Project Summary/Abstract Down Syndrome (DS) is a neurodevelopmental disorder caused by trisomy of chromosome 21, and with age a majority of DS patients develop neuropathological hallmarks associated with Alzheimer’s Disease (AD), including amyloid plaque deposition and astrogliosis, as well as clinical dementia (cooccurrence of DS with AD is DS-AD). Amyloid precursor protein (APP) mutations are linked to AD, and DS patients have triplication of the APP gene, suggesting this as a contributing factor to the overlapping pathology. Research suggests astrocytes are regulators of both DS and AD disease progression. Astrocytes modulate synapses through the release of secreted proteins, and recent work suggests that astrocyte protein secretion is dysregulated in both DS and AD. The Allen lab identified >700 astrocyte-secreted proteins dysregulated in the Ts65Dn mouse model of DS at neonatal timepoints. Of interest, secretion of the pro-growth protein pleiotrophin (Ptn) was >4x down-regulated from Ts65Dn astrocytes, and subsequent investigations of Ptn knockout mice revealed that they phenocopy Ts65Dn mice in many aspects, including decreased dendrite length and spine density. Single nucleus RNA- Sequencing studies show that subsets of “disease-associated” astrocytes in AD down-regulate transcripts encoding for many pro-synaptogenic factors, including Ptn. The main hypotheses of this proposal are that: 1) a network of overlapping astrocyte-secreted proteins is altered in DS-AD and AD, and 2) decreased Ptn secretion from astrocytes contributes to disease progression in DS-AD and AD. This proposal takes an unbiased approach to characterize changes in the astrocyte secretome in DS-AD and AD mouse models, as well as a targeted approach to investigate the potential for Ptn to rescue neuropathological phenotypes. Aim 1/K99 utilizes biotin- mediated proximity labeling (an endoplasmic reticulum localized TurboID virus) to create novel datasets for the in vivo astrocyte-specific secretome in DS-AD and AD mouse models at early, middle and late stages of disease. This aim provides the investigator with extensive training in mass spectrometry technology and quantitative proteomics analysis. Aim 2/R00 employs viral-mediated Ptn overexpression in astrocytes to investigate if Ptn can rescue spine density, astrogliosis and spatial memory impairments in DS-AD and AD mouse models. Additionally, the investigator will utilize the astrocyte-specific secretome datasets from Aim 1 to investigate other protein candidates in their future laboratory. The mentoring team consists of Dr. Nicola Allen, a leader in astrocyte biology; Dr. Alan Saghatelian, who will provide expertise in quantitative proteomics; Dr. Jolene Diedrich, mass spectrometry core director; Dr. Nick Andrews, behavioral core director; and Dr. Douglas Galasko, a neurologist specializing in dementia and Associate Director of the Alzheimer’s Disease Research Center (ADRC) at the University of California San Diego (UCSD). The work will take place at the world class Salk Institute for Biological Studies and establish networking connections at the ADRC and UCSD, providing an essential foundation for Dr. Brandebura’s independent research career focused on astrocyte-secreted proteins in neurodegeneration.

Project Narrative Down Syndrome (DS) is the most common chromosomal disorder with intellectual disability and DS patients acquire neuropathological hallmarks of Alzheimer’s Disease (AD), with a lifetime risk of >80% for developing clinical dementia (DS-AD). The existence of an aging DS population, along with an accelerated age of onset compared to the more general AD population, represents a significant public health concern and a dire need for research focused on the molecular mechanisms that contribute to DS-AD. A growing accumulation of studies highlight astrocyte-secreted proteins as key molecular modulators of characteristic phenotypes in DS and in AD, and this proposal will take both an unbiased approach to characterize astrocyte-specific secretome changes in DS-AD and AD as well as a targeted approach to investigate the potential of one protein candidate, Pleiotrophin, to ameliorate disease progression in DS-AD and AD mouse models.