Precursor mRNA (pre-mRNA) splicing is a critical step in gene expression that results in the removal of intronic sequences from immature mRNA, leading to the production of mature mRNA that can be translated into protein. Alternative pre-mRNA splicing is the process whereby alternative exons and/or introns are selectively included or excluded, generating mature mRNAs that encode proteins that may differ in function. The resulting alterations in the pattern of protein isoform expression can result in changes in protein-protein interaction, subcellular localization, and flux through metabolic pathways. Although basic mechanisms of pre-mRNA splicing of introns and exons are reasonably well characterized, how these mechanisms are regulated remains poorly understood. The goal of this review is to highlight selected recent advances in our understanding of the regulation of pre-mRNA splicing by nutrients and modulation of nutrient metabolism that result from changes in pre-mRNA splicing.

Edited by Jeffrey E. PessinDespite recent advances in therapeutics, diabetic retinopathy remains a leading cause of vision impairment. Improvement in the treatment of diabetic retinopathy requires a better understanding of the molecular mechanisms that cause neurovascular complications, particularly in type 2 diabetes. Recent studies demonstrate that rodents fed a high fat diet exhibit retinal dysfunction concomitant with attenuated Akt phosphorylation. The purpose of the present study was to evaluate the impact of a high fat/high sucrose diet on retinal insulin signaling and evaluate the mechanism(s) responsible for the changes. Mice fed a high fat/sucrose diet exhibited attenuated Akt phosphorylation in the retina as compared with mice fed normal chow. Retinas of mice fed a high fat/sucrose diet also exhibited elevated levels of activated JNK as well as enhanced p70S6K1 autoinhibitory domain phosphorylation. In cells, JNK activation enhanced p70S6K1 phosphorylation and mTORC1-dependent activation of the kinase, as evidenced by enhanced phosphorylation of key substrates. Rictor phosphorylation by p70S6K1 was specifically enhanced by the addition of phosphomimetic mutations in the autoinhibitory domain and was more sensitive to inhibition of the kinase as compared with rpS6. Notably, rictor and IRS-1 phosphorylation by p70S6K1 attenuate insulin action through a negative feedback pathway. Indeed, p70S6K1 inhibition prevented the repressive effect of JNK activation on insulin action in retinas. Overall, the results identify the JNK/S6K1 axis as a key molecular mechanism whereby a high fat/sucrose diet impairs insulin action in retina.Diabetic retinopathy is the leading cause of vision loss in working age Americans. Almost everyone with type 1 diabetes and most people with type 2 diabetes develop retinopathy within two decades of disease onset (1). The incidence of type 2 diabetes is rapidly on the rise and accounts for up to 95% of diabetes prevalence (2). However, the overwhelming majority of experimental studies on diabetic retinopathy have been performed using animal models of type 1 diabetes. Development of type 2 diabetes is associated with consumption of a diet containing high fat and sucrose content, which has become extremely common in Western populations (3). Thus, in the present study we set out to investigate potential mechanisms whereby a Western diet contributes to diabetes-induced vision loss.Diabetic retinopathy is caused by a combination of hyperglycemia and a reduction in insulin-mediated signaling, which results in neurovascular changes in the retina. Insulin provides a protective neurotrophic stimulus to retina by activating the protein kinase Akt through a signaling pathway involving the insulin receptor substrate 1/2 (IRS1/2) 2 (4). In response to insulin, the activation loop of Akt is phosphorylated at Thr-308 by phosphoinositide-dependent kinase 1 (PDK1) (5), whereas the C-terminal hydrophobic motif is phosphorylated at Ser-473 by mTORC2 (mammalian target of rapamycin (mTOR) in complex 2) (6...

Abstract Background: The chronic activation of the mechanistic (mammalian) target of rapamycin in complex 1 (mTORC1) in response to excess nutrients contributes to obesity-associated pathologies. Objective: To understand the initial events that ultimately lead to obesity-associated pathologies, the present study assessed mTORC1 responses in the liver after a relatively short exposure to a high-fat diet (HFD). Methods: Male, obesity-prone rats were meal-trained to consume either a control (CON; 10% of energy from fat) diet or an HFD (60% of energy from fat) for 2 wk. Livers were collected and analyzed for mTORC1 signaling [assessed by changes in phosphorylation of 70-kDa ribosomal protein S6 kinase 1 (p70S6K1) and eukaryotic initiation factor 4E binding protein 1 (4E-BP1)] and potential regulatory mechanisms, including changes in the association of Ras-related GTP binding (Rag) A and RagC with mechanistic target of rapamycin (mTOR) and expression of Sestrin1, Sestrin2, and Sestrin3. Results: Feeding-induced activation of mTORC1 was blunted in the livers of rats fed the HFD compared with those fed the CON diet (p70S6K1 phosphorylation, 19% of CON; 4E-BP1 phosphorylation, 61% of CON). The attenuated response was not due to a change in a kinase also referred to as protein kinase B (Akt) signaling but rather to resistance to amino acid–induced activation of mTORC1, as evidenced by a reduction in the interaction of RagA (69% of CON) and RagC (66% of CON) with mTOR and enhanced expression of the mTORC1 repressors Sestrin2 (132% of CON) and Sestrin3 (143% of CON). The consumption of an HFD led to impaired amino acid–induced activation of mTORC1 as assessed in livers perfused in situ with medium containing various concentrations of amino acids. Conclusions: These results in rats support a model in which the initial response of the liver to an HFD is an attenuation of, rather than the expected activation of, mTORC1. The initial response likely represents a counterregulatory mechanism to handle the onset of excess nutrients and is caused by enhanced expression of Sestrin2 and Sestrin3, which, in turn, leads to impaired Rag signaling, resulting in resistance to amino acid–induced activation of mTORC1.