Hanlin Zhang

Mitochondria serve as critical signaling hubs that monitor the intracellular environment to orchestrate cellular stress responses during pathogen infection. The mitochondrial unfolded protein response (UPR ^MT ), a key surveillance pathway, is widely considered to be activated by pathogen-derived toxins that directly damage host mitochondria. In contrast to this prevailing model, we demonstrate that E. faecalis, which lacks specialized mitochondrial toxins, activates the UPR ^MT in C. elegans via a combination of metabolic and oxidative stress. As both C. elegans and E. faecalis are heme auxotrophs, E. faecalis -ingested animals result in severe heme deficiency, leading to the disruption of the electron transport chain and activation of the UPR ^MT . During live infection, this primary mitochondrial stress is further amplified due to the Fenton reaction, driven by host NADPH oxidase BLI-3-generated hydrogen peroxide. Ultimately, our findings position the UPR ^MT as a fundamental homeostatic sensor that monitors metabolic and oxidative imbalances at the host-pathogen interface.

Glycans regulate cellular function, yet how aging affects the glycocalyx remains unclear. Here, we investigate changes in immune cell glycocalyx with age and find that α2,6-linked sialic acid, a glycan epitope associated with inhibitory signaling, is down-regulated in T cells from old animals. This reduction is tightly correlated with age-associated accumulation of effector T cells, which have little to no α2,6-linked sialic acid. To understand how α2,6-linked sialic acid affects T cell physiology, we generated a mouse model with T cell–specific deletion of sialyltransferase gene St6gal1 . The lack of α2,6-linked sialic acid leads to reduced responsiveness in naïve T cells, leading to impaired T cell responses against Listeria monocytogenes infection and tumor growth. PD-1 pathway blockade partially restores St6gal1 -deficient T cells’ ability to control tumor growth. These findings suggest that α2,6-linked sialic acid is critical for maintaining long-term T cell responsiveness, and its loss may contribute to decreased T cell function with age.

Cell surface glycans, termed the glycocalyx, are essential regulators of cellular signaling and thus cellular development and functions, but how aging impacts the glycocalyx remains poorly understood. Here, using immune cells as a model system for studying the relationship between aging and glycocalyx remodeling, we show that α2,6-linked sialic acid – a terminal glycan epitope typically associated with inhibitory signaling – becomes downregulated in T cells from older animals. This downregulation is tightly correlated with age-associated accumulation of effector T cells, which are decorated with little to no α2,6-linked sialic acids. T cell aging renders older individuals more vulnerable to infections and cancers. To understand the role of α2,6-linked sialic acids in T cell physiology, we generated a mouse model with T cell-specific deletion of the sialyltransferase gene St6gal1 . The chronic depletion of α2,6-linked sialic acids led to naïve T (T _N ) cells expansion in the periphery and premature T cell exhaustion. As a result, these mice were less able to control acute Listeria infection and chronic tumor growth. Blockade of the PD-1 pathway can partially restore the ability of St6gal1 -deficient T cells to control tumor growth. Together, these data suggest that α2,6-linked sialic acids are critical for maintaining long-term T cell responsiveness, and the loss of α2,6-linked sialic acids may directly contribute to age-related T cell exhaustion.

The central nervous system coordinates peripheral cellular stress responses, including the unfolded protein response of the mitochondria (UPR ^MT ); however, the contexts for which this regulatory capability evolved are unknown. UPR ^MT is up-regulated upon pathogenic infection and in metabolic flux, and the olfactory nervous system has been shown to regulate pathogen resistance and peripheral metabolic activity. Therefore, we asked whether the olfactory nervous system in Caenorhabditis elegans controls the UPR ^MT cell nonautonomously. We found that silencing a single inhibitory olfactory neuron pair, AWC, led to robust induction of UPR ^MT and reduction of oxidative phosphorylation dependent on serotonin signaling and parkin -mediated mitophagy. Further, AWC ablation confers resistance to the pathogenic bacteria Pseudomonas aeruginosa partially dependent on the UPR ^MT transcription factor atfs-1 and fully dependent on mitophagy machinery. These data illustrate a role for the olfactory nervous system in regulating whole-organism mitochondrial dynamics, perhaps in preparation for postprandial metabolic stress or pathogenic infection.

