Deregulation of Cdk5 in a mouse model of ALS: toxicity alleviated by perikaryal neurofilament inclusions

Recent studies suggest that increased activity of cyclin-dependent kinase 5 (Cdk5) may contribute to neuronal death and cytoskeletal abnormalities in Alzheimer's disease. We report here such deregulation of Cdk5 activity associated with the hyperphosphorylation of tau and neurofilament (NF) proteins in mice expressing a mutant superoxide dismutase (SOD1(G37R)) linked to amyotrophic lateral sclerosis (ALS). A Cdk5 involvement in motor neuron degeneration is supported by our analysis of three SOD1(G37R) mouse lines exhibiting perikaryal inclusions of NF proteins. Our results suggest that perikaryal accumulations of NF proteins in motor neurons may alleviate ALS pathogenesis by acting as a phosphorylation sink for Cdk5 activity, thereby reducing the detrimental hyperphosphorylation of tau and other neuronal substrates.

Abnormal behavioral phenotypes of serotonin transporter knockout mice: parallels with human anxiety and depression

Evidence of a link between genetic variation of the serotonin transporter and depression and anxiety prompted the generation of serotonin transporter knockout mice. Loss of serotonin reuptake function in knock-outs causes reduced clearance of extracellular serotonin and associated alterations in serotonin neuronal firing and receptor function. Behavioral phenotyping function in knock-outs revealed genetic background-related abnormalities, including increased anxiety-like behaviors, reduced aggression, and exaggerated stress responses. Ongoing studies focus on identifying environmental, genetic, and developmental factors interacting with the htt mutation to produce these abnormalities. Serotonin transporter null mutant mice provide a model system to study how genetic variation in serotonin transporter function affects risk for neuropsychiatric disease.

Hepatocyte growth factor/scatter factor is a motogen for interneurons migrating from the ventral to dorsal telencephalon

Cortical interneurons arise from the proliferative zone of the ventral telencephalon, the ganglionic eminence, and migrate into the developing neocortex. The spatial patterns of migratory interneurons reflect the complementary expression of hepatocyte growth factor/scatter factor (HGF/SF) and its receptor, MET, in the forebrain. Scatter assays on forebrain explants demonstrate regionally specific motogenic activity due to HGF/SF. In addition, exogenous ligand disrupts normal cell migration. Mice lacking the urokinase-type plasminogen activator receptor (u-PAR), a key component of HGF/SF activation, exhibit deficient scatter activity in the forebrain, abnormal interneuron migration from the ganglionic eminence, and reduced interneurons in the frontal and parietal cortex. The data suggest that HGF/SF motogenic activity, which is essential for normal development of other organ systems, is a conserved mechanism that regulates trans-telencephalic migration of interneurons.

EphB forward signaling controls directional branch extension and arborization required for dorsal-ventral retinotopic mapping

We report that EphB receptors direct unique axonal behaviors required for mapping the dorsal-ventral (D-V) retinal axis along the lateral-medial (L-M) axis of the superior colliculus (SC). EphBs are expressed in a D-V gradient, ephrin-B1 in a L-M gradient in SC, and ephrin-B3 at its midline. EphBs and ephrin-Bs are expressed in countergradients in retina and SC. Developmental analyses reveal that retinal axons lack D-V ordering along the L-M axis, but directionally extend branches along it to establish ordered arbors. Directed branch extension is disrupted in EphB2; EphB3-deficient mice resulting in lateral ectopic arbors. Mice with kinase-inactive EphB2 have similar D-V mapping defects indicating that forward signaling dominates over reverse signaling. Our data suggest that branches of EphB expressing axons are attracted medially by ephrin-B1, and provide molecular mechanisms for D-V mapping in visual centers.

Epileptogenesis and enhanced prepulse inhibition in GABA(B1)-deficient mice

The recent cloning of two GABA(B) receptor subunits, GABA(B1) and GABA(B2), has raised the possibility that differences in GABA(B) receptor subunit composition may give rise to pharmacologically or functionally distinct receptors. If present, such molecular diversity could permit the selective targeting of GABA(B) receptor subtypes specifically involved in pathologies such as drug addiction, spasticity, pain, and epilepsy. To address these issues we have developed a GABA(B1) subunit knockout mouse using gene targeting techniques. In the brains of GABA(B1) null mice, all pre- and postsynaptic GABA(B) receptor function was absent demonstrating that the GABA(B1) subunit is essential for all GABA(B) receptor-mediated mechanisms. Despite this, GABA(B1) null mice appeared normal at birth, although by postnatal week four their growth was retarded and they developed a generalized epilepsy that resulted in premature death. In addition, GABA(B1) heterozygote animals showed enhanced prepulse inhibition responses compared to littermate controls, suggesting that GABA(B1) deficient mice exhibit increased sensorimotor gating mechanisms. These data suggest that GABA(B) receptor antagonists may be of benefit in the treatment of psychiatric and neurological disorders in which attentional processing is impaired.

