Impaired lipid metabolism in astrocytes underlies degeneration of cortical projection neurons in hereditary spastic paraplegia

Yongchao Mou,Su-Chun Zhang,Xue-Jun Li,Yi Dong,Craig Blackstone,Zhenyu Chen,Kyle R Denton,Michael O Duff

doi:10.1186/s40478-020-01088-0

Abstract

Hereditary spastic paraplegias (HSPs) are caused by a length-dependent axonopathy of long corticospinal neurons, but how axons of these cortical projection neurons (PNs) degenerate remains elusive. We generated isogenic human pluripotent stem cell (hPSC) lines for two ATL1 missense mutations associated with SPG3A, the most common early-onset autosomal dominant HSP. In hPSC-derived cortical PNs, ATL1 mutations resulted in reduced axonal outgrowth, impaired axonal transport, and accumulated axonal swellings, recapitulating disease-specific phenotypes. Importantly, ATL1 mutations dysregulated proteolipid gene expression, reduced lipid droplet size in astrocytes, and unexpectedly disrupted cholesterol transfer from glia to neurons, leading to cholesterol deficiency in SPG3A cortical PNs. Applying cholesterol or conditioned medium from control astrocytes, a major source of cholesterol in the brain, rescued aberrant axonal transport and swellings in SPG3A cortical PNs. Furthermore, treatment with the NR1H2 agonist GW3965 corrected lipid droplet defects in SPG3A astrocytes and promoted cholesterol efflux from astrocytes, leading to restoration of cholesterol levels and rescue of axonal degeneration in SPG3A cortical PNs. These results reveal a non-cell autonomous mechanism underlying axonal degeneration of cortical PNs mediated by impaired cholesterol homeostasis in glia.

Highlights

Hereditary spastic paraplegias (HSPs) are a large and diverse group of inherited neurodegenerative diseases with the common feature of a length-dependent axonopathy of the corticospinal axons, resulting in spasticity of lower-limb muscles and gait abnormalities [3, 22, 67]
We corrected the heterozygous ATL1 P342S pathogenic SPG3A mutation in iPSCs we previously generated from patientderived cells [77], yielding another isogenic pair (Fig. 1c and d) referred to ATL1-P342S and ATL1-342-Cor iPSCs
With prolonged culture, we found that the Tau+ axons of cortical projection neurons (PNs) derived from SPG3A iPSCs were thinner and exhibited many more areas of enlargement than the isogenic corrected cells (Fig. 1h); the number of axonal swellings per axonal segment was significantly increased in ATL1-P342S cortical PNs as compared to those derived from both wild-type (WT) and isogenic control iPSCs (Fig. 1i)

Summary

Introduction

Hereditary spastic paraplegias (HSPs) are a large and diverse group of inherited neurodegenerative diseases with the common feature of a length-dependent axonopathy of the corticospinal axons, resulting in spasticity of lower-limb muscles and gait abnormalities [3, 22, 67]. At the level of ER, ATL1 mutations impair the proper formation of tubules, vesicles and polygonal networks [27, 38, 46, 52] by inhibiting the formation of three-way ER tubule junctions [50, 75] These findings suggest that impaired ER structure and function are associated with an axonopathy of cortical PNs. Recently, several HSP proteins, including atlastin-1 (SPG3A), spastin (SPG4), seipin (SPG17), spartin (SPG20), and REEP1 (SPG31)—which together are mutated in over 50% of HSP patients –regulate the size and/or number of lipid droplets (LDs) in HEK293, COS7, and HeLa cells in vitro, as well as in fat bodies of worms and flies in vivo [14, 15, 19, 30, 51, 65]. It remains unknown if LDs and lipid metabolism are altered in SPG3A brain and if lipid abnormalities underlie the axonal phenotypes of cortical PNs in HSP

Methods

Results

Conclusion