miR-8 microRNAs regulate the response to osmotic stress in zebrafish embryos

MicroRNAs (miRNAs) are highly conserved small RNAs that act as translational regulators of gene expression, exerting their influence by selectively targeting mRNAs bearing complementary sequence elements. These RNAs function in diverse aspects of animal development and physiology. Because of an ability to act as rapid responders at the level of translation, miRNAs may also influence stress response. In this study, we show that the miR-8 family of miRNAs regulates osmoregulation in zebrafish embryos. Ionocytes, which are a specialized cell type scattered throughout the epidermis, are responsible for pH and ion homeostasis during early development before gill formation. The highly conserved miR-8 family is expressed in ionocytes and enables precise control of ion transport by modulating the expression of Nherf1, which is a regulator of apical trafficking of transmembrane ion transporters. Ultimately, disruption of miR-8 family member function leads to an inability to respond to osmotic stress and blocks the ability to properly traffic and/or cluster transmembrane glycoproteins at the apical surface of ionocytes

Characterization of the Proteostasis Roles of Glycerol Accumulation, Protein Degradation and Protein Synthesis during Osmotic Stress in C. elegans

Exposure of C. elegans to hypertonic stress-induced water loss causes rapid and widespread cellular protein damage. Survival in hypertonic environments depends critically on the ability of worm cells to detect and degrade misfolded and aggregated proteins. Acclimation of C. elegans to mild hypertonic stress suppresses protein damage and increases survival under more extreme hypertonic conditions. Suppression of protein damage in acclimated worms could be due to 1) accumulation of the chemical chaperone glycerol, 2) upregulation of protein degradation activity, and/or 3) increases in molecular chaperoning capacity of the cell. Glycerol and other chemical chaperones are widely thought to protect proteins from hypertonicity-induced damage. However, protein damage is unaffected by gene mutations that inhibit glycerol accumulation or that cause dramatic constitutive elevation of glycerol levels. Pharmacological or RNAi inhibition of proteasome and lyosome function and measurements of cellular protein degradation activity demonstrated that upregulation of protein degradation mechanisms plays no role in acclimation. Thus, changes in molecular chaperone capacity must be responsible for suppressing protein damage in acclimated worms. Transcriptional changes in chaperone expression have not been detected in C. elegans exposed to hypertonic stress. However, acclimation to mild hypertonicity inhibits protein synthesis 50–70%, which is expected to increase chaperone availability for coping with damage to existing proteins. Consistent with this idea, we found that RNAi silencing of essential translational components or acute exposure to cycloheximide results in a 50–80% suppression of hypertonicity-induced aggregation of polyglutamine-YFP (Q35::YFP). Dietary changes that increase protein production also increase Q35::YFP aggregation 70–180%. Our results demonstrate directly for the first time that inhibition of protein translation protects extant proteins from damage brought about by an environmental stressor, demonstrate important differences in aging- versus stress-induced protein damage, and challenge the widely held view that chemical chaperones are accumulated during hypertonic stress to protect protein structure/function

Ribosome-Associated Vesicles promote activity-dependent local translation

Local protein synthesis in axons and dendrites underpins synaptic plasticity. However, the composition of the protein synthesis machinery in distal neuronal processes and the mechanisms for its activity-driven deployment to local translation sites remain unclear. Here, we employed cryo-electron tomography, volume electron microscopy, and live-cell imaging to identify Ribosome-Associated Vesicles (RAVs) as a dynamic platform for moving ribosomes to distal processes. Stimulation via chemically-induced long-term potentiation causes RAV accumulation in distal sites to drive local translation. We also demonstrate activity-driven changes in RAV generation and dynamics , identifying tubular ER shaping proteins in RAV biogenesis. Together, our work identifies a mechanism for ribosomal delivery to distal sites in neurons to promote activity-dependent local translation

Characterization of hypertonic stress-induced protein damage and the cellular mechanisms for defense and repair in C. elegans

Proteostasis is maintained by a complex network of genes and processes which includes core synthesis and degradation machineries as well as chemical and protein chaperones. Much of what is known about the function and organization of the proteostasis network stems from analyzing how cells respond to genetic or environmental perturbation of proteomic integrity. Recent evidence points to a critical role for the proteostasis network in survival of hypertonic environments, but the proteotoxic effects of hypertonic stress remain largely undescribed. Employing the many experimental advantages of the nematode C. elegans, we provide the first detailed description of the nature and extent of protein damage caused by hypertonic stress. Misfolding and aggregation of diverse reporters and endogenous proteins are rapid and widespread in vivo. Additionally, we demonstrate that acclimation of C. elegans to a mild hypertonic environment activates unknown proteostasis activities capable of preventing aggregation during extreme hypertonic stress. To define novel aspects of the hypertonic stress response and extend our understanding of cellular proteostasis strategies, we employ genetic and pharmacological approaches in determining the mechanism by which hypertonic acclimation enhances proteostasis. We hypothesize that chemical chaperones, protein chaperones, proteolysis machineries, and/or protein synthesis must be involved. Surprisingly, hypertonicity- or mutation-induced accumulation of glycerol, an organic osmolyte widely believed to act as a chemical chaperone in vivo, does not directly ameliorate protein damage during stress or aging. Protein chaperone expression is not transcriptionally upregulated. Further, hypertonic stress actually reduces protein degradation, an effect not reversed by acclimation. We demonstrate for the first time that suppression of protein translation during an environmental stress directly enhances proteostasis by preventing aggregation of extant proteins. Combined with recent observations that inhibition of translation extends lifespan and occurs naturally in response to other proteotoxic stressors, this finding suggests that translational reprogramming represents a conserved mechanism by which cells reduce the population of nascent, damage-prone proteins to enhance the availability and effectiveness of pre-existing chaperones

AMPK at the Nexus of Energetics and Aging

When energy supply is low, organisms respond by slowing aging and increasing resistance to diverse age-related pathologies. Targeting the mechanisms underpinning this response may therefore treat multiple disorders through a single intervention. Here, we discuss AMP-activated protein kinase (AMPK) as an integrator and mediator of several pathways and processes linking energetics to longevity. Activated by low energy, AMPK is both prolongevity and druggable, but its role in some pathologies may not be beneficial. As such, activating AMPK may modulate multiple longevity pathways to promote healthy aging, but unlocking its full potential may require selective targeting toward substrates involved in longevity assurance