Mice lacking both subunits of lysosomal beta-hexosaminidase display gangliosidosis and mucopolysaccharidosis

Mouse model of GM2 activator deficiency manifests cerebellar pathology and motor impairment

The GM2 activator deficiency (also known as the AB variant), Tay-Sachs disease, and Sandhoff disease are the major forms of the GM2 gangliosidoses, disorders caused by defective degradation of GM2 ganglioside. Tay-Sachs and Sandhoff diseases are caused by mutations in the genes (HEXA and HEXB) encoding the subunits of beta-hexosaminidase A. The GM2 activator deficiency is caused by mutations in the GM2A gene encoding the GM2 activator protein. For degradation of GM2 ganglioside by beta-hexosamindase A, the GM2 activator protein must participate by forming a soluble complex with the ganglioside. In each of the disorders, GM2 ganglioside and related lipids accumulate to pathologic levels in neuronal lysosomes, resulting in clinically similar disorders with an onset in the first year of life, progressive neurodegeneration, and death by early childhood. We previously have described mouse models of Tay-Sachs (Hexa -/-) and Sandhoff (Hexb -/-) diseases with vastly different clinical phenotypes. The Hexa -/- mice were asymptomatic whereas the Hexb -/- mice were severely affected. Through gene disruption in embryonic stem cells we now have established a mouse model of the GM2 activator deficiency that manifests an intermediate phenotype. The Gm2a -/- mice demonstrated neuronal storage but only in restricted regions of the brain (piriform, entorhinal cortex, amygdala, and hypothalamic nuclei) reminiscent of the asymptomatic Tay-Sachs model mice. However, unlike the Tay-Sachs mice, the Gm2a -/- mice displayed significant storage in the cerebellum and defects in balance and coordination. The abnormal ganglioside storage in the Gm2a -/- mice consisted of GM2 with a low amount of GA2. The results demonstrate that the activator protein is required for GM2 degradation and also may indicate a role for the GM2 activator in GA2 degradation.

Evidence for the involvement of Glu-355 in the catalytic action of human beta-hexosaminidase B

In a previous study the photoactivable affinity probe, 3-azi-1-[([6-3H]2-acetamido-2-deoxy-1-beta-D-galactopyranosyl)thio ]-b utane, was used to identify the active site of beta-hexosaminidase B, a beta-subunit dimer (Liessem, B., Glombitza, G. J., Knoll, F., Lehmann, J., Kellermann, J., Lottspeich, F., and Sandhoff, K. (1995) J. Biol. Chem. 270, 23693-23699). The probe predominately labeled Glu-355, a highly conserved residue among hexosaminidases. To determine if Glu-355 has a role in catalysis, beta-subunit mutants were prepared with the Glu-355 codon altered to either Ala, Gln, Asp, or Trp. After expression of mutant proteins using recombinant baculovirus, the enzyme activity associated with the beta-subunits was found to be reduced to background levels. Although catalytic activity was lost, the mutations did not otherwise affect the folding or assembly of the subunits. The mutant beta-subunits could be isolated using substrate affinity chromatography, indicating they contained intact substrate binding sites. As shown by cross-linking with disuccinimidyl suberate, the mutant beta-subunits were properly assembled. They could also participate in the formation of functional beta-hexosaminidase A activity as indicated by activator-dependent GM2 ganglioside degradation activity produced by co-expression of the mutant beta-subunits with the alpha-subunit. Finally, the mutant subunits showed normal lysosomal processing in COS-1 cells, demonstrating that a transport-competent protein conformation had been attained. Collectively the results provide strong support for the intimate involvement of Glu-355 in beta-hexosaminidase B-mediated catalysis.