemistry. PET scannings were performed on mice after the administration of [¹⁸F]-FDG or [¹⁸F]-F-DOPA. Glucose metabolism was evaluated in basal conditions and after the induction of a hyperdopaminergic state.

Results

Mutant animals show reduced glucose metabolism in prefrontal cortex, amygdala, and nucleus reuniens under the hyperdopaminergic state. They also show reduced [¹⁸F]-F-DOPA uptake in prefrontal cortex, substantia nigra reticulata, raphe nucleus, and ventral striatum but increased [¹⁸F]-F-DOPA uptake in dorsal striatum. Mutant animals also show reduced tyrosine hydroxylase expression on midbrain neurons.

Conclusions

Dopamine D2 mutant animals show reduced glucose metabolism and impaired presynaptic dopaminergic functioning, in line with reports from human studies. This mouse line may be a valuable model of schizophrenia, useful to test novel tracers for PET scanning diagnostic.

Convergent evolution of two mammalian neuronal enhancers by sequential exaptation of unrelated retroposons

The proopiomelanocortin gene (POMC) is expressed in a group of neurons present in the arcuate nucleus of the hypothalamus. Neuron-specific POMC expression in mammals is conveyed by two distal enhancers, named nPE1 and nPE2. Previous transgenic mouse studies showed that nPE1 and nPE2 independently drive reporter gene expression to POMC neurons. Here, we investigated the evolutionary mechanisms that shaped not one but two neuron-specific POMC enhancers and tested whether nPE1 and nPE2 drive identical or complementary spatiotemporal expression patterns. Sequence comparison among representative genomes of most vertebrate classes and mammalian orders showed that nPE1 is a placental novelty. Using in silico paleogenomics we found that nPE1 originated from the exaptation of a mammalian-apparent LTR retrotransposon sometime between the metatherian/eutherian split (147 Mya) and the placental mammal radiation (≈ 90 Mya). Thus, the evolutionary origin of nPE1 differs, in kind and time, from that previously demonstrated for nPE2, which was exapted from a CORE-short interspersed nucleotide element (SINE) retroposon before the origin of prototherians, 166 Mya. Transgenic mice expressing the fluorescent markers tomato and EGFP driven by nPE1 or nPE2, respectively, demonstrated coexpression of both reporter genes along the entire arcuate nucleus. The onset of reporter gene expression guided by nPE1 and nPE2 was also identical and coincidental with the onset of Pomc expression in the presumptive mouse diencephalon. Thus, the independent exaptation of two unrelated retroposons into functional analogs regulating neuronal POMC expression constitutes an authentic example of convergent molecular evolution of cell-specific enhancers.

Generation of interneuron diversity in the mouse cerebral cortex

Gamma-aminobutyric acid-containing (GABAergic) interneurons play an important role in the function of the cerebral cortex. Through mostly inhibitory mechanisms, interneurons control hyperexcitability, and synchronize and shape the spatiotemporal dynamics of cortical activity underlying various brain functions. Their influence on cortical function is remarkably diverse, a reflection of the large variety of interneuronal populations that exist in the mammalian cortex. Research over the past few years has rapidly transformed our understanding of their mechanisms underlying the generation of different classes of interneurons. In this review, we summarize recent progress on this process, progress which holds the promise of providing a rational framework for their classification, as well as means to understand their role in cortical processing.

Impaired sperm fertilizing ability in mice lacking Cysteine-RIch Secretory Protein 1 (CRISP1)

Mammalian fertilization is a complex multi-step process mediated by different molecules present on both gametes. Epididymal protein CRISP1, a member of the Cysteine-RIch Secretory Protein (CRISP) family, was identified by our laboratory and postulated to participate in both sperm-zona pellucida (ZP) interaction and gamete fusion by binding to egg-complementary sites. To elucidate the functional role of CRISP1 in vivo, we disrupted the Crisp1 gene and evaluated the effect on animal fertility and several sperm parameters. Male and female Crisp1(-/-) animals exhibited no differences in fertility compared to controls. Sperm motility and the ability to undergo a spontaneous or progesterone-induced acrosome reaction were neither affected in Crisp1(-/-) mice. However, the level of protein tyrosine phosphorylation during capacitation was clearly lower in mutant sperm than in controls. In vitro fertilization assays showed that Crisp1(-/-) sperm also exhibited a significantly reduced ability to penetrate both ZP-intact and ZP-free eggs. Moreover, when ZP-free eggs were simultaneously inseminated with Crisp1(+/+) and Crisp1(-/-) sperm in a competition assay, the mutant sperm exhibited a greater disadvantage in their fusion ability. Finally, the finding that the fusion ability of Crisp1(-/-) sperm was further inhibited by the presence of CRISP1 or CRISP2 during gamete co-incubation, supports that another CRISP cooperates with CRISP1 during fertilization and might compensate for its lack in the mutant mice. Together, these results indicate that CRISP proteins are players in the mammalian fertilization process. To our knowledge this is the first knockout mice generated for a CRISP protein. The information obtained might have important functional implications for other members of the widely distributed and evolutionarily conserved CRISP family.

