Psychological Stress as a Risk Factor for Accelerated Cellular Aging and Cognitive Decline: The Involvement of Microglia-Neuron Crosstalk

The relationship between the central nervous system (CNS) and microglia is lifelong. Microglia originate in the embryonic yolk sac during development and populate the CNS before the blood-brain barrier forms. In the CNS, they constitute a self-renewing population. Although they represent up to 10% of all brain cells, we are only beginning to understand how much brain homeostasis relies on their physiological functions. Often compared to a double-edged sword, microglia hold the potential to exert neuroprotective roles that can also exacerbate neurodegeneration once compromised. Microglia can promote synaptic growth in addition to eliminating synapses that are less active. Synaptic loss, which is considered one of the best pathological correlates of cognitive decline, is a distinctive feature of major depressive disorder (MDD) and cognitive aging. Long-term psychological stress accelerates cellular aging and predisposes to various diseases, including MDD, and cognitive decline. Among the underlying mechanisms, stress-induced neuroinflammation alters microglial interactions with the surrounding parenchymal cells and exacerbates oxidative burden and cellular damage, hence inducing changes in microglia and neurons typical of cognitive aging. Focusing on microglial interactions with neurons and their synapses, this review discusses the disrupted communication between these cells, notably involving fractalkine signaling and the triggering receptor expressed on myeloid cells (TREM). Overall, chronic stress emerges as a key player in cellular aging by altering the microglial sensome, notably via fractalkine signaling deficiency. To study cellular aging, novel positron emission tomography radiotracers for TREM and the purinergic family of receptors show interest for human study.

Exercise Intervention Promotes the Growth of Synapses and Regulates Neuroplasticity in Rats With Ischemic Stroke Through Exosomes

Background: Stroke is the leading cause of death and disability. Exercise produces neuroprotection by improving neuroplasticity. Exercise can induce exosome production. According to several studies, exosomes are involved in repairing brain function, but the relationship and mechanism of exercise, exosomes, and neuroprotection have not been elucidated. This study intends to explore the relationship and potential mechanism by observing the changes in the exosome level, infarct volume, neurological function and behavioral scores, synapses, and corticospinal tract (CST).Methods: Rats were randomly divided into four groups: a sham operation (SHAM) group, middle cerebral artery occlusion (MCAO) with sedentary intervention (SED-MCAO) group, MCAO with exercise intervention (EX-MCAO) group, and MCAO with exercise intervention and exosome injection (EX-MCAO-EXO) group. The exercise intervention was started 1 day after MCAO and lasted for 4 weeks. All rats were assessed using the modified neurological severity score (mNSS). The levels of exosomes in serum and brain, gait analysis, and magnetic resonance scan were performed 1 and 4 weeks after the intervention. After 4 weeks of intervention, the number of synapses, synaptophysin (Syn), and postsynaptic density protein 95(PSD-95) expression was detected.Results: After 4 weeks of intervention, (1) the EX-MCAO and EX-MCAO-EXO groups showed higher serum exosome (pEX−MCAO = 0.000, pEX−MCAO−EXO = 0.000) and brain exosome (pEX−MCAO = 0.001, pEX−MCAO−EXO = 0.000) levels than the SED-MCAO group, of which the EX-MCAO group had the highest serum exosome (p = 0.000) and the EX-MCAO-EXO group had the highest brain exosome (p = 0.03) levels. (2) The number of synapses in the EX-MCAO (p = 0.032) and EX-MCAO-EXO groups (p = 0.000) was significantly higher than that in the SED-MCAO group. The EX-MCAO-EXO group exhibited a greater number of synapses than the EX-MCAO (p = 0.000) group. (3) The synaptic plasticity-associated proteins were expressed significantly higher in the EX-MCAO (pSyn = 0.010, pPSD−95 = 0.044) and EX-MCAO-EXO (pSyn = 0.000, pPSD−95 = 0.000) groups than in the SED-MCAO group, and the EX-MCAO-EXO group (pSyn = 0.000, pPSD−95 = 0.046) had the highest expression. (4) Compared with the SED-MCAO group, the EX-MCAO group had significantly improved infarct volume ratio (p = 0.000), rFA value (p = 0.000), and rADC (p = 0.000). Compared with the EX-MCAO group, the EX-MCAO-EXO group had a significantly improved infarct volume ratio (p = 0.000), rFA value (p = 0.000), and rADC value (p = 0.001). (5) Compared with the SED-MCAO group, the EX-MCAO group (p = 0.001) and EX-MCAO-EXO group (p = 0.000) had significantly lower mNSS scores and improved gait. (6) The brain exosome levels were negatively correlated with the mNSS score, infarct volume ratio, and rADC value and positively correlated with the rFA value, Syn, and PSD-95 expression. The serum and brain exosome levels showed a positive correlation.Conclusions: Exercise intervention increases the serum exosome level in MCAO rats, which are recruited into the brain, leading to improved synaptic growth and CST integrity, a reduced infarct volume, and improved neurological function and gait.

