Biology subjects

Sreedharan, S.

Publications and source records attributed to Sreedharan, S..

7 recordsLinked to original sources

bioRxiv2026

A conserved Pho4-Pho84 axis ubiquitously maintains intracellular phosphate homeostasis in yeast

Inorganic phosphate (Pi) is central to fundamental cellular processes and the metabolic economy, and is constantly acquired in order to maintain intracellular Pi levels. While much is known about cellular adaptation during Pi starvation, how intracellular Pi is maintained in Pi-replete conditions remains unclear. Here, using Saccharomyces cerevisiae, we uncover an essential role for the Pho4 transcription factor in maintaining intracellular Pi under Pi-replete conditions, via the high affinity Pho84 transporter. Basal Pho4-dependent output is required for intracellular Pi maintenance, and the loss of Pho4 results in decreased intracellular Pi. We uncover that the Pho4 dependent, high affinity Pi transporter Pho84 is the primary transporter required for this intracellular Pi maintenance in phosphate replete conditions, and is not compensated by other transporters. The loss of Pho4 or Pho84 decreases intracellular Pi, with reduced ATP and glycolysis, and decreased growth. Through comparative genomic and phylogenetic analyses we establish that Pho84 is universally conserved across fungi, and Pho84 alone is orthologous to the plant high-affinity phosphate transporter PHT1. Thus, Pho84 is a primary determinant of intracellular Pi homeostasis during phosphate replete growth, and Pi acquisition in replete conditions is built around high-affinity phosphate transport. These findings reiterate the importance of Pi acquisition via high-affinity transport for metabolic homeostasis, with implications for microbial fermentation-based applications.

systems biology↗

bioRxiv2022

Aedes aegypti eggs use rewired polyamine and lipid metabolism to survive extreme desiccation

Upon extreme water loss, some organisms pause their life cycles and escape death, in a process called anhydrobiosis. While widespread in microbes, this is uncommon in animals. Mosquitoes of the Aedes genus are vectors for several viral diseases in humans. These mosquitoes lay eggs that survive extreme desiccation and this property greatly enhances geographical expansion of these insects. The molecular principles of egg survival and hatching post-desiccation in these insects remain obscure. In this report, we find that eggs of Aedes aegypti, in contrast to those of Anopheles stephensi, are true anhydrobiotes. Aedes embryos acquire desiccation tolerance at a late developmental stage. We uncover unique proteome-level changes in Aedes embryos during desiccation. These changes reflect a metabolic state with reduced central carbon metabolism, and precise rewiring towards polyamine production, altered lipid levels and enhanced lipid utilization for energy. Using inhibitor-based approaches targeting these processes in blood-fed mosquitoes that lay eggs, we infer a two-step process of anhydrobiosis in Aedes eggs, where polyamine accumulation as well as lipid breakdown confer desiccation tolerance, and rapid lipid breakdown fuels energetic requirements enabling the revival of mosquito larvae post rehydration.

biochemistry↗

bioRxiv2020

LOCAL REGULATION AND FUNCTION OF IMPORTIN-β1 DURING TRANSCRIPTION-DEPENDENT PLASTICITY

Activity-dependent transcription is critical for the encoding of long-term memories. Regulated nuclear entry of soluble proteins is one method to relay synaptic signals to the nucleus to couple neuronal excitation with transcription. To date, the role of importin-{beta}1 in nuclear shuttling of proteins during activity-dependent transcription has always been inferred but not directly investigated. In this study, we demonstrate activity-dependent nuclear accumulation of importin-{beta}1 from the soma and the synapto-dendritic compartments. Importantly, inhibition of importin-{beta}1 mediated nuclear import during synaptic stimulation impairs long-term plasticity. We show evidence that importin-{beta}1 mRNA-ribosome complex is distributed throughout the synapto-dendritic compartment and synaptic stimulation induces importin-{beta}1 local protein synthesis. Finally, we identified candidate proteins that associate with importin-{beta}1 at the synapse and characterize NDRG1 as an importin-{beta}1 interactor that undergoes activity-dependent translocation into the nucleus. Collectively, our results highlight the crucial role of importin-{beta}1 in the nuclear import of soluble proteins during long-term plasticity.

neuroscience↗

bioRxiv2020

Metaplastic reinforcement of long-term potentiation in rat hippocampal area CA2 by cholinergic receptor activation

