Sovan Sarkar

Phagosome maturation arrest (PMA) imposed by Mycobacterium tuberculosis (Mtb) is a classic tool that helps Mtb evade macrophage anti-bacterial responses. The exclusion of RAB7, a small GTPase, from Mtb-phagosomes causes PMA. Here, we report an unexpected mechanism that triggers crosstalk between the mitochondrial quality control (MQC) and the phagosome maturation pathways that reverses the PMA. CRISPR-mediated p62/SQSTM1 depletion (p62KD) does not appear to impact mitochondrial quality. The p62KD cells are restrictive to Mtb growth, triggered by an increasingly oxidative environment and increased lysosomal targeting. The lysosomal targeting of Mtb is facilitated by enhanced TOM20+ mitochondria-derived vesicles (MDVs) biogenesis, a key MQC mechanism. In p62KD cells, TOM20+-MDVs biogenesis is MIRO1/MIRO2-dependent and gets delivered to lysosomes for degradation in a RAB7-dependent manner. Upon infection in p62KD cells, TOM20+-MDVs get extensively targeted to Mtb-phagosomes, inadvertently facilitating RAB7 recruitment, PMA reversal and lysosomal targeting of Mtb; the phenotype also replicated in p62/SQSTM1 knockout cells. Triggering MQC collapse in p62KDcells further diminishes Mtb survival, signifying cooperation between redox- and lysosome-mediated mechanisms. The MQC-anti-bacterial pathway crosstalk could be exploited for host-directed anti-tuberculosis therapies.

The electron shuttle and coenzyme nicotinamide adenine nucleotide (NAD) is essential for cellular metabolism and homeostasis. NAD levels significantly fluctuate in cells, whilst ageing and age-related diseases are associated with depletion of this metabolite. However, how NAD changes are monitored by nutrient/energy sensing signalling pathways remains poorly understood. We found that at physiological concentrations NAD controls the activity of the AMP-activated protein kinase (AMPK) in vitro and in human cells. Mechanistically, NAD binds gamma subunit of AMPK, and mutagenesis of the putative binding site renders the holoenzyme insensitive to NAD inhibition. Hyperactivation of AMPK in response to NAD depletion suppresses metabolic pathways including mammalian Target of Rapamycin Complex I (mTORC1) and autophagy. These results demonstrate that in addition to monitoring cellular energy levels AMPK functions as a NAD sensor, providing novel insight into how cells and tissues detect and respond to metabolic fluctuations with implications for stress resistance and ageing.

Niemann-Pick type C1 (NPC1) disease is a neurodegenerative lysosomal storage disorder caused by loss-of-function mutations in the NPC1 gene. NPC1 deficit primarily disrupts lipid homeostasis and subsequently drives cellular degeneration through mechanisms involving impaired autophagy and mitophagy, mitochondrial dysfunction, and, recently demonstrated NAD depletion that links autophagy impairment to neuronal death. Emerging evidence also highlights the activation of innate immune signaling leading to neuroinflammation. In this review, we synthesize current mechanistic insights and describe how these molecular deficits are interconnected to drive neuronal death in NPC1 disease. We also discuss how these pathological processes parallel those observed in major age-related neurodegenerative pathologies such as Alzheimer’s and Parkinson’s disease. Finally, we highlight emerging therapeutic strategies that can potentially ameliorate these cellular deficits, offering avenues for mitigating neurodegeneration in NPC1 disease and other related neurodegenerative disorders.

Autophagy is an evolutionarily conserved catabolic process that plays a central role in maintaining cellular homeostasis by degrading and recycling damaged or surplus proteins, organelles, and other cellular macromolecules and components. A growing body of evidence highlights a bidirectional relationship between autophagy and nicotinamide adenine dinucleotide (NAD ⁺ ), a vital metabolic cofactor involved in numerous cellular processes, including energy metabolism, genomic maintenance, stress resistance, and cell survival. Autophagy supports NAD ⁺ homeostasis by recycling metabolic precursors, while NAD ⁺ -dependent enzymes such as sirtuins and PARPs regulate autophagy initiation and lysosomal function. Disruption of this autophagy–NAD ⁺ axis has emerged as a common feature in several neurodegenerative diseases, where impaired cellular clearance and metabolic dysfunction contribute to neuronal vulnerability. In this review, we summarize the advances of the molecular links between autophagy and NAD ⁺ metabolism, with a particular focus on their roles in mitochondrial quality control, bioenergetic regulation, and cellular resilience. We also discuss the therapeutic potential of targeting the autophagy–NAD ⁺ axis to promote neuroprotection in neurodegenerative disease.

Impairment of autophagy leads to an accumulation of misfolded proteins and damaged organelles and has been implicated in plethora of human diseases. Loss of autophagy in actively respiring cells has also been shown to trigger metabolic collapse mediated by the depletion of nicotinamide adenine dinucleotide (NAD) pools, resulting in cell death. Here we found that the deficit in the autophagy-NAD axis underpins the loss of viability in cell models of a neurodegenerative lysosomal storage disorder, Niemann-Pick type C1 (NPC1) disease. Defective autophagic flux in NPC1 cells resulted in mitochondrial dysfunction due to impairment of mitophagy, leading to the depletion of both the reduced and oxidised forms of NAD as identified via metabolic profiling. Consequently, exhaustion of the NAD pools triggered mitochondrial depolarisation and apoptotic cell death. Our chemical screening identified two FDA-approved drugs, celecoxib and memantine, as autophagy activators which effectively restored autophagic flux, NAD levels, and cell viability of NPC1 cells. Of biomedical relevance, either pharmacological rescue of the autophagy deficiency or NAD precursor supplementation restored NAD levels and improved the viability of NPC1 patient fibroblasts and induced pluripotent stem cell (iPSC)-derived cortical neurons. Together, our findings identify the autophagy-NAD axis as a mechanism of cell death and a target for therapeutic interventions in NPC1 disease, with a potential relevance to other neurodegenerative disorders.

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