Motor dysfunction and cerebellar Purkinje cell firing impairment in Ebf2 null mice

Maintenance of neuronal identity in C. elegans and beyond: Lessons from transcription and chromatin factors

Neurons are remarkably long-lived, non-dividing cells that must maintain their functional features (e.g., electrical properties, chemical signaling) for extended periods of time - decades in humans. How neurons accomplish this incredible feat is poorly understood. Here, we review recent advances, primarily in the nematode C. elegans, that have enhanced our understanding of the molecular mechanisms that enable post-mitotic neurons to maintain their functionality across different life stages. We begin with "terminal selectors" - transcription factors necessary for the establishment and maintenance of neuronal identity. We highlight new findings on five terminal selectors (CHE-1 [Glass], UNC-3 [Collier/Ebf1-4], LIN-39 [Scr/Dfd/Hox4-5], UNC-86 [Acj6/Brn3a-c], AST-1 [Etv1/ER81]) from different transcription factor families (ZNF, COE, HOX, POU, ETS). We compare the functions of these factors in specific neuron types of C. elegans with the actions of their orthologs in other invertebrate (D. melanogaster) and vertebrate (M. musculus) systems, highlighting remarkable functional conservation. Finally, we reflect on recent findings implicating chromatin-modifying proteins, such as histone methyltransferases and Polycomb proteins, in the control of neuronal terminal identity. Altogether, these new studies on transcription factors and chromatin modifiers not only shed light on the fundamental problem of neuronal identity maintenance, but also outline mechanistic principles of gene regulation that may operate in other long-lived, post-mitotic cell types.

Deficits in Cerebellum-Dependent Learning and Cerebellar Morphology in Male and Female BTBR Autism Model Mice

Recently, there has been increased interest in the role of the cerebellum in autism spectrum disorder (ASD). To better understand the pathophysiological role of the cerebellum in ASD, it is necessary to have a variety of mouse models that have face validity for cerebellar disruption in humans. Here, we add to the literature on the cerebellum in mouse models of autism with the characterization of the cerebellum in the idiopathic BTBR T + Itpr3tf/J (BTBR) inbred mouse strain, which has behavioral phenotypes that are reminiscent of ASD in patients. When we examined both male and female BTBR mice in comparison to C57BL/6J (C57) controls, we noted that both sexes of BTBR mice showed motor coordination deficits characteristic of cerebellar dysfunction, but only the male mice showed differences in delay eyeblink conditioning, a cerebellum-dependent learning task that is known to be disrupted in ASD patients. Both male and female BTBR mice showed considerable expansion of, and abnormal foliation in, the cerebellum vermis—including a significant expansion of specific lobules in the anterior cerebellum. In addition, we found a slight but significant decrease in Purkinje cell density in both male and female BTBR mice, irrespective of the lobule. Finally, there was a marked reduction of Purkinje cell dendritic spine density in both male and female BTBR mice. These findings suggest that, for the most part, the BTBR mouse model phenocopies many of the characteristics of the subpopulation of ASD patients that have a hypertrophic cerebellum. We discuss the significance of strain differences in the cerebellum as well as the importance of this first effort to identify both similarities and differences between male and female BTBR mice with regard to the cerebellum.

NURR1-deficient mice have age- and sex-specific behavioral phenotypes

The transcription factor NURR1 is essential to the generation and maintenance of midbrain dopaminergic (mDA) neurons and its deregulation is involved in the development of dopamine (DA)‐associated brain disorders, such as Parkinson's disease (PD). The old male NURR1 heterozygous knockout (NURR1‐KO) mouse has been proposed as a model of PD due to its altered motor performance that was, however, not confirmed in a subsequent study. Based on these controversial results, we explored the effects of the NURR1 deficiency on locomotor activity, motor coordination, brain and plasma DA levels, blood pressure and heart rate of old mice, also focusing on the potential effect of sex. As a probable consequence of the role of NURR1 in DA pathway, we observed that the old NURR1‐KO mouse is characterized by motor impairment, and increased brain DA level and heart rate, independently from sex. However, we also observed an alteration in spontaneous locomotor activity that only affects males. In conclusion, NURR1 deficiency triggers sex‐ and age‐specific alterations of behavioral responses, of DA levels and cardiovascular abnormalities. Further studies in simplified systems will be necessary to dissect the mechanism underlying these observations.

