Diabetic phenotype in mouse and humans reduces the number of microglia around ß-amyloid plaques.

BACKGROUND: Alzheimer's disease (AD) is the most common neurodegenerative disease and type 2 diabetes (T2D) plays an important role in conferring the risk for AD. Although AD and T2D share common features, the common molecular mechanisms underlying these two diseases remain elusive. METHODS: Mice with different AD- and/or tauopathy-linked genetic backgrounds (APPswe/PS1dE9, Tau P301L and APPswe/PS1dE9/Tau P301L) were fed for 6 months with standard diet or typical Western diet (TWD). After behavioral and metabolic assessments of the mice, the effects of TWD on global gene expression as well as dystrophic neurite and microglia pathology were elucidated. Consequently, mechanistic aspects related to autophagy, cell survival, phagocytic uptake as well as Trem2/Dap12 signaling pathway, were assessed in microglia upon modulation of PI3K-Akt signaling. To evaluate whether the mouse model-derived results translate to human patients, the effects of diabetic phenotype on microglial pathology were assessed in cortical biopsies of idiopathic normal pressure hydrocephalus (iNPH) patients encompassing ß-amyloid pathology. RESULTS: TWD led to obesity and diabetic phenotype in all mice regardless of the genetic background. TWD also exacerbated memory and learning impairment in APPswe/PS1dE9 and Tau P301L mice. Gene co-expression network analysis revealed impaired microglial responses to AD-related pathologies in APPswe/PS1dE9 and APPswe/PS1dE9/Tau P301L mice upon TWD, pointing specifically towards aberrant microglial functionality due to altered downstream signaling of Trem2 and PI3K-Akt. Accordingly, fewer microglia, which did not show morphological changes, and increased number of dystrophic neurites around ß-amyloid plaques were discovered in the hippocampus of TWD mice. Mechanistic studies in mouse microglia revealed that interference of PI3K-Akt signaling significantly decreased phagocytic uptake and proinflammatory response. Moreover, increased activity of Syk-kinase upon ligand-induced activation of Trem2/Dap12 signaling was detected. Finally, characterization of microglial pathology in cortical biopsies of iNPH patients revealed a significant decrease in the number of microglia per ß-amyloid plaque in obese individuals with concomitant T2D as compared to both normal weight and obese individuals without T2D. CONCLUSIONS: Collectively, these results suggest that diabetic phenotype in mice and humans mechanistically associates with abnormally reduced microglial responses to ß-amyloid pathology and further suggest that AD and T2D share overlapping pathomechanisms, likely involving altered immune function in the brain.

Enhanced polyamine catabolism disturbs hematopoietic lineage commitment and leads to a myeloproliferative disease in mice overexpressing spermidine/spermine N¹-acetyltransferase.

Spermidine/spermine N(1)-acetyltransferase (SSAT) regulates intracellular polyamine levels by catabolizing spermidine and spermine which are essential for cell proliferation and differentiation. Hematological characterization of SSAT overexpressing mice (SSAT mice) revealed enhanced myelopoiesis and thrombocytopoiesis leading to increased amounts of myeloid cells in bone marrow, peripheral blood, and spleen compared to wild-type animals. The level of SSAT activity in the bone marrow cells was associated with the bone marrow cellularity and spleen weight which both were significantly increased in SSAT mice. The result of bone marrow transplantations indicated that both the intrinsic SSAT overexpression of bone marrow cells and bone marrow microenvironment had an impact on the observed hematopoietic phenotype. The Lineage-negative Sca-1(+) c-Kit(+) hematopoietic stem cell (HSC) compartment in SSAT mice, showed enhanced proliferation, increased proportion of long-term HSCs and affected expression of transcription factors associated with lineage priming and myeloid differentiation. The proportions of common myeloid and megakaryocytic/erythroid progenitors were decreased and the proportion of granulocyte-macrophage progenitors was increased in SSAT bone marrow. The data suggest that SSAT overexpression and the concomitantly accelerated polyamine metabolism in hematopoietic cells and bone marrow microenvironment affect lineage commitment and lead to the development of a mouse myeloproliferative disease in SSAT mice.

Proteomic analysis of livers from a transgenic mouse line with activated polyamine catabolism.

We have generated a transgenic mouse line that over expresses the rate-controlling enzyme of the polyamine catabolism, spermidine/spermine N (1)-acetyltransferase, under the control of a heavy metal inducible promoter. This line is characterized by a notable increase in SSAT activity in liver, pancreas and kidneys and a moderate increase in the rest of the tissues. SSAT induction results in an enhanced polyamine catabolism manifested as a depletion of spermidine and spermine and an overaccumulation of putrescine in all tissues. To study how the activation of polyamine catabolism affects other metabolic pathways, protein expression pattern of the livers of transgenic animals was analyzed by two-dimensional polyacrylamide gel electrophoresis and mass spectrometry. A total of 23 proteins were shown to be differentially expressed in the transgenic from the wild-type animals. Many of the identified proteins showed expression patterns associated with polyamine catabolism activation. However, the expression pattern of other proteins, such as repression of GST pi and selenium-binding protein 2 and 60 kDa heat-shock protein, could be explained by the overexpression of peroxisome proliferator-activated receptor gamma co-activator 1alpha in response to depleted ATP pools. The activation of the latter proteins is thought to lead to the improved insulin sensitivity seen in the MT-SSAT animals.

Disturbed keratinocyte differentiation in transgenic mice and organotypic keratinocyte cultures as a result of spermidine/spermine N-acetyltransferase overexpression.

Overexpression of the rate-limiting enzyme in polyamine catabolism spermidine/spermine N1-acetyltransferase (SSAT) in transgenic (Tg) mouse leads to accumulation of putrescine in the skin and permanent hair loss at the age of 3 wk. The hair follicles of these mice are replaced by dermal cysts and epidermal utriculi. Increased putrescine production is also seen in hyperproliferative cutaneous disorders such as in psoriasis. These disorders are characterized by delayed onset of epidermal differentiation characterized as reduced expression of terminal differentiation markers such as cytokeratins 1/10, and filaggrin and persisting expression of basal cell cytokeratins 5/14 in the suprabasal layers. The use of these markers in immunohistological analysis of SSAT Tg skin clearly showed signs of disturbed differentiation. To exclude the possibility that changes in differentiation originated from underlying connective tissue, we introduced SSAT gene into an established rat epidermal cell line. Organotypic cultures derived from the transfected cells displayed similar changes in their differentiation pattern as keratinocytes in Tg skin. The role of accumulated putrescine in cutaneous changes of SSAT Tg mice was verified by an experiment in which putrescine level was reduced by systemic putrescine biosynthesis inhibition. The putrescine reduction was sufficient to alleviate the cutaneous changes to such an extent that distinct hair regrowth could be seen. These results suggest that the cutaneous changes of SSAT Tg animals are due to disorders of the keratinocyte differentiation. Moreover, they strengthen the view that the proper regulation of polyamine metabolism plays an important role in the keratinocyte maturation.