2026-07-06 | Activity-dependent CO2 production in the axon triggers opening of Connexin32 in the Schwann cell paranode.

Loss of function mutations of Cx32, which is expressed in Schwann cells, cause X-linked Charcot-Marie-Tooth disease, a slowly progressive peripheral neuropathy. Action potential propagation causes Cx32 hemichannels in the Schwann cell paranode to open. As Cx32 hemichannels are directly sensitive to CO2, we have tested whether CO2 produced in the axon, as a consequence of the energetic demands of action potential propagation, might gate Cx32 hemichannels. Using isolated sciatic nerve from the mouse, we found that the critical components required for intercellular CO2 signaling are present (nodal mitochondria, the source of CO2; a CO2-permeable aquaporin, AQP1; paranodal Cx32; and carbonic anhydrase). We have used a membrane impermeant fluorescent dye, FITC, to demonstrate the opening of Cx32 in Schwann cells in response to an external CO2 stimulus or during action potential propagation in the isolated nerve. Pharmacological manipulations of AQP1 or carbonic anhydrase activity altered Cx32 gating during action potential firing. Expression of a modified Cx32 subunit, Cx32DN, that coassembles with Cx32WT, revealed that the activity-dependent dye loading of Schwann cells depended upon CO2 binding to Cx32. CO2 can, therefore, mediate neuron-to-glia signaling via connexins. CO2 permeable aquaporins and carbonic anhydrase are key components of this signaling mechanism.

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2026-07-02 | A class of deep intronic IGHMBP2 variants activate a shared cryptic splice donor, enabling correction of select variants with a single antisense oligonucleotide.

Biallelic disease-causing variants in IGHMBP2 cause spinal muscular atrophy with respiratory distress type I (SMARD1) and Charcot-Marie-Tooth type 2S (CMT2S). We present 12 unrelated patients with clinically suspected IGHMBP2-related-disease, each carrying a variant deep in intron 8 of IGHMBP2 (c.1235+1076G>A (n=6), c.1235+450G>A (n=5), and c.1235+894C>A (n=1)), along with a known deleterious variant in trans. To assess aberrant pathogenic splicing induced by these deep intronic variants in a relevant model, patient-derived induced pluripotent stem cells were differentiated into motor neurons (iMNs). Long-read RNA sequencing revealed introduction of different pseudoexons by each variant: c.1235+450G>A (626bp), c.1235+1076G>A (112bp and 77bp) and c.1235+894C>A (182bp). Although each variant utilizes a unique splice acceptor site, they all activate the same cryptic donor site, enabling a therapeutic approach to redirect aberrant splicing for all the variants using a single shared antisense oligonucleotide (ASO). Treatment of iMNs with this single ASO restored full-length IGHMBP2 protein in c.1235+894G>A and c.1235+1076G>A by decreasing the use of the novel acceptor site. In contrast, ASO treatment did not correct the splicing in c.1235+450G>A, suggesting that additional splice correction will be needed for this specific variant. A CRISPR interference screen of IGHMBP2 loss-of-function in iMNs identified ribonucleoprotein complex biogenesis (RNP), and rRNA and tRNA processing as top pathways implicated in motor neuron vulnerability. Proteomics and transcriptomics analysis of successfully treated patient iMNs revealed correction of RNP biogenesis and rRNA processing defects. This study highlights the importance of characterizing deep intronic variants in disease-relevant cells to assist the diagnostic process and inform therapeutics development.

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