Genetic and age related models of neurodegeneration in mice: dystrophic axons

The DST gene in neurobiology

DST is a gene whose alternative splicing yields epithelial, neuronal, and muscular isoforms. The autosomal recessive Dstdt (dystonia musculorum) spontaneous mouse mutation causes degeneration of spinocerebellar tracts as well as peripheral sensory nerves, dorsal root ganglia, and cranial nerve ganglia. In addition to Dstdt mutants, axonopathy and neurofilament accumulation in perikarya are features of two other murine lines with spontaneous Dst mutations, targeted Dst knockout mice, DstTg4 transgenic mice carrying two deleted Dst exons, DstGt mice with trapped actin-binding domain-containing isoforms, and conditional Schwann cell-specific Dst knockout mice. As a result of nerve damage, Dstdt mutants display dystonia and ataxia, as seen in several genetically modified models and their motor coordination deficits have been quantified along with the spontaneous Dst nonsense mutant, the conditional Schwann cell-specific Dst knockout, the conditional DstGt mutant, and the Dst-b isoform specific Dst mutant. Recent findings in humans have associated DST mutations of the Dst-b isoform with hereditary sensory and autonomic neuropathies type 6 (HSAN-VI). These data should further encourage the development of genetic techniques to treat or prevent ataxic and dystonic symptoms.

Brain and spinal cord lesions in 28 inbred strains of aging mice

Brain and spinal cord histopathology findings in male and female 20-month-old mice in a large-scale aging study of 28 inbred Jackson Laboratory mouse strains from 7 genetic families are described. Brain sections from selected strains at 12 and 24 months of age or older were also reviewed. Common lesions include axonal dystrophy in the gracile and/or cuneate nucleus in the sensory tract of the dorsal medulla and in the spinal cord in all strains. Hirano-like bodies were seen in 24/28 strains, and mineralization was observed in the thalamus of 9/28 strains. Less common lesions were also seen in the cerebellum, cerebral cortex, and other brain areas. No brain or spinal cord tumors were found. Evidence of an impairment of the ubiquitin-proteasome system (UPS) and/or suspected autophagy was manifested as medullary axonal dystrophy with intra-axonal granular eosinophilic bodies and LC3B immunohistochemistry in most strains. RIIIS/J, the most severely affected strain, showed moderate axonal dystrophy at 12 months, which progressed to severe lesions at 20 months. Comparative pathology in various species is discussed.

A vertebrate adaptive radiation is assembled from an ancient and disjunct spatiotemporal landscape

To investigate the origins and stages of vertebrate adaptive radiation, we reconstructed the spatial and temporal histories of adaptive alleles underlying major phenotypic axes of diversification from the genomes of 202 Caribbean pupfishes. On a single Bahamian island, ancient standing variation from disjunct geographic sources was reassembled into new combinations under strong directional selection for adaptation to the novel trophic niches of scale-eating and molluscivory. We found evidence for two longstanding hypotheses of adaptive radiation: hybrid swarm origins and temporal stages of adaptation. Using a combination of population genomics, transcriptomics, and genome-wide association mapping, we demonstrate that this microendemic adaptive radiation of novel trophic specialists on San Salvador Island, Bahamas experienced twice as much adaptive introgression as generalist populations on neighboring islands and that adaptive divergence occurred in stages. First, standing regulatory variation in genes associated with feeding behavior (prlh, cfap20, and rmi1) were swept to fixation by selection, then standing regulatory variation in genes associated with craniofacial and muscular development (itga5, ext1, cyp26b1, and galr2) and finally the only de novo nonsynonymous substitution in an osteogenic transcription factor and oncogene (twist1) swept to fixation most recently. Our results demonstrate how ancient alleles maintained in distinct environmental refugia can be assembled into new adaptive combinations and provide a framework for reconstructing the spatiotemporal landscape of adaptation and speciation.

