The Beta Cell in Its Cluster: Stochastic Graphs of Beta Cell Connectivity in the Islets of Langerhans

Pancreatic islets of Langerhans consist of endocrine cells, primarily α, β and δ cells, which secrete glucagon, insulin, and somatostatin, respectively, to regulate plasma glucose. β cells form irregular locally connected clusters within islets that act in concert to secrete insulin upon glucose stimulation. Due to the central functional significance of this local connectivity in the placement of β cells in an islet, it is important to characterize it quantitatively. However, quantification of the seemingly stochastic cytoarchitecture of β cells in an islet requires mathematical methods that can capture topological connectivity in the entire β-cell population in an islet. Graph theory provides such a framework. Using large-scale imaging data for thousands of islets containing hundreds of thousands of cells in human organ donor pancreata, we show that quantitative graph characteristics differ between control and type 2 diabetic islets. Further insight into the processes that shape and maintain this architecture is obtained by formulating a stochastic theory of β-cell rearrangement in whole islets, just as the normal equilibrium distribution of the Ornstein-Uhlenbeck process can be viewed as the result of the interplay between a random walk and a linear restoring force. Requiring that rearrangements maintain the observed quantitative topological graph characteristics strongly constrained possible processes. Our results suggest that β-cell rearrangement is dependent on its connectivity in order to maintain an optimal cluster size in both normal and T2D islets.     

Functional complementation of nuclear targeting-defective mutants of simian virus 40 structural proteins.

Structural proteins of simian virus 40 (SV40), Vp2 and Vp3 (Vp2/3) and Vp1, carry individual nuclear targeting signals, Vp3(198-206) (Vp2(316-324) and Vp1(1-8), respectively, which are encoded in different reading frames of an overlapping region of the genome. How signals coordinate nuclear targeting during virion morphogenesis was examined by using SV40 variants in which there is only one structural gene for Vp1 or Vp2/3, nuclear targeting-defective mutants thereof, Vp2/3(202T) and Vp1 delta N5, or nonoverlapping SV40 variants in which the genes for Vp1 and Vp2/3 are separated, and mutant derivatives of the gene carrying either one or both mutations. Nuclear targeting was assessed immunocytochemically following nuclear microinjection of the variant DNAs. When Vp2/3 and Vp1 mutants with defects in the nuclear targeting signals were expressed individually, the mutant proteins localized mostly to the cytoplasm. However, when mutant Vp2/3(202T) was coexpressed in the same cell along with wild-type Vp1, the mutant protein was effectively targeted to the nucleus. Likewise, the Vp1 delta N5 mutant protein was transported into the nucleus when wild-type Vp2/3 was expressed in the same cells. These results suggest that while Vp1 and Vp2/3 have independent nuclear targeting signals, additional signals, such as those defining protein-protein interactions, play a concerted role in nuclear localization along with the nuclear targeting signals of the individual proteins.     

Nuclear targeting of SV40 and adenovirus

Import o f viral DNA into the nucleus is essential for the successful replication o f DNA tumour viruses. To achieve this goal, viruses have adapted strategies to traverse the barriers between the plasma membrane and the nucleus o f a host cell. Two DNA tumour viruses, simian virus 40 and adenovirus, achieve the nuclear-entry step in slightly different ways. SV40 DNA enters the nucleus through the nuclear pore complexes (NPCs) in apparently intact virions. By contrast, adenovirus particles dissociate near the NPC before the viral DNA is imported into the nucleus. In both cases, karyophilic protein components o f the viruses appear to mediate nuclear entry o f the viral genomes. In this article, we discuss how an understanding o f the cell biology o f virus entry can help us understand the process o f nuclear transport.     

Essential role of the Vp2 and Vp3 DNA-binding domain in simian virus 40 morphogenesis.

