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Susan L Ackerman

Publications and source records attributed to Susan L Ackerman.

16 recordsLinked to original sources
PubMed2006

Editing-defective tRNA synthetase causes protein misfolding and neurodegeneration.

Misfolded proteins are associated with several pathological conditions including neurodegeneration. Although some of these abnormally folded proteins result from mutations in genes encoding disease-associated proteins (for example, repeat-expansion diseases), more general mechanisms that lead to misfolded proteins in neurons remain largely unknown. Here we demonstrate that low levels of mischarged transfer RNAs (tRNAs) can lead to an intracellular accumulation of misfolded proteins in neurons. These accumulations are accompanied by upregulation of cytoplasmic protein chaperones and by induction of the unfolded protein response. We report that the mouse sticky mutation, which causes cerebellar Purkinje cell loss and ataxia, is a missense mutation in the editing domain of the alanyl-tRNA synthetase gene that compromises the proofreading activity of this enzyme during aminoacylation of tRNAs. These findings demonstrate that disruption of translational fidelity in terminally differentiated neurons leads to the accumulation of misfolded proteins and cell death, and provide a novel mechanism underlying neurodegeneration.

Acetylation↗
PubMed2006

EUK-8, a superoxide dismutase and catalase mimetic, reduces cardiac oxidative stress and ameliorates pressure overload-induced heart failure in the harlequin mouse mutant.

OBJECTIVES: The purpose of this study was to identify apoptosis-inducing factor (AIF) as a cardiac mitochondrial antioxidant and assess the efficacy of EUK-8, a salen-manganese catalytic free radical scavenger, to protect the AIF-deficient myocardium against pressure overload. BACKGROUND: Oxidative stress has been postulated to provoke cell death and pathologic remodeling in heart failure. We recently characterized the apoptosis-inducing factor-deficient harlequin (Hq) mouse mutant to display excessive pressure overload-induced oxidative stress, cell death, accelerated progression to heart failure, and a reduced capacity of subsarcolemmal mitochondria to scavenge free radicals, suggesting a role for AIF as a novel mitochondrial antioxidant. METHODS: Oxidative stress-sensitized Hq mutant mice and their wild-type (WT) counterparts were given low-dose EUK-8 (25 mg/kg/day), an antioxidant with superoxide dismutase, catalase, and oxyradical scavenging properties, or vehicle for 4 weeks, and subjected to pressure overload (transverse aortic constriction) for 4 weeks. Myocardial geometry and function was serially assessed by echocardiography. RESULTS: EUK-8 ameliorated survival in Hq and WT mice subjected to pressure overload. EUK-8 also improved left ventricular end-systolic dimensions and fractional shortening, prevented myocardial oxidant stress, attenuated necrotic and apoptotic cell death, and attenuated cardiac hypertrophy and fibrosis in both mutant and WT mice. CONCLUSIONS: The protection against pressure overload-induced heart failure in Hq mice by EUK-8 substantiates the notion that AIF functions as an important mitochondrial antioxidant in the heart. Furthermore, because antioxidant treatment protected both the oxidative stress-prone Hq mouse model and WT mice against pressure overload-induced maladaptive left ventricular remodeling and cardiac decompensation, it may be useful as a novel therapeutic tool in the treatment of human heart failure.

Animals↗
PubMed2006

Endoplasmic reticulum stress in health and disease.

Unfolded proteins and other conditions affecting endoplasmic reticulum (ER) homeostasis cause ER stress. The cell reacts to ER stress by activation of the unfolded protein response (UPR), which induces profound changes in cellular metabolism including general translation attenuation, transcriptional upregulation of molecular chaperone genes, and activation of ER-associated degradation. However, prolonged or acute ER stress results in cell death. Recent progress suggests that ER stress and UPR play key roles in the immune response, diabetes, tumor growth under hypoxic conditions, and in some neurodegenerative diseases. Further research on ER stress and UPR will greatly enhance the understanding of these physiological and pathological processes, and provide novel avenues to potential therapies.

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PubMed2006

Motor axon guidance of the mammalian trochlear and phrenic nerves: dependence on the netrin receptor Unc5c and modifier loci.

