Elisabetta Menna

Acid sphingomyelinase activity triggers microparticle release from glial cells.

We have earlier shown that microglia, the immune cells of the CNS, release microparticles from cell plasma membrane after ATP stimulation. These vesicles contain and release IL‐1β, a crucial cytokine in CNS inflammatory events. In this study, we show that microparticles are also released by astrocytes and we get insights into the mechanism of their shedding. We show that, on activation of the ATP receptor P2X7, microparticle shedding is associated with rapid activation of acid sphingomyelinase, which moves to plasma membrane outer leaflet. ATP‐induced shedding and IL‐1β release are markedly reduced by the inhibition of acid sphingomyelinase, and completely blocked in glial cultures from acid sphingomyelinase knockout mice. We also show that p38 MAPK cascade is relevant for the whole process, as specific kinase inhibitors strongly reduce acid sphingomyelinase activation, microparticle shedding and IL‐1β release. Our results represent the first demonstration that activation of acid sphingomyelinase is necessary and sufficient for microparticle release from glial cells and define key molecular effectors of microparticle formation and IL‐1β release, thus, opening new strategies for the treatment of neuroinflammatory diseases.

Brain-derived neurotrophic factor is an anterograde survival factor in the rat visual system

BACKGROUND The neurotrophins, which include nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), NT-4/5 and NT-6, are a family of proteins that play fundamental roles in the differentiation, survival and maintenance of peripheral and central neurons. Much research has focused on the role of neurotrophins as target-derived, retrogradely transported trophic molecules. Although there is recent evidence that BDNF and NT-3 can be transported in an anterograde direction along peripheral and central axons, there is as yet no conclusive evidence that these anterograde factors have direct post-synaptic actions. RESULTS We report that BDNF travels in an anterograde direction along the optic nerve. The anterogradely transported BDNF had rapid effects on retinal target neurons in the superior colliculus and lateral geniculate nucleus of the brain. When endogenous BDNF within the developing superior colliculus was neutralised, the rate of programmed neuronal death increased. Conversely, provision of an afferent supply of BDNF prevented the degeneration of geniculate neurons after removal of their cortical target. CONCLUSIONS BDNF released from retinal ganglion cells acts as a survival factor for post-synaptic neurons in retinal target fields.

A novel pathway for presynaptic mitogen-activated kinase activation via AMPA receptors

AMPA-type glutamate receptors play a key role in mediating postsynaptic responses of excitatory neurotransmitters. It is now well accepted that AMPA receptors are also present at the presynapse, where they are thought to modulate neurotransmitter release. However, the mechanisms through which they control synaptic vesicle traffic have remained elusive. We used cultured hippocampal neurons and growth cone particles prepared from fetal rat brain to investigate the functional role of presynaptic AMPA receptors. We show here that stimulation of presynaptic AMPA receptors induces activation of mitogen-activated protein kinase (MAPK) through a nonreceptor tyrosine kinase-dependent and Na+/Ca2+-independent mechanism. This pathway is activated predominantly in axonal growth cones compared with the somatodendritic compartment. After stimulation of presynaptic AMPA receptors, synapsin I is phosphorylated at MAPK-specific sites. These events are paralleled by a prominent increase in evoked synaptic vesicle recycling that is blocked by the specific MAPK inhibitor 2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one. Similarly, in synaptosomes isolated from adult brain, AMPA stimulation induces MAPK activation and phosphorylation of synapsin I at MAPK-dependent sites and enhances significantly synaptic vesicle recycling. These results reveal a novel pathway for activation of presynaptic MAPK and suggest a role of this pathway in the regulation of short-term presynaptic plasticity.

The anterogradely transported BDNF promotes retinal axon remodeling during eye specific segregation within the LGN

Neurotrophins have been implicated in regulating many aspects of neuronal development and plasticity, including dendritic and axonal elaboration, by acting primarily as target derived trophic factors. Recently, we have shown that brain-derived neurotrophic factor (BDNF) is produced by retinal ganglion cells (RGCs) and travels in an anterograde direction along the optic nerve in neonatal rats. Here, we have assessed whether the anterogradely transported BDNF plays a role in shaping the retinogeniculate connectivity during development. We used intraocular injections of antisense oligonucleotides to suppress selectively retinal synthesis and anterograde transport of BDNF in rat pups. We found that in the absence of endogenous BDNF, RGC axons retract from their target in the dorsal lateral geniculate nucleus (dLGN). The blockade of BDNF action at the retinal level with the tyrosine kinase inhibitor, K252a, failed to produce this effect, suggesting an anterograde action of the endogenous BDNF. Moreover, the effects of BDNF removal on RGC fibers were evident only during a narrow temporal window coincident with the critical period for the retinothalamic refinement, indicating a role for BDNF on growth and elaboration of RGC axons rather than on their maintenance. Altogether these results propose a novel role for BDNF in the elaboration of retinogeniculate axons.

A Role for Retinal Brain-Derived Neurotrophic Factor in Ocular Dominance Plasticity

Visual deprivation is a classical tool to study the plasticity of visual cortical connections. After eyelid closure in young animals (monocular deprivation, MD), visual cortical neurons become dominated by the open eye, a phenomenon known as ocular dominance (OD) plasticity . It is commonly held that the molecular mediators of OD plasticity are cortically derived and that the retina is immune to the effects of MD . Recently, it has been reported that visual deprivation induces neurochemical, structural, and functional changes in the retina , but whether these retinal changes contribute to the effects of MD in the cortex is unknown. Here, we provide evidence that brain-derived neurotrophic factor (BDNF) produced in the retina influences OD plasticity. We found a reduction of BDNF expression in the deprived retina of young rats. We compensated this BDNF imbalance between the two eyes by either injecting exogenous BDNF in the deprived eye or reducing endogenous BDNF expression in the nondeprived eye. Both treatments were effective in counteracting the OD shift induced by MD. Retinal BDNF could also influence OD distribution in normal animals. These results show for the first time that OD plasticity is modulated by BDNF produced in the retina.

