Granulin is a protein that in humans is encoded by the GRN gene. Each granulin protein is cleaved from the precursor progranulin, a 593 amino-acid-long and 68.5 kDa protein. While the function of progranulin and granulin have yet to be determined, both forms of the protein have been implicated in development, inflammation, cell proliferation and protein homeostasis. The 2006 discovery of the GRN mutation in a population of patients with frontotemporal dementia has spurred much research in uncovering the function and involvement in disease of progranulin in the body. While there is a growing body of research on progranulin's role in the body, studies on specific granulin residues are still limited.
Progranulin Progranulin is the precursor protein for granulin. Cleavage of progranulin produces a variety of active 6 kDa granulin peptides. These smaller cleavage products are named granulin A, granulin B, granulin C, etc. Epithelins 1 and 2 are synonymous with granulins A and B, respectively. Cleavage of progranulin into granulin occurs either in the extracellular matrix or the lysosome. Elastase, proteinase 3 and matrix metalloproteinase are proteases capable of cleaving progranulin into individual granulin peptides. Progranulin and granulin can be further differentiated by their hypothesized opposing roles in the cell. While progranulin is associated with anti-inflammation, cleaved granulin peptides have been implicated in pro-inflammatory behavior. A C. elegans study showed that granulin peptides may also participate in toxic activity.
Expression Progranulin is expressed in a wide variety of cell types both in the periphery and in the central nervous system. Progranulin expression is low in early development, but increases as cells mature. Cell types expressing progranulin include neurons, microglia, astrocytes and endothelial cells. Progranulin has been found to be highly expressed in microglia and up-regulated during injury Within the brain, progranulin mRNA is highly expressed in pyramidal, hippocampal and Purkinje cells cells.
Structure Each individual granulin domain peptide is 60 amino acids in length. Granulin peptides are cysteine rich and capable of forming 6 disulfide bonds per residue. The disulfide bonds form a central rod-like core that shuttles each individual granulin peptide into a stacked β-sheet configuration. The structure of the granulin protein is similar to the structure of proteins from protein families that consist of hormones, growth factors, ion channel modulators and enzyme inhibitors. Because of progranulin's structural similarities to these proteins, much research was done to interrogate progranulin's potential role as a growth factor. When progranulin is secreted into the extracellular matrix, it is often glycosylated at four confirmed and one tentative N-linked glycosylation sites. The n-terminus of progranulin is hypothesized to be involved in the secretion of progranulin via secretory vesicles. Specifically, progranulin may be involved in regulating exosome excretion. The C-terminus of progranulin is hypothesized to be the primary binding partner of SORT1, a receptor of extracellular progranulin. The structural differences between each individual granulin peptide have yet to be characterized.
Interacting partners In the extracellular matrix, progranulin binds to receptors on several cell types resulting in either activation of a signal transduction pathway or engulfment into the cell. Several studies have shown progranulin's involvement in the binding of SORT1 and the subsequent trafficking of bounded progranulin to the lysosome. One recent study has shown that progranulin may actually mediate prosaposin trafficking to the lysosome via SORT1. However, the absence of SORT1 does not prevent exogenous progranulin from promoting neurite outgrowth or enhancing cell survival of GRN knockout cells, suggesting that other receptors are involved in mediating extracellular progranulin function For example, SORT1 -/- neuronal cells are still able to bind progranulin. Other studies have suggested tumor necrosis factor and EPH receptor A2 as potential progranulin facilitators. After binding to the receptor, progranulin may induce and modulate signaling pathways such as MAPK/ERK, PI3K/Akt, and FAK. Gene ontology enrichment analysis reveals an association between progranulin and notch receptor signaling. Granulin has also been shown to interact with Cyclin T1 and TRIB3.
Function
Development Although progranulin expression increases as cells mature, they are still involved in the development of multiple cell types. Progranulin is hypothesized to be a neurotrophic factor involved in corticogenisis. Induced pluripotent stem cell lines (IPSC) harboring the GRN mutation show a decrease in cortical neuronal differentiation ability. A recent mice study suggests that progranulin may be involved in regulating the early development of cerebellar tissue by selecting for individual climbing fibers as they intersect and form synapses with Purkinje cells. In addition, several studies implicate progranulin in synaptic pruning, a microglial process that occurs during development of the neural network. Cytokines, a neuronal marker for synapse elimination, is found to be upregulated in neurons with the GRN mutation. Increased cytokine tagging results in an increase in microglial density and activity around synapses. Progranulin may also be involved in sexual determination during embryonic development.
Inflammation and wound healing Progranulin levels are elevated when tissue is inflamed. After wounding, keratinocytes, macrophages and neutrophils increase production of progranulin. Neutrophils are capable of secreting elastase into the extracellular matrix that is capable of cleaving progranulin into granulin peptides, that promote further promote inflammation. SLPI, inhibitors of elastase, are also released by neutrophils and macrophages to modulate progranulin cleavage. Addition of granulin B in cultured epithelial cells causes cells to secrete IL-8, a chemical that attracts monocytes and neutrophils, which further suggests the involvement of granulin peptides in promoting inflammation. The addition of exogenous SLPI and progranulin is able to alleviate the enhanced inflammatory response of mice that are unable to inhibit the cleavage of progranulin.
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