The effect of background strain on the behavioral phenotypes of the MDGA2+/- mouse model of autism spectrum disorder

The membrane-associated mucin (MAM) domain containing glycosylphosphatidylinositol anchor 2 protein single knock-out mice (MDGA2^+/- ) are models of ASD. We examined the behavioral phenotypes of male and female MDGA2^+/- and wildtype mice on C57BL6/NJ and C57BL6/N backgrounds at 2 months of age and measured MDGA2, neuroligin 1 and neuroligin 2 levels at 7 months. Mice on the C57BL6/NJ background performed better than those on the C57BL6/N background in visual ability and in learning and memory performance in the Morris water maze and differed in measures of motor behavior and anxiety. Mice with the MDGA2^+/- genotype differed from WT mice in motor, social and repetitive behavior and anxiety, but most of these effects involved interactions between MDGA2^+/- genotype and background strain. The background strain also influenced MDGA2 levels and NLGN2 association in MDGA2^+/- mice. Our findings emphasize the importance of the background strain used in studies of genetically modified mice.

NURR1 deficiency is associated to ADHD-like phenotypes in mice

The transcription factor NURR1 regulates the dopamine (DA) signaling pathway and exerts a critical role in the development of midbrain dopaminergic neurons (mDA). NURR1 alterations have been linked to DA-associated brain disorders, such as Parkinson's disease and schizophrenia. However, the association between NURR1 defects and the attention-deficit hyperactivity disorder (ADHD), a DA-associated brain disease characterized by hyperactivity, impulsivity and inattention, has never been demonstrated. To date, a comprehensive murine model of ADHD truly reflecting the whole complex human psychiatric disorder still does not exist. NURR1-knockout (NURR1-KO) mice have been reported to exhibit increased spontaneous locomotor activity, but their complete characterization is still lacking. In the present study a wide-ranging test battery was used to perform a comprehensive analysis of the behavioral phenotype of the male NURR1-KO mice. As a result, their hyperactive phenotype was confirmed, while their impulsive behavior was reported for the first time. On the other hand, no anxiety and alterations in motor coordination, sociability and memory were observed. Also, the number of mDA expressing tyrosine hydroxylase, a rate-limiting enzyme of catecholamines biosynthesis, and DA level in brain were not impaired in NURR1-KO mice. Finally, hyperactivity has been shown to be recovered by treatment with methylphenidate, the first line psychostimulant drug used for ADHD. Overall, our study suggests that the NURR1 deficient male mouse may be a satisfactory model to study some ADHD behavioral phenotypes and to test the clinical efficacy of potential therapeutic agents.

An ancient role for collier/Olf/Ebf (COE)-type transcription factors in axial motor neuron development

BACKGROUND

Mammalian motor circuits display remarkable cellular diversity with hundreds of motor neuron (MN) subtypes innervating hundreds of different muscles. Extensive research on limb muscle-innervating MNs has begun to elucidate the genetic programs that control animal locomotion. In striking contrast, the molecular mechanisms underlying the development of axial muscle-innervating MNs, which control breathing and spinal alignment, are poorly studied.

METHODS

Our previous studies indicated that the function of the Collier/Olf/Ebf (COE) family of transcription factors (TFs) in axial MN development may be conserved from nematodes to simple chordates. Here, we examine the expression pattern of all four mouse COE family members (mEbf1-mEbf4) in spinal MNs and employ genetic approaches in both nematodes and mice to investigate their function in axial MN development.

RESULTS

We report that mEbf1 and mEbf2 are expressed in distinct MN clusters (termed "columns") that innervate different axial muscles. Mouse Ebf1 is expressed in MNs of the hypaxial motor column (HMC), which is necessary for breathing, while mEbf2 is expressed in MNs of the medial motor column (MMC) that control spinal alignment. Our characterization of Ebf2 knock-out mice uncovered a requirement for Ebf2 in the differentiation program of a subset of MMC MNs and revealed for the first time molecular diversity within MMC neurons. Intriguingly, transgenic expression of mEbf1 or mEbf2 can rescue axial MN differentiation and locomotory defects in nematodes (Caenorhabditis elegans) lacking unc-3, the sole C. elegans ortholog of the COE family, suggesting functional conservation among mEbf1, mEbf2 and nematode UNC-3.

