Synaptosomal glutamate uptake declines progressively in the spinal cord of a mutant mouse with motor neuron disease

Treatment of the Ppt1(-/-) mouse model of infantile neuronal ceroid lipofuscinosis with the N-methyl-D-aspartate (NMDA) receptor antagonist memantine

The neuronal ceroid lipofuscinoses, a family of neurodegenerative lysosomal storage disorders, represent the most common cause of pediatric-onset neurodegeneration. The infantile form has a devastatingly early onset and one of the fastest-progressing disease courses. Despite decades of research, the molecular mechanisms driving neuronal loss in infantile neuronal ceroid lipofuscinosis remain unknown. We have previously shown that N-methyl-d-aspartate (NMDA)-type glutamate receptors in the Ppt1(-/-) mouse model of this disease exhibit a hyperfunctional phenotype and postulate that aberrant glutamatergic activity may contribute to neural pathology in both the mouse model and human patients. To test this hypothesis, we treated Ppt1(-/-) mice with the NMDA receptor antagonist memantine and assessed their response to the drug using an accelerating rotarod. At 20 mg/kg, memantine treatment induced a delayed but notable improvement in Ppt1(-/-) mice. Much remains to be assessed before moving to patient trials, but these results suggest memantine has potential as a treatment.

Cell biology and function of neuronal ceroid lipofuscinosis-related proteins

Neuronal ceroid lipofuscinoses (NCL) comprise a group of inherited lysosomal disorders with variable age of onset, characterized by lysosomal accumulation of autofluorescent ceroid lipopigments, neuroinflammation, photoreceptor- and neurodegeneration. Most of the NCL-related genes encode soluble and transmembrane proteins which localize to the endoplasmic reticulum or to the endosomal/lysosomal compartment and directly or indirectly regulate lysosomal function. Recently, exome sequencing led to the identification of four novel gene defects in NCL patients and a new NCL nomenclature currently comprising CLN1 through CLN14. Although the precise function of most of the NCL proteins remains elusive, comprehensive analyses of model organisms, particularly mouse models, provided new insight into pathogenic mechanisms of NCL diseases and roles of mutant NCL proteins in cellular/subcellular protein and lipid homeostasis, as well as their adaptive/compensatorial regulation at the transcriptional level. This review summarizes the current knowledge on the expression, function and regulation of NCL proteins and their impact on lysosomal integrity. This article is part of a Special Issue entitled: The Neuronal Ceroid Lipofuscinoses or Batten Disease.

Functional biology of the neuronal ceroid lipofuscinoses (NCL) proteins

Neuronal ceroid lipofucinoses (NCLs) are a group of severe neurodegenerative disorders characterized by accumulation of autofluorescent ceroid lipopigment in patients' cells. The different forms of NCL share many similar pathological features but result from mutations in different genes. The genes affected in NCLs encode both soluble and transmembrane proteins and are localized to ER or to the endosomes/lysosomes. Due to selective vulnerability of the central nervous system in the NCL disorders, the corresponding proteins are proposed to have important, tissue specific roles in the brain. The pathological similarities of the different NCLs have led not only to the grouping of these disorders but also to suggestion that the NCL proteins function in the same biological pathway. Despite extensive research, including the development of several model organisms for NCLs and establishment of high-throughput techniques, the precise biological function of many of the NCL proteins has remained elusive. The aim of this review is to summarize the current knowledge of the functions, or proposed functions, of the different NCL proteins.

The development of behavioral abnormalities in the motor neuron degeneration (mnd) mouse

The motor neuron degeneration (mnd) mouse, which has widespread abnormal accumulating lipoprotein and neuronal degeneration, has a mutation in CLN8, the gene for human progressive epilepsy with mental retardation (EPMR). EPMR is one of the neuronal ceroid lipofuscinoses (NCLs), a group of neurological disorders characterized by autofluorescent lipopigment accumulation, blindness, seizures, motor deterioration, and dementia. The human phenotype of EPMR suggests that, in addition to the motor symptoms previously categorized, various types of progressive behavioral abnormalities would be expected in mnd mice. We have therefore examined exploratory behavior, fear conditioning, and aggression in 2-3 month and 4-5 month old male mnd mice and age-matched C57BL/6 (B6) controls. The mnd mice displayed increased activity with decreased habituation in the activity monitor, poor contextual and cued memory, and heightened aggression relative to B6 controls. These behavioral deficits were most prominent at 4-5 months of age, which is prior to the onset of gross motor symptoms at 6 months. Our results provide a link from the mutation via pathology to a quantifiable multidimensional behavioral phenotype of this naturally occurring mouse model of NCL.

