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Granulin is a protein that in humans is that is encoded by the GRN gene. Each granulin residue is cleaved from the pre-cursor protein progranulin, a 593 amino acid long and approximately 68.5 kDa protein. Progranulin's function spans a multitude of cellular processes throughout the body, evidenced by its expression in a wide variety of cell types. Recently discovered GRN mutations implicating progranulin in disease pathogenesis has spurred much research in uncovering progranulin's role in the body. Progranulin's involvement in a spectrum of disease can be shown by the identification of a family consisting of FTD-GRN parents and NCL-GRN daughter.

Structure
Granulins are a family of secreted, glycosylated peptides that are cleaved from a single precursor protein with 7.5 repeats of a highly conserved 12-cysteine granulin/epithelin motif. Each individual granulin domain, 60 amino acid in length, consists of 2 (G) or 4 double cysteine motifs, and 4 single cysteine motifs, enabling the formation of 6 disulfide bonds. The presence of cysteine allows for the formation of disulfide bonds, 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 belong to a family of proteins that function as hormones, growth factors, ion channel modulators and enzyme inhibitors. When progranulin secreted into the extracellular matrix, it is often glycosylated at 4 confirmed and 1 tentative N-linked glycosylation sites. The n-terminus is also associated with the secretion of progranulin within secretory vesicles. The tail end of the progranulin, the c-terminus, functions as a primary binding partner to SORT1, a potential receptor of extracellular progranulin. The structural differences between each individual granulin peptide have yet to be characterized.

Progranulin 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 microglial cells and up-regulated during injury Within the brain, progranulin mRNA is highly expressed in pyramidal, hippocampal and purkinje cells.

Progranulin vs. Granulin
The 88 kDa precursor protein, progranulin, is also called proepithelin and prostate cancer (PC) cell-derived growth factor. Cleavage of the signal peptide progranulin produces mature granulin which can be further cleaved into 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. Elastase, proteinase 3 and matrix metalloproteinase are proteases capable of cleaving progranulin into individual granulin peptides. 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.

Progranulin Pathways
Progranulin is translated in the endoplasmic reticulum, where it is modified by disulfide isomerases before trafficking through the golgi apparatus and excretion through transport vesicles. Progranulin may be involved in regulating exosome excretion. In the extracellular matrix, progranulin binds to receptors on several cell types, resulting in activation of a signaling pathway. 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 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 EPHa2 as potential progranulin facilitators. After binding to the receptor, progranulin may induce and modulate signaling pathways such as MAPK/ERK, PI3K/Akt, and FAK. GO enrichment analysis reveals an association between progranulin and notch receptor signaling. Granulin has been shown to interact with Cyclin T1 and TRIB3.

Function
Both the peptides and intact granulin protein regulate cell growth. However, different members of the granulin protein family may act as inhibitors, stimulators, or have dual actions on cell growth. Granulin family members are important in normal development, wound healing, and tumorigenesis.

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 are also released by neutrophils and macrophages to modulate progranulin cleavage by inhibiting elastase cleavage. Addition of granulin B in cultured epithelial cells causes cells to secrete IL-8, a chemoattractant for 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.

Cell Proliferation
Progranulin is highly expressed in cells that are highly proliferative in nature. Several studies implicate progranulin in tumorigenesis and neuronal outgrowth. Progranulin promotes mitogenesis in epithelial cultures. When two epithelial lines were cultured in media with recombinant PGRN, proliferation was stimulated. Knockout of both progranulin homologues in a zebrafish model reduces axonal outgrowth. In primary cortical and motor neurons, progranulin regulates neuronal outgrowth and survival. In primary motor neurons, progranulin has been shown to increase neurite outgrowth by regulating the glycogen synthase kinase-3 beta. Progranulin may function as an autocrine growth factor in tumorigenesis.

