Expression of TMEM106B, the frontotemporal lobar degeneration-associated protein, in normal and diseased human brain

Abstract

Background: Frontotemporal lobar degeneration (FTLD) is the second most common cause of dementia in individuals under 65 years old and manifests as alterations in behavior, personality, or language secondary to degeneration of the frontal and/or temporal lobes. FTLD-TDP, the largest neuropathological subset of FTLD, is characterized by hyperphosphorylated, ubiquitinated TAR DNA-binding protein 43 (TDP-43) inclusions. Mutations in progranulin (GRN), a neuroprotective growth factor, are one of the most common Mendelian genetic causes of FTLD-TDP. Moreover, a recent genome-wide association study (GWAS) identified multiple SNPs within the uncharacterized gene TMEM106B that significantly associated with FTLD-TDP, suggesting that TMEM106B genotype confers risk for FTLD-TDP. Indeed, TMEM106B expression levels, which correlate with TMEM106B genotype, may play a role in the pathogenesis of disease.

Results: Since little is known about TMEM106B and its expression in human brain, we performed immunohistochemical studies of TMEM106B in postmortem human brain samples from normal individuals, FTLD-TDP individuals with and without GRN mutations, and individuals with other neurodegenerative diseases. We find that TMEM106B protein is cytoplasmically expressed in both histopathologically affected and unaffected areas of the brain by neurons, glia, and endothelial cells/pericytes. Furthermore, we demonstrate that TMEM106B expression may differ among neuronal subtypes. Finally, we show that TMEM106B neuronal expression is significantly more disorganized in FTLD-TDP cases with GRN mutations, compared to normal and disease controls, including FTLD-TDP cases without GRN mutations.

Conclusions: Our data provide an initial neuropathological characterization of the newly discovered FTLD-TDP-associated protein TMEM106B. In addition, we demonstrate that FTLD-TDP cases with GRN mutations exhibit a loss of neuronal TMEM106B subcellular localization, adding to evidence that TMEM106B and progranulin may be pathophysiologically linked in FTLD-TDP.

Figure 1

TMEM106B expression in neurons, glial and endothelial cells or pericytes in cortical specimens from normal controls. TMEM106B is normally cytoplasmically expressed in neurons (a), glia (b), and endothelial cells or pericytes (c). (a) Neuronal staining in cortices from normal human controls demonstrated a perikaryal, polarized cytoplasmic distribution mainly in the cell body and variably extending into processes. (b) Glial distribution of TMEM106B similarly demonstrated an asymmetric cytoplasmic distribution. (c) A subset of endothelial cells or pericytes demonstrated intense cytoplasmic expression of TMEM106B. Sections were stained with the anti-TMEM106B polyclonal antibody N2077 [24]. Scale bar represents 50 um.

Figure 2

TMEM106B protein expression in brain tissue from normal controls. TMEM106B protein expression is found throughout all layers of neocortex. Frontal cortex is shown in (a). TMEM106B neuronal and glial staining pattern is similar in both histopathologically affected areas of the brain (b, frontal cortex) and in areas of the brain that are relatively spared of TDP-43 pathology (c, occipital cortex). In the hippocampus, the pyramidal neurons of Ammon’s horn show positive TMEM106B immunoreactivity (d), whereas those of the dentate gyrus do not (e). Lentiform nucleus sections demonstrated very rare neuronal staining (f). There was minimal staining of the Purkinje cells of the cerebellum; rarely, cells demonstrated a cytoplasmic, granular staining, as highlighted in (g). Neurons of the deep cerebellar nuclei demonstrated diffuse cytoplasmic TMEM106B (h). Sections were stained with the anti-TMEM106B polyclonal antibody N2077 [24]. Scale bar represents 50 um.

Figure 3

Scoring of neuronal TMEM106B protein expression. (a) Scoring schema used to grade severity of disorganization of neuronal TMEM106B expression. Scores of 0 were assigned to sections in which almost all neurons displayed cytoplasmic TMEM106B expression with a vesicular pattern exhibiting a polarized quality. Nuclear boundaries were clear. Scores of 1 were assigned to sections in which a sizeable number of neurons displayed more diffuse TMEM106B staining dispersed more widely in the cytoplasm, but still delimited to the soma. Polarity was still usually maintained. Scores of 2 were assigned to sections in which most neurons recapitulated the characteristics of a score of 1. However, these sections also contained rare, non-degenerating neurons which displayed highly disorganized and diffuse TMEM106B staining throughout the cytoplasm with extension into processes. Scores of 3 were assigned to sections in which numerous neurons displayed highly disorganized and diffuse TMEM106B staining, with extension into processes. Scale bar represents 30 um. (b) Shown is the average scoring of the degree of diffuse neuronal TMEM106B expression by two independent, blinded scorers for N2077-stained human frontal cortical samples. Normal cases n = 7; Alzheimer’s disease n = 5; FTLD-tau n = 6, GRN (−) FTLD-TDP n = 5, GRN (+) FTLD-TDP n = 6. The colors in the dot plot correspond to the groups delineated in the bar graph. Weighted kappa = 0.44. GRN (+) FTLD-TDP cases demonstrated more disorganized patterns of TMEM106B expression (p = 0.005 for Mann–Whitney test).

Figure 4

TMEM106B expression is more disorganized in neurons from GRN (+) FTLD-TDP cases, despite comparable levels of TDP-43 pathology. Representative frontal cortical sections from a normal control (a), GRN (−) FTLD-TDP (b), and GRN (+) FTLD-TDP (c). Both the N2077 antibody (top row) [24] and a different polyclonal antibody (middle row) raised against the N-terminus (amino acids 1–96) of TMEM106B [23] show similar patterns of immunoreactivity on serial sections from the same cases. GRN (+) FTLD-TDP cases showed more disorganized TMEM106B expression than GRN (−) FTLD-TDP cases, despite similar degrees of TDP-43 pathology, as indicated by staining against pathological, phosphorylated forms of TDP-43 (bottom row). Scale bar represents 50 um.