A Simple Method for Estimating the Spatiotemporal Distribution of Phenoloxidase Proteins in Insect Tissues

Background

Insects exhibit a remarkable diversity of body coloration patterns, showing extensive variation both among and within species. For example, the ladybird beetle Harmonia axyridis displays approximately 200 distinct color patterns within one species [1]. These patterns often function as warning coloration or mimicry and play important roles in ecological interactions and evolutionary adaptation. Insect color patterns are composed of various pigments, among which melanin pigments are the most widely used in color pattern formation [2]. In insects, the melanin biosynthesis pathway produces four major products—dopa melanin (black), dopamine melanin (brown), NBAD (yellow), and NADA (colorless)—through a highly conserved biochemical process [3].

Previous studies have shown that Lac2, a member of the PO family, catalyzes the reactions leading to the formation of the four major products of the melanin biosynthesis pathway and is essential for cuticle pigmentation and sclerotization [2]. Therefore, identifying the timing and spatial distribution of Lac2 protein expression is crucial for understanding the molecular mechanisms underlying insect melanin pattern formation. Conventional studies of PO activity have largely relied on enzymatic assays using hemolymph or homogenized tissues, typically quantifying total PO activity with substrates such as L-DOPA or dopamine [4,5]. However, these approaches do not provide spatial information on where PO proteins are distributed or potentially functional within tissues.

Identification of Lac2 protein distribution is technically challenging because Lac2 expression peaks immediately before adult eclosion, when the cuticle has already thickened and begun to harden [6]. At this stage, detection of mRNA or protein localization by in situ hybridization or immunostaining is difficult, and transgenic reporter approaches are not established for most non-model insects.

The molecular mechanisms underlying the activation of PO proteins have been well characterized. In insects, PO proteins are synthesized as an inactive precursor, proPOs, and are irreversibly activated through proteolytic cleavage at the N-terminal region by a protease cascade centered on serine proteases, which exposes the catalytic active site [7]. This activation process is tightly regulated in a developmental stage- and tissue-specific manner and is closely associated with immune responses and cuticle sclerotization [7–9]. In contrast, an in vitro method for artificial activation has been developed, in which isopropanol induces conformational changes in proPOs that expose the active site and activate the enzyme [10]. This activation mechanism differs from the physiological in vivo process but can be easily induced experimentally. Taking advantage of this property, True et al. [11] employed proPO as a transgenic reporter gene and established a method in which insect tissues are incubated in a staining solution containing isopropanol and dopamine, leading to artificial activation of proPO and subsequent dopamine melanin formation, thereby visualizing the spatial pattern of PO reporter activity. However, this approach has not been adapted to estimate the distribution of endogenous PO proteins.

Here, we established a protocol to estimate the distribution of endogenous PO proteins. This protocol is based on the method developed by True et al. [11] and allows estimation of the distribution of endogenous PO proteins based on the spatial pattern of dopamine melanin formed by PO activity staining. A major advantage of this protocol is that it does not require antibodies, transgenic lines, or specialized equipment, and clear staining patterns can be detected within 1–2 h. Because dopamine can be oxidized by multiple phenoloxidase/laccase-type enzymes [12], it is challenging to determine which enzymes are responsible. At late pupal stages, the genes encoding laccase-type enzymes (e.g., Lac2) are highly expressed in the epithelial tissues [5,6,13], but contributions from other oxidases cannot be excluded without genetic or biochemical validation. By applying this protocol across multiple developmental stages, the developmental dynamics of staining patterns can be tracked, which may allow estimation of the onset of spatially defined color pattern formation [1]. Furthermore, we have previously demonstrated that RNAs can be extracted from stained tissues [1]. By separating stained and unstained regions, the samples can be used for region-specific gene expression analysis, thereby facilitating the identification of genes expressed in a pattern-specific manner [1]. This approach enables the identification of genes involved not only in melanin biosynthesis but also in pattern-specific gene regulation.

The protocol has been validated in multiple non-model insect species, including coleopterans such as H. axyridis and Propylea japonica, as well as the lepidopteran Eurema hecabe. Therefore, this method represents a broadly applicable and versatile tool for advancing research on insect pigmentation, color pattern formation, and the evolution of color patterns.