Molecular mechanisms of AD pathogenesis

Pathological manifestation of Alzheimer’s disease (AD) precedes more than a decade before the appearance of clinical symptoms (e.g., cognitive impairment). Comparative studies on cognition and the presence of blood, cerebrospinal fluid (CSF), and neuroimaging biomarkers have demonstrate a 10 to 20-year preclinical phase that precedes the manifestation of symptomatic AD. During this preclinical phase both intra- and extra-neuronal pathologies including β-amyloid (Aβ) plaques, neurofibrillary tangles, dystrophic neurites (DNs), mitochondrial degeneration, and glial activation (Fig. 1) advance the onset of clinical symptoms. Currently, there is no effective drug to prevent or cure AD. Large numbers of failed clinical trials and the complexity of disease mechanism suggest that AD diagnosis and disease modifying strategies need to be targeted at early pre-clinical stage. Hence, identifying early diagnosis biomarkers and understanding the cellular/molecular mechanism of etiopathophysiology are prerequisite for effective AD drug development.

The cellular homeostasis relies on spontaneous degradation of its undesired components through autophagy and endo-lysosomal pathways; both processes ultimately are targeted to the lysosomes. Increasing evidence suggests that autophagy-lysosomal dysfunctions contribute to the initiation and progression of AD by disrupting the degradation of toxic molecules such as Aβ and tau. We reported that accumulation of dysfunctional lysosomes and ER-mitochondria inclusion in Aβ core surrounding DNs occurs during Aβ plaque initiation in AD brains. The accumulation of cellular organelles and proteins in DN is a distinguishing feature in AD brains because diffused Aβ plaques those devoid of such DN formation are commonly found in non-demented elderly brain.

Our long-term research goal is to determine novel pathways that are associated with neuronal/glial homeostatic dysfunction in AD and to target DN forming organelles/proteins (Fig. 1) for developing early AD diagnostic biomarkers and delivering Aβ targeted drugs by focusing on specific areas of research.

Fig 1. A neuritic plaque in AD brain is composed with an Aβ-deposited core that is surrounded by dysfunctional organelles /proteins accumulated DNs and activated glial cells. Our research focuses to understand organelle dysfunction mechanism during plaque initiation and to utilize accumulated organelles in DNs for identifying early diagnostic biomarkers and delivering Aβ-targeted drugs.

Areas of Research

Lysosomal clustering in DNs

Lysosomal clustering in AD brain-therapeutic intervention and diagnostic potential

The accumulation of diversified proteins and organelles in DN surrounding amyloid plaques correlate with the dysfunction of various neuronal processes during AD progression. We have shown that hydrolase deficient, but saposin enriched, lysosomes are massively clustered in DNs during neuritic plaque formation in AD brain. Our current studies aim to determine whether:

a. clustered lysosomes in AD brain and lysosomal proteins in bio-fluids can be used as AD early diagnostic biomarkers.
b. lysosomal accumulation can be targeted for AD drug development and drug delivery.
c. a saposin/ferritin-mediated pathway plays a key role in glial/neuronal dysfunction in AD.

Fig:Saposin (SAP)-C and prosaposin (PSAP) enriched lysosomes are accumulated in thioflavin-S (Thi-S) labeled Aβ-plaque surrounding area in APP knock-in (APPNL-G-F), 5xFAD, and APP-PS1delE9 (PA) mouse brains.

Mitochondria dysfunction in AD

Efficient trafficking and clearance of aged mitochondria in neurons is critical for providing energy during synaptic activity. We found that mitochondrial density is increased at the synaptic site, which is likely to provide sufficient energy for the synapse. However, to continue the required energy delivery, aged mitochondria need to be transported back to the neuronal soma for clearance via mitophagy. We report that axonal mitochondria are trapped and degenerated in a tubular endoplasmic reticulum (ER) clustered area of AD brain. A dynactin pointed end subunit, ACTR10, mediate retrograde trafficking of mitochondria in neurons, while Nipsnap1, a mitochondrial matrix protein facilitates mitophagy by accumulating on the mitochondrial surface upon depolarization of mitochondria. Our research aims to elucidate the role of Nipsnap1 and ACTR10 in mitochondrial degeneration in AD.