Altered distribution, aggregation, and protease resistance of cellular prion protein following intracranial inoculation

Prion protein (PrP^C) is a protease-sensitive and soluble cell surface glycoprotein expressed in almost all mammalian cell types. PrP^Sc, a protease-resistant and insoluble form of PrP^C, is the causative agent of prion diseases, fatal and transmissible neurogenerative diseases of mammals. Prion infection is initiated via either ingestion or inoculation of PrP^Sc or when host PrP^C stochastically refolds into PrP^Sc. In either instance, the early events that occur during prion infection remain poorly understood. We have used transgenic mice expressing mouse PrP^C tagged with a unique antibody epitope to monitor the response of host PrP^C to prion inoculation. Following intracranial inoculation of either prion-infected or uninfected brain homogenate, we show that host PrP^C can accumulate both intra-axonally and within the myelin membrane of axons suggesting that it may play a role in axonal loss following brain injury. Moreover, in response to the inoculation host PrP^C exhibits an increased insolubility and protease resistance similar to that of PrP^Sc, even in the absence of infectious prions. Thus, our results raise the possibility that damage to the brain may be one trigger by which PrP^C stochastically refolds into pathogenic PrP^Sc leading to productive prion infection.

Mutations in a P-type ATPase gene cause axonal degeneration

Neuronal loss and axonal degeneration are important pathological features of many neurodegenerative diseases. The molecular mechanisms underlying the majority of axonal degeneration conditions remain unknown. To better understand axonal degeneration, we studied a mouse mutant wabbler-lethal (wl). Wabbler-lethal (wl) mutant mice develop progressive ataxia with pronounced neurodegeneration in the central and peripheral nervous system. Previous studies have led to a debate as to whether myelinopathy or axonopathy is the primary cause of neurodegeneration observed in wl mice. Here we provide clear evidence that wabbler-lethal mutants develop an axonopathy, and that this axonopathy is modulated by Wld^s and Bax mutations. In addition, we have identified the gene harboring the disease-causing mutations as Atp8a2. We studied three wl alleles and found that all result from mutations in the Atp8a2 gene. Our analysis shows that ATP8A2 possesses phosphatidylserine translocase activity and is involved in localization of phosphatidylserine to the inner leaflet of the plasma membrane. Atp8a2 is widely expressed in the brain, spinal cord, and retina. We assessed two of the mutant alleles of Atp8a2 and found they are both nonfunctional for the phosphatidylserine translocase activity. Thus, our data demonstrate for the first time that mutation of a mammalian phosphatidylserine translocase causes axon degeneration and neurodegenerative disease.

Axonal degeneration is an important pathological feature of many neurodegenerative diseases, such as Alzheimer disease, Parkinson's disease, and amyotrophic lateral sclerosis. In most of these disease conditions, molecular mechanisms of axonal degeneration remain largely unknown. Spontaneous mouse mutants are important in human disease studies. Identification of a disease-causing gene in mice can lead to the identification of the human ortholog as the disease gene in humans. This approach has the power to identify unexpected genes and pathways involved in disease. Our study centered on wabbler lethal (wl) mutant mice, which display axonal degeneration in both the central and peripheral nervous systems. We identified the disease-causing gene in mice with different wl mutations. The mutations are in Atp8a2, a gene encoding a phosphatidylserine translocase. This protein functions to keep phosphatidylserine enriched to the inner leaflet of the plasma membrane. Our study demonstrates a new role for phospholipid asymmetry in maintaining axon health, and it also reveals a novel function for phosphatidyleserine translocase in neurodegenerative diseases.

Untethering the nuclear envelope and cytoskeleton: biologically distinct dystonias arising from a common cellular dysfunction

Most cases of early onset DYT1 dystonia in humans are caused by a GAG deletion in the TOR1A gene leading to loss of a glutamic acid (ΔE) in the torsinA protein, which underlies a movement disorder associated with neuronal dysfunction without apparent neurodegeneration. Mutation/deletion of the gene (Dst) encoding dystonin in mice results in a dystonic movement disorder termed dystonia musculorum, which resembles aspects of dystonia in humans. While torsinA and dystonin proteins do not share modular domain architecture, they participate in a similar function by modulating a structural link between the nuclear envelope and the cytoskeleton in neuronal cells. We suggest that through a shared interaction with the nuclear envelope protein nesprin-3α, torsinA and the neuronal dystonin-a2 isoform comprise a bridge complex between the outer nuclear membrane and the cytoskeleton, which is critical for some aspects of neuronal development and function. Elucidation of the overlapping roles of torsinA and dystonin-a2 in nuclear/endoplasmic reticulum dynamics should provide insights into the cellular mechanisms underlying the dystonic phenotype.

