Revisiting LAMP1 as a marker for degradative autophagy-lysosomal organelles in the nervous system

Abstract

Lysosomes serve as the degradation hubs for macroautophagic/autophagic and endocytic components, thus maintaining cellular homeostasis essential for neuronal survival and function. LAMP1 (lysosomal associated membrane protein 1) and LAMP2 are distributed among autophagic and endolysosomal organelles. Despite widespread distribution, LAMP1 is routinely used as a lysosome marker and LAMP1-positive organelles are often referred to as lysosomal compartments. By applying immuno-electron microscopy (iTEM) and confocal imaging combined with Airyscan microscopy, we expand on the limited literature to provide a comprehensive and quantitative analysis of LAMP1 distribution in various autophagic and endolysosomal organelles in neurons. Our study demonstrates that a significant portion of LAMP1-labeled organelles lack major lysosomal hydrolases. BSA-gold pulse-chase assay further shows heterogeneous degradative capacities of LAMP1-labled organelles. In addition, LAMP1 intensity is not a sensitive readout to assess lysosomal deficits in familial amyotrophic lateral sclerosis-linked motor neurons in vivo. Our study thus calls for caution when interpreting LAMP1-labeled organelles in the nervous system where LAMP1 intensity, trafficking, and distribution do not necessarily represent degradative lysosomes or autolysosomes under physiological and pathological conditions.

Abbreviations: ALS: amyotrophic lateral sclerosis; BSA: bovine serum albumin; DRG: dorsal root ganglion; IGF2R/CI-M6PR: insulin like growth factor 2 receptor; iTEM: immuno-transmission electron microscopy; LAMP1/2: lysosomal associated membrane protein 1/2; P80: postnatal day 80; sMNs: spinal motor neurons.

Keywords: Autophagy; cathepsin; degradative lysosome; endolysosome; lysosomal hydrolase.

Figure 1.

LAMP1/2 are distributed among a heterogenous population of autophagic and endolysosomal organelles. Lysosomes serve as the degradation hubs for autophagic and endocytic components. Newly synthesized LAMP1/2 exit the trans-Golgi network and enter the plasma membrane and endolysosomal pathway, where they are in dynamic equilibrium between endosomes, lysosomes, amphisomes, and autolysosomes. Endolysosomal trafficking, from early endosomes to late endosomes and finally into mature lysosomes (route 1), is essential for delivering endocytosed materials for degradation. Autophagosomes deliver bulk cytoplasmic components and damaged organelles to lysosomes through a stepwise maturation by fusing with endosomes into intermediate amphisomes or with lysosomes into degradative autolysosomes (route 2). To achieve effective degradation, both endocytic and autophagic organelles undergo dynein-driven retrograde transport from distal regions toward the soma, where degradative lysosomes are relatively enriched. MLB, multilamellar body; MVB, multivesicular body.