Timely regulated sorting from early to late endosomes is required to maintain cerebellar long-term depression

Emerging Lysosomal Functions for Photoreceptor Cell Homeostasis and Survival

Lysosomes are membrane-bound cell organelles that respond to nutrient changes and are implicated in cell homeostasis and clearance mechanisms, allowing effective adaptation to specific cellular needs. The relevance of the lysosome has been elucidated in a number of different contexts. Of these, the retina represents an interesting scenario to appreciate the various functions of this organelle in both physiological and pathological conditions. Growing evidence suggests a role for lysosome-related mechanisms in retinal degeneration. Abnormal lysosomal activation or inhibition has dramatic consequences on photoreceptor cell homeostasis and impacts extensive cellular function, which in turn affects vision. Based on these findings, a series of therapeutic methods targeting lysosomal processes could offer treatment for blindness conditions. Here, we review the recent findings on membrane trafficking, subcellular organization, mechanisms by which lysosome/autophagy pathway impairment affects photoreceptor cell homeostasis and the recent advances on developing efficient lysosomal-based therapies for retinal disorders.

Sushi domain-containing protein 4 controls synaptic plasticity and motor learning

Fine control of protein stoichiometry at synapses underlies brain function and plasticity. How proteostasis is controlled independently for each type of synaptic protein in a synapse-specific and activity-dependent manner remains unclear. Here, we show that Susd4, a gene coding for a complement-related transmembrane protein, is expressed by many neuronal populations starting at the time of synapse formation. Constitutive loss-of-function of Susd4 in the mouse impairs motor coordination adaptation and learning, prevents long-term depression at cerebellar synapses, and leads to misregulation of activity-dependent AMPA receptor subunit GluA2 degradation. We identified several proteins with known roles in the regulation of AMPA receptor turnover, in particular ubiquitin ligases of the NEDD4 subfamily, as SUSD4 binding partners. Our findings shed light on the potential role of SUSD4 mutations in neurodevelopmental diseases.

On the dynamical aspects of local translation at the activated synapse

Background

The key role in the dynamic regulation of synaptic protein turnover belongs to the Fragile X Mental Retardation Protein, which regulates the efficiency of dendritic mRNA translation in response to stimulation of metabotropic glutamate receptors at excitatory synapses of the hippocampal pyramidal cells. Its activity is regulated via positive and negative regulatory loops that function in different time ranges, which is an absolute factor for the formation of chaotic regimes that lead to disrupted proteome stability. The indicated condition may cause a number of neuropsychiatric diseases, including autism and epilepsy. The present study is devoted to a theoretical analysis of the local translation system dynamic properties and identification of parameters affecting the chaotic potential of the system.

Results

A mathematical model that describes the maintenance of a specific pool of active receptors on the postsynaptic membrane via two mechanisms – de novo synthesis of receptor proteins and restoration of protein function during the recycling process – has been developed. Analysis of the model revealed that an increase in the values of the parameters describing the impact of protein recycling on the maintenance of a pool of active receptors in the membrane, duration of the signal transduction via the mammalian target of rapamycin pathway, influence of receptors on the translation activation, as well as reduction of the rate of synthesis and integration of de novo synthesized proteins into the postsynaptic membrane – contribute to the reduced complexity of the local translation system dynamic state. Formation of these patterns significantly depends on the complexity and non-linearity of the mechanisms of exposure of de novo synthesized receptors to the postsynaptic membrane, the correct evaluation of which is currently problematic.

Conclusions

The model predicts that an increase of “receptor recycling” and reduction of the rate of synthesis and integration of de novo synthesized proteins into the postsynaptic membrane contribute to the reduced complexity of the local translation system dynamic state. Herewith, stable stationary states occur much less frequently than cyclic states. It is possible that cyclical nature of functioning of the local translation system is its “normal” dynamic state.

Sevoflurane Impairs Short-Term Memory by Affecting PSD-95 and AMPA Receptor in the Hippocampus of a Mouse Model

Objective

To explore the effects of sevoflurane on the latency and error times of the passive avoidance and levels of PSD-95 and AMPA receptors in the hippocampus. We evaluated the effects of sevoflurane on short-term memory in adult mice and explored the possible mechanism.