Netrin signaling is important to guide migrating neurons and axons in many systems. Experiments with vertebrate CNS explants suggested netrin is bifunctional, attracting some axons and repelling others. Netrin1-expressing cells attracted spinal commissural axons and repelled trochlear cranial nerve axons in these experiments. Subsequent genetic studies demonstrated that multiple axon types, including those of the spinal commissural neurons, are attracted to netrin in vivo; however, an in vivo role for netrin signaling in trochlear nerve repulsion has not been observed. Here, we demonstrate that mice with a null mutation in the netrin receptor Unc5c on the inbred C57BL/6J (B6) genetic background have ventral/ipsilateral trochlear nerve misprojections. These misprojections are attenuated on a hybrid B6 x SJL background. In addition, B6.Unc5c(-/-) mice die as neonates of apparent respiratory distress and have incomplete phrenic nerve innervation of the diaphragm muscle. Neither the trochlear nerve misprojections nor the phrenic nerve phenotype was observed in B6 embryos lacking the netrin receptors DCC or Neogenin1, or the ligand netrin1, indicating these signaling molecules are dispensable for guidance of these axons. Like the trochlear nerve, the phrenic nerve phenotype is modified in a B6 x SJL hybrid background. To identify these modifier loci, we performed genome scans of the hybrid Unc5c(-/-) mice and found a major SJL-derived suppressor locus on Chromosome 17. Our results provide the first evidence that genes involved in netrin signaling are necessary for proper mammalian spinal motor axon development and trochlear axon guidance. In addition, they demonstrate the importance of modifier genes in vertebrate axonal guidance.

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PubMed2006

In vivo magnetic resonance imaging and semiautomated image analysis extend the brain phenotype for cdf/cdf mice.

Magnetic resonance imaging and computer image analysis in human clinical studies effectively identify abnormal neuroanatomy in disease populations. As more mouse models of neurological disorders are discovered, such an approach may prove useful for translational studies. Here, we demonstrate the effectiveness of a similar strategy for mouse neuroscience studies by phenotyping mice with the cerebellar deficient folia (cdf) mutation. Using in vivo multiple-mouse magnetic resonance imaging for increased throughput, we imaged groups of cdf mutant, heterozygous, and wild-type mice and made an atlas-based segmentation of the structures in 15 individual brains. We then performed computer automated volume measurements on the structures. We found a reduced cerebellar volume in the cdf mutants, which was expected, but we also found a new phenotype in the inferior colliculus and the olfactory bulbs. Subsequent local histology revealed additional cytoarchitectural abnormalities in the olfactory bulbs. This demonstrates the utility of anatomical magnetic resonance imaging and semiautomated image analysis for detecting abnormal neuroarchitecture in mutant mice.

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PubMed2006

Dorsally derived netrin 1 provides an inhibitory cue and elaborates the 'waiting period' for primary sensory axons in the developing spinal cord.

Dorsal root ganglion (DRG) neurons extend axons to specific targets in the gray matter of the spinal cord. During development, DRG axons grow into the dorsolateral margin of the spinal cord and projection into the dorsal mantle layer occurs after a ;waiting period' of a few days. Netrin 1 is a long-range diffusible factor expressed in the ventral midline of the developing neural tube, and has chemoattractive and chemorepulsive effects on growing axons. Netrin 1 is also expressed in the dorsal spinal cord. However, the roles of dorsally derived netrin 1 remain totally unknown. Here, we show that dorsal netrin 1 controls the correct guidance of primary sensory axons. During the waiting period, netrin 1 is transiently expressed or upregulated in the dorsal spinal cord, and the absence of netrin 1 results in the aberrant projection of sensory axons, including both cutaneous and proprioceptive afferents, into the dorsal mantle layer. Netrin 1 derived from the dorsal spinal cord, but not the floor plate, is involved in the correct projection of DRG axons. Furthermore, netrin 1 suppresses axon outgrowth from DRG in vitro. Unc5c(rcm) mutant shows abnormal invasion of DRG axons as observed in netrin 1 mutants. These results are the first direct evidence that netrin 1 in the dorsal spinal cord acts as an inhibitory cue for primary sensory axons and is a crucial signal for the formation of sensory afferent neural networks.

Animals↗
PubMed2005

Protein accumulation and neurodegeneration in the woozy mutant mouse is caused by disruption of SIL1, a cochaperone of BiP.

Endoplasmic reticulum (ER) chaperones and ER stress have been implicated in the pathogenesis of neurodegenerative disorders, such as Alzheimer and Parkinson diseases, but their contribution to neuron death remains uncertain. In this study, we establish a direct in vivo link between ER dysfunction and neurodegeneration. Mice homozygous with respect to the woozy (wz) mutation develop adult-onset ataxia with cerebellar Purkinje cell loss. Affected cells have intracellular protein accumulations reminiscent of protein inclusions in both the ER and the nucleus. In addition, upregulation of the unfolded protein response, suggestive of ER stress, occurs in mutant Purkinje cells. We report that the wz mutation disrupts the gene Sil1 that encodes an adenine nucleotide exchange factor of BiP, a crucial ER chaperone. These findings provide evidence that perturbation of ER chaperone function in terminally differentiated neurons leads to protein accumulation, ER stress and subsequent neurodegeneration.

