The islets of Langerhans are highly complex endocrine mini-organs within the pancreas that secrete insulin and glucagon to control glucose homeostasis and are the target of multiple metabolic diseases including type 1 diabetes (T1D) and type 2 diabetes (T2D). Islet dysfunction and autoimmunity cause severe morbidity in more than 300 million people worldwide and are associated with health care costs in the hundreds of billions of dollars annually.
In T1D, the insulin-producing beta cells of the islet are destroyed by an auto-immune attack. Our research interests lie at the interface of biomaterials engineering and the biology and treatment of diseases of the pancreatic islets such as T1D.
Immunostaining for insulin, glucagon, GAD65, and giantin
Current strategies for islet-compatible biomaterials are heavily focused on islet encapsulation technologies using mostly bio-inert non-degradable materials to shield transplanted cells from the recipient immune system. While islet encapsulation is advantageous to avoid short-term immunological rejection, islet viability and function degrade over time, limiting long-term outcomes. Integration of transplanted islets with host tissues can lead to better long-term islet function, especially if the challenging problem of immune-rejection can be addressed through emerging immunomodulatory technologies.
Synthetic hydrogel systems that incorporate bioactive motifs into an engineered polymeric material are well-suited to define properties supportive of islet cell activities. In the past, we have developed such systems to deliver angiogenic growth factors to improve revascularization of transplanted pancreatic islets. Such a strategy is designed to improve islet survival after transplantation to treat T1D. We are currently designing a new generation of three-dimensional microenvironments centered around meeting the requirements for long-term maintenance of islet and beta cell identity and function.
From a basic science perspective, engineered microenvironments are excellent tools for studying factors influencing islet and beta cell biology in a controlled system.
Bio-artificial hydrogels designed to induce a vascularization response in vivo
Michael-type addition hydrogel reaction scheme: PEG-macromers are first functionalized with biomolecules followed by cross-linking with a thiol-fanked enzyme-degradable peptide. This plug-and-play design scheme enables a high degree of flexibility to tailor the properties of the system to suit different therapeutic design criteria.
Engineered microenvironments allow for controlled spatial and temporal patterning of bioactive cues.
Immunostaining for insulin, cell nuclei, CD3, and glucagon
Type 1 diabetes (T1D) is an autoimmune disease characterized by circulating autoantibodies and T cell mediated destruction of insulin-producing beta cells. Several promising strategies have emerged for manipulating the mechanisms of central and peripheral immune tolerance to treat autoimmunity. However, a majority of clinically-translated immunotherapies for T1D have focused on systemic delivery of immunomodulatory molecules or immune cells, and have not yet proven particularly effective at preventing the onset of T1D in humans.
We are developing engineered microenvironments to combat the problem of autoimmune destruction of beta cell mass both as a preventative treatment for T1D and in the context of recurrence of autoimmunity following islet transplantation. To promote tolerance to beta cell antigens we are investigating strategies which tumors use to evade the immune system. In one approach, we use biomaterial matrices and/or engineered particles to deliver immunomodulatory proteins in combination with beta cell-derived autoantigens to prime draining lymph nodes for a tolerogenic immune response. To protect transplanted beta cell mass from recurring autoimmune destruction, we are designing delivery matrices to locally present immunosuppressive modulators that discourage beta cell destruction.
Type 1 Diabetes (T1D) in humans develops following the autoimmune destruction of pancreatic beta cells in the islet of Langerhans. Multiple beta cell membrane proteins are the target of autoantibodies and auto-reactive T cells associated with T1D. One of the targeted beta cell membrane proteins in human diabetes is GAD65, the smaller isoform of the GABA synthesizing enzyme glutamic acid decarboxylase (GAD).
The initiation of autoimmunity against beta cell antigens has been linked to endoplasmic reticulum (ER) stress in beta cells. ER stress is induced when the demand for protein folding in a cell exceeds the capacity of the protein folding machinery. Beta cells are particularly susceptible to ER stress due to their enormous capacity to synthesize and secrete insulin. We recently discovered that induction of ER stress in beta cells causes a disruption of the regulation of lipid-membrane trafficking dynamics of GAD65 and accumulation of an immunogenic form of GAD65 in the Golgi compartment. A similar accumulation of GAD65 in Golgi membranes was observed in human pancreatic sections obtained from T1D patients but not from healthy donors.
We have also shown that during ER stress, beta cell autoantigens GAD65, proinsulin and IA-2 can be released from beta cells in the form of extracellular vesicles called exosomes together with danger-associated molecular pattern molecules (DAMPs), which can strongly activate immune responses. We are pursuing a strategy of engineered exosome-mimetic particles displaying GAD65 and DAMPs to further examine the immunogenic potential of islet-derived exosomes.
A variety of post-translational modifications to beta cell antigens have been identified that increase their potency to activate an autoimmune response. ER stress may be a major contributing factor to triggering autoimmunity in susceptible individuals.
(A) Schematic model of GAD65 cycling between the cytosol, the ER-Golgi, and a vesicular pathway. Newly synthesized hydrophilic and soluble GAD65 undergoes an irreversible hydrophobic modification in the cytosol. The resulting hydrophobic GAD65 reversibly associates with ER and Golgi membranes, establishing an equilibrium between membrane and cytosolic pools. In Golgi membranes, the protein undergoes a double palmitoylation and is targeted through TGN to cytosolic vesicles. A depalmitoylation along this pathway results in a reversal trafficking back to a weakly membrane associated form.
(B) When beta cells are treated to induced ER stress GAD65 accumulated in the Golgi compartment.
(C) A similar Golgi accumulation of GAD65 was observed in the islets of prediabetic GAD65-autoantibody positive and T1D pancreas.
T1D is a complex multi-faceted disease, with large knowledge gaps in comprehending the causative factors, immunopathology, and possibilities for intervention. Complicating the understanding of this disease are extreme differences between the physiology of rodent and human islets and between the immunopathology of diabetes in the NOD mouse model and human autoimmune diabetes.
Pancreatic beta cells share many intriguing similarities with neurons including the secretion of neurotransmitters and expression of neurotransmitter receptors. We are particularly interested in deciphering the functional role of the neurotransmitter GABA in both healthy and diabetic pancreatic islets. GABA has been previously shown to play a critical role in beta cell survival, helps control islet function, and has immuno-modulatory properties.
For this research we use advanced technical tools to study the cell biology of islet endocrine cells such as super-resolution optical microscopy, proteomics/transcriptomics and live functional imaging.
We use super-resolution optical microscopy to peer deep inside beta cells to see details more clearly than can be resolved. This image shows an example of stimulated emission depletion (STED) microscopy performed on primary human beta cells grown on the surface of glass coverslips.