Stabilization of Growing Retinal Axons by the Combined Signaling of Nitric Oxide and Brain-Derived Neurotrophic Factor

118

113

The mechanisms by which neurotrophins regulate growth cone motility are unclear. We investigated the role of the p75 neurotrophin receptor (p75 NTR ) in mediating neurotrophin-induced increases in filopodial length. Our data demonstrate that neurotrophin binding to p75 NTR is necessary and sufficient to regulate filopodial dynamics. Furthermore, retinal and dorsal root ganglion growth cones from p75 mutant mice are insensitive to neurotrophins but display enhanced filopodial lengths comparable with neurotrophin-treated wild-type growth cones. This suggests unoccupied p75 NTR negatively regulates filopodia length. Furthermore, p75 NTR regulates RhoA activity to mediate filopodial dynamics. Constitutively active RhoA blocks neurotrophin-induced increases in filopodial length, whereas inhibition of RhoA enhances filopodial lengths, similar to neurotrophin treatment. BDNF treatment of retinal neurons results in reduced RhoA activity. Furthermore, p75 mutant neurons display reduced levels of activated RhoA compared with wild-type counterparts, consistent with the enhanced filopodial lengths observed on mutant growth cones. These observations suggest that neurotrophins regulate filopodial dynamics by depressing the activation of RhoA that occurs through p75 NTR signaling.

Section: Methodsmentioning

confidence: 99%

p75 Neurotrophin Receptor Signaling Regulates Growth Cone Filopodial Dynamics through Modulating RhoA Activity

Gehler

Gallo²,

Veien³

et al. 2004

118

113

“…Explants were cultured on glass coverslips coated with 25 g/ml laminin. Retinal explants were cultured in F12 medium supplemented with additives, as described previously (Ernst et al, 2000).…”

Section: Methodsmentioning

confidence: 99%

Brain-Derived Neurotrophic Factor Regulation of Retinal Growth Cone Filopodial Dynamics Is Mediated through Actin Depolymerizing Factor/Cofilin

Gehler¹,

Shaw²,

Sarmiere³

et al. 2004

110

The molecular mechanisms by which neurotrophins regulate growth cone motility are not well understood. This study investigated the signaling involved in transducing BDNF-induced increases of filopodial dynamics. Our results indicate that BDNF regulates filopodial length and number through a Rho kinase-dependent mechanism. Additionally, actin depolymerizing factor (ADF)/cofilin activity is necessary and sufficient to transduce the effects of BDNF. Our data indicate that activation of ADF/cofilin mimics the effects of BDNF on filopodial dynamics, whereas ADF/cofilin inactivity blocks the effects of BDNF. Furthermore, BDNF promotes the activation of ADF/ cofilin by reducing the phosphorylation of ADF/cofilin. Although inhibition of myosin II also enhances filopodial length, our results indicate that BDNF signaling is independent of myosin II activity and that the two pathways result in additive effects on filopodial length. Thus, filopodial extension is regulated by at least two independent mechanisms. The BDNF-dependent pathway works via regulation of ADF/cofilin, independently of myosin II activity.

“…Additional molecules involved in the cascade of events induced by interruption of trophic support, or by alteration in the neurotrophic factor level itself, participate jointly with NO in the control of bouton dynamics. On this aspect, Ernst et al (2000) demonstrated that growth cone collapse and axon retraction caused by NO can be blocked by pretreatment with brain-derived neurotrophic factor (BDNF). Furthermore, increasing BDNF levels enhanced axon arborization and synapse formation during development (Alsina et al, 2001) and inhibited NOS-I expression in axotomized motoneurons (Wu et al, 2003).…”

Section: Induced Neuronal No Is Necessary For Synaptic Stripping Butmentioning

confidence: 99%

Nitric Oxide-Directed Synaptic Remodeling in the Adult Mammal CNS

Sunico¹,

Portillo²,

González-Forero³

et al. 2005

118

In adult mammals, learning, memory, and restoration of sensorimotor lost functions imply synaptic reorganization that requires diffusible messengers-mediated communication between presynaptic and postsynaptic structures. A candidate molecule to accomplish this function is the gaseous intercellular messenger nitric oxide (NO), which is involved in synaptogenesis and projection refinement during development; however, the role of NO in synaptic reorganization processes in adulthood remains to be established. In this work, we tested the hypothesis that this free radical is a mediator in the adult mammal CNS synaptic remodeling processes using a model of hypoglossal axonal injury recently developed by us. Axonal injury-induced disconnection of motoneurons from myocytes produces withdrawal of synaptic inputs to motoneurons and concomitant upregulation of the neuronal isoform of NO synthase (NOS-I). After recovery of the neuromuscular function, synaptic coverage is reestablished and NOS-I is downregulated. We also report, by using functional and morphological approaches, that chronic inhibition of the NO/cGMP pathway prevents synaptic withdrawal evoked by axon injury, despite the persistent muscle disconnection. After successful withdrawal of synaptic boutons, inhibition of NO synthesis, but not of cGMP, accelerated the recovery of synaptic coverage, although neuromuscular disconnection was maintained. Furthermore, protein S-nitrosylation was upregulated after nerve injury, and this effect was reversed by NOS-I inhibition. Our results suggest that during synaptic remodeling in the adult CNS, NO acts as a signal for synaptic detachment and inhibits synapse formation by cGMP-dependent and probably S-nitrosylation-mediated mechanisms, respectively. We also suggest a feasible role of NO in neurological disorders coursing with NOS-I upregulation.