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

Absence of Stress Response in Dorsal Raphe Nucleus in Modulator of Apoptosis 1-Deficient Mice

Modulator of apoptosis 1 (MOAP-1) is a Bcl-2-associated X Protein (BAX)-associating protein that plays an important role in regulating apoptosis. It is highly enriched in the brain but its function in this organ remains unknown. Studies on BAX^-/- mice suggested that disruption of programmed cell death may lead to abnormal emotional states. We thus hypothesize that MOAP-1^-/- mice may also display stress-related behavioral differences and perhaps involved in stress responses in the brain and investigated if a depression-like trait exists in MOAP-1^-/- mice, and if so, whether it is age related, and how it relates to central serotonergic stress response in the dorsal raphe nucleus. Young MOAP-1^-/- mice exhibit depression-like behavior, in the form of increased immobility time when compared to age-matched wild-type mice in the forced swimming test, which is abolished by acute treatment of fluoxetine. This is supported by data from the tail suspension and sucrose preference tests. Repeated forced swimming stress causes an up-regulation of tryptophan hydroxylase 2 (TPH2) and a down-regulation of brain-derived neurotrophic factor (BDNF) in the dorsal raphe nucleus (DRN) in young wild-type (WT) control mice. In contrast, TPH2 up-regulation was not observed in aged WT mice. Interestingly, such a stress response appears absent in both young and aged MOAP-1^-/- mice. Aged MOAP-1^-/- and WT mice also have similar immobility times on the forced swimming test. These data suggest that MOAP-1 is required in the regulation of stress response in the DRN. Crosstalk between BDNF and 5-HT appears to play an important role in this stress response.

Animal models for bipolar disorder: from bedside to the cage

Bipolar disorder is characterized by recurrent manic and depressive episodes. Patients suffering from this disorder experience dramatic mood swings with a wide variety of typical behavioral facets, affecting overall activity, energy, sexual behavior, sense of self, self-esteem, circadian rhythm, cognition, and increased risk for suicide. Effective treatment options are limited and diagnosis can be complicated. To overcome these obstacles, a better understanding of the neurobiology underlying bipolar disorder is needed. Animal models can be useful tools in understanding brain mechanisms associated with certain behavior. The following review discusses several pathological aspects of humans suffering from bipolar disorder and compares these findings with insights obtained from several animal models mimicking diverse facets of its symptomatology. Various sections of the review concentrate on specific topics that are relevant in human patients, namely circadian rhythms, neurotransmitters, focusing on the dopaminergic system, stressful environment, and the immune system. We then explain how these areas have been manipulated to create animal models for the disorder. Even though several approaches have been conducted, there is still a lack of adequate animal models for bipolar disorder. Specifically, most animal models mimic only mania or depression and only a few include the cyclical nature of the human condition. Future studies could therefore focus on modeling both episodes in the same animal model to also have the possibility to investigate the switch from mania-like behavior to depressive-like behavior and vice versa. The use of viral tools and a focus on circadian rhythms and the immune system might make the creation of such animal models possible.

Hyperactivity and depression-like traits in Bax KO mice

The Bax gene is a member of the Bcl-2 gene family and its pro-apoptotic Bcl-associated X (Bax) protein is believed to be crucial in regulating apoptosis during neuronal development as well as following injury. With the advent of mouse genomics, mice lacking the pro-apoptotic Bax gene (Bax KO) have been extensively used to study how cell death helps to determine synaptic circuitry formation during neurodevelopment and disease. Surprisingly, in spite of its wide use and the association of programmed neuronal death with motor dysfunctions and depression, the effects of Bax deletion on mice spontaneous locomotor activity and depression-like traits are unknown. Here we examine the behavioral characteristics of Bax KO male mice using classical paradigms to evaluate spontaneous locomotor activity and depressive-like responses. In the open field, Bax KO animals exhibited greater locomotor activity than their control littermates. In the forced swimming test, Bax KO mice displayed greater immobility times, a behavior despair state, when compared to controls. Collectively, our findings corroborate the notion that a fine balance between cell survival and death early during development is critical for normal brain function later in life. Furthermore, it points out the importance of considering depressive-like and hyperactivity behavioral phenotypes when conducting neurodevelopmental and other studies using the Bax KO strain.