Animals↗
PubMed2005

Downregulation of apoptosis-inducing factor in harlequin mutant mice sensitizes the myocardium to oxidative stress-related cell death and pressure overload-induced decompensation.

Apoptosis-inducing factor (AIF), or programmed cell death 8 (Pdcd8), is a highly conserved, ubiquitous flavoprotein localized in the mitochondrial intermembrane space. In vivo, AIF provides protection against neuronal apoptosis induced by oxidative stress. Conversely, in vitro, AIF has been demonstrated to have a proapoptotic role when, on induction of the mitochondrial death pathway, AIF translocates to the nucleus where it facilitates chromatin condensation and large scale DNA fragmentation. To determine the role of AIF in myocardial apoptotic processes, we examined cardiomyocytes from an AIF-deficient mouse mutant, Harlequin (Hq). Hq mutant cardiomyocytes demonstrated increased sensitivity to H2O2-induced cell death. Further, Hq hearts subjected to ischemia/reperfusion revealed more cardiac damage and, unlike wild-type mice, the amount of damage increased with the age of the animal. Aortic banding caused enhanced hypertrophy, increased cardiomyocyte apoptotic and necrotic cell death, and accelerated progression toward maladaptive left ventricular remodeling in Hq mutant mice compared with wild-type counterparts. These findings correlated with a reduced capacity of subsarcolemmal mitochondria from Hq mutant hearts to scavenge free radicals. Together, these data demonstrate a critical role for AIF as a cardiac antioxidant in the protection against oxidative stress-induced cell death and development of heart failure induced by pressure overload.

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PubMed2004

Genomewide two-generation screens for recessive mutations by ES cell mutagenesis.

Forward genetic mutation screens in mice are typically begun by mutagenizing the germline of male mice with N-ethyl-N-nitrosourea (ENU). Genomewide recessive mutations transmitted by these males can be rendered homozygous after three generations of breeding, at which time phenotype screens can be performed. An alternative strategy for randomly mutagenizing the mouse genome is by chemical treatment of embryonic stem (ES) cells. Here we demonstrate the feasibility of performing genome-wide mutation screens with only two generations of breeding. Mice potentially homozygous for mutations were obtained by crossing chimeras derived from ethylmethane sulfonate (EMS)-mutagenized ES cells to their daughters, or by intercrossing offspring of chimeras. This strategy was possible because chimeras transmit variations of the same mutagenized diploid genome, whereas ENU-treated males transmit numerous unrelated genomes. This also results in a doubling of screenable mutations in a pedigree compared to germline ENU mutagenesis. Coupled with the flexibility to treat ES cells with a variety of potent mutagens and the ease of producing distributable, quality-controlled, long-term supplies of cells in a single experiment, this strategy offers a number of advantages for conducting forward genetic screens in mice.

Animals↗
PubMed2004

Netrin 1-mediated chemoattraction regulates the migratory pathway of LHRH neurons.

Luteinizing hormone-releasing hormone (LHRH) neurons migrate from the vomeronasal organ (VNO) to the forebrain in all mammals studied. In mice, the direction of LHRH neuron migration is dependent upon axons that originate in the VNO, but bypass the olfactory bulb and project caudally into the basal forebrain. Thus, factors that guide this unique subset of vomeronasal axons that comprise the caudal vomeronasal nerve (cVNN) are candidates for regulating the migration of LHRH neurons. We previously showed that deleted in colorectal cancer (DCC) is expressed by neurons that migrate out of the VNO during development [Schwarting et al. (2001) J. Neurosci., 21, 911-919]. We examined LHRH neuron migration in Dcc-/- mice and found that trajectories of the cVNN and positions of LHRH neurons are abnormal. Here we extend these studies to show that cVNN trajectories and LHRH cell migration in netrin 1 (Ntn1) mutant mice are also abnormal. Substantially reduced numbers of LHRH neurons are found in the basal forebrain and many LHRH neurons migrate into the cerebral cortex of Ntn1 knockout mice. In contrast, migration of LHRH cells is normal in Unc5h3rcm mutant mice. These results are consistent with the idea that the chemoattraction of DCC+ vomeronasal axons by a gradient of netrin 1 protein in the ventral forebrain guides the cVNN, which, in turn, determines the direction of LHRH neuron migration in the forebrain. Loss of function through a genetic deletion in either Dcc or Ntn1 results in the migration of many LHRH neurons to inappropriate destinations.

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PubMed2002

The netrin 1 receptors Unc5h3 and Dcc are necessary at multiple choice points for the guidance of corticospinal tract axons.