CONCLUSIONS

These findings support the hypothesis that genetic programs controlling axial MN development are deeply conserved across species, and further advance our understanding of such programs by revealing an essential role for Ebf2 in mouse axial MNs. Because human mutations in COE orthologs lead to neurodevelopmental disorders characterized by motor developmental delay, our findings may advance our understanding of these human conditions.

Mice harbouring a SCA28 patient mutation in AFG3L2 develop late-onset ataxia associated with enhanced mitochondrial proteotoxicity

Spinocerebellar ataxia 28 is an autosomal dominant neurodegenerative disorder caused by missense mutations affecting the proteolytic domain of AFG3L2, a major component of the mitochondrial m-AAA protease. However, little is known of the underlying pathogenetic mechanisms or how to treat patients with SCA28. Currently available Afg3l2 mutant mice harbour deletions that lead to severe, early-onset neurological phenotypes that do not faithfully reproduce the late-onset and slowly progressing SCA28 phenotype. Here we describe production and detailed analysis of a new knock-in murine model harbouring an Afg3l2 allele carrying the p.Met665Arg patient-derived mutation. Heterozygous mutant mice developed normally but adult mice showed signs of cerebellar ataxia detectable by beam test. Although cerebellar pathology was negative, electrophysiological analysis showed a trend towards increased spontaneous firing in Purkinje cells from heterozygous mutants with respect to wild-type controls. As homozygous mutants died perinatally with evidence of cardiac atrophy, for each genotype we generated mouse embryonic fibroblasts (MEFs) to investigate mitochondrial function. MEFs from mutant mice showed altered mitochondrial bioenergetics, with decreased basal oxygen consumption rate, ATP synthesis and mitochondrial membrane potential. Mitochondrial network formation and morphology was altered, with greatly reduced expression of fusogenic Opa1 isoforms. Mitochondrial alterations were also detected in cerebella of 18-month-old heterozygous mutants and may be a hallmark of disease. Pharmacological inhibition of de novo mitochondrial protein translation with chloramphenicol caused reversal of mitochondrial morphology in homozygous mutant MEFs, supporting the relevance of mitochondrial proteotoxicity for SCA28 pathogenesis and therapy development.

Cognitive function is preserved in aged mice following long-term β-hydroxy β-methylbutyrate supplementation

β-hydroxy β-methylbutyrate (HMB) is a nutritional supplement purported to enhance skeletal muscle mass and strength, as well as cognitive function in older adults. The purpose of this study was to determine the potential for long-term HMB supplementation to preserve muscle function and cognition in aged mice, as well as provide evidence of a link between vessel-associated pericyte function and outcomes. Four- (Adult/Ad) and 17 month-old (Aged/Ag) C57BL/6J mice consumed chow containing 600 mg/kg BW/day of either Ca-HMB (Ad, n=16; Ag, n=17) or Ca-Lactate (Ad, n=16; Ag, n=17) for 6 months. HMB did not prevent age-related reductions in muscle mass, strength and coordination (Age main effect, P<0.05). The rate of muscle protein synthesis decreased within the mitochondrial fraction (age main effect, P<0.05), and this decline was not prevented with HMB. Despite no change in muscle mass or function, an age-dependent reduction in active avoidance learning was attenuated with HMB (Age and HMB main effects, P<0.05). Age detrimentally impacted muscle-resident pericyte gene expression with no recovery observed with HMB, whereas no changes in brain-resident pericyte quantity or function were observed with age or HMB. The findings from this study suggest that prolonged HMB supplementation starting in adulthood may preserve cognition with age.

