Stigmoid Bodies Contain Type I Receptor Proteins SorLA/LR11 and Sortilin: New Perspectives on Their Function

Abstract

S U M M A R Y Stigmoid bodies (SBs) are structures in the cytoplasm of neurons. SBs are mostly found in the hypothalamic region of the rat and contain a protein called huntingtin-associated protein 1 (HAP1). In a recent publication, large cytoplasmic structures were shown to be immunoreactive for a type I receptor called SorLA/LR11. By light microscopic analysis, these structures appeared similar to SBs in size and in brain regional and subcellular localization. To determine whether these large puncta correspond to HAP1-containing SBs, we used antibodies specific to various domains of the apolipoprotein receptor LR11 to perform immunocytochemistry in rat and mouse brain tissue. Transfection studies using HeLa cells were conducted to demonstrate the specificity of the antibodies. We found that, in both species, antibodies to the domain II (or VSP10 for vacuolar sorting protein 10 domain) of LR11 immunoreact with large cytoplasmic structures. Co-localization immunolabeling experiments in rat brain tissue sections and in neuron cultures showed that these LR11-immunoreactive structures correspond to HAP1-positive SBs. Electron microscopy was performed in rat hypothalamus and further demonstrated the presence of LR11 in SBs and its co-localization with HAP1. LR11-containing SBs were most abundant in the hypothalamus but were also found in many brainstem nuclei, thalamus, and hippocampus. Our data also show that sortilin, another transmembrane protein containing a VPS10 domain, localizes to large cytoplasmic puncta and is found in LR11-positive and Hap1-positive SBs in hypothalamic neuron cultures.

Huntingtin-associated protein 1 regulates postnatal neurogenesis and neurotrophin receptor sorting

“…Such diverse functions are likely modulated and specified by proteins that associate with the sortilin-receptor complex. Hap1 has been found to colocalize with sortilin in the cytoplasmic stigmoid bodies in adult hypothalamic neurons (60), and our findings suggest that association of Hap1 with the sortilin-TrkB complex plays an important role in postnatal hypothalamic neurogenesis by regulating TrkB level and its anterograde trafficking. Hap1 is known to interact with kinesin and dynactin p150 (14,(61)(62)(63), which are involved in anterograde and retrograde transport, respectively.…”

Precursor of Brain-derived Neurotrophic Factor (proBDNF) Forms a Complex with Huntingtin-associated Protein-1 (HAP1) and Sortilin That Modulates proBDNF Trafficking, Degradation, and Processing

“…To test this hypothesis, we firstly investigated whether HAP1 associated with sortilin, because 90% sortilin is found in trans-Golgi network (39,42). Sortilin is also reported to co-localize with HAP1 in stigmoid bodies, a type of protein aggregation structure of unknown function found in neurons of healthy animals (35). We found that ϳ91% of endogenous HAP1 and sortilin were co-localized in cortical neurons (Fig.…”

“…HAP1 interacts with a number of proteins including the GABA A receptor (31,32) and neurotrophic factor receptors (33,34). HAP1 and sortilin co-exist in stigmoid-like bodies, a type of protein aggregation structure of unknown function found in neurons of healthy animals, in vivo (35). More emerging evidence suggests that both sortilin and carboxypeptidase E play significant roles in post-translational Golgi sorting of BDNF to the regulated secretory pathway (36,37).…”

Huntingtin associated protein 1 and its functions

“…The SBs are structures found in the cytoplasm of various types of neurons in the central and peripheral nervous system. [94][95][96] They are distinct, spherical-to-ovoidal and non-membrane-bound neuronal cytoplasmic inclusions (~0.5-3 μm in diameter) with a granulo-fuzzy texture and moderate-to-low electron density, 97 and are found abundantly in the preoptic, hypothalamic and limbic 473 and 582) both bind the conserved C-terminal TBP domain. 74 When HAP1 or TBP were expressed independently in COS-7, 293, or Neuro-2a cells, all TBP localizes to the nucleus and all HAP1 assembles into cytoplasmic stigmoid-like bodies (STLBs).…”

“…11,15 SBs containing HAP1 also are found to contain the unknown human placental antigen complex X-P2 (hPAX-P2) and apolipoprotein E receptor, SorLA/LR11 and sortilin (two members of the vacuolar protein sorting 10 (VPS10) domaincontaining family). [94][95][96] HAP1 is a core component of the SBs and important for fetal and early postnatal neural development, particularly in the hypothalamic or limbic networks and HAP1/SBs has been assumed to play a protective role against neurodegeneration in HD. 96,99 Whether HAP1 plays any role in the pathological protein aggregation such as in Alzheimer disease, Parkinson disease and Huntington disease is not clear.…”