Palmoplantar keratoderma along with neuromuscular and metabolic phenotypes in Slurp1-deficient mice

Abstract

Mutations in SLURP1 cause mal de Meleda, a rare palmoplantar keratoderma (PPK). SLURP1 is a secreted protein that is expressed highly in keratinocytes but has also been identified elsewhere (e.g., spinal cord neurons). Here, we examined Slurp1-deficient mice (Slurp1(-/-)) created by replacing exon 2 with β-gal and neo cassettes. Slurp1(-/-) mice developed severe PPK characterized by increased keratinocyte proliferation, an accumulation of lipid droplets in the stratum corneum, and a water barrier defect. In addition, Slurp1(-/-) mice exhibited reduced adiposity, protection from obesity on a high-fat diet, low plasma lipid levels, and a neuromuscular abnormality (hind-limb clasping). Initially, it was unclear whether the metabolic and neuromuscular phenotypes were due to Slurp1 deficiency, because we found that the targeted Slurp1 mutation reduced the expression of several neighboring genes (e.g., Slurp2, Lypd2). We therefore created a new line of knockout mice (Slurp1X(-/-) mice) with a simple nonsense mutation in exon 2. The Slurp1X mutation did not reduce the expression of adjacent genes, but Slurp1X(-/-) mice exhibited all of the phenotypes observed in the original line of knockout mice. Thus, Slurp1 deficiency in mice elicits metabolic and neuromuscular abnormalities in addition to PPK.

Figure 1. Palmoplantar keratoderma and increased stratum corneum lipid droplets in Slurp1^−/− mice

(A) Paws from wild-type and Slurp1^−/− mice, revealing markedly thickened skin in Slurp1^−/− mice. (B–C) Hematoxylin and eosin–stained sections revealing a thickened epidermis in the paw skin of a Slurp1^−/− mouse, along with many tiny lipid droplets in the stratum corneum (arrowheads). Scale bars = 50 µm and 10 µm for B and C respectively. (D) BODIPY 493/503 (green) staining showing lipid droplets in the stratum corneum (SC) of the paw skin of a Slurp1^−/− mouse. DNA is stained with DAPI (blue). Scale bar = 50 µm. (E) Electron micrograph showing lipid droplets in the stratum corneum of Slurp1^−/− paw skin (arrowheads). Scale bar = 1 µm. (F) Increased BrdU incorporation into DNA of paw keratinocytes of Slurp1^−/− mice (green). DNA is stained with DAPI (red). Scale bar = 50 µm.

Figure 2. Immunochemical detection of SLURP1

(A) Western blot of extracts of paw skin from wild-type (Slurp1^+/+) and Slurp1^−/− mice with an antibody against a mouse SLURP1 peptide. CHO cells transfected with a flag-tagged mouse SLURP1 expression vector were used as a control. SLURP1, like nearly all other Ly6 proteins, has an N-linked glycosylation site, and the two SLURP1 bands likely reflect glycosylated and nonglycosylated versions of the protein (Beigneux et al., 2008). The SLURP1 antibody did not cross-react with SLURP2. (B) The specificity of the human SLURP1 antibody (green) in these studies was assessed by immunocytochemistry. CHO-K1 cells were transiently transfected with an empty vector or an S-protein–tagged human SLURP1 expression vector. Cells that had been transiently transfected with the S-protein–tagged SLURP1 vector were identified with an S-protein–specific antibody (red). (C) Detection of SLURP1 (green) in human skin by immunohistochemistry with a human SLURP1 antibody. Keratin 14 (red) was used as a marker of basal keratinocytes; DNA was stained with DAPI (blue). Scale bars = 20 µm.

Figure 3. Evaporative water loss from the paw, ear, and back skin of Slurp1^+/+ and Slurp1^−/− mice

Measurements of evaporative water loss were performed with an RG1 evaporimeter (n = 6/group). TEWL, Transepidermal water loss. *P < 0.05; **P < 0.01; ***P < 0.001.

Figure 4. “Non-skin” phenotypes in Slurp1^−/− mice

(A) Hind limb clasping in male Slurp1^−/− mice when picked up by the tail (similar results were observed with female Slurp1^−/− mice). (B) Quantification of the hind limb clasping phenotype (0 for no hind-limb retraction, 1 for unilateral retraction, 2 for bilateral retraction) (n = 16 for Slurp1^+/+; n = 19 for Slurp1^+/−; and n = 17 for Slurp1^−/− mice). ***P < 0.001 (Slurp1^+/+ vs. Slurp1^−/−). (C) Weight gain in chow-fed female Slurp1^+/+ and Slurp1^−/− mice (4–15 weeks of age; n = 11/group). Similar results were observed with male mice (D) Reduced adiposity in 7-month-old chow-fed female Slurp1^−/− mice, as judged by NMR (n = 5/group). (E) Food consumption in male Slurp1^+/+ and Slurp1^−/− mice over 24 h. Similar results were observed in two independent experiments (n = 3 mice/group/experiment). (F) Slurp1^−/− mice are resistant to diet-induced obesity. Female Slurp1^−/− (n = 4) and Slurp1^+/+ mice (n = 3) were fed a high-fat diet for 11 weeks and weight gain was analyzed. Similar results were observed with male mice. *P < 0.05; **P < 0.01; ***P < 0.001. (G) Consumption of the high-fat diet (HFD) by Slurp1^+/+ and Slurp1^−/− mice over 24 h (n = 3/group).

Figure 5. Altered metabolic parameters in Slurp1^−/− mice

Plasma levels of cholesterol and triglycerides (males, n = 10/group) (A), insulin (males, n = 9 wild-type mice; n = 10 Slurp1^−/− mice) (B), and leptin (males, n = 6 wild-type mice; n = 4 Slurp1^−/− mice) (C) in wild-type and Slurp1^−/− mice. *P < 0.05; **P < 0.01; ***P < 0.001. (D) Metabolic cage studies revealing increased oxygen consumption (VO₂) in male Slurp1^−/− mice. (E) CO₂ production. (F) Respiratory quotient measurements during two dark and light cycles. (G) Decreased activity in male Slurp1^−/− mice. Bar graph showing total activity (the sum of X Total, X Ambulatory, and Z Total). X Total, total counts of beam interference in the X dimension, including stationary positions; X Ambulatory, multiple beam breaks in the X dimension; Z Total, line breaks in the Z dimension (rearing up) (n = 3/group). Similar results were observed in two independent experiments. *P < 0.05; **P < 0.01; ***P < 0.001.

Figure 6. Slurp1X^−/− mice exhibit PPK and the same “non-skin” disease phenotypes observed in the original line of Slurp1 knockout mice

(A–B) Paws of male Slurp1X^+/+ and Slurp1X^−/− mice at 12 weeks of age. (C–D) Hematoxylin and eosin–stained sections revealing a thickened epidermis in the paw skin of a Slurp1X^−/− mouse. Higher-powered images revealed many small lipid droplets in the stratum corneum (arrowhead). Scale bar = 50 µm. (E–F) Hind limb clasping in a male Slurp1X^−/− mouse. Similar results were observed with female mice. (G) Reduced body weight in Slurp1X^−/− mice. Serial measurements of body weight revealed a 15–20% reduction in body weight in 9-week-old Slurp1X^−/− female mice on chow diet, very similar to findings in the original line of Slurp1 knockout mice. *P < 0.05; ***P < 0.001.