Cell therapies for Parkinson’s disease (PD) have most often involved the implantation of dopaminergic neurons in the striatum, with the goal of reintroducing a supply of dopamine for striatal neurons. While this technique has received substantial research interest and has immense potential, it is fundamentally a shortcut. Dopamine loss in PD actually occurs after significant degeneration of an entire brain pathway called the nigrostriatal pathway. The real location of the dopaminergic neurons is in a region called the substantia nigra, from where they project long axons that ultimately reach and connect with neurons in the striatum. Implanting neurons directly into the striatum ignores this anatomy, and may have lead to some of the problems observed with prior human transplantation studies. In the human brain, substantia nigra pars compacta dopaminergic neurons receive primarily inhibitory input from the striatum, globus pallidus and substantia nigra pars reticulata, which decreases the release of dopamine, and primarily excitatory input from the ipsilateral subthalamic nucleus and pedunculopontine nucleus, serving to decrease the output of these neurons. For optimal clinical benefit, it may be essential to have well-regulated dopamine release in the motor circuit of the brain. A different method of cell therapy that restores the nigrostriatal pathway could adequately address one of the main causes of motor symptoms in PD, improve outcomes for patients, and minimize the potential for side effects.

What if we could build engineered neural tissue in the lab that resembles the nigrostriatal pathway and that can then be implanted as a replacement for the degenerated neurons and axons? That is precisely what our team in Center for Neurotrauma, Neurodegeneration and Restoration at the Michael J. Crescenz VA Medical Center and the University of Pennsylvania is investigating, as described in recent publications (Gordían-Vélez et al., 2021, Brain Res. Bull., https://doi.org/10.1016/j.brainresbull.2021.07.016). The team has worked for several years in fabricating, characterizing, optimizing, and testing the tissue-engineered nigrostriatal pathways (TE-NSPs), which currently have the following properties:

1. A mass or aggregate of dopaminergic neurons derived from human stem cells located on one end of a three-dimensional tubular hydrogel.

2. The hydrogel encases the tissue and protects it during and after implantation and is comprised of hyaluronic acid (HA), a key part of the extracellular matrix in the brain.

3. A lumen filled with crosslinked collagen and laminin to ensure that the tissue can survive and grow.

4. Long axonal projections from the aggregate throughout the diameter and length of the matrix within the hydrogel.

5. Growth of axons to lengths up to at least 1 cm after 30 days of culture.

6. Expression of several markers of mature dopaminergic neurons.

7. The capacity to release dopamine upon electrical stimulation to levels that may be therapeutic in rats.

8. The axons of the TE-NSPs can physically connect with and release dopamine within an aggregate of striatal neurons.

9. The versatility to be fabricated with various hydrogel dimensions, including inner/outer diameters of 160/345 µm and 500/973 µm, and cell densities (3,000-100,000 cells per aggregate) according to the application.

We believe these properties make the TE-NSPs an ideal strategy to replace and restore the nigrostriatal pathway as a regenerative medicine-based treatment for PD. Our recent work has focused on evaluating these TE-NSPs after implantation in the brains of rats lesioned with the 6-OHDA neurotoxin in order to exhibit nigrostriatal degeneration and deficits similar to those of PD. We observed that after 3 months, human TE-NSPs survived and had well-preserved axon tracts. Brains implanted with TE-NSPs had significantly higher dopaminergic innervation and dopamine concentration in specific locations within the dorsal striatum, as measured with histology and voltammetry with live brain slices, respectively. We also implanted human TE-NSPs with a much higher cell dose closer to what would be needed in human brains as a preliminary test for the translatability of the TE-NSPs. These scaled-up TE-NSPs survived, had well-defined axon tracts, grew into the dorsal striatum, and remained functional within the brain even 6 months after implantation.

These results were promising and suggested potential avenues for future studies and optimization avenues. More work remains to improve the reproducibility and scalability of our fabrication method, to further optimize our human stem cell source, and to increase the capacity of the TE-NSPs to grow after implantation and directly reinnervate the striatum. An important challenge will be obtaining TE-NSPs with long enough axons (> 2.5 cm) for implantation in human brains while minimizing the duration of culture in the lab. We also envision testing these scaled-up TE-NSPs in large animal models such as lesioned swine. We are also actively planning and considering steps needed to turn the TE-NSPs into clinical products through our collaboration with a spinoff company from the lab called Innervace. We hope the sum of our efforts will result in a novel regenerative medicine alternative for PD patients that is more tailored to restoring circuit-level deficits and maximizing therapeutic benefits than current conventional treatments and cell therapies.

Wisberty Gordian-Velez, PhD is a Postdoctoral Fellow in Bioengineering and Neurosurgery at the University of Pennsylvania.     

John E. Duda, MD is the Director, Parkinson’s disease Research, Education and Clinical Center (PADRECC) at the Cpl. Michael J. Crescenz VA Medical Center and Professor of Neurology at the University of Pennsylvania School of Medicine. Dr. Duda has spoken at World Parkinson Congresses.

Dr. Kacy Cullen, PhD is the Director, Center for Neurotrauma, Neurodegeneration & Restoration, Corporal Michael J. Crescenz VA Medical Center and Associate Professor of Neurosurgery & Bioengineering at the University of Pennsylvania.                         

Ideas and opinions expressed in this post reflect that of the author(s) solely. They do not necessarily reflect the opinions or positions of the World Parkinson Coalition®