Hu-Bcl-2 transgenic mice with supernumerary neurons exhibit timing impairment in a complex motor task

Developmental Cell Death in the Cerebral Cortex

In spite of the high metabolic cost of cellular production, the brain contains only a fraction of the neurons generated during embryonic development. In the rodent cerebral cortex, a first wave of programmed cell death surges at embryonic stages and affects primarily progenitor cells. A second, larger wave unfolds during early postnatal development and ultimately determines the final number of cortical neurons. Programmed cell death in the developing cortex is particularly dependent on neuronal activity and unfolds in a cell-specific manner with precise temporal control. Pyramidal cells and interneurons adjust their numbers in sync, which is likely crucial for the establishment of balanced networks of excitatory and inhibitory neurons. In contrast, several other neuronal populations are almost completely eliminated through apoptosis during the first two weeks of postnatal development, highlighting the importance of programmed cell death in sculpting the mature cerebral cortex.

Cerebellar synapse properties and cerebellum-dependent motor and non-motor performance in Dp71-null mice

Recent emphasis has been placed on the role that cerebellar dysfunctions could have in the genesis of cognitive deficits in Duchenne muscular dystrophy (DMD). However, relevant genotype-phenotype analyses are missing to define whether cerebellar defects underlie the severe cases of intellectual deficiency that have been associated with genetic loss of the smallest product of the dmd gene, the Dp71 dystrophin. To determine for the first time whether Dp71 loss could affect cerebellar physiology and functions, we have used patch-clamp electrophysiological recordings in acute cerebellar slices and a cerebellum-dependent behavioral test battery addressing cerebellum-dependent motor and non-motor functions in Dp71-null transgenic mice. We found that Dp71 deficiency selectively enhances excitatory transmission at glutamatergic synapses formed by climbing fibers (CFs) on Purkinje neurons, but not at those formed by parallel fibers. Altered basal neurotransmission at CFs was associated with impairments in synaptic plasticity and clustering of the scaffolding postsynaptic density protein PSD-95. At the behavioral level, Dp71-null mice showed some improvements in motor coordination and were unimpaired for muscle force, static and dynamic equilibrium, motivation in high-motor demand and synchronization learning. Dp71-null mice displayed altered strategies in goal-oriented navigation tasks, however, suggesting a deficit in the cerebellum-dependent processing of the procedural components of spatial learning, which could contribute to the visuospatial deficits identified in this model. In all, the observed deficits suggest that Dp71 loss alters cerebellar synapse function and cerebellum-dependent navigation strategies without being detrimental for motor functions.

Summary: Dp71 is the most prominent dystrophin gene product in the adult brain. Here, multiple approaches including behavioral tests and electrophysiology are adopted to explore the role of Dp71 in the cerebellum.

Mature Purkinje cells require the retinoic acid-related orphan receptor-α (RORα) to maintain climbing fiber mono-innervation and other adult characteristics

Neuronal maturation during development is a multistep process regulated by transcription factors. The transcription factor RORα (retinoic acid-related orphan receptor α) is necessary for early Purkinje cell (PC) maturation but is also expressed throughout adulthood. To identify the role of RORα in mature PCs, we used Cre-lox mouse genetic tools in vivo that delete it specifically from PCs between postnatal days 10-21. Up to 14 d of age, differences between mutant and control PCs were not detectable: both were mono-innervated by climbing fibers (CFs) extending along their well-developed dendrites with spiny branchlets. By week 4, mutant mice were ataxic, some PCs had died, and remaining PC soma and dendrites were atrophic, with almost complete disappearance of spiny branchlets. The innervation pattern of surviving RORα-deleted PCs was abnormal with several immature characteristics. Notably, multiple functional CF innervation was reestablished on these mature PCs, simultaneously with the relocation of CF contacts to the PC soma and their stem dendrite. This morphological modification of CF contacts could be induced even later, using lentivirus-mediated depletion of rora from adult PCs. These data show that the late postnatal expression of RORα cell-autonomously regulates the maintenance of PC dendritic complexity, and the CF innervation status of the PC (dendritic vs somatic contacts, and mono-innervation vs multi-innervation). Thus, the differentiation state of adult neurons is under the control of transcription factors; and in their absence, adult neurons lose their mature characteristics and acquire some characteristics of an earlier developmental stage.

