Sertoli cells require TDP-43 to support spermatogenesis†.

TAR DNA binding protein of 43 kD (TDP-43) is an evolutionarily conserved, ubiquitously expressed transcription factor and RNA-binding protein with major human health relevance. TDP-43 is present in Sertoli and germ cells of the testis and is aberrantly expressed in the sperm of infertile men. Sertoli cells play a key role in spermatogenesis by offering physical and nutritional support to male germ cells. The current study investigated the requirement of TDP-43 in Sertoli cells. Conditional knockout (cKO) of TDP-43 in mouse Sertoli cells caused failure of spermatogenesis and male subfertility. The cKO mice showed decreased testis weight, and low sperm count. Testis showed loss of germ cell layers, presence of vacuoles, and sloughing of round spermatids, suggesting loss of contact with Sertoli cells. Using a biotin tracer, we found that the blood-testis barrier (BTB) was disrupted as early as postnatal day 24 and worsened in adult cKO mice. We noted aberrant expression of the junction proteins connexin-43 (gap junction) and N-cadherin (ectoplasmic specialization). Oil Red O staining showed a decrease in lipid droplets (phagocytic function) in tubule cross-sections, Sertoli cells cytoplasm, and in the lumen of seminiferous tubules of cKO mice. Finally, qRT-PCR showed upregulation of genes involved in the formation and/or maintenance of Sertoli cell junctions as well as in the phagocytic pathway. Sertoli cells require TDP-43 for germ cell attachment, formation and maintenance of BTB, and phagocytic function, thus indicating an essential role for TDP-43 in the maintenance of spermatogenesis.

Loss of TDP-43 in male germ cells causes meiotic failure and impairs fertility in mice.

Meiotic arrest is a common cause of human male infertility, but the causes of this arrest are poorly understood. Transactive response DNA-binding protein of 43 kDa (TDP-43) is highly expressed in spermatocytes in the preleptotene and pachytene stages of meiosis. TDP-43 is linked to several human neurodegenerative disorders wherein its nuclear clearance accompanied by cytoplasmic aggregates underlies neurodegeneration. Exploring the functional requirement for TDP-43 for spermatogenesis for the first time, we show here that conditional KO (cKO) of the Tardbp gene (encoding TDP-43) in male germ cells of mice leads to reduced testis size, depletion of germ cells, vacuole formation within the seminiferous epithelium, and reduced sperm production. Fertility trials also indicated severe subfertility. Spermatocytes of cKO mice showed failure to complete prophase I of meiosis with arrest at the midpachytene stage. Staining of synaptonemal complex protein 3 and γH2AX, markers of the meiotic synaptonemal complex and DNA damage, respectively, and super illumination microscopy revealed nonhomologous pairing and synapsis defects. Quantitative RT-PCR showed reduction in the expression of genes critical for prophase I of meiosis, including Spo11 (initiator of meiotic double-stranded breaks), Rec8 (meiotic recombination protein), and Rad21L (RAD21-like, cohesin complex component), as well as those involved in the retinoic acid pathway critical for entry into meiosis. RNA-Seq showed 1036 upregulated and 1638 downregulated genes (false discovery rate <0.05) in the Tardbp cKO testis, impacting meiosis pathways. Our work reveals a crucial role for TDP-43 in male meiosis and suggests that some forms of meiotic arrest seen in infertile men may result from the loss of function of TDP-43.

Temperature affects the host hematological and cytokine response following experimental ranavirus infection in red-eared sliders (Trachemys scripta elegans).

Pathogen-host interactions are important components of epidemiological research, but are scarcely investigated in chelonians. Red-eared sliders (Trachemys scripta elegans), are recognized as a model for frog virus-3 infection (FV3), a ranavirus in the family Iridoviridae that infects multiple classes of ectothermic vertebrates. Previous challenge studies observed differences in disease outcome based on environmental temperature in this species, but the host response was minimally evaluated. We challenged red-eared sliders with an FV3-like ranavirus at both 28°C and 22°C. We monitored several host response variables for 30 days, including: survival (binary outcome and duration), clinical signs, total and differential leukocytes, and select cytokine transcription in the buffy coat (IL-1ß, TNFα, IFYg, IL-10). After 30 days, 17% of challenged turtles survived at 28°C (Median survival time [MST]: 15 days, range: 10-30 days) and 50% survived (MST: 28.5 days, range: 23-30 days) at 22°C (range 23-30 days). The most common clinical signs were injection site swelling, palpebral swelling, and lethargy. The heterophil/lymphocyte ratio at 22°C and interleukin-1 beta (IL1ß) transcription at both 22°C and 28°C were significantly greater on days 9, 16, and 23 in FV3 challenged groups. Tumor necrosis factor alpha and interleukin-10 were transcribed at detectable levels, but did not display significant differences in mean relative transcription quantity over time. Overall, evidence indicates an over-robust immune response leading to death in the challenged turtles. FV3 remains a risk for captive and free-ranging chelonian populations, and insight to host/pathogen interaction through this model helps to elucidate the timing and intensity of the host response that contribute to mortality.

