S-acylation of the Wnt receptor Frizzled-5 by zDHHC5 controls its cellular localization and synaptogenic activity in the rodent hippocampus.

Proper localization of receptors for synaptic organizing factors is crucial for synapse formation. Wnt proteins promote synapse assembly through Frizzled (Fz) receptors. In hippocampal neurons, the surface and synaptic localization of Fz5 is regulated by neuronal activity, but the mechanisms involved remain poorly understood. Here, we report that all Fz receptors can be post-translationally modified by S-acylation and that Fz5 is S-acylated on three C-terminal cysteines by zDHHC5. S-acylation is essential for Fz5 localization to the cell surface, axons, and presynaptic sites. Notably, S-acylation-deficient Fz5 is internalized faster, affecting its association with signalosome components at the cell surface. S-acylation-deficient Fz5 also fails to activate canonical and divergent canonical Wnt pathways. Fz5 S-acylation levels are regulated by the pattern of neuronal activity. In vivo studies demonstrate that S-acylation-deficient Fz5 expression fails to induce presynaptic assembly. Our studies show that S-acylation of Frizzled receptors is a mechanism controlling their localization and function.

Characterization of the structure and control of the blood-nerve barrier identifies avenues for therapeutic delivery.

The blood barriers of the nervous system protect neural environments but can hinder therapeutic accessibility. The blood-brain barrier (BBB) is well characterized, consisting of endothelial cells with specialized tight junctions and low levels of transcytosis, properties conferred by contacting pericytes and astrocytes. In contrast, the blood-nerve barrier (BNB) of the peripheral nervous system is poorly defined. Here, we characterize the structure of the mammalian BNB, identify the processes that confer barrier function, and demonstrate how the barrier can be opened in response to injury. The homeostatic BNB is leakier than the BBB, which we show is due to higher levels of transcytosis. However, the barrier is reinforced by macrophages that specifically engulf leaked materials, identifying a role for resident macrophages as an important component of the BNB. Finally, we demonstrate the exploitation of these processes to effectively deliver RNA-targeting therapeutics to peripheral nerves, indicating new treatment approaches for nervous system pathologies.

Wnt Signaling Mediates LTP-Dependent Spine Plasticity and AMPAR Localization through Frizzled-7 Receptors.

The structural and functional plasticity of synapses is critical for learning and memory. Long-term potentiation (LTP) induction promotes spine growth and AMPAR accumulation at excitatory synapses, leading to increased synaptic strength. Glutamate initiates these processes, but the contribution from extracellular modulators is not fully established. Wnts are required for spine formation; however, their impact on activity-mediated spine plasticity and AMPAR localization is unknown. We found that LTP induction rapidly increased synaptic Wnt7a/b protein levels. Acute blockade of endogenous Wnts or loss of postsynaptic Frizzled-7 (Fz7) receptors impaired LTP-mediated synaptic strength, spine growth, and AMPAR localization at synapses. Live imaging of SEP-GluA1 and single-particle tracking revealed that Wnt7a rapidly promoted synaptic AMPAR recruitment and trapping. Wnt7a, through Fz7, induced CaMKII-dependent loss of SynGAP from spines and increased extrasynaptic AMPARs by PKA phosphorylation. We identify a critical role for Wnt-Fz7 signaling in LTP-mediated synaptic accumulation of AMPARs and spine plasticity.

Low extracellular sodium causes neuronal distress independently of reduced osmolality in an experimental model of chronic hyponatremia.

There is evidence that chronic hyponatremia, even when mild, may cause neurological signs and symptoms. These have been traditionally associated with water movement into nervous cells, as a result of the hypotonic state. The aim of the present study was to determine whether low extracellular sodium directly exerts negative effects on human neuronal cells, independently of reduced osmolality. We exposed neuroblastoma SK-N-AS and SH-SY5Y cells to sustained low extracellular sodium, thus mimicking a condition of chronic hyponatremia, both in the presence of reduced and in the presence of unaltered osmolality. We found that very low sodium (i.e., 115 mmol/L in SK-N-AS and 90 mmol/L in SH-SY5Y) significantly reduced cell viability. However, intermediate low sodium was able to cause cell distress, as assessed by the altered expression of anti-apoptotic genes and the reduced ability to differentiate into a mature neuronal phenotype. Noteworthy, these effects were observed also in the presence of unaltered osmolality. Moreover, we performed a comprehensive microarray analysis in cells maintained in normal sodium or in low sodium and unaltered osmolality, and we found that the most altered pathway included genes involved in "cell death and survival." Among the more than 40 differentially expressed genes, the Heme oxygenase gene, which represents a transcriptional response to oxidative stress, showed the highest increase in the expression level. This study demonstrates that low extracellular sodium causes detrimental effects in neuronal cells that are at least in part independent of reduced osmolality. These findings further support the recommendation to effectively correct hyponatremia, even when mild and chronic.