Activated microglia in cortex of mouse models of mucopolysaccharidoses I and IIIB

Abstract

Alpha-N-acetylglucosaminidase deficiency (mucopolysaccharidosis IIIB, MPS IIIB) and alpha-l-iduronidase deficiency (MPS I) are heritable lysosomal storage diseases; neurodegeneration is prominent in MPS IIIB and in severe cases of MPS I. We have obtained morphologic and molecular evidence for the involvement of microglia in brain pathology of mouse models of the two diseases. In the cortex, a subset of microglia (sometimes perineuronal) consists of cells that are probably phagocytic; they have large storage vacuoles, react with MOMA-2 (monoclonal antibody against macrophages) and Griffonia simplicifolia isolectin IB(4), and stain intensely for the lysosomal proteins Lamp-1, Lamp-2, and cathepsin D as well as for G(M3) ganglioside. MOMA-2-positive cells appear at 1 and 6 months in MPS IIIB and MPS I mice, respectively, but though their number increases with age, they remain sparse. However, a profusion of cells carrying the macrophage CD68/macrosialin antigen appear in the cortex of both mouse models at 1 month. mRNA encoding CD68/macrosialin also increases at that time, as shown by microarray and Northern blot analyses. Ten other transcripts elevated in both mouse models are associated with macrophage functions, including complement C4, the three subunits of complement C1q, lysozyme M, cathepsins S and Z, cytochrome b558 small subunit, macrophage-specific protein 1, and DAP12. An increase in IFN-gamma and IFN-gamma receptor was observed by immunohistochemistry. These functional increases may represent activation of resident microglia, an influx and activation of blood monocytes, or both. They show an inflammatory component of brain disease in the two MPS, as is known for many neurodegenerative disorders.

Figure 1

A microglial cell with large inclusions, juxtaposed to a neuron in the cortex of a 6-month-old MPS III B mouse. M and N identify the nucleus of the microglia and of the neuron, respectively. Note the large, nearly empty inclusions in the microglia (**) and the much smaller and denser inclusions in the neuron (*). Arrowheads trace the membrane of the neuron. (Scale bar = 5 μm.)

Figure 2

Histochemical and immunohistochemical characterization of microglia in cortex. (A) G. simplicifolia isolectin IB₄-positive microglia apposed to neuron; MPS IIIB, 7 months (×1,750). (B) MOMA-2-positive microglia apposed to neuron; MPS IIIB, 3 months (×1,150). (C and D) MOMA-2-positive cells seen at low power; MPS IIIB 3 months and MPS I, 14 months, respectively (×140). (E–G) Confocal images of sections stained with antibody against Lamp-1 (green), MOMA-2 (red), and merged; MPS IIIB, 6 months (×350). (H–J) Fluorescent images using antibody against ganglioside G_M3 (green), MOMA-2 (red), and merged; MPS IIIB, 6 months (×260). (K–M) Images of sections stained with antibody against CD68/macrosialin, MPS IIIB, MPS I, and control, respectively, 3 months (×65). (N and O) Images of sections stained with antibody against IFN-γ; MPS IIIB, and control, respectively, 3 months (×250). (P and Q) Images of sections stained with antibody against IFN-γ receptor; MPS IIIB and control, respectively, 7 months (×250).

Figure 3

Increase of MOMA-2-positive microglia with age. (A) Comparison of the number of MOMA-2-positive cells in cortex of MPS IIIB (−) and wild-type control (+) mice at 0.5, 1, 3, and 6 months. Each bar represents the number of MOMA-2-positive cells in a low-power field of the cortex of one mouse. (B) Comparison of density of MOMA-2-positive cells in a low-power field in the cortex of MPS I and MPS IIIB mice of different ages; each bar represents one mouse, with SD derived from counting cells in three low-power fields.