Heterozygote galactocerebrosidase (GALC) mutants have reduced remyelination and impaired myelin debris clearance following demyelinating injury

Abstract

Genome-wide association studies are identifying multiple genetic risk factors for several diseases, but the functional role of these changes remains mostly unknown. Variants in the galactocerebrosidase (GALC) gene, for example, were identified as a risk factor for Multiple Sclerosis (MS); however, the potential biological relevance of GALC variants to MS remains elusive. We found that heterozygote GALC mutant mice have reduced myelin debris clearance and diminished remyelination after a demyelinating insult. We found no histological or behavioral differences between adult wild-type and GALC +/- animals under normal conditions. Following exposure to the demyelinating agent cuprizone, however, GALC +/- animals had significantly reduced remyelination during recovery. In addition, the microglial phagocytic response and elevation of Trem2, both necessary for clearing damaged myelin, were markedly reduced in GALC +/- animals. These altered responses could be corrected in vitro by treatment with NKH-477, a compound discovered as protective in our previous studies on Krabbe disease, which is caused by mutations in both GALC alleles. Our data are the first to show remyelination defects in individuals with a single mutant GALC allele, suggesting such carriers may have increased vulnerability to myelin damage following injury or disease due to inefficient myelin debris clearance. We thus provide a potential functional link between GALC variants and increased MS susceptibility, particularly due to the failure of remyelination associated with progressive MS. Finally, this work demonstrates that genetic variants identified through genome-wide association studies may contribute significantly to complex diseases, not by driving initial symptoms, but by altering repair mechanisms.

Differential Roles of TREM2+ Microglia in Anterograde and Retrograde Axonal Injury Models

“…In our study, upregulation of TREM2 expression could be observed in both models of axonal injury, being coincident with the previous general in vivo data in chronic (Jay et al, 2017a;Sayed et al, 2018) and acute CNS inflammatory injuries (Saber et al, 2017;Scott-Hewitt et al, 2017;Wu et al, 2017), as well as in other axonal injury models (Kobayashi et al, 2016;Tay et al, 2018). Additionally, an increase in sTREM2 could be found at the peak of TREM2 microglial expression after PPT, which was not detected after FNA.…”

“…Certain specificity of TREM2 for phagocytosis of myelin debris and apoptotic neurons has already been suggested based also on TREM2 ligands (Gervois and Lambrichts, 2019). In the case of myelin phagocytosis, similarly to our observations, TREM2 increases its expression correlating to myelin debris appearance in demyelinating models (Piccio et al, 2007;Takahashi et al, 2007;Scott-Hewitt et al, 2017). Overexpression of TREM2 in administered bone-marrow-derived cells improved myelin clearance in EAE-induced mice (Takahashi et al, 2005), while TREM2 blockade increased demyelination (Piccio et al, 2007).…”

Cell-autonomous expression of the acid hydrolase galactocerebrosidase

“…We see clear evidence of macrophages in the sciatic nerve at 16 wk, and the reduced debris in the interaxonal space could be from their infiltration and clearance of myelin debris. It has been shown previously that GALC-sufficient macrophages are critical to eliminate myelin debris in an induced model of multiple sclerosis (20). However, we believe that it is unlikely that the infiltration of GALCLAMP1sufficient macrophages alone accounts for the transient improvement in motor function.…”

Targeted resequencing reveals rare variants enrichment in multiple sclerosis susceptibility genes

“…Fourteen candidate genes previously associated with MS by GWAS (Beecham et al, 2013; Patsopoulos et al, 2019; Sawcer et al, 2011) were selected based on the accomplishment of one or two of the following criteria: (a) their differential gene expression between MS patients and healthy controls in microarray studies performed by our group in peripheral blood mononuclear cells (PBMC; unpublished data; criterion 1; see Supporting Information Text); (b) their potential contribution to the pathogenesis of MS according to the literature beyond its association with MS risk in the GWAS study (criterion 2). The 14 genes selected for DNA resequencing were the following: Fc receptor like 1 ( FCRL1 ; criterion 2: Comabella et al, 2016), regulator of G protein signaling 1 ( RGS1 ; criterion 1: upregulated in MS patients vs. controls; criterion 2: Tran et al, 2010), translocase of inner mitochondrial membrane domain containing 1 ( TIMMDC1 ; criterion 1: downregulated in MS patients vs. controls), hematopoietically expressed homeobox ( HHEX ; criterion 1: downregulated in MS patients vs. controls ) , C–X–C motif chemokine receptor 5 ( CXCR5 ; criterion 2: Klimatcheva et al, 2015), lymphotoxin beta receptor ( LTBR ; criterion 2: Inoue et al, 2016), Ts translation elongation factor, mitochondrial ( TSFM ; criterion 2: Alcina et al, 2013), galactosylceramidase ( GALC ; criterion 2: Scott‐Hewitt et al, 2017), TNF receptor associated factor 3 ( TRAF3 ; criterion 1: upregulated in MS patients vs. controls), signal transducer and activator of transcription 3 ( STAT3; criterion 2: Benveniste, Liu, McFarland, & Qin, 2014), TNF superfamily member 14 ( TNFSF14 ; criterion 1: downregulated in MS patients vs. controls; criterion 2: Malmeström et al, 2012), IFI30, lysosomal thiol reductase ( IFI30 ; criterion 2: West & Cresswell, 2013), CD40 molecule ( CD40 ; criterion 2: Chen et al, 2016) and cytochrome P450 family 24 subfamily A member 1 ( CYP24A1 ; criterion 2: Ramasamy et al, 2014).…”