Both a DNA-binding domain and a Vp1 interactive determinant have been mapped to the carboxy-terminal 40 residues of the simian virus 40 (SV40) minor capsid proteins, Vp2 and Vp3 (Vp2/3), with the last 13 residues being necessary for these activities. The role of this DNA-binding domain in SV40 morphogenesis and the ability to separate these two signals were investigated by mutagenesis and assessment of the activity and viability of the mutants. The carboxy-terminal 40 residues of Vp2/3 were expressed as a polyhistidine fusion protein, and five basic residues at the extreme carboxy terminus (Vp3 residues K226, R227, R228, R230, and R233) were mutagenized. The wild-type fusion protein bound DNA with a Kd of 3 x 10(-8) identical to that of the full-length Vp3. Mutant proteins containing either one to three or four amino acid substitutions bound DNA 4- to 7-fold or 20- to 30-fold less well, respectively, than the wild-type protein did. The most severe point mutants showed residual DNA binding similar to that of a truncated protein which lacks the entire 13 carboxy-terminal residues. All of the point mutants were able to interact with Vp1, indicating that the two signals within this region are mediated by different residues. When the mutations were placed into the context of the viral DNA and introduced into cells, all the structural proteins were expressed and localized correctly. Not all, however, were viable: mutant genomes whose Vp2/3 bound DNA with intermediate affinities formed plaques just as well as wild-type SV40 DNA did, but three mutants showing greatly reduced DNA binding failed to form plaques at all. These results are consistent with the hypothesis that Vp2/3 plays an essential role in SV40 virion assembly in the nucleus.     

GPAT2, a mitochondrial outer membrane protein,in piRNA biogenesis in germline stem cells

piRNA (PIWI-interacting RNA) is a germ cell–specific small RNA in which biogenesis PIWI (P-element wimpy testis) family proteins play crucial roles. MILI (mouse Piwi-like), one of the three mouse PIWI family members, is indispensable for piRNA production, DNA methylation of retrotransposons presumably through the piRNA, and spermatogenesis. The biogenesis of piRNA has been divided into primary and secondary processing pathways; in both of these MILI is involved in mice. To analyze the molecular function of MILI in piRNA biogenesis, we utilized germline stem (GS) cells, which are derived from testicular stem cells and possess a spermatogonial phenotype. We established MILI-null GS cell lines and their revertant, MILI-rescued GS cells, by introducing the Mili gene with Sendai virus vector. Comparison of wild-type, MILI-null, and MILI-rescued GS cells revealed that GS cells were quite useful for analyzing the molecular mechanisms of piRNA production, especially the primary processing pathway. We found that glycerol-3-phosphate acyltransferase 2 (GPAT2), a mitochondrial outer membrane protein for lysophosphatidic acid, bound to MILI using the cells and that gene knockdown of GPAT2 brought about impaired piRNA production in GS cells. GPAT2 is not only one of the MILI bound proteins but also a protein essential for primary piRNA biogenesis.     

Flavonoids isolated from the leaves of Melicope triphylla and their extracellular-superoxide dismutase-inducing activity

Two novel flavonoids, named meliflavones A (1) and B (2), were isolated from the leaves of Melicope triphylla (Lam.) Merr., along with thirteen known compounds (3–15). Four of the polymethoxyflavonoids bearing a prenyloxy (3-methylbut-2-enyloxy) function (1, 3–5) induced the expression of extracellular-superoxide dismutase (EC-SOD) in a human leukemic U937 cell-based assay.     

Human cathelicidin antimicrobial peptide LL-37 promotes lymphangiogenesis in lymphatic endothelial cells through the ERK and Akt signaling pathways

Molecular Biology Reports - LL-37, the only member of the cathelicidin family of cationic antimicrobial peptides in humans has been shown to exhibit a wide variety of biological actions in addition...     

Mother–calf interactions and social behavior development in Commerson’s dolphins (Cephalorhynchus commersonii)

Mother–calf interactions and the behaviors of mothers during separation from their calves were examined in four Commerson’s dolphin (Cephalorhynchus commersonii) mother–calf pairs. Four infants were observed: 56.8 h over 30 days from birth to 263 days of age, 36.9 h over 20 days from birth to 149 days of age, 10.4 h over 3 days from birth to 2 days of age, and 15.0 h over 3 days from birth to 2 days of age. All four pairs shared common characteristics in the rate and frequencies of mother–calf interactions and the behaviors of mothers during the first week of life. After the first week, individual differences in changes in the frequency of each behavior were observed. The three behaviors considered representative of maternal care (parallel swimming, synchronous breathing, and body-to-body contact) were frequently performed in the first week; thereafter, the frequencies declined. Separate behaviors of mothers were infrequent during the first week and increased with an increase in infants’ age. Bumping by infants increased with time, suggesting an increase in soliciting by calves and conflict between mothers and calves. The frequency of flipper-to-body rubbing also changed but in a complex manner, probably because the calves needed to learn how to perform this behavior from their mothers and because initiator and recipient of this behavior can be changed quickly.