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Biomedical subjects

R E Burke

Publications and source records attributed to R E Burke.

At least 19 recordsLinked to original sources
PubMed2001

The expression of mRNAs for the proteasome complex is developmentally regulated in the rat mesencephalon.

The proteasome is a large protease complex that recognizes, unfolds and degrades ubiquitinated proteins. Evidence is now accumulating that the ubiquitin-proteasome system may play an important role in neuronal apoptosis. However, little is known about the involvement of the proteasome in neuronal death in vivo, and there has been no prior analysis of the developmental expression of proteasome subunits in brain during periods of natural and inducible apoptotic death. We therefore studied the mRNA expression levels, using Northern analysis, of a subunit from each of the three key components of the proteasome in the rat mesencephalon from E21 through development and in adulthood. We measured mRNA expression for RC6 (a subunit of 20S), p112 (a subunit of 19S) and PA28-alpha (a subunit of 11S). The expression of PA28-alpha in rat mesencephalon was highest at the earliest times studied, and then decreased at PND 21, 28 and adult, in comparison to E21 (P<0.05) and PND 2, 4 and 7 (P<0.01). The expression of RC6 was lower in adult in comparison to PND 2, 4 and 21 (P<0.05) and PND 14 (P<0.01). There were no significant differences in the mRNA levels of p112 at various times studied. In situ hybridization at PND 7 indicated that all the subunits studied are particularly abundant in the SNpc. Thus, PA28-alpha and RC6 are developmentally regulated, and they may therefore play a role in developmental cell death or differentiation in neurons of the SN.

Animals↗
PubMed2001

Synuclein-1 is selectively up-regulated in response to nerve growth factor treatment in PC12 cells.

Mutations in the alpha-synuclein gene have recently been identified in families with inherited Parkinson's disease and the protein product of this gene is a component of Lewy bodies, indicating that alpha-synuclein is involved in Parkinson's disease pathogenesis. A role for normal alpha-synuclein in synaptic function, apoptosis or plasticity responses has been suggested. We show here that in rat pheochromocytoma PC12 cells synuclein-1, the rat homolog of human alpha-synuclein, is highly and selectively up-regulated at the mRNA and protein levels after 7 days of nerve growth factor treatment. Synuclein-1 expression appears neither sufficient nor necessary for the neuritic sprouting that occurs within 1-2 days of nerve growth factor treatment. Rather, it likely represents a component of a late neuronal maturational response. Synuclein-1 redistributes diffusely within the cell soma and the neuritic processes in nerve growth factor-treated PC12 cells. Cultured neonatal rat sympathetic neurones express high levels of synuclein-1, with a diffuse intracellular distribution, similar to neuronal PC12 cells. These results suggest that levels of synuclein-1 may be regulated by neurotrophic factors in the nervous system and reinforce a role for alpha-synuclein in plasticity-maturational responses. In contrast, there is no correlation between synuclein expression and apoptotic death following trophic deprivation.

Animals↗
PubMed2001

Expression of cyclin-dependent kinase 5 and its activator p35 in models of induced apoptotic death in neurons of the substantia nigra in vivo.

Cyclin-dependent kinase 5 is predominantly expressed in postmitotic neurons and plays a role in neurite elongation during development. It has also been postulated to play a role in apoptosis in a variety of cells, including neurons, but little is known about the generality and functional significance of cdk5 expression in neuronal apoptosis in living brain. We have therefore examined its expression and that of its known activators, p35, p39 and p67, in models of induced apoptosis in neurons of the substantia nigra. We find that cdk5 is expressed in apoptotic profiles following intrastriatal injection of 6-hydroxydopamine and axotomy. It is expressed exclusively in profiles which are in late morphologic stages of apoptosis. In these late stages, derivation of the profiles from neurons, and localization of expression to the nucleus, can be demonstrated by co-labeling with a neuron-specific nuclear marker, NeuN. In another model of induced apoptotic death in nigra, produced by developmental striatal lesion, kinase activity increases in parallel with cell death. While mRNAs for all three cdk5 activators are expressed in nigra during development, only p35 protein is expressed in apoptotic profiles. We conclude that cdk5/p35 expression is a general feature of apoptotic neuron death in substantia nigra neurons in vivo.

Animals↗
PubMed2001

Short-term synaptic depression in the neonatal mouse spinal cord: effects of calcium and temperature.