Formation of multiple hearts in mice following deletion of beta-catenin in the embryonic endoderm

Using Cre/loxP, we conditionally inactivated the beta-catenin gene in cells of structures that exhibit important embryonic organizer functions: the visceral endoderm, the node, the notochord, and the definitive endoderm. Mesoderm formation was not affected in the mutant embryos, but the node was missing, patterning of the head and trunk was affected, and no notochord or somites were formed. Surprisingly, deletion of beta-catenin in the definitive endoderm led to the formation of multiple hearts all along the anterior-posterior (A/P) axis of the embryo. Ectopic hearts developed in parallel with the normal heart in regions of ectopic Bmp2 expression. We provide evidence that ablation of beta-catenin in embryonic endoderm changes cell fate from endoderm to precardiac mesoderm, consistent with the existence of bipotential mesendodermal progenitors in mouse embryos.

GPRC6A null mice exhibit osteopenia, feminization and metabolic syndrome

Background

GPRC6A is a widely expressed orphan G-protein coupled receptor that senses extracellular amino acids, osteocalcin and divalent cations in vitro. The physiological functions of GPRC6A are unknown.

Methods/Principal Findings

In this study, we created and characterized the phenotype of GPRC6A^−/− mice. We observed complex metabolic abnormalities in GPRC6A^−/− mice involving multiple organ systems that express GPRC6A, including bone, kidney, testes, and liver. GPRC6A^−/− mice exhibited hepatic steatosis, hyperglycemia, glucose intolerance, and insulin resistance. In addition, we observed high expression of GPRC6A in Leydig cells in the testis. Ablation of GPRC6A resulted in feminization of male GPRC6A^−/− mice in association with decreased lean body mass, increased fat mass, increased circulating levels of estradiol, and reduced levels of testosterone. GPRC6A was also highly expressed in kidney proximal and distal tubules, and GPRC6A^−/− mice exhibited increments in urine Ca/Cr and PO₄/Cr ratios as well as low molecular weight proteinuria. Finally, GPRC6A^−/− mice exhibited a decrease in bone mineral density (BMD) in association with impaired mineralization of bone.

Conclusions/Significance

GPRC6A^−/− mice have a metabolic syndrome characterized by defective osteoblast-mediated bone mineralization, abnormal renal handling of calcium and phosphorus, fatty liver, glucose intolerance and disordered steroidogenesis. These findings suggest the overall function of GPRC6A may be to coordinate the anabolic responses of multiple tissues through the sensing of extracellular amino acids, osteocalcin and divalent cations.

An 'oligarchy' rules neural development

Recent reports show that Olig genes, which encode the basic helix-loop-helix Olig transcription factors, are essential for development of oligodendrocytes. Surprisingly, Olig function is also required for formation of somatic motor neurons. These findings alter our views of how the oligodendrocyte lineage is generated and raise further questions about the underlying developmental relationships between neurons and glia.

Involvement of sst2 somatostatin receptor in locomotor, exploratory activity and emotional reactivity in mice

Somatostatin (SRIF) controls many physiological and pathological processes in the central nervous system but the respective roles of the five receptor isotypes (sst1-5) that mediate its effects are yet to be defined. In the present study, we attempted to identify functions of the sst2 receptor using mice with no functional copy of this gene (sst2 KO mice). In contrast with control 129Sv/C57Bl6 mice, sst2 mRNA was no longer detectable in the brain of sst2 KO mice; 125I-labeled Tyr0DTrp8-SRIF14 binding was also greatly reduced in almost all brain structures except for the hippocampal CA1 area, demonstrating that sst2 accounts for most SRIF binding in mouse brain. Invalidation of this subtype generated an increased anxiety-related behaviour in a number of behavioural paradigms, while locomotor and exploratory activity was decreased in stress-inducing situations. No major motor defects could be detected. sst2 KO mice also displayed increased release of pituitary ACTH, a main regulator of the stress response. Thus, somatostatin, via sst2 receptor isotype pathways, appears involved in the modulation of locomotor, exploratory and emotional reactivity in mice.