Disrupted membrane homeostasis and accumulation of ubiquitinated proteins in a mouse model of infantile neuroaxonal dystrophy caused by PLA2G6 mutations

Mutations in the PLA2G6 gene, which encodes group VIA calcium-independent phospholipase A2 (iPLA(2)beta), were recently identified in patients with infantile neuroaxonal dystrophy (INAD) and neurodegeneration with brain iron accumulation. A pathological hallmark of these childhood neurodegenerative diseases is the presence of distinctive spheroids in distal axons that contain accumulated membranes. We used iPLA(2)beta-KO mice generated by homologous recombination to investigate neurodegenerative consequences of PLA2G6 mutations. iPLA(2)beta-KO mice developed age-dependent neurological impairment that was evident in rotarod, balance, and climbing tests by 13 months of age. The primary abnormality underlying this neurological impairment was the formation of spheroids containing tubulovesicular membranes remarkably similar to human INAD. Spheroids were strongly labeled with anti-ubiquitin antibodies. Accumulation of ubiquitinated protein in spheroids was evident in some brain regions as early as 4 months of age, and the onset of motor impairment correlated with a dramatic increase in ubiquitin-positive spheroids throughout the neuropil in nearly all brain regions. Furthermore accumulating ubiquitinated proteins were observed primarily in insoluble fractions of brain tissue, implicating protein aggregation in this pathogenic process. These results indicate that loss of iPLA(2)beta causes age-dependent impairment of axonal membrane homeostasis and protein degradation pathways, leading to age-dependent neurological impairment. iPLA(2)beta-KO mice will be useful for further studies of pathogenesis and experimental interventions in INAD and neurodegeneration with brain iron accumulation.

Spontaneous murine neuroaxonal dystrophy: a model of infantile neuroaxonal dystrophy

The neuroaxonal dystrophies (NADs) in human beings are fatal, inherited, neurodegenerative diseases with distinctive pathological features. This report describes a new mouse model of NAD that was identified as a spontaneous mutation in a BALB/c congenic mouse strain. The affected animals developed clinical signs of a sensory axonopathy consisting of hindlimb spasticity and ataxia as early as 3 weeks of age, with progression to paraparesis and severe morbidity by 6 months of age. Hallmark histological lesions consisted of spheroids (swollen axons), in the grey and white matter of the midbrain, brain stem, and all levels of the spinal cord. Ultrastructural analysis of the spheroids revealed accumulations of layered stacks of membranes and tubulovesicular elements, strongly resembling the ultrastructural changes seen in the axons of human patients with endogenous forms of NAD. Mouse NAD would therefore seem a potentially valuable model of human NADs.

A quantitative survey of gravity receptor function in mutant mouse strains

The purpose of this research was to identify vestibular deficits in mice using linear vestibular evoked potentials (VsEPs). VsEP thresholds, peak latencies, and peak amplitudes from 24 strains with known genetic mutations and 6 inbred background strains were analyzed and descriptive statistics generated for each strain. Response parameters from mutant homozygotes were compared with heterozygote and/or background controls and all strain averages were contrasted to normative ranges. Homozygotes of the following recessive mutations had absent VsEPs at the ages tested: Espn(je), Atp2b2dfw-2J, Spnb4qv-lnd2J, Spnb4qv-3J, Myo7ash1, Tmie(sr), Myo6sv, jc, Pcdh15av-J, Pcdh15av-2J, Pcdh15av-3J, Cdh23v-2J, Sans(js), hr, Kcne1pkr and Pou3f4del. These results suggest profound gravity receptor deficits for these homozygotes, which is consistent with the structural deficits that have been documented for many of these strains. Homozygotes of Catna2cdf, Grid2ho4J, Wnt1sw, qk, and Mbpshi strains and heterozygotes of Grid2lc had measurable VsEPs but one or more response parameters differed from the respective control group (heterozygote or background strain) or were outside normal ranges. For example, qk and Mbpshi homozygotes showed significantly prolonged latencies consistent with the abnormal myelin that has been described for these strains. Prolonged latencies may suggest deficits in neural conduction; elevated thresholds suggest reduced sensitivity, and reduced amplitudes may be suggestive for reduced neural synchrony. One mutation, Otx1jv, had all VsEP response parameters within normal limits--an expected finding because the abnormality in Otxljv is presumably restricted to the lateral semicircular canal. Interestingly, some heterozygote groups also showed abnormalities in one or more VsEP response parameters, suggesting that vestibular dysfunction, although less severe, may be present in some heterozygous animals.