Eps8 controls dendritic spine density and synaptic plasticity through its actin‐capping activity

Actin‐based remodelling underlies spine structural changes occurring during synaptic plasticity, the process that constantly reshapes the circuitry of the adult brain in response to external stimuli, leading to learning and memory formation. A positive correlation exists between spine shape and synaptic strength and, consistently, abnormalities in spine number and morphology have been described in a number of neurological disorders. In the present study, we demonstrate that the actin‐regulating protein, Eps8, is recruited to the spine head during chemically induced long‐term potentiation in culture and that inhibition of its actin‐capping activity impairs spine enlargement and plasticity. Accordingly, mice lacking Eps8 display immature spines, which are unable to undergo potentiation, and are impaired in cognitive functions. Additionally, we found that reduction in the levels of Eps8 occurs in brains of patients affected by autism compared to controls. Our data reveal the key role of Eps8 actin‐capping activity in spine morphogenesis and plasticity and indicate that reductions in actin‐capping proteins may characterize forms of intellectual disabilities associated with spine defects.

Reduced SNAP‐25 alters short‐term plasticity at developing glutamatergic synapses

SNAP‐25 is a key component of the synaptic‐vesicle fusion machinery, involved in several psychiatric diseases including schizophrenia and ADHD. SNAP‐25 protein expression is lower in different brain areas of schizophrenic patients and in ADHD mouse models. How the reduced expression of SNAP‐25 alters the properties of synaptic transmission, leading to a pathological phenotype, is unknown. We show that, unexpectedly, halved SNAP‐25 levels at 13–14 DIV not only fail to impair synaptic transmission but instead enhance evoked glutamatergic neurotransmission. This effect is possibly dependent on presynaptic voltage‐gated calcium channel activity and is not accompanied by changes in spontaneous quantal events or in the pool of readily releasable synaptic vesicles. Notably, synapses of 13–14 DIV neurons with reduced SNAP‐25 expression show paired‐pulse depression as opposed to paired‐pulse facilitation occurring in their wild‐type counterparts. This phenotype disappears with synapse maturation. As alterations in short‐term plasticity represent a new mechanism contributing to cognitive impairments in intellectual disabilities, our data provide mechanistic clues for neuronal circuit alterations in psychiatric diseases characterized by reduced expression of SNAP‐25.

The Eps8/IRSp53/VASP Network Differentially Controls Actin Capping and Bundling in Filopodia Formation

There is a body of literature that describes the geometry and the physics of filopodia using either stochastic models or partial differential equations and elasticity and coarse-grained theory. Comparatively, there is a paucity of models focusing on the regulation of the network of proteins that control the formation of different actin structures. Using a combination of in-vivo and in-vitro experiments together with a system of ordinary differential equations, we focused on a small number of well-characterized, interacting molecules involved in actin-dependent filopodia formation: the actin remodeler Eps8, whose capping and bundling activities are a function of its ligands, Abi-1 and IRSp53, respectively; VASP and Capping Protein (CP), which exert antagonistic functions in controlling filament elongation. The model emphasizes the essential role of complexes that contain the membrane deforming protein IRSp53, in the process of filopodia initiation. This model accurately accounted for all observations, including a seemingly paradoxical result whereby genetic removal of Eps8 reduced filopodia in HeLa, but increased them in hippocampal neurons, and generated quantitative predictions, which were experimentally verified. The model further permitted us to explain how filopodia are generated in different cellular contexts, depending on the dynamic interaction established by Eps8, IRSp53 and VASP with actin filaments, thus revealing an unexpected plasticity of the signaling network that governs the multifunctional activities of its components in the formation of filopodia.

From filopodia to synapses: The role of actin-capping and anti-capping proteins

Actin‐capping and anti‐capping proteins are crucial regulators of actin dynamics. Recent studies have indicated that these proteins may be heavily involved in all stages of synaptogenesis, from the emergence of filopodia, through neuritogenesis and synaptic contact stabilization, to the structural changes occurring at the synapse during potentiation phenomena. In this review, we focus on recent evidence pointing to an active role of actin‐capping and anti‐capping proteins in orchestrating the processes controlling neuronal connectivity and plasticity.

SNAP-25, a Known Presynaptic Protein with Emerging Postsynaptic Functions

A hallmark of synaptic specializations is their dependence on highly organized complexes of proteins that interact with each other. The loss or modification of key synaptic proteins directly affects the properties of such networks, ultimately impacting synaptic function. SNAP-25 is a component of the SNARE complex, which is central to synaptic vesicle exocytosis, and, by directly interacting with different calcium channels subunits, it negatively modulates neuronal voltage-gated calcium channels, thus regulating intracellular calcium dynamics. The SNAP-25 gene has been associated with distinct brain diseases, including Attention Deficit Hyperactivity Disorder (ADHD), schizophrenia and bipolar disorder, indicating that the protein may act as a shared biological substrate among different “synaptopathies”. The mechanisms by which alterations in SNAP-25 may concur to these psychiatric diseases are still undefined, although alterations in neurotransmitter release have been indicated as potential causative processes. This review summarizes recent work showing that SNAP-25 not only controls exo/endocytic processes at the presynaptic terminal, but also regulates postsynaptic receptor trafficking, spine morphogenesis, and plasticity, thus opening the possibility that SNAP-25 defects may contribute to psychiatric diseases by impacting not only presynaptic but also postsynaptic functions.