Fig:Distribution of mitochondria in a synapse participating axon participating in synapsis was reconstructed from a series of 3-dimentional electron microscopy (3D-EM) images obtained from a 6-month-old wild type mouse brain. Mitochondria were clustered at the synaptic site in the axon.

Serum/plasma Aβ aggregates and peripheral monocytes

Utilizing serum/plasma and peripheral monocytes for AD diagnosis and understanding pathogenesis mechanism - The presence of higher level of plasma matrix proteins such as albumin alpha2-macroglobulin, lipoprotein, etc., hinders the accuracy of immunoassays to detect the amount of Aβ in serum/plasma samples. Evidence suggests that plasma Aβ42 and Aβ40 levels are associated with the levels of CSF Aβ42/Aβ40 and cerebral amyloid deposition. Regardless of the amount of monomeric Aβ42 or Aβ40, we aim to determine the potential Aβ aggregates in serum using seed extension methods to diagnose AD using noninvasive methods.

Fig: Aβ aggregation progresses via two steps, nucleation and fibril extension. During nucleation, smaller aggregates are formed by assembly of multiple monomeric species. During the extension phase, monomers are attached to existing oligomers that extend the fibrils. The nucleation (seed formation/lag) phase can be omitted by adding preformed oligomers/fibrils/seeds in the reaction mixture.

It has been shown that significant portion of brain macrophages are migrated from peripheral monocytes upon acute or chronic brain injuries. We aim to identify subsets of peripheral monocytes that potentially participate in inflammatory response and migrate to brain in AD patients.

Additional Resources

Featured Publications

View a complete list of research studies led by Dr. Golam Sharoar.

Md. Golam Sharoar, John Zou, Marc Benoit, Wanxia He and Riqiang Yan. DCTN6 deficiency enhanced aging associated dystrophic neurites formation in mouse brains. Neurobiology of Aging, 107: 21-29; Nonmember, 2021. (Corresponding author). doi.org/10.1016/j.neurobiolaging.
2021.07.006.
Md Golam Sharoar, Sarah Palko, Yingying Ge, Takaomi C Saido and Riqiang Yan. Accumulation of saposin in dystrophic neurites is linked to impaired lysosomal functions in Alzheimer’s disease brains. Molecular Neurodegeneration, 16(1):45; Jul 2021. doi: 10.1186/s13024-021-00464-1.
Md Golam Sharoar, Xiangyou Hu, Xin-Ming Ma, Xiongwei Zhu and Riqiang Yan. Sequential formation of different layers of dystrophic neurites in Alzheimer’s disease brains. Molecular Psychiatry, 24(9):1369-1382; Sep 2019. doi: 10.1038/s41380-019-0396-2. Featured figure: Dystrophic neurites labeled by pre-autophagosomal proteins in Alzheimer's brains. Molecular Psychiatry, 24(9):1247; Sep 2019. doi: 10.1038/s41380-019-0476-3.
Md. Golam Sharoar, Qi Shi, Yingying Ge, Wanxia He, Xiangyou Hu, George Perry, Xiongwei Zhu and Riqiang Yan. Dysfunctional tubular endoplasmic reticulum induces formation of dystrophic neurites in Alzheimer’s disease. Molecular Psychiatry, 21:1263-1271; Sep 2016. doi: 10.1038/mp.2015.181.
Md. Golam Sharoar, ArjunThapa, Md. Shahnawaz, Vijay Samkar Ramasamy, Eun-Rhan Woo, Song Yub Shin & Il-Seon Park. Keampferol-3-O-rhamnoside abrogates amyloid beta toxicity by modulating monomers and remodeling oligomers and fibrils to non-toxic aggregates. Journal of Biomedical Science, 19:104; Dec 2012. doi: 10.1186/1423-0127-19-104 (Corresponding author).

Lab Members

Md Golam Sharoar, PhD
Assistant Professor, Internal Medicine & Alzheimer’s Disease Research Program

Zakia Zaman
Research Assistant