Lysosomal function is intimately linked to exocytosis, and multiple LSDs such as MLIV, NPC, and Sialidosis have been shown to have impaired exocytosis [49
]. Exocytosis is the removal of cellular cargo by fusion of vesicles with the plasma membrane (). Synaptic vesicles and lysosomes both have exocytic activity. Activated exocytosis (eg. neurotransmitter release) and constitutive exocytosis (eg. of lysosomal contents) both result in secretion (eg. of signaling molecules such as neurotransmitters, or toxic metabolites) and plasma membrane expansion.
Recent data show that lysosomes and synaptic vesicles originate from a common early endosome, and preventing the maturation of either vesicle induces mistrafficking of their respective proteins to the opposite organelle [52
]. Mechanisms for exocytic activation are also similar between the two types of vesicles. During synaptic vesicle release, neuronal action potentials reach the presynaptic terminal and activate voltage regulated Ca2+
channels causing an influx of Ca2+
into the presynaptic cytoplasm, vesicle fusion with the plasma membrane, and neurotransmitter release [53
]. Synaptic vesicles are decorated with the Ca2+
sensor synaptotagmin and the Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) synaptobrevin-2, which bind to the corresponding plasma membrane SNAREs syntaxin-1A and synaptosomal-associated protein 25 (SNAP-25) [54
]. Lysosomal exocytosis utilizes Ca2+
influx into the cell, which drives interaction between the vesicular and target SNAREs synaptotagmin 7 (SYT7) and syntaxin 4 [55
]. In cultures of embryonic fibroblast cells from mouse models of MSD and MPS IIIA, cholesterol accumulation was shown to result in SNARE mislocalization, preventing lysosome fusion and vesicle cycling. Reducing cholesterol levels by SRT with methyl-β-cyclodextran restored SNARE localization and lysosome function [56
]. Cholesterol storage can also inhibit Rab GTPases, which promote membrane recycling; overexpressing Rab4 in cultured fibroblast cells derived from NPC1 deficient mice led to an activation of exocytosis and reduced lysosomal accumulation [57
]. Thus SRT can not only reduce the storage burden, but also improve secondary phenotypes.
Advances in understanding exocytosis have emerged from defining the function of proteins deficient in lysosomal storage diseases. Recent data show that the ultimate fusogenic Ca2+
signal for lysosomes does not originate from voltage dependent Ca2+
channels, but rather the protein lysosomal transient receptor potential mucolipin-1 (TRPML1) [58
]. Mutations in TRPML1 cause MLIV (). In the presence of the Ca2+
ionophore ionomycin, TRPML1 activates exocytosis [59
]. When key residues of TRPML1 were mutated to create a constitutively active channel, enhanced endosomal and lysosomal Ca2+
mobilization induced LAMP-1 relocalization to the plasma membrane [60
]. Whether LAMP-1 itself is important for exocytosis, or its presence on the plasma membrane is merely a result of lysosomal membrane fusion, is becoming clearer. In Sialidosis, which is caused by a deficiency in α-acetyl-neuraminidase, terminal sialic acid residues of LAMP-1 are not removed and its half-life at the plasma membrane is elevated [51
]. The elevated exocytosis in these cells can be blocked by LAMP-1 knock-down [51
Enhancing exocytosis could reduce storage in the short term, but a major concern with these approaches is that the products may not be adequately cleared in the brain, since even scavenger cells would harbor the enzymatic defect. Alternatively, autophagolysosome contents could be exocytosed and transported to the cerebrospinal fluid (CSF) and plasma. It would be interesting to test if the CSF from LSD patients contains elevated levels of accumulated storage material, or if enhancing exocytosis would cause accumulation in the neuropil and/or the CSF. LSD patients with CNS deficits do secrete storage products in urine [13
], but whether this occurs via exocytosis of autophagosome constituents from peripheral tissues or the brain is currently unknown.
The specialized vasculature system within the CNS may hinder extracellular clearance of exocytosed materials, and thus, contribute to cell death in this way. The familiar view of the blood brain barrier (BBB) is as a physical barrier that comprises the neurovascular system, and which blocks foreign particle entry. The BBB poses a major hurdle for drug delivery to brain, and it could also be a limiting force in substance removal after lysosomal exocytosis. An alternative, less neurocentric view to LSDs, is that defects within the neurovascular system may precede neuronal dysfunction [61
]. Certainly, there is vascular remodeling within the CNS of LSD animal models [62
] and obvious morphological changes in all cells of the neurovascular unit. Thus, drugs or other therapies capable of restoring BBB function may alleviate or slow disease progression, and as a bonus, would not be required to cross it.