Antagonistic actions of PAK1 and NF2/Merlin drive myelin membrane expansion in oligodendrocytes.

In the central nervous system, the formation of myelin by oligodendrocytes (OLs) relies on the switch from the polymerization of the actin cytoskeleton to its depolymerization. The molecular mechanisms that trigger this switch have yet to be elucidated. Here, we identified P21-activated kinase 1 (PAK1) as a major regulator of actin depolymerization in OLs. Our results demonstrate that PAK1 accumulates in OLs in a kinase-inhibited form, triggering actin disassembly and, consequently, myelin membrane expansion. Remarkably, proteomic analysis of PAK1 binding partners enabled the identification of NF2/Merlin as its endogenous inhibitor. Our findings indicate that Nf2 knockdown in OLs results in PAK1 activation, actin polymerization, and a reduction in OL myelin membrane expansion. This effect is rescued by treatment with a PAK1 inhibitor. We also provide evidence that the specific Pak1 loss-of-function in oligodendroglia stimulates the thickening of myelin sheaths in vivo. Overall, our data indicate that the antagonistic actions of PAK1 and NF2/Merlin on the actin cytoskeleton of the OLs are critical for proper myelin formation. These findings have broad mechanistic and therapeutic implications in demyelinating diseases and neurodevelopmental disorders.

SMARCA4-deficient lung carcinoma is an aggressive tumor highly infiltrated by FOXP3+ cells and neutrophils.

INTRODUCTION: SMARCA4/BRG1 loss of expression occurs in 5-10% of non-small cell lung carcinomas (NSCLC). We investigated the pathological, molecular and immune environment characteristics of this deficiency among NSCLC, its impact on overall survival (OS) of resected patients and the sensitivity to anti-PD1 inhibitors in metastatic patients. MATERIALS AND METHODS: BRG1 expression was assessed by immunohistochemistry to identify SMARCA4-deficient NSCLC (SD-NSCLC) from the cancer tissue collection of Cochin Hospital (Paris, France). Molecular profiles were analyzed by targeted NGS covering 28 genes in 63 resected SD-NSCLC. The balance of immune cells between CD8+, FOXP3+ cells and neutrophils (CD66b+) was characterized by multiplex immunohistochemistry and compared to non-SD NSCLC. Clinical outcome after anti-PD-1 therapy was evaluated in 7 SD-NSCLC out of 77 NSCLC patients. RESULTS: SD-NSCLCs were more commonly found in TTF1-negative high-grade adenocarcinomas and pleomorphic carcinomas. They were associated with few targetable alterations (KRAS G12C and MET amplification). Their immune environment was characterized by an increased of FOXP3+ cell and neutrophil densities, but not of CD8+ T cells, compared to non-SD NSCLC. SD-NSCLC patients had a significantly shorter OS in early stages of resected patients and in metastatic patients treated by anti-PD1 treatment. CONCLUSION: BRG1-loss in NSCLC confers a poor prognosis and is associated with an immunosuppressive environment that could be responsible of limited efficacy to anti-PD1 inhibitors. The identification of SD-NSCLC by BRG1 immunohistochemistry is desirable for an optimal management of NSCLC patients.

Proposal for a Combined Histomolecular Algorithm to Distinguish Multiple Primary Adenocarcinomas from Intrapulmonary Metastasis in Patients with Multiple Lung Tumors.

INTRODUCTION: Multiple nodules in the lung are being diagnosed with an increasing frequency thanks to high-quality computed tomography imaging. In patients with lung cancer, this situation represents up to 10% of patients who have an operation. For clinical management, it is important to classify the disease as intrapulmonary metastasis or multiple primary lung carcinoma to define TNM classification and optimize therapeutic options. In the present study, we evaluated the respective and combined input of histological and molecular classification to propose a classification algorithm for multiple nodules. METHODS: We studied consecutive patients undergoing an operation with curative intent for lung adenocarcinoma (N = 120) and harboring two tumors (N = 240). Histological diagnosis according to the WHO 2015 classification and molecular profiling using next-generation sequencing targeting 22 hotspot genes allowed classification of samples as multiple primary lung adenocarcinomas or as intrapulmonary metastasis. RESULTS: Next-generation sequencing identified molecular mutations in 91% of tumor pairs (109 of 120). Genomic and histological classification showed a fair agreement when the κ test was used (κ = 0.43). Discordant cases (30 of 109 [27%]) were reclassified by using a combined histomolecular algorithm. EGFR mutations (p = 0.03) and node involvement (p = 0.03) were significantly associated with intrapulmonary metastasis, whereas KRAS mutations (p = 0.00005) were significantly associated with multiple primary lung adenocarcinomas. EGFR mutations (p = 0.02) and node involvement (p = 0.004) were the only independent prognostic factors. CONCLUSION: We showed that combined histomolecular algorithm represents a relevant tool to classify multifocal lung cancers, which could guide adjuvant treatment decisions. Survival analysis underlined the good prognosis of EGFR-mutated adenocarcinoma in patients with intrapulmonary metastasis.