Insect Endocrinology (eBook)

1.5 The Prothoracic Gland

In this section, an overview is presented of developmental changes in the prothoracic glands and factors, aside from conventional PTTH, that affect prothoracic gland function. As will be seen from the brief descriptions in the following sections, the field of peptide regulators of prothoracic gland function is rapidly expanding beyond what can be covered in a single chapter. Several chapters in this book including Chapter 2, Chapter 4 and Chapter 10, address important aspects of ecdysteroid secretion. A recent comprehensive review providing a broad perspective on prothoracic gland function can also be found in Marchal et al. (2010).

1.5.1 Developmental Changes in the Prothoracic Gland

A number of components in the PTTH transduction cascade change as the fifth larval instar progresses, as discussed in the previous section. These include an increase in cAMP phosphodiesterase activity, a decrease in the ERK phosphorylation response, an increase in the amount of the regulatory subunit of PKA, and a change in calmodulin sensitivity, rendering pupal adenylyl cyclase less responsive than the larval enzyme. The prothoracic glands undergo many other developmental changes. First, prothoracic gland cell size changes dynamically and rapidly accompanied by a number of structural changes. In Manduca and Spodoptera, cell size (i.e., diameter) is roughly correlated with ecdysteroidogenic capacity during the last larval instar with large-celled glands the most productive (Sedlak et al., 1983; Zimowska et al., 1985; Hanton et al., 1993). While the prothoracic gland cells do not divide, DNA synthesis occurs, and prothoracic gland cells have been identified as polyploid (Samia, Oberlander et al., 1965; Oncopeltus, Dorn and Romer, 1976; Bombyx, Gu and Chow, 2001, 2005a). As expected, as cell size increases, the protein content of the gland also increases. In Manduca, total extractable and particulate protein increases from a minimum at the beginning of the fifth instar of ≈3 μg/gland to ≈20 μg/gland on V4; protein levels stay high through V7 before reaching a low of 10–12 μg/gland on P0 (Smith and Pasquarello, 1989; Meller et al., 1990; Lee et al., 1995; Rybczynski and Gilbert, 1995b). Protein content rises again through at least P4 (Rybczynski and Gilbert, 1995b). Although size matters, the ecdysteroidogenic potential of prothoracic glands is not a simple function of cell size or protein content. An analysis of protein content and ecdysteroidogenesis in V1 to V3 Manduca fifth instar prothoracic glands revealed that basal and PTTH-stimulated synthesis per microgram protein increased during this period; that is, ecdysteroid synthesis per gland increased faster than did the gland’s total protein content (Rybczynski and Gilbert, 1994). A re-analysis of data gathered throughout the fifth instar of Manduca (provided by Smith and Pasquarello, 1989) confirms this observation, and revealed that a peak of basal ecdysteroid synthesis per unit protein occurred on V4. Levels after V4 declined but never reached the low seen on V1. PTTH-stimulated synthesis per microgram protein exhibited a different pattern. A peak was again seen on V4, but levels were high and relatively similar from V2 through V7, followed by a dip on V8 and V9, and then a sharp rise to near V4 levels on P0. Note that PTTH-stimulated synthesis per microgram protein was essentially the same on V2 and V3 using the data from Smith and Pasquarello (1989), while Rybczynski and Gilbert (1994) found V2 levels to be considerably lower than V3 levels.

An additional developmental change in prothoracic glands has been described for Bombyx. Bombyx prothoracic glands from the early fifth instar do not respond to PTTH stimulation with either increased ecdysteroid or cAMP synthesis, and are also distinguished by an exceedingly low basal level of steroid synthesis in vitro (Sakurai, 1983; Gu et al., 1996; Gu and Chow, 2005b). However, these glands apparently respond with increased ecdysteroid synthesis to dbcAMP, suggesting that there is an upstream developmental “lesion” in one or more components of the PTTH signaling pathway (Gu et al., 1996, 1997; Gu and Chow, 2005b). In contrast, Takaki and Sakurai (2003) found that dbcAMP did not elicit ecdysteroid synthesis in early fifth instar glands. The reason for the discrepancy in results is not known but might lie in methodological or strain-specific differences among the studies. Further work indicated that early fifth instar glands from Bombyx possess an adenylyl cyclase that is much less responsive to Ca2+/calmodulin stimulation than that characterized later in the instar when the gland is responsive to PTTH (Chen et al., 2001). It is not known if this cyclase difference is sufficient to explain fully the refractory nature of early fifth instar glands, nor is it known what underlies the cyclase behavior.