Spontaneous and induced mouse mutations with cerebellar dysfunctions: behavior and neurochemistry

Grid2(Lc) (Lurcher), Grid2(ho) (hot-foot), Rora(sg) (staggerer), nr (nervous), Agtpbp1(pcd) (Purkinje cell degeneration), Reln(rl) (reeler), and Girk2(Wv) (Weaver) are spontaneous mutations with cerebellar atrophy, ataxia, and deficits in motor coordination tasks requiring balance and equilibrium. In addition to these signs, the Dst(dt) (dystonia musculorum) spinocerebellar mutant displays dystonic postures and crawling. More recently, transgenic models with human spinocerebellar ataxia mutations and alterations in calcium homeostasis have been shown to exhibit cerebellar anomalies and motor coordination deficits. We describe neurochemical characteristics of these mutants with respect to regional brain metabolism as well as amino acid and biogenic amine concentrations, uptake sites, and receptors.

Primary neurologic screening and motor coordination of Dstdt-J mutant mice (dystonia musculorum) with spinocerebellar atrophy

The autosomal recessive dystonia musculorum (Dst(dt-J)) mutation causes degenerative lesions of peripheral and central sensory pathways. A test battery of motor, sensory, postural, and autonomic functions was used to compare young control and homozygous Dst(dt-J) mice. The Dst(dt-J) mutants were severely impaired for muscle strength, limb coordination, and postural reflexes. As a result of a loss in motor control, the mutants were hypoactive in the open-field and fell quickly from the stationary beam. In sensory tests, the acoustic startle response was impaired, but not tactile reflexes and contact righting, attesting to preserved labyrinthine function and non-lemniscal pathways. Dst(dt-J) mutants were also distinguishable from controls on the basis of tremor, a paler skin, piloerection, and half-open eyes, as well as low body weight and fecal boli. Grooming episodes were less frequent in the mutants but without any reduction in grooming time. The neurologic screening battery delineated the functional integrity of some sensorimotor pathways in a spinocerebellar mutant whose severe phenotype prevents a more elaborate evaluation.

Stathmin-deficient mice develop an age-dependent axonopathy of the central and peripheral nervous systems

Stathmin is a cytosolic protein that binds tubulin and destabilizes cellular microtubules, an activity regulated by phosphorylation. Despite its abundant expression in the developing mammalian nervous system and despite its high degree of evolutionary conservation, stathmin-deficient mice do not exhibit a developmental phenotype.(1) Here we report that aging stathmin(-/-) mice develop an axonopathy of the central and peripheral nervous systems. The pathological hallmark of the early axonal lesions was a highly irregular axoplasm predominantly affecting large, heavily myelinated axons in motor tracts. As the lesions progressed, degeneration of axons, dysmyelination, and an unusual glial reaction were observed. At the functional level, electrophysiology recordings demonstrated a significant reduction of motor nerve conduction velocity in stathmin(-/-) mice. At the molecular level, increased gene expression of SCG 10-like protein, a stathmin-related gene with microtubule destabilizing activity, was detected in the central nervous system of aging stathmin(-/-) mice. Together, these findings suggest that stathmin plays an essential role in the maintenance of axonal integrity.

