Glial-derived mitochondrial signals affect neuronal proteostasis and aging.

The nervous system plays a critical role in maintaining whole-organism homeostasis; neurons experiencing mitochondrial stress can coordinate the induction of protective cellular pathways, such as the mitochondrial unfolded protein response (UPRMT), between tissues. However, these studies largely ignored nonneuronal cells of the nervous system. Here, we found that UPRMT activation in four astrocyte-like glial cells in the nematode, Caenorhabditis elegans, can promote protein homeostasis by alleviating protein aggregation in neurons. Unexpectedly, we find that glial cells use small clear vesicles (SCVs) to signal to neurons, which then relay the signal to the periphery using dense-core vesicles (DCVs). This work underlines the importance of glia in establishing and regulating protein homeostasis within the nervous system, which can then affect neuron-mediated effects in organismal homeostasis and longevity.

Glial-derived mitochondrial signals impact neuronal proteostasis and aging.

The nervous system plays a critical role in maintaining whole-organism homeostasis; neurons experiencing mitochondrial stress can coordinate the induction of protective cellular pathways, such as the mitochondrial unfolded protein response (UPRMT), between tissues. However, these studies largely ignored non-neuronal cells of the nervous system. Here, we found that UPRMT activation in four, astrocyte-like glial cells in the nematode, C. elegans, can promote protein homeostasis by alleviating protein aggregation in neurons. Surprisingly, we find that glial cells utilize small clear vesicles (SCVs) to signal to neurons, which then relay the signal to the periphery using dense-core vesicles (DCVs). This work underlines the importance of glia in establishing and regulating protein homeostasis within the nervous system, which can then impact neuron-mediated effects in organismal homeostasis and longevity.

Glia of C. elegans coordinate a protective organismal heat shock response independent of the neuronal thermosensory circuit.

Aging organisms lose the ability to induce stress responses, becoming vulnerable to protein toxicity and tissue damage. Neurons can signal to peripheral tissues to induce protective organelle-specific stress responses. Recent work shows that glia can independently induce such responses. Here, we show that overexpression of heat shock factor 1 (hsf-1) in the four astrocyte-like cephalic sheath cells of Caenorhabditis elegans induces a non-cell-autonomous cytosolic unfolded protein response, also known as the heat shock response (HSR). These animals have increased lifespan and heat stress resistance and decreased protein aggregation. Glial HSR regulation is independent of canonical thermosensory circuitry and known neurotransmitters but requires the small clear vesicle release protein UNC-13. HSF-1 and the FOXO transcription factor DAF-16 are partially required in peripheral tissues for non-cell-autonomous HSR, longevity, and thermotolerance. Cephalic sheath glial hsf-1 overexpression also leads to pathogen resistance, suggesting a role for this signaling pathway in immune function.

Divergent Nodes of Non-autonomous UPRER Signaling through Serotonergic and Dopaminergic Neurons.

In multicellular organisms, neurons integrate a diverse array of external cues to affect downstream changes in organismal health. Specifically, activation of the endoplasmic reticulum (ER) unfolded protein response (UPRER) in neurons increases lifespan by preventing age-onset loss of ER proteostasis and driving lipid depletion in a cell non-autonomous manner. The mechanism of this communication is dependent on the release of small clear vesicles from neurons. We find dopaminergic neurons are necessary and sufficient for activation of cell non-autonomous UPRER to drive lipid depletion in peripheral tissues, whereas serotonergic neurons are sufficient to drive protein homeostasis in peripheral tissues. These signaling modalities are unique and independent and together coordinate the beneficial effects of neuronal cell non-autonomous ER stress signaling upon health and longevity.

Lysosomal recycling of amino acids affects ER quality control.

Recent work has highlighted the fact that lysosomes are a critical signaling hub of metabolic processes, providing fundamental building blocks crucial for anabolic functions. How lysosomal functions affect other cellular compartments is not fully understood. Here, we find that lysosomal recycling of the amino acids lysine and arginine is essential for proper ER quality control through the UPRER. Specifically, loss of the lysine and arginine amino acid transporter LAAT-1 results in increased sensitivity to proteotoxic stress in the ER and decreased animal physiology. We find that these LAAT-1-dependent effects are linked to glycine metabolism and transport and that the loss of function of the glycine transporter SKAT-1 also increases sensitivity to ER stress. Direct lysine and arginine supplementation, or glycine supplementation alone, can ameliorate increased ER stress sensitivity found in laat-1 mutants. These data implicate a crucial role in recycling lysine, arginine, and glycine in communication between the lysosome and ER.

