The Larimore Lab

The BLOC-1 (Biogenesis of Lysosome Related Organelles Complex 1) is implicated in neurodevelopment disorders including Autism Spectrum Disorders (ASDs) and Schizophrenia (SZ), both of which have well-documented affects on neuronal morphology in the hippocampus and the cortex. BLOC-1 regulates vesicle trafficking from early endosomes to late endosomes and in neurons, it regulates trafficking from the cell body to the synaptic terminal. BLOC-1 may regulate the trafficking of integral membrane proteins that regulate neuronal differentiation. Currently, the lab is exploring the role of BLOC-1 in neuronal differentiation and branching as that could directly impact function of parvalbumin interneurons in the hippocampus.

 

The Research Project: Parvalbumin neuron development in the GABAergic phenotype of BLOC-1 and MeCP2 deficient mice

Research in our lab sponsored by a previously funded IRSF grant demonstrates a common molecular pathway, the Dysbindin-BLOC-1 endosomal trafficking pathway, is disrupted in both Rett Syndrome (RTT) and Schizophrenia (SZ)(Larimore et al., 2013). One regulator of endosomal trafficking is the Biogenesis of Lysosome Related Organelles -1 (BLOC-1) complex. Subunits of this complex, pallidin and dysbindin, are targeted from the endosome to the synapse and their expression is reduced in the brain of patients with SZ (Larimore et al., 2011; Talbot et al., 2010; Tang et al., 2009). The DTNBP1 gene, which encodes the dysbindin subunit of the BLOC-1 complex, is strongly associated with SZ risk (Tang et al., 2009). Previously, our lab has demonstrated that the BLOC-1 complex participates in the generation of vesicle carriers that deliver selected synaptic membrane proteins from cell body endosomes to nerve terminals (Larimore et al., 2011). Through IRSF funded research, our lab has also demonstrated an alteration of BLOC-1 in the hippocampus of a mouse model of RTT during early development and in the symptomatic animal (Larimore et al., 2013).  These data support the hypothesis of a common molecular pathway disrupted in both RTT and SZ.

Neurodevelopmental disorders Rett Syndrome (RTT) and schizophrenia (SZ) exhibit alterations in GABAergic transmission in the prefrontal cortex and the hippocampus. Whether these phenotypic similarities stem from a molecular mechanism common to both disorders is presently unexplored. Notably, we have demonstrated that genetic defects in the Dysbindin-BLOC-1 endosomal trafficking pathway also impair the expression of GABAergic markers (Figure 1 and Larimore et al., 2014). Our findings suggest that the Dysbindin-BLOC-1 pathway could be a common defective mechanism shared by patients with RTT or SZ. Understanding how the BLOC-1 endosomal pathway may contribute to GABAergic transmission in the prefrontal cortex and hippocampus may uncover potential therapeutic targets to restore normal GABA function in these developmental disorders.

Rett syndrome (RTT) is a neurodevelopmental disorder that results from a variety of mutations in the MECP2 gene located on the X chromosome. Several phenotypes observed in the mouse model for RTT include alterations in the levels of endosomal proteins, such as several subunits of the BLOC-1 complex, and diverse GABAergic markers (Larimore et al., 2013, Chahrour et al., 2008). A current hypothesis in RTT research suggests an imbalance of excitatory and inhibitory signals in the developing RTT brain. Similarly, one of the schizophrenia (SZ) pathogenesis hypotheses postulates a defect in GABAergic neurotransmission yet the causes of GABAergic defects in these two disorders remain unknown. Here we propose that genetic defects in the gene encoding the BLOC-1 subunit dysbindin, DTNBP1, underlie the GABAergic neurotransmission defects observed in SZ and RTT.

GABAergic transmission is necessary to maintain proper excitatory/inhibitory balance within the brain. The underlying phenotypes associated with loss of functional MeCP2 in the hippocampus include alterations in hippocampal excitability (Calfa et al., 2011; Chao et al., 2010).  Research conducted in the lab of our collaborator, Lucas Pozzo-Miller at The University of Alabama at Birmingham, demonstrates a decrease in the firing frequency of parvalbumin-expressing fast-spiking interneurons in area CA3 of the hippocampus of symptomatic male Mecp2 knockout mice, which is due to a reduced excitatory drive (personal communication). Alteration in GABAergic transmission has also been observed in mice lacking dysbindin. In these BLOC-1 null mice, there is a significant decrease in the excitability of the fast spiking interneurons of the pre-frontal cortex in young adolescent mice. In young adult mice lacking dysbindin, there is a decrease in the number of the parvalbumin positive GABAergic interneurons in CA1-CA3 regions of the hippocampus (Carlson et.al, 2001). In the hippocampus, our lab has demonstrated a decrease in mRNA of GABA-related genes, including parvalbumin (Figure 1, Larimore et al., 2014).

These fundamental observations lead us to hypothesize that impaired parvalbumin interneuron development contributes to the GABAergic phenotype observed in BLOC-1 deficient or Mecp2 deficient mice.

There are several mechanisms that may impact parvalbumin interneuron development. Of those, we will explore two mechanisms in our specific aims: migration of the interneuron precursors or a decrease in the population of interneurons once they have reached their target tissue. These mechanisms of potential regulation may occur independently of each other or simultaneously, and, as such, each should be considered. These potential mechanisms are described in the specific aims.

1. Parvalbumin interneurons fail to migrate properly during development in mice of lacking functional BLOC-1 or functional MeCP2.

Hippocampal interneurons arise from the medial ganglionic eminence (MGE) and caudal ganglionic eminence (CGE) and are produced during E9-12 and E12-E16 respectively. The MGE is where Parvalbumin interneurons are generated. Another mechanism that may impact parvalbumin interneurons is regulation of migration into the hippocampus between E13-E16. Using immunofluorescent confocal microscopy we will examine the hippocampi isolated from several developmental stages of mice lacking the BLOC-1 complex or functional MeCP2 and determine if interneuron migration is occurring properly.

Parvalbumin interneurons generated in the MGE migrate to the cortex starting at E12.5. A second wave of migration occurs at E14-15, and a final migration occurs prior to E18, with the majority of migration occurring prior to birth. Using immunofluorescent confocal microscopy we will examine the prefrontal cortex isolated from several developmental stages of mice lacking the BLOC-1 complex or functional MeCP2 and determine if interneuron migration is occurring properly.

2. The number of parvalbumin interneurons is decreased in mice lacking functional BLOC-1 or functional Mecp2.

Our preliminary data demonstrate an alteration in the GABAergic markers at P7 in mice lacking dysbindin (Larimore et al., 2014). We will examine hippocampus and prefrontal cortex isolated from several developmental stages from mice lacking the BLOC-1 complex or functional MeCP2. With these samples we will perform qt-PCR and western blot analysis to determine the mRNA and protein levels of the parvalbumin interneurons. Using immunofluorscent confocal microscopy we will examine the localization and the number of the parvalbumin interneurons.