Mutant beta-spectrin 4 causes auditory and motor neuropathies in quivering mice

The autosomal recessive mouse mutation quivering (qv), which arose spontaneously in 1953, produces progressive ataxia with hind limb paralysis, deafness and tremor. Six additional spontaneous alleles, qvJ, qv2J, qv3J, qv4J, qvlnd and qvlnd2J, have been identified. Ear twitch responses (Preyer's reflex) to sound are absent in homozygous qv/qv mice, although cochlear morphology seems normal and cochlear potentials recorded at the round window are no different from those of control mice. However, responses from brainstem auditory nuclei show abnormal transmission of auditory information, indicating that, in contrast to the many known mutations causing deafness originating in the cochlea, deafness in qv is central in origin. Here we report that quivering mice carry loss-of-function mutations in the mouse beta-spectrin 4 gene (Spnb4) that cause alterations in ion channel localization in myelinated nerves; this provides a rationale for the auditory and motor neuropathies of these mice.

Isolation and characterization of a bovine neural specific protein (CRMP-2) cDNA homologous to unc-33, a C. elegans gene implicated in axonal outgrowth and guidance

We have cloned the cDNA encoding bovine CRMP-2 from bovine brains. A full length cDNA encoding bovine CRMP-2 was isolated and sequenced. The deduced amino acid sequence reveals that the gene encodes a protein of 572 amino acids and is highly homologous to Caenorhabditis elegans unc-33, which controls the guidance and outgrowth of neuronal axons. The CRMP-2 transcript was present in bovine brains but not non-neural tissues, and its protein product existed in both soluble and membrane-bound forms. The expression of CRMP-2 protein and mRNA was upregulated during neuronal differentiation of rat PC12 cells. Immunoprecipitation of PC12 cell extracts shows that CRMP-2 was co-immunoprecipitated with a 190 kDa protein (p190). Both CRMP-2 and p190 were phosphorylated on serine residues in vivo and in vitro in a kinase assay of CRMP-2 immune complexes. Immunocytochemistry shows that CRMP-2 was exclusively localized in both the central and peripheral nervous systems in mouse embryos and detectable in the adult brain although the level of CRMP-2 decreased. The protein was expressed in the axon, dendrite, and cytoplasm of postmitotic neurons and in the cytoplasm of oligodendrocytes and astrocytes. The CRMP-2 gene maps to the region of mouse chromosome 14 syntenic with human chromosome 8p21. Taken together, these data suggest multi-functional roles for CRMP-2 in developing and adult nervous systems, and the biological activity of CRMP-2 could be regulated by phosphorylation reaction.

Molecular characterization of two mammalian bHLH-PAS domain proteins selectively expressed in the central nervous system

Here we describe two mammalian transcription factors selectively expressed in the central nervous system. Both proteins, neuronal PAS domain protein (NPAS) 1 and NPAS2, are members of the basic helix-loop-helix-PAS family of transcription factors. cDNAs encoding mouse and human forms of NPAS1 and NPAS2 have been isolated and sequenced. RNA blotting assays demonstrated the selective presence of NPAS1 and NPAS2 mRNAs in brain and spinal cord tissues of adult mice. NPAS1 mRNA was first detected at embryonic day 15 of mouse development, shortly after early organogenesis of the brain. NPAS2 mRNA was first detected during early postnatal development of the mouse brain. In situ hybridization assays using brain tissue of postnatal mice revealed an exclusively neuronal pattern of expression for NPAS1 and NPAS2 mRNAs. The human NPAS1 gene was mapped to chromosome 19q13.2-q13.3, and the mouse Npas1 gene to chromosome 7 at 2 centimorgans. Similarly, the human NPAS2 gene was assigned to chromosome 2p11.2-2q13, and the mouse Npas2 gene to chromosome 1 at 21-22 centimorgans. The chromosomal regions to which human NPAS1 and NPAS2 map are syntenic with those containing the mouse Npas1 and Npas2 genes, indicating that the mouse and human genes are true homologs.

