Phenotypic and genetic analysis of the cerebellar mutant tmgc26, a new ENU-induced ROR-alpha allele

Melatonin and Melatonergic Influence on Neuronal Transcription Factors: Implications for the Development of Novel Therapies for Neurodegenerative Disorders

Melatonin is a multifunctional signalling molecule that is secreted by the mammalian pineal gland, and also found in a number of organisms including plants and bacteria. Research has continued to uncover an ever-increasing number of processes in which melatonin is known to play crucial roles in mammals. Amongst these functions is its contribution to cell multiplication, differentiation and survival in the brain. Experimental studies show that melatonin can achieve these functions by influencing transcription factors which control neuronal and glial gene expression. Since neuronal survival and differentiation are processes that are important determinants of the pathogenesis, course and outcome of neurodegenerative disorders; the known and potential influences of melatonin on neuronal and glial transcription factors are worthy of constant examination. In this review, relevant scientific literature on the role of melatonin in preventing or altering the course and outcome of neurodegenerative disorders, by focusing on melatonin's influence on transcription factors is examined. A number of transcription factors whose functions can be influenced by melatonin in neurodegenerative disease models have also been highlighted. Finally, the therapeutic implications of melatonin's influences have also been discussed and the potential limitations to its applications have been highlighted.

Intrauterine growth restriction compromises cerebellar development by affecting radial migration of granule cells via the JamC/Pard3a molecular pathway

Intrauterine growth restriction (IUGR) affects ~10% of human pregnancies, results in infants born small for gestational age (SGA), and is associated with motor and cognitive deficits. Human studies suggest that some deficits in SGA patients originate in the cerebellum, a major motor-coordination and cognitive center, but the underlying mechanisms remain unknown. To identify the cerebellar developmental program affected by IUGR, we analyzed the pig as a translational animal model in which some fetuses spontaneously develop IUGR due to early-onset chronic placental insufficiency. Similar to humans, SGA pigs revealed small cerebella, which contained fewer mature granule cells (GCs) in the internal granule cell layer (IGL). Surprisingly, newborn SGA pigs had increased proliferation of GC precursors in the external granule cell layer (EGL), which was associated with an increased density of Purkinje cells, known to non-autonomously promote the proliferation of GCs. However, the GCs of SGA pigs did not properly initiate exit from the EGL to IGL, which was associated with a decreased density of guiding Bergmann glial fibers, reduced expression of pro-migratory genes Pard3a, JamC and Sema6a, and increased apoptosis. While proliferation spontaneously normalized during postnatal development, accumulation of pre-migratory GCs and apoptosis in the EGL were long-lasting consequences of IUGR. Using organotypic cerebellar slice cultures, we showed that normalizing expression of Pard3a and JamC, which operate in the same molecular pathway in GCs, was sufficient to rescue both migratory and, at a later time point, apoptotic defects of IUGR. Thus, a decreased exit of GCs from the EGL, due to disrupted Pard3a/JamC radial migration initiation pathway, is a major mechanism of IUGR-related cerebellar pathology.

Preterm birth disrupts cerebellar development by affecting granule cell proliferation program and Bergmann glia

Preterm birth is a leading cause of long-term motor and cognitive deficits. Clinical studies suggest that some of these deficits result from disruption of cerebellar development, but the mechanisms that mediate cerebellar abnormalities in preterm infants are largely unknown. Furthermore, it remains unclear whether preterm birth and precocious exposure to the ex-utero environment directly disrupt cerebellar development or indirectly by increasing the probability of cerebellar injury, including that resulting from clinical interventions and protocols associated with the care of preterm infants. In this study, we analyzed the cerebellum of preterm pigs delivered via c-section at 91% term and raised for 10 days, until term-equivalent age. The pigs did not receive any treatments known or suspected to affect cerebellar development and had no evidence of brain damage. Term pigs sacrificed at birth were used as controls. Immunohistochemical analysis revealed that preterm birth did not affect either size or numbers of Purkinje cells or molecular layer interneurons at term-equivalent age. The number of granule cell precursors and Bergmann glial fibers, however, were reduced in preterm pigs. Preterm pigs had reduced proliferation but not differentiation of granule cells. qRT-PCR analysis of laser capture microdissected external granule cell layer showed that preterm pigs had a reduced expression of Ccnd1 (Cyclin D1), Ccnb1 (Cyclin B1), granule cell master regulatory transcription factor Atoh1, and signaling molecule Jag1. In vitro rescue experiments identified Jag1 as a central granule cell gene affected by preterm birth. Thus, preterm birth and precocious exposure to the ex-utero environment disrupt cerebellum by modulating expression of key cerebellar developmental genes, predominantly affecting development of granule precursors and Bergmann glia.

