Two amino acids, Phe 16 and Ala 776, on the polyprotein are most likely to be responsible for the diabetogenicity of encephalomyocarditis virus

Viruses in type 1 diabetes: brief review

Type 1 diabetes results from the progressive destruction of insulin-producing pancreatic beta cells. Although the etiology of type 1 diabetes is believed to have a major genetic component, studies on the risk of developing type 1 diabetes suggest that environmental factors, such as viruses, may be important etiological determinants. Among the viruses, the most clear and unequivocal evidence that a virus induces type 1 diabetes in animals comes from studies on the D variant of encephalomyocarditis (EMC-D) virus in mice and Kilham rat virus (KRV) in rats. A high titer of EMC-D viral infection results in the development of diabetes within 3 days, primarily due to the rapid destruction of beta cells by viral replication within the cells. A low titer of EMC-D viral infection results in the recruitment of macrophages to the islets. Soluble mediators produced by the activated macrophages such as interleukin-1Beta, tumor necrosis factor-alpha, and nitric oxide play a critical role in the destruction of residual beta cells. KRV causes autoimmune type 1 diabetes in diabetes resistant-BioBreeding rats by breakdown of immune balance, including the preferential activation of effector T cells, such as Th1-like CD45RC+CD4+ T cells and CD8+ T cells, and down-regulation of Th2-like CD45RC-CD4+ and CD4+CD25+ T cells, rather than by direct infection of pancreatic beta cells.

Oral administration of branched chain amino acids improves virus-induced glucose intolerance in mice

We investigated the therapeutic effect of branched chain amino acids (BCAA) on mice with glucose intolerance induced by encephalomyocarditis virus (EMCV). Male DBA/2 mice were divided into three groups: treated with BCAA, (such as valine, leucine, and isoleucine), untreated, and control. BCAA-treated and -untreated groups were inoculated intraperitoneally with the NDK25 variant of EMCV at 200 plaque-forming units per mouse. The BCAA-treated group was administered orally 0.9 g/kg/day of each BCAA from the day after viral inoculation. The control group neither received virus inoculation nor was treated with BCAA. One week after inoculation, oral glucose tolerance tests (OGTT) were performed. After the glucose loading at 1.5 g/kg of body weight, blood glucose levels in the untreated group were 92.0+/-10.0 mg/dl at baseline, 224.6+/-10.9 mg/dl at 30 min, and 169.4+/-21.4 mg/dl at 60 min, which were significantly (P<0.05) higher than those in the control group (62. 7+/-3.6 mg/dl, 167.2+/-16.4, and 83.8+/-6.0 mg/dl, respectively). Blood glucose levels in the BCAA-treated group were 54.5+/-3.7 mg/dl at baseline, 145.2+/-8.7 mg/dl at 30 min, and 128.7+/-18.3 mg/dl at 60 min after the glucose loading, which were not significantly higher than those in the control group. Immunoreactive insulin levels at 30 min after the glucose loading were lower in the untreated group than in the control group at 1 week after virus inoculation. Histological investigations showed that the grade of insulitis in the pancreas of mice of the BCAA-treated group was lower than that of the mice of the untreated group. These results suggest that oral administration of BCAA is able to improve glucose intolerance induced by EMCV.

Possible role of macrophage-derived soluble mediators in the pathogenesis of encephalomyocarditis virus-induced diabetes in mice