Neuropeptides and neurotrophin receptor mRNAs in primary sensory neurons of aged rats

Neuropeptides and neurotrophin receptors are regulated in primary sensory neurons in response to axonal injury, and axonal lesions are characteristic stigmata of aging primary sensory neurons. We have therefore examined the expression of neuropeptides and neurotrophin receptor mRNAs in 30-month-old (median survival age) Sprague-Dawley rats to see if similar adaptive mechanisms operate in senescence. The content of neuropeptides was examined with immunohistochemistry (IHC) and in situ hybridization (ISH), and the cellular mRNA expression of neurotrophin receptors was studied with ISH. All of the aged rats had symptoms of hind limb incapacity (posterior paralysis), but fore limbs did not seem affected. The size-distribution of neuronal profiles in cervical and lumbar dorsal root ganglia (DRGs) was similar in aged and young adult (2-3 months old) rats. In aged rats, the DRG neurons showed an increase in both immunolabelling and mRNA content of neuropeptide tyrosine (NPY), as well as an increased cellular expression of galanin mRNA. In the same animals, there were decreased cellular levels of calcitonin gene-related peptide (CGRP; IHC and ISH) and substance P (SP; IHC and ISH), while the difference in neuronal somatostatin (IHC and ISH) was small. The distribution of neuropeptide immunoreactivities in the dorsal horn of the corresponding spinal cord segments revealed a decreased labelling for CGRP-, SP-, and somatostatin-like immunoreactivities (LI) in the aged rats at both cervical and lumbar levels. NPY- and galanin-LI had a similar distribution in aged and young adult rats. NPY-immunoreactive fibers were also encountered in the dorsal column of aged but not young adult rats. ISH revealed that most of the primary sensory neurons express mRNA for the p75 low-affinity neurotrophin receptor (p75-LANR) and that there was no discernible difference between young adult and aged rats. The labelling intensity for mRNA encoding high-affinity tyrosine kinase receptors (TrkA, TrkB, and TrkC) was decreased in aged rat DRG neurons, while the percentage of neuronal profiles expressing mRNA for TrkA/B/C was similar in young adult and aged rats. The changed pattern of neuropeptide expression in primary sensory neurons of aged rats resembled that seen in young adult rats subjected to axonal injury of peripheral sensory nerves and may, thus, indicate aging-related lesions of sensory fibers. Since NPY is primarily present in large and galanin in small DRG neurons, the stronger effect on NPY as compared to galanin expression may indicate that aging preferentially affects neurons associated with mechanoreception (A alpha and A beta fibers) as compared to nociceptive units (A delta and C fibers). Furthermore, the observed changes in neuropeptide expression were most pronounced in lumbar DRGs, that harbors the sensory neurons supplying the affected hindlimbs of the rats.

Exploration and motor coordination in dystonia musculorum mutant mice

Dystonia musculorum (dt) mutant mice, characterized by degeneration of spinocerebellar tracts, were impaired in terms of horizontal and vertical motor activity, hole poking, exploration, and motor coordination. In tests of motor coordination, their deficits were more severe than those of previously tested mutant mice with degeneration of cerebellar cells. However, unlike other cerebellar mutants, dt mutants alternated above chance levels in a two-trial spontaneous alternation test, which is a test of inhibitory tendencies and spatial orientation, and so dt mutants may not be impaired in these functions.

Vestibular morphology in relation to age and circling behavior

The DBA/2 strain of mice have genetically induced vestibular dysfunction that presents grossly at an early age as circling behavior and abnormal righting reflexes. The vestibular morphology of this strain has not previously been examined. DBA/2 mice of different ages that showed circling behavior were sacrificed and then had their inner ears immediately removed and fixed in glutaraldehyde. The specimens were prepared for light, transmission and scanning electron microscopy. Additional specimens at 10 months of age were fixed with paraformaldehyde for immunohistochemical investigation and labelling of alpha-actinin. Non-circling litter mates served as controls. The morphology and immunohistochemistry of the vestibular end-organs were evaluated as a function of age and circling behavior. The sensory epithelium of the ampulla and utricle in the circling mice showed softening of the cuticle, hair cell cytoplasmic herniation, expelled cellular debris, fused stereocilia and giant hair cells that progressively increased in severity with age. The non-circling litter mates showed similar but less severe pathology of the vestibular sensory epithelium. The immunohistochemical analysis showed no differences at a magnification of 400 x.