We have studied short-term synaptic depression of excitatory postsynaptic potentials (EPSPs) in lumbosacral motoneurons in the isolated, in vitro spinal cord of neonatal mice at 2-4 days postnatal age. We used 2-amino-5-phosphonovaleric acid (AP5; 100 microM) to suppress spontaneous and stimulus-evoked polysynaptic activity. Monosynaptic EPSPs were generated by trains of 10 pulses stimuli delivered to a dorsal root at eight frequencies between 0.125 and 16 Hz. The amplitudes of the second (R2), third (R3), and the average of R8, R9, and R10 (tail) EPSPs, normalized by the first EPSP (R1), defined the shapes of synaptic depression curves. Tail responses were increasingly depressed as stimulation frequency increased but R2 and R3 exhibited relative facilitation at frequencies >1 Hz. Control experiments indicated that the depression curves were not explained by presynaptic activation failure. Lowering external Ca(2+) concentration ([Ca(2+)](o)) from 2.0 to 0.8 mM without changing [Mg(2+)](o) reduced average R1 amplitudes and R2 depression with little change in tail depression. Conversely, increasing [Ca(2+)](o) to 4.0 mM increased average R1 amplitude and R2 depression but again did not change tail depression. Increasing the bath temperature from 24 to 32 degrees C produced little change in R1 amplitudes but markedly reduced the depression of all responses at most frequencies. We developed an empirical model, based on mechanisms described in more accessible synaptic systems, that assumes: transmitter is released from a constant fraction, f, of release-ready elements in two presynaptic compartments (N and S) that are subsequently renewed by independent processes with exponential time constants (tau(N) and tau(S)); an activation-dependent facilitation of transmitter release with constant increment and fast exponential decay; and a more slowly decaying, activation-dependent augmentation of the rate of renewal (tau(N)) of N. The model gave satisfactory fits to data from all [Ca(2+)](o) conditions and implied that f and the increments of the facilitation and augmentation processes were all changed in the same direction as [Ca(2+)](o), without changing the time constants. In contrast, model fits to the 32 degrees C data implied that the process time constants all decreased by 40-45% while the presumably Ca(2+)-related weighting factors were unchanged. The model also successfully matched the normalized amplitudes of EPSPs during trains with irregular intervals.

2-Amino-5-phosphonovalerate↗
PubMed2001

Patterns of locomotor drive to motoneurons and last-order interneurons: clues to the structure of the CPG.

We have examined the linkage between patterns of activity in several hindlimb motor pools and the modulation of oligosynaptic cutaneous reflex pathways during fictive locomotion in decerebrate unanesthetized cats to assess the notion that such linkages can shed light on the structure of the central pattern generator (CPG) for locomotion. We have concentrated attention on the cutaneous reflex pathways that project to the flexor digitorum longus (FDL) motor pool because of that muscle's unique variable behavior during normal and fictive locomotion in the cat. Differential locomotor control of last-order excitatory interneurons in pathways from low-threshold cutaneous afferents in the superficial peroneal and medial plantar afferents to FDL motoneurons is fully documented for the first time. The qualitative patterns of differential control are shown to remain the same whether the FDL muscle is active in early flexion, as usually found, or during the extension phase of fictive locomotion, which is less common during fictive stepping. The patterns of motor pool activity and of reflex pathway modulation indicate that the flexion phase of fictive locomotion has distinct early versus late components. Observations during "normal" and unusual patterns of fictive stepping suggest that some aspects of locomotor pattern formation can be separated from rhythm generation, implying that these two CPG functions may be embodied, at least in part, in distinct neural organizations. The results are discussed in relation to a provisional circuit diagram that could explain the experimental findings.

Animals↗
PubMed2000

Developmental cell death in dopaminergic neurons of the substantia nigra of mice.