Pancreatic islets from DBA/2 mice infected with the D variant of encephalomyocarditis (EMC-D) virus revealed lymphocytic infiltration with moderate to severe destruction of pancreatic beta cells. Our previous studies showed that the major population of infiltrating cells at the early stages of infection is macrophages. The inactivation of macrophages prior to viral infection resulted in the prevention of diabetes, whereas activation of macrophages prior to viral infection resulted in the enhancement of beta-cell destruction. This investigation was initiated to determine whether macrophage-produced soluble mediators play a role in the destruction of pancreatic beta cells in mice infected with a low dose of EMC-D virus. When we examined the expression of the soluble mediators interleukin-1 beta (IL-1beta), tumor necrosis factor alpha (TNF-alpha), and inducible nitric oxide synthase (iNOS) in the pancreatic islets, we found that these mediators were clearly expressed at an early stage of insulitis and that this expression was evident until the development of diabetes. We confirmed the expression of these mediators by in situ hybridization with digoxigenin-labelled RNA probes or immunohistochemistry in the pancreatic islets. Mice treated with antibody against IL-1beta or TNF-alpha or with the iNOS inhibitor aminoguanidine exhibited a significant decrease in the incidence of diabetes. Mice treated with a combination of anti-IL-1beta antibody, anti-TNF-alpha antibody, and aminoguanidine exhibited a greater decrease in the incidence of disease than did mice treated with one of the antibodies or aminoguanidine. On the basis of these observations, we conclude that macrophage-produced soluble mediators play an important role in the destruction of pancreatic beta cells, resulting in the development of diabetes in mice infected with a low dose of EMC-D virus.

Complete nucleotide sequence of a strain of coxsackie B4 virus of human origin that induces diabetes in mice and its comparison with nondiabetogenic coxsackie B4 JBV strain

The E2 strain of coxsackie B4 virus (CB4), which is of human origin, can induce a diabetes-like syndrome in mice. The cDNA of the genome of the E2 strain was cloned and sequenced. The E2 viral genome was found to comprise 7,396 bases, which appear to encode a polyprotein of 2,183 amino acids with an overall similarity of 94.91% to nondiabetogenic CB4 prototype JBV strain. The E2 genome is organized like other enteroviruses. It has a 5' noncoding region of 744 nucleotides, a single long open translational reading frame starting at nucleotide 745 and extending to nucleotide 7293, a 3' noncoding region of 100 nucleotides, and a poly (A) tract. Genomic sequence comparison of the E2 and JBV strains showed 1,369 nucleotide substitutions in the genome of the E2 strain, most of which are single and silent. There were 111 resultant amino acid changes arising from some of these substitutions, including 82 amino acid changes in the noncapsid proteins, and 29 amino acid changes in the capsid proteins VP1, VP2, VP3, and VP4, which showed 11, 13, 4, and 1 substitution(s), respectively. Noncapsid protein P2-C showed eight amino acid substitutions. On the basis of the sequence comparison of E2 and JBV strains of CB4, we suggest that some of the amino acid changes in the capsid and noncapsid proteins of the E2 strain may be involved in the determination of its diabetogenicity.

Complete nucleotide sequence of a beta-cell tropic variant of coxsackievirus B4

A mouse pancreas-adapted variant of coxsackievirus B4 (P-CB4) has been shown to replicate in, and cause an excessive release of insulin from, pancreatic beta cells cultured in vitro. The prototype CB4 strain (JVB Benschoten), from which the adapted variant was derived, although able to replicate in cultured islets does not cause a similar release of insulin from the beta cells. The pancreas-adapted virus has also been shown to cause host cell protein synthesis shut-off in beta cells and to inhibit (pro)insulin biosynthesis. These metabolic changes occur in the absence of cytolytic damage [Szopa et al.: Bioscience Reports 5:63-69, 1985 and Cell Biochemistry and Function 4:181-187, 1986]. To investigate the genetic basis for this beta cell tropism, the complete nucleotide sequence of P-CB4 has been determined and compared to that of the previously published sequence of the prototype CB4 strain (JVB Benschoten) [Jenkins et al.: Journal of General Virology 68:1835-1848, 1987]. Twenty-five nucleotide sequence differences were observed. Of these, six occur in the 5' noncoding region of the genome and 19 in the coding region (resulting in seven amino acid changes). The possible significance of these changes in relation to the beta cell tropism of the pancreas-adapted virus is discussed.