The expression of the glutamate re-uptake transporter excitatory amino acid transporter 1 (EAAT1) in the normal human CNS and in motor neurone disease: an immunohistochemical study

A monoclonal antibody to excitatory amino acid transporter 1 (EAAT1) has been generated which robustly stains paraffin-embedded, formaldehyde-fixed as well as snap-frozen human post-mortem brain tissue. We have used this antibody to map the distribution of EAAT1 throughout normal human CNS tissue. In addition this antibody has been used to perform a semi-quantitative immunohistochemical analysis of the expression of EAAT1 in motor cortex and cervical cord tissue taken from motor neurone disease cases (n=17) and neurologically normal controls (n=12). By comparing the relative optical density measurements of identical regions of motor cortex and cervical spinal cord an increase in the expression levels of EAAT1 was observed in motor neurone disease tissue compared to the control tissue and in both motor cortex and cervical spinal cord (9-17% and 13-33% increases respectively). EAAT1 was observed to be the most abundant transporter in more "caudal" brain regions such as the diencephalon and brainstem and its expression in other regions was frequently more uniform than that of EAAT2. In the motor cortex, EAAT1 immunoreactivity was present in all grey matter laminae, with some staining of individual astrocytes in the white matter. In spinal cord, EAAT1 immunoreactivity was strongest in the substantia gelatinosa. In the ventral horn, motor neurones were surrounded with a dense rim of perisomatic EAAT1 immunoreactivity, and the neuropil showed diffuse staining. Additional studies using double-labelling immunocytochemistry demonstrated that astrocytic co-localisation of EAAT1 and EAAT2 may occasionally be seen, but was not widespread in the human CNS and that in general astrocytes were positive for either EAAT1 or EAAT2. These results demonstrate that the EAAT1 has a widespread abundance throughout all regions of the human CNS examined and that there exist discrete populations of astrocytes that are positive solely for either EAAT1 or EAAT2. Furthermore, there is evidence to suggest that altered EAAT1 expression in motor neurone disease follows a different pattern to the reported changes of EAAT2 expression in this condition, indicating that the role of glutamate transporters in the pathogenesis of motor neurone disease appears more complex than previously appreciated.

Onset and progression of motor deficits in motor neuron degeneration (mnd) mice are unaltered by the glycine/NMDA receptor antagonist L-701,324 or the MAO-B inhibitor R(-)-deprenyl

NMDA-mediated neurotoxicity and oxidative stress have been implicated in the etiology of a number of degenerative diseases including motor neuron disease. The present study examined the effect of chronic administration of the glycine/NMDA receptor antagonist L-701,324 and the monoamine oxidase B inhibitor (r)-deprenyl on the onset and rate of progression of neurological impairment in the motor neuron degeneration (mnd) mouse, a murine model of neurodegeneration. Neurological assessment of mnd mice revealed an onset of motor deficits at 6 months of age as observed by the loss of hindlimb reflex extension. By 7 months, balance was also markedly impaired as measured by deficits in rotarod performance and ability to remain on balancing beams. At 8 months of age mnd mice exhibited gross abnormalities in walking pattern; animals were unable to flex their hindlimbs and tended to walk in small labored movements. Daily administration of L-701,324 (10 mg/kg p.o.) or r(-)-deprenyl (1 mg/kg p.o.) to mnd mice from 4 to 8 months of age failed to delay the onset of symptoms or slow the rate of deterioration of motor performance. These findings suggest that excessive activation of NMDA receptors may not be involved in the pathological process leading to motor neuron dysfunction in mnd mice and do not suggest a protective effect of deprenyl on motor neurons in these mice.

Batten disease: four genes and still counting

The neuronal ceroid lipofuscinoses (NCLs, also known as Batten disease) are the most common childhood neurodegenerative disease. They are a group of inherited neurodegenerative disorders characterized by the accumulation of autofluorescent storage material in many cell types. Clinical features include seizures, psychomotor deterioration, and blindness, the ages and order of onset of which differ for each NCL type. An increasing number of subtypes caused by mutations in different genes are now recognized. With the advent of molecular genetics the basic genetic defect underlying each NCL phenotype is being determined, thus shedding light on the molecular basis of the NCLs and opening the way for the development of effective treatment. Four genes have been identified to date. The function of two of these is known and suggests that the primary defect in the NCLs lies in lysosomal proteolysis, the first example of this type of disease. However, since the function of the other two genes remains elusive, and at least four more genes remain to be identified, the molecular basis underlying the NCLs may be more complex than originally predicted.

Blood glutamate levels in patients with motor neuron disease

This study was undertaken to evaluate the role of excitatory amino acid glutamate (Glu) in the pathophysiology of motor neuron disease (MND). It was observed that blood Glu levels were significantly higher in MND patients with respect to healthy controls. The data indicate that Glu homeostasis is altered in the patients with MND.

