Pharmacological chaperone-rescued cystic fibrosis CFTR-F508del mutant overcomes PRAF2-gated access to endoplasmic reticulum exit sites

Identification of potential pharmacological chaperones that selectively stabilize mutated Aspartoacylases in Canavan disease

Canavan disease is caused by mutations in the ASPA gene, leading to diminished catalytic activity of aspartoacylase in the brain. Clinical missense mutations are found throughout the enzyme structure, with many of these mutated enzymes having not only decreased activity but also compromised stability. High-throughput screening of a small molecule library has identified several compounds that significantly increase the thermal stability of the E285A mutant enzyme, the most predominant clinical mutation in Canavan disease, while having a negligible effect on the native enzyme. Based on the initial successes, some structural analogs of these initial hits were selected for further examination. Glutathione, NAAG and patulin were each confirmed to be competitive inhibitors, indicating the binding of these compounds at the dimer interface or near the active site of the E285A enzyme. The experimental results were theoretically examined with the help of the docking analysis method. The structure activity-guided optimization of these compounds can potentially lead to potential pharmacological chaperones that could alleviate the detrimental effect of ASPA mutations in Canavan patients.

COPII cage assembly factor Sec13 integrates information flow regulating endomembrane function in response to human variation

How information flow is coordinated for managing transit of 1/3 of the genome through endomembrane pathways by the coat complex II (COPII) system in response to human variation remains an enigma. By examining the interactome of the COPII cage-assembly component Sec13, we show that it is simultaneously associated with multiple protein complexes that facilitate different features of a continuous program of chromatin organization, transcription, translation, trafficking, and degradation steps that are differentially sensitive to Sec13 levels. For the trafficking step, and unlike other COPII components, reduction of Sec13 expression decreased the ubiquitination and degradation of wild-type (WT) and F508del variant cargo protein cystic fibrosis transmembrane conductance regulator (CFTR) leading to a striking increase in fold stability suggesting that the events differentiating export from degradation are critically dependent on COPII cage assembly at the ER Golgi intermediate compartment (ERGIC) associated recycling and degradation step linked to COPI exchange. Given Sec13's multiple roles in protein complex assemblies that change in response to its expression, we suggest that Sec13 serves as an unanticipated master regulator coordinating information flow from the genome to the proteome to facilitate spatial covariant features initiating and maintaining design and function of membrane architecture in response to human variation.

Tetrahydrobiopterin (BH4) treatment stabilizes tyrosine hydroxylase: Rescue of tyrosine hydroxylase deficiency phenotypes in human neurons and in a knock-in mouse model

Proteostatic regulation of tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine biosynthesis, is crucial for maintaining proper brain neurotransmitter homeostasis. Variants of the TH gene are associated with tyrosine hydroxylase deficiency (THD), a rare disorder with a wide phenotypic spectrum and variable response to treatment, which affects protein stability and may lead to accelerated degradation, loss of TH function and catecholamine deficiency. In this study, we investigated the effects of the TH cofactor tetrahydrobiopterin (BH4) on the stability of TH in isolated protein and in DAn- differentiated from iPSCs from a human healthy subject, as well as from THD patients with the R233H variant in homozygosity (THDA) and R328W and T399M variants in heterozygosity (THDB). We report an increase in TH and dopamine levels, and an increase in the number of TH+ cells in control and THDA cells. To translate this in vitro effect, we treated with BH4 a knock-in THD mouse model with Th variant corresponding to R233H in patients. Importantly, treatment with BH4 significantly improved motor function in these mice, as demonstrated by increased latency on the rotarod test and improved horizontal activity (catalepsy). In conclusion, our study demonstrates the stabilizing effects of BH4 on TH protein levels and function in THD neurons and mice, rescuing disease phenotypes and improving motor outcomes. These findings highlight the therapeutic potential of BH4 as a treatment option for THDA patients with specific variants and provide insights into the modulation of TH stability and its implications for THD management.

Control of CCR5 Cell-Surface Targeting by the PRAF2 Gatekeeper

The cell-surface targeting of neo-synthesized G protein-coupled receptors (GPCRs) involves the recruitment of receptors into COPII vesicles budding at endoplasmic reticulum exit sites (ERESs). This process is regulated for some GPCRs by escort proteins, which facilitate their export, or by gatekeepers that retain the receptors in the ER. PRAF2, an ER-resident four trans- membrane domain protein with cytoplasmic extremities, operates as a gatekeeper for the GB1 protomer of the heterodimeric GABAB receptor, interacting with a tandem di-leucine/RXR retention motif in the carboxyterminal tail of GB1. PRAF2 was also reported to interact in a two-hybrid screen with a peptide corresponding to the carboxyterminal tail of the chemokine receptor CCR5 despite the absence of RXR motifs in its sequence. Using a bioluminescence resonance energy transfer (BRET)-based subcellular localization system, we found that PRAF2 inhibits, in a concentration-dependent manner, the plasma membrane export of CCR5. BRET-based proximity assays and Co-IP experiments demonstrated that PRAF2/CCR5 interaction does not require the presence of a receptor carboxyterminal tail and involves instead the transmembrane domains of both proteins. The mutation of the potential di-leucine/RXR motif contained in the third intracellular loop of CCR5 does not affect PRAF2-mediated retention. It instead impairs the cell-surface export of CCR5 by inhibiting CCR5's interaction with its private escort protein, CD4. PRAF2 and CD4 thus display opposite roles on the cell-surface export of CCR5, with PRAF2 inhibiting and CD4 promoting this process, likely operating at the level of CCR5 recruitment into COPII vesicles, which leave the ER.