Leslie O. Goodwin

Role of glutamine depletion in directing tissue-specific nutrient stress responses to L-asparaginase

l-Asparaginase is important in the induction regimen for treating acute lymphoblastic leukemia. Cytotoxic complications are clinically significant problems lacking mechanistic insight. To reveal tissue-specific molecular responses to this drug, mice were administered asparaginase from either Escherichia coli (clinically used) or Wolinella succinogenes (novel, glutaminase-free form). Both enzymes abolished serum asparagine, but only the E. coli form reduced circulating glutamine. E. coli asparaginase reduced protein synthesis in liver and spleen but not pancreas via increased phosphorylation of the translation factor eIF2. In contrast, treatment with Wolinella caused no untoward changes in protein synthesis in any tissue examined. Treating mice deleted for the eIF2 kinase, GCN2, with the E. coli enzyme showed eIF2 phosphorylation to be GCN2-dependent, but only initially. Furthermore, although eIF2 phosphorylation was not increased in the pancreas or by Wolinella asparaginase, expression of the amino acid stress response genes, asparagine synthetase and CHOP/GADD153, increased as a result of both enzymes, even in tissues demonstrating no change in eIF2 phosphorylation. Finally, signaling downstream of the mammalian target of rapamycin kinase was repressed in liver and pancreas by E. coli but not Wolinella asparaginase. These data demonstrate that the nutrient stress response to asparaginase is tissue-specific and exacerbated by glutamine depletion. Importantly, increased expression of asparagine synthetase and CHOP does not require eIF2 phosphorylation, signifying alternate or auxiliary means of inducing gene expression under conditions of amino acid depletion in the whole animal.

Voltage-dependent calcium channels in mammalian spermatozoa revisited.

The last few years have seen an explosion in the number of voltage-dependent ion channel sequences detected in sperm and testes. The complex structural paradigm of these channels is now known to include a pore-forming alpha1 subunit(s) whose electrophysiological properties are modulated by an intracellular beta subunit, a disulfide-linked complex of a membrane-spanning delta subunit with an extracellular alpha2 subunit, and a transmembrane gamma subunit. Many of these are alternatively spliced. Furthermore, the known number of genes coding each subtype has expanded significantly (10 alpha1, 4 beta, 4 alpha2delta, 8 gamma). Recently, the CatSper gene family has been characterized based on similarity to the voltage-dependent calcium channel alpha1 subunit. From among this multiplicity, a wide cross-section is active in sperm, including many splice variants. For example, expression of the various alpha1 subunits appears strictly localized in discrete domains of mature sperm, and seems to control distinct physiological roles such as cellular signaling pathways. These include alpha1 alternative splicing variants that are regulated by ions passed by channels in developing sperm. Various combinations of ion channel sequence variants have been studies in research models and in a variety of human diseases, including male infertility. For example, rats that are genetically resistant to testes damage by lead seem to respond to lead ions by increasing alpha1 alternative splicing. In contrast, in varicocele-associated male infertility, the outcome from surgical correction correlates with suppression of alpha1 alternative splicing, Ion channel blockers remain attractive model contraceptive drugs because of their ability to modulate cholesterol levels. However, the large number of sperm ion channel variants shared with other cell types make ion channels less attractive targets for male contraceptive development than a few years ago. In this review, the genetics, structure and function of voltage-dependent calcium channels and related CatSper molecules will be discussed, and several practical clinical applications associated with these channels will be reported.