Control of cell number in the sexually dimorphic brain and spinal cord

The hormonal control of cell death is currently the best-established mechanism for creating sex differences in cell number in the brain and spinal cord. For example, males have more cells than do females in the principal nucleus of the bed nucleus of the stria terminalis (BNSTp) and spinal nucleus of the bulbocavernosus (SNB), whereas females have a cell number advantage in the anteroventral periventricular nucleus (AVPV). In each case, the difference in cell number in adulthood correlates with a sex difference in the number of dying cells at some point in development. Mice with over- or under-expression of cell death genes have been used to test more directly the contribution of cell death to neural sex differences, to identify molecular mechanisms involved, and to determine the behavioural consequences of suppressing developmental cell death. Bax is a pro-death gene of the Bcl-2 family that is singularly important for apoptosis in neural development. In mice lacking bax, the number of cells in the BNSTp, SNB and AVPV are significantly increased, and sex differences in total cell number in each of these regions are eliminated. Cells rescued by bax gene deletion in the BNSTp express markers of differentiated neurones and the androgen receptor. On the other hand, sex differences in other phenotypically identified populations, such as vasopressin-expressing neurones in the BNSTp or dopaminergic neurones in AVPV, are not affected by either bax deletion or bcl-2 over-expression. Possible mechanisms by which testosterone may regulate cell death in the nervous system are discussed, as are the behavioural effects of eliminating sex differences in neuronal cell number.

Deletion of the Bax gene disrupts sexual behavior and modestly impairs motor function in mice

Cell death is a nearly ubiquitous feature of the developing nervous system, and differential death in males and females contributes to several well studied sex differences in neuron number. Nonetheless, the functional importance of neuronal cell death has been subjected to few direct tests. Bax, a pro-apoptotic protein, is required for cell death in many neural regions. Deletion of the Bax gene in mice increases neuron number in several areas and eliminates sex differences in cell number in the brain and spinal cord. Here, sexual and motor behaviors were examined in Bax-/- mice and their wild-type siblings to test the functional consequences of preventing Bax-dependent cell death. Animals were gonadectomized in adulthood and provided with ovarian hormones or with testosterone for tests of feminine and masculine sexual behaviors, respectively. Wild-type mice exhibited a sex difference in feminine sexual behavior, with high lordosis scores in females and low scores in males. This sex difference was eliminated by Bax deletion, with very low receptivity exhibited by both male and female Bax-/- mice. Masculine sexual behavior was not sexually dimorphic among wild-type mice, but mounts and pelvic thrusts were nearly eliminated in Bax-/- mice of both sexes. Motor strength and performance at low speeds on a RotaRod apparatus did not differ by sex or Bax gene status. However, Bax-/- animals exhibited impairments on the RotaRod at higher speeds. Thus, developmental cell death may be required for masculine and feminine sexual behaviors and the fine tuning of motor coordination.

Neuromuscular development in the absence of programmed cell death: phenotypic alteration of motoneurons and muscle

The widespread, massive loss of developing neurons in the central and peripheral nervous system of birds and mammals is generally considered to be an evolutionary adaptation. However, until recently, models for testing both the immediate and long-term consequences of preventing this normal cell loss have not been available. We have taken advantage of several methods for preventing neuronal death in vivo to ask whether rescued neurons [e.g., motoneurons (MNs)] differentiate normally and become functionally incorporated into the nervous system. Although many aspects of MN differentiation occurred normally after the prevention of cell death (including the expression of several motoneuron-specific markers, axon projections into the ventral root and peripheral nerves, ultrastructure, dendritic arborization, and afferent axosomatic synapses), other features of the neuromuscular system (MNs and muscle) were abnormal. The cell bodies and axons of MNs were smaller than normal, many MN axons failed to become myelinated or to form functional synaptic contacts with target muscles, and a subpopulation of rescued cells were transformed from alpha- to gamma-like MNs. Additionally, after the rescue of MNs in myogenin glial cell line-derived neurotrophic factor (MyoGDNF) transgenic mice, myofiber differentiation of extrafusal skeletal muscle was transformed and muscle physiology and motor behaviors were abnormal. In contrast, extrafusal myofiber phenotype, muscle physiology, and (except for muscle strength tests) motor behaviors were all normal after the rescue of MNs by genetic deletion of the proapoptotic gene Bax. However, there was an increase in intrafusal muscle fibers (spindles) in Bax knock-out versus both wild-type and MyoGDNF mice. Together, these data indicate that after the prevention of MN death, the neuromuscular system becomes transformed in novel ways to compensate for the presence of the thousands of excess cells.

