Granin

Granins are a family of proteins found in secretory granules of neuroendocrine cells. The granin family includes several members, such as chromogranin A (CgA), chromogranin B (CgB), and secretogranin II (SgII), among others. These proteins are characterized by their ability to bind calcium and aggregate in the presence of this ion, which is crucial for their role in the storage and release of hormones and neurotransmitters.

Function

Granins play a pivotal role in the formation of secretory granules and in the sorting and storage of hormones and neurotransmitters. They are involved in the calcium-dependent aggregation of granule contents, which is essential for the proper packaging of these signaling molecules. Upon cell stimulation, granins are co-released with the hormones and neurotransmitters they store, suggesting they may also play a role in the regulation of secretion. Furthermore, some granins are cleaved into smaller, biologically active peptides that have been implicated in various physiological processes, including the regulation of blood pressure, glucose metabolism, and the immune response.

Structure

Granins are characterized by a high proportion of acidic amino acids, which confer a net negative charge at physiological pH, and the presence of several conserved regions, including phosphorylation sites and a motif for binding calcium. Their ability to bind calcium is critical for their function in secretory granule formation and hormone/neurotransmitter storage.

Clinical Significance

Alterations in the expression levels of granins and their derived peptides have been associated with several diseases, including neuroendocrine tumors, heart disease, and neurodegenerative diseases. For example, chromogranin A is a well-established marker for neuroendocrine tumors. Elevated levels of granin-derived peptides have been implicated in the pathophysiology of heart disease and hypertension, suggesting these peptides could serve as biomarkers or therapeutic targets.

Research

Research on granins has focused on elucidating their precise roles in secretory granule biology, the mechanisms governing their processing into bioactive peptides, and the physiological functions of these peptides. Understanding the complex roles of granins and their derived peptides in health and disease could lead to the development of novel diagnostic and therapeutic strategies.

See Also

• Neuroendocrine cell
• Secretory granule
• Calcium in biology
• Hormone
• Neurotransmitter