Diabetes mellitus due to viruses--some recent developments

Many different viruses belonging to several genera have the potential to damage beta cells. The mechanisms they employ are varied, and infection may result in either a direct destruction of islets and rapid insulin deficiency, or in a more gradual loss of functioning islets with the onset of diabetes many years later. Several case histories involving extensive cytolysis of beta cells can be directly linked to viral infection, whilst an example of diabetes occurring many years after viral infection is found in individuals who had a congenital infection with rubella virus. Here, the virus induces an autoimmune reaction against beta cells. Autoimmune phenomena have also been observed in islets following infections with viruses other than rubella, and thus activation of autoimmune mechanisms leading to beta-cell destruction may be a relatively frequent occurrence. Recent evidence shows that picornaviruses are not exclusively lytic, and can induce more subtle, long-term changes in beta cells, which may be important in the aetiology of diabetes. The exact mechanisms involved are not known, but it is clear that several viruses can directly inhibit insulin synthesis and induce the expression of other proteins such as interferons, and the HLA antigens. Strain differences in viruses are important since not all variants are tropic for the beta cells. Several laboratories are in the process of identifying the genetic determinants of tropism and diabetogenicity, especially amongst the Coxsackie B (CB) virus group. The sequence of one such diabetogenic CB4 strain virus has been determined.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of a recombinant RNA technique for the construction of chimeric RNA with a long poly(C) tract

The murine cardioviruses and bovine aphthoviruses are distinguished from other (+) strand RNA viruses by their long poly(C) tract in the 5'-noncoding region. The presence of this poly(C) tract has long hampered the construction of full-length cDNA with the complete poly(C) tract, because long poly(dC-dG) homopolymer-containing plasmids are difficult to amplify in bacterial systems. To overcome this problem, we constructed a chimeric RNA by joining the poly(C) region of the viral RNA to the 5'-truncated RNA transcript of the encephalomyocarditis (EMC) virus cDNA. The non-chimeric, recombinant EMC virus with a short poly(C) tract produces recombinant progeny virus, but this is not pathogenic in vivo. On the other hand, the EMC viral RNA chimera with the complete poly(C) tract produces recombinant progeny virus that is pathogenic in vivo. This method of viral RNA construction will be invaluable for functional studies of other cardioviruses and aphthoviruses, as well as for recombinant RNA manipulations.

Role of viruses in the pathogenesis of IDDM

Insulin-dependent diabetes mellitus (IDDM), also known as type I diabetes, results from the destruction of pancreatic beta cells. During the past few decades, genetic factors, autoimmunity and viral infections have been extensively studied as the possible cause of beta cell destruction. The evidence for virus-induced diabetes comes largely from experiments in animals, but several studies in humans also point to viruses as a trigger of this disease in some cases. There are at least two possible mechanisms for the involvement of viruses in the pathogenesis of IDDM: (a) cytolytic infection of beta cells may result in destruction of the cells without the induction of autoimmunity, or may be a final insult leading to the clinical onset of diabetes in individuals with an already decreased beta cell mass resulting from an autoimmune process; and (b) persistent viral infection (e.g. retrovirus, rubella virus, cytomegalovirus) may result in the triggering of autoimmune IDDM in certain circumstances. Regarding the latter possibility, viruses may insert, expose, or alter antigens in the plasma membrane of the beta cell, which may initiate autoimmunity leading to the destruction of the cells.

Predominant binding of Theiler's viruses to a 34-kilodalton receptor protein on susceptible cell lines

Western immunoblots of BHK-21 cell lysates probed with the highly virulent GDVII and the less virulent BeAn strains of Theiler's murine encephalomyelitis virus (TMEV) revealed predominant binding to a 34-kDa membrane protein and much lower levels of binding to 100- and 18-kDa membrane proteins. Complete inhibition of virus binding to both the 34- and 18-kDa membrane species by excess unlabeled TMEV demonstrated specificity of binding. Virus binding was also blocked by wheat germ agglutinin, which specifically binds to sialic acid residues and blocks TMEV binding to whole BHK-21 cells. Radiolabeled TMEV also bound to 100-, 34-, and 18-kDa membrane proteins expressed on other TMEV permissive cell lines but not on the nonpermissive cell lines tested. These data suggest that a 34-kDa cellular protein may be the primary determinant of susceptibility to TMEV infection by mediating the binding of GDVII and BeAn viruses to susceptible cells.