Delineation of multiple subpallial progenitor domains by the combinatorial expression of transcriptional codes

The mammalian telencephalon is considered the most complex of all biological structures. It comprises a large number of functionally and morphologically distinct types of neurons that coordinately control most aspects of cognition and behavior. The subpallium, for example, not only gives rise to multiple neuronal types that form the basal ganglia and parts of the amygdala and septum but also is the origin of an astonishing diversity of cortical interneurons. Despite our detailed knowledge on the molecular, morphological, and physiological properties of most of these neuronal populations, the mechanisms underlying their generation are still poorly understood. Here, we comprehensively analyzed the expression patterns of several transcription factors in the ventricular zone of the developing subpallium in the mouse to generate a detailed molecular map of the different progenitor domains present in this region. Our study demonstrates that the ventricular zone of the mouse subpallium contains at least 18 domains that are uniquely defined by the combinatorial expression of several transcription factors. Furthermore, the results of microtransplantation experiments in vivo corroborate that anatomically defined regions of the mouse subpallium, such as the medial ganglionic eminence, can be subdivided into functionally distinct domains.

The dopamine D4 receptor is essential for hyperactivity and impaired behavioral inhibition in a mouse model of attention deficit/hyperactivity disorder

The dopamine D4 receptor (D4R) is a candidate gene for attention deficit/hyperactivity disorder (ADHD) based on genetic studies reporting that particular polymorphisms are present at a higher frequency in affected children. However, the direct participation of the D4R in the onset or progression of ADHD has not been tested. Here, we generated a mouse model with high face value to screen candidate genes for the clinical disorder by neonatal disruption of central dopaminergic pathways with 6-hydroxydopamine (6-OHDA). The lesioned mice exhibited hyperactivity that waned after puberty, paradoxical hypolocomotor responses to amphetamine and methylphenidate, poor behavioral inhibition in approach/avoidance conflict tests and deficits in continuously performed motor coordination tasks. To determine whether the D4R plays a role in these behavioral phenotypes, we performed 6-OHDA lesions in neonatal mice lacking D4Rs (Drd4(-/-)). Although striatal dopamine contents and tyrosine hydroxylase-positive midbrain neurons were reduced to the same extent in both genotypes, Drd4(-/-) mice lesioned with 6-OHDA did not develop hyperactivity. Similarly, the D4R antagonist PNU-101387G prevented hyperactivity in wild-type 6-OHDA-lesioned mice. Furthermore, wild-type mice lesioned with 6-OHDA showed an absence of behavioral inhibition when tested in the open field or the elevated plus maze, while their Drd4(-/-) siblings exhibited normal avoidance for the unprotected areas of these mazes. Together, our results from a combination of genetic and pharmacological approaches demonstrate that D4R signaling is essential for the expression of juvenile hyperactivity and impaired behavioral inhibition, relevant features present in this ADHD-like mouse model.

Transgenic mice engineered to target Cre/loxP-mediated DNA recombination into catecholaminergic neurons

To introduce restricted DNA recombination events into catecholaminergic neurons using the Cre/loxP technology, we generated transgenic mice carrying the Cre recombinase gene driven by a 9 kb rat tyrosine hydroxylase (TH) promoter. Immunohistochemistry performed on transgenic mouse brain sections revealed a high number of cells expressing Cre in areas where TH is normally expressed, including the olfactory bulb, hypothalamic and midbrain dopaminergic neurons, and the locus coeruleus. Double immunohistochemistry and immunofluorescence indicated that colocalization of TH and Cre is greater than 80%. Cre expression was also found in TH-positive amacrine neurons of the retina, chromaffin cells of the adrenal medulla, and sympathetic ganglia. We crossbred TH-Cre mice with the floxed reporter strain Z/AP and observed efficient Cre-mediated recombination in all areas expressing TH, indicating that transgenic Cre is functional. Therefore, we have generated a valuable transgenic mouse strain to induce specific mutations of "floxed" genes in catecholaminergic neurons.

Absence of dopamine D4 receptors results in enhanced reactivity to unconditioned, but not conditioned, fear

The prefrontal cortex receives a major dopaminergic input from the ventral tegmental area, which plays an important role in the integration of neuronal signals influencing behavioural responses to stressful environmental stimuli. The dopamine D4 receptor (D4R) is expressed at highest levels in the prefrontal cortex and is the predominant D2-like receptor localized in this brain area. To investigate the functional significance of D4Rs in dopamine-mediated responses we have analysed a strain of mice lacking this receptor subtype (Drd4-/-). Wild-type and Drd4-/- mice were challenged in two different approach/avoidance conflict paradigms: the elevated plus maze and the light/dark preference exploration test. By these behavioural measures Drd4-/- mice showed heightened avoidance to the more fear-provoking areas of each maze as demonstrated by reduced exploration of the open arms of the plus maze and longer latencies to explore the illuminated compartment of the light/dark shuttle box. These exaggerated avoidance behaviours were further enhanced by an additional handling stress but completely prevented by anxiolytic agents such as the benzodiazepine midazolam and ethanol. Although Drd4-/- mice displayed heightened anxiety, they exhibited normal ethanol preference and consumption in a two-bottle choice test. Learned fear responses evaluated by contextual, cued and instrumental fear-conditioning tests showed no difference between wild-type and Drd4-/- mice. Taken together these results indicate that the absence of D4Rs increases avoidance behaviour to unconditioned stimuli and does not impair behavioural reactions to Pavlovian fear-conditioning, suggesting that the D4R could play a key role in the dopaminergic modulation of cortical signals triggered by environmental stimuli.