Endosomal phosphatidylinositol 3‐phosphate controls synaptic vesicle cycling and neurotransmission

Neural circuit function requires mechanisms for controlling neurotransmitter release and the activity of neuronal networks, including modulation by synaptic contacts, synaptic plasticity, and homeostatic scaling. However, how neurons intrinsically monitor and feedback control presynaptic neurotransmitter release and synaptic vesicle (SV) recycling to restrict neuronal network activity remains poorly understood at the molecular level. Here, we investigated the reciprocal interplay between neuronal endosomes, organelles of central importance for the function of synapses, and synaptic activity. We show that elevated neuronal activity represses the synthesis of endosomal lipid phosphatidylinositol 3‐phosphate [PI(3)P] by the lipid kinase VPS34. Neuronal activity in turn is regulated by endosomal PI(3)P, the depletion of which reduces neurotransmission as a consequence of perturbed SV endocytosis. We find that this mechanism involves Calpain 2‐mediated hyperactivation of Cdk5 downstream of receptor‐ and activity‐dependent calcium influx. Our results unravel an unexpected function for PI(3)P‐containing neuronal endosomes in the control of presynaptic vesicle cycling and neurotransmission, which may explain the involvement of the PI(3)P‐producing VPS34 kinase in neurological disease and neurodegeneration.     

Synaptic dysregulation and hyperexcitability induced by intracellular amyloid beta oligomers

Intracellular amyloid beta oligomer (iAβo) accumulation and neuronal hyperexcitability are two crucial events at early stages of Alzheimer''s disease (AD). However, to date, no mechanism linking iAβo with an increase in neuronal excitability has been reported. Here, the effects of human AD brain‐derived (h‐iAβo) and synthetic (iAβo) peptides on synaptic currents and action potential firing were investigated in hippocampal neurons. Starting from 500 pM, iAβo rapidly increased the frequency of synaptic currents and higher concentrations potentiated the AMPA receptor‐mediated current. Both effects were PKC‐dependent. Parallel recordings of synaptic currents and nitric oxide (NO)‐associated fluorescence showed that the increased frequency, related to pre‐synaptic release, was dependent on a NO‐mediated retrograde signaling. Moreover, increased synchronization in NO production was also observed in neurons neighboring those dialyzed with iAβo, indicating that iAβo can increase network excitability at a distance. Current‐clamp recordings suggested that iAβo increased neuronal excitability via AMPA‐driven synaptic activity without altering membrane intrinsic properties. These results strongly indicate that iAβo causes functional spreading of hyperexcitability through a synaptic‐driven mechanism and offers an important neuropathological significance to intracellular species in the initial stages of AD, which include brain hyperexcitability and seizures.     

Age‐related changes in hippocampal‐dependent synaptic plasticity and memory mediated by p75 neurotrophin receptor

The plasticity mechanisms in the nervous system that are important for learning and memory are greatly impacted during aging. Notably, hippocampal‐dependent long‐term plasticity and its associative plasticity, such as synaptic tagging and capture (STC), show considerable age‐related decline. The p75 neurotrophin receptor (p75^NTR) is a negative regulator of structural and functional plasticity in the brain and thus represents a potential candidate to mediate age‐related alterations. However, the mechanisms by which p75^NTR affects synaptic plasticity of aged neuronal networks and ultimately contribute to deficits in cognitive function have not been well characterized. Here, we report that mutant mice lacking the p75^NTR were resistant to age‐associated changes in long‐term plasticity, associative plasticity, and associative memory. Our study shows that p75^NTR is responsible for age‐dependent disruption of hippocampal homeostatic plasticity by modulating several signaling pathways, including BDNF, MAPK, Arc, and RhoA‐ROCK2‐LIMK1‐cofilin. p75^NTR may thus represent an important therapeutic target for limiting the age‐related memory and cognitive function deficits.     

Tau protein is involved in morphological plasticity in hippocampal neurons in response to BDNF

Tau protein, a microtubule-associated protein involved in a number of neurological disorders such as Alzheimer's disease (AD), may undergo modifications under both physiological and pathological conditions. However, the signaling pathways that couple tau protein to neuronal physiology such as synaptic plasticity have not yet been elucidated. Here we report that tau protein is involved in morphological plasticity in response to brain derived neurotrophic factor (BDNF). Stimulation of the cultured rat hippocampal neurons with BDNF resulted in increased tau protein expression, as detected by Western blotting. Furthermore, tau protein accumulated in the distal region of the neurite when treated with taxol or taxol plus BDNF. The increased tau protein also protected neurons against nocodazole-induced dendrite loss. Moreover, BDNF promoted spine growth as well as tau protein over-expression. Knockdown of tau protein using specific short-hairpin RNA (shRNA) significantly decreased the spine density. And BDNF could not increase the spine density of tau-knockdown neurons. These results highlight a possible role for tau protein in the dynamic rearrangement of cytoskeletal fibers vital for BDNF-induced synaptic plasticity.     

