Transcriptomic plasticity of the hypothalamic osmoregulatory control centre of the Arabian dromedary camel (Camelus dromedarius)

Transcriptomic plasticity of the hypothalamic osmoregulatory control centre of the Arabian dromedary camel (Camelus dromedarius)
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The dromedary camel (Camelus dromedarius) has considerable economic and cultural importance in the Middle East and North Africa. The “ship of the desert” has been the most important domesticated species in these regions for millennia.  As an adaptation to long-term drought conditions in the desert environment, the camel has evolved robust mechanisms to maintain water homeostasis. At the level of the kidney, the camel is capable of producing a low volume of highly concentrated urine via efficient water reabsorption, especially when challenged by water deprivation. The water reabsorption from the pre-urine is mediated by the hormones arginine vasopressin (AVP) and oxytocin (OXT), two key osmo-regulators that are produced in the hypothalamic supraoptic nucleus (SON) and paraventricular nucleus (PVN) in the brain.

One-humped Arabian camels

To reveal the physiological mechanisms underlying water homeostasis under the tremendous pressures of the hot desert environment, our lab decided to look at the nature’s water conserving wonder, the camel, to seek answers to specialisations that ensure survival in desert using omics techniques and advanced bioinformatics. Thus, an exciting collaborative project developed between Professor Abdu Adem (United Arab Emirates University) and Professor David Murphy (University of Bristol). The mutual goal for the two laboratories was to study the kidney and brain of the Arabian dromedary camel challenged by long-term water deprivation and subsequent rapid rehydration. The physiological experiments were carried out with ranch-housed camel outside Al Ain, United Arab Emirates, during the hot months (April and May) of 2016, under careful veterinary supervision to ensure animal welfare. Having reported transcriptomic and proteomic adaptations to water deprivation in the camel kidney in 2021 (please visit this nature portfolio blog by Fernando Alvira Iraizoz for more information), we turned our attention to the neuroendocrine mechanisms that orchestrate the response to water deprivation in the camel hypothalamus. This is the first time that the camel hypothalamus has been so extensively studied in terms of its adaptations to the desert environment.

My enthusiasm in camels was developed during my past residence in the northwest China, where I encountered free-range farming of Bactrian camels when traveling to the Kumtag Desert and Gobi deserts. Similar to the important roles played by dromedary camels to provide milk, meat, transportation and entertainment to people in North and East Africa, the Arabian Peninsula and Iran and many Middle Eastern countries, the Bactrian camel is a very important livestock for people from northwest China and Mongolia as well. Additionally, I am concerned about the conservation of one of the critically endangered species in the world - the wild Bactrian camel – which inhabits the deserts of northwest China and southwest Mongolia. How these domestic and wild camels cope with drastic environmental change such as desertification and climate change has become my major interest over the past few years. As an earlier stage researcher, I had background in microbiology as well as applied biosciences and biotechniques before joining the Murphy Lab, so basically, I had limited experience in animal physiology and neuroendocrinology. Luckily, by working together with many amazing colleagues and collaborators, I was equipped with relevant knowledge and techniques that enabled me to vigorously pursue my interest in the Camelid family.  

Figure 1: Mapping of the dromedary camel SON. A screenshot of Movie 1 showing a three-dimensional model of camel SON constructed using RNAscope images to demonstrate spatially relative locations of rostral and caudal SON subdivisions. This model was built using 12 RNAscope images (interpolated to 96 planes) of a partial hypothalamus containing SON (in coronal sections). The locations of AVP (red) and OXT (green) mRNA in the SON of a WD camel indicate the organization of SON. SON: supraoptic nucleus, 3V: third ventricle.

Movie 1

This project studying the transcriptomic plasticity of SON – the hypothalamic osmoregulatory control centre of the dromedary camel – started with three-dimensional mapping of the camel SON (Figure 1, Movie 1) based on the expression of the AVP and OXT mRNAs to facilitate SON sampling. Different from the rodent SONs, the camel SON revealed a distinct spatial structure that including two separate subpopulations of magnocellular cells that make these hormones. We named them the rostral SON and caudal SON, regarding their relative location along the rostral-caudal axis in the brain.

a Experimental workflow studying the camel SON (created with BioRender.com). After acclimatisation, hypothalamic samples were collected from 19 camels divided into 3 groups; control (water ad libitum, n=5), WD (water deprivation for 20 days, n=8) and rehydrated (water deprivation for 20 days followed by water ad libitum for three days, n=6). b Principal component analysis (PCA) showing separation between control and WD conditions. Control sample: red; WD sample: turquoise. PC: principal component. PC1 (46%) and PC2 (24%) are the most and second underlying variation between samples. c Volcano plot of statistical significance (-log10 padj) against log2 fold change (LFC) of differentially expressed genes (DEGs) (padj≤0.05) in WD. Red: upregulated DEGs; blue: downregulated DEGs; grey: unchanged genes. Selected DEGs labeled by gene symbols. Over-representation analysis of pathways were performed based on all camel DEGs: d Over-represented GO: biological processes. e Over-represented GO: KEGG pathways. Benjamini-Hochberg correction (padj≤0.05) was used for multiple comparison correction. Dot plots illustrate the enriched pathways by WD and their associated DEGs. Significantly enriched pathways are listed along the y-axis by padj value from top to bottom in ascending order. Pathway-associated DEGs are denoted by colored dots. Dot color and size represent LFC and transcript abundance measured by average normalized read counts aligned to each gene across all samples (basemean), respectively.

