viruses Review Astrovirus Pathogenesis Cydney Johnson 1,†, Virginia Hargest 1,2,†, Valerie Cortez 1, Victoria A. Meliopoulos 1 and Stacey Schultz-Cherry 1,* 1 Department of Infectious Diseases, St. Jude Children’s Research Hospital, Memphis, TN 38105, USA; [email protected] (C.J.); [email protected] (V.H.); [email protected] (V.C.); [email protected] (V.A.M.) 2 Department of Microbiology, Immunology, and Biochemistry, University of Tennessee Health Science Center, Memphis, TN 38163, USA * Correspondence: [email protected]; Tel.: +1-901-595-6629 † These authors have contributed equally to this paper. Academic Editor: Eric O. Freed Received: 9 December 2016; Accepted: 17 January 2017; Published: 22 January 2017 Abstract: Astroviruses are a major cause of diarrhea in the young, elderly, and the immunocompromised. Since the discovery of human astrovirus type 1 (HAstV-1) in 1975, the family Astroviridae has expanded to include two more human clades and numerous mammalian and avian-specific genotypes. Despite this, there is still little known about pathogenesis. The following review highlights the current knowledge of astrovirus pathogenesis, and outlines the critical steps needed to further astrovirus research, including the development of animal models of cell culture systems. Keywords: astrovirus; pathogenesis; animal models; cell culture 1. Introduction Infectious diarrhea is a leading cause of childhood morbidity and mortality worldwide. Human astroviruses (HAstV)—small, non-enveloped positive-sense single-stranded RNA viruses in the Astroviridae family—are a leading cause of diarrhea in children, the elderly, and immunocompromised people. They are extremely prevalent, and around 90% of the population 9 years and older has antibodies against HAstV type 1 (HAstV-1) [1–3]. Since the initial discovery of HAstV-1 in humans in 1975 [4,5], additional astroviruses have been described and reported in humans [6] as well as in domesticated and wild animals [7–25]. This broad host range has become increasingly evident, with the viruses divided into two main genera—Avastrovirus and Mamastrovirus—based on their ability to infect avian and mammalian species, respectively (Figure1). Astrovirus infections were thought to be species-specific, yet turkey, chicken, and duck astroviruses share genetic features, indicating that cross-species transmission may be frequent [26]. Poultry abattoir workers were three times more likely to test positive for antibodies against turkey astrovirus than people with no contact with poultry [27], and perhaps the most compelling evidence was the detection of astrovirus strains previously shown to be limited to human infections in non-human primates (NHP) by real-time polymerase chain reaction (RT-PCR) [28]. These studies and others suggest that astroviruses can cross species barriers; however, whether these transmissions result in disease remains unclear. Despite the high prevalence of astrovirus [29] and the advances made in the identification of novel genotypes, there is still little known about HAstV pathogenesis—especially among the different HAstV genotypes. Previous studies demonstrated that HAstVs increase epithelial cell permeability by disrupting cellular tight junctional complexes [30]. Since the intestinal tract depends on tight junctions to separate the lumen from the basal lamina, the loss of integrity increases ion, solute, and water trafficking across the compartments, reducing the ability of the intestine to reabsorb water Viruses 2017, 9, 22; doi:10.3390/v9010022 www.mdpi.com/journal/viruses Viruses 2017, 9, 22 2 of 10 solute, and water trafficking across the compartments, reducing the ability of the intestine to reabsorb Viruses 2017, 9, 22 2 of 10 water and nutrients, leading to diarrhea. Additionally, extra‐gastrointestinal astrovirus‐associated disease has been reported in animals and humans [31–33], which could result from the increased intestinaland nutrients,permeability. leading The definite to diarrhea. mechanisms Additionally, underlying extra-gastrointestinal diarrhea and systemic astrovirus-associated spread remain diseasean area hasof continual been reported research; in animals increasing and our humans understanding [31–33], which of astrovirus could result disease from will the increasedbe integral intestinal for futurepermeability. treatment and The prevention definite mechanisms strategies. underlying Recently, studies diarrhea in and both systemic humans spread and animals remain anhave area of helpedcontinual to elucidate research; the molecular increasing and our cellular understanding attributes ofof astrovirus disease. disease willThe following be integral review for future highlightstreatment these and findings, prevention and