Cortical malformations and epilepsy

Brain malformations, resulting from aberrant patterns of brain development, are highly correlated with childhood seizure syndromes, as well as with cognitive disabilities and other neurological disorders. The structural malformations, often referred to as cortical dysplasia, are extremely varied, reflecting diverse underlying processes and critical timing of the developmental aberration. Recent studies have revealed a genetic basis for many forms of dysplasia. Gene mutations responsible for such common forms of dysplasia as lissencephaly and tuberous sclerosis have been identified, and investigators are beginning to understand how these gene mutations interrupt and/or misdirect the normal developmental pattern. Laboratory investigations, using animal models of cortical dysplasia, are beginning to elucidate how these structural malformations give rise to epilepsy and other functional pathologies.

Callosal axon guidance defects in p35(-/-) mice

Mice lacking p35, an activator of cdk5 in the central nervous system (CNS), exhibit defects in a variety of CNS structures, most prominently characterized by a disruption in the laminar structure of the neocortex (Chae et al., 1997). In addition, alterations of certain axonal fiber tracts are found in the cortex of p35 mutant mice. Notably, the corpus callosum appears bundled at the midline, but dispersed lateral to the midline. Tracer injection experiments in adult p35 mutant mice reveal that projecting cortical axons fail to assimilate into the corpus callosum, and take oblique paths to the midline. After crossing the midline, cortical axons defasciculate prematurely from the corpus callosum and take similarly oblique paths through the cortex. This callosal phenotype is not detected in reeler mice, which also exhibit defects in cortical lamination, suggesting that the lack of fasciculation of callosal axons is not an inherent manifestation of a disruption of cortical lamination. The embryonic callosal axon tract is defasciculated before crossing the midline, suggesting that axon guidance may be affected during embryonic development of the corpus callosum. In addition, embryonic thalamocortical afferents also exhibit a defasciculated phenotype. These results suggest that defective axonal fasciculation and guidance may be primary responses to the loss of p35 in the cortex. Furthermore, this study postulates a role for the p35/cdk5 kinase in molecular signaling pathways necessary for proper guidance of selective axons during embryonic development.

trkA modulation of developing somatosensory neurons in oro-facial tissues: tooth pulp fibers are absent in trkA knockout mice

To investigate the nerve growth factor requirement of developing oro-facial somatosensory afferents, we have studied the survival of sensory fibers subserving nociception, mechanoreception or proprioception in receptor tyrosine kinase (trkA) knockout mice using immunohistochemistry. trkA receptor null mutant mice lack nerve fibers in tooth pulp, including sympathetic fibers, and showed only sparse innervation of the periodontal ligament. Ruffini endings were formed definitively in the periodontal ligament of the trkA knockout mice, although calcitonin gene-related peptide- and substance P-immunoreactive fibers were reduced in number or had disappeared completely. trkA gene deletion had also no obvious effect on the formation of Meissner corpuscles in the palate. In the vibrissal follicle, however, some mechanoreceptive afferents were sensitive for trkA gene deletion, confirming a previous report [Fundin et al. (1997) Dev. Biol. 190, 94-116]. Moreover, calretinin-positive fibers innervating longitudinal lanceolate endings were completely lost in trkA knockout mice, as were the calretinin-containing parent cells in the trigeminal ganglion.These results indicate that trkA is indispensable for developing nociceptive neurons innervating oral tissues, but not for developing mechanoreceptive neurons innervating oral tissues (Ruffini endings and Meissner corpuscles), and that calretinin-containing, trkA dependent neurons in the trigeminal ganglion normally participate in mechanoreception through longitudinal lanceolate endings of the vibrissal follicle.