Figure 2: Experimental design and analysis of the transcriptomes of the dromedary camel SON during long-term water deprivation. a Experimental workflow studying the camel SON (created with BioRender.com). After acclimatisation, hypothalamic samples were collected from 19 camels divided into 3 groups; control (water ad libitum, n=5), WD (water deprivation for 20 days, n=8) and rehydrated (water deprivation for 20 days followed by water ad libitum for three days, n=6). b Principal component analysis (PCA) showing separation between control and WD conditions. Control sample: red; WD sample: turquoise. PC: principal component. PC1 (46%) and PC2 (24%) are the most and second underlying variation between samples. c Volcano plot of statistical significance (-log10 padj) against log2 fold change (LFC) of differentially expressed genes (DEGs) (padj≤0.05) in WD. Red: upregulated DEGs; blue: downregulated DEGs; grey: unchanged genes. Selected DEGs labeled by gene symbols. Over-representation analysis of pathways were performed based on all camel DEGs: d Over-represented GO: biological processes. e Over-represented GO: KEGG pathways. Benjamini-Hochberg correction (padj≤0.05) was used for multiple comparison correction. Dot plots illustrate the enriched pathways by WD and their associated DEGs. Significantly enriched pathways are listed along the y-axis by padj value from top to bottom in ascending order. Pathway-associated DEGs are denoted by colored dots. Dot color and size represent LFC and transcript abundance measured by average normalized read counts aligned to each gene across all samples (basemean), respectively.

a Venn diagram comparing WD camel and rat DEGs. b Over-represented GO: biological processes based on the common DEGs between camel and rat. c Over-represented GO: KEGG pathways based on the common DEGs between camel and rat. d Over-represented GO: biological processes based on the camel-unique DEGs. e Over-represented GO: KEGG pathways based on the camel-unique DEGs. Benjamini-Hochberg correction (padj≤0.05) was used for multiple comparison correction. Dot plots illustrate the enriched pathways by WD and their associated genes. Significantly enriched pathways are listed along the y-axis by padj value from top to bottom in ascending order. Pathway-associated genes are denoted by colored dots. Dot color and size represent log2 fold change (LFC) and transcript abundance measured by average normalized read counts aligned to each gene across all samples (basemean), respectively.

Figure 3: Comparison of SON differentially expressed genes (DEGs) in water deprived dromedary camels and rats.a Venn diagram comparing WD camel and rat DEGs. b Over-represented GO: biological processes based on the common DEGs between camel and rat. c Over-represented GO: KEGG pathways based on the common DEGs between camel and rat. d Over-represented GO: biological processes based on the camel-unique DEGs. e Over-represented GO: KEGG pathways based on the camel-unique DEGs. Benjamini-Hochberg correction (padj≤0.05) was used for multiple comparison correction. Dot plots illustrate the enriched pathways by WD and their associated genes. Significantly enriched pathways are listed along the y-axis by padj value from top to bottom in ascending order. Pathway-associated genes are denoted by colored dots. Dot color and size represent log2 fold change (LFC) and transcript abundance measured by average normalized read counts aligned to each gene across all samples (basemean), respectively.

We then compared the transcriptomes of the camel SON under control and water deprived conditions. 209 genes were identified to be significantly changed in expression by water deprivation (Figure 2). By further comparing the WD camel SON transcriptome to our previously published rat transcriptome (Pauža et al., 2021), 80 common differentially expressed genes while 129 were uniquely changed in the camel SON (Figure 3). Further, we identified core gene pathways that are commonly changed in the WD camel and rat (Figure 3), including the pathway “Protein processing in endoplasmic reticulum”. Same as rat, the camel SON may undergo enhanced protein processing that is associated with increased demand of neuropeptide secretion, endoplasmic reticulum stress and unfolded protein response due to the accumulation of unfolded/misfolded protein during WD. Other genes and pathways that are uniquely changed in the WD camel SON (Figure 3) and as such might be indispensable for life in the arid desert were also identified, suggesting that the camel SON may undergo additional structural remodelling in extracellular matrix to promote the synthesis and release of AVP and OXT. The upregulated expression of cellular stress sensor protein IRE1 uniquely in the camel supports the concept that the unfolded protein response is activated in the SON perhaps as a protective mechanism for neurons in chronic WD. 

To support the growing interest in osmoregulatory processes inclassic rodent models and more unusual desert animals, my colleague Ben Gillard has set up a multi species/tissue expression analysis app (please visit https://bengillard.shinyapps.io/MultiSpeciesExpression/), where the transcriptomic data of rat, dromedary camel and jerboa (a desert rodent model) is easily accessible. The expression of genes of interest can be retrieved by browsing by gene name in this app. We are keen to receive feedback on this app!

Apart from the camel hypothalamic samples, many other organs and tissues from these animals were collected. These samples are being studied by numerous groups around the globe. Thus, over the coming years, we will present a comprehensive picture of the overall physiological and molecular responses of the camel to WD and subsequent rehydration. This will provide valuable information in the context of desertification and climate change and could be used to forecast and evaluate how different species will adapt to constantly changing environments.

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