MORTALITY INVESTIGATION OF MONITORED EASTERN BOX TURTLES (TERRAPENE CAROLINA CAROLINA) IN CENTRAL ILLINOIS, USA, FROM 2016-18.

Mortality events in eastern box turtles (Terrapene carolina carolina) threaten conservation efforts across the species range. These events are often under-diagnosed and, when observed, predictive health factors are unavailable prior to death. At Kickapoo State Park in central Illinois, USA, ranaviruses caused observed mortality events in amphibians and chelonians in 2014 and 2015. Following these outbreaks, eastern box turtles (n=36) were affixed with radio transmitters and temperature data loggers to obtain repeated location and temperature data from spring 2016-spring 2018. Bimonthly, samples of blood and oral and cloacal swabs were collected to investigate health parameters (hematology and cytokine transcription) and presence of multiple pathogens. Deaths of instrumented turtles occurred in 2016 (n=5), 2017 (n=15), and 2018 (n=2). The largest single die-off occurred in February 2017 (n=7). Seventeen turtles were necropsied and multiple pathologic processes were identified, most frequently decreased adipose stores (n=6). Two turtles had pathologic findings consistent with multisystemic inflammation. In addition, infectious pathogens were identified in turtles prior to death, but no single agent was associated with each mortality event. Ranavirus was not detected in any turtle. Hot spot analysis revealed spatial clustering at the center and edges of the study area for body temperature as well as for relative cytokine transcription of interleukin-1 beta, tumor necrosis factor alpha, and interleukin-10 associated with turtle death. Though no single causal factor could be identified, the information from this mortality event can direct future chelonian mortality investigations by providing baseline longitudinal data prior to death and in surviving turtles.

Development of reverse-transcriptase quantitative PCR assays for detection of the cytokines IL-1ß, TNF-α, and IL-10 in chelonians.

In response to viral pathogens, a host releases pro-inflammatory cytokines such as interleukin-1 beta (IL-1ß) and tumor necrosis factor alpha (TNF-α) and anti-inflammatory cytokines such as interleukin-10 (IL-10). While several approaches exist to measure cytokine responses, evaluating gene transcription through reverse transcription quantitative polymerase chain reaction (RT-qPCR) provides a fast, reproducible, and sensitive method for quantifying this response. The objective of this study was to develop an effective and sensitive RT-qPCR assay for the quantification of red-eared slider (Trachemys scripta elegans) and eastern box turtle (Terrapene carolina carolina) cytokines: IL-1ß, TNFα, IL-10 and the reference gene ß-actin. RNA was isolated from the buffy coat layer of whole blood, comprised mainly of circulating leukocytes, and complimentary DNA (cDNA) was produced. Conventional PCR was performed to obtain cytokine mRNA sequences, products were sequenced, and a hydrolysis probe-based RT-qPCR assay was designed for each cytokine. Standard curves were generated using the target gene sequences cloned within a plasmid. Efficiencies for each assay were between of 85-110%, R2 > 0.98, and limits of detection of 10-100 copies per reaction. The initial samples used to identify the novel target sequences were then used to evaluate the performance of the qPCR assays. Consistent transcription of beta actin across individuals in both species and measurable transcription of IL-1ß, TNF-α, and IL-10 transcript targets in individuals of both species were observed. The assays are a novel technique in chelonians to evaluate host innate immune response.

EXPERIMENTAL TRANSMISSION OF FROG VIRUS 3-LIKE RANAVIRUS IN JUVENILE CHELONIANS AT TWO TEMPERATURES.

The pathogenicity of frog virus 3 (FV3)-like ranavirus varies in adult chelonian species at different environmental temperatures, but differences in pathogenicity at different temperatures has yet to be determined in juveniles. Our objective was to determine the susceptibility to FV3-like ranavirus in four species of juvenile chelonians: red-eared sliders (RES; Trachemys scripta elegans), Mississippi map turtles ( Graptemys pseudogeographica kohnii), false map turtles (FMT; Graptemys pseudogeographica), and eastern river cooters ( Pseudemys concinna concinna) at two environmental temperatures. Two simultaneous trials ( n=8 treatment and n=4 controls of each species) were conducted in separate temperature-controlled rooms with animals maintained at 22 C or 27 C. All of the inoculated animals of each species at each temperature died, but no mortality was observed in control animals. Median survival times varied between 8 d and 11 d, based on species and temperature, with RES in the 27 C trial surviving the shortest time and the FMT in the 22 C trial surviving the longest. Combining all species, turtles in the 27 C trial survived for fewer days than those housed at 22 C, despite all turtles in both trials having similar viral copies detected in postmortem tissues. Lesions in inoculated turtles resembled those noted in natural and experimental FV3-like ranavirus infections and included vasculitis, thrombosis, hemorrhage in multiple organs, renal tubular necrosis, and hepatic necrosis. Myositis was not present in any juvenile, infected turtles in this study. This study confirmed that juvenile chelonians have a high susceptibility to ranaviral disease.