Mask, the Drosophila Ankyrin Repeat and KH domain-containing protein, affects microtubule stability

Proper regulation of microtubule (MT) stability and dynamics is vital for essential cellular processes, including axonal transportation and synaptic growth and remodeling in neurons. Here, we demonstrate that Mask negatively affects MT stability in both fly larval muscles and motor neurons. In larval muscles, loss-of-function of mask increases MT polymer length, and in motor neurons, loss of mask function results in overexpansion of the presynaptic terminal at the larval neuromuscular junctions (NMJs). mask genetically interacts with stathmin (stai), a neuronal modulator of MT stability, in the regulation of axon transportation and synaptic terminal stability. Our structure/function analysis on Mask revealed that its Ankyrin Repeats domain-containing N-terminal portion is sufficient to mediate Mask's impact to MT stability. Furthermore, we discovered that Mask negatively regulates the abundance of the microtubule-associated protein Jupiter in motor neuron axons, and that neuronal knocking down of Jupiter partially suppresses mask loss-of-function phenotypes at the larval NMJs. Together, our studies demonstrated that Mask is a novel regulator for microtubule stability, and such a role of Mask requires normal function of Jupiter.

Local regulation of extracellular vesicle traffic by the synaptic endocytic machinery

Neuronal extracellular vesicles (EVs) carry cargoes that are important in intercellular signaling and disease, but how and where cargoes are sorted into EVs remains unclear. Here, we identified a new role for canonical clathrin-mediated endocytic machinery in controlling EV cargo traffic in Drosophila neurons. Endocytic mutants, including nervous wreck (nwk), Shibire/Dynamin, and AP-2, exhibit local depletion of multiple cargoes in presynaptic EV donor terminals as well as in EVs. Accordingly, nwk mutants phenocopy synaptic plasticity defects associated with loss of the EV cargo Synaptotagmin-4, and suppress lethality upon overexpression of the EV cargo Amyloid Precursor Protein. These EV defects are genetically separable from canonical functions of endocytic proteins in synaptic vesicle recycling and synaptic growth. Nwk opposes the endosomal retromer complex to regulate EV cargo levels, and acts upstream of dynactin-mediated retrograde axonal transport. Our data suggest a novel molecular mechanism that protects EV cargoes from local depletion at synapses.

Glia-neuron signaling mediated by two different BMP ligands impacts synaptic growth

ABSTRACTThe nervous system is a complex network of cells whose interactions provide circuitry necessary for an organism to perceive and move through its environment. Revealing the molecular basis of how neurons and non-neuronal glia communicate is essential for understanding neural development, behavior, and abnormalities of the nervous system. BMP signaling in motor neurons, activated in part by retrograde signals from muscle expressed Gbb (BMP5/6/7) has been implicated in synaptic growth, function and plasticity inDrosophila melanogaster. Through loss-of-function studies, we establish Gbb as a critical mediator of glia to neuron signaling important for proper synaptic growth. Furthermore, the BMP2/4 ortholog, Dpp, expressed in a subset of motor neurons, acts by autocrine signaling to also facilitate neuromuscular junction (NMJ) growth at specific muscle innervation sites. In addition to signaling from glia to motor neurons, autocrine Gbb induces signaling in larval VNC glia which strongly express the BMP type II receptor, Wit. In addition to Dpp’s autocrine motor neuron signaling, Dpp also engages in paracrine signaling to adjacent glia but not to neighboring motor neurons. In one type of dorsal midline motor neuron, RP2,dpptranscription is under tight regulation, as its expression is under autoregulatory control in RP2 but not aCC neurons. Taken together our findings indicate that bi-directional BMP signaling, mediated by two different ligands, facilitates communication between glia and neurons. Gbb, prominently expressed in glia, and Dpp acting from a discrete set of neurons induce active Smad-dependent BMP signaling to influence bouton number during neuromuscular junction growth.