strategies.outlines necessary Recently, and studies critical in both steps humans needed and to animalsfurther haveastrovirus helped to research.elucidate the molecular and cellular attributes of astrovirus disease. The following review highlights these findings, and outlines necessary and critical steps needed to further astrovirus research. Figure 1. A brief phylogenetic tree of the Astroviridae family. Phylogenetic tree with representative FigureAvastrovirus 1. A brief (phylogeneticred) and Mamastrovirus tree of the( blackAstroviridae) genotypes. family. Using Phylogenetic full genome tree sequences, with representative the evolutionary Avastrovirushistory (red was) and inferred Mamastrovirus using the (black Maximum) genotypes. Likelihood Using full method genome based sequences, on the the Kimura evolutionary 2-parameter historymodel. was inferred Trees were using constructed the Maximum using Likelihood 500 bootstrapped method based replicates, on the Kimura with values2‐parameter above model. 70 shown. Trees Branchwere constructed lengths representusing 500 bootstrapped the number replicates, of substitutions with values per above site. 70 HAstV: shown. Branch Human lengths astrovirus; representMLB: the Melbourne; number of VA: substitutions Virginia. per site. HAstV: Human astrovirus; MLB: Melbourne; VA: Virginia. 2. Astrovirus Disease 2. Astrovirus Disease Within the years following the discovery of HAstV, eight distinct genotypes (HAstV-1–8, alsoWithin referred the years to asfollowing classical the or discovery canonical of human HAstV, clades) eight distinct were identified, genotypes with (HAstV infections‐1–8, also caused referredby to HAstV-1 as classical shown or canonical to be the human most clades) prevalent were worldwideidentified, with [34 –infections38]. In the caused late by 2000s, HAstV through‐1 shownpathogen to be the discovery most prevalent investigations worldwide of diarrheal [34–38]. In outbreaks, the late 2000s, two divergent through pathogen HAstV genotypes discovery were investigationsdiscovered: of diarrheal HAstV-MLB outbreaks, (Melbourne) two divergent clade containing HAstV genotypes at least three were strainsdiscovered: (MLB1, HAstV MLB2,‐MLB MLB3) (Melbourne)and the HAstV-VA/HMOclade containing at (Virginia/Human-Mink-Ovine-like) least three strains (MLB1, MLB2, MLB3) clade and containing the HAstV at least‐VA/HMO five strains (Virginia/Human(VA1, VA2, VA3,‐Mink VA4,‐Ovine VA5).‐like) The clade HAstV-MLB containing and at HAstV-VA/HMOleast five strains (VA1, viruses VA2, have VA3, been VA4, designated VA5). as The non-canonicalHAstV‐MLB and human HAstV genotypes.‐VA/HMO HAstV-MLB1 viruses have was been initially designated discovered as non in pediatric‐canonical stool human samples genotypes.from Australia HAstV‐MLB1 [39–41 was], whereas initially HAstV-MLB2 discovered in andpediatric HAstV-MLB3 stool samples were from discovered Australia in India[39–41], [42 ,43]. whereasThe HAstV HAstV-VA1–VA3‐MLB2 and virusesHAstV‐ wereMLB3 first were discovered discovered in in an India outbreak [42,43]. of The gastroenteritis HAstV‐VA1–VA3 in Virginia, virusesUSA were [44 ,first45]; HAstV-VA4discovered in aan cohort outbreak of Indian of gastroenteritis children with in diarrhea Virginia, [43 USA]; and [44,45]; HAstV-VA5 HAstV in‐VA4 a cohort in a cohortin Gambia of Indian [46]. The children prevalence with diarrhea of the non-canonical [43]; and HAstV viruses‐VA5 varies in a cohort greatly in according Gambia to[46]. geographic The location [6], and their association with clinical disease remains somewhat of a mystery in comparison to the canonical strains. Viruses 2017, 9, 22 3 of 10 Signs of HAstV last 2 to 4 days, and consist of watery diarrhea that can be less commonly accompanied by fever, headaches, abdominal pain, and anorexia [1,47,48]. However, many of the infections in healthy children and adults tend to be asymptomatic [37]. The consequence of these asymptomatic cases on the epidemiology and transmission of the virus remain unclear. Further research is needed to understand the clinical disease associated with the different human astrovirus genotypes. Astrovirus infections are of clinical concern in the immunocompromised population due to their increased severity of symptoms and extra-gastrointestinal involvement [6]. HAstV has also been suggested to be the causative agent in encephalitis and meningitis, which was brought to light in a case report of a 15-year old boy with X-linked agammaglobulinemia by Quan et al. [33]. The patient was admitted to a psychiatric ward due to cognitive decline, became comatose, and died 71 days post admission. RNA was extracted from the patient’s biopsy and postmortem samples, and sequenced and amplified with virus-specific