Abnormal thalamocortical pathfinding and terminal arbors lead to enlarged barrels in neonatal GAP-43 heterozygous mice

GAP-43 has been implicated in axonal pathfinding and sprouting, synaptic plasticity, and neurotransmitter release. However, its effect on cortical development in vivo is poorly understood. We have previously shown that GAP-43 knockout (-/-) mice fail to develop whisker-related barrels or an ordered whisker map in the cortex. Here we used cytochrome oxidase (CO) histochemistry to demonstrate that GAP-43 heterozygous (+/-) mice develop larger than normal barrels at postnatal day 7 (P7), despite normal body and brain weight. Using serotonin transporter (5HT-T) histochemistry to label thalamocortical afferents (TCAs), we found no obvious abnormalities in other somatosensory areas or primary visual cortex of GAP-43 (+/-) mice. However, TCA projections to (+/-) primary auditory cortex were not as clearly defined. To clarify the mechanism underlying the large-barrel phenotype, we used lipophilic (DiI) axon labeling. We found evidence for multiple pathfinding abnormalities among GAP-43 (+/-) TCAs. These axons show increased fasciculation within the internal capsule, as well as abnormal turning and branching in the subcortical white matter. These pathfinding errors most likely reflect failures of signal recognition and/or transduction by ingrowing TCAs. In addition, many DiI-labeled (+/-) TCAs exhibit widespread, sparsely branched terminal arbors in layer IV, reflecting the large-barrel phenotype. They also resemble those found in rat barrel cortex deprived of whisker inputs from birth, suggesting a failure of activity-dependent synaptogenesis and/or synaptic stabilization in (+/-) cortex. Our findings suggest that reduced GAP-43 expression can alter the fine-tuning of a cortical map through a combination of pathfinding and synaptic plasticity mechanisms.

Impairment of spermatogenesis leading to infertility

Highly differentiated spermatozoa are generated through multiple cellular and molecular processes maintained by Sertoli cells. The cellular events associated with germ cells include proliferation, protein folding and transportation, as well as sequential changes in chromatin and cell organelles. These processes are strictly controlled by the expression of specific genes, including transcription and DNA replication/repair. This complex spermatogenesis is impaired by a mutation such as gene knockout, which leads to a variety of morphological and functional abnormalities found in mature spermatozoa. An overview of spermatogenesis impairment induced by gene knockout is provided in the present review.

A compendium of mouse knockouts with inner ear defects

Genetically engineered strains of mice, modified by gene targeting (knockouts), are increasingly being employed as alternative effective research tools in elucidating the genetic basis of human deafness. An impressive array of auditory and vestibular mouse knockouts is already available as a valuable resource for studying the ontogenesis, morphogenesis and function of the mammalian inner ear. This article provides a current catalog of mouse knockouts with inner ear morphogenetic malformations and hearing or balance deficits resulting from ablation of genes that are regionally expressed in the inner ear and/or within surrounding tissues, such as the hindbrain, neural crest and mesenchyme.

Respiratory function in adult mice lacking the mu-opioid receptor: role of delta-receptors

Mice lacking the mu-opioid receptor (MOR) provide a unique model to determine whether opioid receptors are functionally interactive. Recent results have shown that respiratory depression produced by delta-opioid receptor agonists is suppressed in mice lacking the mu-opioid receptor. Here we investigated the involvement of mu- and delta-opioid receptors in the control of ventilation and mu/delta receptor interactions in brainstem rhythm-generating structures. Unrestrained MOR-/- and wild-type mice showed similar ventilatory patterns at rest and similar chemosensory responses to hyperoxia (100% O2), hypoxia (10% O2) or hypercapnia (5%CO2-95%O2). Blockade of delta-opioid receptors with naltrindole affected neither the ventilatory patterns nor the ventilatory responses to hypoxia in MOR-/- and wild-type mice. In-vitro, respiratory neurons were recorded in the pre-Bötzinger complex of thick brainstem slices of MOR-/- and wild-type young adult mice. Respiratory frequency was not significantly different between these two groups. The delta2 receptor agonist deltorphin II (0.1-1.0 microM) decreased respiratory frequency in both groups whereas doses of the delta1 receptor agonist enkephalin[D-Pen2,5] (0.1-1.0 microM) which were ineffective in wild-type mice significantly decreased respiratory frequency in MOR-/- mice. We conclude that deletion of the mu-opioid receptor gene has no significant effect on ensuing respiratory rhythm generation, ventilatory pattern, or chemosensory control. In MOR-/- mice, the loss of respiratory-depressant effects of delta2-opioid receptor agonists previously observed in vivo does not result from a blunted response of delta receptors in brainstem rhythm-generating structures. These structures show an unaltered response to delta2-receptor agonists and an augmented response to delta1-receptor agonists.