Migrating axons require the correct presentation of guidance molecules, often at multiple choice points, to find their target. Netrin 1, a bifunctional cue involved in both attracting and repelling axons, is involved in many cell migration and axon pathfinding processes in the CNS. The netrin 1 receptor DCC and its Caenorhabditis elegans homolog UNC-40 have been implicated in directing the guidance of axons toward netrin sources, whereas the C. elegans UNC-6 receptor, UNC-5 is necessary for migrations away from UNC-6. However, a role of vertebrate UNC-5 homologs in axonal migration has not been demonstrated. We demonstrate that the Unc5h3 gene product, shown previously to regulate cerebellar granule cell migrations, also controls the guidance of the corticospinal tract, the major tract responsible for coordination of limb movements. Furthermore, we show that corticospinal tract fibers respond differently to loss of UNC5H3. In addition, we observe corticospinal tract defects in mice homozygous for a spontaneous mutation that truncates the Dcc transcript. Postnatal day 0 netrin 1 mutant mice also demonstrate corticospinal tract abnormalities. Last, interactions between the Dcc and Unc5h3 mutations were observed in gene dosage experiments. This is the first evidence of an involvement in axon guidance for any member of the vertebrate unc-5 family and confirms that both the cellular and axonal guidance functions of C. elegans unc-5 have been conserved in vertebrates.

Animals↗
PubMed2002

The harlequin mouse mutation downregulates apoptosis-inducing factor.

Harlequin (Hq) mutant mice have progressive degeneration of terminally differentiated cerebellar and retinal neurons. We have identified the Hq mutation as a proviral insertion in the apoptosis-inducing factor (Aif) gene, causing about an 80% reduction in AIF expression. Mutant cerebellar granule cells are susceptible to exogenous and endogenous peroxide-mediated apoptosis, but can be rescued by AIF expression. Overexpression of AIF in wild-type granule cells further decreases peroxide-mediated cell death, suggesting that AIF serves as a free radical scavenger. In agreement, dying neurons in aged Hq mutant mice show oxidative stress. In addition, neurons damaged by oxidative stress in both the cerebellum and retina of Hq mutant mice re-enter the cell cycle before undergoing apoptosis. Our results provide a genetic model of oxidative stress-mediated neurodegeneration and demonstrate a direct connection between cell cycle re-entry and oxidative stress in the ageing central nervous system.

Aging↗
PubMed2002

Deletion in Catna2, encoding alpha N-catenin, causes cerebellar and hippocampal lamination defects and impaired startle modulation.

Mice homozygous for the cerebellar deficient folia (cdf) mutation are ataxic and have cerebellar hypoplasia and abnormal lobulation of the cerebellum. In the cerebella of cdf/cdf homozygous mice, approximately 40% of Purkinje cells are located ectopically in the white matter and inner granule-cell layer. Many hippocampal pyramidal cells are scattered in the plexiform layers, and those that are correctly positioned are less densely packed than are cells in wild-type mice. We show that fear conditioning and prepulse inhibition of the startle response are also disrupted in cdf/cdf mice. We identify a deletion on chromosome 6 that removes approximately 150 kb in the cdf critical region. The deletion includes part of Catna2, encoding alpha N-catenin, a protein that links the classical cadherins to the neuronal cytoskeleton. Expression of a Catna2 transgene in cdf/cdf mice restored normal cerebellar and hippocampal morphology, prepulse inhibition and fear conditioning. The findings suggest that catenin cadherin cell-adhesion complexes are important in cerebellar and hippocampal lamination and in the control of startle modulation.

Animals↗
PubMed2002

The cerebellar deficient folia (cdf) gene acts intrinsically in Purkinje cell migrations.

Cerebellar deficient folia (cdf) is a recently identified mouse mutation causing ataxia and cerebellar abnormalities including lobulation defects and abnormal placement of a specific subset of Purkinje cells. To understand the etiology of the cerebellar defects in cdf mutant mice, we examined postnatal development of the cdf/cdf cerebellum. Our results demonstrate that Purkinje cell ectopia and foliation defects are apparent at birth, suggesting the cdf mutation disrupts the positioning of many, but not all, Purkinje cells during development. In addition to cerebellar abnormalities, we observed lamination defects in the hippocampus of cdf mutant mice, although neocortical defects were not seen. Furthermore, ectopic Purkinje cells in cdf/cdf mice express an increased level of Dab1 protein, as previously observed in mice with mutations in genes in the reelin signaling pathway. Lastly, analysis of cdf <-->ROSA26 chimeric mice demonstrated that the cdf mutation is intrinsic to Purkinje cells. We suggest that the cdf gene product is required in a subset of Purkinje cells, possibly to respond to Reelin signals.

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