Download Richardson Final miR Commentary Diabetes 2016

miR, miR in the cell, does the virus control them all?
Sarah J Richardson1, Marc S Horwitz2
1 University of Exeter Medical School, Devon, UK
2 University of British Columbia, Vancouver, Canada
Correspondence to:
Marc Horwitz
Associate Professor, Sauder Chair of Pediatric Virology
Co-leader, Infection, Inflammation & Immunity Research Group
Department of Microbiology and Immunology
University of British Columbia
3551-2530 Health Sciences Mall, Vancouver, BC V6T 1Z3, Canada
Tel: 604-822-6298
FAX: 604-822-6041
E-mail: [email protected]
Dr Sarah Richardson
Lecturer in Biomedical Sciences (Education & Research)
Institute of Biomedical and Clinical Sciences
University of Exeter Medical School
RILD Building (Level 4), Barrack Road, Exeter, EX2 5DW, UK
Tel: +44 (0)1392 408225
E-mail: [email protected]
Word Count : 1000
Figures: 1
As if the pathway of regulation leading islet cells toward dysfunction and diabetes was not difficult
enough to grasp, here come viruses to further complicate matters. In the past few years, we have
come to recognize that a class of small noncoding RNAs termed microRNAs (miRNAs) has a powerful
ability to regulate most, if not all, of the key processes in cell biology (1,2). In fact, there is evidence
that these key molecules can be transported from cell to cell, thereby influencing gene expression
not only within the host cell but also in neighboring cells and in distant cell subsets when the
miRNAs travel in vesicles via the blood and lymph (3–5). The evidence that miRNAs play pivotal roles
in b-cell biology, including regulating their differentiation, glucose responsiveness, insulin
production, apoptosis, and inflammation, is rapidly accumulating and was recently discussed by
Filios and Shalev (4) in an issue of Diabetes. As one might expect given their range of functions
within the b-cells, these same molecules are hypothesized to have key roles in the pathogenesis of
type 1 diabetes (4,6–13). Yet, it is still not clear whether or not they are central to the induction of
disease or if they are merely a biomarker of active b-cell dysfunction. Type 1 diabetes is often
described as a chronic autoimmune disease characterized by the destruction of insulin producing
islet b-cells, and it is a widely held belief that a complex blend of genetic and environmental factors
is required to drive disease. In terms of clinically relevant environmental factors, the culprits that are
most often mentioned and have the strongest evidentiary support are coxsackieviruses (CVBs).
Evidence for CVB infection has often been described in recent-onset patients, and CVBs have a clear
propensity for infecting human islet b-cells and can induce diabetes in mouse models (14–16).
Whereas many viruses encode their own miRNAs to control both viral and cellular processes, other
viruses simply usurp cellular miRNAs to their own advantage (3,5,17). Small RNA viruses such as
CVBs seem to lack miRNAs, as none have been identified in their genomes to date. Previously,
several groups have examined the impact of cytokines on miRNAs in mouse and human b-cells
(7,13,18), but the study by Kim et al. (19) in this issue of Diabetes is the first study to examine the
impact of an enterovirus infection on miRNA expression in human pancreatic islets. Taking into
consideration the important role of miRNAs in normal b-cell function, Kim et al. asked whether a
virus infection could be in part responsible for the changes in miRNA expression within the infected
b-cell that can subsequently contribute to a type 1 diabetes phenotype. These changes could
promote b-cell dysfunction and/or apoptosis, could alter the sensing and subsequent antiviral host
cell immune response, and could impact the ability of infiltrating and peripheral immune cells to
respond to an infection, potentially promoting increased inflammation, surveillance, or antigen
presentation. Kim et al. (19) assessed the cellular expression of miRNAs within human islets infected
with CVB5. In total, the expression of 754 miRNAs was assessed in the infected islets using a highthroughput quantitative PCR array, with the expression of 33 miRNAs found to be significantly
altered when compared with the control islets. Strikingly, these 33 miRNAs were predicted to target
57 of the 72 known type 1 diabetes risk genes. The majority of miRNAs were downregulated postinfection but 6 were upregulated (Fig. 1). Key immune modulatory miRNAs miR-155-5p (increased
fourfold) and miR-181a-3p (decreased 4.4-fold) were found to be dysregulated by 24 h after
infection, and intriguingly, miR-34a-5p demonstrated the greatest increase in expression (more than
eightfold) of any miRNA by 4 days post-infection. miR-34a-5p is pluripotent and has been shown to
target genes resulting in diminishing insulin secretion (VAMP2) and increasing apoptosis (BclII) (18).