Primary olfactory axons form ectopic glomeruli in mice lacking p75NTR

The restricted expression of the low affinity nerve growth factor receptor p75NTR by olfactory ensheathing cells suggests that this molecule is involved in the development of the olfactory nerve pathway. To begin to understand the role of p75NTR, we examined the development of the primary olfactory system in p75NTR(-/-) and wild-type mice. Our results demonstrate that, although p75NTR is not essential for the initial assembly of the olfactory nerve, it plays an important role in the postnatal maturation of the olfactory bulb. In the absence of p75NTR, there is exuberant growth of some primary olfactory axons into the olfactory bulb. These axons either aberrantly bypass the glomerular layer and project into deeper lamina or grow into an abnormal bleb of tissue protruding from the medial surface of the dorsocaudal olfactory bulb. These blebs become apparent in neonatal mice and contain axons expressing olfactory marker protein that form ectopic glomerular-like tufts. Histochemical staining with the plant lectin Dolichos biflorus agglutinin revealed that axons sorted out and selectively converged on glomeruli within these blebs. Our results suggest that p75NTR indirectly influences axon growth but not glomerular targeting and plays a role in the postnatal maturation of laminar cytoarchitecture in the olfactory bulb.

Homeobox gene mutations and brain-stem developmental disorders: learning from knockout mice

Analysis of mice that carry targeted inactivations of Hox, Nkx and Phox2 homeobox genes revealed their involvement in regional patterning of brain-stem territories, in specification of neuronal identity, in establishment of appropriate patterns of connectivity and in control of neurotransmission. The specific abnormalities generated by such mutations may provide clues to the genetic basis and cellular mechanisms that are involved in human brain-stem developmental disorders.

Abnormal inspiratory depth in Phox2a haploinsufficient mice

Mutations of genes encoding Phox2a or Phox2b transcription factors induce modifications of different brainstem neuronal networks. Such modifications are associated with defects in breathing behavior at birth. In particular, an abnormal breathing frequency is observed in Phox2a-/- mutant mice, resulting from abnormal development of the locus coeruleus (LC) nucleus. However, the role of Phox2a proteins in the establishment of respiratory neuronal pathways is unknown, largely because mutants die shortly after birth. In the present study, we examined the effects of a haploinsufficiency of the Phox2a gene. Phox2a heterozygotes survive and exhibit a significantly larger inspiratory volume both during normoxic breathing and in response to hypoxia and a delayed maturation of inspiratory duration compared to wild-type animals. This phenotype accompanied by an unaltered frequency is evident at birth and persists until at least postnatal day 10. Morphological analyses of Phox2a+/- animals revealed no anomaly in the LC region, but highlighted an increase in the number of cells expressing tyrosine hydroxylase enzyme, a marker of chemoafferent neurons, in the petrosal sensory ganglion. These data indicate that Phox2a plays a critical role in the ontogeny of the reflex control of inspiration.

Rumpshaker-like proteolipid protein (PLP) ratio in a mouse model with unperturbed structural and functional integrity of the myelin sheath and axons in the central nervous system

The gene plp on the X chromosome encodes the isoforms proteolipid protein (PLP) and DM(20), two dominant integral membrane proteins of central nervous system (CNS) myelin. DM(20) results from the activation of the cryptic splice site in exon III of the PLP gene. We inserted a sense-orientated loxP flanked neomycin-gene into intron III of the plp sequence, using homologous recombination in embryonic stem cells and generated the homozygous neoS mouse line. Unlike the previously described complete PLP/DM(20) ablation (plp(-/-)), which has been obtained by introducing a neo-gene in antisense-orientation in the same position of intron III, the plp expression surprisingly revealed reduced mRNA levels. The PLP isoform was reduced to 50%, but DM(20) expression was unaffected. This protein pattern resembles the expression profile of the PLP isoforms in the natural occurring rumpshaker mutant. Electron microscopic examination revealed a normal compaction of CNS-myelin and maintenance of axon integrity. PLP expression levels of the wt control were recovered by Cre excision of the neo-selection gene after intercrossing neoS mice and oligodendrocyte-specific Cre-mice. These data strongly hint at different functions of intron III in PLP/DM(20)-specific splicing and mRNA stability. Furthermore evidence is provided for functionally affected translation products of the PLP gene in the rumpshaker mutant, whereas no PLP-isoform occur in plp(-/-) mice generated by introducing a selectable marker into intron III in antisense orientation.