All multicellular organisms use signaling molecules to convey information between cells. Neurons and endocrine tissues- such as the brain, pituitary, pancreas and adrenal gland- make especially important signaling molecules: peptidergic neurotransmitters and peptide hormones.
Our research questions center on the secretory pathway in which these signaling molecules are made. We are especially interested in the synthesis, identification, and characterization of secreted signaling molecules by their synthesizing enzymes, the proprotein convertases.
Many different disease processes are influenced by variations in convertase sequences or reduced convertase activity. For example, the first enzyme which controls neuropeptide and peptide hormone production, PC1/3, exists naturally as variants which are strongly associated with human obesity risk. Our studies on convertase-mediated opioid peptide synthesis are relevant to the control of pain pathways, while our work on two secretory chaperones (first identified through their potent blockade of convertase aggregation) is of potential interest to patients with Alzheimer’s and many other neurodegenerative diseases involving protein aggregation.
In brain, peptidergic signaling is required for the function of many different neuronal circuits; aberrant peptidergic signaling caused by defects in peptide synthesis thus likely contributes to mental disorders. In diabetes, peptide hormone synthesis (e.g. insulin and glucagon) is highly relevant to glycemic control. Lastly, the proprotein convertase furin participates in a variety of pathogenic processes, such as cancer; and bacterial and viral infections.
Alzheimer’s and Parkinson’s Diseases
Alzheimer’s disease is a devastating neurodegenerative disease presently affecting as many as 5.1 million Americans; the prevalence of the disease increases radically after age 65. The Census Bureau has estimated that the number of people over 65 will increase by 2050 to 88.5 million; thus, Alzheimer’s disease presents an enormous future public health problem which was recently recognized by Congress with the allocation of 300 million in additional NIH research funding.
Alzheimer’s disease involves the aberrant aggregation of certain proteins, such as A-beta and tau, into insoluble formations in brain tissues,plaques and tangles. Increasing evidence suggests that protein chaperones are involved in the formation and disposition of these insoluble aggregates. We are interested in the role of secreted chaperones, such as clusterin, 7B2, and proSAAS, in blocking the formation of protein aggregates in neurodegenerative disease. This work involves behavior, immunohistochemistry (of brain tissue from mouse models of Alzheimer’s), cell culture, and in vitro biochemistry. We are also interested in the effect of 7B2 and proSAAS on Parkinson’s disease protein aggregates, known as Lewy bodies, which are formed from synuclein protein within brain stem areas that control movement, and have recently shown that proSAAS overexpression blocks cell death in neuronal cell cultures.
Diabetes and Obesity
Diabetes, a disease of glucose misregulation, is increasingly prevalent both in the world population and in the US. The American Diabetes Association has estimated that in 2012, 29.1 million Americans, or 9.3% of the population, live with this chronic disease. Many forms of diabetes involve impaired synthesis of peptide hormones such as insulin and glucagon; these important signaling molecules are synthesized through the action of enzymes known as prohormone convertases. We study the cell biology, binding proteins, structure and regulation of these important enzymes, using neuroendocrine cell lines as model systems. We study naturally occurring mutations in prohormone convertases which are associated with obesity, and have shown that inactivating mutations can lead to dominant-negative effects on peptide hormone biology in mice bearing only one mutant allele. We are now developing animal models that recapitulate human obesity mutations in an effort to determine the cellular factors that underlie obesity.
Biochemistry of Prohormone Convertases
Establishing the three-dimensional structure of the prohormone convertases. Using recombinant protein expression we can produce milligram quantities of recombinant convertases, as well as of their two endogenous binding proteins. Our work on mouse furin resulted in the publication of the structure of the first mammalian convertase in mid-2003 (Henrich et al, Nature Structural Biology 10,520-526). We have recently made great progress in obtaining recombinan enzymes and plan obtain the structure of other convertases, such as PC1/3, as well as of convertase-inhibitor complexes (PC2 and 7B2). This work is vital to understanding the control of peptide hormone production.
Model of the Furin Substrate Binding Site, Courtesy of Stefan Henrich and Manuel Than
Bone diseases, either genetic (X-linked hypophosphatemia) or dietary (rickets), represent an essential failure of the bone cell secretory system to maintain a stable calcium phosphate-mineralized collagen scaffold. Bone maintenance is a highly complex and dynamic process which is under the control of various hormones, growth factors, and vitamins. Our laboratory has studied the role of various proprotein convertases in the degradation and cell biology of FGF-23, a peptide hormone secreted by bone cells which controls phosphate metabolism via its action in the kidney.
Cancer, the uncontrolled growth and migration of a small population of cells within a given tissue, is one of the leading causes of death in the United States in older adults. The expression of the proprotein convertase furin is strongly associated with increased metastasis (migration) of cancer cells; this is thought to be due to its ability to activate the cell surface enzymes responsible for extracellular matrix breakdown. In collaboration with several drug companies and academic laboratories, we developed furin inhibitors active both in the test tube and in cell culture. We have used a variety of cell models to test toxicity, cell penetration, and efficacy of furin inhibitors, and have identified small molecule inhibitors selective for cell surface furin action as well as inhibitors which also work intracellularly.
Optimization of convertase inhibitors and activators.
We discovered a potent small molecule inhibitor of furin which has proven useful both in bacterial diseases where furin activation is critical to toxin activation as well as in cancer pathogenesis. We have screened a variety of different libraries to obtain new inhibitors for PC1/3, PC2 and furin, and have optimized our current leads through chemical modification. We are also interested in therapeutic application of convertase inhibitors, which we test in cell-based assays (see recent publications).
Diseases potentially amenable to convertase inhibitor therapy include diseases of excess hormone production such as ectopic peptide production in small cell carcinoma; furin inhibition would be beneficial in blocking cancer pathogenesis. Blocking the production of glucagon- largely a PC2-mediated process- could also be of benefit in diabetes, as glucagon acts in opposition to insulin.
Identification of convertase activators. Recent evidence supports the idea that small molecules and basic peptides can act allosterically to stimulate the prohormone convertases; we plan to characterize these compounds as to their mechanism of action, and examine their possible in vivo use in controlling peptide production.Through these experiments we hope to identify new small-molecule activators of convertases which can be used to stimulate prohormone convertase-mediated cleavage of unprocessed precursors Stimulation of PC1/3 might act to increase bioactive peptide production, for example in certain forms of diabetes associated with high circulating proinsulin.
To connect with Dr. Iris Lindberg and her team, please see the Contact page.
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