Four glial cells regulate ER stress resistance and longevity via neuropeptide signaling in C. elegans.

The ability of the nervous system to sense cellular stress and coordinate protein homeostasis is essential for organismal health. Unfortunately, stress responses that mitigate disturbances in proteostasis, such as the unfolded protein response of the endoplasmic reticulum (UPRER), become defunct with age. In this work, we expressed the constitutively active UPRER transcription factor, XBP-1s, in a subset of astrocyte-like glia, which extended the life span in Caenorhabditis elegans Glial XBP-1s initiated a robust cell nonautonomous activation of the UPRER in distal cells and rendered animals more resistant to protein aggregation and chronic ER stress. Mutants deficient in neuropeptide processing and secretion suppressed glial cell nonautonomous induction of the UPRER and life-span extension. Thus, astrocyte-like glial cells play a role in regulating organismal ER stress resistance and longevity.

UPRER promotes lipophagy independent of chaperones to extend life span.

Longevity is dictated by a combination of environmental and genetic factors. One of the key mechanisms to regulate life-span extension is the induction of protein chaperones for protein homeostasis. Ectopic activation of the unfolded protein response of the endoplasmic reticulum (UPRER) specifically in neurons is sufficient to enhance organismal stress resistance and extend life span. Here, we find that this activation not only promotes chaperones but also facilitates ER restructuring and ER function. This restructuring is concomitant with lipid depletion through lipophagy. Activation of lipophagy is distinct from chaperone induction and is required for the life-span extension found in this paradigm. Last, we find that overexpression of the lipophagy component, ehbp-1, is sufficient to deplete lipids, remodel ER, and promote life span. Therefore, UPR induction in neurons triggers two distinct programs in the periphery: the proteostasis arm through protein chaperones and metabolic changes through lipid depletion mediated by EH domain binding protein 1 (EHBP-1).

Spatial regulation of the actin cytoskeleton by HSF-1 during aging.

There are many studies suggesting an age-associated decline in the actin cytoskeleton, and this has been adopted as common knowledge in the field of aging biology. However, a direct identification of this phenomenon in aging multicellular organisms has not been performed. Here, we express LifeAct::mRuby in a tissue-specific manner to interrogate cytoskeletal organization as a function of age. We show for the first time in Caenorhabditis elegans that the organization and morphology of the actin cytoskeleton deteriorate at advanced age in the muscles, intestine, and hypodermis. Moreover, hsf-1 is essential for regulating cytoskeletal integrity during aging, so that knockdown of hsf-1 results in premature aging of actin and its overexpression protects actin cytoskeletal integrity in the muscles, the intestine, and the hypodermis. Finally, hsf-1 overexpression in neurons alone is sufficient to protect cytoskeletal integrity in nonneuronal cells.

Two Conserved Histone Demethylases Regulate Mitochondrial Stress-Induced Longevity.

Across eukaryotic species, mild mitochondrial stress can have beneficial effects on the lifespan of organisms. Mitochondrial dysfunction activates an unfolded protein response (UPR(mt)), a stress signaling mechanism designed to ensure mitochondrial homeostasis. Perturbation of mitochondria during larval development in C. elegans not only delays aging but also maintains UPR(mt) signaling, suggesting an epigenetic mechanism that modulates both longevity and mitochondrial proteostasis throughout life. We identify the conserved histone lysine demethylases jmjd-1.2/PHF8 and jmjd-3.1/JMJD3 as positive regulators of lifespan in response to mitochondrial dysfunction across species. Reduction of function of the demethylases potently suppresses longevity and UPR(mt) induction, while gain of function is sufficient to extend lifespan in a UPR(mt)-dependent manner. A systems genetics approach in the BXD mouse reference population further indicates conserved roles of the mammalian orthologs in longevity and UPR(mt) signaling. These findings illustrate an evolutionary conserved epigenetic mechanism that determines the rate of aging downstream of mitochondrial perturbations.