Biomedical subjects

Peter G Noakes

Publications and source records attributed to Peter G Noakes.

11 recordsLinked to original sources

PubMed2006

Neural agrin: a synaptic stabiliser.

Neural agrin is a heparan sulphate proteoglycan first defined by its ability to induce the clustering of acetylcholine receptors (AChRs) on cultured muscle cells. Neural agrin activates the transmembrane Muscle Specific Kinase (MuSK) on the postsynaptic muscle cell to stabilise the developing neuromuscular synapse. Three biological mechanisms for agrin/MuSK signalling are briefly discussed: selective transcription of synaptic genes such as MuSK itself, to reinforce developing postsynaptic clusters of AChRs; initiation of second messenger signalling pathways that can induce the formation of AChR clusters and retrograde signalling downstream of agrin/MuSK that may transform the growth cone of the motor axon into a stable differentiated nerve terminal, specialised for regulated exocytosis of neurotransmitter. Here we briefly review some key mechanisms through which neural agrin acts to foster the formation of mature neuromuscular synapses.

Agrin↗

PubMed2005

Heterozygote effects in mice with partial truncations in the growth hormone receptor cytoplasmic domain: assessment of growth parameters and phenotype.

The GH receptor (GHR) is essential for normal postnatal growth and development, and the molecular basis of GHR action has been studied intensively. Clinical case studies and more recently mouse models have revealed the extensive phenotype of impaired GH action. We recently reported two new mouse models, possessing cytoplasmic truncations at position 569 (plus Y539/545-F) and 391, which were created to identify functional subdomains within the cytoplasmic signaling domain. In the homozygous state, these animals show progressively impaired postnatal growth coupled with complex changes in gene expression. We describe here an extended phenotype analysis encompassing the heterozygote state to identify whether single copies of these mutant receptors bring about partial or dominant-negative phenotypes. It appears that the retention of the ubiquitin-dependent endocytosis motif in the N-terminal cytoplasmic domain permits turnover of these mutant receptors because no dominant-negative phenotype is seen. Nonetheless, we do observe partial impairment of postnatal growth in heterozygotes supporting limited haploinsufficiency. Reproductive function is impaired in these models in a progressive manner, in parallel with loss of signal transducer and activator of transcription-5 activation ability. In summary, we describe a more comprehensive phenotypic analysis of these mouse models, encompassing overall and longitudinal body growth, reproductive function, and hormonal status in both the heterozygote and homozygote state. Our results suggest that patients expressing single copies of similarly mutated GHRs would not display an obvious clinical phenotype.

Animals↗

PubMed2005

Glycinergic and GABAergic synaptic activity differentially regulate motoneuron survival and skeletal muscle innervation.

GABAergic and glycinergic synaptic transmission is proposed to promote the maturation and refinement of the developing CNS. Here we provide morphological and functional evidence that glycinergic and GABAergic synapses control motoneuron development in a region-specific manner during programmed cell death. In gephyrin-deficient mice that lack all postsynaptic glycine receptor and some GABA(A) receptor clusters, there was increased spontaneous respiratory motor activity, reduced respiratory motoneuron survival, and decreased innervation of the diaphragm. In contrast, limb-innervating motoneurons showed decreased spontaneous activity, increased survival, and increased innervation of their target muscles. Both GABA and glycine increased limb-innervating motoneuron activity and decreased respiratory motoneuron activity in wild-type mice, but only glycine responses were abolished in gephyrin-deficient mice. Our results provide genetic evidence that the development of glycinergic and GABAergic synaptic inputs onto motoneurons plays an important role in the survival, axonal branching, and spontaneous activity of motoneurons in developing mammalian embryos.

Animals↗

PubMed2005

In vivo analysis of growth hormone receptor signaling domains and their associated transcripts.

The growth hormone receptor (GHR) is a critical regulator of postnatal growth and metabolism. However, the GHR signaling domains and pathways that regulate these processes in vivo are not defined. We report the first knock-in mouse models with deletions of specific domains of the receptor that are required for its in vivo actions. Mice expressing truncations at residue m569 (plus Y539/545-F) and at residue m391 displayed a progressive impairment of postnatal growth with receptor truncation. Moreover, after 4 months of age, marked male obesity was observed in both mutant 569 and mutant 391 and was associated with hyperglycemia. Both mutants activated hepatic JAK2 and ERK2, whereas STAT5 phosphorylation was substantially decreased for mutant 569 and absent from mutant 391, correlating with loss of IGF-1 expression and reduction in growth. Microarray analysis of these and GHR(-/-) mice demonstrated that particular signaling domains are responsible for the regulation of different target genes and revealed novel actions of growth hormone. These mice represent the first step in delineating the domains of the GHR regulating body growth and composition and the transcripts associated with these domains.

