Involvement of Leptinotarsa hormone receptor 38 in the larval-pupal transition

Highlights

• Knockdown of LdHR38 inhibits ecdysteroidogenesis in third instar larvae.

• Silencing LdHR38 intensify the expression of early ecdysone genes but represses LdFTZ-F1.

• Depletion of LdHR38 impairs larval-pupal ecdysis.

• Silencing LdHR38 at the fourth instar causes similar but less severe defects.

Abstract

In insects, nuclear receptors (NRs) including EcR (NR1H1), USP (NR2B4), E75 (NR1D3), HR3 (NR1F), HR4 (NR6) and FTZ-F1 (NR5A3) mediate the 20-hydroxyecdysone (20E) signaling cascade to play a critical role during larval metamorphosis. In this present paper, we focused on hormone receptor 38 (HR38) in Leptinotarsa decemlineata, the only insect homolog of the NR4A subclass. RNA interference (RNAi) of LdHR38 in the penultimate (third) instar larvae reduced the expression of an ecdysteroidogenesis gene and declined the titer of 20E. Knockdown of LdHR38 intensified the expression of LdUSP, LdE75, LdE74, LdE93, LdBroad and LdHR3, whereas repressed the transcription of LdFTZ-F1. Disruption of 20E signaling inhibited chitin biosynthesis in the larval cuticle. Approximately 25% of the LdHR38 RNAi larvae died, around 40% of the resultant larvae remained as prepupae or become deformed pupae. The body surface of the HR38 depleted abnormal prepupae and pupae looked wet, just like the cuticle being covered with a layer of liquid. Moreover, the increase of larval mortality, and the impairment of pupation and emergence exhibited dose-dependent manners. Furthermore, silencing LdHR38 at the final (fourth) instar caused similar but less severe impairment of pupation. Dietary supplement with 20E for the third instar larvae did not rescue the high larval death and only slightly alleviated the low pupation rate in the LdHR38 RNAi hypomorphs. Accordingly, we propose that HR38 is necessary for tune of ecdysteroidogenesis and for mediation of 20E signaling during metamorphosis in L. decemlineata.

Introduction

Nuclear receptors (NRs) are a family of ligand-regulated transcription factors defined by a conserved zinc finger DNA-binding domain (DBD) and a ligand-binding domain (LBD) (Chawla et al., 2001). In insect species, seven NR subfamilies are currently recognized (NR0 through NR6) and members of all subfamilies have been identified based on structural as well as functional data (Laudet, 1997, Aranda and Pascual, 2001).

In holometabolan insects, NRs are involved in the mediation of the 20-hydroxyecdysone (20E) signaling cascade, a critical hormonal way that elicits larval-pupal transition (Nijhout and Williams, 1974, Nijhout, 1976, Meng et al., 2018, Meng et al., 2019). In Leptinotarsa decemlineata, as well as in other insect species, a heterodimer of ecdysone receptor (EcR, NR1H1) and ultraspiracle (USP, NR2B4) (Xu et al., 2019a, Xu et al., 2019c), upon binding the ligand 20E, triggers the expression of a cascade of genes, including ecdysone-induced protein 75 (E75, NR1D3), hormone receptor 3 (HR3, NR1F) and hormone receptor 4 (HR4, NR6) (Guo et al., 2015, Guo et al., 2016, Xu et al., 2018b). HR3 plays a key role during metamorphosis by repressing early genes, and directly induces the fushi tarazu factor-1 (FTZ-F1) gene that encodes a prepupal regulator FTZ-F1 (NR5A3) (Liu et al., 2014, Guo et al., 2015). HR4 is involved in the regulation of 20E biosynthesis and mediation of 20E signaling during larval-pupal transition (Xu et al., 2018b).

In this study, we focused on hormone receptor 38 (HR38), the only member of the NR4A subclass (Fisk and Thummel, 1995, Sutherland et al., 1995, Zhu et al., 2000). In Drosophila melanogaster, some DHR38 homozygotes die during larval (Kozlova et al., 1998, Kozlova et al., 2009), or pharate adult (Kozlova et al., 2009) stages. In Tribolium castaneum, RNAi of HR38 causes mortality during larval-pupal-adult metamorphosis (Tan and Palli, 2008a). These results demonstrate the role of HR38 in metamorphosis (Kozlova et al., 1998, Kozlova et al., 2009). However, the specific roles of HR38 during larval-pupal-adult molts are not fully determined.

Insect HR38 is the ortholog of the mammalian nerve growth factor induced protein B (NGFI-B) subfamily (Sutherland et al., 1995), which includes NGFI-B (NR4A1), Nurr1 (NR4A2), and NOR1 (NR4A3) (Close et al., 2013, Kurakula et al., 2014). Mammalian NR4A members are associated with the regulation of steroidogenesis (Maxwell and Muscat, 2006). For example, Nur77 and Nurr1 play key roles in regulating basal and hormone-induced gene expressions in steroidogenic cells (Maxwell and Muscat, 2006). In Bombyx mori, treatment of prothoracic glands (PGs) with prothoracicotropic hormone (PTTH) in vitro rapidly and transiently induces HR38 expression and pretreatment with cycloheximide does not block subsequent induction by PTTH. Injection of PTTH into day-5 last instar larvae also greatly elevates HR38 expression (Gu et al., 2016). Is HR38 involved in the regulation of ecdysteroidogenesis in larvae?

In D. melanogaster, HR38 can form heterodimer with either EcR (Zoglowek et al., 2012) or USP (Sutherland et al., 1995, Baker et al., 2003). Moreover, HR38s from B. mori and Aedes aegypti interact strongly with USP (Sutherland et al., 1995, Zhu et al., 2000). In D. melanogaster and A. aegypti, both HR38-USP and EcR-USP heterodimers can bind to EcRE of ng-1 and ng-2 intermolt genes (Crispi et al., 1998, Zhu et al., 2000). This suggests that HR38 may interfere with the 20E signaling pathway not only by disrupting the formation of the EcR-USP heterodimer, but also by competing for binding to EcRE sites. Does knockdown of HR38 disturb in vivo 20E signal to impair larval-pupal metamorphosis?

Drosophila HR38 may play an important role in cuticle formation and melanization (Kozlova et al., 1998, Sekine et al., 2011). Its ectopic expression intensifies the transcription of a cuticle gene (Acp65A) (Bruey-Sedano et al., 2005, Cui et al., 2009) and downregulates the expression of Ddc (encoding dopa decarboxylase) (Davis et al., 2007). In contrast, HR38 depletion diminishes the expression of 3 cuticle genes (Cpr92A, CG6240; Cpr49Ae, CG8505; Acp65A, CG10297) (Kozlova et al., 2009), but upregulates the transcription of Ddc (Kozlova et al., 2009). Thus, Drosophila HR38 appears to regulate genes that direct the deposition of both flexible and rigid components of the cuticle (Bruey-Sedano et al., 2005, Karouzou et al., 2007). However, the role of HR38 in cuticle formation and melanization has not been confirmed in any non-Drosophila insect species.

In L. decemlineata, the main ecdysteroidogenesis (Wan et al., 2013, Kong et al., 2014) and 20E signaling (Ogura et al., 2005, Guo et al., 2015, Guo et al., 2016, Xu et al., 2018a, Xu et al., 2018b) genes have been identified. In the present paper, we intended to address the three issues. We silenced HR38 gene and compared the defective phenotypes. Our results suggested the roles of HR38 in tune of ecdysteroidogenesis and mediation of 20E signaling in L. decemlineata.