A cAMP Sensor Based on Ligand-Dependent Protein Stabilization.

cAMP is a ubiquitous second messenger with many functions in diverse organisms. Current cAMP sensors, including Föster resonance energy transfer (FRET)-based and single-wavelength-based sensors, allow for real time visualization of this small molecule in cultured cells and in some cases in vivo. Nonetheless the observation of cAMP in living animals is still difficult, typically requiring specialized microscopes and ex vivo tissue processing. Here we used ligand-dependent protein stabilization to create a new cAMP sensor. This sensor allows specific and sensitive detection of cAMP in living zebrafish embryos, which may enable new understanding of the functions of cAMP in living vertebrates.

Calcineurin Activity Is Increased in Charcot-Marie-Tooth 1B Demyelinating Neuropathy.

Schwann cells produce a considerable amount of lipids and proteins to form myelin in the PNS. For this reason, the quality control of myelin proteins is crucial to ensure proper myelin synthesis. Deletion of serine 63 from P0 (P0S63del) protein in myelin forming Schwann cells causes Charcot-Marie-Tooth type 1B neuropathy in humans and mice. Misfolded P0S63del accumulates in the ER of Schwann cells where it elicits the unfolded protein response (UPR). PERK is the UPR transducer that attenuates global translation and reduces ER stress by phosphorylating the translation initiation factor eIF2alpha. Paradoxically, Perk ablation in P0S63del Schwann cells (S63del/PerkSCKO ) reduced the level of P-eIF2alpha, leaving UPR markers upregulated, yet unexpectedly improved S63del myelin defects in vivo We therefore investigated the hypothesis that PERK may interfere with signals outside of the UPR and specifically with calcineurin/NFATc4 pro-myelinating pathway. Using mouse genetics including females and males in our experimental setting, we show that PERK and calcineurin interact in P0S63del nerves and that calcineurin activity and NFATc4 nuclear localization are increased in S63del Schwann cells, without altering EGR2/KROX20 expression. Moreover, genetic manipulation of the calcineurin subunits appears to be either protective or toxic in S63del in a context-dependent manner, suggesting that Schwann cells are highly sensitive to alterations of calcineurin activity.SIGNIFICANCE STATEMENT Our work shows a novel activity and function for calcineurin in Schwann cells in the context of ER stress. Schwann cells expressing the S63del mutation in P0 protein induce the unfolded protein response and upregulate calcineurin activity. Calcineurin interacts with the ER stress transducer PERK, but the relationship between the UPR and calcineurin in Schwann cells is unclear. Here we propose a protective role for calcineurin in S63del neuropathy, although Schwann cells appear to be very sensitive to its regulation. The paper uncovers a new important role for calcineurin in a demyelinating diseases.

Deletion of Calcineurin in Schwann Cells Does Not Affect Developmental Myelination, But Reduces Autophagy and Delays Myelin Clearance after Peripheral Nerve Injury.

In the PNS, myelination occurs postnatally when Schwann cells (SCs) contact axons. Axonal factors, such as Neuregulin-1 Type III, trigger promyelinating signals that upregulate myelin genes. Neuregulin-1 Type III has been proposed to activate calcineurin signaling in immature SCs to initiate differentiation and myelination. However, little is known about the role of calcineurin in promyelinating SCs after birth. By creating a SC conditional KO of calcineurin B (CnBscko), we assessed the effects of CnB ablation on peripheral myelination after birth in both male and female mice. Surprisingly, CnBscko mice have minimal myelination defects, no alteration of myelin thickness, and normal KROX20 expression. In contrast, we did find a unique role for calcineurin in SCs after nerve injury. Following nerve crush, CnBscko mice have slower degeneration of myelin compared with WT mice. Furthermore, absence of CnB in primary SCs delays clearance of myelin debris. SCs clear myelin via autophagy and recent literature has demonstrated that calcineurin can regulate autophagy via dephosphorylation of transcription factor EB (TFEB), a master regulator of lysosomal biogenesis and autophagy. We demonstrate that loss of CnB reduces autophagic flux in primary SCs, indicating a possible mechanism for impaired myelin clearance. In addition, ablation of CnB impairs TFEB translocation to the nucleus 3 d after crush, suggesting that calcineurin may regulate autophagy in SCs via TFEB activation. Together, our data indicate that calcineurin is not essential for myelination but has a novel role in myelin clearance after injury.SIGNIFICANCE STATEMENT Our data offer a novel mechanism for activation of autophagy after peripheral nerve injury. Efficient clearance of myelin after injury by Schwann cells is important for axonal regrowth and remyelination, which is one reason why the PNS is significantly better at recovery compared with the CNS. Improved understanding of myelin clearance allows for the identification of pathways that are potentially accessible to increase myelin clearance and improve remyelination and recovery. Finally, this paper clarifies the role of calcineurin in Schwann cells and myelination.