Methods

144 Kunming mice (2-3 months, 30-35 g) were randomly divided into two groups A (n = 64) and B (n = 80) and received the dark-avoidance (DA) and step-down avoidance (SA) tests, respectively. The groups DA and SA were further divided into control (inhaled 40% O₂ 2 h) and sevoflurane (3.3% sevoflurane and 40% O₂ 2 h) subgroups. Before inhalation intervention, all mice were trained to be familiar with the Morris water maze (MWM). According to the test points of behavioral indicators, 8 mice were randomly selected from each subgroup at point 12 h (T1), 24 h (T2), 48 h (T3), and 72 h (T4) after inhalation intervention. The step-through latency and error times were measured in 5 min. After the behavioral test, the mice were killed and the tissues of the hippocampus were taken for hematoxylin and eosin (H&E) staining. The expression level of PSD-95 and AMPA receptors in the hippocampus was detected by immunohistochemistry and Western Blot. The changes of synaptic transmission were measured via electrophysiology analysis of hippocampal slices.

Results

The mice in the control subgroups found the platform in a shorter pathway than those in the sevoflurane subgroups during an MWM test. The step-through latency of T1 and T2 in the sevoflurane subgroup was shorter than baseline time, and the error times were increased in 5 min and higher than baseline time when compared with the control subgroup (P < 0.05) in the A and B groups. Compared with the control subgroup, the expression level of PSD-95 and AMPA receptors in the hippocampus was decreased at T1 and T2 in the sevoflurane subgroup (P < 0.05). The nerve cells were partially swelling. Electrophysiology analysis showed that the levels of PSD-95 and AMPA receptor expression were associated with synaptic transmission.

Conclusion

Sevoflurane impaired short-term memory in adult mice by inhibiting the expression of PSD-95 and AMPA receptors in the hippocampus, which led to the decrease in synaptic transmission.

The Endolysosomal System and Proteostasis: From Development to Degeneration

How do neurons adapt their endolysosomal system to address the particular challenge of membrane transport across their elaborate cellular landscape and to maintain proteostasis for the lifetime of the organism? Here we review recent findings that address this central question. We discuss the cellular and molecular mechanisms of endolysosomal trafficking and the autophagy pathway in neurons, as well as their role in neuronal development and degeneration. These studies highlight the importance of understanding the basic cell biology of endolysosomal trafficking and autophagy and their roles in the maintenance of proteostasis within the context of neurons, which will be critical for developing effective therapies for various neurodevelopmental and neurodegenerative disorders.

Persistent cytosolic Ca2+ increase induced by angiotensin II at nanomolar concentrations in acutely dissociated subfornical organ (SFO) neurons of rats

It is known that angiotensin II (AII) is sensed by subfornical organ (SFO) to induce drinking behaviors and autonomic changes. AII at picomolar concentrations have been shown to induce Ca²⁺ oscillations and increase in the amplitude and frequency of spontaneous Ca²⁺ oscillations in SFO neurons. The present study was conducted to examine effects of nanomolar concentrations of AII using the Fura-2 Ca²⁺-imaging technique in acutely dissociated SFO neurons. AII at nanomolar concentrations induced an initial [Ca²⁺]_i peak followed by a persistent [Ca²⁺]_i increase lasting for longer than 1 hour. By contrast, [Ca²⁺]_i responses to 50 mM K⁺, maximally effective concentrations of glutamate, carbachol, and vasopressin, and AII given at picomolar concentrations returned to the basal level within 20 min. The AII-induced [Ca²⁺]_i increase was blocked by the AT1 antagonist losartan. However, losartan had no effect when added during the persistent phase. The persistent phase was suppressed by extracellular Ca2+ removal, significantly inhibited by blockers of L and P/Q type Ca²⁺ channels , but unaffected by inhibition of Ca²⁺ store Ca²⁺ ATPase. The persistent phase was reversibly suppressed by GABA and inhibited by CaMK and PKC inhibitors. These results suggest that the persistent [Ca²⁺]_i increase evoked by nanomolar concentrations of AII is initiated by AT1 receptor activation and maintained by Ca²⁺ entry mechanisms in part through L and P/Q type Ca²⁺ channels, and that CaMK and PKC are involved in this process. The persistent [Ca²⁺]_i increase induced by AII at high pathophysiological levels may have a significant role in altering SFO neuronal functions.

