Functions of disordered regions in mammalian early base excision repair proteins

Reactive oxygen species, generated endogenously and induced as a toxic response, produce several dozen oxidized or modified bases and/or single-strand breaks in mammalian and other genomes. These lesions are predominantly repaired via the conserved base excision repair (BER) pathway. BER is initiated with excision of oxidized or modified bases by DNA glycosylases leading to formation of abasic (AP) site or strand break at the lesion site. Structural analysis by experimental and modeling approaches shows the presence of a disordered segment commonly localized at the N- or C-terminus as a characteristic signature of mammalian DNA glycosylases which is absent in their bacterial prototypes. Recent studies on unstructured regions in DNA metabolizing proteins have indicated their essential role in interaction with other proteins and target DNA recognition. In this review, we have discussed the unique presence of disordered segments in human DNA glycosylases, and AP endonuclease involved in the processing of glycosylase products, and their critical role in regulating repair functions. These disordered segments also include sites for posttranslational modifications and nuclear localization signal. The teleological basis for their structural flexibility is discussed.     

Galanin – 25 years with a multitalented neuropeptide

Neuroanatomical localization and physiological properties of galanin suggest that the peptide may be involved in the regulation of seizures. Indeed, administration of galanin receptor agonists into brain areas pertinent to the initiation and propagation of epileptic activity attenuated seizure responses under conditions of animal models of epilepsy; pharmacological blocking of galanin receptors exerted proconvulsant effects. Functional deletion of both galanin and galanin type 1 receptor genes produced transgenic mice with either spontaneous seizure phenotype, or with enhanced susceptibility to seizure stimuli. At the same time, overexpression of galanin in seizure pathways, using both transgenic and virus vector transfection techniques, hindered the epileptic process. Galanin exerts anticonvulsant effects through both type 1 and type 2 receptors, with distinct downstream signaling cascades. Several synthetic agonists of galanin receptors with optimized bioavailability have been synthesized and inhibited experimental seizures upon systemic administration, thus opening an opportunity for the development of galanin-based antiepileptic drugs.     

Neuroanatomy of flying reptiles and implications for flight, posture and behaviour

Comparison of birds and pterosaurs, the two archosaurian flyers, sheds light on adaptation to an aerial lifestyle. The neurological basis of control holds particular interest in that flight demands on sensory integration, equilibrium, and muscular coordination are acute. Here we compare the brain and vestibular apparatus in two pterosaurs based on high-resolution computed tomographic (CT) scans from which we constructed digital endocasts. Although general neural organization resembles birds, pterosaurs had smaller brains relative to body mass than do birds. This difference probably has more to do with phylogeny than flight, in that birds evolved from nonavian theropods that had already established trends for greater encephalization. Orientation of the osseous labyrinth relative to the long axis of the skull was different in these two pterosaur species, suggesting very different head postures and reflecting differing behaviours. Their enlarged semicircular canals reflect a highly refined organ of equilibrium, which is concordant with pterosaurs being visually based, aerial predators. Their enormous cerebellar floccular lobes may suggest neural integration of extensive sensory information from the wing, further enhancing eye- and neck-based reflex mechanisms for stabilizing gaze.     

The endonuclease activity of Mili fuels piRNA amplification that silences LINE1 elements

Piwi proteins and Piwi-interacting RNAs (piRNAs) have conserved functions in transposon silencing. The murine Piwi proteins Mili and Miwi2 (also called Piwil2 and Piwil4, respectively) direct epigenetic LINE1 and intracisternal A particle transposon silencing during genome reprogramming in the embryonic male germ line. Piwi proteins are proposed to be piRNA-guided endonucleases that initiate secondary piRNA biogenesis; however, the actual contribution of their endonuclease activities to piRNA biogenesis and transposon silencing remain unknown. To investigate the role of Piwi-catalysed endonucleolytic activity, we engineered point mutations in mice that substitute the second aspartic acid to an alanine in the DDH catalytic triad of Mili and Miwi2, generating the Mili(DAH) and Miwi2(DAH) alleles, respectively. Analysis of Mili-bound piRNAs from homozygous Mili(DAH) fetal gonadocytes revealed a failure of transposon piRNA amplification, resulting in the marked reduction of piRNA bound within Miwi2 ribonuclear particles. We find that Mili-mediated piRNA amplification is selectively required for LINE1, but not intracisternal A particle, silencing. The defective piRNA pathway in Mili(DAH) mice results in spermatogenic failure and sterility. Surprisingly, homozygous Miwi2(DAH) mice are fertile, transposon silencing is established normally and no defects in secondary piRNA biogenesis are observed. In addition, the hallmarks of piRNA amplification are observed in Miwi2-deficient gonadocytes. We conclude that cycles of intra-Mili secondary piRNA biogenesis fuel piRNA amplification that is absolutely required for LINE1 silencing.