Dopaminergic neurons in the substantia nigra pars compacta (SNpc) undergo natural cell death during development in rats. Controversy exists as to the occurrence of this phenomenon in SNpc dopaminergic neurons in the developing mouse. Herein, by using an array of morphologic techniques, we show that many SNpc neurons fulfill the criteria for apoptosis and that the number of apoptotic neurons in the SNpc vary in a time-dependent manner from postnatal day 2 to 32. These dying neurons also show evidence of DNA fragmentation, of activated caspase-3, and of cleavage of beta-actin. Some, but not all of the SNpc apoptotic neurons still express their phenotypic marker tyrosine hydroxylase, confirming their dopaminergic nature. Consistent with the importance of target-derived trophic support in modulating developmental cell death, we demonstrate that destruction of intrinsic striatal neurons by a local injection of quinolinic acid (QA) dramatically enhances the magnitude of SNpc apoptosis and results in a lower number of adult SNpc dopaminergic neurons. Strengthening the apoptotic nature of the observed SNpc developmental cell death, we demonstrate that overexpression of the anti-apoptotic protein Bcl-2 attenuates both natural and QA-induced SNpc apoptosis. The present study provides compelling evidence that developmental neuronal death with a morphology of apoptosis does occur in the SNpc of mice and that this process plays a critical role in regulating the adult number of dopaminergic neurons in the SNpc.

Age Factors↗
PubMed2000

Upregulation of cytosolic branched chain aminotransferase in substantia nigra following developmental striatal target injury.

We have previously shown that apoptotic cell death can be induced in substantia nigra (SN) by developmental striatal target lesion. In this model, only a portion of nigral neurons dies, so it provides a paradigm to examine not only the molecular basis of cell death, but also the cellular responses of adjacent neurons which survive. Using a differential display approach, we have found that cytosolic branched chain aminotransferase (BCATc) mRNA is upregulated in SN in this model. This upregulation is associated with an increased number of BCATc-positive neuronal profiles, demonstrated by immunostaining. BCATc-positive neurons show normal morphology and rarely contain apoptotic chromatin. We conclude that BCATc is upregulated in neurons, which are likely to survive, and plays a role in either maintenance of viability or restoration of normal function.

Animals↗
PubMed1999

Synuclein expression is decreased in rat substantia nigra following induction of apoptosis by intrastriatal 6-hydroxydopamine.

We have previously shown that synuclein is upregulated in the substantia nigra following developmental lesion of the target striatum. In this model, synuclein is preferentially expressed in normal-appearing neurons, rather than those which undergo apoptosis. It has been proposed, however, that synuclein may mediate apoptosis in other contexts, such as that induced by neurotoxins. To examine this possibility, we have studied a model in which apoptosis is induced in dopamine neurons by intrastriatal injection of 6-hydroxydopamine. In this model synuclein mRNA and protein expression are not upregulated at any time point, but instead diminish as dopamine neurons die. We observe a complete dissociation between apoptotic morphology and synuclein protein expression. We conclude that synuclein is unlikely to play a direct role in apoptotic death in dopamine neurons and is more likely, in the target injury model, to play a role in protection or restoration of neurons which survive.

Animals↗
PubMed1999

Alpha-synuclein expression in substantia nigra and cortex in Parkinson's disease.

Mutations in the human alpha-synuclein gene have been identified in several families of European descent with early-onset Parkinson's disease (PD). We sequenced the complete alpha-synuclein cDNA from substantia nigra and cortex from nine patients with PD and eight control subjects. No mutations were found. We then analyzed alpha-synuclein mRNA levels using a ribonuclease protection assay. Two major protected bands of alpha-synuclein mRNA, possibly representing two splice variants of the gene, were observed. Alpha-synuclein mRNA was significantly diminished in the substantia nigra of patients with PD compared with control subjects but not in the cortex. Our findings suggest that decreased synuclein mRNA may be an early alteration in the SN in PD, and imply that decreased levels of the protein may play a role in the pathogenesis of sporadic cases of the disease.

Adult↗
PubMed1999

The use of state-dependent modulation of spinal reflexes as a tool to investigate the organization of spinal interneurons.

This review examines the proposition that state-dependent modulation of transmission through spinal reflex pathways can be used as an investigative tool to reveal details about the organization of spinal interneurons into functional circuits. The first set of examples includes the use of spinal and supraspinal lesions, as well as the administration of the drug l-dihydroxyphenylalanine (l-DOPA), to produce different, relatively stable "states" of the central nervous system (CNS), revealing previously unsuspected spinal pathways activated by the flexor reflex afferents (FRA). The second set of examples deals with the use of fictive locomotion and scratching to investigate the organization of oligosynaptic excitatory and inhibitory reflex pathways from cutaneous and muscle afferents. As in the first set of examples, several hitherto unknown reflex pathways have been found only during the flexion or extension phases of rhythmic locomotion, which are regarded as different CNS states. Differences in the patterns of control can be used to infer the existence of distinct sets of reflex pathway interneurons that have remarkably precise input/output relations.