ADAPTIVE ROLES OF PROGRAMMED CELL DEATH DURING NERVOUS SYSTEM DEVELOPMENT

The programmed cell death (PCD) of developing cells is considered an essential adaptive process that evolved to serve diverse roles. We review the putative adaptive functions of PCD in the animal kingdom with a major focus on PCD in the developing nervous system. Considerable evidence is consistent with the role of PCD in events ranging from neurulation and synaptogenesis to the elimination of adult-generated CNS cells. The remarkable recent progress in our understanding of the genetic regulation of PCD has made it possible to perturb (inhibit) PCD and determine the possible repercussions for nervous system development and function. Although still in their infancy, these studies have so far revealed few striking behavioral or functional phenotypes.

Role of programmed cell death in normal neuronal development and function

The consequences of eliminating the process of programmed cell death during the development of the nervous system is examined by reviewing studies in the genetic model organisms Caenorhabditis elegans, Drosophila melanogaster, Danio rerio and Mus musculus, where mutations of cell death genes have eliminated or reduced programmed cell death in the nervous system. In many cases, genetic elimination of cell death leads to embryonic mortality or gross anatomical malformations; however, there are cases where animals develop normally but with excess neurons and glia in the nervous system. Undead cells either differentiate and function as working neurons, in some instances being of smaller size, or fail to differentiate and lack normal connections with their targets. Changes in motor control and sensory processing are generally not observed, except for during the most complex of behaviors. Examination of organisms where death genes have been genetically eliminated reveals that programmed cell death may play an important role in sculpting gross brain structure during early development of the neural tube. In contrast, the consequences of preventing neuronal cell death at later developmental stages (e.g. during vertebrate synapse formation) are just beginning to be understood.

Experimental basis for the putative role of GluR6/kainate glutamate receptor subunit in Huntington's disease natural history

Age of onset of Huntington's disease (HD) statistically correlates with the length of expanded CAG repeats in the IT15 gene. However, other factors such as polymorphism in the 3' untranslated region of the GluR6 kainate receptor gene subunit may contribute to variability in the age at onset. To investigate this issue, we studied the motor disorder and related striatal damage induced by 3-nitropropionic acid (3-NP) subacute administration in GluR6 knockout mice (GluR6(-/-)) as compared to wild-type mice. In two different age groups (6 months and 1 year), we observed that GluR6(-/-) mice did not display more motor impairment nor more striatal histopathological damage than GluR6(+/+) mice, although 1-year-old GluR6(-/-) mice displayed reduced activity parameters either at baseline or after 3-NP administration compared to GluR6(+/+). In both age groups, GluR6(-/-) mice died earlier and displayed earlier motor symptoms during 3-NP-induced metabolic compromise, suggesting that GluR6-containing kainate receptors may be implicated during neurodegeneration, such as in HD, rather than in the final outcome.

Transgenic mice expressing F3/contactin from the transient axonal glycoprotein promoter undergo developmentally regulated deficits of the cerebellar function

We have shown that transgenic transient axonal glycoprotein (TAG)/F3 mice, in which the mouse axonal glycoprotein F3/contactin was misexpressed from a regulatory region of the gene encoding the transient axonal glycoprotein TAG-1, exhibit a transient disruption of cerebellar granule and Purkinje cell development [Development 130 (2003) 29]. In the present study we explore the neurobehavioural consequences of this mutation. We report on assays of reproductive parameters (gestation length, litter size and offspring viability) and on somatic and neurobehavioural end-points (sensorimotor development, homing performance, motor activity, motor coordination and motor learning). Compared with wild-type littermates, TAG/F3 mice display delayed sensorimotor development, reduced exploratory activity and impaired motor activity, motor coordination and motor learning. The latter parameters, in particular, were affected also in adult mice, despite the apparent recovery of cerebellar morphology, suggesting that subtle changes of neuronal circuitry persist in these animals after development is complete. These behavioural deficits indicate that the finely coordinated expression of immunoglobulin-like cell adhesion molecules such as TAG-1 and F3/contactin is of key relevance to the functional, as well as morphological maturation of the cerebellum.

