Mucolipidosis type IV: the effect of increased lysosomal pH on the abnormal lysosomal storage.

Mucolipidosis type IV (MLIV) is a neurodegenerative channelopathy that is caused by the deficiency of TRPML1 activity, a nonselective cation channel. TRPML1 is a lysosomal membrane protein, and thus, MLIV is a lysosomal storage disorder. The basic, specific function of TRPML1 has not been yet clarified. A recent report (Soyombo AA, Tjon-Kon-Sang S, Rbaibi Y, Bashllari E, Bisceglia J, Muallem S, Kiselyov K: J Biol Chem 281:7294-7301, 2006) indicated that TRPML1 functions as an outwardly proton channel whose function is the prevention of overacidification of these organelles. Thus, in MLIV the lysosomal pH is lower than normal. Furthermore, attempts by these investigators to increase slightly the lysososmal pH with either Nigericin or Chloroquine suggested corrective effect of the abnormal storage in MLIV cells. We investigated this approach using these agents with cultured fibroblasts from severely affected and milder patients. Our data indicated that there was no reduction in the total number of storage vesicles by either agent, although Nigericin resulted in a change in the nature of the storage materials, reducing the presence of lamellated substances (lipids) so that the storage vesicles contained predominantly granulated substances. On the other hand, transfection with the normal MCOLN1 cDNA (the gene coding for TRPML1) resulted in the removal of almost all the storage materials.