chapter 1 - Bentham Science

96 HPFP: Recent Advances in Insects and Other Arthropods Vol. 1 Makio Takeda
In mammalian intestinal tract, Notch proteins comprise a family of 4 transmembrane receptors that interact
with cell surface ligands from the neighboring cells. The Notch ligand binding induces cleavage of the
Notch intracellular domain (NICD) that nuclear-translocates, as in neuroblast differentiation, in association
with DNA-binding protein, RBP-Kκ or delta 1. The latter is a transcription factor that activates HES, bHLH
transcription repressors by binding to the N-box of its promoter. Hes1 inhibits bHLH factors such as Math1
and then NGN3, and BETA2/NeuroD that are required for differentiation of paraneurons. These
transcription factors are homologs of Drosophila’s atonal. NGN3 is a downstream target of MAS1.Several
transcription factors belonging to the bHLH, homeodomain, i.e., HTH and paired box homeodomain
families are responsible for determining the several types of peptide-producing cells. BETA2, a
downstream target of NGN3 binds to the E-box of CCK, and secretin genes. Pdx, homeodomain protein
regulates Insulin, Gastrin and Somatostatin genes in the endocrine pancreas. Pax 4 and 6 are paired box
transcription factors and affect differentiation of Glucagon related genes. BETA2 is also involved in cell
cycle arrest, increasing p21, an inhibitor of CDK. It is not known whether these mechanisms occur also in
differentiating insect midgut paraneurons.
The formation of the small intestine of mammals and the midgut of Drosophila share some common
features [20]. Similarities are also observed between the mammalian large intestine and the insect hindgut
[20]. Mammalian epithelial cells are classified either as differentiated cell such as (1) enterocytes (EC), (2)
goblet cells, (3) enteroendocrine (ee) and (4) Paneth cells that are immune cells or (5) stem cells such as
CBC(crypt base columnar) cells at the bottom of the crypt and (6) TA (transient amplifying) cells above +4
position. These multiply at different rates and fulfill two distinct needs for the repair and fidelity of
proliferation. Drosophila midgut has only one type of stem cell, called ISCs (intestinal stem cells) located
immediately adjacent to the basement membrane. An ISC asymmetrically divides into two cells in a semiconservative
way producing a new ISC and an immature daughter cell called an enteroblast. ISCs have
small diploid nuclei expressing Delta, the ligand of Notch. Enteroblasts (EBs) also have small diploid
nuclei expressing suppressor of Hairless, Su(H), as shown by Su(H) GBE-lacZ, a transcriptional reporter of
the Notch signaling pathway. The Hedgehog-BMP relay signal promotes stem cell differentiation in
mammalian small intestine but stimulates cell proliferation via interactions with mesenchyme [21]. The
Wnt (a mammalian homolog of Wg) signal is a master regulator of stem cell-renewal and stem cell
maintenance [20]. The Wnt pathway is activated both in CBC cells and Paneth cells. This condition may be
conserved between the two lineages, fly and mammals [22]. Fly EBs differentiate to ECs that have a large
polyploid nucleus, expressing prospero.
PDM-1-, NPC- and Myosin IA-expressing cells correspond to enteric paraneurons in Drosophila [23]. Pros
are also expressed in neural elements when ganglional mother cells undergo neuron/glial asymmetric
division [24-25]. Miranda associates with pros in the asymmetric division in the midgut also. However,
vertebrate enteric paraneurons are of endodermal origin but Pros/miranda are known as neural-specific
markers [26]. Therefore, these suggest that pros+cells are probably paraneurons in the midgut. I should note
that Marcia Loeb produced antibodies to MDFs and one of them [at least MDF4] cross-reacted with a part
of the cell population that expressed Prospero-ir in Drosophila midgut (unpublished data).
Table 1 shows that immunohistochemical reactivity against a variety of mammalian-like neuropeptides,
neurotransmitters, growth factors etc., both in the epithelium and basal mucosa of insects belonging to
almost all orders with references. Early works employed antibodies against mammalian peptides but the
insect antigens are often distinct peptides belonging to the same superfamily and therefore new
nomenclature has been applied recently. It has been determined that some of the relationships overlap. For
example BPP-like immunohistochemical reactivity (-ir) and NPF-ir are the same and so are Substance-P-ir
and Tachykinin (TK)-ir.
Numerous mammalian peptides-irs have been demonstrated in the midgut of insects [39, 28, 50, 46]. The
immunopositive areas were specific to particular sites within each cell, as in the case of mammalian models
[53, 45-46] even in the case of three allatostatins, A, B and C, both in adults and maggots of D.
melanogaster. Regional specificities can also shift drastically between larval and adult stages [45-46].
Using 7 antibodies, Veenstra [45] could specify immunopositive regions and cell types in D. melanogaster.