Animals↗

PubMed2004

Neuregulin potentiates agrin-induced acetylcholine receptor clustering in myotubes.

Agrin and neuregulin (HRG-beta1) play complementary roles in synapse formation. While HRG-beta1 induces transcriptional up-regulation of postsynaptic proteins, here we present evidence that it can potentiate agrin-induced acetylcholine receptor (AChR) clustering in C2 myotubes. Agrin induced maximal AChR clustering in 4 h. HRG-beta1 treatment for 4 h produced no increase over basal AChR cluster numbers. When myotubes were treated for 4 h with 100 pM agrin, HRG-beta1 augmented AChR cluster numbers by 2-fold compared to myotubes treated with 100 pM agrin alone. Thus, HRG-beta1 can potentiate agrin-induced AChR clustering.

Agrin↗

PubMed2004

Postnatal changes in TASK-1 and TREK-1 expression in rat brain stem and cerebellum.

Developmental changes in expression of two-pore domain K+ channels, TASK-1 and TREK-1, were investigated in the juvenile (postnatal day 13; P13) and adult (P105) rat brain stem and cerebellum using immunohistochemistry. In the juvenile, extensive TASK-1-like immunoreactivity (TASK-1-LIR) was seen among glial cells in the white matter (e.g., radial glia), which showed marked reduction in the adult. In contrast, TASK-1-LIR in neurons including cerebellar Purkinje and granule cells, hypoglossal and facial motoneurons, and ventrolateral medulla neurons was increased in the adult. TASK-1-LIR in neuroglia surrounding peripheral axons of cranial nerves was persistent. TREK-1-LIR was similar between ages, although TREK-1-LIR was neuronal and present only in juvenile cerebellar external germinal layer. Present results suggest roles for TASK-1 and K+ homeostasis in neuro-glial interaction, neurogenesis, differentiation, migration, axon guidance, synaptogenesis and myelination.

Aging↗

PubMed2004

Developmental expression of two-pore domain K+ channels, TASK-1 and TREK-1, in the rat cochlea.

Developmental expression of two-pore domain potassium (2P K) channels, TASK-1 and TREK-1, was investigated in the rat cochlea at onset of hearing and after maturity using RT-PCR and immunocytochemistry. TASK-1 and TREK-1 mRNAs were detected by RT-PCR at postnatal day (P) 9-12. TASK-1 like immunoreactivity (LIR) in the P13 cochlea was observed in Deiters', pillar, Claudius' and outer sulcus cells, spiral limbus fibrocytes, and neuroglia. At P13, TREK-1-LIR was more wide-spread, and included sensory and supporting cells of the organ of Corti, spiral ganglion, stria vascularis, Reissner's membrane, inner and outer sulcus cells, connective and support tissues surrounding modiolus. By P105 the pattern of TASK-1- and TREK-1-LIR became limited to a subset of the above structures, suggesting developmental regulation. During postnatal development, TASK-1 may be important in the onset (around P11) and maturation (by P22) of endocochlear potential and hearing. The distribution of TASK-1 and TREK-1 suggest a role in K cycling and homeostasis. As TASK-1 and TREK-1 are inhibited by local anesthetics at doses used to treat tinnitus, 2P K channels may also be important in cochlear dysfunction.

Animals↗

PubMed2003

Neuromuscular synapses mediate motor axon branching and motoneuron survival during the embryonic period of programmed cell death.

The embryonic period of motoneuron programmed cell death (PCD) is marked by transient motor axon branching, but the role of neuromuscular synapses in regulating motoneuron number and axonal branching is not known. Here, we test whether neuromuscular synapses are required for the quantitative association between reduced skeletal muscle contraction, increased motor neurite branching, and increased motoneuron survival. We achieved this by comparing agrin and rapsyn mutant mice that lack acetylcholine receptor (AChR) clusters. There were significant reductions in nerve-evoked skeletal muscle contraction, increases in intramuscular axonal branching, and increases in spinal motoneuron survival in agrin and rapsyn mutant mice compared with their wild-type littermates at embryonic day 18.5 (E18.5). The maximum nerve-evoked skeletal muscle contraction was reduced a further 17% in agrin mutants than in rapsyn mutants. This correlated to an increase in motor axon branch extension and number that was 38% more in agrin mutants than in rapsyn mutants. This suggests that specializations of the neuromuscular synapse that ensure efficient synaptic transmission and muscle contraction are also vital mediators of motor axon branching. However, these increases in motor axon branching did not correlate with increases in motoneuron survival when comparing agrin and rapsyn mutants. Thus, agrin-induced synaptic specializations are required for skeletal muscle to effectively control motoneuron numbers during embryonic development.