Domestication of the dog from the wolf was promoted by enhanced excitatory synaptic plasticity: a hypothesis

Dogs shared a much closer relationship with humans than any other domesticated animals, probably due to their unique social cognitive capabilities, which were hypothesized to be a by-product of selection for tameness toward humans. Here, we demonstrate that genes involved in glutamate metabolism, which account partially for fear response, indeed show the greatest population differentiation by whole-genome comparison of dogs and wolves. However, the changing direction of their expression supports a role in increasing excitatory synaptic plasticity in dogs rather than reducing fear response. Because synaptic plasticity are widely believed to be cellular correlates of learning and memory, this change may alter the learning and memory abilities of ancient scavenging wolves, weaken the fear reaction toward humans, and prompt the initial interspecific contact.

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.

Dexamethasone induces apoptosis in the developing rat amygdala in an age-, region-, and sex-specific manner

Exposure to glucocorticoids (GCs) in early development can lead to long-term changes in brain function and behavior, although little is known about the underlying neural mechanisms. Perinatal exposure to GCs alters adult anxiety and neuroendocrine responses to stress. Therefore, we investigated the effects of either late gestational or neonatal exposure to the GC receptor agonist dexamethasone (DEX), on apoptosis within the amygdala, a region critical for emotional regulation. DEX was administered to timed-pregnant rat dams from gestational day 18 until parturition, or postnatal day 4-6. Offspring were sacrificed the day following the last DEX treatment, and tissue was processed for immunohistochemical detection of cleaved caspase-3, a marker for apoptotic cells. Prenatal DEX treatment significantly increased the number of cleaved caspase-3-positive cells in the amygdala of both sexes, largely due to increases within the medial and basomedial subregions. Postnatal DEX treatment also increased cleaved caspase-3 immunoreactivity within the amygdala, although effects reached significance only in the central nucleus of females. Overall, DEX induction of cleaved caspase-3 in the amygdala was greater following prenatal compared with postnatal treatment, yet in both instances, elevations in cleaved caspase-3 correlated with an increase in pro-apoptotic Bax mRNA expression. Dual-label immunohistochemistry of cleaved caspase-3 and the neuronal marker NeuN confirmed that virtually all cleaved caspase-3-positive cells in the amygdala were neurons, and a subset of these cells (primarily following postnatal treatment) expressed a GABAergic calcium-binding protein phenotype (calbindin or calretinin). Together these results indicate that early developmental GC exposure induces neuronal apoptosis within the amygdala in an age-, sex-, and region-dependent manner.

Early detection of age-related memory deficits in individual mice

To date, no consensus has been reached concerning the age of the earliest onset of age-related cognitive deficits in rodents. Our aim was to develop a behavioral model allowing early and individual detection of age-related cognitive impairments. We tested young (3 months), middle-aged (10 months) and aged (17 months) C57Bl/6 mice in the starmaze, a task allowing precise analysis of the search pattern of mice via standardized calculation of two navigation indices. We performed mouse-per-mouse analyses and compared each mouse's performance to a threshold based on young mice's performances. Using this method we identified impaired mice from the age of 10 months old. Their deficits were independent of any sensorimotor dysfunctions and were associated with an alteration of the maintenance of the hippocampal CA1 late-LTP. This study develops reliable methodology for early detection of age-related memory disorders and provides evidence that memory can decline in some individuals as early as from the age of 10 months.

NMDA receptors promote survival in somatosensory relay nuclei by inhibiting Bax-dependent developmental cell death

Naturally occurring cell death is a universal feature of developing nervous systems that plays an essential role in determining adult brain function. Yet little is known about the decisions that select a subset of CNS neurons for survival and cause others to die. We report that postnatal day 0 NMDA receptor subunit 1 (NMDAR1) knockout mice display an approximately 2-fold increase in cell death in the brainstem trigeminal complex (BSTC), including all four nuclei that receive somatosensory inputs from the face (principalis, oralis, interpolaris, and caudalis). Treatment with the NMDA receptor antagonist dizocilpine maleate (MK-801) for 24 h before birth also caused an increase in cell death that reached statistical significance in two of the four nuclei (oralis and interpolaris). The neonatal sensitivity to NMDA receptor hypofunction in the BSTC, and in its main thalamic target, the ventrobasal nucleus (VB), coincides with the peak of naturally occurring cell death and trigeminothalamic synaptogenesis. At embryonic day 17.5, before the onset of these events, NMDAR1 knockout does not affect cell survival in either the BSTC or the VB. Immunostaining for active caspase-3 and the neuronal marker Hu specifically confirms the presence of dying neurons in the BSTC and the VB of NMDAR1 knockout neonates. Finally, genetic deletion of Bax rescues these structures from the requirement for NMDA receptors to limit naturally occurring cell death. Taken together, the results indicate that NMDA receptors play a survival role for somatosensory relay neurons during synaptogenesis by inhibiting Bax-dependent developmental cell death.

