Abstract:
BACKGROUND:
Concerted efforts to identify the pathogenesis and mechanism(s) involved in pansteatitis, (a
generalized inflammation of the adipose tissue), that was attributed to the recent crocodile
die off in the Olifants River and Loskop Dam in Kruger National Park, Mpumalanga, South
Africa have been in the forefront of research in recent time. As part of the efforts, molecular
characterization of healthy and pansteatitis adipose tissue was carried out by RNA sequencing (RNA-Seq) using Next Generation Sequencing (NGS) and de novo assembly of the adipose transcriptome, followed by differential gene expression analysis.
METHODOLOGY:
Healthy adipose tissue consisting of fifty samples was collected from the subcutaneous, visceral, intermuscular adipose tissues and the abdominal fat body of ten 4 years old juvenile
crocodiles from a local crocodile farm in Pretoria, South Africa. Ten pansteatitis samples
were collected from visceral and intermuscular adipose tissues of five crocodiles that were
dying of pansteatitis.
RESULTS:
Forty-two thousand, two hundred and one (42,201) transcripts were assembled, out of
which 37, 835 had previously been characterized. The de novo assembled transcriptome
had an N50 (average sequence) of 436 bp, percentage GC content of 43.92, which compared well with previously assembled transcripts in the saltwater crocodile. Seventy genes
were differentially expressed and upregulated in pansteatitis. These included genes coding
for extracellular matrix (ECM) signaling ligands, inflammatory cytokines and tumour necrosis factor alpha (TNFα) receptors, fatty acid synthase and fatty acid binding proteins, peroxisome proliferator-activated receptor gamma (PPARγ), nuclear factor and apoptosis
signaling ligands, and mitogen activated protein kinase enzymes among others. Majority
(88.6%) of the upregulated genes were found to be involved in hypoxia inducible pathways
for activation of NFkβ and inflammation, apoptosis, Toll-like receptor pathway and PPARγ.
Bicaudal homologous 2 Drosophila gene (BICD2) associated with spinal and lower extremity muscle atrophy was also upregulated in pansteatitis while Sphingosine -1-phosphate
phosphatase 2 (SGPP2) involved in Sphingosine -1- phosphate metabolism was downregulated. Futhermore, Doublesex–mab-related transcription factor 1 (DMRT1) responsible for
sex gonad development and germ cell differentiation was also downregulated.
CONCLUSION:
Thus, from the present study, based on differentially expressed genes in pansteatitis,
affected Nile crocodiles might have died partly due to their inability to utilize stored triglycerides as a result of inflammation induced insulin resistance, leading to starvation in the midst
of plenty. Affected animals may have also suffered muscular atrophy of the lower extremities and poor fertility.
