Transgenic isolation of skeletal muscle and kidney defects in lamininβ2 mutant mice: implications for Pierson syndrome

Abstract

Pierson syndrome is a recently defined disease usually lethal within the first postnatal months and caused by mutations in the gene encoding lamininβ2 (LAMB2). The hallmarks of Pierson syndrome are congenital nephrotic syndrome accompanied by ocular abnormalities, including microcoria(small pupils), with muscular and neurological developmental defects also present. Lamb2-/- mice are a model for Pierson syndrome;they exhibit defects in the kidney glomerular barrier, in the development and organization of the neuromuscular junction, and in the retina. Lamb2-/- mice fail to thrive and die very small at 3 weeks of age, but to what extent the kidney and neuromuscular defects each contribute to this severe phenotype has been obscure, though highly relevant to understanding Pierson syndrome. To investigate this, we generated transgenic mouse lines expressing rat laminin β2 either in muscle or in glomerular epithelial cells (podocytes) and crossed them onto the Lamb2-/- background. Rat β2 was confined in skeletal muscle to synapses and myotendinous junctions, and in kidney to the glomerular basement membrane. In transgenic Lamb2-/- mice, β2 deposition in only glomeruli prevented proteinuria but did not ameliorate the severe phenotype. By contrast, β2 expression in only muscle restored synaptic architecture and led to greatly improved health, but the mice died from kidney disease at 1 month. Rescue of both glomeruli and synapses was associated with normal weight gain, fertility and lifespan. We conclude that muscle defects in Lamb2-/- mice are responsible for the severe failure to thrive phenotype, and that renal replacement therapy alone will be an inadequate treatment for Pierson syndrome.

Myotendinous Junction Defects and Reduced Force Transmission in Mice that Lack α7 Integrin and Utrophin

“…For example, mice lacking laminin ␤2 showed structural and functional disruptions at the MTJ, while ␣-dystrobrevin knockout mice have shorter folds at the MTJ. 44,45 Although the pathology observed in ␣7/utr Ϫ/Ϫ mice is similar to ␣7 Ϫ/Ϫ mice, there is a distinct difference at the MTJ, with the former having more severe structural defects. These observations suggest that utrophin compensates for loss of ␣7 integrin in the ␣7 Ϫ/Ϫ mice resulting in milder MTJ abnormalities and that the additional loss of utrophin results in severe MTJ structural defects.…”

Laminin β2 Gene Missense Mutation Produces Endoplasmic Reticulum Stress in Podocytes

“…19,31 All animal experiments conformed to the National Institutes of Health Guide for the Care and Use of Laboratory Animals and were approved by the Washington University Animal Studies Committee.…”

“…31 For CHOP staining, frozen sections were fixed with 4% paraformaldehyde in PBS for 10 minutes and permeabilized with 1% Triton X-100 for 5 minutes at room temperature. For BiP staining, kidneys were fixed by transcardiac perfusion with PBS containing 4% paraformaldehyde, and paraffin-embedded sections were used.…”

A Missense LAMB2 Mutation Causes Congenital Nephrotic Syndrome by Impairing Laminin Secretion

“…21,22 Briefly, transgenic expression of the full length wild-type rat ␤2 cDNA in podocytes (via the nephrin promoter) is sufficient to prevent protein- The proposed mechanisms of the R246Q missense mutation causing congenital nephrotic syndrome. It may inhibit laminin secretion from podocytes, eventually leading to degradation intracellularly.…”

“…(In all cases these mice also carried a muscle-specific wild-type rat ␤2 transgene (MCK-B2) that rescues the otherwise lethal neuromuscular junction defects. 21 ) Because proteinuria is the earliest and most sensitive clinical indicator of glomerular dysfunction, proteinuria was closely monitored. The three lines of Lamb2 Ϫ/Ϫ ;NEPH-R246Q-LAMB2 mice (hereafter referred to as Tg mutants) were tested weekly during the first month, then monthly thereafter, for albumin/protein and creatinine in the urine.…”