RESUMEN
Loss of spectrin or ankyrin in the presynaptic motoneuron disrupts the synaptic microtubule cytoskeleton and leads to disassembly of the neuromuscular junction (NMJ). Here, we demonstrate that NMJ disassembly after loss of alpha-spectrin can be suppressed by expression of a Wld(S) transgene, providing evidence for a Wallerian-type degenerative mechanism. We then identify a second signaling system. Enhanced MAPK-JNK-Fos signaling suppresses NMJ disassembly despite loss of presynaptic alpha-spectrin or ankyrin2-L. This signaling system is activated after an acute cytoskeletal disruption, suggesting an endogenous role during neurological stress. This signaling system also includes delayed, negative feedback via the JNK phosphatase puckered, which inhibits JNK-Fos to allow NMJ disassembly in the presence of persistent cytoskeletal stress. Finally, the MAPK-JNK pathway is not required for baseline NMJ stabilization during normal NMJ growth. We present a model in which signaling via JNK-Fos functions as a stress response system that is transiently activated after cytoskeletal disruption to enhance NMJ stability, and is then shut off allowing NMJ disassembly during persistent cytoskeletal disruption.
Asunto(s)
Ancirinas/metabolismo , Citoesqueleto/fisiología , Proteínas Asociadas a Microtúbulos/metabolismo , Espectrina/metabolismo , Sinapsis/fisiología , Animales , Animales Modificados Genéticamente , Ancirinas/genética , Citoesqueleto/genética , Citoesqueleto/metabolismo , Drosophila/genética , Drosophila/metabolismo , Técnica del Anticuerpo Fluorescente Directa , Inmunohistoquímica , Proteínas Asociadas a Microtúbulos/genética , Mutación , Unión Neuromuscular/crecimiento & desarrollo , Unión Neuromuscular/metabolismo , Unión Neuromuscular/fisiología , Interferencia de ARN , Transducción de Señal/genética , Espectrina/genética , Sinapsis/genética , Sinapsis/metabolismo , TransgenesRESUMEN
Reelin regulates nervous system development and modulates synaptic plasticity in the adult brain. Several findings suggest that alterations in Reelin signaling may contribute to neuronal dysfunction associated with Alzheimer's disease (AD). Cell surface receptors for Reelin, including integrins and very-low-density lipoprotein receptor/apolipoprotein E2 receptor, may be targets of amyloid-beta (Abeta) peptides presumed to play key roles in the pathogenesis of AD. Reelin also regulates the extent of tau phosphorylation. Finally, increased amounts of Reelin fragments have been found in CSF from AD patients, suggesting altered processing of Reelin. We therefore hypothesized that Reelin levels might be altered in the brains of human amyloid precursor protein (hAPP) transgenic mice, particularly in brain regions vulnerable to AD such as hippocampus and entorhinal cortex. Compared with nontransgenic controls, hAPP mice had significantly fewer Reelin-expressing pyramidal cells in the entorhinal cortex, the major population of glutamatergic neurons expressing Reelin in the brain. Western blot analysis of the hippocampus, which receives projections from the entorhinal cortex, revealed significant reductions in Reelin levels. In contrast, the number of Reelin-expressing GABAergic interneurons was not altered in either the entorhinal cortex or the hippocampus. Thus, neuronal expression of hAPP/Abeta is sufficient to reduce Reelin expression in a specific population of entorhinal cortical pyramidal neurons in vivo. Underscoring the relevance of these findings, we found qualitatively similar reductions of Reelin-expressing pyramidal neurons in the entorhinal cortex of AD brains. We conclude that alterations in Reelin processing or signaling may be involved in AD-related neuronal dysfunction.
