Activation of Glutamate Transporter-1 (GLT-1) Confers Sex-Dependent Neuroprotection in Brain Ischemia

Stroke is one of the leading causes of long-term disability. During ischemic stroke, glutamate is released, reuptake processes are impaired, and glutamate promotes excitotoxic neuronal death. Astrocytic glutamate transporter 1 (GLT-1) is the major transporter responsible for removing excess glutamate from the extracellular space. A translational activator of GLT-1, LDN/OSU 0212320 (LDN) has been previously developed with beneficial outcomes in epileptic animal models but has never been tested as a potential therapeutic for ischemic strokes. The present study evaluated the effects of LDN on stroke-associated brain injury. Male and female mice received LDN or vehicle 24 h before or 2 h after focal ischemia was induced in the sensorimotor cortex. Sensorimotor performance was determined using the Rung Ladder Walk and infarct area was assessed using triphenyltetrazolium chloride staining. Males treated with LDN exhibited upregulated GLT-1 protein levels, significantly smaller infarct size, and displayed better sensorimotor performance in comparison to those treated with vehicle only. In contrast, there was no upregulation of GLT-1 protein levels and no difference in infarct size or sensorimotor performance between vehicle- and LDN-treated females. Taken together, our results indicate that the GLT-1 translational activator LDN improved stroke outcomes in young adult male, but not female mice.

The role of cytosolic polyadenylation element binding protein 2 alternative splicing in hypoxia

HIF1 (Hypoxia-inducible Factor 1) is a transcription factor that plays a crucial role in the hypoxia stress response. Its primary function is to return the cell to its homeostatic state following oxygen deprivation. However, chronic hypoxia exposure can cause irreversible physiological changes that can lead to pulmonary hypertension (PH) and the need for therapeutics to ameliorate these conditions is great and unmet. Previous studies in our lab have demonstrated that CPEB2 (cytoplasmic polyadenylation element binding protein 2) is a translational repressor of one of the HIF1 subunits: HIF1α. Our lab demonstrated that the alternatively spliced CPEB2A isoform of CPEB2 is a repressor of translation, while the CPEB2B isoform is a translational activator of HIF1α during hypoxia, suggesting a major regulatory role for CPEB2 AS in the pulmonary hypoxic response. Although it is well established that during hypoxia, HIF1α levels are dramatically upregulated due to a decrease in the degradation of this factor, we propose that during chronic hypoxia, the expression of HIF1α is maintained via a translational mechanism, likely alongside a decrease in proteolytic degradation. In this study we demonstrate that depletion of the CPEB2B splice isoform has an inhibitory effect on the translation of nascent HIF1α protein during chronic hypoxia, but not the acute phase. We further demonstrate that this pathway is dependent on the initiation factor eIF3H. Finally, we show data which indicate that CPEB2A and CPEB2B bind differentially to cytoplasmic polyadenylation element consensus sequences depending on surrounding sequence context. These findings are important, since they provide evidence for potential of CPEB2 to act as a therapeutic target for treating chronic hypoxia-related pulmonary diseases.

Fmr1 translationally activates stress-sensitive mRNAs encoding large proteins in oocytes and neurons

Mutations in Fmr1 are the leading heritable cause of intellectual disability and autism spectrum disorder. We previously found that Fmr1 acts as a ∼2-fold activator of translation of large proteins in Drosophila oocytes, in contrast to its proposed role as a repressor of translation elongation. Here, we show that genes associated with autism spectrum disorders tend to be dosage-sensitive and encode proteins that are larger than average. Reanalysis of Fmr1 KO mouse cortex ribosome profiling data demonstrates that autism-associated mRNAs encoding large proteins exhibit a concordant reduction in ribosome footprints, consistent with a general role for Fmr1 as a translational activator. We find no evidence that differential ribosomal pausing affects translational output in Fmr1-deficient Drosophila oocytes or mouse cortex. Furthermore, long Fmr1 target transcripts are preferentially enriched in stress granules upon acute stress. Our data thus identify a critical role for Fmr1 in promoting the translation of long, stress-sensitive, autism-associated mRNAs.

Modular assembly of yeast mitochondrial ATP synthase and cytochrome oxidase

The respiratory pathway of mitochondria is composed of four electron transfer complexes and the ATP synthase. In this article, we review evidence from studies of Saccharomyces cerevisiae that both ATP synthase and cytochrome oxidase (COX) are assembled from independent modules that correspond to structurally and functionally identifiable components of each complex. Biogenesis of the respiratory chain requires a coordinate and balanced expression of gene products that become partner subunits of the same complex, but are encoded in the two physically separated genomes. Current evidence indicates that synthesis of two key mitochondrial encoded subunits of ATP synthase is regulated by the F1 module. Expression of COX1 that codes for a subunit of the COX catalytic core is also regulated by a mechanism that restricts synthesis of this subunit to the availability of a nuclear-encoded translational activator. The respiratory chain must maintain a fixed stoichiometry of the component enzyme complexes during cell growth. We propose that high-molecular-weight complexes composed of Cox6, a subunit of COX, and of the Atp9 subunit of ATP synthase play a key role in establishing the ratio of the two complexes during their assembly.