Regardless of the site of refractoriness, Bombyx glands must regain steroidogenic capacity and ability to respond to PTTH. It appears that an absence of JH and ecdysteroids leads to this change, with no requisite involvement of a trophic factor beyond the glands themselves (Sakurai, 1983). Direct evidence for this was obtained by Mizoguchi and Kataoka (2005) using long-term cultures of Bombyx prothoracic glands. In this study, glands from fourth instar larvae exposed to 20E for 24h in culture gradually lost ecdysone secretory activity, while those not exposed to 20E remained active. Newly ecdysed prothoracic glands were inactive for 24h in culture and then began to spontaneously secrete ecdysone. Glands co-cultured with a source of JH, in the form of corpora allata, remained inactive for 8 days. Yamanaka et al. (2007) also saw increased steroidogenic cytochrome P450 gene expression in long-term prothoracic gland cultures. Gene expression was inhibited by co-culture with CA, similar to the acquisition of steroidogenic capacity and the sensitivity to JH seen by Mizoguchi and Kataoka (2005).

The prothoracic glands are sensitive to changes in nutritional state (see Section 1.5.2.1.). Based on abundant evidence from Drosophila, the insect insulin-like hormones have taken center stage as systemic nutrient signals, regulating growth of the glands as well as other tissues (see Chapter 2 and Section 1.5.2.1.). The nutritional regulation of prothoracic gland growth in Lepidoptera is also well documented, although the factors responsible are not well defined. Chen and Gu (2006, 2008) found that starvation inhibits DNA synthesis in the prothoracic glands of Bombyx until day 3 of the fifth instar. Starvation also blocks protein synthesis (Chen and Gu, 2006) and ecdysone secretion (Gu and Chow, 2005b). After day 3, ecdysteroidogenesis occurs in glands from starved larvae, despite low protein levels, and is actually accelerated by about a day relative to controls. This is not accompanied by an increase in PTTH release, assessed by bioassay. Responsiveness to PTTH and exogenously added cAMP analogs are not affected. Drosophila similarly accelerates pupation upon starvation after acquiring an appropriate body weight (Mirth et al., 2005) as does the beetle Onthophagus (Shafiei et al., 2001). The underlying regulatory mechanisms have not been identified.

As an activator of receptor tyrosine kinase activity, PTTH is well suited to promote glandular growth. Functionally homologous vertebrate peptide hormones, acting alone or in concert with other factors, often have a differentiation and mitogenic effect on their target tissues in addition to their steroidogenic activity (see Adashi, 1994; Richards, 1994; Richards et al., 2002). PTTH was shown to stimulate two categories of protein synthesis in the Manduca prothoracic glands: one class of proteins comprised a small group of molecules whose translation appears to be linked to PTTH-stimulated ecdysteroidogenesis (see Section 1.4.4.), while the second class of translation products was less well defined and thought to include housekeeping proteins in the prothoracic gland (Rybczynski and Gilbert, 1994). Synthesis of the second class of products was enhanced by PTTH, but not by Ca2+ and/or cAMP. Given that the PTTH preparation used in these experiments was not purified, it is likely that factors other than PTTH contributed to the observed response (Rybczynski and Gilbert, 1994).

Vertebrate steroidogenic hormones enhance the transcription of steroidogenic enzymes and associated proteins (see Simpson and Waterman, 1988; Orme-Johnson, 1990), and the same has proven true for PTTH. Many of the enzymes responsible for conversion of cholesterol to 20E have now been identified (see Chapter 4 and Huang et al., 2008). Briefly, cholesterol is converted to 7-dehydrocholesterol by the action of a non-rate-limiting dehydrogenase. An enzyme encoded by a gene termed neverland (nvd) has been implicated in this step. Then 7-dehydrocholesterol is converted to a diketol by reactions termed the “black box.” The enzymes encoded by the cytochrome P450 Halloween genes, spook and spookier (spo), have been implicated here. The diketol is...