Cerebellar Purkinje cell loss in aging Hu-Bcl-2 transgenic mice

The number of cerebellar Purkinje cells is increased by over 40% in young transgenic mice that overexpress a human Bcl-2 transgene (Hu-Bcl-2). To determine whether the Bcl-2-mediated rescue of Purkinje cells persists through life, the numbers of Purkinje cells were estimated in 6-, 12-, 18-, and 24-month-old Hu-Bcl-2 transgenic mice and age-matched controls. In addition, the expression of four markers for Purkinje cell differentiation, calbindin (CaBP), the 67-kDa isoform of glutamic acid decarboxylase (GAD67), gamma-aminobutyric acid transaminase (GABA-T), and the NMDA-R1 receptor subtype (NMDA-NR1) was analyzed in 6-month-old Hu-Bcl-2 transgenics and controls to determine whether overexpression of Bcl-2 and rescue from naturally occurring cell death affects the normal differentiation of Purkinje cells. The estimates of Purkinje cell numbers showed that the number of Purkinje cells in the Hu-Bcl-2 transgenics declines after 6 months to approach wild-type values by 18 months. Although the exogenous human BCL-2 is still expressed in Purkinje cells at 24 months, the expression levels of human BCL-2 appear to decline significantly after 6 months, suggesting that survival of the supernumary Purkinje cells depends on the sustained overexpression of Bcl-2. All the Purkinje cells in the Hu-Bcl-2 transgenic mice appeared to express normal levels of the differentiation markers analyzed so there was no evidence for a class of Purkinje cells that do not differentiate normally when rescued from naturally occurring cell death.

Motor behaviour deficits and their histopathological and functional correlates in the nigrostriatal system of dopamine transporter knockout mice

Chronic dysregulation of dopamine homeostasis has been shown to induce behavioural impairment in dopamine transporter knockout mutant mice arising from the dysfunction of the mesolimbic and hypothalamo-infundibular system. Here, we assessed whether there are also any motor consequences of a chronic and constitutive hyperdopaminergia in the nigrostriatal system in dopamine transporter knockout mutant mice. For this, we analysed motor performances using tests assessing balance, coordinated motor skills (rotarod, pole test), stride lengths and locomotor activity. Dopamine transporter knockout mutant mice were markedly hyperactive in the open field with central compartment avoidance, as previously shown. However, sensorimotor integration was also found to be altered in dopamine transporter knockout mutant mice which displayed a reduced fore- and hind-limb mean stride length, impaired motor coordination on the pole test and reduced rearings in the open field. Moreover, dopamine transporter knockout mutant mice showed a slower task acquisition on the rotarod. Six-week-old dopamine transporter knockout wild type mice having the same femur size as adult dopamine transporter knockout mutant mice ruled out a possible size-effect bias. Whilst there was no significant difference in the striatal volume, we found a slight but significant reduction in neuronal density in the striatum but not in the nucleus accumbens of dopamine transporter knockout mutant mice. There was a reduced binding in the striatum and nucleus accumbens of dopamine(1) receptors ([(3)H]SCH 23390) and dopamine(2) receptors ([(3)H]YM-09151-2). There was no significant difference in the number of dopaminergic neurons in the substantia nigra between dopamine transporter knockout mutant mice and dopamine transporter knockout wild type mice. These results suggest an impaired functioning of the nigrostriatal system in dopamine transporter knockout mutant hyperdopaminergic mice, as illustrated by motor and sensorimotor integration deficits, despite their apparent hyperactivity. These dysfunctions may arise from combined striatal cell loss and/or functional changes of dopaminergic neurotransmission.

p53 inactivation leads to impaired motor synchronization in mice

We have combined genetic and pharmacological approaches to investigate the behavioural consequences of inactivation of the murine p53 protein. Our behavioural analysis revealed that p53-null mice (p53KO) exhibit a very specific and significant motor deficit in rapid walking synchronization. This deficit, observed using the rotarod test, was the only behavioural defect of p53KO mice. We demonstrated that it was not due to an increase in neuronal number or abnormal connectivity in the olivo-cerebellar system, thought to control motor synchronization. In order to test the role of p53 in the central nervous system, we injected a pharmacological inhibitor of p53 activation, pifithrin-alpha, into the cerebellum of wild-type mice. This treatment mimicked the walking synchronization deficit of p53KO mice, suggesting that presence of p53 protein in the cerebellum is necessary to execute this synchronization of walking. Our investigation reveals a functional role of cerebellar p53 protein in adult walking synchronization.

Subacute systemic 3-nitropropionic acid intoxication induces a distinct motor disorder in adult C57Bl/6 mice: behavioural and histopathological characterisation