Changes in lipid metabolism are associated with aging and age-related diseases, including proteopathies. The endoplasmic reticulum (ER) is uniquely a major hub for protein and lipid synthesis, making its function essential for both protein and lipid homeostasis. However, it is less clear how lipid metabolism and protein quality may impact each other. Here, we identified let-767, a putative hydroxysteroid dehydrogenase in Caenorhabditis elegans, as an essential gene for both lipid and ER protein homeostasis. Knockdown of let-767 reduces lipid stores, alters ER morphology in a lipid-dependent manner, and blocks induction of the Unfolded Protein Response of the ER (UPR^ER). Interestingly, a global reduction in lipogenic pathways restores UPR^ER induction in animals with reduced let-767. Specifically, we find that supplementation of 3-oxoacyl, the predicted metabolite directly upstream of let-767, is sufficient to block induction of the UPR^ER. This study highlights a novel interaction through which changes in lipid metabolism can alter a cell’s response to protein-induced stress.

Mitochondria function as a hub of the cellular metabolic network. Mitochondrial stress is closely associated with aging and a variety of diseases, including neurodegeneration and cancer. Cells autonomously elicit specific stress responses to cope with mitochondrial stress to maintain mitochondrial homeostasis. Interestingly, mitochondrial stress responses may also be induced in a non-autonomous manner in cells or tissues that are not directly experiencing such stress. Such non-autonomous mitochondrial stress responses are mediated by secreted molecules called mitokines. Due to their significant translational potential in improving human metabolic health, there has been a surge in mitokine-focused research. In this review, we summarize the findings regarding inter-tissue communication of mitochondrial stress in animal models. In addition, we discuss the possibility of mitokine-mediated intercellular mitochondrial communication originating from bacterial quorum sensing.

Patients with multiple myeloma, an incurable malignancy of plasma cells, frequently develop osteolytic bone lesions that severely impact quality of life and clinical outcomes. Eliglustat, a U.S. Food and Drug Administration-approved glucosylceramide synthase inhibitor, reduced osteoclast-driven bone loss in preclinical in vivo models of myeloma. In combination with zoledronic acid, a bisphosphonate that treats myeloma bone disease, eliglustat provided further protection from bone loss. Autophagic degradation of TRAF3, a key step for osteoclast differentiation, was inhibited by eliglustat as evidenced by TRAF3 lysosomal and cytoplasmic accumulation. Eliglustat blocked autophagy by altering glycosphingolipid composition whilst restoration of missing glycosphingolipids rescued autophagy markers and TRAF3 degradation thus restoring osteoclastogenesis in bone marrow cells from myeloma patients. This work delineates both the mechanism by which glucosylceramide synthase inhibition prevents autophagic degradation of TRAF3 to reduce osteoclastogenesis as well as highlighting the clinical translational potential of eliglustat for the treatment of myeloma bone disease.

Changes in lipid metabolism are associated with aging and age-related diseases, including proteopathies. The endoplasmic reticulum (ER) is uniquely a major hub for protein and lipid synthesis, making its function essential for both protein and lipid homeostasis. However, it is less clear how lipid metabolism and protein quality may impact each other. Here, we identity let-767 , a putative hydroxysteroid dehydrogenase, as an essential gene for both lipid and ER protein homeostasis. Knockdown of let-767 reduces lipid stores, alters ER morphology in a lipid-dependent manner, and also blocks induction of the Unfolded Protein Response of the ER (UPR ^ER ). Interestingly, a global reduction in lipogenic pathways restores UPR ^ER induction in animals with reduced let-767 . Specifically, we find that supplementation of 3-oxoacyl, the predicted metabolite directly upstream of let-767 , is sufficient to block induction of the UPR ^ER . This study highlights a novel interaction through which changes in lipid metabolism can alter a cell’s response to protein-induced stress.

SUMMARY: The deleterious potential to generate oxidative stress and damage is a fundamental challenge to metabolism. The oxidative stress response transcription factor, SKN-1/NRF2, can sense and respond to changes in metabolic state, although the mechanism and physiological consequences of this remain unknown. To explore this connection, we performed a genetic screen in C. elegans targeting amino acid catabolism and identified multiple metabolic pathways as regulators of SKN-1 activity. We found that genetic perturbation of the conserved amidohydrolase T12A2.1/amdh-1 activates a unique subset of SKN-1 regulated detoxification genes. Interestingly, this transcriptional program is independent of canonical P38-MAPK signaling components but requires the GATA transcription factor ELT-3, nuclear hormone receptor NHR-49, and mediator complex subunit MDT-15. This activation of SKN-1 is dependent on upstream histidine catabolism genes HALY-1 and Y51H4A.7/UROC-1 and may occur through accumulation of a catabolite, 4-imidazolone-5-propanoate (IP). Triggering SKN-1 activation results in a physiological trade off of increased oxidative stress resistance but decreased survival to heat stress. Together, our data suggest that SKN-1 is a key surveillance factor which senses and responds to metabolic perturbations to influence physiology and stress resistance.

Cross-species genetic screens for mitochondrial regulators revealed EPS-8 as a link between actin stability and mitochondria.