AP2 regulates Thickveins trafficking through Rab11 to attenuate NMJ growth signaling in Drosophila

ABSTRACTCompromised endocytosis in neurons leads to synapse overgrowth and altered organization of synaptic proteins. However, the molecular players and the signaling pathways which regulate the process remains poorly understood. Here we show that σ2-adaptin, one of the subunits of the AP2-complex, genetically interacts with BMP type I receptor, Thickveins (Tkv), and Daughter against decapentaplegic (Dad), two of the components of BMP signaling. We found that mutations in σ2-adaptin lead to an accumulation of Tkv receptors at the NMJ and results in a significant reduction in Tkv-positive early endosomes in the presynaptic terminals. Interestingly, the level of small GTPase Rab11 was significantly reduced in the σ2-adaptin mutant synapses. Consistent with the role of σ2-adaptin and Rab11 in the regulation of the same signaling pathway, a mutation in Rab11 or overexpression of a GDP-locked form of Rab11 (Rab11S25N) phenocopies the morphological and signaling defects of the σ2-adaptin mutants. Finally, we demonstrate that σ2-adaptin mutants show an accumulation of large vesicles and massive membranous structures, akin to endosomes at the synapse. Thus, we propose a model in which AP2 regulates Tkv internalization and recycling through a process that requires Rab11 activity to control the synaptic growth.

Fragile X Premutation rCGG Repeats Impairs Synaptic Growth and Synaptic Transmission at Drosophila larval Neuromuscular Junction

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a progressive neurodegenerative disease manifesting in the premutation (PM) carriers of the FMR1 gene with alleles bearing 55-200 CGG repeats. The discovery of a broad spectrum of clinical and cell developmental abnormalities among PM carriers with or without FXTAS, and in model systems suggests that neurodegeneration seen in FXTAS could be the inevitable end-result of pathophysiological processes set during early development. Hence, it is imperative to trace early pathological abnormalities. Our previous studies have shown that transgenic Drosophila carrying human-derived fragile X premutation-length CGG repeats are sufficient to cause neurodegeneration. Here, we used the same transgenic Drosophila model to understand the effects of fragile X premutation-length CGG repeats on the structure and function of the developing nervous system. We show that presynaptic expression of the premutation length CGG repeats restricts synaptic growth, reduces the number of synaptic boutons, leads to aberrant presynaptic varicosities, and impairs synaptic transmission at the larval neuromuscular junctions (NMJs). The postsynaptic analysis shows both glutamate receptor and subsynaptic reticulum proteins are normal. However, a high percentage of boutons show the reduced density of Bruchpilot protein, a key component of presynaptic active zones required for vesicle release. The electrophysiological analysis shows a significant reduction in the quantal content, a measure of total synaptic vesicles released per excitation potential. Together these findings endorse that synapse perturbation caused by rCGG repeats mediate presynaptically during larval NMJ development.

Developmental demands contribute to early neuromuscular degeneration in CMT2D mice

Dominantly inherited, missense mutations in the widely expressed housekeeping gene, GARS1, cause Charcot-Marie-Tooth type 2D (CMT2D), a peripheral neuropathy characterised by muscle weakness and wasting in limb extremities. Mice modelling CMT2D display early and selective neuromuscular junction (NMJ) pathology, epitomised by disturbed maturation and neurotransmission, leading to denervation. Indeed, the NMJ disruption has been reported in several different muscles; however, a systematic comparison of neuromuscular synapses from distinct body locations has yet to be performed. We therefore analysed NMJ development and degeneration across five different wholemount muscles to identify key synaptic features contributing to the distinct pattern of neurodegeneration in CMT2D mice. Denervation was found to occur along a distal-to-proximal gradient, providing a cellular explanation for the greater weakness observed in mutant Gars hindlimbs compared with forelimbs. Nonetheless, muscles from similar locations and innervated by axons of equivalent length showed significant differences in neuropathology, suggestive of additional factors impacting on site-specific neuromuscular degeneration. Defective NMJ development preceded and associated with degeneration, but was not linked to a delay of wild-type NMJ maturation processes. Correlation analyses indicate that muscle fibre type nor synaptic architecture explain the differential denervation of CMT2D NMJs, rather it is the extent of post-natal synaptic growth that predisposes to neurodegeneration. Together, this work improves our understanding of the mechanisms driving synaptic vulnerability in CMT2D and hints at pertinent pathogenic pathways.

