Autophagy, mitochondria and cell death in lysosomal storage diseases

Abstract

Lysosomal storage diseases (LSDs) are debilitating genetic conditions that frequently manifest as neurodegenerative disorders. They severely affect eye, motor and cognitive functions and, in most cases, abbreviate the lifespan. Postmitotic cells such as neurons and mononuclear phagocytes rich in lysosomes are most often affected by the accumulation of undegraded material. Cell death is well documented in parts of the brain and in other cells of LSD patients and animal models, although little is known about mechanisms by which death pathways are activated in these diseases, and not all cells exhibiting increased storage material are affected by cell death. Lysosomes are essential for maturation and completion of autophagy-initiated protein and organelle degradation. Moreover, accumulation of effete mitochondria has been documented in postmitotic cells whose lysosomal function is suppressed or in aging cells with lipofuscin accumulation. Based upon observations in the literature and our own data showing similar mitochondrial abnormalities in several LSDs, we propose a new model of cell death in LSDs. We suggest that the lysosomal deficiencies in LSDs inhibit autophagic maturation, leading to a condition of autophagic stress. The resulting accumulation of dysfunctional mitochondria showing impaired Ca2+ buffering increases the vulnerability of the cells to pro-apoptotic signals.

Fig 1. Mitochondrial fragmentation in LSDs

In the normal cells, mitochondrial fragments are cleared from the cytoplasm by autophagy followed by lysosomal degradation (Top). In LSD-affected cells, autophagy is suppressed through unknown mechanisms; as a result, mitochondrial fragments with altered functional capacities accumulate (Bottom).

Fig 2. Inhibition of μ-calpain suppresses Ca²⁺-dependent cell death in MLIV fibroblasts

Cells were pretreated with 1 μM of calpain II (μ-calpain) inhibitor (Calbiochem) for 15 min before experiments and apoptosis was induced as in . Note that apoptosis rates of pretreated cells are significantly lower than those of untreated cells. The results represent 3 separate experiments; more than 20 cells were analyzed in each experiment under each condition.

Fig 3. A model of mitochondrial involvement in cell death in LSDs

In a normal cells during Ca²⁺ stimulation, perimitochondrial Ca²⁺ is rapidly cleared into mitochondria. Mitochondria in LSDs-affected cells fail to buffer intracellular increases in Ca²⁺, potentiating activation of μ-calpain and triggering cell death through caspse-8-dependent mechanisms.

Fig 4. Mitochondrial abnormalities in LSDs

Electron microscopic images of mitochondria in control, MLIV and MLIII fibroblasts. In order to better illustrate the inner mitochondrial structure, large mitochondria were chosen in control and LSDs-affected cells. Please note dilated, distorted cristae (arrows) and dark matrix space in mitochondria from LSDs-affected cells.