Basic Science and Pathogenesis.

Area of Science:

• Immunology
• Neuroscience
• Genetics

Background:

• The sialic-acid binding immunoglobulin-like lectin 3 receptor (Siglec-3 / CD33) is a gene strongly associated with Alzheimer's disease (AD) risk.
• Specific AD-risk alleles correlate with increased CD33 expression, particularly the long CD33^M splice form.
• The precise molecular mechanisms underlying CD33's immuno-inhibitory function remain largely unknown.

Purpose of the Study:

• To investigate the dimerization of CD33 isoforms and its functional implications in AD.
• To elucidate the molecular basis of CD33's role in AD pathology.
• To explore potential therapeutic strategies targeting CD33 in AD.

Main Methods:

• Blue Native Gel electrophoresis and co-immunoprecipitation to detect CD33 dimers.
• Flow cytometry to quantify cell-surface CD33 expression.
• Single-molecule fluorescence resonance energy transfer (smFRET) with TIRF imaging to visualize CD33 dimers.
• Western Blotting to assess CD33 phosphorylation and downstream signaling.

Main Results:

• CD33^M and CD33^m isoforms form both homodimers and heterodimers.
• CD33^M isoforms are preferentially transported to the cell surface.
• smFRET confirmed the presence of CD33 dimers on the cell surface, and CD33^M homodimers were shown to be functional.

Conclusions:

• CD33^M plays a critical role in AD pathology through the formation of functional dimers.
• Only CD33^M isoforms are trafficked to the cell surface and form active dimers.
• These findings provide crucial insights into CD33's immuno-inhibitory function and its link to AD, suggesting new therapeutic avenues.