AP-4 mediated ATG9A sorting underlies axonal and autophagosome biogenesis defects in a mouse model of AP-4 deficiency syndrome

Abstract

Adaptor protein (AP) complexes have critical roles in transmembrane protein sorting. AP-4 remains poorly understood in the brain despite its loss of function leading to a hereditary spastic paraplegia termed AP-4 deficiency syndrome. Here we demonstrate that knockout (KO) of AP-4 in a mouse model leads to thinning of the corpus callosum and ventricular enlargement, anatomical defects previously described in patients. At the cellular level, we find that AP-4 KO leads to defects in axonal extension and branching, in addition to aberrant distal swellings. Interestingly, we show that ATG9A, a key protein in autophagosome maturation, is critically dependent on AP-4 for its sorting from the trans-golgi network.Failure of AP-4 mediated ATG9A sorting results in its dramatic retention in the trans-golgi network in vitro and in vivo leading to a specific reduction of the axonal pool of ATG9A. As a result, autophagosome biogenesis is aberrant in the axon of AP-4 deficient neurons. The specific alteration to axonal integrity and axonal autophagosome maturation in AP-4 knockout neurons may underpin the pathology of AP-4 deficiency.

AP4S1 splice-site mutation in a case of spastic paraplegia type 52 with polymicrogyria

“…Anatomical changes similar to those observed in patients have been reported in an AP-4 complex knockout mouse model: enlargement of the lateral ventricles and thinning of the corpus callosum. 7 Similar changes have also been seen in the patient described here, together with febrile and afebrile seizures. When exome sequencing was performed and analyzed, the patient did not show hypertonia in the lower limbs.…”

Novel insights into the clinical and molecular spectrum of congenital disorders of autophagy

“…52 Recent studies in cultured cells have demonstrated that the autophagy protein ATG9A is a cargo of AP-4 and that loss of AP-4 leads to a mislocalization of ATG9A, potentially impacting the transport and function of ATG9A. [53][54][55][56] Dysregulation of autophagy has been reported in AP-4 knockout cells and Ap4e1 knockout mice. 53,54…”

“…AP‐4 is composed of four subunits (β4, ε, μ4, σ4) forming an obligate complex and has been shown to be involved in protein trafficking from the trans‐Golgi network to early and late endosomes . Recent studies in cultured cells have demonstrated that the autophagy protein ATG9A is a cargo of AP‐4 and that loss of AP‐4 leads to a mislocalization of ATG9A, potentially impacting the transport and function of ATG9A . Dysregulation of autophagy has been reported in AP‐4 knockout cells and Ap4e1 knockout mice …”