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Basic Research Papers

TBC1D20 mediates autophagy as a key regulator of autophagosome maturation

, , , &
Pages 1759-1775 | Received 29 Jul 2015, Accepted 01 Jun 2016, Published online: 03 Aug 2016
 

ABSTRACT

In humans, loss of TBC1D20 (TBC1 domain family, member 20) protein function causes Warburg Micro syndrome 4 (WARBM4), an autosomal recessive disorder characterized by congenital eye, brain, and genital abnormalities. TBC1D20-deficient mice exhibit ocular abnormalities and male infertility. TBC1D20 is a ubiquitously expressed member of the family of GTPase-activating proteins (GAPs) that increase the intrinsically slow GTP-hydrolysis rate of small RAB-GTPases when bound to GTP. Biochemical studies have established TBC1D20 as a GAP for RAB1B and RAB2A. However, the cellular role of TBC1D20 still remains elusive, and there is little information about how the functional loss of TBC1D20 causes clinical manifestations in WARBM4-affected children. Here we evaluate the role of TBC1D20 in cells carrying a null mutant allele, as well as TBC1D20-deficient mice, which display eye and testicular abnormalities. We demonstrate that TBC1D20, via its RAB1B GAP function, is a key regulator of autophagosome maturation, a process required for maintenance of autophagic flux and degradation of autophagic cargo. Our results provide evidence that TBC1D20-mediated autophagosome maturation maintains lens transparency by mediating the removal of damaged proteins and organelles from lens fiber cells. Additionally, our results show that in the testes TBC1D20-mediated maturation of autophagosomes is required for autophagic flux, but is also required for the formation of acrosomes. Furthermore TBC1D20-deficient mice, while not mimicking severe developmental brain abnormalities identified in WARBM4 affected children, display disrupted neuronal autophagic flux resulting in adult-onset motor dysfunction. In summary, we show that TBC1D20 has an essential role in the maturation of autophagosomes and a defect in TBC1D20 function results in eye, testicular, and neuronal abnormalities in mice implicating disrupted autophagy as a mechanism that contributes to WARBM4 pathogenesis.

Abbreviations

ABCD3=

ATP-binding cassette sub-family D (ALD), member 3

ACTB/β-actin=

actin, β

bs=

blind-sterile

COX4I1=

cytochrome c oxidase subunit IV isoform 1

EGFP=

enhanced green fluorescent protein

EGFR=

epidermal growth factor receptor

ER=

endoplasmic reticulum

GAPs=

GTPase activating proteins

GFP=

green fluorescent protein

LDs=

lipid droplets

LAMP1=

lysosomal-associated membrane protein 1

LAMP2=

lysosomal-associated membrane protein 2

MAP1LC3/LC3=

microtubule-associated protein 1 light chain 3

MEFs=

mouse embryonic fibroblasts

OMIM=

Online Mendelian Inheritance in Man

SQSTM1=

sequestosome 1

TBC1D20=

TBC1 domain family, member 20

TEM=

transmission electron microscopy

TOMM20=

translocase of outer mitochondrial membrane 20 homolog (yeast)

WARBM4=

Warburg Micro syndrome 4

WT=

wild type

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.

Funding

This work was supported by National Institutes of Health grants EY018872, P30EY001931 (D.J.S.), AI104928 (W.T.J.), and a Dr. Michael J. Dunn Summer Medical Student Research Fellowship Award, Medical College of Wisconsin (A.K.P).

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