Spinal cord GLT-1 glutamate transporter and blood glutamic acid alterations in motor neuron degeneration (Mnd) mice

This study characterizes for the first time neurochemical mechanisms in Mnd mice, initially described as a model of motor neuron disease and more recently proposed as a model for neuronal ceroid lipofuscinosis. A selective decrease (-30%) of [3H]glutamate uptake was found in spinal cord but not cortical synaptosomes of Mnd mice aged 28 weeks, when they show histopathological alterations, complete blindness and moderate neurological deficits. In spite of the widespread presence of stored material in neurons in many brain regions and spinal cord, the active transport of [3H]serotonin, [3H]dopamine and depolarization-induced [3H]serotonin release were not affected. Spinal EAAC1 glutamate transporter protein was significantly decreased in some but not all aged mice by 36% on average, possibly due to the loss of motor neurons. GLT-1 immunoreactivity was reduced by 34% in 28-week-old Mnd mice, while GLAST immunoreactivity was not affected. In Mnd mice aged 14 weeks, when there was no apparent alteration of motor function, the defect in the glial transporter protein GLT-1 was similar to that in 28-week-old mice (25%). Blood glutamic acid concentration was increased in Mnd mice aged 14-22 weeks. We suggest that the early decrease of GLT-1 protein might raise the extrasynaptic glutamic acid concentration, and contribute to the loss of motor neurons in affected mice, resulting in low [3H]glutamate uptake, low EAAC1 immunoreactivity and neurological deficits.

Alteration of enzymatic activities implicating neuronal degeneration in the spinal cord of the motor neuron degeneration mouse during postnatal development

Oxidative stress is suggested as a significant causative factor for pathogenesis of neuronal degeneration on spinal cord of human ALS. We measured some enzymic activities implicating neuronal degeneration process, such as cytochrome c oxidase (CO), superoxide dismutase (SOD), and transglutaminase (TG) in spinal cord of an animal model of ALS, motor neuron degeneration (Mnd) mouse, a mutant that exhibits progressive degeneration of lower spinal neurons during developmental growth, and compared them with age-matched control C57BL/6 mice. CO activity in Mnd spinal cord decreased during early postnatal period, while SOD activity reduced in later stage. In Mnd tissue, TG activity in lumbar cord was increasing during early stage, but tended to decline in later period gradually. These biochemical alterations became evident prior to the appearance of clinical motor dysfunction which were observed in later stages of development in Mnd spinal cord.

Expression of the glial glutamate transporter EAAT2 in the human CNS: an immunohistochemical study

Glutamate transporters play an essential role in terminating the excitatory glutamatergic signal at post-synaptic receptors and in protecting neurones from excitotoxic effects, as well as replenishing the neurotransmitter supply at glutamatergic synapses. The distribution and density of glutamate transporters may be important determinants of vulnerability to glutamate-mediated injury. There is emerging evidence that glutamate transporter dysfunction may be present in motor neurone disease (MND). In this study, a monoclonal antibody, suitable for immunohistochemistry (IHC) in human post-mortem tissue, was produced to the human astrocytic glutamate transporter EAAT2 (excitatory amino acid transporter 2). Western blotting of homogenates of human cortical tissue with the EAAT2 antibody produced a discrete band at 66 kDa. Detailed IHC analysis of the expression of the EAAT2 protein in the human CNS was undertaken. EAAT2 was exclusively localised to astrocytes, with preferential expression in the caudate nucleus, nucleus basalis of Meynert, spinal ventral horn, cerebral cortex and hippocampus, but with lower levels of expression throughout many other CNS regions. Motor neurone groups vulnerable to neurodegeneration in MND appeared distinctive in being surrounded by extensive, coarse, strongly immunoreactive perisomatic glial profiles. Motor neurone groups which tend to be spared in MND, such as those present in the oculomotor nucleus, showed a lower expression of EAAT2, with fewer perisomatic profiles. The EAAT2 antibody will provide a useful tool for increasing our understanding of the role of EAAT2 in excitatory neurotransmission in health and disease states.