Hippocampal CA2, an inconspicuously positioned area between the well-studied CA1 and CA3 subfields, has captured research interest in recent years due to its role in the formation of social memory. The effects of synaptic depression for subsequent long-term potentiation (LTP) of synaptic transmission at entorhinal cortical (EC)-CA2 and Schaffer collateral (SC)-CA2 synapses have not been previously explored. Here we show that cholinergic receptor activation with the non-selective cholinergic agonist carbachol (CCh) triggers a long-term synaptic depression (CCh-LTD) of field excitatory postsynaptic potentials at EC- and SC-CA2 synapses in the hippocampus of adult rats. The activation of muscarinic acetylcholine receptors (mAChRs) is critical for the induction of an early phase (<100 min) of CCh-LTD, with a strong dependency upon M3 mAChR activation and a weaker one by M1 mAChRs. Interestingly, muscarinic M2 and nicotinic receptor activation are crucially involved in the late phase (>100 min) of CCh-LTD. Importantly, CCh priming lowers the threshold, in a protein synthesis-dependent manner, for the late maintenance of LTP that can be subsequently induced by high-frequency electrical stimulation at EC- or SC-CA2 pathways. The results demonstrate that CA2 synaptic learning rules are modified in a metaplastic manner, wherein synaptic modifications triggered by cholinergic stimulation can dictate the outcome of future plasticity events. Moreover, the observed enabling of late LTP at EC inputs to CA2 following the priming stimulus co-exists with concurrent sustained CCh-LTD at SC-CA2 and is dynamically scaled by modulation of SC-CA2 synaptic transmission. Significance StatementThe release of the neuromodulator acetylcholine is critically involved in processes of hippocampus-dependent memory formation. Cholinergic afferents originating in the medial septum and diagonal bands of Broca terminating in the hippocampal area CA2 might play an important role in the modulation of area-specific synaptic plasticity. Our findings demonstrate that cholinergic receptor activation induces a long-term depression of synaptic transmission at entorhinal cortical- and Schaffer collateral-CA2 synapses. This cholinergic activation-mediated long-term depression displays a bidirectional metaplastic switch to long-term potentiation on a future timescale. This suggests that such bidirectional synaptic modifications triggered by the dynamic modulation of tonic cholinergic receptor activation may support the formation of CA2-dependent memories given the increased hippocampal cholinergic tone during active wakefulness observed in exploratory behaviour.

neuroscience↗

bioRxiv2020

mTOR S-nitrosylation inhibits autophagy and lysosomal proteolysis

Mammalian Target of Rapamycin (mTOR) is a master regulator of autophagy and lysosomes, and its downstream kinase-dependent pathways have been extensively characterized. Here, we report an unexpected kinase-independent regulation of autophagy and lysosomes by S-nitrosylation at Cys423 position of mTOR that resulted in suppression of VPS34 and PIKfyve-dependent phosphoinositide synthesis. Physiologically, S-nitrosylation of mTOR reduced basal lysosomal proteolysis via nitric oxide synthase (NOS)-mediated synthesis of NO from lysosomal arginine precursor, a marker of cellular nutrition status. Significantly, we found increased lysosomal NOS-mTOR complexes in APP-PS1 Alzheimers disease (AD) murine model, and increased mTOR S-nitrosylation in AD patient-derived fibroblasts. Lastly, we demonstrated that pharmacological inhibition of NOS or overexpression of mTORCys423Ala mutant reversed lysosomal and autophagic dysfunction in AD patient-derived fibroblasts, suggesting novel therapeutic strategies for autophagosome-lysosomal activation.

cell biology↗

bioRxiv2020

Methylated PP2A stabilizes Gcn4 to enable a methionine-induced anabolic program

Methionine, through S-adenosylmethionine, activates multifaceted growth programs where ribosome biogenesis, carbon metabolism, amino acid and nucleotide biosynthesis are induced. This growth program requires activity of the Gcn4 transcription factor (called ATF4 in mammals), which enables metabolic precursor supply essential for anabolism. Here, we discover how the Gcn4 protein is induced by methionine, despite conditions of high translation and anabolism. This induction mechanism is independent of transcription, as well as the conventional Gcn2/eIF2 mediated increased translation of Gcn4. Instead, when methionine is abundant, Gcn4 ubiqitination and therefore degradation is reduced, due to the decreased phosphorylation of this protein. This Gcn4 stabilization is mediated by the activity of the conserved methyltransferase, Ppm1, which specifically methylates the catalytic subunit of protein phosphatase PP2A when methionine is abundant. This methylation of PP2A shifts the balance of Gcn4 to a dephosphorylated state, which stabilizes the protein. The loss of Ppm1, or PP2A-methylation destabilizes Gcn4 when methionine is abundant, and the Gcn4-dependent anabolic program collapses. These findings reveal a novel signaling and regulatory axis, where methionine directs a conserved methyltransferase Ppm1, via its target phosphatase PP2A, to selectively stabilize Gcn4. Thereby, when methionine is abundant, cells conditionally modify a major phosphatase in order to stabilize a metabolic master-regulator and drive anabolism.

biochemistry↗

bioRxiv2020

Inactive variants of death receptor p75NTR reduce Alzheimer's neuropathology by interfering with APP internalization

A prevalent model of Alzheimers disease (AD) pathogenesis postulates the generation of neurotoxic fragments derived from the amyloid precursor protein (APP) after its internalization to endocytic compartments. However, the molecular pathways that regulate APP internalization and intracellular trafficking in neurons are unknown. Here we report that 5xFAD mice, an animal model of AD, expressing signaling-deficient variants of the p75 neurotrophin receptor (p75NTR) show greater neuroprotection from AD neuropathology than animals lacking this receptor. p75NTR knock-in mice lacking the death domain or transmembrane Cys259 showed lower levels of A{beta} species, amyloid plaque burden, gliosis, mitochondrial stress and neurite dystrophy than global knock-outs. Strikingly, long-term synaptic plasticity and memory, which are completely disrupted in 5xFAD mice, were fully recovered in the knock-in mice. Mechanistically, we found that p75NTR interacts with APP and regulates its internalization in hippocampal neurons. Inactive p75NTR variants internalized much slower and to lower levels than wild type p75NTR, favoring non-amyloidogenic APP cleavage by reducing APP internalization and colocalization with BACE1, the critical protease for generation of neurotoxic APP fragments. These results reveal a novel pathway that directly and specifically regulates APP internalization, amyloidogenic processing and disease progression, and suggest that inhibitors targeting the p75NTR transmembrane domain may be an effective therapeutic strategy in AD.

neuroscience↗