Additionally, miR-155-5p has also been shown by others to affect several other diabetes risk genes
such as GLIS3 (b-cell survival) and SOCS1 (negative regulator of the type I interferon [antivirus]
pathway) (8,9). Other dysregulated miRNAs were predicted to target a number of pathogen
recognition receptors, such as IFIHI, TLR7, and TLR8; cytokines, including IL2, IL10, IL17D, and IL21;
and regulators of cytokine apoptosis, such as BACH2. Many of these miRNA–target gene interactions
have been validated in both adult pancreatic b-cells as well as b-cells from prediabetic mice (NOD
and db/db) (18), but others require further verification. Clearly though, CVB5 infection alters cellular
miRNA expression to focus the cell’s material and energy to promote virus replication while at the
same time attempting to downregulate pathways that would inhibit virus propagation, including
those affecting antivirus responses. The production of proteins that are not required for the virus life
cycle, such as insulin and apoptosis pathways, appear to be downregulated. An example of the latter
is miR-21, which is increased more than sevenfold in the infected cells and targets the tumor
suppressor PDCD4, which has been shown to block pancreatic cell death in mouse models (12).
Several miRNAs, including some identified by Kim et al. (19) (miR-21, miR-181, and miR-29a), were
identi- fied as dysregulated in the serum of children with type 1 diabetes (10,11). Additionally,
multiple reports have identified exosomes containing miRNAs that are exchanged between immune
cells and result in changes in cellular expression, cytokine expression, and antigen presentation. An
entire cluster of miRNA loci has been shown to target the expression of type 1 diabetes autoantigens
IA-2, IA-2b, and GAD65 (6). It seems that virus infection reflects miRNA expression back onto the cell
in an effort to prioritize replication of virus and on toward surrounding cells in an effort to minimize
detection. In a susceptible individual, can this induce islet cell dysfunction and diabetes? This is the
premise of the study by Kim et al. (19), yet a number of questions are raised. Prior work has
demonstrated that different strains and substrains of CVB are associated with diabetes induction
and protection (20). As such, the most strongly associated diabetogenic strains are CVB1 and CVB4,
and their miRNA expression pattern may reveal further specific diabetes-associated miRNAs and
targets. Likewise, the virus-mediated mechanism for the induction of diabetes remains elusive, yet
two viable hypotheses endure and suggest that the disease could either be induced following an
acute infection that is subsequently resolved or via a more chronic, persistent infection of b-cells.
The expression of miRNA during acute and latent herpesvirus infection has been well described to
change over the life cycle of the virus, and it would be expected that similar results would occur with
CVB (5). Kim et al. describe the early events of acute infection with exhibited changes in miRNA
expression within the first 7 days. Later time points during persistent infection would likely reveal a
novel pattern of miRNA expression. Furthermore, exosome analysis of the intercommunication
between cells could uncover communication within the islet to neighboring endocrine cells as well as
with the immune cells responding to infection. Clearly, longitudinal studies of in vivo tissue from
mice and humans will add to the confirmation of potential viral etiology. In summary, CVB infection
of human islets dramatically alters the profile of miRNA expression. Many of the affected miRNAs
target the type 1 diabetes risk genes, suggesting that viral infection has another means by which it
could contribute to the development of a type 1 diabetes phenotype. Further array analysis for
cellular and vesicular miRNAs will likely expose a number of potential miRNAs as therapies
themselves or as therapeutic targets.
Funding. This work was partially funded by a JDRF Career Development Award (5-CDA-2014-221-AN) to S.J.R.
Duality of Interest. No potential conflicts of interest relevant to this article were reported.
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Figure 1—How a viral infection could impact on the development of autoimmunity by altering host
miRNA expression. ① The enterovirus infects b-cells within an islet. ② The enterovirus replicates,
and viral proteins are produced. ③ The virus induces upregulation and downregulation of specific
host miRNAs. ④ miR-155-5p at early time points and miR-21-3p at later time points impact on the
host cell apoptotic responses. ⑤ Several of the miRNAs impact on the expression levels of key
pathogen recognition receptors (PRRs) and SOCS1, which could alter host response to infection. ⑥
miR-34a and miR-21-3p alter expression of key genes involved in the regulation of b-cell exocytosis
(VAMP2, OC2), thus potentially impacting on host cell insulin secretion. ⑦ Furthermore, miRNAs
can be packaged into microvesicles and/or endosomes and released from the infected cells. ⑧
Neighboring islet cells can take up secreted miRNAs, which can then subsequently impact on their
gene expression, either making them more or less susceptible to infection or apoptosis, which can
alter their insulin secretion. ⑨ Secreted miRNAs can also be taken up by infiltrating and peripheral
immune cells, which could then significantly impact on their regulatory or effector cell functions.