Insulin-like material from the digestive tract of the tunicate Pyura pachydermatina (sea tulip)

New Neuronal Subtypes With a "Pre-Pancreatic" Signature in the Sea Urchin Stongylocentrotus purpuratus

Neurons and pancreatic endocrine cells have a common physiology and express a similar toolkit of transcription factors during development. To explain these common features, it has been hypothesized that pancreatic cells most likely co-opted a pre-existing gene regulatory program from ancestral neurons. To test this idea, we looked for neurons with a "pre-pancreatic" program in an early-branched deuterostome, the sea urchin. Only vertebrates have a proper pancreas, however, our lab previously found that cells with a pancreatic-like signature are localized within the sea urchin embryonic gut. We also found that the pancreatic transcription factors Xlox/Pdx1 and Brn1/2/4 co-localize in a sub-population of ectodermal cells. Here, we find that the ectodermal SpLox+ SpBrn1/2/4 cells are specified as SpSoxC and SpPtf1a neuronal precursors that become the lateral ganglion and the apical organ neurons. Two of the SpLox+ SpBrn1/2/4 cells also express another pancreatic transcription factor, the LIM-homeodomain gene islet-1. Moreover, we find that SpLox neurons produce the neuropeptide SpANP2, and that SpLox regulates SpANP2 expression. Taken together, our data reveal that there is a subset of sea urchin larval neurons with a gene program that predated pancreatic cells. These findings suggest that pancreatic endocrine cells co-opted a regulatory signature from an ancestral neuron that was already present in an early-branched deuterostome.

The comparative anatomy of islets

In the past 20 years, numerous publications on a variety of mammalian and non-mammalian species have appeared in the literature to supplement the excellent comparative work performed in the 70s and 80s by the Falkmer, Epple, and Youson groups. What emerges is that islets are much more complex than once thought and show a lot of similarities in rodents and higher primates. The diversity of lifestyles, metabolic demands, and diets has most likely influenced the great diversity in both structure and cell-type content of islets in lower vertebrate species. In this chapter, I try to provide an overview of the evolution from endocrine cell types in invertebrates to the higher mammals and focus on what has been reported in the literature and some of our own experiences and also include a description of other hormones reported to be found in islets.

Endocrinology of protochordates

Large-scale gene duplications occurred early in the vertebrate lineage after the split with protochordates. Thus, protochordate hormones and their receptors, transcription factors, and signaling pathways may be the foundation for the endocrine system in vertebrates. A number of hormones have been identified including cionin, a likely ancestor of cholecytokinin (CCK) and gastrin. Both insulin and insulin-like growth hormone (IGF) have been identified in separate cDNAs in a tunicate, whereas only a single insulin-like peptide was found in amphioxus. In tunicates, nine distinct forms of gonadotropin-releasing hormone (GnRH) are shown to induce gamete release, even though a pituitary gland and sex steroids are lacking. In both tunicates and amphioxus, there is evidence of some components of a thyroid system, but the lack of a sequenced genome for amphioxus has slowed progress in the structural identification of its hormones. Immunocytochemistry has been used to tentatively identify a number of hormones in protochordates, but structural and functional studies are needed. For receptors, protochordates have many vertebrate homologs of nuclear receptors, such as the thyroid, retinoic acid, and retinoid X receptors. Also, tunicates have cell surface receptors including the G-protein-coupled type, such as β-adrenergic, putative endocannabinoid, cionin (CCK-like), and two GnRH receptors. Several tyrosine kinase receptors include two epidermal growth factor (EGF) receptors (tunicates) and an insulin/IGF receptor (amphioxus). Interestingly, neither steroid receptors nor a full complement of enzymes for synthesis of sex steroids are encoded in the Ciona genome. Tunicates appear to have some but not all of the necessary molecules to develop a vertebrate-like pituitary or complete thyroid system.

Some of my not so favorite things about insulin and insulin-like growth factors in fish

Several topics concerning insulin and IGFs rarely discussed among comparative physiologists/endocrinologists are reviewed. The topics addressed include interpretation of radioimmunoassays for insulin and IGFs, the major sites of synthesis of these peptides, signal transduction via peptide-specific, and possibly via related peptide receptors. Finally, some thoughts are included on the development of the insulin superfamily of peptides in two phyletic lines of evolution, proto- and deuterostomian animals. The author considers all the above topics open to further exploration. If this review initiates more discussion leading to unorthodox input to the field, its goal is achieved.

Ancient divergence of insulin and insulin-like growth factor

Studies on the evolutionary pathway of the insulin gene family suggest that insulin and insulin-like growth factor (IGF) became distinct molecules only after the vertebrates arose. A single molecule with identity to both insulin and IGF was reported in amphioxus. To study the origin of insulin, we selected tunicates because their ancestors are thought to be a nodal point in the evolution of vertebrates. This is the first report of separate insulin and IGF molecules from invertebrates. Two cDNAs were isolated from the tunicate Chelyosoma productum: One cDNA encodes a distinct preproinsulin with B, C, and A domains, whereas the other encodes tunicate preproIGF, including all five domains in their proper sequence. Both mRNAs are expressed in the nervous system, digestive tract, heart, and possibly the gonad but not in branchial basket or tunic. Hence, insulin and igf genes have similar expression patterns. In situ methods confirm the polymerase chain reaction evidence that tunicate insulin and igf mRNAs are expressed in cortical cells of the neural ganglion. We conclude that insulin and IGF have maintained separate gene lineages in both vertebrate and protochordate evolution and, thus, a distinct evolutionary history of more than 600 million years.

Insulin-like immunoreactivity in the brain of two hagfishes, Eptatretus stouti and Myxine glutinosa

Immunocytochemical investigation of the brains of hagfish Myxine glutinosa and Eptatretus stouti with antisera raised against salmon insulin revealed the presence of groups of immunoreactive cells discretely localized in the mid- and hindbrain of both species. Subpopulations of these cells reacted weakly with antisera against Myxine islet insulin and equivocally with anti-bovine insulin serum. Extracts prepared from Myxine brain were subject to gel filtration and were found by radioimmunoassay to contain two forms of insulin-like material, one of large molecular weight (less than 66 kDa) and another smaller molecule (6 kDa). The relationship of these molecules to the insulin-related growth factor family of neurohormonal peptides is discussed and their potential function assessed in terms of a possible homology with Muller-type cells and involvement in axonal regeneration phenomena.