Data on motor behavioural disorders induced by systemic 3-nitropropionic acid, an irreversible inhibitor of mitochondrial succinate dehydrogenase and their histopathological correlates in mice, are sparse. We thus further characterised the subacute 3-nitropropionic-acid-induced motor disorder and its time course in C57Bl/6 mice using standard behavioural tests, histopathological correlates and in vivo magnetic resonance imaging. Firstly, we studied two intoxication paradigms (340 and 560 mg 3-nitropropionic acid/kg, 7 days) compared to controls. The low-dose regimen induced only slight motor changes (reduced hindlimb stride length and rearing). The high-dose regimen induced significant (P<0.05) behavioural and sensorimotor integration deficits (pole test, rotarod, stride length, open-field spontaneous activity) but with 37.5% lethality at week one. The clinical motor disorder consisted of hindlimb clasping and dystonia, truncal dystonia, bradykinesia and impaired postural control. Histopathologically, there were discrete lesions of the dorsolateral striatum in 62.5% of mice together with a 32% reduction (P<0.0001) of the striatal volume, reduced caldbindin-D28K immunoreactivity in the lateral striatum, and met-enkephalin and substance P in the striatal output pathways. There was also a significant (P<0.05) 30-40% dopaminergic cell loss within the substantia nigra pars compacta. Secondly, we validated a semi-quantitative behavioural scale to describe the time course of the motor deficits and to predict the occurrence of striatal damage. We sought to determine whether it could also be disclosed in vivo by magnetic resonance imaging. The scale correlated with the striatal volume reduction (r(2)=0.57) and striatal cell loss (r(2)=0.87) but not with the loss of striatal dopaminergic terminals (dopamine transporter binding). Increased T2-signal intensity within the striatal lesion correlated with the cell loss (r(2)=0.66). We conclude that systemic administration of 3-nitropropionic acid in C57Bl/6 mice induces a distinct motor disorder and dose-dependent striatonigral damage, which are potentially useful to model human diseases of the basal ganglia.

Dopamine transporter knock-out mice are hypersensitive to 3-nitropropionic acid-induced striatal damage

Evidence suggests that dopamine is involved in the modulation of striatal excitotoxic processes. To further investigate this issue, we studied the effects of systemic 'low-dose' (total dose, 340 mg/kg in 7 days) 3-nitropropionic acid (3-NP) intoxication in dopamine transporter knock-out mice (DAT-/-) compared to wildtype (DAT+/+) mice. Systemic 'low-dose' 3-NP induced a significant impairment in a rotarod task only in DAT-/- mice. Histopathology also demonstrated a significant reduction of the striatal volume (-7%, P < 0.05), neuronal density (-12.5%, P < 0.001) and absolute number estimates of striatal neurons (-11.5%, P < 0.001) in DAT-/- compared to DAT+/+ mice, with increased glial activation, independent of the degree of succinate dehydrogenase inhibition. These findings strengthen the hypothesis for dopamine modulation of excitotoxicity within the nigrostriatal system.

To die or not to die, does it change the function? Behavior of transgenic mice reveals a role for developmental cell death

In humans, perturbations in the developmental neuronal death leading to an excess of neurons could be associated with developmental neuropsychiatric disorders. Hu-bcl-2 transgenic mice appear to be a valuable tool to study the functional role of developmental programmed cell death. Indeed, the over-expression of the anti-apoptotic gene bcl-2 decreases developmental neuronal death and Hu-bcl-2 mice present supernumerary neurons in several brain regions. A detailed behavioral analysis of these mice revealed selective deficits. Hu-bcl-2 mice have normal vision, general activity and motor skills. Only the most complex behavior like anxiety and learning abilities are impaired in these mice.

Transgenic mice with neuronal overexpression of bcl-2 gene present navigation disabilities in a water task

In the CNS, Bcl-2 is an antiapoptotic gene involved in the regulation of neuronal death. Transgenic mice overexpressing the human gene Bcl-2 (Hu-bcl-2 mice) showed delayed acquisition in two tasks requiring them to find a hidden platform starting from either a random or a constant starting location. The same mice were not deficient in another task requiring them to find a visible platform suggesting that the delay observed was not due to motor, visual or motivational deficits in the water. The delay observed in Hu-bcl-2 mice was more important in the random starting test in which the allocentric demand for navigation was stronger. The results suggested that allocentric navigation is particularly sensitive to abnormal CNS maturation following the overexpression of the bcl-2 gene. The specific deficits (motor learning, fear-related behavior and allocentric navigation) observed in Hu-bcl-2 mice suggest that the regulation of developmental neuronal death is crucial for multisensorial learning and emotional behavior.

Chasing fate and function of new neurons in adult brains

Neuron production, migration and differentiation are major developmental events that continue, on a smaller scale, into adult life in a wide range of species from insects to mammals. Recent reports of adult neurogenesis in primates, including humans, have led to explosive scientific and public attention. During the last two years, significant discoveries have revealed that the generation, recruitment and survival of new neurons in adult brains are governed by principles similar to those that shape the developing brain, such as neuronal death, sensory experience, activity levels, and learning. Similarly, many factors implicated in embryonic neurogenesis are increasingly found to regulate adult neurogenesis and survival as well. These findings now allow the first manipulations of the numbers of adult-generated neurons to address their potential behavioral function.