Asunto(s)
Enfermedad de Alzheimer/metabolismo , Precursor de Proteína beta-Amiloide/genética , Moléculas de Adhesión Celular Neuronal/deficiencia , Moléculas de Adhesión Celular Neuronal/genética , Corteza Entorrinal/metabolismo , Proteínas de la Matriz Extracelular/deficiencia , Proteínas de la Matriz Extracelular/genética , Proteínas del Tejido Nervioso/deficiencia , Proteínas del Tejido Nervioso/genética , Serina Endopeptidasas/deficiencia , Serina Endopeptidasas/genética , Enfermedad de Alzheimer/genética , Enfermedad de Alzheimer/patología , Precursor de Proteína beta-Amiloide/biosíntesis , Animales , Moléculas de Adhesión Celular Neuronal/fisiología , Corteza Entorrinal/patología , Proteínas de la Matriz Extracelular/fisiología , Humanos , Ratones , Ratones Endogámicos C57BL , Ratones Transgénicos , Proteínas del Tejido Nervioso/fisiología , Neuronas/patología , Procesamiento Proteico-Postraduccional/genética , Células Piramidales/metabolismo , Células Piramidales/patología , Proteína Reelina , Serina Endopeptidasas/fisiología , Transducción de Señal/genéticaRESUMEN
Human amyloid precursor protein (hAPP) transgenic mice with high levels of amyloid-beta (Abeta) develop behavioral deficits that correlate with the depletion of synaptic activity-related proteins in the dentate gyrus. The tyrosine kinase Fyn is altered in Alzheimer's disease brains and modulates premature mortality and synaptotoxicity in hAPP mice. To determine whether Fyn also modulates Abeta-induced behavioral deficits and depletions of synaptic activity-dependent proteins, we overexpressed Fyn in neurons of hAPP mice with moderate levels of Abeta production. Compared with nontransgenic controls and singly transgenic mice expressing hAPP or FYN alone, doubly transgenic FYN/hAPP mice had striking depletions of calbindin, Fos, and phosphorylated ERK (extracellular signal-regulated kinase), impaired neuronal induction of Arc, and impaired spatial memory retention. These deficits were qualitatively and quantitatively similar to those otherwise seen only in hAPP mice with higher Abeta levels. Surprisingly, levels of active Fyn were lower in high expresser hAPP mice than in NTG controls and lower in FYN/hAPP mice than in FYN mice. Suppression of Fyn activity may result from dephosphorylation by striatal-enriched phosphatase, which was upregulated in FYN/hAPP mice and in hAPP mice with high levels of Abeta. Thus, increased Fyn expression is sufficient to trigger prominent neuronal deficits in the context of even relatively moderate Abeta levels, and inhibition of Fyn activity may help counteract Abeta-induced impairments.
Asunto(s)
Enfermedad de Alzheimer/enzimología , Trastornos del Conocimiento/enzimología , Modelos Animales de Enfermedad , Proteínas Proto-Oncogénicas c-fyn/biosíntesis , Sinapsis/enzimología , Enfermedad de Alzheimer/genética , Precursor de Proteína beta-Amiloide/biosíntesis , Precursor de Proteína beta-Amiloide/deficiencia , Precursor de Proteína beta-Amiloide/genética , Animales , Trastornos del Conocimiento/genética , Femenino , Humanos , Masculino , Ratones , Ratones Endogámicos C57BL , Ratones Transgénicos , Proteínas Proto-Oncogénicas c-fyn/deficiencia , Proteínas Proto-Oncogénicas c-fyn/genética , Sinapsis/genéticaRESUMEN
Activity-induced expression of Arc is necessary for maintenance of long-term potentiation and for memory consolidation. In transgenic (TG) mice with neuronal production of human amyloid precursor protein (hAPP) and hAPP-derived amyloid-beta (Abeta) peptides, basal Arc expression was reduced primarily in granule cells of the dentate gyrus. After exploration of a novel environment, Arc expression in these neurons was unaltered in hAPP mice but increased markedly in nontransgenic controls. Other TG neuronal populations showed no or only minor deficits in Arc expression, indicating a special vulnerability of dentate granule cells. The phosphorylation states of NR2B and ERK1/2 were reduced in the dentate gyrus of hAPP mice, suggesting attenuated activity in NMDA-dependent signaling pathways that regulate synaptic plasticity as well as Arc expression. Arc reductions in hAPP mice correlated with reductions in the actin-binding protein alpha-actinin-2, which is located in dendritic spines and, like Arc, fulfills important functions in excitatory synaptic activity. Reductions in Arc and alpha-actinin-2 correlated tightly with reductions in Fos and calbindin, shown previously to reflect learning deficits in hAPP mice. None of these alterations correlated with the extent of plaque formation, suggesting a plaque-independent mechanism of hAPP/Abeta-induced neuronal deficits. The brain region-specific depletion of factors that participate in activity-dependent modification of synapses may critically contribute to cognitive deficits in hAPP mice and possibly in humans with Alzheimer's disease.