The p75 Neurotrophin Receptor Mediates Neuronal Apoptosis and Is Essential for Naturally Occurring Sympathetic Neuron Death

Abstract. To determine whether the p75 neurotrophin receptor (p75NTR) plays a role in naturally occurring neuronal death, we examined neonatal sympathetic neurons that express both the TrkA tyrosine kinase receptor and p75NTR. When sympathetic neuron survival is maintained with low quantities of NGF or KCl, the neurotrophin brain-derived neurotrophic factor (BDNF), which does not activate Trk receptors on sympathetic neurons, causes neuronal apoptosis and increased phosphorylation of c-jun. Function-blocking antibody studies indicate that this apoptosis is due to BDNF-mediated activation of p75NTR. To determine the physiological relevance of these culture findings, we examined sympathetic neurons in BDNF−/− and p75NTR−/− mice. In BDNF−/− mice, sympathetic neuron number is increased relative to BDNF+/+ littermates, and in p75NTR−/− mice, the normal period of sympathetic neuron death does not occur, with neuronal attrition occurring later in life. This deficit in apoptosis is intrinsic to sympathetic neurons, since cultured p75NTR−/− neurons die more slowly than do their wild-type counterparts. Together, these data indicate that p75NTR can signal to mediate apoptosis, and that this mechanism is essential for naturally occurring sympathetic neuron death.     

From modulator to mediator: rapid effects of BDNF on ion channels

Neurotrophins (NTs) are {?AUTHOR} a family of structurally related, secreted proteins that regulate the survival, differentiation and maintenance of function of different populations of peripheral and central neurons. 1 , 2 Among these, BDNF (brain‐derived neurotrophic factor) has drawn considerable interest because both its synthesis and secretion are increased by physiological levels of activity, indicating a unique role of this neurotrophin in coupling neuronal activity to structural and functional properties of neuronal circuits. In addition to its classical neurotrophic effects, which are evident within hours or days and which usually result from changes in cellular gene expression, BDNF exerts acute effects on synaptic transmission and is involved in the induction of long‐term potentiation. Many of these rapid effects of BDNF are mediated by its modulation of ion channel properties following TrkB‐mediated activation of intracellular second messenger cascades and protein phosphorylation. However, recent reports have shown that BDNF not only acts as a modulator of ion channels, but can also directly and rapidly gate a Na⁺ channel, thereby assigning BDNF the properties of a classical excitatory transmitter. Thus, BDNF, in addition its role as a potent neuromodulator, emerges as an excitatory transmitter‐like substance which acutely controls resting membrane potential, neuronal excitability, synaptic transmission and participates in the induction of synaptic plasticity. BioEssays 26:1185–1194, 2004. © 2004 Wiley Periodicals, Inc.     

MeCP2 prevents age‐associated cognitive decline via restoring synaptic plasticity in a senescence‐accelerated mouse model

Age‐related cognitive decline in neurodegenerative diseases, such as Alzheimer''s disease (AD), is associated with the deficits of synaptic plasticity. Therefore, exploring promising targets to enhance synaptic plasticity in neurodegenerative disorders is crucial. It has been demonstrated that methyl‐CpG binding protein 2 (MeCP2) plays a vital role in neuronal development and MeCP2 malfunction causes various neurodevelopmental disorders. However, the role of MeCP2 in neurodegenerative diseases has been less reported. In the study, we found that MeCP2 expression in the hippocampus was reduced in the hippocampus of senescence‐accelerated mice P8 (SAMP8) mice. Overexpression of hippocampal MeCP2 could elevate synaptic plasticity and cognitive function in SAMP8 mice, while knockdown of MeCP2 impaired synaptic plasticity and cognitive function in senescence accelerated‐resistant 1 (SAMR1) mice. MeCP2‐mediated regulation of synaptic plasticity may be associated with CREB1 pathway. These results suggest that MeCP2 plays a vital role in age‐related cognitive decline by regulating synaptic plasticity and indicate that MeCP2 may be promising targets for the treatment of age‐related cognitive decline in neurodegenerative diseases.     

Centrosome‐mediated microtubule remodeling during axon formation in human iPSC‐derived neurons

Axon formation critically relies on local microtubule remodeling and marks the first step in establishing neuronal polarity. However, the function of the microtubule‐organizing centrosomes during the onset of axon formation is still under debate. Here, we demonstrate that centrosomes play an essential role in controlling axon formation in human‐induced pluripotent stem cell (iPSC)‐derived neurons. Depleting centrioles, the core components of centrosomes, in unpolarized human neuronal stem cells results in various axon developmental defects at later stages, including immature action potential firing, mislocalization of axonal microtubule‐associated Trim46 proteins, suppressed expression of growth cone proteins, and affected growth cone morphologies. Live‐cell imaging of microtubules reveals that centriole loss impairs axonal microtubule reorganization toward the unique parallel plus‐end out microtubule bundles during early development. We propose that centrosomes mediate microtubule remodeling during early axon development in human iPSC‐derived neurons, thereby laying the foundation for further axon development and function.