Afferent Pathways↗
PubMed1999

Expression of c-fos, c-jun, and c-jun N-terminal kinase (JNK) in a developmental model of induced apoptotic death in neurons of the substantia nigra.

The transcription factors c-fos and c-jun have been proposed to play a role in the initiation of programmed cell death in neurons. We have shown that programmed cell death, with the morphology of apoptosis, occurs in dopamine neurons of the substantia nigra (SN) during normal postnatal development and that this death event can be induced by early striatal target injury. We have investigated the relationship between c-fos and c-jun protein expression and induced death in neurons of the SN. Although c-fos is induced, it is unlikely to play a role in cell death, because its expression is not well correlated with apoptotic death either temporally or at a cellular level. Expression of c-jun, however, is both temporally and regionally correlated with induction of death, and, at a cellular level, it colocalizes with apoptotic morphology. The increased expression of c-jun is likely to be functionally significant, because it is associated with increased c-jun N-terminal kinase (JNK) and phosphorylated c-jun expression. JNK expression also colocalizes with apoptotic morphology. We conclude that c-jun is likely to play a role in the initiation of apoptotic cell death in these neurons.

Animals↗
PubMed1999

Activation of caspase-3 in developmental models of programmed cell death in neurons of the substantia nigra.

Programmed cell death has been proposed to play a role in the death of neurons in acute and chronic degenerative neurologic disease. There is now evidence that the caspases, a family of cysteine proteases, mediate programmed cell death in various cells. In neurons, caspase-3 (CPP32/Yama/apopain), in particular, has been proposed to play a role. We examined the expression of caspase-3 in three models of programmed cell death affecting neurons of the substantia nigra in the rat: natural developmental neuron death and induced developmental death following either striatal target injury with quinolinic acid or dopamine terminal lesion with intrastriatal injection of 6-hydroxydopamine. Using an antibody to the large (p17) subunit of activated caspase-3, we have found that activated enzyme is expressed in apoptotic profiles in all models. Increased p17 immunostaining correlated with increased enzyme activity. The subcellular distribution of activated caspase-3 differed among the models: In natural cell death and the target injury model, it was strictly nuclear, whereas in the toxin model, it was also cytoplasmic. We conclude that p17 immunostaining is a useful marker for programmed cell death in neurons of the substantia nigra.

Animals↗
PubMed1999

Increased expression of rat synuclein in the substantia nigra pars compacta identified by mRNA differential display in a model of developmental target injury.

Human alpha-synuclein was identified on the basis of proteolytic fragments derived from senile plaques of Alzheimer's disease, and it is the locus of mutations in some familial forms of Parkinson's disease. Its normal function and whether it may play a direct role in neural degeneration remain unknown. To explore cellular responses to neural degeneration in the dopamine neurons of the substantia nigra, we have developed a rodent model of apoptotic death induced by developmental injury to their target, the striatum. We find by mRNA differential display that synuclein is up-regulated in this model, and thus it provides an opportunity to examine directly whether synuclein plays a role in the death of these neurons or, alternatively, in compensatory responses. Up-regulation of mRNA is associated with an increase in the number of neuronal profiles immunostained for synuclein protein. At a cellular level, synuclein is almost exclusively expressed in normal neurons, rather than apoptotic profiles. Synuclein is up-regulated throughout normal postnatal development of substantia nigra neurons, but it is not further up-regulated during periods of natural cell death. We conclude that up-regulation of synuclein in the target injury model is unlikely to mediate apoptotic death and propose that it may be due to a compensatory response in neurons destined to survive.

Amino Acid Sequence↗
PubMed1999

Revisiting the notion of 'motor unit types'.

Like many areas of biology, the study of motor units and their types has progressed from apparent simplicity to intimations of deep complexities as information has accumulated from different muscles, species, and methodolgical approaches. In my own (not unbiassed) view, the general notion of motor units 'types' has been amply validated by over two decades of work from many laboratories. From the standpoint of function, it still seems valid to think of three basic types in mammalian limb muscles. However, it is clear that the details of how these types are best recognized, and how their mechanical properties are mapped onto muscle fiber biochemistry, are still open questions. The organization of motor units clearly remains an exciting and interesting field.

