Role of astrocyte-secreted pleiotrophin in dendritic spine phenotypes in Down Syndrome

Mouse models of Down Syndrome (DS) and DS human postmortem tissue display an overall decrease in dendritic spine density with an increased number of spines with immature morphology.

Although the spine deficits observed in DS were traditionally viewed as a failure of neuronal cell intrinsic signaling, more recent studies suggest that astrocyte-neuron signaling contributes to the spine phenotypes.

One seminal study utilizing co-cultures of human astrocytes and rat primary neurons demonstrated that wildtype (WT) neurons cultured in the presence of DS astrocytes possessed a significantly decreased total spine density with a shift towards a higher ratio of thin filopodia-like spines (immature morphology) when compared to WT neurons cultured with WT astrocytes.

These results echo the findings of a multitude of studies demonstrating that proteins secreted from astrocytes can modulate neuronal synaptogenesis, maturation and maintenance.

In order to identify the full complement of changes in the astrocyte secretome in DS, the lab conducted an unbiased mass spectrometry screen on the conditioned media of cultured astrocytes isolated from trisomic Ts65Dn mice (a well-characterized mouse model of DS) compared to their euploid controls on the same background.

Pleiotrophin (Ptn) was identified among the top 10 proteins decreased in DS (over 4-fold decrease).

Ptn is an attractive protein candidate to investigate for a role in spine phenotypes in DS due to (i) its highly enriched mRNA expression pattern in astrocytes during cortical synaptogenesis and (ii) a recently demonstrated role for Ptn in neuronal spinogenesis in dentate gyrus.

There are many intriguing questions that remain regarding the involvement of Ptn in various aspects of cortical synapse and spine formation, maturation and maintenance, the role of Ptn in DS pathology, as well as which receptor(s) may be transducing effects on neuronal synaptic architecture.

The goals of this proposal are to investigate the role of Ptn in regulating synapse and spine density, spine morphology and spine stability in normal cortical development, to determine if restoring physiological levels of Ptn secretion in DS can rescue the spine phenotypes observed, and to define the receptor(s) that mediate the effects of Ptn on dendritic spines.

The main hypothesis is that Ptn regulates spine density, morphology and stability in cortical development and that down-regulation of secreted Ptn is a key mechanism leading to spine phenotypes in DS.

Aim 1 will investigate if Ptn regulates cortical synapse and spine density, spine morphology and spine stability using Ptn knockout mice. Aim 2 asks if up-regulation of Ptn can rescue spine phenotypes in DS utilizing in vitro and in vivo approaches. Aim 3 investigates which neuronal receptor(s) mediate effects of Ptn utilizing an in vitro siRNA screen.

These experiments will expand the current knowledge of the role of Ptn in cortical development and have significant clinical relevance to elucidate a mechanism by which dysregulation of Ptn levels leads to spine dysgenesis in DS.

The proposed work will take place at the Salk Institute for Biological Studies, a high caliber, collaborative research environment which provides access to all necessary equipment and training.