AMPA receptor-mediated slow neuronal death in the rat spinal cord induced by long-term blockade of glutamate transporters with THA

Excitotoxicity secondary to the loss of glutamate transporters (GluT) has been proposed as a possible pathogenetic mechanism for neuronal degeneration in amyotrophic lateral sclerosis. We therefore investigated whether prolonged in vivo pharmacologic inhibition of GluT would result in neuronal damage in the rat. DL-Threo-beta-hydroxyaspartate (THA), a potent GluT inhibitor, and glutamate were continuously infused into the rat spinal subarachnoid space by using a mini-osmotic pump. Animals that received both THA and glutamate, but not those received either singly, displayed tail paralysis with or without hind-limb paralysis and urinary incontinence after the third postoperative day. Pathologically, symptomatic animals exhibited neuronal loss with a variable extent of gliosis preferentially involving the dorsal horn of the lumbosacral cord. In the rostral spinal segments adjacent to those regions of intense pathologic changes, small neurons in the dorsal horn were selectively destroyed, a pattern similar to the late-onset neuronal damage induced by continuous intrathecal administration of 1-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) [R. Nakamura et al., Brain Res. 654 (1994) 279-285]. These behavioral and pathologic changes were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), suggesting that pharmacologic blockade of GluT causes selective neuronal damage in vivo by AMPA receptor activation.

Studies of the coding region of the neuronal glutamate transporter gene in amyotrophic lateral sclerosis

Studies of the coding region of the neuronal glutamate transporter of 6 amyotrophic lateral sclerosis (ALS) patients and 10 controls show an identical pattern of four reported amino acid variations. No mutations and polymorphisms were detected in 5 sporadic ALS patients and a single patient with the familial form of the disease.

Neurotransmitter transporters

In the past year, our knowledge of neurotransmitter transporters has increased significantly. Recently, new members of two families of plasma membrane uptake carriers have been cloned, and the stoichiometries, physiological function and mechanisms of modulation of some of these transporters are now better understood. These developments highlight the possible role of neurotransmitter transporters in disease states, in the development of the nervous system, and as targets for therapeutic drugs.

Excitatory amino acids, free radicals and the pathogenesis of motor neuron disease

The cause of motor neuron disease (MND) remains unknown, but the pathogenic involvement of excitatory amino acid (EAA) neurotransmitters and related exogenous compounds has been proposed. We discuss current concepts of the mechanisms of action of EAAs and the evidence for links between these neurotransmitters and free radical hypotheses of neuronal damage. These concepts are especially pertinent following reports of mutations in the gene encoding the free radical scavenging enzyme, copper-zinc superoxide dismutase, in familial MND. New approaches to treatment are suggested by advances in understanding of the disease.

(-)-Deprenyl alters the survival of adult murine facial motoneurons after axotomy: increases in vulnerable C57BL strain but decreases in motor neuron degeneration mutants

The effect of (-)-deprenyl on the survival of axotomized adult murine facial motoneurons was investigated. Previously, (-)-deprenyl was shown to increase the number of rat facial motoneurons (FMns) surviving after axotomy at postnatal day 14, apparently by compensating for the loss of muscle-derived trophic factor. In the present study, three different strains of adult mice--A/J, C57BL/6J, and a congenic substrain of the C57BL/6J mice, the C57BL/Mnd mutants--underwent unilateral facial nerve transection. FMns were counted from serial sections taken through the entire length of the facial nuclei ipsilateral and contralateral to the facial nerve transections in animals sacrificed 21 days after axotomy. Subgroups of C57BL/6J and Mnd mutants were treated with either saline or 1.0 mg/kg (-)-deprenyl for 21 days. Another subgroup of Mnd mutants were treated with the metabolites of (-)-deprenyl, a mixture of (-)-amphetamine and (-)-methamphetamine, at a dosage equimolar to 1.0 mg/kg (-)-deprenyl. The number of surviving facial motoneurons in the A/J strain was 90% of unlesioned, control values which supports previous findings that adult FMns receive adequate trophic support and thus can survive loss of muscle-derived trophic support. In the C57BL/6J strain, the facial motoneuron survival was 35% and (-)-deprenyl increased the survival to 50.5%. Mnd mutants showed 62.4% survival; however, (-)-deprenyl decreased the number of motoneurons to 54.9% and amphetamine and methamphetamine treatment further decreased the motoneuron survival to 41.1%. These findings show that FMns in the Mnd mutants and their parental strain, C57BL/6J mice, show greater vulnerability to axotomy as compared to other adult strains of mice. The vulnerability is similar to that found in early postnatal life. (-)-Deprenyl increases the survival of the axotomized C57BL/6J FMns but its major metabolites, (-)-methamphetamine and (-)-amphetamine, further decrease FMn survival in the C57BL/Mnd mutants, possibly due to the induction of neurotoxic proteins causing programmed neuronal death. The efficacy of (-)-deprenyl in increasing the survival of damaged neurons would be expected to decrease as dosage increased above the dosage sufficient to induce maximum neuronal rescue (approximately 0.01 mg/kg) but would decrease as the dosage exceeded that necessary to produce toxic concentrations of the metabolites of (-)-deprenyl (1.0 mg/kg in this study).