Developmental demands contribute to early neuromuscular degeneration in CMT2D mice

Dominantly inherited, missense mutations in the widely expressed housekeeping gene, GARS1, cause Charcot-Marie-Tooth type 2D (CMT2D), a peripheral neuropathy characterised by muscle weakness and wasting in limb extremities. Mice modelling CMT2D display early and selective neuromuscular junction (NMJ) pathology, epitomised by disturbed maturation and neurotransmission, leading to denervation. Indeed, the NMJ disruption has been reported in several different muscles; however, a systematic comparison of neuromuscular synapses from distinct body locations has yet to be performed. We therefore analysed NMJ development and degeneration across five different wholemount muscles to identify key synaptic features contributing to the distinct pattern of neurodegeneration in CMT2D mice. Denervation was found to occur along a distal-to-proximal gradient, providing a cellular explanation for the greater weakness observed in mutant Gars hindlimbs compared to forelimbs. Nonetheless, muscles from similar locations and innervated by axons of equivalent length showed significant differences in neuropathology, suggestive of additional factors impacting on site-specific neuromuscular degeneration. Defective NMJ development preceded and associated with degeneration, but was not linked to a delay of wild-type NMJ maturation processes. Correlation analyses indicate that muscle fibre type nor synaptic architecture explain the differential denervation of CMT2D NMJs, rather it is the extent of post-natal synaptic growth that predisposes to neurodegeneration. Together, this work improves our understanding of the mechanisms driving synaptic vulnerability in CMT2D and hints at pertinent pathogenic pathways.

Mask, the Drosophila Ankyrin Repeat and KH domain-containing protein, regulates microtubule dynamics

Proper regulation of microtubule (MT) dynamics is vital for essential cellular processes and many neuronal activities, including axonal transport and synaptic growth and remodeling. Here we demonstrate that Mask negatively regulates MT stability and maintains a balanced MT length and architecture in both fly larval muscles and motor neurons. In larval muscles, loss of mask increases MT length, and altering mask genetically modifies the Tau-induced MT fragmentation. In motor neurons, loss of mask function reduces the number of End-Binding Protein 1 (EB1)-positive MT plus-ends in the axons and results in overexpansion of the presynaptic terminal at larval neuromuscular junctions (NMJ). mask shows strong genetic interaction with stathmin (stai), a neuronal modulator of MT dynamics, in regulation of axon transportation and synaptic terminal stability. The structure/function analysis on Mask suggests that Mask’s action in regulating MT stability does not depend on the nucleotide-binding function of its KH domain. Furthermore, through a proteomic approach, we found that Mask physically interacts with Jupiter, an MT stabilizing factor. The MT localization of Jupiter in the axons inversely correlates with Mask levels, suggesting that Mask may modulate MT stability by inhibiting the association of Jupiter to MTs.Author SummaryMicrotubules (MT) are part of the cytoskeleton of the cells that provides essential structural basis for critical processes and functions of the cells. A complex factors are required to orchestrate the assembly and disassembly of MT. Here we identified Mask as a novel regulator for MT dynamics in fruit flies. Mask negatively regulates MT stability. It shows prominent interplay with two important modulators of MT, Tau and Stathmin (Stai), both genes are linked to human neurodegenerative disorders. These findings not only support the role of Mask as a novel microtubule regulator, but also provide foundation to explore future therapeutic strategies in mitigating deficit related to dysfunction of Tau and Stathmin. Our further analysis on Mask protein demonstrate that Mask can physically interacts with another MT stabilizing factor named Jupiter. Jupiter can bind to MT, but its localization to the MTs in the axons is negatively affected by Mask, implying a possible underlying mechanism that Mask may modulate MT stability by inhibiting the association of Jupiter to MTs.