Postsynaptic Stability and Variability Described by a Stochastic Model of Endosomal Trafficking

Neurons undergo dynamic processes of constitutive AMPA-type glutamate receptor (AMPAR) trafficking, such as the insertion and internalization of AMPARs by exocytosis and endocytosis, while stably maintaining synaptic efficacy. Studies using advanced imaging techniques have suggested that the frequency of these constitutive trafficking processes, as well as the number of AMPARs that are involved in a particular event highly fluctuate. In addition, mechanisms that trigger some forms of synaptic plasticity have been shown to include not only these processes but also additional fluctuating processes, such as the sorting of AMPARs to late endosomes (LEs). Thus, the regulation of postsynaptic AMPARs by the endosomal trafficking system appears to have superficially conflicting properties between the stability or organized control of plasticity and highly fluctuating or stochastic processes. However, it is not clear how the endosomal trafficking system reconciles and utilizes such conflicting properties. Although deterministic models have been effective to describe the stable maintenance of synaptic AMPAR numbers by constitutive recycling, as well as the involvement of endosomal trafficking in synaptic plasticity, they do not take stochasticity into account. In this study, we introduced the stochasticity into the model of each crucial machinery of the endosomal trafficking system. The specific questions we solved by our improved model are whether stability is accomplished even with a combination of fluctuating processes, and how overall variability occurs while controlling long-term synaptic depression (LTD). Our new stochastic model indeed demonstrated the stable regulation of postsynaptic AMPAR numbers at the basal state and during LTD maintenance, despite fast fluctuations in AMPAR numbers as well as high variability in the time course and amounts of LTD. In addition, our analysis suggested that the high variability arising from this stochasticity is beneficial for reproducing the relatively constant timing of LE sorting for LTD. We therefore propose that the coexistence of stability and stochasticity in the endosomal trafficking system is suitable for stable synaptic transmission and the reliable induction of synaptic plasticity, with variable properties that have been observed experimentally.

Degradation of dendritic cargos requires Rab7-dependent transport to somatic lysosomes

Neurons are large and long lived, creating high needs for regulating protein turnover. Disturbances in proteostasis lead to aggregates and cellular stress. We characterized the behavior of the short-lived dendritic membrane proteins Nsg1 and Nsg2 to determine whether these proteins are degraded locally in dendrites or centrally in the soma. We discovered a spatial heterogeneity of endolysosomal compartments in dendrites. Early EEA1-positive and late Rab7-positive endosomes are found throughout dendrites, whereas the density of degradative LAMP1- and cathepsin (Cat) B/D-positive lysosomes decreases steeply past the proximal segment. Unlike in fibroblasts, we found that the majority of dendritic Rab7 late endosomes (LEs) do not contain LAMP1 and that a large proportion of LAMP1 compartments do not contain CatB/D. Second, Rab7 activity is required to mobilize distal predegradative LEs for transport to the soma and terminal degradation. We conclude that the majority of dendritic LAMP1 endosomes are not degradative lysosomes and that terminal degradation of dendritic cargos such as Nsg1, Nsg2, and DNER requires Rab7-dependent transport in LEs to somatic lysosomes.

Neuronal lysosomes

Lysosomes support diverse cellular functions by acting as sites of macromolecule degradation and nutrient recycling. The degradative abilities of lysosomes are conferred by a lumen that is characterized by an acidic pH and which contains numerous hydrolases that support the breakdown of major cellular macromolecules to yield cellular building blocks (amino acids, nucleic acids, sugars, lipids and metals) that are transported into the cytoplasm for their re-use. In addition to these important hydrolytic and recycling functions, lysosomes also serve as a signaling platform that integrates nutrient and metabolic cues to control signaling via the mTORC1 pathway. Due to their extreme longevity, polarity, demands of neurotransmission and metabolic activity, neurons are particularly sensitive to perturbations in lysosome function. The dependence of neurons on optimal lysosome function is highlighted by insights from human genetics that link lysosome dysfunction to a wide range of both rare and common neurological diseases. How then is lysosome function adapted to the unique demands of neurons? This review will focus on the roles played by lysosomes in distinct neuronal sub-compartments, the regulation of neuronal lysosome sub-cellular localization and the implications of such neuronal lysosome regulation for both physiology and disease.