Bcl-2 over-expression fails to prevent age-related loss of calretinin positive neurons in the mouse dentate gyrus

BACKGROUND

Cognitive performance declines with increasing age. Possible cellular mechanisms underlying this age-related functional decline remain incompletely understood. Early studies attributed this functional decline to age-related neuronal loss. Subsequent studies using unbiased stereological techniques found little or no neuronal loss during aging. However, studies using specific cellular markers found age-related loss of specific neuronal types. To test whether there is age-related loss of specific neuronal populations in the hippocampus, and subsequently, whether over-expression of the B-cell lymphoma protein-2 (Bcl-2) in these neurons could delay possible age-related neuronal loss, we examined calretinin (CR) positive neurons in the mouse dentate gyrus during aging.

RESULT

In normal mice, there was an age-related loss of CR positive cells in the dentate gyrus. At the same region, there was no significant decrease of total numbers of neurons, which suggested that age-related loss of CR positive cells was due to the decrease of CR expression in these cells instead of cell death. In the transgenic mouse line over-expressing Bcl-2 in neurons, there was an age-related loss of CR positive cells. Interestingly, there was also an age-related neuronal loss in this transgenic mouse line.

CONCLUSION

These data suggest an age-related loss of CR positive neurons but not total neuronal loss in normal mice and this age-related neuronal change is not prevented by Bcl-2 over-expression.

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.

Cellular plasticity cascades: genes-to-behavior pathways in animal models of bipolar disorder

BACKGROUND

Despite extensive research, the molecular/cellular underpinnings of bipolar disorder (BD) remain to be fully elucidated. Recent data has demonstrated that mood stabilizers exert major effects on signaling that regulate cellular plasticity; however, a direct extrapolation to mechanisms of disease demands proof that manipulation of candidate genes, proteins, or pathways result in relevant behavioral changes.

METHODS

We critique and evaluate the behavioral changes induced by manipulation of cellular plasticity cascades implicated in BD.

RESULTS

Not surprisingly, the behavioral data suggest that several important signaling molecules might play important roles in mediating facets of the complex symptomatology of BD. Notably, the protein kinase C and extracellular signal-regulated kinase cascades might play important roles in the antimanic effects of mood stabilizers, whereas glycogen synthase kinase (GSK)-3 might mediate facets of lithium's antimanic/antidepressant actions. Glucocorticoid receptor (GR) modulation also seems to be capable to inducing affective-like changes observed in mood disorders. And Bcl-2, amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors, and inositol homeostasis represent important pharmacological targets for mood stabilizers, but additional behavioral research is needed to more fully delineate their behavioral effects.

CONCLUSIONS

Behavioral data support the notion that regulation of cellular plasticity is involved in affective-like behavioral changes observed in BD. These findings are leading to the development of novel therapeutics for this devastating illness.

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.

Alterations in mitochondrial morphology are associated with hyperthermia-induced apoptosis in early postimplantation mouse embryos

BACKGROUND

Previously, we showed that teratogens such as hyperthermia activate the mitochondrial apoptotic pathway in day nine mouse embryos. Activation of this pathway involves an initial release of cytochrome c from intermembranous spaces of the mitochondria into the cytoplasm. Cytoplasmic cytochrome c then activates a caspase cascade resulting in the orderly demise of the cell. In addition, we showed that teratogens activate the mitochondrial pathway in cells of the neuroepithelium, but not the heart.

METHODS

To further investigate the role of the mitochondrion in teratogen-induced apoptosis, we used transmission electron microscopy (TEM) to compare mitochondrial morphology in cells of the neuroepithelium and heart of control and hyperthermia-treated embryos. Because we know that the apoptotic pathway is activated some time during the first 5 hr after teratogen exposure is initiated, we assessed mitochondrial morphology at 1, 2.5, and 5 hr after day nine mouse embryos were exposed to hyperthermia (43 degrees C, 15 min).

RESULTS

In neuroepithelial cells of the prosencephalon, abnormally-shaped mitochondria were observed at the 1 hr time point and thereafter, whereas loss of cristae and shrunken mitochondria were noted at the 5 hr time point. In contrast, no obvious changes in mitochondria of heart cells were observed at any of the time points monitored.

CONCLUSIONS

These results indicate that teratogen-induced cell death in neuroepithelial cells is temporally correlated with alterations in mitochondrial morphology, whereas the absence of cell death in the heart is correlated with a corresponding lack of change in mitochondrial morphology. Birth Defects Research (Part A), 2003.