Kishore BK, Fuming Lu, Maldague P, Tulkens PM, Courtoy PJ
Unite de Pharmacologie Cellulaire et Moleculaire, Universite Catholique de Louvain, Brussels, Belgium.
In the companion paper, we report that a single injection of poly-D-glutamic
acid causes an acute lysosomal storage condition and apparently impairs
the lysosomal fission dynamics. The present paper addresses the mechanisms
of these two alterations using a combination of in vivo and in vitro biochemical
approaches. After a single intravenous injection, 14C-poly-D-glutamic acid
was rapidly cleared from the plasma and appeared in the urine. Yet, a small
but sizable fraction of the injected polymer was taken up by the kidney
cortex through a saturable process (Kuptake, 150 mg/kg body wt; uptakemax
96 micrograms/g cortex). Analytical subcellular fractionation of cortex
homogenates demonstrated that at initial stages, the 14C label was predominantly
associated with subcellular particles of intermediate size and low equilibrium
density, and was therefore slowly transferred to larger particles equilibrating
at high density, then codistributing with the lysosomal hydrolases. At
a concentration of 10 mg/ml (equivalent to its estimated concentration
in lysosomes), poly-D-glutamic acid formed micronic aggregates ( > or =
10 microns) when brought to solution at pH < or = 6 in relation to its
decreased ionization (pKa of lateral chains approximately equal to 4.25).
Finally, 1 day after the injection of poly-D-glutamic acid, the activities
of several lysosomal enzymes (hexosaminidase, cathepsin B, acid sphingomyelinase,
and sulfatase B), but not of all of them (eg, acid phosphatase), were increased
in the kidney cortex. We propose that poly-D-glutamic acid reaches lysosomes
by adsorptive endocytosis and becomes concentrated within these organelles
because its withstands hydrolysis until it forms aggregates or precipitates,
causing a decrease in the fluidity or the deformability ("gelling") of
the lysosomal matrix. This should alter the dynamics of intercommunication
of these organelles by impairing their fission without a proportionate
effect on their fusion properties. In addition, the data suggest that the
presence of poly-D-glutamic acid directly or indirectly slows down the
degradation of several lysosomal enzymes.
PMID: 8667607, UI: 96249111