Agrin↗

PubMed2003

Functional analysis of neurotransmission at beta2-laminin deficient terminals.

beta2-Laminin is important for the formation of neuromuscular junctions in vertebrates. Previously, we have inactivated the gene that encodes for beta2-laminin in mice and observed predominantly prejunctional structural defects. In this study, we have used both intra- and extracellular recording methods to investigate evoked neurotransmission in beta2-laminin-deficient mice, from postnatal day 8 (P8) through to day 18 (P18). Our results confirmed that there was a decrease in the frequency of spontaneous release, but no change in the postjunctional response to such release. Analysis of evoked neurotransmission showed an increase in the frequency of stimuli that failed to elicit an evoked postjunctional response in the mutants compared to litter mate controls, resulting in a 50 % reduction in mean quantal content at mutant terminals. Compared to littermate controls, beta2-laminin-deficient terminals showed greater synaptic depression when subjected to high frequency stimulation. Furthermore, the paired pulse ratio of the first two stimuli was significantly lower in beta2-laminin mutant terminals. Statistical analysis of the binomial parameters of release showed that the decrease in quantal content was due to a decrease in the number of release sites without any significant change in the average probability of release. This suggestion was supported by the observation of fewer synaptic vesicle protein 2 (SV2)-positive varicosities in beta2-laminin-deficient terminals and by ultrastructural observations showing smaller terminal profiles and increased Schwann cell invasion in beta2-laminin mutants; the differences between beta2-laminin mutants and wild-type mice were the same at both P8 and P18. From these results we conclude that beta2-laminin plays a role in the early structural development of the neuromuscular junction. We also suggest that transmitter release activity may act as a deterrent to Schwann cell invasion in the absence of beta2-laminin.

Action Potentials↗

PubMed2003

Genetic disruption of the growth hormone receptor does not influence motoneuron survival in the developing mouse.

In the rodent central nervous system (CNS) during the five days prior to birth, both growth hormone (GH) and its receptor (GHR) undergo transient increases in expression to levels considerably higher than those found postnatally. This increase in expression coincides with the period of neuronal programmed cell death (PCD) in the developing CNS. To evaluate the involvement of growth hormone in the process of PCD, we have quantified the number of motoneurons in the spinal cord and brain stem of wild type and littermate GHR-deficient mice at the beginning and end of the neuronal PCD period. We found no change in motoneuron survival in either the brachial or lumbar lateral motor columns of the spinal cord or in the trochlear, trigeminal, facial or hypoglossal nuclei in the brain stem. We also found no significant differences in spinal cord volume, muscle fiber diameter, or body weight of GHR-deficient fetal mice when compared to their littermate controls. Therefore, despite considerable in vitro evidence for GH action on neurons and glia, genetic disruption of GHR signalling has no effect on prenatal motoneuron number in the mouse, under normal physiological conditions. This may be a result of compensation by the signalling of other neurotrophic cytokines.

Animals↗

PubMed2002

Elucidating the molecular mechanisms that underlie the target control of motoneuron death.

Approximately half of the motoneurons generated during normal embryonic development undergo programmed cell death. Most of this death occurs during the time when synaptic connections are being formed between motoneurons and their target, skeletal muscle. Subsequent muscle activity stemming from this connection helps determine the final number of surviving motoneurons. These observations have given rise to the idea that motoneuron survival is dependent upon access to muscle derived trophic factors, presumably through intact neuromuscular synapses. However, it is not yet understood how the muscle regulates the supply of such trophic factors, or if there are additional mechanisms operating to control the fate of the innervating motoneuron. Recent observations have highlighted target independent mechanisms that also operate to support the survival of motoneurons, such as early trophic-independent periods of motoneuron death, trophic factors derived from Schwann cells and selection of motoneurons during pathfinding. Here we review recent investigations into motoneuron cell death when the molecular signalling between motoneurons and muscle has been genetically disrupted. From these studies, we suggest that in addition to trophic factors from muscle and/or Schwann cells, specific adhesive interactions between motoneurons and muscle are needed to regulate motoneuron survival. Such interactions, along with intact synaptic basal lamina, may help to regulate the supply and presentation of trophic factors to motoneurons.

Agrin↗