Bag Cell Peptides

Definition
Bag cell peptides (BCPs) are a class of small neuropeptides secreted by the bag cell neurons in the marine mollusk Aplysia1. They trigger a series of reproductive behavior in this mollusk that finally culminates in egg-laying1.

Discovery
BCPs were originally identified in the bag cell extracts of abdominal ganglion of mature Aplysia that were obtained in Venice, CA2. They were isolated based on their ability to cause bag cell excitation2.

Classification
Bag cell specific gene that encodes for the precursor form of the egg laying hormone also contains sequences that encode several small BCPs: a-BCP, b-BCP, g-BCP2, d-BCP3,4 and e-BCP5.

Structural Characteristics
a-BCP is a 9 amino acid (Ala-Pro-Arg-Leu-Arg-Phe-Tyr-Ser-Leu) neuro tramsmittor whose Phe6-Tyr7 is necessary and sufficient for its function6. It does not undergo any NH2 or COOH modifications. b-BCP (Arg-Leu-Arg-Phe-His)7 and g-BCP are five amino acid peptides while d-BCP and e-BCP contain 39 and 19 amino acid residues respectively5.

Mode of action
Excitation of Bag Cell neurons triggers an afterdischarge response that results in the secretion of BCPs8. In addition, once the afterdischarge is underway, peptide release occurs in response to both calcium influx from the extracellular space and calcium release from intracellular stores8. Upon secretion the BCPs are packaged into discrete vescicles that are distributed throughout the central nervous system of Aplysia where they exert their functions4.

Functions
Release of egg-laying hormone and BCPs mediates the egg laying behavior in Aplysia4. Egg laying behavior is a result of several neuronal responses that include burst augmentation of cell R15, prolonged excitation of left lower quadrant neurons and inhibition of left upper quadrant neurons. BCPs have several functions in the bag cells2. a-BCP inhibits the left upper quardant cells, b-BCP excites L1, R1 and bag cells in the abdominal ganglion of Aplysia, g-BCP excites bag cells and d-BCP stimulates calcium release from the mitochondria5. This elecrophysiological chnages to the bag cells by the BCPs and other hormones culminates in egg-laying.

References
1.Rothman BS, Weir G, Dudek FE (1983a). Egg-laying hormone: direct action on the ovitestis Aplysia. Gen Comp Endocrinol, 52,134-141.
2.Rothman BS, Mayeri E, Brown RO, Yuan PM, Shively JE (1983b). Primary structure and neuronal effects of alpha-bag cell peptide, a second candidate neurotransmitter encoded by a single gene in bag cell neurons of Aplysia. Proc Natl Acad Sci, 80, 5753-5757.
3.Fisher JM, Sossin W, Newcomb R, Scheller RH (1988). Multiple neuropeptides derived from a common precursor are differentially packaged and transported. Cell, 54,813-822.
4.Hatcher NG, Sweedler JV (2008). Aplysia bag cells function as a distributed neurosecretory network. J. Neurophysiol.. 99, 333-343.
5.Gregg TN, Sherry DP, and James EB (1989). The Egg-Laying Hormone Family: Precursors, Products, and Functions. Bio Bull, 177, 210-217.
6.Owens DF, Menon JG, Rotham BS (1992). Structure-activity relationship of the neurotransmitter alpha-bag cell peptide on aplysia LUQ neurons: implications regarding its inactivation in the extra cellular space. J of Neurobiology, 6, 650-70.
7.Rothman BS, Dekruyfft S, Talebian SM, Menon JG, Squire CR, Yehll CH, and Lee TD (1992). Aplysia Peptide Neurotransmitters & Bag Cell Peptide, Phe-Met-Arg-Phe- amide, and Small Cardioexcitatory Peptide B Are Rapid Degraded by a Leucine Aminopeptidase-like Activity in Hemolymph, J Biol. Chem., 287, 35, 25135-140.
8.Karen JL, Ronald JK, John AC and Leonard KK (2004). Hyperosmotic media inhibit voltage-dependent calcium influx and peptide release in Aplysia neurons, J Memb. Biol., Vol 128, 41-52.