Menin Immunoreactivity in Secretory Granules of Human Pancreatic Islet Cells

Abstract

The protein product of the Multiple Endocrine Neoplasia Type I (MEN1) gene is thought to be involved in predominantly nuclear functions; however, immunohistochemistry (IHC) data on cellular localization are conflicting. To further investigate menin expression, we analyzed human pancreas (an MEN1 target organ) using IHC and six antibodies raised against full-length menin or its peptides. In 10 normal pancreas specimens, two independently raised antibodies showed unexpected cytoplasmic immunoreactivity in peripheral cells in each islet examined (over 100 total across all 10 patients). The staining exhibited a distinct punctate pattern and subsequent immunoelectron microscopy indicated the target antigen was in secretory granules. Exocrine pancreas and pancreatic stroma were not immunoreactive. In MEN1 patients, unaffected islets stained similar to those in normal samples but with a more peripheral location of positive cells, whereas hyperplastic islets and tumorlets showed increased and diffuse cytoplasmic staining, respectively. Endocrine tumors from MEN1 patients were negative for menin, consistent with a two-hit loss of a tumor suppressor gene. Secretory granule localization of menin in a subset of islet cells suggests a function of the protein unique to a target organ of familial endocrine neoplasia, although the IHC data must be interpreted with some caution due to the possibility of antibody cross reaction. The identity, cellular trafficking, and role of this putative secretory granule-form of menin warrant additional investigation.

Tumor suppressor menin is required for subunit-specific nAChR α5 transcription and nAChR-dependent presynaptic facilitation in cultured mouse hippocampal neurons

“…However, calpain expression is ubiquitous and the appearance of a C-terminal menin immunoreactive fragment has been described in multiple experimental preparations and tissues, suggesting that the function of C-menin may not be limited to the clustering of nAChRs in neurons. For instance, C-terminal menin immunoreactivity has been found to localize to the membrane of pancreatic islets 40 , and pancreatic β-cells have been reported to express nAChRs 41 . In line with our present findings, this localization could be indicative of the C-menin fragment associated with nAChRs, and suggests that the functional clustering of nAChRs by the C-menin proteolytic fragment may also occur in non-neuronal tissues.…”

Menin localization in cell membrane compartment

“…Although menin is primarily a nuclear protein, 27 interacting with multiple nuclear proteins such as Smad protein, JunD, NFkB, FancD2, and MLL1/2, 25,[28][29][30] it is also detectable in cell membrane fraction. 10 However, little is known as to how menin associates with the cell membrane fraction. Coupled with the fact that menin participates in various cell membrane-related functions such as regulating Ras signaling on the cell surface.…”

“…Consistent with our observation, Debelenko et al recently reported that menin is also detected within the secretory vesicles in b cells using electron microscopy. 10 It is also possible that palmitoylated menin binds other molecules or undergoes other chemical modifications.…”

“…Consistently, menin is recently found to reside within the insulin-containing secretory vesicles. 10 However, our inability at this stage to definitively identify the palmitoylation sites and to show its glycosylation in the secretory hampers our functional analysis of the 2 modes of menin association with the cell compartment.…”

“…7,8 Menin also regulates the interplay with multiple signaling pathways that traverse the cell surface membrane. 9 While we and others have shown that menin is also detectable in cell membrane fraction, 10 precise nature regarding to how menin associates with membrane is poorly understood. Understanding of how menin associates with the cell membrane compartment will likely provide new insights into how menin can regulate various cell membrane-traversing signaling pathways.…”