The translational activator Sov1 coordinates mitochondrial gene expression with mitoribosome biogenesis

Mitoribosome biogenesis is an expensive metabolic process that is essential to maintain cellular respiratory capacity and requires the stoichiometric accumulation of rRNAs and proteins encoded in two distinct genomes. In yeast, the ribosomal protein Var1, alias uS3m, is mitochondrion-encoded. uS3m is a protein universally present in all ribosomes, where it forms part of the small subunit (SSU) mRNA entry channel and plays a pivotal role in ribosome loading onto the mRNA. However, despite its critical functional role, very little is known concerning VAR1 gene expression. Here, we demonstrate that the protein Sov1 is an in bona fide VAR1 mRNA translational activator and additionally interacts with newly synthesized Var1 polypeptide. Moreover, we show that Sov1 assists the late steps of mtSSU biogenesis involving the incorporation of Var1, an event necessary for uS14 and mS46 assembly. Notably, we have uncovered a translational regulatory mechanism by which Sov1 fine-tunes Var1 synthesis with its assembly into the mitoribosome.

USP29 is a novel non-canonical Hypoxia Inducible Factor-α activator

Hypoxia Inducible Factor (HIF) is the master transcriptional regulator that orchestrates cellular adaptation to low oxygen. HIF is tightly regulated via the stability of its α-subunit, which is subjected to oxygen-dependent proline hydroxylation by Prolyl-Hydroxylase Domain containing proteins (PHDs/EGLNs), and ultimately targeted for proteasomal degradation through poly-ubiquitination by von-Hippel-Lindau protein (pVHL). However, sustained HIF-α signalling is found in many tumours independently of oxygen availability pointing towards the relevance of non-canonical HIF-α regulators. In this study, we establish the Ubiquitin Specific Protease 29 (USP29) as direct post-translational activator of HIF-α in a variety of cancer cell lines. USP29 binds to HIF-α, decreases poly-ubiquitination and thus protects HIF-α from proteasomal degradation. Deubiquitinating activity of USP29 is essential to stabilise not only HIF-1α but also HIF-2α, via their C-termini in an oxygen/PHD/pVHL-independent manner. Furthermore, in prostate cancer samples the expression of USP29 correlates with the HIF-target gene CA9 (carbonic anhydrase 9) as well as disease progression and severity.

Makorin 1 controls embryonic patterning by alleviating Bruno1-mediated repression ofoskartranslation

Makorins are evolutionary conserved proteins that contain C3H-type zinc finger modules and a RING E3 ubiquitin ligase domain. InDrosophilamaternal Makorin 1 (Mkrn1) has been linked to embryonic patterning but the mechanism remained unsolved. Here, we show that Mkrn1 is essential for axis specification and pole plasm assembly by translational activation ofoskar. We demonstrate that Mkrn1 interacts with poly(A) binding protein (pAbp) and bindsosk3’ UTR in a region adjacent to A-rich sequences. This binding site overlaps with Bruno1 (Bru1) responsive elements (BREs), which regulateosktranslation. We observe increased association of the translational repressor Bru1 withoskmRNA upon depletion of Mkrn1, indicating that both proteins compete foroskbinding. Consistently, reducing Bru1 dosage partially rescues viability and Osk protein level in ovaries fromMkrn1females. We conclude that Mkrn1 controls embryonic patterning and germ cell formation by specifically activatingosktranslation by displacing Bru1 from its 3’ UTR.Author SummaryTo ensure accurate development of theDrosophilaembryo, proteins and mRNAs are positioned at specific sites within the embryo. Many of these proteins and mRNAs are produced and localized during the development of the egg in the mother. One protein essential for this process that has been heavily studied is Oskar (Osk), which is positioned at the posterior pole. During the localization ofoskmRNA, its translation is repressed by the RNA-binding protein Bruno1 (Bru1), ensuring that Osk protein is not present outside of the posterior where it is harmful. At the posterior pole,oskmRNA is activated through mechanisms that are not yet understood. In this work, we show that the conserved protein Makorin 1 (Mkrn1) is a novel factor involved in the translational activation ofosk. Mkrn1 binds specifically tooskmRNA in a region that overlaps with the binding site of Bru1, thus alleviating the association of Bru1 withosk. Moreover, Mkrn1 is stabilized by poly(A) binding protein, a translational activator that bindsoskmRNA in close proximity to Mkrn1. Our work thus helps to answer a long-standing question in the field, providing insight about the function of Mkrn1 and more generally into embryonic patterning in animals.