Animals↗
PubMed1998

Programmed cell death: does it play a role in Parkinson's disease?

In recent years, the possibility that programmed cell death (PCD), which is mediated by genetic programs intrinsic to the cell, may underlie the degeneration of neurons that occurs in Parkinson's disease (PD) and allied disorders has become an important hypothesis. Although PCD was originally identified in tissues as a normal developmental phenomenon, there is no question that it can also occur in neurologic disease and models thereof. The possibility that PCD could occur in dopamine neurons in degenerative disease is made plausible by the observations that natural cell death, with the morphology of apoptosis, does occur in these neurons and that this event is regulated by developmental target interactions. In addition, it has been shown that apoptotic death can be induced in these neurons in some animal models of parkinsonism. We have shown, for example, that apoptosis can be induced during development by intrastriatal injection of the neurotoxin 6-hydroxydopamine. Other investigators have shown that apoptosis can be induced in a chronic model of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyride toxicity. However, investigations in human PD brains have yielded mixed results thus far, with some investigators identifying evidence of apoptotic death but others not. Further investigation of human postmortem tissue will benefit from a more complete understanding of the molecular basis of PCD in dopamine neurons, such that its molecular features can be investigated, rather than strictly relying on the morphologic markers presently available.

Animals↗
PubMed1998

Glial cell line-derived neurotrophic growth factor inhibits apoptotic death of postnatal substantia nigra dopamine neurons in primary culture.

Glial cell line-derived neurotrophic factor (GDNF) was identified on the basis of its ability to enhance the development of embryonic mesencephalic dopamine neurons. It remains unknown whether GDNF is a physiologically relevant trophic factor for these neurons. We have shown that natural cell death among dopamine neurons of the substantia nigra occurs largely postnatally. To investigate whether GDNF may have the ability to support these neurons during their period of natural cell death, we have used a postnatal primary culture model. We find that GDNF is able to support the viability of postnatal nigral dopamine neurons by inhibiting apoptotic death. This ability of GDNF shows both regional specificity for the nigra and cellular specificity for the dopamine phenotype. Among eight other neurotrophic factors previously reported to support embryonic dopamine neurons, GDNF was unique in this ability. Thus, GDNF meets this criterion for a physiologically relevant trophic factor for dopamine neurons of the substantia nigra.

Animals↗
PubMed1998

Current concepts on the clinical features, aetiology and management of idiopathic cervical dystonia.

Idiopathic cervical dystonia (ICD) is the most common form of adult-onset focal dystonia. Previously, disagreement existed about whether ICD was a psychiatric illness, but the disorder is now viewed as a neurological illness and large clinical series have clarified the clinical features of the disease. At the time of diagnosis, extracervical dystonia is found in approximately 20% of patients, and there may be a concomitant head or hand tremor. Importantly, adult-onset ICD does not become generalized, although there may be segmental spread and pain may increase independently of the dystonia. While 10-20% of patients may experience remission, nearly all patients relapse within 5 years and are left with persistent disease. The aetiology of ICD is unknown, but there has been much progress in clarifying the genetic abnormality in families with inherited adult-onset cervical dystonia; linkage to chromosome 18p has been demonstrated in one family, and the DYT1 locus has been excluded in two other families. Painful trauma may be involved in the pathogenesis of ICD. Painful stimuli are received and processed by the basal ganglia, and the synaptic changes provoked by pain may lead to the abnormal physiology underlying dystonia. Consistent with this idea are experiments which demonstrate that altered sensory input leads to plasticity of the motor cortex, and those that explore the 'tonic vibration reflex' in patients with dystonia. Another theory suggests that a primary vestibular abnormality is responsible for ICD. Botulinum toxin is the most effective treatment for ICD. Roughly 75% of patients improve, and a response is generally seen within the first week. However, many questions remain regarding the optimal technique of administration. The development of neutralizing antibodies occurs in at least 5-10% of patients, and appears to be related both to dosage and to the interval between treatments. Side-effects are generally mild and result from the action of the toxin in the periphery. If the response to botulinum toxin is not adequate, anticholinergics, benzodiazepines, baclofen and other medications are used as adjunctive therapy. Surgical therapies are available for the treatment of ICD but are reserved for patients refractory to conservative measures.

Diagnosis, Differential↗