Unsaturated glycoceramides as molecular carriers for mucosal drug delivery of GLP-1

Abstract

The incretin hormone Glucagon-like peptide 1 (GLP-1) requires delivery by injection for the treatment of Type 2 diabetes mellitus. Here, we test if the properties of glycosphingolipid trafficking in epithelial cells can be applied to convert GLP-1 into a molecule suitable for mucosal absorption. GLP-1 was coupled to the extracellular oligosaccharide domain of GM1 species containing ceramides with different fatty acids and with minimal loss of incretin bioactivity. When applied to apical surfaces of polarized epithelial cells in monolayer culture, only GLP-1 coupled to GM1-ceramides with short- or cis-unsaturated fatty acids trafficked efficiently across the cell to the basolateral membrane by transcytosis. In vivo studies showed mucosal absorption after nasal administration. The results substantiate our recently reported dependence on ceramide structure for trafficking the GM1 across polarized epithelial cells and support the idea that specific glycosphingolipids can be harnessed as molecular vehicles for mucosal delivery of therapeutic peptides.

Introduction

One of the major difficulties in effectively treating Type II diabetes and other chronic diseases involves patient compliance for lifelong management by frequent injections of therapeutic peptides or proteins. Two of the most potent therapeutic peptides used to control blood glucose in Type II diabetes, insulin and analogs of the incretin hormone GLP-1, are impermeant to epithelial barriers at mucosal surfaces and require delivery by subcutaneous (SC) injection. Optimal control of blood sugar levels by these agents entails frequent daily injections, which can be sometimes missed by patients due to lifestyle and the social stigma of self-injection in public. Thus, much effort has gone into the development of technologies to render these therapeutic peptides absorbable across mucosal surfaces for efficient delivery without self-injection and with improved patient compliance. Here, we test if our recent discoveries on glycosphingolipid trafficking in polarized epithelial cells [1], [2] can be applied to convert GLP-1 into a molecule suitable for mucosal absorption.

The gastrointestinal, respiratory, and genitourinary tracts represent vast mucosal surfaces where host tissues are separated from the environment only by a delicate but highly effective single layer of columnar epithelial cells, joined by tight junctions that are impermeable to proteins and even small peptides. Proteins non-specifically taken up into the epithelial cell by endocytosis are generally transported to lysosomes for degradation, establishing an effective barrier to uncontrolled entry of these materials into the body and the maintenance of systemic homeostasis. So far, the lack of rational and efficient methods to circumvent this barrier has prevented the application of most therapeutic proteins for efficient mucosal drug delivery.

Glucagon-like peptide-1 (GLP-1) is an incretin hormone derived from the transcription product of the pro-glucagon gene and is primarily released from L-cells of the intestinal mucosa [3]. This hormone has lasting and potent antihyperglycemic actions by inducing glucose-dependent stimulation of insulin secretion while suppressing glucagon secretion, as well as restoring glucose sensitivity of pancreatic β-cells while it inhibits gastric secretion and motility to protract carbohydrate absorption and contribute to satiety [4]. Once in the circulation, GLP-1 is rapidly degraded (with a half-life of less than 2 min). Thus, the majority of pharmaceutical approaches have been focused on developing longer-acting analogs of GLP-1 [5]. Exenatide, a synthetic form of the peptide exendin-4 from lizard saliva, is a potent GLP-1 receptor (GLP-1R) agonist on the market for Type 2 diabetes [6] and even has been developed into a once-weekly formulation by encapsulation in Poly-(d,l-Lactide-Co-Glycolide) Microspheres [7], [8]. Another marketed example is liraglutide, an acylated human GLP-1 analog [9]. Although these degradation-resistant GLP-1R agonists are excellent agents for the treatment of diabetes, they still require daily to weekly delivery by injection. Developing an oral long-acting GLP-1 formulation would allow patients to manage their blood glucose levels by a non-parenteral form of administration.

We recently discovered that the structure of the ceramide domain of the glycosphingolipid GM1, the receptor for cholera toxin that mediates cell entry, dictates intracellular trafficking and the subcellular localization of this lipid in mammalian cells [1], [2]. GM1 is comprised of two major domains: the hydrophilic oligosaccharide head group that protrudes from the cell membrane into the extra-cellular space; and the hydrophobic ceramide lipid tail that anchors the molecule within the membrane bilayer. Native GM1 ceramides in human cells show heterogeneity in structure, most typically in length of the fatty acid carbon chain and degree of saturation [10].

When exogenously applied to epithelial cells, the GM1-ceramide species that contain a short chain C12:0 or an unsaturated C16:1 fatty acid in the ceramide domain enter the early sorting/recycling endosome from where they are sorted to other destinations [1]. The GM1-ceramide species with long, saturated C18:0 fatty acyl chains are transported instead to the lysosome, presumably for degradation. In simple polarized epithelial cells that form the single-cell thick monolayer lining mucosal surfaces, exogenously added GM1 species with short or unsaturated ceramide domains are transported from the apical (luminal) membrane across the cell to the basolateral (serosal) membrane by transcytosis, following a transcellular pathway leading to absorption [2].

Here, we test if GM1 can be used as a molecular carrier to deliver analogs of GLP-1 across epithelial barriers by harnessing these properties of endogenous sphingolipid trafficking. We linked the GLP-1 peptide to the extracellular domain of different GM1 species with ceramide domains of known structure, synthesized as described before [1]. We found that ceramide-directed trafficking remained intact after GM1 attachment to a therapeutic analog of GLP-1. Fusion molecules with short- or cis-unsaturated ceramide domains entered epithelial cells by endocytosis, trafficked to the recycling endosome, and reached the basolateral membrane of polarized cells by transcytosis; suggesting that specific glycosphingolipids may, indeed, be harnessed as molecular vehicles for mucosal delivery of therapeutic peptides.