Huntingtin phosphorylation governs BDNF homeostasis and improves the phenotype of Mecp2 knockout mice

Abstract

Mutations in the X‐linked MECP2 gene are responsible for Rett syndrome (RTT), a severe neurological disorder for which there is no treatment. Several studies have linked the loss of MeCP2 function to alterations of brain‐derived neurotrophic factor (BDNF) levels, but non‐specific overexpression of BDNF only partially improves the phenotype of Mecp2‐deficient mice. We and others have previously shown that huntingtin (HTT) scaffolds molecular motor complexes, transports BDNF‐containing vesicles, and is under‐expressed in Mecp2 knockout brains. Here, we demonstrate that promoting HTT phosphorylation at Ser421, either by a phospho‐mimetic mutation or inhibition of the phosphatase calcineurin, restores endogenous BDNF axonal transport in vitro in the corticostriatal pathway, increases striatal BDNF availability and synaptic connectivity in vivo, and improves the phenotype and the survival of Mecp2 knockout mice—even though treatments were initiated only after the mice had already developed symptoms. Stimulation of endogenous cellular pathways may thus be a promising approach for the treatment of RTT patients.

Calcineurin and huntingtin form a calcium-sensing machinery that directs neurotrophic signals to the nucleus

“…To determine whether endogenous HTT S421 phosphorylation influences TrkB transport specifically in axons in response to synaptic BDNF, we took advantage of two lines of homozygous knock-in mice: one in which HTT S421 is replaced by an aspartic acid ( Htt S421D/S421D or HTT-SD), mimicking constitutive phosphorylation, and one in which S421 is converted to an alanine, which is unphosphorylatable ( Htt S421A/S421A or HTT-SA) ( 16 , 38 ). To measure kinesin-1 recruitment to vesicles, we performed subcellular fractionation of HTT-SD and HTT-SA mouse brains.…”

“…It will be interesting to examine the functioning of the BDNF-TrkB-CaN-HTT pathway in HD and other diseases involving impaired neurotrophin signaling. These include Rett syndrome, in which dynein- and HTT-dependent trafficking of BDNF is disrupted ( 16 ). Dysregulation of endosomal signaling could also be relevant for Alzheimer’s disease, since β-amyloid oligomers reduce the velocity of BDNF-signaling endosomes moving in the retrograde direction ( 60 ).…”

“…BDNF can be transported in either an anterograde or retrograde direction, depending on whether the protein that scaffolds it, huntingtin (HTT), links up with dynein or kinesin proteins. HTT phosphorylation at S421 by the serine/threonine kinase Akt recruits kinesin, whereas dephosphorylation by calcineurin (CaN) or the protein phosphatase PP2B causes kinesin to be released from the complex, allowing dynein to carry out retrograde transport of vesicles, including those containing BDNF and amyloid precursor protein (APP) ( 14 – 16 ). The machinery that carries out this retrograde transport is rather involved.…”

Regulation of sensorimotor gating via Disc1/Huntingtin-mediated Bdnf transport in the cortico-striatal circuit

“…Currently accepted putative targets of lithium include inositol monophosphatase and glycogen synthase kinase-3 ( 48 ), yet we have not integrated these known pathways in addressing the lithium-mediated PPI rescuing effect in this study. In addition, calcineurin, a phosphatase, is associated with schizophrenia ( 49 , 50 ) and negatively regulates Htt (Ser-421) phosphorylation and Bdnf transport ( 51 , 52 ); however, whether calcineurin plays a role in lithium-mediated PPI regulation remains to be studied. Although we acknowledge these as a potential limitation of the present study, further studies are warranted to address possible involvement of these known targets in lithium-mediated rescuing effects on BDNF transport and PPI, taking into account the cellular and circuit-wide functions of this compound.…”

Huntingtin-KIF1A-mediated axonal transport of synaptic vesicle precursors influences synaptic transmission and motor skill learning in mice

“…Because the direction of HTT-mediated transport is dictated by S421 phosphorylation (Bruyere et al, 2020; Colin et al, 2008; Ehinger et al, 2020; Vitet et al, 2020), we investigated how phosphorylation of HTT affects axonal transport of SVPs using a reconstituted neuronal circuit on-a-chip. We then studied the effects of S421 mutants that are either constitutively phosphorylated or unphosphorylatable on physiology and behavior in mice and find that phosphorylated HTT recruits the molecular motor KIF1A to promote SVP transport.…”