Galanin

Discovery

Galanin was first identified from porcine intestinal extracts in 1978 by Professor Viktor Mutt and colleagues at the Karolinska Institute, Sweden using a chemical assay technique that detects peptides according to its C-terminal alanine amide structure. Galanin is so-called because it contains an N-terminal glycine residue and a C-terminal alanine. The structure of galanin was determined in 1983 by the same team, and the cDNA of galanin was cloned from a rat anterior pituitary library in 1987.

Tissue distribution

Galanin is located predominantly in the central nervous system and gastrointestinal tract. Within the central nervous system, highest concentrations are found in the hypothalamus, with lower levels in the cortex and brainstem. Gastrointestinal galanin is most abundant in the duodenum, with lower concentrations in the stomach, small intestine, and colon.

Structure

Galanin is a peptide consisting of a chain of 29 amino acids (30 amino acids in humans) produced from the cleavage of a 123-amino acid protein known as prepro galanin, which is encoded by the GAL gene. The sequence of this gene is highly conserved among mammals, showing over 85% homology between rat, mouse, porcine, bovine, and human sequences. In these animal forms, the first 15 amino acids from the N-terminus are identical, but amino acids differ at several positions on the C-terminal end of the protein.

These slight differences in protein structure have far-reaching implications on their function. For example, porcine and rat galanin inhibit glucose-induced insulin secretion in rats and dogs but have no effect on insulin secretion in humans. This demonstrates that it is essential to study the effects of galanin and other regulatory peptides in their autologous species.

The galanin family of protein consists of four proteins, of which GAL was the first to be identified. The second was galanin message-associated protein (GMAP), a 59- or 60-amino acid peptide also formed from the cleavage of prepro galanin. The other two peptides, galanin-like peptide (GALP) and alarin, were identified relatively recently and are both encoded for in the same gene, the prepro GALP gene. GALP and alarin are produced by different post-transcriptional splicing of this gene.

Receptors

Galanin signalling occurs through three classes of receptors, GALR1, GALR2, and GALR3, which are all part of the G protein-coupled receptor (GPCR) superfamily. Galanin receptors are expressed in the central nervous system, in the pancreas, and on solid tumours. The level of expression of the different receptors varies at each location, and this distribution changes after injury to neurons. Experiments into the function of the receptor subtypes involve mostly genetic knockout mice. The location of the receptor and the combination of receptors that are inhibited or stimulated heavily affect the outcome of galanin signalling.

Alzheimer's disease

One of the pathological features of the brain in the later stages of Alzheimer's disease is the presence of overgrown GAL-containing fibres innervating the surviving cholinergic neurons. Another feature is an increase in the expression of GAL and GAL receptors, in which increases of up to 200% have been observed in postmortem brains of Alzheimer's patients. The cause and role of this increase is poorly understood.

It has been suggested that the hyper-innervation acts to promote the death of these neurons and that the inhibitory effect of galanin on cholinergic neurons worsened the degeneration of cognitive function in patients by decreasing the amount of acetylcholine available to these neurons.

A second hypothesis has been generated based on data that suggest GAL is involved in protecting the hippocampus from excitotoxic damage and the neurons in the cholinergic basal forebrain from amyloid toxicity. It is interesting to note that studies of gene expression of CBF tissue suggests that the hyperinnervation of cholinergic neurons by GAL up regulates the transcription of factors that promote neuron function and survival. It is still unclear as to whether galanin acts to protect cholinergic neurons and promote their firing or whether it worsens the symptoms of this disease.

Epilepsy

Galanin in the hippocampus is an inhibitor of glutamate but not of GABA. This means that galanin is capable of increasing the seizure threshold and, therefore, is expected to act as an anticonvulsant. To be specific, GalR1 has been linked to the suppression of spontaneous seizures. An agonist antiepileptic drug candidate is NAX 5055.

In development

It has been shown that galanin plays a role in the control of the early post-natal neural development of the dorsal root ganglion (DRG). Galanin-mutant animals show a 13% decrease in the number of adult DRG cells as well as a 24% decrease in the percentage of cells expressing substance P. This suggests that the cell loss by apoptosis that usually occurs in the developing DRG is regulated by galanin and that the absence of galanin results in an increase in the number of cells that die.

After injury

In vitro experiments show that DRG cells removed from galanin mutants have impaired abilities to extend neurites in culture, in that the number of cells producing neurites is decreased by a third and the mean length of these processes was halved when compared to wild-type controls. In vivo, many of the actions of galanin in the brain after an injury are similar to those observed in the developing DRG. Adult mutant animals have been shown to be 35% less capable of regenerating the sciatic nerve after crush injury, which is linked to long-term functional problems.

Parental role in mice

Galanin-expressing neurons in the medial preoptic area of the brain would be responsible for regulating aggression towards pups by male mice.