Purkinje Cell Signaling Deficits in Animal Models of Ataxia

Purkinje cell (PC) dysfunction or degeneration is the most frequent finding in animal models with ataxic symptoms. Mutations affecting intrinsic membrane properties can lead to ataxia by altering the firing rate of PCs or their firing pattern. However, the relationship between specific firing alterations and motor symptoms is not yet clear, and in some cases PC dysfunction precedes the onset of ataxic signs. Moreover, a great variety of ionic and synaptic mechanisms can affect PC signaling, resulting in different features of motor dysfunction. Mutations affecting Na⁺ channels (Na_V1.1, Na_V1.6, Na_Vβ4, Fgf14 or Rer1) reduce the firing rate of PCs, mainly via an impairment of the Na⁺ resurgent current. Mutations that reduce Kv3 currents limit the firing rate frequency range. Mutations of Kv1 channels act mainly on inhibitory interneurons, generating excessive GABAergic signaling onto PCs, resulting in episodic ataxia. Kv4.3 mutations are responsible for a complex syndrome with several neurologic dysfunctions including ataxia. Mutations of either Cav or BK channels have similar consequences, consisting in a disruption of the firing pattern of PCs, with loss of precision, leading to ataxia. Another category of pathogenic mechanisms of ataxia regards alterations of synaptic signals arriving at the PC. At the parallel fiber (PF)-PC synapse, mutations of glutamate delta-2 (GluD2) or its ligand Crbl1 are responsible for the loss of synaptic contacts, abolishment of long-term depression (LTD) and motor deficits. At the same synapse, a correct function of metabotropic glutamate receptor 1 (mGlu1) receptors is necessary to avoid ataxia. Failure of climbing fiber (CF) maturation and establishment of PC mono-innervation occurs in a great number of mutant mice, including mGlu1 and its transduction pathway, GluD2, semaphorins and their receptors. All these models have in common the alteration of PC output signals, due to a variety of mechanisms affecting incoming synaptic signals or the way they are processed by the repertoire of ionic channels responsible for intrinsic membrane properties. Although the PC is a final common pathway of ataxia, the link between specific firing alterations and neurologic symptoms has not yet been systematically studied and the alterations of the cerebellar contribution to motor signals are still unknown.

Motor Deficits and Cerebellar Atrophy in Elovl5 Knock Out Mice

Spino-Cerebellar-Ataxia type 38 (SCA38) is caused by missense mutations in the very long chain fatty acid elongase 5 gene, ELOVL5. The main clinical findings in this disease are ataxia, hyposmia and cerebellar atrophy. Mice in which Elovl5 has been knocked out represent a model of the loss of function hypothesis of SCA38. In agreement with this hypothesis, Elovl5 knock out mice reproduced the main symptoms of patients, motor deficits at the beam balance test and hyposmia. The cerebellar cortex of Elovl5 knock out mice showed a reduction of thickness of the molecular layer, already detectable at 6 months of age, confirmed at 12 and 18 months. The total perimeter length of the Purkinje cell (PC) layer was also reduced in Elovl5 knock out mice. Since Elovl5 transcripts are expressed by PCs, whose dendrites are a major component of the molecular layer, we hypothesized that an alteration of their dendrites might be responsible for the reduced thickness of this layer. Reconstruction of the dendritic tree of biocytin-filled PCs, followed by Sholl analysis, showed that the distribution of distal dendrites was significantly reduced in Elovl5 knock out mice. Dendritic spine density was conserved. These results suggest that Elovl5 knock out mice recapitulate SCA38 symptoms and that their cerebellar atrophy is due, at least in part, to a reduced extension of PC dendritic arborization.

In vivo detection of reduced Purkinje cell fibers with diffusion MRI tractography in children with autistic spectrum disorders

Postmortem neuropathology studies report reduced number and size of Purkinje cells (PC) in a majority of cerebellar specimens from persons diagnosed with autism spectrum disorders (ASD). We used diffusion weighted MRI tractography to investigate whether structural changes associated with reduced number and size of PC, could be detected in vivo by measuring streamlines connecting the posterior-lateral region of the cerebellar cortex to the dentate nucleus using an independent component analysis with a ball and stick model. Seed regions were identified in the cerebellar cortex, and streamlines were identified to two sorting regions, the dorsal dentate nucleus (DDN) and the ventral dentate nucleus (VDN), and probability of connection and measures of directional coherence for these streamlines were calculated. Tractography was performed in 14 typically developing children (TD) and 15 children with diagnoses of ASD. Decreased numbers of streamlines were found in the children with ASD in the pathway connecting cerebellar cortex to the right VDN (p-value = 0.015). Reduced fractional anisotropy (FA) values were observed in pathways connecting the cerebellar cortex to the right DDN (p-value = 0.008), the right VDN (p-value = 0.010) and left VDN (p-value = 0.020) in children with ASD compared to the TD group. In an analysis of single subjects, reduced FA in the pathway connecting cerebellar cortex to the right VDN was found in 73% of the children in the ASD group using a threshold of 3 standard errors of the TD group. The detection of diffusion changes in cerebellum may provide an in vivo biomarker of Purkinje cell pathology in children with ASD.