Absence of beta-amyloid deposition in the central nervous system of a transgenic mouse model of distal myopathy with rimmed vacuoles

Distal myopathy with rimmed vacuoles (DMRV) is an autosomal recessive disorder characterised by early adultonset (15–40 years), slowly progressive myopathy with preferential weakness of the tibialis anterior and sparing of the quadriceps femoris muscles [1]. The muscle biopsy shows rimmed vacuoles containing deposits immunoreactive for b-amyloid (Ab) and other proteins. DMRV is due to mutations in the GNE gene, which encodes a bifunctional enzyme (uridine diphosphate-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase) in the sialic acid synthetic pathway. A recently developed mouse model of DMRV that expresses the human GNE D176V mutation in a Gne knockout background reproduces the clinical, pathological and biochemical features of human DMRV [2]. As in the human disease the model exhibited muscle atrophy and weakness, and low levels of sialic acid in serum and solid organs. Moreover, Abwas also found to be associated with the rimmed vacuoles and myofibrillar degeneration in the muscle fibres [2]. Because Alzheimer’s disease (AD) is characterised by Ab deposition in the brain, we examined the central nervous system of the DMRV mouse model for evidence of Ab deposition and other pathological abnormalities to determine if there is any link between DMRV and AD. Brain and skeletal muscle tissues from 30 DMRV mice older than 40 weeks were harvested, formalin-fixed, processed and paraffin embedded. Sixteen spinal cords were also available for examination. Tissues sections were stained by hematoxylin and eosin for light microscopy. Immunohistochemistry (IHC) to detect Ab was performed using a mouse monoclonal primary antibody (clone 6E/10; Covance, Princeton, NJ) and the Envision method with minor modifications [3]. As a positive IHC control, we used tissues from an established mouse model of AD, the Tg2576 transgenic mouse (Taconic, Germantown, NY) that expresses the Swedish mutation of amyloid precursor protein. Brain tissue sections from a human case of AD were also included as a positive control. For negative controls, sections were incubated with normal goat serum to replace the primary antibody. In the 30 aged DMRV mice examined there was no evidence of Ab deposits in the brains or spinal cords (Figure 1B). However, Ab deposits were detected in the skeletal muscles of all these animals. The deposits were usually linear, beaded and found in the central part of the fibres (Figure 1C, D). Occasionally, these deposits were associated with vacuoles (Figure 1C) corresponding to rimmed vacuoles as shown by the hematoxylin and eosin stains (data not shown). The three AD mouse brain positive controls showed plaque-like amyloid deposits in the cerebral cortex (Figure 1A). The human AD brain tissues showed Ab deposition in the brain neuropil and around blood vessels (data not shown). There were no light microscopic abnormalities such as neurofibrillary tangles, amyloid angiopathy and other features of AD in the central nervous system of all test and control mice. Although the aged DMRV mouse model demonstrated convincing Ab deposition in skeletal muscles, there was no Ab deposition in the CNS. This suggests that amyloidogenesis in AD and DMRV may be different. Amyloidogenesis in AD is mainly associated with post-translational proteolytic processing of amyloid precursor protein by a-, band g-secretases. However, hyposialylation is an important factor in the pathogenesis of the disease and amyloidogenesis in DMRV. Hyposialylation of several important muscle glycoproteins Keywords