Loss of Ptf1a Leads to a Widespread Cell-Fate Misspecification in the Brainstem, Affecting the Development of Somatosensory and Viscerosensory Nuclei

The brainstem contains diverse neuronal populations that regulate a wide range of processes vital to the organism. Proper cell-fate specification decisions are critical to achieve neuronal diversity in the CNS, but the mechanisms regulating cell-fate specification in the developing brainstem are poorly understood. Previously, it has been shown that basic helix-loop-helix transcription factor Ptf1a is required for the differentiation and survival of neurons of the inferior olivary and cochlear brainstem nuclei, which contribute to motor coordination and sound processing, respectively. In this study, we show that the loss of Ptf1a compromises the development of the nucleus of the solitary tract, which processes viscerosensory information, and the spinal and principal trigeminal nuclei, which integrate somatosensory information of the face. Combining genetic fate-mapping, birth-dating, and gene expression studies, we found that at least a subset of brainstem abnormalities in Ptf1a(-/-) mice are mediated by a dramatic cell-fate misspecification in rhombomeres 2-7, which results in the production of supernumerary viscerosensory and somatosensory neurons of the Lmx1b lineage at the expense of Pax2(+) GABAergic viscerosensory and somatosensory neurons, and inferior olivary neurons. Our data identify Ptf1a as a major regulator of cell-fate specification decisions in the developing brainstem, and as a previously unrecognized developmental regulator of both viscerosensory and somatosensory brainstem nuclei.

SIGNIFICANCE STATEMENT

Cell-fate specification decisions are critical for normal CNS development. Although extensively studied in the cerebellum and spinal cord, the mechanisms mediating cell-fate decisions in the brainstem, which regulates a wide range of processes vital to the organism, remain largely unknown. Here we identified mouse Ptf1a as a novel regulator of cell-fate decisions during both early and late brainstem neurogenesis, which are critical for proper development of several major classes of brainstem cells, including neurons of the somatosensory and viscerosensory nuclei. Since loss-of-function PTF1A mutations were described in human patients, we suggest Ptf1a-dependent cell-fate misspecification as a novel mechanism of human brainstem pathology.

Characterization of Highper, an ENU-induced mouse mutant with abnormal psychostimulant and stress responses

RATIONALE

Chemical mutagenesis in the mouse is a forward genetics approach that introduces random mutations into the genome, thereby providing an opportunity to annotate gene function and characterize phenotypes that have not been previously linked to a given gene.

OBJECTIVES

We report on the behavioral characterization of Highper, an N-ethyl-N-nitrosourea (ENU)-induced mutant mouse line.

METHODS

Highper and B6 control mice were assessed for locomotor activity in the open field and home cage environments. Basal and acute restraint stress-induced corticosterone levels were measured. Mice were tested for locomotor response to cocaine (5, 20, 30, and 45 mg/kg), methylphenidate (30 mg/kg), and ethanol (0.75, 1.25, and 1.75 g/kg). The rewarding and reinforcing effects of cocaine were assessed using conditioned place preference and self-administration paradigms.

RESULTS

Highper mice are hyperactive during behavioral tests but show normal home cage locomotor behavior. Highper mice also exhibit a twofold increase in locomotor response to cocaine, methylphenidate, and ethanol and prolonged activation of the hypothalamic-pituitary-adrenal axis in response to acute stress. Highper mice are more sensitive to the rewarding and reinforcing effects of cocaine, although place preference in Highper mice appears to be significantly influenced by the environment in which the drug is administered.

CONCLUSIONS

Altogether, our findings indicate that Highper mice may provide important insights into the genetic, molecular, and biological mechanisms underlying stress and the drug reward pathway.

New insights into behaviour using mouse ENU mutagenesis

Identifying genes involved in behavioural disorders in man is a challenge as the cause is often multigenic and the phenotype is modulated by environmental cues. Mouse mutants are a valuable tool for identifying novel pathways underlying specific neurological phenotypes and exploring the influence both genetic and non-genetic factors. Many human variants causing behavioural disorders are not gene deletions but changes in levels of expression or activity of a gene product; consequently, large-scale mouse ENU mutagenesis has the advantage over the study of null mutants in that it generates a range of point mutations that frequently mirror the subtlety and heterogeneity of human genetic lesions. ENU mutants have provided novel and clinically relevant functional information on genes that influence many aspects of mammalian behaviour, from neuropsychiatric endophenotypes to circadian rhythms. This review will highlight some of the most important findings that have been made using this method in several key areas of neurological disease research.

Forward genetic approaches to understanding complex behaviors

Assigning function to genes has long been a focus of biomedical research.Even with complete knowledge of the genomic sequences of humans, mice and other experimental organisms, there is still much to be learned about gene function and control. Ablation or overexpression of single genes using knockout or transgenic technologies has provided functional annotation for many genes, but these technologies do not capture the extensive genetic variation present in existing experimental mouse populations. Researchers have only recently begun to truly appreciate naturally occurring genetic variation resulting from single nucleotide substitutions,insertions, deletions, copy number variation, epigenetic changes (DNA methylation,histone modifications, etc.) and gene expression differences and how this variation contributes to complex phenotypes. In this chapter, we will discuss the benefits and limitations of different forward genetic approaches that capture the genetic variation present in inbred mouse strains and present the utility of these approaches for mapping QTL that influence complex behavioral phenotypes.