Ablation of Perk in Schwann Cells Improves Myelination in the S63del Charcot-Marie-Tooth 1B Mouse.

In factory cells, the accumulation of misfolded protein provokes the unfolded protein response (UPR). For example, deletion of serine 63 (S63del) in myelin protein zero (P0) induces P0 accumulation in the endoplasmic reticulum (ER) of Schwann cells and a persistent UPR associated with Charcot-Marie-Tooth 1B (CMT1B) demyelinating peripheral neuropathy in human and mouse. PERK (protein kinase RNA-like ER kinase) is the ER stress sensor that attenuates global translation by phosphorylating eIF2α. Inhibition of the eIF2α holophosphatase GADD34:PP1, increases the phosphorylation of eIF2α in Schwann cells and largely rescues S63del neuropathy. Nonetheless, reducing phosphorylation of eIF2α, by Perk haploinsufficiency, also ameliorates the myelin defects of S63del nerves. This contradictory finding prompted us to investigate whether the beneficial effect of Perk deficiency on myelination could derive from neurons. To test this hypothesis, we generated and compared Schwann cell- and neuron-specific ablation of Perk in S63del nerves. Our data suggest that the detrimental effect of Perk in CMT1B derives primarily from Schwann cells. Furthermore, we show that Perk loss of function in Schwann cells restores myelination without diminishing accumulation of P0 or markers of ER stress, suggesting that Perk may modulate myelination through a pathway independent of the UPR. SIGNIFICANCE STATEMENT: In many endoplasmic reticulum (ER) stress-related disorders, activation of the unfolded protein sensor protein kinase RNA-like ER kinase (PERK) kinase is beneficial. Nonetheless, in Charcot-Marie-Tooth 1B neuropathy mice, we show that activation of PERK in Schwann cells, but not in neurons, is detrimental for myelination. PERK may interfere with myelination, independent of its role in ER stress.

Perk Ablation Ameliorates Myelination in S63del-Charcot-Marie-Tooth 1B Neuropathy.

In peripheral nerves, P0 glycoprotein accounts for more than 20% of myelin protein content. P0 is synthesized by Schwann cells, processed in the endoplasmic reticulum (ER) and enters the secretory pathway. However, the mutant P0 with S63 deleted (P0S63del) accumulates in the ER lumen and induces a demyelinating neuropathy in Charcot-Marie-Tooth disease type 1B (CMT1B)-S63del mice. Accumulation of P0S63del in the ER triggers a persistent unfolded protein response. Protein kinase RNA-like endoplasmic reticulum kinase (PERK) is an ER stress sensor that phosphorylates eukaryotic initiation factor 2 alpha (eIF2alpha) in order to attenuate protein synthesis. We have shown that increasing phosphophorylated-eIF2alpha (P-eIF2alpha) is a potent therapeutic strategy, improving myelination and motor function in S63del mice. Here, we explore the converse experiment:Perkhaploinsufficiency reduces P-eIF2alpha in S63del nerves as expected, but surprisingly, ameliorates, rather than worsens S63del neuropathy. Motor performance and myelin abnormalities improved in S63del//Perk+/- compared with S63del mice. These data suggest that mechanisms other than protein translation might be involved in CMT1B/S63del neuropathy. In addition,Perkdeficiency in other cells may contribute to demyelination in a non-Schwann-cell autonomous manner.