518
success, and decreased embryonic and larval survival (Whitehead et al. 2010, 2011;
Lanham et al. 2014).
Activation of the AHR pathway has been consistently observed in developing
embryos and larvae exposed to oil. When fertilized mahi-mahi embryos were
exposed to slick oil from the DWH oil spill, cyp1a1 was the most strongly upregulated gene at 24, 48, and 96 hours post fertilization (hpf), and downstream AHR
pathway genes such as cyp1b1, udp-glucuronosyltransferase (ugt), and arylhydrocarbon receptor repressor (ahrr) were also upregulated (Xu et al. 2016a). In
fact, a total of 127 AHR-responsive genes were differentially expressed at 96 hpf
(Ibid); cyp1a1, ahrr, and cyp3a4 were also among the most highly upregulated
genes in slick-oil-exposed red drum embryos; and gene ontology analysis predicted
significant activation of AHR pathways at both 24 and 72 hpf (Xu et al. 2017). Oilexposed Atlantic haddock embryos also showed upregulation of many stress
response genes mediated by the AHR pathway at all time points from 2 days post
fertilization to 10 days post hatch (Sørhus et al. 2017).
Induction of AHR and stress response pathways also occurs in adult fish. In a
field study of Gulf killifish, livers from fish at sites that had measurable PAH concentrations showed upregulation of ahr1b and ahr2 receptors compared to nonoiled sites (Garcia et al. 2012), as well as upregulation of cyp1a1 in the gills. Other
AHR pathway genes such as cytochrome p450s and ugts were also upregulated in
the livers of killifish from contaminated sites. Upregulation of ugt has been observed
in other Fundulus studies and in the gills of juvenile Japanese flounder, Paralichthys
olivaceus (Whitehead et al. 2012a; Pilcher et al. 2014; Zhu et al. 2016). HEWAF
exposed Japanese flounder also showed significant effects on stress genes related to
AHR pathway activation (Zhu et al. 2016). Together, these studies have shown that
upregulation of AHR-mediated genes, in particular cyp1a1, is consistent transcriptomic responses to oil in many species and life stages and supports previous research
indicating the use of CYP as a biomarker for field studies assessing oil (Adams
1990; Schlenk et al. 2008b).
Transcriptomic studies performed in larval fish have also provided evidence that
changes in gene expression not associated with the AHR pathway may in part
explain abnormalities during early development. Altered cardiac gene expression,
particularly for genes involved in intracellular Ca
2+
homeostasis and excitationcontraction coupling, may directly contribute to cardiotoxicity in developing
embryos and larvae. In mahi-mahi embryos at 48 and 96 hpf, transcripts of calsequestrins, which are involved in Ca
2+
homeostasis, were upregulated after slick oil
exposure (Xu et al. 2016a). Downregulation of T-type receptor 2 (tnnt2) transcripts
was also observed. Tnnt2 regulates intracellular Ca
2+
concentrations (Metzger and
Westfall 2004). Diminished expression of transcripts encoding L-type and P/Q-type
voltage-dependent calcium channels was also observed following oil exposure, further suggesting alterations in Ca
2+
signaling (Xu et al. 2016a). Similarly, in haddock, transcripts of the cardiac sarcoplasmic reticulum Ca
2+
ATPase (atp2a2) and
ryr2, a ryanodine receptor that mediates cellular calcium release, were downregulated, as was the gene encoding the delayed rectifying K
+
channel, kcnh2, which is
D. S. Portnoy et al.
success, and decreased embryonic and larval survival (Whitehead et al. 2010, 2011;
Lanham et al. 2014).
Activation of the AHR pathway has been consistently observed in developing
embryos and larvae exposed to oil. When fertilized mahi-mahi embryos were
exposed to slick oil from the DWH oil spill, cyp1a1 was the most strongly upregulated gene at 24, 48, and 96 hours post fertilization (hpf), and downstream AHR
pathway genes such as cyp1b1, udp-glucuronosyltransferase (ugt), and arylhydrocarbon receptor repressor (ahrr) were also upregulated (Xu et al. 2016a). In
fact, a total of 127 AHR-responsive genes were differentially expressed at 96 hpf
(Ibid); cyp1a1, ahrr, and cyp3a4 were also among the most highly upregulated
genes in slick-oil-exposed red drum embryos; and gene ontology analysis predicted
significant activation of AHR pathways at both 24 and 72 hpf (Xu et al. 2017). Oilexposed Atlantic haddock embryos also showed upregulation of many stress
response genes mediated by the AHR pathway at all time points from 2 days post
fertilization to 10 days post hatch (Sørhus et al. 2017).
Induction of AHR and stress response pathways also occurs in adult fish. In a
field study of Gulf killifish, livers from fish at sites that had measurable PAH concentrations showed upregulation of ahr1b and ahr2 receptors compared to nonoiled sites (Garcia et al. 2012), as well as upregulation of cyp1a1 in the gills. Other
AHR pathway genes such as cytochrome p450s and ugts were also upregulated in
the livers of killifish from contaminated sites. Upregulation of ugt has been observed
in other Fundulus studies and in the gills of juvenile Japanese flounder, Paralichthys
olivaceus (Whitehead et al. 2012a; Pilcher et al. 2014; Zhu et al. 2016). HEWAF
exposed Japanese flounder also showed significant effects on stress genes related to
AHR pathway activation (Zhu et al. 2016). Together, these studies have shown that
upregulation of AHR-mediated genes, in particular cyp1a1, is consistent transcriptomic responses to oil in many species and life stages and supports previous research
indicating the use of CYP as a biomarker for field studies assessing oil (Adams
1990; Schlenk et al. 2008b).
Transcriptomic studies performed in larval fish have also provided evidence that
changes in gene expression not associated with the AHR pathway may in part
explain abnormalities during early development. Altered cardiac gene expression,
particularly for genes involved in intracellular Ca
2+
homeostasis and excitationcontraction coupling, may directly contribute to cardiotoxicity in developing
embryos and larvae. In mahi-mahi embryos at 48 and 96 hpf, transcripts of calsequestrins, which are involved in Ca
2+
homeostasis, were upregulated after slick oil
exposure (Xu et al. 2016a). Downregulation of T-type receptor 2 (tnnt2) transcripts
was also observed. Tnnt2 regulates intracellular Ca
2+
concentrations (Metzger and
Westfall 2004). Diminished expression of transcripts encoding L-type and P/Q-type
voltage-dependent calcium channels was also observed following oil exposure, further suggesting alterations in Ca
2+
signaling (Xu et al. 2016a). Similarly, in haddock, transcripts of the cardiac sarcoplasmic reticulum Ca
2+
ATPase (atp2a2) and
ryr2, a ryanodine receptor that mediates cellular calcium release, were downregulated, as was the gene encoding the delayed rectifying K
+
channel, kcnh2, which is
D. S. Portnoy et al.