Lysosomal Function
The discovery of GRN mutation leading to lysosomal storage disorder led to many studies that explored progranulin's role in regulating protein homeostasis via the lysosomal pathway. Transcriptional gene network interference study suggests that progranulin is highly involved in lysosomal function and organization. Imaging studies have shown co-localization of progranulin and lysosomal marker LAMP-1. Progranulin expression is regulated by TFEB, a transcription factor that mediates proteins involved in lysosomal biosynthesis. Progranulin may be involved in regulating protease activity. Proteases that could be regulated by progranulin include prosaposin, which is cleaved into saposin peptides in the lysosome, and cathepsin D, the primary protease involved in aggregate break down. GRN mutation shares similar neuropathology and clinical phenotype with CHMP2B and VCP mutations, genes that are both involved in the trafficking and breakdown of proteins involved in lysosomal function.

Frontal Temporal Lobar Demantia
Mutations in the GRN gene have been implicated in up to 25% of frontotemporal lobar degeneration, inherited in an autosomal dominant fashion with high penetrance. Several loss-of-function mutations disease-causing mutations in GRN have been identified. These mutations include frameshift, splice site,nonsense signal peptide, Kozak sequence disruptions and missense mutations, which result in either nonsense mediated decay or the production of non-functional protein; however, FTD-GRN is caused by progranulin haploinsufficiency and not a gain of function mutant protein. Patients with FTD-GRN exhibit asymmetric brain atrophy, although age of onset, disease progression and clinical symptoms vary, suggesting that other genetic or environmental factors may be involved in disease expression. Pathological indicators include cytosolic ubiquitin deposits enriched in hyperphosphorylated TDP-43, autophagy-related protein aggregates, ubiquitin-binding protein p62, lentiform intranuclear inclusions, dystrophic neurites and inflammation. Patients with the heterozygote mutation exhibit a reduction of 70-80% serum progranulin levels when compared to controls. Reprogrammed stem cells restore GRN mRNA levels to 50%, further suggesting that some other genetic or environmental factor is involved in regulating FTD disease expression. Mice exhibit reduced autophagic flux and autophagy-dependent clearance. Human FTLD-GRN derived fibroblasts show decrease lysosomal protease activity and lymphoblasts containing neuronal ceroid lipofuscinosis-like storage material. FTLD-GRN IPSC cortical neurons have enlarged vesicles, lipofuscin accumulation and cathepsin D deficiency.

Neuronal Ceroid Lipofuscinosis
Homozygous mutation of the GRN gene causes a subpopulation patients with neuronal ceroid lipofuscinosis characterized by an accumulation of autofluorescent lipofuscin, enlarged vacuoles, impairment in lysosomal activity, retinal & brain degeneration, exaggerated inflammatory responses, microgliosis, astrogliosis and behavioral dysfunction such as OCD-like and disinhibition-like behavior. Aged GRN double mutant mice have lipofuscin deposits and enlarge lysosomes, while one group found phosphorylated TDP-43.

Cancer
Progranulin's role in cell proliferation strongly implicates its role in cancer development. Progranulin can promote cyclin D1 expression in breast cancer lines and phosphorylation of proteins through extracellular regulated kinase signaling pathways. Progranulin is highly expressed in ovarian, adrenal carcinomas and immortalized epithelial cells. There is a correlation between Progranulin concentration and cancer severity. Granulin release by macrophages has been associated with fibrotic hepatic metastasis in pancreatic cancer.

Metabolic Disease
There is an increase in serum and plasma progranulin levels in patients with type 2 diabetes and visceral obesity, which may cause an increase in IL-6 or inhibition of the insulin-signaling cascade Progranulin may act as an adipokine, a signaling molecule released by adipose tissue that acts on the immune system. Progranulin may recruit macrophages to adipose tissue in obese and type 2 diabetes patients. Progranulin may stimulate angiogenesis and growth of adipose tissue. Progranulin may also be involved in energy homeostasis.

Other Diseases
The human liver fluke (Opisthorchis viverrini) contributes to the development of bile duct (liver) cancer by secreting a granulin-like growth hormone. Mutations in this gene have been associated with hereditary spastic paraplegia. Progranulin modulates migration of vascular smooth-muscle cells toward the innermost arterial layer to promote the progression of atherosclerosis. While progranulin may be anti-atherogenic, granulins may be pro-atherogenic. Elevated serum progranulin levels may be an indicator of atherosclerosis.