Graduation Year

2012

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Medical Sciences

Major Professor

Steven W. Barger

Keywords

autism, behavior, immune, sAPP-á, T-cell

Abstract

Autism is a heterogeneous neurodevelopmental disorder that is characterized by impaired social interaction and communication as well as restricted and repetitive behaviors and focused interests. In addition to the cardinal behavioral characteristics observed for diagnosis, autistic individuals exhibit a number of comorbid conditions as well as a variety of aberrant immunological features. Recent studies report that plasma from autistic children contained elevated levels of secreted amyloid precursor protein alpha (sAPP-á), the á-secretase cleavage product of the amyloid precursor protein (APP) which is ubiquitously expressed in the brain, spleen, thymus and other organs. Interestingly, the sAPP-á fragment functions as a neurotrophic and neuroprotective factor in the central nervous system (CNS) and is also implicated in T-lymphocyte activation in the periphery. These functions of sAPP-á, alongside the observation of the fragment at elevated levels in autism patients, led us to hypothesize that sAPP-á over-expression is associated with autism and may be involved in its pathophysiology by impacting the brain or the immune system, leading to autistic-like behavior. In order to determine how the sAPP-á fragment may affect autism pathophysiology, we generated transgenic mice that overexpress human sAPP-á and used molecular biology, immunohistochemistry and behavioral techniques to study them. Additionally, we examined sAPP-á levels in plasma from a unique cohort of autism patients, and levels of both sAPP-á and the á-C-terminal fragment (á-CTF) in post mortem brains from an unrelated cohort of autistic patients. Resulting data from studies on autistic patient tissue samples confirmed the increase in plasma sAPP-á seen in autism patients in an earlier study and identified elevations in the sAPP-á and á-CTF fragments of post mortem insular cortex brain homogenates from autistic children. Data from molecular biology studies on the human sAPP-á transgenic (TgsAPP-á) mice demonstrate that these mice mimic the human autistic condition, expressing elevated levels of hsAPP-á in both the brain and plasma. They also show increased synaptophysin expression, suggesting increased synaptic density, in the brain. Behavior tests showed that while they do not display anxiety, lack of motor coordination/balance or impaired cognition, TgsAPP-á mice are hypoactive and the males exhibit decreased preference for social interaction. Molecular biology, immunohistochemistry and flow cytometry experiments focusing on immune function in TgsAPP-á mice unearthed aberrant development and function of T-cells. Compared to those from wild-type littermates, splenocytes from TgsAPP-á mice consisted of decreased B-cell and increased T-cell populations and secreted increased interferon-ã (IFN-ã), interleukin-2 (IL-2) and IL-4 after T-cell mitogen challenge. We discovered alterations in T-cell and thymocyte populations within TgsAPP-á mouse thymi, which may be explained by data portraying reduced apoptosis and decreased pro-apoptotic signaling. Finally, the splenocytes from TgsAPP-á mice immunized with myelin oligodendrocyte protein displayed impaired recall memory function which may be influenced by reduced phosphorylation of æ-chain-associated protein kinase 70 (ZAP-70), an apparent trait in TgsAPP-á mice irrespective of immunization. Altogether, the data leads us to conclude that overexpression of the sAPP-á fragment is indeed associated with autism, and sAPP-á overexpression leads to hypoactivity and impaired social behavior as well as aberrant T-lymphocyte development and function. For autism, the data implies that sAPP-á overexpression may contribute to autism pathophysiology through its effects on the immune system, leading to the characteristic behaviors. The data also provides evidence for a likely association between sAPP-á over-expression and aberrant T-cell populations and function in autism patients. Studies of sAPP-á interaction with the immune system and in the brain at the molecular level are needed to further clarify the purpose of sAPP-á overexpression in autism patients.

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