The 5'-untranslated region of picornaviral genomes

Picornaviruses are small naked icosahedral viruses with a single-stranded RNA genome of positive polarity. According to current taxonomy, the family includes four genera: Enterouirus (polioviruses, coxsackieviruses, echoviruses, and other enteroviruses), Rhinovirus, Curdiouirus [encephalomyocarditis virus (EMCV), mengovirus, Theiler's murine encephalomyelitis virus (TMEV)], and Aphthouirus [foot-and-mouth disease viruses (FMDV)]. There are also some, as yet, unclassified picornaviruses [e.g., hepatitis A virus (HAW] that should certainly be assessed as a separate genus. Studies on the molecular biology of picornaviruses might be divided into two periods: those before and after the first sequencing of the poliovirus genome. The 5'-untranslated region (5-UTR) of the viral genome was one of the unexpected problems. This segment proved to be immensely long: about 750 nucleotides or ∼10% of the genome length. There were also other unusual features (e.g., multiple AUG triplets preceding the single open reading frame (ORF) that encodes the viral polyprotein). This chapter shows that the picornaviral 5-UTRs are not only involved in such essential events as the synthesis of viral proteins and RNAs that could be expected to some extent, although some of the underlying mechanisms appeared to be quite a surprise, but also may determine diverse biological phenotypes from the plaque size or thermosensitivity of reproduction to attenuation of neurovirulence. Furthermore, a close inspection of the 5-UTR structure unravels certain hidden facets of the evolution of the picornaviral genome. Finally, the conclusions drawn from the experiments with the picornaviral5-UTRs provide important clues for understanding the functional capabilities of the eukaryotic ribosomes.

Role of macrophages in the pathogenesis of encephalomyocarditis virus-induced diabetes in mice

Pancreatic islets from SJL/J mice infected with the D variant of encephalomyocarditis virus (EMC-D virus) showed lymphocytic infiltration with moderate to severe destruction of beta cells. Immunohistochemical staining of the islet sections with several monoclonal antibodies, anti-Mac-1, anti-Mac-2, and F4/80 for macrophages, anti-L3T4 for helper/inducer T cells, and anti-Lyt2 for cytotoxic/suppressor T cells revealed that the major population of infiltrating cells at the early stage of viral infection was Mac-2-positive macrophages. In contrast, macrophages detected by anti-Mac-1 and F4/80 monoclonal antibodies were not found at the early stage of viral infection but were found at intermediate and late stages of viral infection. Helper/inducer T cells and cytotoxic/suppressor T cells also infiltrated the islets at intermediate and late stages of viral infection. Short-term treatment of mice with silica prior to viral infection resulted in an enhancement of beta-cell destruction, leading to the development of diabetes. In contrast, long-term treatment of mice with silica resulted in complete prevention of diabetes caused by a low dose of viral infection and a significant decrease in the incidence of diabetes caused by an intermediate or high dose of viral infection. Furthermore, depletion of macrophages by a specific monoclonal antibody (anti-Mac-2) resulted in a much greater decrease in the incidence of diabetes caused by an intermediate dose of viral infection. However, suppression of helper/inducer T cells and cytotoxic/suppressor T cells, by anti-L3T4 and anti-Lyt2 antibodies, respectively, did not alter the incidence of diabetes. On the basis of these data, it is concluded that macrophages, particularly Mac-2-positive macrophages, play a crucial role in the process of pancreatic beta-cell destruction at the early stage of encephalomyocarditis D virus infection in SJL/J mice.