Molecular insights into the regulation of host unfolded protein response and angiogenesis during Mycobacterium tuberculosis pathogenesis

Abstract

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), continues to co-evolve with the human population, rendering it one of the most significant global health threats among infectious diseases. The lungs are the primary site of Mtb infection, where the bacillus is phagocytosed by immune cells such as macrophages, dendritic cells, and neutrophils. Upon infection, Mtb modulates a range of host cellular processes that are otherwise critical for bacterial clearance. While TB is potentially curable, the rise of drug-resistant Mtb strains and its co-infection with other pathogens have compounded the complexity of disease management. The prolonged chemotherapeutic regimens, lack of effective vaccines, and limited understanding of the disease’s molecular pathology have propelled TB to remain the leading cause of death among infectious diseases globally. In this context, delineating the immune-evasion strategies employed by Mtb and identifying key host signaling nodes subverted by the pathogen are imperative for the development of effective host-directed therapies (HDTs). The current study aims to dissect three distinct host cellular outcomes upon Mtb infection. In this regard, we identify specific host signaling pathways and epigenetic regulators manipulated by Mtb to enhance its pathogenesis.

Part I: Molecular insights into the regulation of host unfolded protein response upon Mtb infection The unfolded protein response (UPR) is a fine-tuned signaling response to maintain cellular proteostasis. Our data reveal that Mtb infection induces a robust UPR in host macrophages, which corroborates with the existing findings. Interestingly, lipid peroxidation was found to play a pivotal role in UPR induction. In this line, two key regulatory axes were identified: the Wnt-15-LOX/ACSL4 axis promoting lipid hydroperoxide synthesis, and the SIRT1-PGC1α-GPX4 axis driving its reduction. Both axes were found to be critical in UPR regulation, as perturbing either in Mtb-infected macrophages altered the expression and localization of UPR markers. Importantly, dual pharmacological inhibition of UPR (using 4-Phenylbutyric acid, an FDA-approved drug) and lipid peroxidation (using Ferrostatin-1) in Mtb-infected mice demonstrated a synergistic effect in substantially reducing the bacterial burden and improving lung pathology. These findings highlight the therapeutic potential of targeting UPR and lipid peroxidation in TB. Part II: Elucidation of CHD4-driven regulation of host angiogenesis upon Mtb infection In parallel, we explored the role of the chromatin remodeling factor CHD4 (Chromodomain-helicase-DNA-binding protein 4) in TB pathogenesis. CHD4 is well-established as a core ATPase motor-containing subunit of the mammalian nucleosome remodeling and histone deacetylase (NuRD) complex. It is known to regulate the DNA damage response, cell cycle, vascular integrity, immune cell development, cancer progression, genome architecture stability, neuronal connectivity, etc. It has been proposed as a prognostic marker and a therapeutic target in a variety of cancers. In our study, CHD4 demonstrated increased recruitment over the promoters of angiogenesis-associated genes, including Pdgfrb in Mtb-infected macrophages. Under the same scenario, CHD4 was found to regulate angiogenesis in a PDGFRβ-dependent manner. Inhibition of PDGFR in a therapeutic mouse model of TB not only reduced infection-induced angiogenesis but also significantly decreased Mtb burden and improved lung pathology. Since angiogenesis has been shown to impede drug delivery to granuloma cores, targeting PDGFR may enhance the efficacy of frontline TB therapies. Moreover, PDGF-BB is elevated in TB patients and has been proposed as a biomarker for distinguishing active from latent disease. These findings suggest PDGFR inhibition as a promising HDT strategy. Part III: Understanding the role for PDGFR signaling in host ribosome biogenesis during Mtb pathogenesis In this part of the study, we examined the orchestration between PDGFR signaling and ribosome biogenesis. The latter, as the name suggests, is the process of synthesis and assembly of various components of the ribosome complex. Dysregulation of ribosome biogenesis has been implicated in various diseases, including cancer, cardiovascular disorders, aging, neurodegenerative conditions and COVID-19. In this regard, we conducted preliminary investigations in the setting of Mtb infection. Bioinformatic analysis of the published host transcriptome data in the context of Mtb infection revealed a significant downregulation of genes involved in rRNA processing and ribosomal subunit maturation. Mtb-induced PDGFR signaling was found to downregulate rRNA expression. This suppression was mediated through the upregulation of the lncRNA Papas, which is known to facilitate CHD4 recruitment to the rDNA locus, thereby repressing rRNA transcription. Inhibition of PDGFR signaling reversed these effects, restoring rRNA levels and implicating a PDGFR–lncPapas axis in regulating ribosome biogenesis. Furthermore, a significant reduction in global protein translation was observed in Mtb-infected macrophages, indicating that Mtb may manipulate host protein synthesis through coordinated control of ribosome biogenesis. In summary, our study unravels novel regulatory circuits by which Mtb attunes host cell biology to facilitate its survival and persistence. Targeting host UPR, lipid peroxidation and angiogenesis opens new avenues for the development of HDTs against TB.