Essential role of Stasimon in motor circuit development and disease

Motor circuits control fundamental behaviors such as swallowing, breathing and locomotion. Spinal motor neurons are the key mediators translating motor commands generated within the central nervous system to peripheral muscle targets. Motor neurons are activated by a precisely regulated pattern of synaptic activity from sensory neurons, local spinal interneurons and descending pathways from the brain. Additionally, synaptic activity received by motor neurons during early development shapes their functional properties. In contrast, gene mutations that induce perturbations in either neuronal wiring or synaptic drive received by motor neurons often result in motor system disorders, although the primary cellular targets and the precise molecular events remain largely elusive. Thus, understanding the principles of neural circuit development and function as well as the mechanisms of synaptic dysfunction and selective neuronal death in human disease represent outstanding challenges in neurobiology. A prominent example of this situation is spinal muscular atrophy (SMA)-an inherited neuromuscular disease caused by ubiquitous deficiency in the survival motor neuron (SMN) protein. SMA pathogenesis involves alterations of multiple components of the motor circuit leading to abnormalities in spinal reflexes, motor neuron loss and skeletal muscle atrophy. However, the molecular and cellular mechanisms underlying motor circuit dysfunction in SMA remain poorly understood. In our previous work we have identified Stasimon as a novel transmembrane protein that localizes at contacts sites between ER and mitochondria membranes and contributes to motor dysfunction in animal models of SMA through undefined mechanisms. Furthermore, our preliminary studies revealed that Stasimon’s conditional depletion in neural circuits severely disrupts motor function in mouse models, pointing to an essential requirement for normal motor system development and function. Building on these findings, our goal is to define the neural circuit components and cellular pathway(s) in which Stasimon functions that underlie its essential role in the motor circuit and contribution to human disease. To do so, we will employ newly developed conditional mice for cell type-­specific restoration of Stasimon in vivo to study whether Stasimon dysfunction induced by SMN deficiency acts cell autonomously to promote death of SMA motor neurons and non-­cell autonomously to alter motor neuron firing through dysfunction of proprioceptive sensory neurons (Aim 1). We will also investigate the temporal and spatial requirement of Stasimon for normal development and function of the sensory-­motor circuit using novel conditional knockout mice we have recently developed (Aim 2). Lastly, we will use both cellular and mouse models to characterize the molecular function of Stasimon at the ER-­mitochondria contacts and its requirement for motor circuit function in health and disease (Aim 3). The successful accomplishment of the objectives of this proposal will characterize novel aspects of synaptic transmission and motor circuit function as well as the underlying mechanisms of SMA.

Motor  circuits  control  fundamental  behaviors  such  as  swallowing,  breathing  and  locomotion.  Spinal  motor  neurons  are  the  key  mediators  translating  motor  commands  generated  within  the  central  nervous  system  to  peripheral muscle targets. Motor neurons are activated by a precisely regulated pattern of synaptic activity from  sensory  neurons,  local  spinal  interneurons  and  descending  pathways  from  the  brain.  Additionally,  synaptic  activity received by motor neurons during early development shapes their functional properties. In contrast, gene  mutations that induce perturbations in either neuronal wiring or synaptic drive received by motor neurons often  result in motor system disorders, although the primary cellular targets and the precise molecular events remain  largely  elusive.  Thus,  understanding  the  principles  of  neural  circuit  development  and  function  as  well  as  the  mechanisms  of  synaptic  dysfunction  and  selective  neuronal  death  in  human  disease  represent  outstanding  challenges in neurobiology. A prominent example of this situation is spinal muscular atrophy (SMA)—an inherited  neuromuscular  disease  caused  by  ubiquitous  deficiency  in  the  survival  motor  neuron  (SMN)  protein.  SMA  pathogenesis involves alterations of multiple components of the motor circuit leading to abnormalities in spinal  reflexes,  motor  neuron  loss  and  skeletal  muscle  atrophy.  However,  the  molecular  and  cellular  mechanisms  underlying motor circuit dysfunction in SMA remain poorly understood. In our previous work we have identified  Stasimon  as  a  novel  transmembrane  protein  that  localizes  at  contacts  sites  between  ER  and  mitochondria  membranes  and  contributes  to  motor  dysfunction  in  animal  models  of  SMA  through  undefined  mechanisms.  Furthermore,  our  preliminary  studies  revealed  that  Stasimon’s  conditional  depletion  in  neural  circuits  severely  disrupts  motor  function  in mouse  models,  pointing  to  an  essential  requirement  for  normal  motor  system  development  and  function.  Building on  these  findings,  our  goal  is  to  define  the neural  circuit  components  and  cellular pathway(s) in which Stasimon functions that underlie its essential role in the motor circuit and contribution  to human disease. To do so, we will employ newly developed conditional mice for cell type-­specific restoration  of Stasimon in vivo to study whether Stasimon dysfunction induced by SMN deficiency acts cell autonomously  to  promote  death  of  SMA  motor  neurons  and  non-­cell  autonomously  to  alter  motor  neuron  firing  through  dysfunction  of  proprioceptive  sensory  neurons  (Aim  1).  We  will  also  investigate  the  temporal  and  spatial  requirement  of  Stasimon  for  normal  development  and  function  of  the  sensory-­motor  circuit  using  novel  conditional  knockout  mice  we  have  recently  developed  (Aim  2).  Lastly,  we  will  use  both  cellular  and  mouse  models to characterize the molecular function of Stasimon at the ER-­mitochondria contacts and its requirement  for motor circuit function in health and disease (Aim 3).  The successful accomplishment of the objectives of this  proposal  will  characterize  novel  aspects  of  synaptic  transmission  and  motor  circuit  function  as  well  as  the  underlying mechanisms of SMA.